Abstract:
A method of processing a pulse train output signal provided by a non-contact sensor that senses the rotational speed and angular position of a rotating wheel includes comparing the amplitudes of the pulses with a variable switching threshold value, and adjusting the value of the variable switching threshold value if the difference between the amplitudes of two successive pulses exceeds a maximum value.

Description:
PRIORITY INFORMATION 
   This application is a continuation of Ser. No. 09/747,503 filed Dec. 20, 2000 now U.S. Pat. No. 6,965,227. 

   BACKGROUND OF THE INVENTION 
   The invention relates to a technique for detecting the rotational speed (e.g., RPMs) and the angular position of a rotating wheel with a non-contact sensing device. 
   For electronic control systems in motor vehicles (e.g., an ABS braking system or an electronic ignition system) it is necessary to measure parameters such as rotational speed, the relative angular position of a rotating wheel or the crankshaft and/or their angular acceleration or instantaneous angular velocity. This is often performed by a non-contact sensor (e.g., an optical sensor or a Hall sensor) that scans the rotating wheel or a wheel seated on the crankshaft. The rotating wheel typically includes scan marks (e.g., lines or teeth). 
   The accuracy of the rotational speed and angular position measurement is often impaired by external noise signals and/or thermal noise. In a measurement of angular position, the thermal noise is often noticeable as jitter. If a digital measurement system is used to measure the rotational speed and the angular position, there is another error source in addition to the thermal noise that garbles the measurement result. This error source is determined by the minimum digital resolution of the least significant bit (LSB) or by the clock pulse rate. Usually the measurement error caused by the minimum digital resolution of the LSB is greater than the measurement error caused by thermal noise. For example, if the position of the threshold changes within one revolution, the scanning times between the individual scan marks on the wheel also change, so that the valuation device sees an instantaneous change of the rotational velocity, although the rotational velocity has not changed. 
   Other error sources are the offset of the sensor and the long-term drift due to aging of the sensor, or a gradual change of temperature in the measurement environment (e.g., by the warming of the engine). 
   Therefore, there is a need for a technique for compensating for various error sources associated with sensing rotational speed and/or angular position. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to process a pulse train output signal provided by a non-contact sensor that senses the rotational speed and angular position of a rotating wheel, in such a way that the offset and long-term drift errors due to aging of the sensor or the offset error due to temperature change are compensated for accuracy. 
   To detect the rotational speed and the angular position of the wheel, the non-contact sensor (e.g., an optical sensor or a Hall sensor) scans marks on the wheel, and generates a pulse train indicative of the rotational speed and angular position of the wheel. The amplitude of the pulses is compared in a comparator with a variable switching threshold. To compensate for offset and long-term drift of the sensor, the variable switching threshold is adjusted based upon one or more characteristics of the pulse train (i.e., threshold adjustment criteria). 
   In one embodiment, the amplitude of the pulses from the sensor are compared in the comparator with the variable switching threshold value, which can be adjusted so the difference between the amplitudes of the pulses and the switching threshold does not exceed a fixable first maximum value. For example, when the difference between the pulse amplitude and the switching threshold of the comparator exceeds the fixable first maximum value, the switching threshold is adjusted so that this difference falls below the fixable first value. If the amplitude of the pulses generated by the sensor is greater than the switching threshold value, the switching threshold value is increased; on the other hand, the value of the switching threshold is decreased if the amplitude of the pulses is less than the switching threshold value. 
   In another embodiment, the positive and negative amplitude of the pulses from the sensor, or their maxima and minima, are compared in the comparator with the variable switching threshold, which is adjusted so that the difference between the extremes or the amplitudes of the pulses and the variable switching threshold does not exceed a fixable second maximum value. 
   In yet another embodiment, the pulses from the sensor are compared in the comparator with the variable switching threshold, which is adjusted if the difference between the amplitudes of two successive pulses exceeds a fixable third maximum value. For example, if the evaluation circuit determines that the amplitude of the second pulse is greater than that of the first pulse, the switching threshold is increased. Initially, if the amplitude of the second pulse is less than that of the first pulse, the value of the switching threshold is decreased. 
   In still another embodiment, the amplitude of the pulses is compared in the comparator with the variable switching threshold, which is adjusted if the difference of the frequencies of successive pulse trains exceeds a fixable fourth maximum value. 
   The variable switching threshold is preferably positioned by the evaluation circuit as stably as possible and is situated in the middle between the minimum and maximum sensor value. 
   The threshold adjustment techniques of the present invention may be used alone or in combination with one another. In addition, the switching threshold of the comparator is preferably only adjusted during the synchronization signal. It is contemplated that in some embodiments, the various individual threshold adjustment criteria may be combined with one another in order to define the criteria when the evaluation circuit may adjust the threshold. For example, consider one embodiment where the following conditions must be fulfilled simultaneously in order for the evaluation circuit to adjust the threshold:
         1. The difference between the pulse amplitudes and the switching threshold exceeds a first fixable threshold value.   2. The difference of the amplitudes of successive pulses exceeds a second fixable maximum.   3. The difference of the frequencies of successive pulses exceeds the third fixable maximum.   4. The RPM of the wheel is greater than a fixable minimum.   5. A synchronization signal is generated.       

