Abstract:
A method for determining a rotational speed of a rotating object of a vehicle includes providing information unrelated to the rotational speed of the object of the vehicle using an electronic device and providing information related to the rotational speed of the object of the vehicle using the electronic device. One example of a system employing this method is a tire pressure sensor system. The tire pressure sensor provides information regarding the pressure of the tire of the vehicle. Likewise, a periodicity in a signal transmitted by a transmitter associated with the tire pressure sensor can be used to determine the rotational speed of the wheel of the vehicle to which the tire is attached.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]    The present application is a U.S. national phase filing of PCT/FR02/03744 filed Oct. 30, 2002, which claims priority to French Application No. 01/14140 filed Oct. 31, 2001, the disclosures of which are hereby incorporated by reference. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates to the measurement of the rotational speed of the wheels of automobiles. These rotational speeds may be useful in particular in wheel anti-locking systems or travel control systems.  
           [0003]    There are many methods available which permit the rotational speed of the wheels of automobiles to be measured, the most basic being the simple rev-counter. However, these methods always require specific devices which are therefore costly in terms of assembly and maintenance.  
           [0004]    Wheel tire pressure sensors are also known. These tire pressure sensors are becoming more and more widely used, it could almost be said that manufacturers systematically mount them on their vehicles. FR 2 774 178 discloses that the signal emitted by the emitter of a wheel tire pressure sensor, rotationally driven with the wheel, is modulated in amplitude during rotation as a function of the obstacles and other fixed masking objects due to the portion of the chassis which is disposed between said emitter and the corresponding receiver.  
         SUMMARY  
         [0005]    The Applicant has also noticed that the modulation envelope of the signal from a tire pressure sensor was a periodic signal whose period is equal to the length of one revolution of the wheel.  
           [0006]    To this end, the present invention relates to a method for measuring rotational speeds of the wheels of an automobile. In some embodiments, this is characterized by the fact that the envelopes of the signals from the emitters of the tire pressure sensors of said wheels are detected and their periods are determined so as to deduce therefrom their rotational speeds.  
           [0007]    Of course, the angular rotational speed v, in revolutions/second, and linear rotational speed V, in meters/second, of a wheel having a circumference of length c and on the wheel nm of which the emitter of the sensor emits a signal modulated by the rotation in accordance with an envelope of period T, are given by the following formulae: v=1/T; V=c.v.  
           [0008]    An important feature of the method of an exemplary embodiment is that no additional equipment is necessary, other than the tire pressure monitoring system.  
           [0009]    Another embodiment also relates to a system for measuring the rotational speeds of the wheels of an automobile for implementation of the method, comprising a set of wheel tire pressure sensors having emitters mounted on the wheels, means for processing the signal in order to extract the modulation envelopes from the signals emitted by the emitters, to calculate the period of these envelopes, and to deduce therefrom the rotational speeds.  
           [0010]    The measuring system in accordance with some embodiments may also comprise means for estimating the period of the modulation envelopes before measuring said period, for example means indicating the linear speed of the vehicle.  
           [0011]    Owing thereto, it is possible to measure the rotational speeds only during a minimum time taking into account the speed of the vehicle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 shows the block diagram of the system according to an exemplary embodiment;  
         [0013]    [0013]FIG. 2 shows a typical signal output by a pressure sensor and its modulation envelope;  
         [0014]    [0014]FIG. 3 shows the flow chart of the method for determining the rotational speed of a wheel; and  
         [0015]    [0015]FIG. 4 shows how the method is implemented on the four wheels of a vehicle. 
     
    
       [0016]    The system for measuring the rotational speed of a wheel  11  in accordance with one embodiment will now be described.  
       DETAILED DESCRIPTION  
       [0017]    With reference to FIG. 1, the embodiment comprises an assembly having a pressure sensor  12  and its emitter  13  both mounted on the wheel  11  as well as a fixed receiver  14  intended to process the signal  21  emitted by the emitter  13 .  
