It would therefore be advantageous for the driver to be given early warning of such problems by reliable on-board instrumentation. The problem of detecting tyre pressure changes has received considerable attention in recent years. One of the earliest approaches was to take a pressure tapping through the wheel and axle assembly which then allowed conventional pressure transducers to be mounted on the vehicle. An added advantage of this design was the ability to inflate or deflate the tyres while driving, which was helpful when travelling over mixed terrain, e.g. road= mud=snow. The main drawback was the necessity to provide a seal between rotating and non-rotating components and this fact has restricted the application of this approach to specialized vehicles involved in this type of operation. A more recent approach (2) has resulted in the development of a piston type pressure transducer mounted on each wheel of the vehicle. Changes in pressure are detected by a Hall effect probe mounted on the axle, sensing changes in piston position relative to a datum. The device is relatively inexpensive but long-term reliability in the harsh environment found under wheel arches remains to be proved. Telemetry (3) has been used to provide power to wheel-mounted transducers and to receive signals for onboard display but there is no evidence of the system being widely used, presumably due to high costs and limited reliability. Systems have recently been developed with pressure transducers inside the tyre volume, the signals being transmitted at radio frequencies. The power for these devices is provided by batteries (4, 5). Battery lives of 10 years are being claimed by the manufacturers but long-term reliability remains to be proved on a commercial scale
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