The present invention relates to wheel location techniques used on certain vehicles whose wheels are fitted with detection and transmission modules.
Such modules take measurements on the wheels and transmit a signal across a wireless link (usually radio) to a reception module installed on board the vehicle to report the measurements or specific conditions detected from the measurements.
The most common application is for measuring tire pressure. However, other parameters may also be measured, for example temperature. The purpose of taking such measurements is to alert the vehicle driver to any abnormal conditions which might occur or simply to make him aware of the measured parameters. For this, it is necessary to give him an indication as to which wheel is concerned.
To this end, a known approach is to incorporate an identifier, characteristic of the wheel, in the signal transmitted by the module mounted on that wheel.
One problem which may arise is that the vehicle wheels can be switched in various repair or maintenance operations, which means that the identifier will no longer enable the driver to locate a wheel on the vehicle.
European patent 0 626 911 describes a device for monitoring the tire pressure of vehicle wheels, in which the receiver is also able to operate in a matching mode in which it establishes a link between each identifier incorporated in the signal by a module and a wheel position. It is necessary to switch to matching mode at least whenever a wheel has been changed. The process is relatively laborious: having manually switched the receiver to matching mode, the operator must deflate or re-inflate a tire in order to determine the detection signal whose measurements reflect the change in pressure; the operator indicates the position of the tire he has deflated or re-inflated and a link is stored in memory between this wheel position and the identifier picked up from the determined detection signal. To avoid these operations, which can not generally be performed by the driver but require a mechanic, the number of receivers can be increased, one being positioned facing each wheel: the position of the receiver which picks up a signal carrying a given identifier with the strongest intensity then provides the information about the corresponding wheel position. However, compared with a single receiver, this solution has the disadvantage of increasing the number of circuits required by the system.
In European patent 0 861 159, it has been proposed to take into account the intensities of the signals received from the wheels in order to eliminate any signals which might have been picked up from another vehicle. It is also proposed that any fluctuations in the intensity of the received signal be examined in order to distinguish between the spare wheel or wheels of the vehicle, which give rise to a constant intensity because they are immobile, and the axle-mounted wheels, from which the intensity varies due to the wheel rotation.
French patent 2 774 118 describes a method in which the processing circuits co-operating with the receiver analyse a signature of the received detection signal in order to allocate the signal to a specific wheel position. Typically, this signature is an intensity profile of the received signal, which differs depending on the position at which the wheel is mounted on the vehicle due to the different obstacles to propagation of the waves. Therefore, a pattern recognition associates a signal with a wheel position. This may be performed from time to time to update or validate the link between an identifier and a wheel position, the distinction then being made on the basis of the detected identifier. An initial learning phase, performed on the actual vehicle or on another vehicle of the same model, is used to determine a reference waveform for each wheel position. The signature observed during the recognition phase is then compared with the different reference waveforms in order to estimate the position of the wheel from which the signal was transmitted.
This pattern recognition process is relatively difficult since the detected signals contain noise and are not in phase with the reference waveforms and they exhibit fluctuations depending on the speed of the vehicle upon transmission of the detection signal, which speed may be different from that at which the reference waveforms were learned. This learning process therefore requires a relatively powerful signal processor, which is detrimental to the cost of the device.
An object of the present invention is to propose a signal processing method that will enable the requisite operations to be simplified, thereby making it possible, in particular, to use relatively simple and economic processing circuits.
Accordingly, the invention proposes a method of processing a signal sensed by a receiver installed on board a vehicle having a plurality of wheels fitted with respective detection and transmission modules, each detection and transmission module being capable of transmitting a detection signal including an indication of a state of the wheel fitted with said module, wherein the sensed signal is compared with a plurality of waveforms respectively stored in association with wheel positions to select a wheel position for which the stored waveform matches the sensed signal. According to the invention, after pre-processing the sensed signal and identifying extrema of the pre-processed signal, the comparison of the sensed signal with a waveform comprises applying a time transform to the pre-processed signal to align first and second extrema of the pre-processed signal with first and second extrema of said waveform and computing a distance between the transformed signal and said waveform. The wheel position for which the computed distance is minimum is then selected.
The time transform applied to the signal is determined from a limited number of simple parameters, relating to the extrema of the signal and the stored waveforms. The waveform can then be reliably recognised in a relatively limited number of operations.
There are various possible ways of further limiting this number of operations. For example, the position in time and the type (maximum or minimum) of each identified extremum of the pre-processed signal may be ascertained so that the time transform to be applied to align first and second extrema of the pre-processed signal with first and second extrema of a stored waveform is not determined unless the first extrema are of the same type as well as the second extrema.
The waveforms being stored for a reference speed of the vehicle, it is also possible to obtain an indication of the speed of the vehicle at the time the sensed signal is received, ascertain the position in time of each identified extremum of the pre-processed signal and then determine the time transform to be applied to align first and second extrema of the pre-processed signal with first and second extrema of a stored waveform only if the difference in time between the first and second extrema of the pre-processed signal and that between the first and second extrema of said waveform is compatible with the reference speed and the obtained speed indication according to a predetermined compatibility criterion.
Another aspect of the present invention relates to a device for processing a signal sensed by a receiver installed on board a vehicle having several wheels fitted with respective detection and transmission modules, each detection and transmission module being capable of transmitting a detection signal including an indication of a state of the wheel fitted with said module. The device comprises means for pre-processing the sensed signal, means for identifying extrema of the pre-processed signal, and means for comparing the sensed signal with a plurality of waveforms respectively stored in association with wheel positions to select a wheel position for which the stored waveform matches the sensed signal. The means for comparing the sensed signal with a waveform comprise means for applying a time transform to the pre-processed signal to align first and second extrema of the pre-processed signal with first and second extrema of said waveform and means for computing a distance between the transformed signal and said waveform, the wheel position being selected by minimising the computed distance.
A third aspect of the invention relates to a method of learning waveforms for a device for processing a signal sensed by a receiver installed on board a vehicle having several wheels fitted with respective detection and transmission modules, each detection and transmission module being capable of transmitting a detection signal including an indication of a state of the wheel fitted with said module. This method comprises the following steps for each of said vehicle wheels:
recording a substantially periodic reference signal from a detection and transmission module;
determining the period of the reference signal after a pre-processing;
re-sampling a period of the pre-processed reference signal over N points, N being a predetermined number, and storing the re-sampled signal period;
additionally storing the position in time and the type (maximum or minimum) of each extremum within the stored signal period.
The waveforms thus learned are suitable for implementing the signal processing method applied in the recognition phase.