APOLLO IST-2001-34372 Intelligent Tyre for Accident-free Traffic
Intelligent Tyre Systems – State of the Art and Potential Technologies Deliverable D7
Report Version: 1 Report Preparation Date: 22.05.2003 Classification: Public Contract Start Date:
01.03.2002
Duration: 36 months
Project Co-ordinator: Technical Research Centre of Finland (VTT)
FI
Partners:
DaimlerChrysler AG (DC AG)
D
Helsinki University of Technology (HUT)
FI
Rheinisch Westfälische Technische Hochschule Aachen (RWTH Aachen)
D
IXFIN Magneti Marelli Sistemi Elettronici S.p.A (MM)
I
Nokian Tyres plc (NR)
FI
Pirelli Pneumatici SPA (PIRELLI)
I
Project funded by the European Community under the “Information Society Technology” Programme (1998-2002)
APOLLO Intelligent Tyre for Accident-free Traffic Deliverable D7 Intelligent Tyre Systems – State of the Art and Potential Technologies _____________________________________________________________________________
DELIVERABLES SUMMARY SHEET
Project Number:
IST-2001-34372
Project Acronym:
APOLLO
Title:
Intelligent Tyre for Accident-free Traffic
Deliverable N°:
D7
Due date:
31.12.2002
Delivery Date:
22.05.2003
Short Description:
Intelligent Tyre Systems – State of the Art and Potential Technologies
This document provides an overview on the state of the art of intelligent tyre systems and potential technologies that can be utilised for sensors, wireless data transmission, and batteryless power supply. An accident analysis with an investigation of tyre-related risk factors shows the great benefit of the envisaged system to improve traffic safety. This fits to visions and strategies of vehicle manufacturers, automotive suppliers and tyre manufactures in respect to intelligent tyre/wheel systems that are summarised. The first products that have been introduced in the field of intelligent tyres are Tyre Pressure Monitoring Systems (TPMS). Different system solutions for TPMS and activities of suppliers are described. More sophisticated sensor systems, that are still in the process of research or pre-development, show the high interest in this field. Basic sensor technologies which enhance the realisation potential for innovative monitoring of tyre and tyre-road contact are presented. Radio communication is a technology with a high potential for high performance wireless data transmission. Applications for different systems, future trends, aspects of vehicle integration and standards are discussed. First investigations on inductive transmission and power generation prove the potential of these technologies for the development of a batteryless power supply. Promising results of a pre-study, showing measurements of electromagnetic properties of the tyre/wheel system, are presented. An overview is given on selected patent applications for systems and key components of intelligent tyre/wheel systems. Following the approach of the APOLLO project is a promising perspective to achieve a successful product for an intelligent tyre/wheel system which allows to expect over 4 000 saved lives in all EU countries every year. Partners owning:
The APOLLO consortium
Partners contributed: VTT, DC AG, HUT, RWTH Aachen, MM, NR, PIRELLI Made available to:
IST-2001-34372
European Commission, Information Society Directorate-General
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Table of Contents
1. 2.
Introduction ...................................................................................................................5 Accident Analysis..........................................................................................................8 2.1 Introduction ...............................................................................................................8 2.2 Finland ......................................................................................................................9 2.3 Germany .................................................................................................................11 2.4 Conclusion ..............................................................................................................13 3. Trends and Strategies ................................................................................................15 3.1 Vehicle manufacturers.............................................................................................15 3.2 Automotive electronics suppliers .............................................................................17 3.3 Tyre manufacturers and co-operations....................................................................19 4. Tyre Pressure Monitoring Systems (TPMS)................................................................21 4.1 Indirect measurement..............................................................................................21 4.2 Direct measurement - active sensors ......................................................................22 4.2.1 Battery-operated sensor technology in general.................................................22 4.2.2 Clamp-on-rim sensors.......................................................................................23 4.2.3 Valve-attached sensors.....................................................................................25 4.2.4 Valve-cap-integrated sensors ...........................................................................31 4.3 Direct measurement - passive sensors ...................................................................34 4.3.1 Introduction on batteryless sensor technology ..................................................34 4.3.2 Batteryless TPMS at 2.4 GHz ...........................................................................35 4.3.3 Other systems...................................................................................................36 4.4 Conclusion ..............................................................................................................38 5. Advanced Tyre Sensor Systems.................................................................................39 5.1 Side Wall Torsion sensor ........................................................................................39 5.2 Darmstadt tyre sensor .............................................................................................41 5.3 Surface Acoustic Wave sensor ...............................................................................43 5.4 Conclusion ..............................................................................................................45 6. Basic Sensor Technologies ........................................................................................46 6.1 Introduction .............................................................................................................46 6.2 Acoustic sensor.......................................................................................................46 6.3 Optical sensor .........................................................................................................46 6.4 Vibrating string sensor.............................................................................................49 6.5 Ultra Wide Band technology....................................................................................50 6.6 Capacitive sensor....................................................................................................52 6.6.1 Micromechanical sensor ...................................................................................52 6.6.2 Capacitive displacement sensor .......................................................................53 6.6.3 Measurement of capacitance ............................................................................55 6.7 Conclusion ..............................................................................................................56 7. Basic Technologies for Wireless Data Transmission ..................................................57 7.1 Technology overview...............................................................................................57 7.1.1 Classification of wireless data transmission ......................................................57 7.1.2 Data transmission of passive wireless sensors.................................................58 7.2 Existing wireless vehicle applications......................................................................60
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7.3 Standards and regulations ......................................................................................62 7.4 Trends .....................................................................................................................63 7.4.1 Wireless sensors using active radio communication .........................................63 7.4.2 Vehicle applications using active radio communication.....................................64 7.5 Conclusion ..............................................................................................................65 8. Basic Technologies for Batteryless Power Supply......................................................67 8.1 Introduction .............................................................................................................67 8.2 Inductive power transmission ..................................................................................67 8.3 Power generation ....................................................................................................73 8.3.1 Capacitive generator .........................................................................................74 8.3.2 Piezo generator.................................................................................................77 8.3.3 Summary...........................................................................................................80 8.4 Conclusion ..............................................................................................................81 9. Physical Properties of Tyre/Wheel System .................................................................82 9.1 Tyre attenuation at 434 MHz, 869 MHz, and 2.45 GHz...........................................82 9.2 Electromagnetic properties......................................................................................85 9.2.1 Introduction .......................................................................................................85 9.2.2 Measurement results of permittivity...................................................................85 9.2.3 Attenuation of magnetic field below 100 MHz ...................................................89 10. Patent Overview .........................................................................................................92 10.1 Accelerometer and other sensors ...........................................................................93 10.2 Tyre integration .......................................................................................................93 10.3 Antenna...................................................................................................................95 10.4 Power transmission / generation .............................................................................96 10.5 General aspects of total system ..............................................................................96 10.6 Conclusion ..............................................................................................................98 11. Abbreviations and References....................................................................................99
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APOLLO Intelligent Tyre for Accident-free Traffic Deliverable D7 Intelligent Tyre Systems – State of the Art and Potential Technologies _____________________________________________________________________________
1. INTRODUCTION Objectives The objectives of the APOLLO project producing a prototype for an intelligent tyre/wheel system are listed as follows: 1. To increase traffic safety by adding an intelligent tyre/wheel system to advanced vehicles which provides data on the particular tyre and the given tyre-road contact. 2. To enable improvements for chassis/vehicle control systems, Advanced Driver Assistance Systems (ADAS), and driver information. 3. To enable the introduction of innovative services concerning tyre and road conditions for different user groups outside the vehicle. The term “intelligent tyre/wheel system” does not literally mean that “intelligence” resides inside the tyre/wheel system. The APOLLO project aims at making additional data from the tyre and the tyre-road contact available at a communication interface on the vehicle. The objectives are met by integrating innovative sensors into tyres and developing advanced solutions for wireless data transmission and batteryless power supply. A mechatronic tyre/wheel system is constructed by integrating all electronics into the tyre. [MAE02] State of the art and potential technologies for intelligent tyre systems The main goal of the APOLLO project is to increase traffic safety. To show the relevance of tyre related risk factors in accident scenarios, an accident analysis was performed (Chapter 2). The results with a more detailed description of the situation in the European countries Finland and Germany are presented. The basic technologies to be investigated in the APOLLO project are as follows: •
Sensor technology,
•
Wireless data transmission and
•
Batteryless power supply.
A crucial point of the state of the art study are sensors and sensor technologies (Chapter 4 - 6). In this context, Tyre Pressure Monitoring Systems (TPMS) – the first products that have been introduced in the market -, advanced tyre sensors and basic sensor technologies are described. Already a TPMS contains more components than a single sensor. Therefore, important aspects of system design and system integration into a tyre/wheel system can be derived by studying these systems (Chapter 4). Some advanced tyre sensor systems, aiming e.g. at additional data on tyre forces or friction information, are being developed since nearly 10 years, but products are not available in the market yet. These sensor systems and the status of their development is presented (Chapter 5). In addition, basic sensor technologies, that can be used for future developments of intelligent tyre/wheel systems, are described, including the promising technology of capacitive sensors (Chapter 6).
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Key technologies for intelligent tyre/wheel systems are technologies for wireless data transmission and batteryless power supply. Inductive transmission and radio transmission are the basic technologies used for wireless data transmission (Chapter 7). Radio transmission is already used for passive systems. There are a lot of developments and research activities utilising radio transmission for active systems, too, but still the power consumption needed for this technology is a big hurdle. Aspects of vehicle integration and frequency bands and regulations for short range radio devices are described. Inductive transmission and power generation are basic technologies for batteryless power supply of electronic systems. The principles of inductive power transmission are described, and an overview is given on ongoing developments and research activities on different methods of power generation technologies (Chapter 8). The ongoing evolution and high innovation rate in these technology fields are a strong support for the future product envisaged in the APOLLO project. Constructing a mechatronic tyre/wheel system means the integration of electronics into the tyre/wheel system. This mechatronic system has to operate properly in a vehicle environment under all operating conditions. For the investigation and the selection of appropriate technologies to be used for such a system it is necessary to know the basic electromagnetic properties of a tyre/wheel system. Therefore, a pre-study was performed to measure these parameters (Chapter 9). First measurements and simulations which are presented show promising results indicating that the relevant frequency bands can be used to set up a mechatronic tyre/wheel system. It is not possible to give a complete overview on the subject of patent applications in the field of intelligent tyre/wheel system, because of high and ongoing activities in this field. Therefore, only important patent applications for key systems or key components are summarised (Chapter 10). The state of the art study of sensors and the patent application overview show that there is still a big gap between a lot of inventions and series products which are available on the market. Therefore, it is worthwhile to pursue the approach of the APOLLO project where new technologies are used for the development of an innovative tyre/wheel system which is suitable for the vehicle environment including the production process, vehicle operation and product recycling. Overview This document provides an overview on the state of the art of intelligent tyre systems and potential technologies that can be utilised for sensors, wireless data transmission, and batteryless power supply. An accident analysis with an investigation of tyre-related risk factors shows the great benefit of the envisaged system to improve traffic safety. This fits to visions and strategies of vehicle manufacturers, automotive suppliers and tyre manufactures in respect to intelligent tyre/wheel systems that are summarised. The first products that have been introduced in the field of intelligent tyres are Tyre Pressure Monitoring Systems (TPMS). Different system solutions for TPMS and activities of suppliers are described. More sophisticated sensor systems, that are still in the process of research or predevelopment, show the high interest in this field. Basic sensor technologies which enhance the realisation potential for innovative monitoring of tyre and tyre-road contact are presented.
