Speed sensor fault detection system and method

Abstract: The control method/system for sensing "tooth or pulse drop-out" type faults in the output signals from a rotational speed sensor assembly (10) of the type utilizing a rotor (12) and a relatively fixed sensor (16) to provide an output comprising a plurality of pulses having a frequency indicative of the rotational speed of the rotor is provided.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an apparatus/method for detecting faults in the output signals from rotational speed sensors. In particular, the present invention relates to an apparatus/method for detecting faults, particularly "tooth drop-out" type faults, in rotational speed sensors of the type comprising a relatively rotatable toothed rotor which rotates past a rotationally fixed sensor or stator member. Description of the Prior Art Rotational speed sensors which utilize a relatively rotatable toothed rotor which rotates past a relatively fixed sensor or the like are well known in the prior art. Examples of such speed sensor assemblies may be seen by reference to U.S. Patent Nos. 3,961,215; 4,862,025; 4,862,028 and 4,893,075. Such rotational speed sensors are often used in connection with automated vehicular components/systems such as automatic fuel controls, automated transmission systems and/or vehicular anti-lock brake systems (ABS). Typically, such rotational speed sensor assemblies are electromagnetic in nature and rely on the alignment of rotor teeth or spaces between rotor teeth rotating past a sensor to complete or break, respectively, a magnetic flux path to produce a digital or analog signal indicative of the rotational speed of the rotor. While systems utilizing such rotational speed sensors are well known and widely used, they are not totally satisfactory as misalignments, jolts, damaged rotors or other causes may cause one or more tooth or pulse per measuring cycle not to be properly counted, i.e. "tooth drop-out". When this occurs, a false reading can be generated, usually indicating a much lower than actual speed and/or much greater than actual deceleration rate, which false reading if not detected can result in the automated system operating in an undesirable manner. Patent Abstracts of Japan Vol. 10, No. 215 (P-481) [2271] July 26, 1986, Pub.No. JP-A-61 054456 discloses a correcting device for abnormal value of speed or the like of automobil or the like. The known device comprises a sensor that delivers output pulses corresponding to the rotating speed of a shaft. A counter counts the pulses coming from the sensor and a comparing means evaluates the number of pulses which are counted in subsequent equal time intervals. If the difference in counted pulses exceeds a given thresholdvalue then a correction term is added to the last count wherein the correction term corresponds to the maximum variation that may occur. This amended value is communicated. DE-A-34 26 663 discloses a speed sensor providing pulses to be counted for determining the rotational speed of a wheel. The acceleration of the wheel is further determined based on the rate of change of the wheel speed. If the rate of change of the wheel speed exceeds a predetermined threshold value, a disturbance signal is produced. The wheel speed is calculated by dividing the pulses counted during a measuring interval by the length of the measuring interval. The measuring interval starts with the trailing edge of a detected pulse. Its length is determined at each further trailing edge and the measuring interval ends when its determined length exceeds a predetermined value. SUMMARY OF THE INVENTION In accordance with the present invention, the drawbacks of the prior art have been minimized or overcome by the provision of a fault sensing technique for detecting if a "missed" or "dropped tooth" speed sensor reading has occurred and, if sensed, for going into a "fail safe" or a "fail soft" logic routine. The above is accomplished by calculating the currently indicated rotational speed by the relationship S = a function of (N-1)K/ (C N -C O ) where: N = number of tooth counts or pulses per fixed duration time loop (0.010 sec. for example) ; K = a constant; C N = time of occurrence of the last N count; and C O = time of occurrence of the first N count. If the current change in S, or d S/dt, is greater than a predetermined reference value (REF) selected to represent the greatest expected wheel deceleration, such as -25G to -30G (G = 9,81 m/s²) for