A system for monitoring the pressures of the tires of a motor vehicle comprises a pressure and/or temperature sensor mounted in each wheel of the vehicle, and a central processor unit that collects the data coming from each sensor by means of wireless links. Such a system makes it possible, in particular, to inform the driver of the vehicle whenever the pressure of a tire is abnormally low.
In practice, each sensor comprises a pressure and/or temperature probe, as well as a radio signal transmitter and a power supply battery. Pressure and temperature data is thus transmitted by each sensor and is collected by said central processor unit.
For that purpose, each sensor is provided with means for determining whether or not the wheel in which it is mounted is rotating. Those means may comprise an electronic accelerometer, of the micro-electromechanical system (MEMS) component type, that regularly evaluates the direction of gravity relative to a reference frame that is specific to it, thereby making it possible to determine that the wheel is rotating when the direction of gravity varies relative to the reference frame of the sensor, and that it is not rotating if said direction is constant.
Thus, in general, each sensor transmits pressure and/or temperature data while the wheel in which it is mounted is rotating, and does not transmit any such data whenever it evaluates that the wheel is stationary. That makes it possible to limit the electricity consumption of the sensor significantly throughout its life.
When a sensor is transmitting signals, said signals comprise firstly digital data transmission signals for transmitting digital data such as measured pressure and measured temperature, and secondly a “location” signal that is used by the central processor unit to identify the wheel in which the sensor transmitting that digital data is mounted. For example, the location signal is made up of a series of pulses transmitted at predetermined regular intervals.
In practical terms, the central processor unit identifies the wheels on the basis of their speeds of rotation that differ significantly from one wheel to another, to a small but real extent. These differences in speed are due, in particular, to the difference in the state of the road surface between a left wheel and a right wheel, to a wheel alignment or “tracking” defect, and to other parameters of that type that inevitably give rise to differences in speed of rotation of the wheels.
In practice, on the basis of the location signal, the central processor unit establishes an accurate estimate of the speed of rotation of the wheel carrying the sensor that generated the signal. The estimate is then compared with measurements of speeds of each wheel that are established and updated in parallel by an independent system such as an anti-lock braking system (ABS) with which the vehicle is equipped.
Finally, the central processor unit determines that the wheel from which the location signal is coming is the wheel that has the speed measured by the independent system that is the closest to the speed estimated accurately on the basis of the location signal.
Accurate estimation of the speed of the wheel on the basis of the location signal is, for example, achieved as described in Patent Application FR 2 833 523, i.e. with an algorithm that identifies a periodicity for the envelope curve of the location signal received by the central processor unit: the period of said envelope curve corresponds to the frequency of rotation of the wheel.
Another solution for accurately estimating the speed of the wheel on the basis of the location signal is given in Patent Application FR 2 844 748, that solution being based on the phase shift of that signal.
In practice, it appears that the sensors must transmit signals having relatively high power in order to obtain information transmission that is sufficiently reliable, which significantly limits the lives of the batteries equipping such sensors.
One of the solutions that makes it possible to increase the lives of the sensors consists in adding an additional antenna connected to the central processor unit by positioning it in such a manner as to improve the wireless transmission conditions. That makes it possible to reduce the emission power of the signals generated by the sensors, and thereby to increase the lives of the primary or secondary batteries of such sensors.
Unfortunately, adding an additional antenna gives rise to extra equipment cost and extra integration cost, which costs are prohibitive to implementing a pressure monitoring system.