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Radio communication is a technology with a high potential for high performance wireless data transmission. Applications for passive systems, trends towards radio transmission for active systems, aspects of vehicle integration and standards are discussed. First investigations on inductive transmission and power generation prove the potential of these technologies for the development of a batteryless power supply. Promising results of a pre-study, showing measurements of electromagnetic properties of the tyre/wheel system, are presented. An overview is given on selected patent applications for systems or key components of intelligent tyre/wheel systems. Following the approach of the APOLLO project is a promising perspective to achieve a successful product for an intelligent tyre/wheel system which allows to expect over 4 000 saved lives in all EU countries every year.
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2. ACCIDENT ANALYSIS 2.1
INTRODUCTION
European Transport Safety Council’s (ETSC) general reports on EU fatalities illustrate that each year 42 000 EU citizens are killed and over 3.5 million are injured in transport crashes. These accidents cost over 166 billion Euros and are the leading cause of death and hospital admission for citizens under 45 years. As shown in Figure 2.1-1 the number of deaths varies strongly in ETSC member countries. This is partly due to differences in traffic volume, but also other reasons can be seen. European-wide accident comparison is impossible, because the availability of comparable in-depth data is limited. Compilation of statistics and accident related definitions differ from one country to another. No representative statistical data are available on road traffic accidents in Europe that are caused by a) tyre defects and b) accidents caused by drivers not taking into account adverse road conditions. The following studies give, however, a good idea that we are facing a severe safety problem caused either by defective tyres, adverse road conditions or their combination. Finnish accident analysis shows that defective tyres were either a contributing factor or a main cause in about 16 % of all fatal accidents. According to a report from German Traffic Safety Committee, more than half of the accidents with personal injury are caused by slippery tracks due to rain, ice and snow. Every year 40 people are killed in Germany and over 2 000 are injured due to defective tyres only (German Traffic Safety Committee). [HUT1] For the accident analysis two European countries – Finland and Germany – are selected. Finland represents a Nordic country with a long winter period. Germany represents a country with a high traffic volume and density.
9000 8000 7000 6000 5000 4000 3000 2000
LUX
FIN
IRL
DK
S
A
NL
B
P
GR (1999)
UK
E
I
F
0
D
1000
Figure 2.1-1: EU road deaths in 2000. Source: IRTAD, OECD, 2002 [HUT2]
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2.2
FINLAND
The analysis is based on the reports of VALT (Finnish Motor Insurers’ Centre) and on the investigation database of the Finnish Road Accident Investigation Teams, [HUT4]. The situation in Finland is monitored by 21 teams having approximately 240 members in total. Every fatal road accident is thoroughly investigated by these teams consisting of police-, vehicle engineering-, traffic engineering-, doctor- and behavioural science- members. The lead-up to the accident, risk factors, consequences and circumstances are documented into the album and electronic database. By studying the figures and table presented below, it is evident, that tyre defects have a significant role in fatal accidents. Their role is especially high in slippery and wet road conditions. Defective tyres were either a contributing factor or a main cause in 16 % (483 accidents) of all fatal accidents (2 980 accidents) in Finland in 1991 - 2001. In two out of three tyre related accidents either worn-out tyres or tyres unfit for road conditions were a major contributory factor. Under-inflated tyres were a risk factor in 12 % of tyre related accidents (see Figure 2.2-1). Moreover, it is noteworthy that these figures do not include accidents where adverse road conditions alone - even when tyres have been good - have been a contributory factor which is also a target area for intelligent vehicle control systems.
TYRE RELATED RISK FACTORS IN FATAL ACCIDENTS IN 19912001 (FINLAND) Other tyre risks Under-inflated tyres 5,73% 12,15% Different tyre pressures in tyres 4,17%
Worn-out tyres with studs 19,79%
Worn-out tyres 18,75%
Tyres unfit for road conditions 25,52%
Tyres with different properties Tyres unfit for the 12,67% vehicle 1,22% NOTE: TYRE RISK FACTOR WAS INVOLVED IN 483 ACCIDENTS, WHICH IS 16,2% OF ALL 2980
Figure 2.2-1: Tyre related risk factors by type of the tyre defect. [HUT3] The effect of defective tyres varies strongly according to the road conditions. In snowy or icy conditions the percentage of accidents, where tyre defect has had an effect is significantly high –
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about 38 %. In these conditions, the most common deficiencies are the tyres unfit for road conditions (about 15 %) and worn-out tyres with studs (about 14 %). In wet road conditions defective tyres have been reported in 14 % of all accidents. The most important risk factor in wet road conditions is worn-out tyres (about 8 %) (see Table 2.2-1). Table 2.2-1: Tyre related risk factors by type of the defect and by road conditions. [HUT3] TYRE DEFICIENCIES AS RISK FACTOR IN OCCUPANT-FATAL ACCIDENTS IN 1991-2001 (FINLAND) ROAD SURFACE CONDITIONS BARE, DRY RISK FACTORS OF TYRES Under-inflated tyres 1) Different tyre pressures in tyres 1) Worn-out tyres 1) Tyres with different properties 1) Tyres unfit for the vehicle 1) Tyres unfit for road conditions 1) Worn-out tyres with studs 1) Other tyre risks 1) TOTAL, ACCIDENTS INVOLVING TYRE DEFECT 2) TOTAL, ACCIDENTS INVESTIGATED
WET
SNOWY OTHER TOTAL OR ICY
N % 3) N % 3) N % 3) N % 3) 38 2,2 8 1,7 24 3,0 0 0,0 8 0,5 9 1,9 7 0,9 0 0,0 31 1,8 37 7,8 39 4,9 0 0,0 16 0,9 10 2,1 46 5,8 1 7,1 3 0,2 1 0,2 3 0,4 0 0,0 9 0,5 17 3,6 116 14,5 5 35,7 3 0,2 2 0,4 109 13,7 0 0,0 14 0,8 3 0,6 15 1,9 1 7,1 108 6,4 67 14,1 302 37,8 6 42,9 1692 100 476 100 798 100 14 100
N % 3) 70 2,3 24 0,8 107 3,6 73 2,4 7 0,2 147 4,9 114 3,8 33 1,1 483 16,2 2980 100
1) Number of accidents involving risk factor in question 2) Number of accidents involving tyre related risk factors 3) Percentage of accidents investigated in the category
A more thorough investigation was made using the investigation database of the Finnish Road Accident Investigation Teams. The objective was to study such fatal accidents which are characterised by search parameters that are relevant for tyre monitoring and monitoring of tyreroad contact, and based on this study to describe specific “accident scenarios” for later use in the APOLLO project. These specific “accident scenarios” mean in this context accidents, which possibly could be avoided by using an intelligent tyre/wheel system - the technical objective of the APOLLO project. In this study years 1998 - 2001 were chosen as a reference period. During this period 1 025 fatal accidents were recorded (1998: 254, 1999: 261, 2000: 240 and 2001: 270). The main search parameter was the loss of driving control. Other parameters (e.g. condition of tyres, abnormal weather, road conditions, aquaplaning) varied in different searches. The percentage figure is calculated in such a way that only accidents, where all marked search parameters are relevant, are included (see Table 2.2-2).
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Table 2.2-2: Study of fatal accidents in 1998 - 2001 (Finland). [HUT4]
Year
1998
1999
2000
2001
Search parameters Tyres Dry or only Loss of Percentage of as a Winter slightly Dry driving Aquaplaning all accidents in risk conditions moist weather control that year % factor weather X X X 9,5 X X X 2,4 X X 5,5 X X 0,4 X X X 8,4 X X X 1,9 X X 8,4 X X 0,8 X X X 11,0 X X X 4,2 X X 12,1 X X X 10,0 X X X 3,0 X X 6,3 X X 0,8
The investigation shows, that the percentage value of accidents with the same search parameters stays roughly constant year after year. The road condition is the most significant single parameter causing the loss of control. The icy and snowy roads are naturally strongly represented in casualties in Finland. When investigating casualties on a dry road surface it can be seen, that excessive speed or alcohol is the main cause for the accidents. In this study these both factors are included, because also those accidents might be prevented by the application of an intelligent tyre/wheel system. The most significant factor causing the loss of control directly associated with vehicle is the condition of its tyres. Also this investigation reveals that tyre defects are strongly presented in accidents in adverse road conditions.
2.3
GERMANY
As shown in Figure 2.3-1, the accidents with personal injury referenced to 1 000 vehicles in the Federal Republic of Germany decrease continuously since 1980. The only eye-catching exception in the declining of the rate is the increase of fatalities in 1991, which can be explained by the reunification of Germany. It is worth mentioning that the number of vehicles in Germany increased from 27 million in 1980 to about 51 million vehicles in the year 2000. [BMV99], [SBA02]
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Fat. 0.5
20 18
0.4
16 14
0.3
12
0.2
10 8
0.1
Fatalities per 1000 vehicles Injured per 1000 vehicles
Inj.
6 4
2 0 0 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 year
Figure 2.3-1: Road traffic accident fatalities and injured per 1 000 motor vehicles in Germany 1980 - 2000. The graphs in Figure 2.3-1 prove that it was possible to reduce the rate of injured persons in the last 20 years about 47 %, while the rate of deadly injured persons has fallen about 70 %. The main reasons for that intense reduction is the continuously rising level of technical improvement of vehicles in the field of passive safety (airbag, side impact protection, etc.) as well as active safety (ABS, ESP, etc.).