example, then a single or multiple "tooth drop-out" error is sensed and the system will either shut off ("fail safe") or go into a fault tolerant ("fail soft") mode of operation. The time duration of the loop is selected so that a false reading of (N-1) by a single unit will either not effect the calculated value of S or will be detectable (i.e. will cause dS/dt to exceed the reference value REF). Preferably, the fault check logic is performed prior to any filtering of the sensor input signal. Accordingly, it is an object of the present invention to provide a control apparatus and method for detecting "tooth drop-out" type errors in a tooth count type toothed rotor/fixed sensor rotational speed sensor. This and other objects and advantages of the present invention will become apparent from a reading of the description of the preferred embodiment taken in connection with the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of a toothed rotor type speed sensor assembly. Figure 2 is a graphical illustration of the output signals from a typical prior art rotational speed sensor. Figure 3 is a schematic illustration, in flow chart format, of the control system/method of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT The control apparatus/control method of the present invention is intended for use in connection with rotational speed sensor assemblies of the type typically utilized to provide input signals to the controllers (CPUs) of vehicular automated drivetrain and/or braking systems. Typically, the greatest possible acceleration and/or deceleration of such wheels is a known value. Vehicular ABS and automated drivetrain systems are well known in the prior art and examples thereof may be seen by reference to U.S. Patent Nos. 3,920,284; 3,929382; 4,168,866; 4,478,840; 4,818,035; 4,863,221; 4,361,060; 4,527,447; 4,643,048 and 4,860,861. Speed sensor assemblies for providing input signals to control units, usually microprocessor based central processing units, are well known in the prior art as may be seen by reference to above-mentioned U.S. Patent Nos. 3,961,215; 4,862,025; 4,862,028 and 4,893,075. Typically, as may be seen by reference to Figure 1, such speed sensor assemblies include a toothed ferromagnetic rotor 12 which is keyed to a monitored shaft 14 for rotation therewith and a stator or sensor member 16 rotationally fixed relative to a vehicle component 18 for providing signals indicative of the rotational speed of the rotor. Usually, rotation of one tooth and one tooth space past the stator or sensor 16 will result in the making and then breaking of a magnetic flux path which will result in pulses of increasing and decreasing induced current, the frequency of such pulses or tooth counts being indicative of the rotational speed of the rotor and those members rotating therewith. Of course, the rotor teeth may be replaced by undulations, apertures, etc. While the present invention is particularly well suited for use with toothed rotor type electromagnetic speed sensor assemblies of the type schematically illustrated in Figure 1, the present invention is equally applicable to any type of speed sensor assembly providing an output in the form of pulses the frequency of which is indicative of the rotational speed of the monitored members. Figure 2 is a graphical illustration of the output of a typical pulse type speed sensor assembly and of the use of said pulses to calculate a rotational speed of the monitored shaft 14. As has been indicated above, the basic relationship or formula for calculating the rotational speed of a monitored member such as shaft 14 is S = (N-1)K/(C N -C O ) where: N = number of teeth counts per fixed duration time loop (0.010 second for example); K = a constant; C N = time of occurrence of the last N count; and C O = time of occurrence of the first N count. The constant K depends upon the number of teeth on the rotor (60 tooth and 100 tooth rotors are typical in ABS systems), gear ratios, tire sizes and the like. In the 60 tooth rotor example illustrated in Figure 2, K = 860 cycles per second (Hz) at 160 km/h (100 mph) vehicle speed. Assuming the cycle time 10/1000th of a second (0.010 second), line traces 20 and 22 illustrate proper operation of the sensor assembly 10. At 160 km/h (100 mph) vehicle speed, with a constant equal to 860 Hz at 160 km/h (100 mph) vehicle speed, a tooth pulse should be generated by the sensor assembly every 0.0012 second (1/860). In trace line 20, the first pulse will be received