         [0018]    Generally, the fixed receiver  14  comprises, in series, a receiving antenna  141 , a demodulator  142  outputting an analogue carrier-free signal  22 , a filter  143  and electronics  144  for processing the tire pressure data.  
         [0019]    These devices permit the radio signals to be transformed into digital signals and the tire pressure to be calculated.  
         [0020]    The system has, in this case downstream of the demodulator  142 , a branching  16  of the signal received by the fixed receiver, supplying means  17  for processing the signal in order to extract the modulation envelope from the signal and in order to calculate the rotational speed of the wheel.  
         [0021]    In this case, the signal processing means comprise, in series, the following means:  
         [0022]    filtering means  171  for extracting the modulation envelope  23  from the signal,  
         [0023]    acquisition means  172 ,  173  for sampling and digitising said envelope  23 ,  
         [0024]    a calculating processor  174 ,  178  for determining the period of this envelope.  
         [0025]    The system also comprises at least one clock  176  connected to the acquisition means and to the processor which is itself connected to at least one memory  177  for storing the samples and to the vehicle speed indicator  15 .  
         [0026]    In this example, the system also uses the onboard speed indicator  15  of the vehicle. This could equally be the odometer since it is disposed along the circumference of the wheels.  
         [0027]    With reference to FIG. 2, the method consists of cyclically processing the signal emitted by the emitter of the pressure sensor, the cycle comprising an observation time Θ during which the amplitude maxima P i1 , P i2 , which are substantially equal and successive, and the times separating these maxima are detected.  
         [0028]    These times correspond to the sought period T, from which the rotational speed will be deduced.  
         [0029]    To obtain this result, the signal  22  is recovered at the output of the demodulator  142  and is submitted to low-pass filtering in the filter  143  so as to isolate the envelope.  
         [0030]    This can be effected by an analogue filtering method, choosing a filter cutoff frequency, Fc, which is slightly greater than the maximum rotational frequency of the wheels.  
         [0031]    The filtered signal is then sampled at a predetermined sampling frequency fe. In accordance with a well known signal processing rule, this frequency is at least double the cutoff frequency Fc.  
         [0032]    The observation time Θ is determined from a piece of information regarding the speed of the vehicle, which information is available from other sources e.g. onboard instruments: speedometer, odometer.  
         [0033]    In fact, if U is this speed in meters/second and c is the length in meters of the circumference of the wheel, the period T of rotation of the wheels is estimated by the ratio c/U. This estimation permits the observation time Θ be chosen such that it at least contains the two sought maxima:  
       Θ   =       2      c     U                           
 
         [0034]    The observation time Θ can, from a certain number of cycles, be optimized to a smaller value, taking into account the signal history, the knowledge of the speed of the vehicle and the position of the maximum in the period, up to a value close to, but always greater than, T such that the periodic signal, the period of which is to be determined, is completely located therein.  
         [0035]    Once this difficulty has been removed, the n obtained sampled values P 1 , P 2  . . . , P i , P n  are arranged into an order for example chronological 1, 2, . . . , i, . . . , n from a time to and during the thus determined observation time. Of course, the number n of arranged values is such that  
         n=fe.Θ 
         [0036]    Two successive, substantially equal, maxima P i1  and P i2  are thus sought in the arranged signal, the values of the maxima correspond to the maximums of one and the other of two successive periods, and their arrangement locations i 1  and  12  in this storing time Θ are noted.  
         [0037]    The sought period is deduced therefrom.  
           T =( 12 − i   1 )/ fe    
         [0038]    And the rotational speed of the wheel between the time to and the time to +Θ is finally obtained using one of the above formulae.  
         [0039]    The cycle which has just been described may be repeated in order to obtain successive rotational speeds. A sampling of the momentary rotational speed of the wheel is thus provided having a certain sampling frequency Fe.  
         [0040]    Since a cycle contains at least one observation time to which, in theory, a processing time may be added, the length of the cycle may be longer and, in principle, may be fixed.  