100%
others
90% animals on track
80%
sight obscuration due to fog slipperiness due to ice and snow slipperiness due to rain
70% 60% 50% 40% 30% 20% 10% 0% 1999
2000 year
2001 © Statistisches Bundesamt, Wiesbaden 2002
Figure 2.3-2: General causes of accidents with personal injury. The federal statistical office in Germany has analysed accidents with personal injury in term of its causes. General causes (Figure 2.3-2) and technical faults (Figure 2.3-3) are distinguished as
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causal factors. Looking into the general causes in Figure 2.3-2, it can be seen that slippery roads due to rain, ice and snow cause more than 55 % of the accidents with personal injury. It can be concluded, that the lack of information on the current road conditions is an important influencing factor for these accidents. Therefore, it can be expected that the appropriate use of additional information on road condition in vehicle applications offers great potential in decreasing road traffic accidents. [SBA02]
100% others braking system tyre
80%
60%
40%
20%
0% 1999
2000 year
2001 © Statistisches Bundesamt, Wiesbaden 2002
Figure 2.3-3: Accidents with personal injury caused by technical faults. The second cause, which is mentioned by the federal statistical office, is technical faults of the vehicle. Technical faults of the brake system, the tyre and other technical faults are distinguished. Figure 2.3-3 shows the percentage of accidents caused by technical faults in the period of years 1999 – 2001. About 30 % of all accidents, which are due to technical faults, are caused by defected tyres. [SBA02]
2.4
CONCLUSION
The high potential of accident prevention by using an intelligent tyre/wheel system can be clearly seen through this accident analysis. It has been shown, that adverse road conditions, tyre defects or their combination play an important role in road accidents. Accident analysis reveals, that detecting adverse road conditions is of great interest from traffic safety point of view. The road condition is the most significant single parameter causing the loss of driving control. For example, in Germany 55 % of accidents with personal injury are caused by slippery roads. Giving information about adverse road conditions to the vehicle applications, to the driver and to other road users have high potential in diminishing the impacts of accidents and preventing accidents. With this information drivers have the opportunity to adapt their speed and
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their driving behaviour to the current conditions. Additionally, existing active safety systems such as ABS and ESP are able to work much more effectively, if current road conditions can be used as a parameter in control algorithms. This information is missing today, and deriving this information from tyre-road contact patch is one of the objectives in the APOLLO project. Tyre-related risk factors were recorded in 16 % of all fatal accidents in Finland during the period of 1991 - 2001. In two out of three tyre-related accidents either worn-out tyres or tyres unfit for road conditions were a major contributory factor. Under-inflated tyres were a risk factor in 12 % of tyre related accidents. Defective tyres have a significant role in fatal accidents especially in adverse road conditions. According to the federal statistical office in Germany, defective tyres cause about 30 % of all accidents, which are due to technical faults. These figures show, that monitoring tyre condition and detecting tyre defects are important objectives with respect to accident prevention. There is a clear need for monitoring not only tyre pressure, but also tyre wear and damage. It is impossible to give any exact figure of safety benefits resulting from an intelligent tyre/wheel system. Accident analysis suggests that the decrease in the number of fatalities, provided that the entire car fleet is equipped with intelligent tyre systems, could be according to a conservative estimate at least 10 % of accidents. This would mean that over 4 000 lives could be saved every year in EU countries.
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3. TRENDS AND STRATEGIES 3.1
VEHICLE MANUFACTURERS
The vision of accident-free traffic is one of the most challenging visions in today’s and future automotive market. Therefore, the improvement of traffic safety is a strong motivation for many important innovations in automotive industry. To improve traffic or driving safety is not a specific task of some dedicated systems, but it can only be achieved by an optimised integration of different vehicle systems such as Advanced Driver Assistance System, driver information, power train management, chassis systems and external services (see Figure 3.1-1). In Figure 3.1-1 the tyre itself is added to the group of chassis systems, and all systems that contribute to reach the goal of accident free traffic are shown.
Advanced Driver Assistance System
Infrastructure Peripheral Data
Innovative vehicles for an accident free traffic
Power Train Management
External Services
Steering
Driver Information
Braking
Suspension
Tyre
Chassis and Tyre Systems
Figure 3.1-1: Systems and system integration needed for innovative vehicles and accident-free traffic. Beside the development of automotive systems the improvement of road construction and maintenance and traffic organisation are important tasks to achieve the goal of accident-free traffic. [BAL02], [FIS01], [STE01]
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Important trends in the development of safety relevant vehicle systems (see Figure 3.1-1) are to improve the integration or the co-operation of these systems and to make the systems more intelligent. Intelligent systems in this context means to add new capabilities such as •
gaining information / sensing,
•
actuation,
•
control,
•
evaluation of information and making decisions, or
•
providing information to the systems or presenting information to the driver respectively.
By these means a •
lack of information and an
•
incorrect acting of the driver or a control system
must be avoided to reach the overall goal of accident-free traffic. It is obvious, that an intelligent tyre system providing information on the tyre and the tyre-road contact is an important system, which can provide many additional information to a wide range of vehicle systems or applications. Examples of OEM’s activities and developments on a high level integration of vehicle systems are integrated driver assistance systems, networked chassis control, integrated chassis control and the integration of traction management and stability management. [BAC00], [HIE02], [KON02], [RIE02], [ZIE01] In the field of intelligent tyre systems Tyre Pressure Monitoring Systems (TPMS) have been the first products introduced in the market. This development was mainly driven by vehicle manufacturers. The basic functionality of a TPMS is to monitor tyre inflation pressure and temperature. The range of available solutions and products comprises various indirect and direct systems as well as simple or sophisticated means for the relevant driver information (see Chapter 4). The direct TPMS are using RF technology for transmitting sensor data to the vehicle and they are powered by batteries (see Chapter 7 and 8). Most direct TPMS are installed at the rim or attached to the valve. The TPMS products introduced by OEM’s are more or less proprietary systems. During last years, standardisation activities on these systems have been started, e.g. by European ISO committee or working groups of VDA (German Association of the Automotive Industry). These activities are focused on standards for data exchange and communication technologies. Other upcoming trends are the replacement of batteries by innovative solutions for electrical power supply and the integration of electronics directly to or into the tyre. A standardisation for the components needed at the vehicle is also important to achieve a non proprietary system. The need for standardisation – especially in case of a tyre integrated solution – can be emphasised by the requirement, that the customer should keep the free choice of a wide range of tyres from different suppliers for his vehicle. Another trend is to develop sophisticated sensors which can provide additional information for vehicle applications. These activities are more or less driven by automotive electronic suppliers or research organisations (see Chapter 5 and 6).
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European OEM’s have been the first players who introduced TPMS as a first step towards intelligent tyre/wheel systems in the automotive market. In 2002 a new standard on TPMS was established, that requires the installation of TPMS to warn the driver when the tyre is significantly under-inflated. This standard was published in a paper on final rule by the National Highway Traffic Safety Administration (NHTSA), Department of Transportation (DOT) in United States of America. This standard is called “Federal Motor Vehicle Safety Standards; Tire Pressure Monitoring Systems; Controls and Displays”. It makes TPMS mandatory for passenger cars, trucks, multipurpose vehicles, and buses (a gross vehicle weight rating of 10 000 pounds or less) and contains a short term realisation period from 2003 – 2006 and a long-term realisation period starting in 2006. This legislation is a strong driver for new developments, fast market introduction and standardisation of TPMS. It is very important for European OEM’s, because the automotive market in the USA is the biggest national market world wide with a high market share on exports from European countries. [NHT02] Beside current activities in the field of TPMS there is a big interest of OEM’s in gathering more information on the tyre-road contact, but up to know no solutions or products are available for series production.
3.2
AUTOMOTIVE ELECTRONICS SUPPLIERS
General aspects A wide range of different suppliers is active in the market of automotive electronics. There are small or medium sized companies, acting as component suppliers, and large companies, which are system suppliers for automotive manufactures. There is an ongoing trend of an increase in vertical system integration of system suppliers in automotive industry. In the field of TPMS most products are developed in a co-operation of system supplier and OEM. The system supplier procures single components, e.g. a sensor, from a component supplier. Some suppliers, which are specialised on TPMS, have started to offer their products in the after market. Safety relevant systems such as Electronic Stability Programme (ESP), chassis control systems and ADAS are high level systems, which are of great interest for system suppliers. Additional information on tyre and tyre-road contact becomes more and more important to improve the performance of these systems. For example, the influence of the characteristics of the particular tyres of a vehicle on the performance of ESP and the driving behaviour was presented by Robert Bosch GmbH. Activities on the development of more advanced tyre sensors are running e.g. at Continental AG, but these systems are still in a pre-development phase (see Chapter 5). This company is proclaiming the concept of a global chassis control system, which is comparable to the trend followed by some OEM’s (see Chapter 3.1). The approach of such a global control system uses advanced networking technologies for the integration of different systems. The expected benefits are an increase in system performance and a reduction of engineering efforts for system implementation and enhancements for different applications and vehicles. [ZAN02], [RIE02], [ZIE01]
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Technical aspects Tyre pressure monitoring systems (TPMS) are currently moving from high-end cars towards midrange vehicles with associated increase in volumes. This situation causes strong efforts to integrate the new feature into existing car electronic system architectures in order to lower costs by means of an optimised functions’ mapping and to opening way to new warning/control strategies by means of more efficient sharing of information. For high volume series products of TPMS an easy way of system implementation and cost reductions are necessary. Therefore, existing electronics ought to be used for TPMS’s components such as dedicated antennas, receiver, computing device and display as far as possible. An important trend that can be seen in automotive electronics is to set up a more standardised architecture for electronic and electrical systems. The typical bus (CAN/LIN) and node architecture that is needed for advanced remote keyless entry feature fits well in the new application thanks to the availability of existing RF antenna and receiver (usually located in the body computer) and dashboard visualisation capability (see Figure 3.2-1).
DASHBOARD
ROOF NODE
CLIMATE ECU
DOOR MODULES
ST. WHEEL MODULE
CAN
TELEMATIC NODE BODY COMPUTER
REAR NODE
Figure 3.2-1: Typical bus and node architecture. Standards for communication and electronic architecture as well as synergies by using existing hardware components are possibilities to cut costs for series products. Beside the sensor system, the design of the interface between the tyre/wheel system and the vehicle is very important for the implementation of an intelligent tyre/wheel system. Wireless technologies for data transmission and electrical power supply have to be used, because the wheel is a rotating system. This requirements fit to a current trend of using RF technologies or inductive transmission for data transmission to vehicle systems such as keyless entry, remote sensors or displays. The replacement of batteries by an inductive transmission or a local power generation for those systems is of big interest, too. The main benefits are an increase of reliability and environmental friendliness (recycling) and reduced maintenance efforts. Another trend an intelligent tyre/wheel system can benefit from is the development of robust electronics for a harsh vehicle environment, e.g. engine electronics.
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The introduction of x-by-wire systems is a future trend that can be foreseen for automotive applications. X-by-wire means to replace mechanical parts by electrical or electronic parts. This technology, which is already introduced in aircraft industry, is used for current pre-development activities in automotive industry for different applications, e.g. steer-by-wire system or brake-bywire system. X-by-wire systems as well as an intelligent tyre/wheel system are safety relevant vehicle systems. Even though wires are used for x-by-wire systems, the experience gained from these developments, especially on relevant architectures, standards and the process of product homologation, can be used for the design and development of an intelligent tyre/wheel system utilising wireless technologies.
