Wireless Tyre Pressure Monitoring （3）

Published:2011/8/1 1:10:00 Author:Amy From:SeekIC

By Helmuth Lemme

Quartz crystal sensors

Still another principle has been adopted by IQ Mobil (Munich, Germany) for their tyre pressure monitoring sys­tems. The central part is a quartz volume oscillator pro­ducing a controlled amount of ’ringing’. The interrogator module emits a microwave signal (at 2.45 GHz) which is amplitude-modulated with a frequency between 6 MHz and 10 MHz. Inside the responder, the RF burst is rectified and the resulting voltage used to excite the quartz crystal (within approx. 1 ms), Next, the AM modulation is removed from the carrier. The quartz crystal will continue to oscil­late at its own frequency, but subject to the capacitance pre­sented by the pressure sensor. The result is crystal ’pulling’ to a frequency removed from the excitation frequency. The resultant ringing (oscillation decay, lasting about 2 ms) is mixed with the carrier fre­quency, hence modulated by it and returned as an RF signal. The control module measures the modulation frequency of the responder signal and employs the deviation from the excitation frequency to calcu­late the pressure value. (Fig­ure 4). The entyre interrogate-and-respond sequence is com­pleted within 6 ms. The (small) temperature coefficient of the quartz element responsible for the pressure measurement is compensated. In this way, pressure measurements can be performed at an accuracy of ±0.1 bar (1.45 psi).



The second measurement channel is for temperature and employs its own quartz crys­tal with a different cut angle and a much larger tempera­ture coefficient. In practice, it allows and accuracy of ± 1 K (kelvin) to be achieved. The return carrier is alternately modulated with the two crys­tal resonance frequencies. Extensive tests showed that the system is immune to other RF sources in the 2.4-GHz band (say, from other cars). This is achieved by measures including frequency hopping after each sensor interroga­tion action. The usable range of the RF links is about 1 m. The size of the module is 22 x 22 mm and its weight, approximately 14 g. The sys­tem will be taken into volume production by the Japanese company Alps. Reportedly the unit cost should remain under 1 USS. The chip could also be programmed to contain data on the tyre used. This may be useful for car production logis­tics or even to inform the car computer new tyres have been fitted.

Road contact sensors

Still more security is afforded by an additional tyre parame¬ter which is actually the most important: road contact. Using this real-time information, anti¬blocking and vehicle stabilization systems could be made to work even better. The ’Darm¬stadt tyre sensor’ (Figure 5) is a joint development by Conti¬nental and the University of Darmstadt (Faculty of Vehicle Technology, Prof. Winner). It records deformation of the tyre profile elements when in con¬tact with the supporting sur¬face. The current version employs a surface-wave sen¬sor in the tyre profile and has not yet left the test bench. The adaptation to ’real life’ on the road is problematic because the sensor is expected to measure as well as transmit when it is in contact with the road. At that instant, the RF contact is hindered by the wheel rim. There remains a lot to explore and research. If, however, researchers are suc¬cessful, they can pride them¬selves in having reduced the number of road accidents even further.

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