   Advantageously, inventive adjustment of the switching threshold improves the accuracy of the measurement. Furthermore, the offset and long-term drift of the sensor due to aging are largely compensated for. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  illustrates a rotational sensing system; 
       FIG. 2  illustrates an alternative embodiment rotational sensing system; and 
       FIG. 3  illustrates yet another alternative embodiment rotational sensing system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a rotational sensor system  10 , which includes a sensor  12  that reads scan marks  14  on a rotating wheel  16 , and provides a sensor output signal on a line  17 . The sensor output signal is input to a comparator  18 , which also receives a first threshold signal on a line  20 , and provides a rotational sensing output signal on a line  22 . 
   The sensor scans the scan marks on the wheel. These scan marks can be lines that are scanned (i.e., read) by an optical sensor, or they can be teeth that are scanned by a Hall sensor. The synchronization mark can be a broad line, a missing tooth, or a broad tooth. When scanning a scan mark—a line or a tooth or a tooth gap, the sensor generates a pulse or skips a pulse, for example in the case of a tooth gap. The data read by the sensor are input to an evaluation circuit that determines the relative angular position of the wheel, its instantaneous angular velocity, and/or its angular acceleration. 
   According to an aspect of the present invention, an evaluation circuit  24  receives the sensor output signal on the line  17  and compares the amplitude(s) of the pulses generated by the sensor  12  against the first threshold signal value on the line  20 . If the difference between the amplitude of the pulses and the first threshold signal on the line  20  exceeds a fixable first maximum value, the evaluation circuit  24  adjusts the value of the first threshold signal on the line  20 . For example, when the difference between the pulse amplitude and the switching threshold of the comparator exceeds the fixable first maximum value, the threshold signal on the line  20  is adjusted so that this difference again falls below the fixable first value. If the amplitude of the pulses generated by the sensor is greater than the threshold value on the line, the switching threshold value is increased; on the other hand, the value of the threshold signal on the line  20  is decreased if the amplitude of the pulses is less than the current value of the threshold. 
   In another embodiment, the positive and negative amplitude of the pulses from the sensor, or their maxima and minima, are compared in the comparator with a variable switching threshold. In this embodiment the variable switching threshold on the line  20  is adjusted so the difference between the amplitudes of the pulses and the variable switching threshold does not exceed a fixable second maximum value. 
   The evaluation circuit  24  may also be configured to compare the amplitudes of successive pulses of the sensor output signal on the line  17 . If the difference of the amplitudes of the two successive pulses exceeds a fixable third maximum value, the evaluation circuit  24  adjusts the value of the switching threshold on the line  20 . For example, if the evaluation circuit determines that the amplitude of the second pulse is greater than that of the first pulse, the switching threshold is increased. Initially, if the amplitude of the second pulse is less than that of the first pulse, the value of the switching threshold is decreased. 
   In yet another embodiment, the evaluation circuit  24  adjusts the comparator threshold if the difference of the frequencies of successive pulses exceeds a fixable fourth maximum value. 
   The evaluation circuit  24  may be configured to apply any of these foregoing threshold adjustment criteria either alone or in combination. 
     FIG. 2  illustrates an alternative embodiment rotational sensing system  30 . This system also includes the sensor  12  that cooperates with the rotating wheel  16 , and provides the sensor output signal on the line  17 . The system illustrated in  FIG. 2  is substantially similar to the system disclosed in  FIG. 1 , with a principal exception that the sensing system  30  illustrated in  FIG. 2  includes a synchronization detector  32 . The synchronization detector  32  receives the output signal on the line  22  and provides a sync output signal on a line  34 , which is input to the evaluation circuit  24 . In this embodiment, the evaluation circuit  24  is only allowed to make changes to the threshold signal on the line  20  when the synchronization signal on the line  34  is valid, indicating the presence of the synchronization signal. When the synchronization signal is detected, the evaluation circuit  24  is then allowed to make the adjustments as set forth above. For example, if the evaluation circuit  24  is configured to adjust the threshold when the difference between the amplitude of the pulses and the threshold signal on the line  20  exceeds the fixable first maximum value, this adjustment will only be made when the sync signal on the line  34  is valid. That is, if the sync signal on the line  34  is not valid, the threshold signal will not be adjusted even if the difference between the amplitude of the pulses and the threshold signal on the line  20  exceeds the fixable first maximum value. Of course, any of the adjustment criteria set forth above may be used alone or in combination in the system illustrated in  FIG. 2 . 
     FIG. 3  illustrates yet another alternative embodiment rotational sensing system  50 . This embodiment, is substantially the same as the embodiment illustrated in  FIG. 2 , with the principal exception that the embodiment in  FIG. 3  includes a filter  52 , that receives the sensor output signal on the line  17 . The circuit arrangement shown in  FIG. 3  may adjust the switching threshold in accordance with one or more of the specified threshold adjustment criteria. 
   The invention is especially suited for use in a motor vehicle, where, for example, it accurately measures: (i) the rotational speed and angular position of the crankshaft for an electronic ignition system, or (ii) the rotational speed and the angular position of the individual wheels for an ABS braking system, an anti-skid control, or a vehicle stabilization system. 
   Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.