3.3
TYRE MANUFACTURERS AND CO-OPERATIONS
Tyre manufactures Tyre manufactures as well as vehicle manufactures pay high attention to tyres, because a tyre has a big influence on driving safety, driving behaviour, comfort and vehicle design. Therefore, the tyre turns to be a high-tech product, which is true for the material and the manufacturing technology, even though many customers or drivers are not aware of this context. The introduction of a system such as TPMS and the importance of the tyre for accident-free driving open up new chances for tyre manufacturers to increase the added value of their products and to place new and innovative products in automotive market. Tyre manufactures started product developments to bring intelligence into the tyre. To bring intelligence into the tyre means to enhance tyre functionality in providing additional information to vehicle systems. Therefore, it is necessary to apply sensors and electronics to the tyre/wheel system. The integration of electronics into a tyre/wheel system requires a new approach for the design of a mechatronic tyre which has to consider both the production process and the aspects of recycling. Tyre manufactures are taking part in standardisation activities, too, e.g. European Tyre and Rim Technical Organisation (ETRTO), Scandinavian Tyre an Rim Organisation (STRO). The relevant market segments for tyre manufactures are OEM’s, fleet managers and operators and the aftermarket. A rough estimate of the yearly volume of automotive manufacturing is 40 million pieces, which means less than 200 million tyres. The size of the tyre aftermarket is yearly about 600 million pieces. This means that for a tyre manufacturer it is important to succeed in both of these markets. Thus the “intelligence” in a tyre should allow replacement with a newer model of the tyre used as original equipment and even with a product from competing tyre companies. As an example, Nokian Tyres plc, which has conventionally been an aftermarket player, has formed a new subsidiary called RoadSnoop Ltd. to commercialise intelligent tyre technology. A new TPMS product is being introduced in the aftermarket first, and new products are developed to open a path towards the OEM business with high-tech an relatively high-margin tyre products. Pirelli Pneumatici S.p.A., which is a player in the OEM market segment, is developing intelligent tyre systems and forming a new business field called Tyre Systems. Continental AG, which was focused on tyre business some years ago, developed a new business strategy and formed the company division Continental Automotive System (CAS) with the members Continental Teves
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and Conti Temic. Currently the company is an automotive system supplier and a tyre manufacturer with enlarged capabilities of developing and producing automotive electronics. Co-operation of electronics suppliers and tyre manufacturers The market potential of future intelligent tyre/systems is underlined by the formation of cooperations between automotive electronics suppliers and tyre manufactures. The co-operation of Robert Bosch GmbH and Michelin was announced in 2001. Another co-operation was formed by Siemens VDO Automotive and Goodyear. This trend might also be a hint, that the development of a mechatronic tyre/wheel system is a big challenge, that requires to share know-how in tyre technology as well as automotive electronics.
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4. TYRE PRESSURE MONITORING SYSTEMS (TPMS) 4.1
INDIRECT MEASUREMENT
A simple and low-cost way to build a tyre pressure monitoring system is to utilise the already existing wheel speed sensors and ECU of the ABS system in the car. A tyre’s rolling radius depends on the air pressure inside (Figure 4.1-1). However, this radius also depends on many other variables, which do not make ABS-based TPMS very reliable.
Figure 4.1-1: Tyre rolling radius versus pressure. [NR1] Problems that might occur using indirect tyre pressure monitoring systems are listed as follows: [NR1] •
The system needs calibration to learn what is an appropriate wheel speed relationship, before it can sense differences from that. A long driving time can be necessary for this calibration. During this time the system is not detecting tyre pressure drops.
•
In a test carried out by the US Department of Transportation an indirect TPMS did not detect low tyre pressures for situations as follows: -
Two low-pressure tyres were on the same side.
-
Two low-pressure tyres were on the same axle.
-
All four tyres were low in pressure.
•
Slip at the wheels disturbs the pressure-sensing algorithm.
•
Speed, acceleration, uneven tyre wear and production tolerances affect rolling radius.
•
System does not work if a compact spare tyre or tyre chains are being used.
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•
Most systems warn the driver when pressure in one tyre has dropped 20 % to 30 % below the other tyres.
For example, the tyre manufacturers Continental AG and Dunlop are supplying indirect TPMS products. Continental’s product name is Deflation Detection System DDS (exists in BMW M3) and Dunlop calls its system Warnair (in new Mini). [NR2], [NR3]
4.2 4.2.1
DIRECT MEASUREMENT - ACTIVE SENSORS Battery-operated sensor technology in general
In this context an active sensor means, that the TPMS contains a component for electric power supply. The main components of an active battery operated tyre pressure monitoring system are battery, processor, memory, sensors, radio component and antenna. Some of these components are usually integrated on single chips (ASIC) to save weight and space and to reduce power consumption. The sensors typically measure pressure and temperature. Most manufacturers also use a simple acceleration sensor or a switch that helps determine whether the tyre is rotating or not. For example Motorola (USA) and Sensonor (Norway) manufacture commercially available components for TPMS products (Figure 4.2-1). [NR4], [NR5]
Figure 4.2-1: Integrated pressure and temperature sensor component SP13 for TPMS from Sensonor. [NR5] The most commonly used frequency for transmitting the measured tyre information to the receiver is about 433 MHz. This frequency can be freely used in Europe. In the United States a similar license-free frequency is 315 MHz. A relatively new frequency suitable for TPMS is the band between 868 and 870 MHz. Also the 2.45 GHz ISM frequency is in some of the TPMS manufacturers' roadmaps. These frequencies can of course be used for other applications, which can cause interference. Both, amplitude and frequency modulation are used. The receivers consist of antenna, processor, memory and a user interface. Products for the aftermarket usually have a receiver that contains all of these components in one box. If the TPMS
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is an original equipment system, in other words, installed during the car manufacturing, the receiver is typically integrated into the car and connected with an external display and keyboard, e.g. the control panel of the car. Sometimes the receiver can be more or less integrated in the remote keyless entry system of the car. In some OE-products the receiver uses several antennas, one near each wheel. In such a case, the positions of the tyre sensors under the vehicle can be automatically detected.
4.2.2
Clamp-on-rim sensors
Clamp-on rim sensors can be fixed on the well bed of the rim with a stainless steel clamp. This fixing method can be used in aftermarket products, when the same product must suit a large variety of cars. SmarTire, a company based in Vancouver, Canada, is the oldest player on the TPMS market. The first SmarTire product was presented in the early 1990’s. The newest, third generation product consists of tyre sensors, that are fixed on the rims with stainless steel bands, and a receiver, that is fixed on the dashboard or on the windscreen and connected to the cigarette lighter of the car for power supply. This product is sold on the aftermarket (Figure 4.2-2). [NR6]
Figure 4.2-2: SmarTire’s latest generation TPMS product. [NR6] RoadSnoop Pressure Watch is an aftermarket product from the Finnish tyre manufacturer Nokian Tyres plc. It’s sensors are fixed on the rim with steel bands. The receiver is a small wireless battery-operated device, which can be put on any place inside the car, where it can be easily seen and heard. The receiver gets pressure and temperature data from the tyres and gives a warning, if the pressure drops below a pre-set value. RoadSnoop will be available in the beginning of 2003 (Figure 4.2-3). [NR7]
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Figure 4.2-3: RoadSnoop Pressure Watch. [NR7] Other aftermarket products that have a sensor with steel clamp fixing are TireSafe from Algonquin Scientific (USA) (Figure 4.2-4), Tire-SafeGuard from Topchek System (China) (Figure 4.2-5), The Third Eye from Strong Frontier (Malaysia) (Figure 4.2-6) and TMS from A. M. Bromley (UK) (Figure 4.2-7). However, these products seem not to be available on the market yet. [NR8], [NR9], [NR10], [NR11]
Figure 4.2-4: TireSafe from Algonquin Scientific. [NR8]
Figure 4.2-5: Tire-SafeGuard system components (Topcheck System): TPM-DU = receiver, TPM-S2 = sensor, TPM-MB = fixing clamp. [NR9]
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Figure 4.2-6: The Third Eye product components (Strong Frontier). [NR10]
Figure 4.2-7: TMS product components (A. M. Bromley). [NR11]
4.2.3
Valve-attached sensors
Another sensor fixing method is attaching the sensor casing on the bottom end of the tyre valve. In this case, the sensor is actually located on the very same spot as when using a clamp on the rim well bed. Different rims require different valves, which means that this fixing method is better for the original equipment market, where it only has to fit a specific car model with limited variety of wheel types.
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Schrader Electronics from UK is the manufacturer for the standard TPMS sensors in Renault Laguna II and some other French car models like Peugeot 607 and Citroën C5. Schrader’s sensor is fixed on the metal tyre valve and uses the valve shaft as an antenna (Figure 4.2-8). Schrader has recently released information about their second generation sensor, which ought to be fixed on a snap-on-type rubber valve. [NR12]
Figure 4.2-8: Smart Valves manufactured by Schrader Electronics. [NR12] Beru from Germany is manufacturing perhaps the most sophisticated (and no doubt the most expensive) tyre pressure monitoring system for the original equipment market. The sensor’s valve attachment solution is supplied by Alligator Ventilfabrik, also from Germany. Beru’s receiver has separate antennas for every wheel and the user interface is integrated into the car’s instrument panel (Figure 4.2-9). [NR13]
Figure 4.2-9: Beru product’s main components: wheel house antenna, valve-attached sensor and central processing unit. [NR13] Lots of products similar to the Schrader’s and Beru’s can be found in the Internet. These products also contain valve-fixed sensors and a receiver, which is either a separate unit (aftermarket) or integrated into the car (OE). These Internet presentations come from both, large and well-known
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as well as smaller companies. So far, most of these products are not yet available on the market. Some of these products are summarised as follows: •
The German company Siemens VDO Automotive promised to bring a TPMS product of their own as an OE-product in some car models during 2002 (Figure 4.2-10). [NR14]
Figure 4.2-10: Siemens’ TPM product. [NR14] •
The Japanese companies Pacific Industrial (Figure 4.2-11) and Omron (Figure 4.2-12) are also promoting TPMS suitable for OE market on their Internet sites. [NR15], [NR16]
Figure 4.2-11: Pacific Industrial’s TPMS sensor. [NR15]
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Figure 4.2-12: Omron’s TPMS. [NR16]
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•
In Geneva car exhibition, a valve attached pressure sensor has been spotted in the Japanese tyre manufacturer Bridgestone-Firestone’s stand with their logo on it (Figure 4.2-13). [NR17]
Figure 4.2-13 Bridgestone’s tyre pressure sensor (photographed in Geneva car exhibition 2002). [NR17] •
Another Japanese company in TPMS market is Alps Electric, who has published a cooperation with Schrader Electronics (Figure 4.2-14). [NR18]
Figure 4.2-14: Alps Electric’s TPMS central unit with Schrader’s sensor. [NR18] •
The large global automotive suppliers Johnson Controls and TRW have had their TPMS products presented on their Internet pages for a long time. Both JCI’s PSI Mirror (Figure 4.2-15) and TRW’s Tire Watch uses the rear view mirror as a display for tyre information (Figure 4.2-16). From these two products the PSI Mirror is available on the US market. The manufacturers of the actual tyre sensor modules used in these products are not known, but it can be assumed, that the sensors are supplied by other TPMS manufacturers. [NR19], [NR20]
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Figure 4.2-15: Johnson Controls’ PSI Mirror. [NR19]
Figure 4.2-16: TRW’s Tyre Watch. [NR20]
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•
A third large automotive supplier promoting its TPMS technology is Visteon (Figure 4.2-17). [NR21]
Figure 4.2-17: Visteon’s idea of valve-fixed tyre pressure sensor. [NR21] •
Both the Italian company All-Tech (Figure 4.2-18) and the Taiwanese Cowealth (Figure 4.2-19) have several TPMS products in their portfolio, both types, valve attached and valve cap integrated. None of their products are on the market yet. [NR22], [NR23]
Figure 4.2-18: All-Tech’s TPMS receiver and sensor with valve attachment. [NR22]
Figure 4.2-19: Cowealth’s TPMS receiver and valve-attached sensor. [NR23]
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•
Another Taiwanese company with a presentation of a TPMS on their web site is Lite-On Automotive (Figure 4.2-20). [NR24]
Figure 4.2-20: Lite-On Automotive’s TPMS product. [NR24] A slightly different system meant for heavy vehicles, which seems to be very final comes from the French companies Wabco and Michelin, and it is called IVTM (Figure 4.2-21). In this product, the sensor casing is fixed near the centre of the wheel, using the standard wheel bolts. The sensor casing is connected to the outer end of the tyre valves with tubes. In this case, the measured temperature at the sensor is not the temperature inside the tyre but closer to the temperature of the wheel centre. This could cause problems in compensating the normal pressure changes due to fluctuation of temperature. The manufacturers claim to have solved these problems with advanced algorithms. [NR25]
Figure 4.2-21: IVTM system components (Wabco and Michelin). [NR25]
4.2.4
Valve-cap-integrated sensors
A third sensor fixing possibility is to try and squeeze the sensor electronics inside a valve cap. This fixing method is easy and suitable for aftermarket and especially heavy vehicles. A heavy tyre structure is typically very thick and has a steel cord in it, which makes it more difficult for the radio signal to get out of the tyre. Valve cap sensors require a stiff metal valve and limit the size of the battery so that these sensors typically use a mechanical pressure switch, which turns
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the radio transmitting circuitry on only if an alert is necessary. In other words, continuous tyre pressure information is not available. Tyre pressure monitoring systems with valve-cap-integrated tyre sensors can be found on the Internet-sites of the US companies listed as follows: •
The Sensor Technology International with a product called Run Safe (Figure 4.2-22), [NR26]
•
Fleet Specialties with “Tire Sentry” (Figure 4.2-23), [NR27],
•
Sensatec with “Tire Sentinel” and
•
Advantage Enterprises with “Pressure Pro”.
