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).
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.
Roll off path, based on end of contact patch [m]
Veh. velocity [km/h]
Deflection of tread element, x-direction [mm]
Standard parameter, Contidrom water depth 9 mm
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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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