         [0041]    The method permits the rotational speed of the wheels to be measured in a continuous manner at a frequency  
           Fe= 1/Θ 
         [0042]    This frequency is variable and depends on the speed of the vehicle. The higher the speed, the higher the frequency. Thus a rotational speed of the wheel is obtained more quickly as the speed of the vehicle increases.  
         [0043]    Returning to the embodiment of FIG. 1, the signal  21  emitted by the emitter  13  is subjected to a parasitic modulation in particular owing to the chassis  10  before being received by the receiver  14  and being processed therein so as to provide the pressure of the tires. Thus the signal  22  at the output of the demodulator  142  is filtered by the filter of the receiver  143  for the normal requirements for processing the pressure information.  
         [0044]    The signal  22  is also filtered by the filter  171  of the signal processing means  17 , for example an RC filter.  
         [0045]    At the output of the filter  171 , the signal  23  is constantly sampled and digitised at the frequency fe by a sample-and-hold device  172  which maintains the analogue magnitudes at regular times at the frequency fe and an analog to digital converter  173  which provides at its output  24  digital samples Pi of the signal for which the period is to be determined. These means effect these operations in particular owing to the clock  176 , under the control of the processor  174 .  
         [0046]    With reference to FIG. 3, the processor  174  transmits the control to the program of the memory  178  which acquires and arranges ( 30 ) these samples Pi in the memory  177  from P 1  to P n  from memory M 1  to memory M n  respectively.  
         [0047]    Then the program, or the method, calculates the rotational speed of the wheel which it transmits to the onboard computer  175 . To achieve this, at this stage, it effects the following steps successively:  
         [0048]    searching ( 31 ) for the arrangement addresses i 1  and i 2  of the two largest values Pi in the memory  177 ,  
         [0049]    calculating ( 32 ,  33 ) the time which separate these two rows i.e. the modulation envelope period T=(i 2 −i 1 )/fe,  
         [0050]    calculating ( 34 ) the linear, V=c/T, or angular, v=1/T, rotational speeds and putting these speeds at the disposal of the user means  175 ,  
         [0051]    acquiring ( 35 ) the speed of the vehicle U,  
         [0052]    calculating ( 36 ) a new observation time Θ=2c/U and a new number of samples to be acquired n, in relation to the speed of the vehicle,  
         [0053]    initializing ( 38 ) the following cycle consisting of updating the previously calculated time Θ and number n, and of clearing the memory  177 ,  
         [0054]    waiting ( 37 ), if necessary, for start of the following cycle to +(N+ 1 )Θ,  
         [0055]    starting the next cycle ( 39 ) and then controlling ( 40 ) the sample-and-hold device and the converter  172 ,  173 .  
         [0056]    It is not necessary to wait for the end of the current cycle if the calculations are to end at the same time as the current cycle ends. This is the case when the steps of observing and acquiring the samples and the steps of calculating the speeds are consecutive.  
         [0057]    This is not the case for a more complex version wherein the steps of determining the rotational speed of a wheel effected during the cycle N correspond to samples observed and acquired during cycle N−1.  
         [0058]    For this more complex version, the calculation of the speed at cycle N corresponds, in fact, to the observation of the signal at cycle N−1. Thus during the time from to +N.Θ to +(N+1).Θ the microprocessor, after acquiring the n samples, calculates the rotational speed at which the wheel was running during the time interval [to +(N−1).Θ, to +N.Θ)].  
         [0059]    In the most common example of a measuring system for the set of wheels of a vehicle (FIG. 4) and thus comprising a set of wheel tire pressure sensors, the signal processing means ( 17 ) are common for the processing of the four wheels.  
         [0060]    In the case of a system for measuring the tire pressure comprising four receivers of the ( 14   a ) type, the low-pass filter only needs to be quadrupled and the inputs to the sample-and-hold device and the A/D converter only need to be multiplexed for example. This reduces the amount of necessary equipment accordingly.