The last two of these companies are not showing real pictures of their products on their Internet sites. [NR28], [NR29]
Figure 4.2-22: Run Safe product’s. Receiver display with one valve-cap-integrated tyre pressure sensor on top of it. [NR26]
Figure 4.2-23: Tire Sentry product’s valve-cap-sensor and receiver/display. [NR27]
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A British valve cap integrated TPMS product is called Tyre Shield and comes from A.I.R. Automotive Systems. The actual availability of these systems is not known (Figure 4.2-24). [NR30]
Figure 4.2-24: The receiver and valve cap sensors of Tyre Shield. [NR30]
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4.3 4.3.1
DIRECT MEASUREMENT - PASSIVE SENSORS Introduction on batteryless sensor technology
In the direct TPMS the battery is the most problematic component. It limits the operation time of the sensor. In order to guarantee a life time of at least 5 to 10 years the battery needs to have several hundred mAh capacity, which causes the battery to be relatively big and heavy. The existing batteries have also temperature limitations. In very cold (under -40 °C) and very high environment (over +130 °C) they may not work properly or may even be destroyed completely. The obvious solution would be to replace the battery with a generator that would convert the kinetic energy of a rolling tyre into electrical energy. So far, this kind of generators only exist in research laboratories and patent publications. The main reasons why there are no feasible generators available are probably the complexity, size, weight, limited temperature tolerance and cost of the published technologies. For example, in an US-patent by D. Snyder, a concept of such a generator is presented (Figure 4.3-1). There is a piezoelectric reed included in the tyre sensor unit. The movements of the wheel cause the piezoelectric reed to bend and generate electricity. [NR31]
Figure 4.3-1: Tyre monitoring system with piezoelectric reed power supply. [NR31] Some companies have tried to solve the battery problem by using the so-called transponder principle, in other words passive sensors. This means that the sensor electronics get the necessary energy from the radio signal. The idea is similar to the RFID-technology, which is widely used in authorisation cards in companies to open doors or to record working hours. The signal from the receiver causes the circuitry of the transponder to resonate, which uses this energy to transmit a reply to the receiver. So far, the energy from the radio signal has not been sufficient to operate a sensor for measuring for example pressure and still have enough left for the reply. However, some very promising technologies have been published, and they might come out on the market in a couple of years.
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4.3.2
Batteryless TPMS at 2.4 GHz
The German IQ-Mobil GmbH is developing a batteryless tyre pressure monitoring system, which they call RDKS. It consists of control unit, antenna controller and transponder. The control unit feeds four antennas (one in each wheel arch) (Figure 4.3-2). The system uses the 2.4 GHz ISM band and automatically switches to a free frequency in the 80 MHz wide band to avoid already occupied frequencies. The radio transmission range between the sensor in the tyre and the antenna in the wheel arch is currently from 50 cm to 70 cm. [NR32]
Figure 4.3-2: IQ-Mobil’s sensor attached on a tyre valve and the size of the electronics. [NR32] The sensor in the tyre is irradiated by the antenna in the wheel arch at a carrier frequency of 2.45 GHz. Initially, the signal is amplitude-modulated by the control unit in the range from 5 MHz to 10 MHz.
Figure 4.3-3: The transmission principle of IQ-Mobil’s batteryless TPMS sensor. [NR32]
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The sensor receives the signal and demodulates it by means of a detector diode; the modulated wave is used to stimulate the oscillations of a quartz. Then the modulation is switched off, and the carrier radiated at reduced power. The quartz now vibrates at its natural frequency, which varies with temperature or as influenced by the capacitive pressure sensor. These vibrations are mixed with the remaining carrier signal, which is now reflected to the antenna with its modulation sidebands. The control unit receives the signal and analyses it by means of a digital receiver circuit (Figure 4.3-3). [NR32] The measurable pressure ranges from 0.5 bar to a maximum of 12 bar, depending on the quartz being used. Operating Temperature Range is from -40 °C to +170 °C. Chip Size is 22 x 22 x 6.1 mm³ and weight is 6.1 g, pressure measurement sensitivity is 500 Hz/bar. The transponder is mounted on the tyre valve. [NR32]
4.3.3
Other systems
Fraunhofer Institut in Germany has studied a batteryless tyre pressure transponder. They utilise surface micromachined pressure sensors with low power consumption (e.g. 150 µW). The tag integrates an absolute pressure sensor and a temperature sensor including the necessary readout electronics with the interface for wireless readout on one die. The die is powered by the RF-field of a remote reader unit. The system operates with a RF carrier at 133 kHz. Only an antenna coil with resonance capacitor and an additional resistor are needed as external components. Data of the sensor output and calibration coefficients stored in the on-chip EEPROMs are transmitted in a robust fully digital protocol using amplitude shift keying (ASK) modulation. Depending on the antenna geometry, operation distance up to 1 m with data rates up to 4 kBits/s are achieved. For the tyre pressure monitoring of trucks a 10 bar sensor is used. The sensor can either be placed inside the tyre or the valve cap (Figure 4.3-4). [NR33]
Figure 4.3-4: Fraunhofer Institute’s tyre pressure transponder. [NR33] The US tyre manufacturer Goodyear has announced co-operation with a technology company Phase IV in developing tyre pressure monitoring technology. Goodyear has announced an "intelligent" tyre system that monitors and reports tyre pressure and temperature for tyres on large haul trucks used in mining and other off-highway, heavy-duty applications. The product has been developed with Phase IV and was unveiled at MINExpo 2000 in Las Vegas. Phase IV specialises in passive sensing transponders based on their custom chip and low cost, battery powered temperature and sensing telemetry systems. Goodyear has been working with Phase IV since
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1992 to develop a system that could be embedded in a tyre for identification purposes and monitoring air pressure and tyre temperature (Figure 4.3-5). [NR34]
Figure 4.3-5: PhaseIV Engineering’s passive RFID tag with built in temperature sensor. [NR34] The British technology companies Qinetiq and First Technology have also announced the development of batteryless TPMS technology of their own. A thin element of metallic alloy is at the heart of this system’s in-valve tyre sensors. This advanced material, developed by QinetiQ, changes in characteristic as the tyre deflates, which is then, in turn, detected by the on-board monitoring equipment. The Sensor is powered by harvesting energy from radio waves. These are transmitted from a transceiver located outside the tyre on the vehicle body. The sensor modifies the transmitted radio waves, in response to changes in tyre pressure, and the receiver section of the on-board transceiver detects the reflected signal. Tyre pressure information is transmitted via a frequency-modulated protocol, which allows pressure in the wheels and in the spare tyre to be individually monitored (Figure 4.3-6). [NR35]
Figure 4.3-6: Qinetiq’s and First Technology’s illustration of their batteryless TPMS technology. [NR35]
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4.4
CONCLUSION
TPMS are the first series products in the field of intelligent tyre/wheel systems which are already introduced in the automotive market. The development activities on TPMS are still high. This applies to nearly all aspects of components and technologies used for a TPMS such as indirect measurement principle or direct measurement principle, sensor, power supply, data transmission and data handling. Further improvements for TPMS for direct measurements are expected in an increased robustness of the electronics, an easy way of vehicle integration including vehicle assembly process and a simple handling for service and maintenance purposes. Therefore, many pre-development activities are focused on wireless data transmission and batteryless power supply. These technologies are important for the way of integrating a TPMS into the vehicle and the tyre/wheel system. Different possibilities where to locate and how to attach a TPMS are still investigated and discussed. There is not a clear trend whether future TPMS are valve-attached, rim-attached, attached to the tyre surface or even integrated into the rubber. The legislation in the US by the NHTSA is a strong driver for new developments, fast market introduction and standardisation of TPMS. These activities are supporting the development of more advanced intelligent tyre/wheel systems the APOLLO project is aiming at. The functionality of TPMS can not be replaced by a more sophisticated tyre/wheel system. The tyre inflation pressure and temperature are important data to derive information on forces or friction parameters from more advanced sensors. Therefore, future development might result in a solution for an intelligent tyre/wheel system with an incorporated functionality of TPMS.
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5. ADVANCED TYRE SENSOR SYSTEMS 5.1
SIDE WALL TORSION SENSOR
A vehicle is affected by longitudinal forces during braking and accelerating and by lateral forces while cornering. Both, longitudinal and lateral forces cause global deformation of the tyre, including belt and side walls. The German tyre manufacturer and automotive systems supplier Continental uses two combined sensor devices to measure this deformation in order to derive the acting forces. Two sensors are attached to the chassis. They are set up to measure at different diameters of the tyre side wall and oriented perpendicular to the side wall. The special characteristic of the tyre is that it contains two magnetised stripes on the inner side wall. The magnetised stripes consist of a regular pattern of magnetic material, incorporated into the rubber material. These stripes are arranged at two different diameters of the tyre (Figure 5.1-1). [NR39]
Figure 5.1-1: Continental’s SWT sensor. (1) Magnetic field sensors. (2) Tyre with two magnetised stripes at the side wall. [NR39] To magnetise the tyre side wall, synthetic magnetic fields with alternating north and south poles are installed along the entire circumference of the side wall. The two sensors applied to the chassis are measuring a signal which is proportional to the magnetic field of the individual poles at the tyre side wall. If no longitudinal forces are being applied to the tyres, alternations between the magnetic poles occur simultaneously at both sensors, and the time difference between these signals is zero. However, if longitudinal forces are applied, i.e. during a braking or accelerating operation, the maximum of the magnetic poles on the inner diameter and those of the outer diameter pass the sensors at different times. This effect results in a phase shift between the two sensor signals. The phenomenon is identical for acceleration and deceleration, except that the polarity signs of the readings are reversed. The magnitude of the longitudinal force acting on each tyre is nearly linear to the phase shift between the two sensor signals of each magnetic stripe. This SWT sensor offers potential information for an improvement of slip control systems such as ABS and TCS. In addition the sensor signal can be used to measure the wheel speed. [BEC98], [NR39]
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In cornering conditions, lateral forces act on the tyres due to the centrifugal forces acting on the vehicle mass. This results in lateral deflections of the side wall which cause variations of the distance between the sensors and the magnetised tyre side wall. Depending on this behaviour, the measured strength of the magnetic field changes. The measured amplitudes can be used to derive the acting lateral force. This value is of importance, especially for chassis control systems such as ABS and ESP. Two technical characteristics of this sensor system can be mentioned: All electronic components needed are located at the chassis and the sensor component at the tyre – the magnetised rubber – is a fully passive component. Therefore, it is necessary to develop a special tyre and a special tyre manufacturing process. Magnetising processes are known, tested and stable. Mechanical problems have been essentially eliminated, because the inside wall of the tyre facing away from the curb is magnetised. The SWT sensor allows to measure the forces acting at the tyre-road contact. As a result of this more precise information about the driving states of the tyres can contribute to further optimisation of the electronic vehicle stability systems. The benefits for the driver are a shorter braking distance and improved vehicle control in difficult driving manoeuvres and on difficult road surfaces. [BEC98], [NR39] Following an extensive patent application activity, the development team is now involved in an accelerated effort to introduce the system into series production. These activities are focused on a sensor system for longitudinal forces. A system enhancement is under development to detect a tyre defect and transmit relevant data to the control system. [NR39] One of the most critical design issues of the SWT system is the distance between the sensor at the chassis and the magnetised tyre side wall and the fact that the signals are measured with a phase shift of one half tyre rotation at the opposite of the tyre-road contact. While the longitudinal force measurement works adequately in driving tests, the side force measurements are more complicated, due to the small lateral deformations which occur in the upper region of the tyre, where the sensors are placed. Another reason is the decreasing signal amplitude due to an increasing distance between the sensor and the magnetised side wall. It has to be considered, that the models needed for signal interpretation and calculation of the lateral forces from the sensor data are more complex compared to the calculation of longitudinal forces. Models and algorithms needed for processing the sensor data to derive the relevant forces are not yet published. The vertical force and friction available can not be detected with the SWT sensor. The question how to set up an appropriate process for mass production of series products is still an issue. An important requirement in the automotive market is the free choice of the tyre by customers and vehicle manufactures. This is another issue for commercialisation of a special tyre as it is necessary for the SWT sensor.
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5.2
DARMSTADT TYRE SENSOR
Another innovative application in the field of intelligent tyre technology is the sensor system developed by the researchers in Darmstadt University of Technology, Germany. In this invention a magnet is placed inside the tyre tread block and the movements of this magnet are monitored by a Hall-sensor (Figure 5.2-1). The movements of the tread block are dependent on the friction and the forces at the tyre-road contact, and can be monitored. [NR40]
Figure 5.2-1: Darmstadt tyre sensor: Magnet and Hall sensor. [NR40] The position sensor employs four monolithically integrated Hall crosses and an additional temperature sensor. A permanent magnet is mounted within the tyre rubber in a distance of 1mm from the Hall cross for position and deflection monitoring. The differential signals of two crosses in x- and y-direction deliver the x- and y-deformation output signal. The sum of the four Hall voltages forms the z-signal. The quasi-linear measurement range is 1 mm. For a wireless transmission of the sensor signals from inside the tyre to a measuring device, a miniature low power four-channel telemetry system has been realised. [NR40] A deformation of a tyre element takes place even before it gets in direct contact with the road surface. The x-deformation is, analogue to the brush-model, directed against driving direction in front and into driving direction behind the contact patch. The y-signal stands for the deformation in lateral direction. A deflection can be seen although the tyre runs without lateral force. Test stand experiments have shown the potential to measure tyre pressure (z, x), wheel load (z), forces in circumferential (x) and lateral direction (y). [NR40] The sensor miniaturisation enables the integration of chip and magnet into a single tread lug element. So, standard steel belt tyres can be investigated without any influence of the steel cord on the magnetic field. The power consumption has been successfully reduced to enable the use of a transponder system. An important point is the increase of the possible mechanical resolution. This parameter depends on the Hall cross magnetic field sensitivity, the size of the magnet and the distance of the Hall crosses. [NR40]
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In addition, the sensor signal is influenced by the driving speed or the water depth on the road. It is expected, that this effect can be used to derive a warning on arising aquaplaning (see Figure 5.1-2).
Deflection of tread element, x-direction [mm]
Standard parameter, Contidrom water depth 9 mm
Roll off path, based on end of contact patch [m]
Veh. velocity [km/h]
Figure 5.2-2 Tyre tread block movement in x-direction on a wet surface with different driving speeds. [NR41] Many measurements have been performed with the Darmstadt tyre sensor. The results show dependencies of the longitudinal deformations on the friction available between the tyre and the road. [STR02], [FAC00] Adequate evaluation routines to derive friction data from the measured deflections have not yet been published. These measurements are performed at a laboratory test rig under nearly ideal conditions for the tyre-surface contact and for low velocities. A lot of data have to be processed, because the full signal information from entering the contact patch until leaving it is needed for deriving relevant data for application systems from measured signals. The model which is used to interpret the sensor data for deriving friction information is based on the so called brush model. It is used to describe the behaviour of the tread lug element at the tyre contact patch. Algorithms for the calculation of forces, pressure or friction parameters are not yet published. It has to be considered that the height of the tread lug has a significant influence on the characteristics of the tread lug. [STR02], [FAC00], [BRE98] The aforementioned four channel telemetry system can actually be used only for laboratory purposes. The dimensions (30x30x20 mm³) and the power consumption of the Hall element (ca. 60 mW including sensor electronics) are still too high for system integration into the tyre in serial products.
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5.3
SURFACE ACOUSTIC WAVE SENSOR
Filters based on Surface Acoustic Wave (SAW) technology are widely used in mobile phones and televisions, among others. The SAW filters are used to screen out unwanted frequencies from the signal received by the device. This ceramic component can be designed in such a way that only predefined frequencies can pass the filter. The SAW technology can be used to design sensors, too. The incoming HF signal is transformed into a surface acoustic wave. This wave is reflected on the device and again, transformed back into a HF signal which can be detected by a reader system. To design a sensor means, to make this device sensitive for the desired signal. This can be achieved by selecting the appropriate material and by a specific design of the reflectors on the surface (Figure 5.3-1). In many applications, SAW devices are fully passive components. The energy needed is provided from the input signal which is the output signal of the receiver component or the reader system. [BUL98]
Figure 5.3-1: Functional principle of a SAW component. [NR36] An UK company Transense is developing SAW sensor technology for tyre monitoring purposes. Transense has announced co-operation with the US TPMS manufacturer SmarTire and the French tyre manufacturer Michelin. Transense’s sensor uses the SAW device as a diaphragm between the side of the sensor subjected to tyre pressure and a sealed reference chamber. Changes in tyre pressure cause the resonant frequency of one device to increase whilst the frequency of the second device decreases. The change in the difference between these two frequencies is directly proportional to the difference in pressure of the tyre and the reference chamber. The energy needed is provided from the signal of the receiver component [NR37]
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Figure 5.3-2: Photographs of Transense’s and SmarTire’s SAW sensor (see leaflet given out in Tire Technology Expo 2002 in Hamburg). [NR37] The German company Siemens has a slightly different approach. They also use SAW technology suitable for tyre sensors, but they are researching the possibility of generating the energy for the SAW filter from physical phenomena near the SAW device. Siemens has announced co-operation with the tyre company Continental and the Darmstadt University of Technology. They are aiming at a sensor inside the tread block of the tyre based on a SAW device which can provide data comparable to the Darmstadt tyre sensor (see Chapter 5.2). [NR36], [NR38], [POH99], [KLU02] Siemens has researched piezoelectric crystals connected with a SAW filter. These could be used for example as a wireless light switch in buildings. The piezo crystal generates the necessary energy when pressing the light switch. This energy is lead through the SAW filter’s bar-code-like surface and sent out to a receiver, located in the light bulb to be switched on. Pyroelectric versions of these devices could react to a change in temperature and transmit their own signal to a receiver. These autonomous sensors could be vulcanised into tyres and calibrated to produce a signal for a given reduction in air pressure. Pyroelectric versions could warn of excessive tyre temperature. [NR38]
Figure 5.3-2: Siemens’ SAW sensor with energy generating piezoelectric crystals. [NR38]
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The main issues on SAW sensors for tyre applications are the robustness of the electronics and the way of integrating it into the tyre.
5.4
CONCLUSION
The concepts and prototypes of innovative tyre sensors presented in this chapter prove the interest in the field of intelligent tyre/wheel systems. The idea behind all these systems is to provide more information on the tyre and the tyre-road contact such as forces and friction parameters. A critical issue for all concepts is the way of bridging the distance between the vehicle and the rotating tyre/wheel system and it is not yet solved. Many publications are available describing the sensor and measurements but there is a lack of information on algorithms and models which are necessary for the interpretation and processing of the measured data to derive the relevant information for vehicle applications. In addition, the effort which is needed for these calculations is not described in the publications. For some sensor systems the possibility to monitor several data such as lateral forces, longitudinal force, vertical force, tyre inflation pressure and friction parameters with only one sensor is claimed. This might be possible for laboratory measurements but it seems to be not realistic for a series product. The functional correlation between these data has to be considered in more detail. The innovative tyre sensor systems are published and discussed since about ten years but up to now no product is available and there is no announcement for the market introduction from any company.
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6. BASIC SENSOR TECHNOLOGIES 6.1
INTRODUCTION
Sensor technology is a large subject. Here we consider only those technologies that are relevant for research and development of innovative solutions for intelligent tyre/wheel systems and for automotive industry at large. In the following chapters, an overview is given on the sensor technologies listed as follows: •
Acoustic sensor,
•
Optical Sensor,
•
Vibrating string sensor,
•
Ultra Wide Band Technology and
•
Capacitive Sensor.
6.2
ACOUSTIC SENSOR
A company from the UK Tekgenuity claims to have developed a technology where the “sound signature” of a tyre is measured by microphones and post-processed with special signal processing technologies. The name of this product is Chromasonics. Tekgenuity’s researchers say that changes in this measured sound signature can be interpreted as changes in tyre pressure, temperature, wear status or even available friction on the road. Warnings to the driver or information to electronic control systems of the vehicle could be delivered in real time. [NR44]
6.3
OPTICAL SENSOR
The German company Optimess is developing laser sensors which can be used for tyre manufacturing and testing. The sensor is a non-contact measurement detector, based on triangulation principle. By means of two laser sensors arranged in a traversing way, one above and one below the test material, a running tape's entire profile can be measured. The sensor can also be used to measure the profiles of tyre running treads. [NR45] Non-contact scanning of tyre surfaces offers the possibility of measurement of a tyre's expansion at maximum speed. This can be carried out either at a roller type test stand or directly at the vehicle. Optimess sensors provide the possibility of non-contact detection of tyre deformations under various test conditions, as, for example, wheel pressure, track and inclination. These can be measurements at a roller type test stand but also - due to the small size of the sensors measurements at the test vehicle. A special preparation of the tyre, e.g. erasing of the marking, is not necessary. An extreme mode of application for this sensor is the deformation measurement of
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the contact surface of the tyre inside of a truck tyre. In this case, a specific Optimess sensor has been installed inside the tyre at the wheel rim, that measures in direction of the tyre's surface (see Figure 6.3-1). [NR45]
Figure 6.3-1: Optimess laser measurement instrumentation inside a truck tyre. [NR45] Another area of application is the non-contact distance measurement vehicle - road for performance characterisation of the driving behaviour of a vehicle. In the sector of tyre test running, the laser offers the possibility to scan the tyre's surface on a test device, i.e. the entire tyre surface is measured after fixed driving cycles. Independently of genuine wear and tear, also saw tooth formation, washing out, possible rolling off the rim, etc. can be analysed and documented. [NR45] The laser sensor is used for the detection of road surface conditions while driving, too. This is important for test stand simulation, for which as accurate information as possible about the actual road conditions of test routes are required for carrying out a field-experienced simulation. Furthermore, knowledge about road surfaces is an important parameter for the development of tyres with regard to aquaplaning and noise behaviour. [NR45] The Optimess sensor is not used in series production vehicles.
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An optical sensor concept for the measurement of the water film height on the road surface is shown in Figure 6.3-2. The sensor consists of a infrared source which is directed from the vehicle to the tyre surface and two light receivers which are directed to the tyre, too. These receivers detect the diffused and reflected light. Depending on the percentage of the received diffused and reflected light the water film height is calculated by a specific algorithm. [GOE93]
100
Reflected light reflexed light
Relative light flux [%]
Infrared source
Diffused light receiver Reflected light receiver
Diffused light diffused light
10 Thickness of waterfilm [µm]
Figure 6.3-2: Optical tyre sensor for detection of water film height. [GOE93] The German company Hella built up a prototype of a infrared sensor which directly illuminates the road (see Figure 6.3-3). The sensor is mounted on the front part of the vehicle. The spectral intensities of the reflected light are analysed and an algorithm is used for the detection of road condition such as dry, wet, icy, snowy. The goal is to estimate the friction available and to warn the driver in case of low friction. Up to now the reliability of the system is not sufficient to use the sensor information for vehicle dynamic control systems. [BMW00]
Figure 6.3-3: Optical sensor prototype by Hella. [BMW00]
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6.4
VIBRATING STRING SENSOR
A simple metal wire in a plastic tube and a standard microwave radio transceiver are the basic components of this technology which is proposed by the Swedish company Vibstring. When mechanically excited, the wire is vibrating at its natural frequency which is correlated to the length of the wire, its material properties and the state of strain. The vibration of the wire has been found to modulate a radio signal at the frequency of vibration. The demodulated radio signal can be analysed to determine the frequency of wire vibration, and thereby, the state of strain in the wire can be detected. [NR42] Application of pressure to the sensor increases or decreases the strain in the wire resulting in a corresponding shift in natural frequency. The relationship between the natural frequency and strain in the wire determines the high sensitivity of the sensor. A transceiver directed towards the sensing wire emits a monotone HF signal and receives the HF signal, modulated by the vibrations of the wire. Modulation due to the wire vibration overrides all other vibrational effects due to the antenna nature of the wire, whose length is selected in accordance with the HF signal frequency. Demodulation of the HF signal results in a signal corresponding to the wire vibration. This signal is filtered, amplified and analysed to determine its frequency and thus the corresponding torque on the shaft (Figure 6.4-1). [NR42], [NR43]
Figure 6.4-1: Basic principle of a vibrating string tyre pressure sensor. [NR43] A potential application of a vibrating string sensor is to measure strains in the tyre. Measurement results with a vibrating string embedded in the tyre as well as strategies what signals are measured or how the signals can be interpreted have not yet been published.
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6.5
ULTRA WIDE BAND TECHNOLOGY
Ultra Wide Band radio (UWB) is a revolutionary wireless technology for transmitting digital data over a wide spectrum of frequency bands with very low power. It can transmit data at very high rates e.g. for wireless local area network (WLAN) applications. Within the power limit allowed under current US regulations, UWB radio can not only carry huge amounts of data over a short distance at very low power, but also has the ability to carry signals through doors and other obstacles that tend to reflect signals at more limited bandwidths and a higher power. At higher power levels, UWB signals can be transmitted over significantly greater ranges. Instead of traditional sine waves, ultra wideband radio broadcast digital pulses, that are timed very precisely on a signal across a very wide spectrum at the same time. Transmitter and receiver must be coordinated to send and receive pulses with a high accuracy. Ultra Wide Band can also be used for very high- resolution radars and precision (sub-centimeter) radio location systems. [NR46] UWB-devices can be used for precise measurement of distances or locations and for obtaining the images of objects buried under ground or behind surfaces. UWB devices can also be used for wireless communication, particularly for short-range high-speed data transmissions suitable for broad band access to the Internet. At this time UWB technology does not have regulatory approval outside USA. However, there is significant interest in many countries and steps are being taken to explore a number of foreign markets and regulatory processes. [NR46] UWB wireless is unlike familiar forms of radio communications such as AM/FM, short-wave, police/fire, radio, television, and so forth. These narrow band services which avoid interfering with one another by staying within the confines of their allocated frequency bands, use what is called a carrier wave. Data messages are impressed on the underlying carrier signal by modulating its amplitude, frequency or phase in some way and then are extracted upon reception. Conventional narrow band radio techniques rely on a base "carrier" wave that is altered in a systematic manner (modulated) to embody a coded bit stream. Carrier waves can be modified to incorporate digital data by varying their amplitude, frequency or phase. UWB wireless technology uses no underlying carrier wave, instead modulating individual pulses in some way. In a bipolar modulation scheme, a digital 1 is represented by a positive (rising) pulse and a 0 by an inverted (falling) pulse. In another approach, full-amplitude pulses stand for 1's, whereas half-amplitude pulses stand for 0's. Pulse-position modulation sends identical pulses but alters the transmission timing. Delayed pulses indicate 0's (Figure 6.5-1). [NR47]
Figure 6.5-1: Various modulation techniques in common narrow band and in wide band transmissions. [NR47]
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A system to alert drivers to potential tyre blow-out situations prior to the actual tyre failure has recently been developed by the US company McEvanTechnologies (MET) using a new 24 GHz pulse Doppler radar. The new system can immediately detect tyre abnormalities such as tread delamination, bald spots, side wall ballooning and embedded nails. With spread spectrum emissions to permit four or more units to operate on a single vehicle in an environment crowded with similar sensors, the MET sensor also detects tyre and wheel geometry errors such as out-ofround and run-out. The microRADAR system can measure wheel speed on a non-contact basis, and should find application in sensing and controlling wheel lockup during heavy braking, particularly on large trucks, or wheel slip for traction control systems on SUVs and other fourwheel drive vehicles. Even high performance race cars will benefit from this new technology. Prototype tests were conducted at 24 GHz using MET's DPD-24 range gated motion sensor. [NR48]
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6.6 6.6.1
CAPACITIVE SENSOR Micromechanical sensor
The silicon technology called microelectromechanical systems MEMS has experienced a revolution in recent years. MEMS allows sensors, electronics, and actuators to be fabricated on a single chip. In principle, MEMS sensors can be designed for nearly all quantities (temperature, pressure, acceleration, deformation, electric voltage, current, etc.) but also other electric components can be realised. Figure 6.6-1 shows a MEMS acceleration sensor and Figure 6.6-2 shows an oscillator
Figure: 6.6-1: MEMS acceleration sensor.
Figure 6.6-2: MEMS oscillator. The length of the side of the figure is 42 µm.
MEMS manufacturing processes can be divided into four categories: •
bulk micromachining,
•
surface micromachining,
•
high aspect ratio micromachining (HARM) and
•
silicon on insulator (SOI) micromechanics.
Bulk micromechanics makes micromechanical devices by etching deeply into silicon wafer. The acceleration sensor of Figure 6.6-1 a is made by bulk micromachining. In surface micromachining, thin layers of sacrificial and structural material on the surface of a silicon wafer which acts as a carrier only, is deposited. In addition to airbag acceleration sensors, microphones, flow sensors, etc. have been fabricated with this method. HARM provides ability to fabricate high aspect ratio microstructures that are very tall with aspect ratios larger than 1:10 with relatively low costs. In SOI techniques the active sensor is made by etching monocrystalline silicone that is bonded with an insulating silicon oxide layer to silicon wafer. The insulating layer is sacrificially
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etched. The monocrystallinity of the silicon in SOI gives advantages: negligible residual stresses and possibility to make electrical circuits. Thus standard integrated circuit manufacturing process CMOS can be combined with SOI MEMS. [PII99] The MEMS sensors are so called capacitive sensors. It means that the quantity to be measured acts on the sensing element by bending or deforming it, and the change in the capacitance between the element and some other part of the structure is measured electronically.
6.6.2
Capacitive displacement sensor
Capacitance value of a capacitor can be varied by varying any of the parameters in the capacitance formula for a plate capacitor:
C = ε 0ε r
A d
z
∆C = −ε r ε 0
y
xy ∆z z2
x Figure 6.6-3: Vertical displacement sensor based on plate capacitor.
x z
y
∆C = −ε r ε 0
y ∆x z
x
Figure 6.6-4: Lateral displacement sensor based on plate capacitor. Based on plate capacitors, Figure 6.6-3 show an arrangement to measure vertical displacement. A design for measuring a lateral displacement is described in Figure 6.6-3. Using interdigitated capacitor a lateral displacement sensor can be designed, that provides a better sensitivity than the one shown in Figure 6.6-4 (see Figure 6.6-5).
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l F
C
F
d
w
0.1 m m
3 mm Fig.6.6-5: Lateral displacement sensor based on interdigitated capacitor. The following principles can be used to eliminate the effect of dispalcement in uninteresting directions. Eliminating the effect of vertical displacement in measurement of lateral deformation can be achieved by the special arrangement of two capacitors is shown in Figure 6.6-6.
C1
Vi
V0 C2
Figure 6.6-6: Lateral displacement sensor with elimination of vertical displacement. By measuring alternating voltage V0, only the difference between the two capacitances C1 and C2 can be measured. The both are sensitive to vertical deformation, but, because of the different construction of the plates, only C2 is sensitive to lateral displacement.
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A sensor arrangement for eliminating the effect of lateral displacement in the measurement of vertical displacement is described in Figure 6.6-7. C
Figure 6.6-7: Vertical displacement sensor with elimination of lateral displacement. Capacitor C is formed by two capacitors that are connected in series by a common conducting plane. This plane is large enough to make the structure insensitive to lateral displacement.
6.6.3
Measurement of capacitance
A simple method of direct readout that is suitable for the measurement of capacitance changes, is shown in Figure 6.6-8. The electronics is based on an operational amplifier that is directly connected to the capacitor.
Z
+ _
V
C
VC Figure 6.6-8: A simple electronics for measuring capacitance changes. The electronics shown in Figure. 6.6-8 is suitable for the measurement of capacitance changes only. The method works poorly at low frequency, because of the 1/f-noise of the amplifier.
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The more sophisticated way to convert the change of capacitance into voltage value is to use a complementary-phase AC bridge (see Figure 6.6-9). The capacitance change is monitored with an oscillator, amplifier, mixer, and filter.
+1
C+ C
OSC.
V
C/C
C -1
Figure 6.6-9: A complementary-phase AC bridge for measuring capacitance. Capacitive sensors are used in many applications, and a lot of experience exists in using this sensor technology to solve new measurement tasks by innovative solutions. [ELW00], [FRA1], [SEP95], [TIL93]
6.7
CONCLUSION
From the sensor technologies investigated in this chapter the capacitve sensor has the highest potential to be used for the development of an intelligent tyre/wheel system. The main advantages are robustness, small size, low power consumption and good possibilities for the integration into the electronics of a total system. Therefore, capacitive sensor technology opens up new perspectives for an intelligent tyre/wheel system. It fulfils the requirements on high robustness and low demand on electrical power supply. Series products based on capacitive sensor technology are available on the market and they are used in many industrial applications. This, in addition, offers cost benefits. Acoustic sensors and optical sensors have the potential to detect data on road condition which can be used to derive friction parameters, but they can not be used for force measurement. The low robustness in a harsh environment during vehicle operation is a disadvantage for optical sensors. First investigations are started to use a mechanical sensor such as the vibrating string sensor for pressure measurement. The capability of this technology for more challenging measurement tasks seems to be limited. For the development of an intelligent tyre wheel system further investigations have to be performed to answer the following questions:
•
Which physical phenomena of the tyre/wheel system and the tyre-road contact contain relevant information needed for specified vehicle applications?
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•
Which measurement tasks for a sensor in a tyre/wheel system can be specified to detect the relevant information and which sensor type is the best solution?
•
Which models and algorithms are needed to provide the relevant data for vehicle applications?
•
Which is an appropriate design for an integrated electronics including all components of the total system and for a mechatronical integration into the tyre/wheel system?
7. BASIC TECHNOLOGIES FOR WIRELESS DATA TRANSMISSION 7.1 7.1.1
TECHNOLOGY OVERVIEW Classification of wireless data transmission
Wireless sensors are classified into three types:
•
Active sensors have a power supply, have active components for communication, e.g. RF components, and are able to send e.g. a radio signal.
•
Semipassive sensors have a power supply, but they use it only after a wake-up signal from the reader unit. They use back scattering, i.e. modulate the signal of a base station or reader to send data instead of sending an RF carrier.
•
Passive sensors have no power supply or a battery. They also use back scattering for RF communication. For performing the measurement and transferring the information, passive sensors take energy from the electromagnetic field emitted by the reader.
Wireless sensors are also divided according to the type of electromagnetic coupling to the reader:
•
Inductive uses magnetic field,
•
Capacitive uses electric field or
•
Radiating uses radiating field.
Low-frequency inductively coupled systems are in a widespread use in RFID. Capacitively coupled sensors require in practice a very short reading distance – nearly contact – and do not suit for tyre monitoring.
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7.1.2
Data transmission of passive wireless sensors
The simplest and cheapest wireless sensor is based on an inductor-capacitor resonator. One example of such a sensor is the 8 MHz resonator used in electrical article surveillance systems. The manufacturing price of this kind of a sensor is in the order of few cents in large volumes. The functionality of this sensor is, however, limited. Data can be coded into the resonant frequency or the Q-value of the resonance only. [WDT01] If an integrated circuit (IC) chip is connected to an antenna, the wireless sensor can have much more complicated functions. The IC has a microcontroller or a finite state machine and it communicates to both directions wirelessly. It also contains a circuit, a voltage rectifier which derives power for the sensor from the field of the reader or the base station. Because the power available from this supply gets smaller when the distance between the base station and the sensor is increased, the power consumption must be very small if long operation distances are required. This means that there can be no active RF components or radio transmitter on the chip. Figure 7.1-1 shows the concept of a passive wireless RF sensor system. The antenna of a wireless sensor is fabricated on a laminate or a printed circuit board. The chip, fabricated with CMOS technology, consists of antenna matching circuit, voltage rectifier, detector, logic, sensor, sensor electronics and memory. The reader interrogates the wireless sensor by sending a measurement command and then continues sending a constant RF signal for powering the sensor. The value of the measured quantity is either converted into digital form and stored into the memory or is immediately transmitted by modulating the antenna impedance. At each interrogation the sensor transmits the content of its memory which contains the identification code of the sensor. When the impedance of the sensor antenna is modulated, the back scattering from the antenna is also modulated. The back scattering is then detected by the reader. [WDT01], [WDT12 – WDT15]
WIRELESS SENSOR
READER LAMINATE Prosessor Demodulator
Modulation
Voltage Antenna Matching rectifier and detector
RF electronics
Sensor
Sensor electronics
Figure 7.1-1: Concept of a passive wireless RF sensor system.
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CMOS
Logic
Memory
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The schematic diagram of the reader RF electronics is shown in Fig. 7.1-2. In principle this type of electronics is suitable for reading all the RF sensors using back scattering. The electronics is in effect a sensitive impedance measurement device. When the wireless sensor modulates the impedance of its antenna, these modulations are reflected to the impedance of the antenna of the reader. The back-scattered signal from the sensor is detected by a sensitive impedance measurement of the reader antenna. [WDT14]
Modulation
IF
RF OUT 0
RF
o
LO
PA
VCO 0 PLL
0 IF
o
90
LO
LO
Q out
o
RF
RF IF
0
o
0
Frequency control
o
o
LNA
I out
Figure 7.1-2: Schematic diagram of the RF electronics of a reader unit of a wireless sensor.
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7.2
EXISTING WIRELESS VEHICLE APPLICATIONS
Automotive security systems (Keyless Entry / Keyless Go) use RF wireless data transmission. A typical system topology is presented in Figure 7.2-1.
Figure 7.2-1: System topology of wireless application system. SA: Smart antenna. The smart antenna (Figure 7.2-1) is a mechatronic device consisting of a wirewound coil and electronics (driver, logic and LIN interface). Such a solution gives maximum flexibility for system definition and antenna allocation, allowing reduction of EMI (Electromagnetic Interference). Smart Antennas communicate at 125 kHz with a Custom Identification Device (CID) by means of magnetic near field (inductive coupling) in order to discriminate the position of CID (external, in the passenger compartment or in the trunk) for a maximum distance in the order of 1 meter. They are all transmitting antennas (their function is to communicate a vehicle identification code to the CID for example when a handle is pulled) except for one recovery antenna that is a receiver able to operate at a distance of few centimeters when CID’s battery is exhaust (Easy Go function). Just in this case, there is also a power transmission as for the former anti-theft immobilizer function (transponder in keyfob and reader antenna on ignition block) with a data transmission based on back-scattering principle (Figure 7.2-2). If the CID acknowledges vehicle’s identification code, it sends its CID identification code to the body computer via UHF (434 / 868 MHz, ASK / FSK / OOK modulated). With this strategy the CID is allowed to transmit only when required by the car which it is associated with, thus
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avoiding useless transmission and battery’s energy consumption. The same UHF range is used for conventional remote control and other functions like TPMS.
Figure 7.2-2: Custom Identification Device (CID). The adoption of a RF link to Tyre Pressure Monitoring Systems is required to establish the transmission of on-wheel sensors’ data to the related vehicle electronics without a wired connection. Battery powered sensors’ reading electronics consist of signal conditioning circuits and RF transmitting means, namely transmitter and antenna. On the vehicle the RF components antenna and receiver are needed in order to receive data from the sensor. If low-end TPMS are just requested to give a generic warning about sub-inflated tyres, more performant ones must be able to identify the faulty tyre. In a high volumes context, identification must be automatically done, without the need for time-lossy and expensive end-of-line manual operations. The more obvious approach is to have separate antennas/receivers for each tyre but this would mean high complexity and costs. Applying a different concept the receiver could be centralised and an RF multiplexer might be employed to switch dedicated antennas. Also this architecture is expensive and problematic due to the fact that high frequency cables should be needed in order to feed signals from antennas to the receiver. The more advanced concept of using a bi-directional RF link would allow identification with just one antenna and one receiver in the vehicle. This approach would ease anti-collision strategies and allow better remote power management. Obviously this solution implies the use of a transceiver (transmitter and receiver) both on wheel and on vehicle electronics. In the future, an architecture with bi-directional RF link will be able to manage other information coming from additional and new sensors located inside the tyre, giving an opportunity for more sophisticated control strategies and driver information.
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7.3
STANDARDS AND REGULATIONS
Table 7.3-1 shows the frequency bands and power levels allocated for short-range radio devices (SRD) mainly in Europe but also in some bands in America. No license is needed if the device operating within the given bands and power. The regulations for wireless sensor communication are not well harmonised worldwide. This is a major obstacle for the widespread acceptance of the passive RF sensors today. [WDT13]
Table 7.4-1: Frequency bands power levels generally allocated for SRD in Europe. Some American bands are also given. EIRP = Equivalent Isotropic Radiated Power; ERP = Equivalent Radiated Power FREQUENCY BAND
POWER, LIMITATIONS, REGION
100
QPSK/CCK 40-50 Host/client or peer-to-peer