exactly 0.0012 seconds after the start of the first cycle and thus, assuming substantially constant vehicle speed, eight (8) tooth pulses will be counted during the testing cycle. Accordingly, (N-1) will equal seven (7) and C N -C O will equal 0.0096 - 0.0012, will equal 0.0084 or one pulse every 0.0012 seconds, which corresponds to the calibration of 860 pulses per second equalling 160 km/h (100 mph). Similarly, referring to trace 22 where the first pulse is received at .001 second into the cycle, nine (9) pulses will be received during the 0.010 second measurement cycle, (N-1) will equal eight (8), C N -C O will equal 0.0096 seconds and once again an average pulse frequency of 0.0012 seconds is observed corresponding to the 160 km/h (100 mph) at 860 Hz calibration. Referring to trace 23, assuming vehicle speed has remained at a substantially constant 160 km/h (100 mph) and that the toothed pulse which should have been sensed at 0.0025 seconds into the measurement cycle has "dropped out", the CPU will calculate a (N-1) = seven (7) with a C N -C O equalling.0096. This will result in a pulse being received on average once every .00137 seconds or a frequency of about 729.93 Hz which would, correspond to about a vehicle velocity of 136 km/h (84.9 mph) per hour. This translates into a wheel deceleration of about 24 km/h (15.1 mph) in 0.010 second which would correspond to a wheel deceleration of approximately -60g which is clearly in excess of expected wheel deceleration. In practice, in typical vehicular use, any wheel acceleration exceeding a reference value of approximately -25 or -30g may be considered indicative of an error. In trace line 24, assuming a substantially constant 160 km/h (100 mph) vehicle speed, and a "drop-out" of the tooth pulse which should have been received at approximately .0097 seconds of the test cycle, (N-1) will equal seven (7), and C N -C O will equal .0084 resulting in a frequency of tooth pulse of one every .0012 second which, while not allowing sensing of a tooth drop-out problem, will result in an accurate indication of vehicular speed. Typically, in a situation such as this, the tooth drop-out will eventually occur other than at the very beginning or the very end of the testing cycle and will allow fault detection thereof. At 80 km/h (50 mph) vehicle speed, assuming conditions similar to those in the example of trace 20 above, i.e. K = 860 Hz at 160 km/h (100 mph) vehicle speed and a cycle time of 0.010 second, a tooth pulse should be generated, on average, every 0.0024 second (1/430). Thus N will equal 4, N-1 will equal 3 and C N -C O will equal .0072 (.0096 - .0024) which is a pulse every 0.0024 second. If a tooth or pulse is "dropped", N-1 will equal 2, which indicates a pulse every 0.0036 second or a vehicle deceleration of about 75G which will, of course, exceed the reference REF. In the case of a 100 tooth rotor, K will equal 1400 KHz at 160 km/h (100 mph) vehicle speed (700 KHz at 80 km/h (50 mph) vehicle speed) and a dropped tooth pulse will result in an apparent vehicle acceleration of about -35g at true 160 km/h (100 mph) and about -38g at true 80 km/h (50 mph) vehicle speeds, both of which will exceed the selected reference. It is noted that the apparent vehicle acceleration resulting from a "dropped tooth" is generally constant for a given rotor over a large range of true vehicle speeds. Upon sensing of a tooth drop-out fault, the system will, preferably, indicate the existence of a fault to the vehicle operator and will take appropriate action in the form of either fail safe or fail soft logic. In a fail safe logic situation, the automated system utilizing the sensor assembly 10 will simply discontinue operation which may render the vehicle inoperative. In a fail soft situation, usually associated with non-vehicle safety related equipment, a fault tolerance logic will be utilized. Typically, in for example a transmission system, if the failed sensor is an engine speed sensor the logic may utilize transmission input shaft speed as a substitute therefor. Fault tolerance logic of this nature may be seen by reference to U.S. Patent No. 4,849,899. It is, of course, important to select the cycle time which is short enough such that, in the range of expected speeds, a reading of (N-1) which is at least one less than true value will result in a value for the change in S or dS/dt exceeding the predetermined reference value (REF). It is also important that the tooth drop-out fault testing occur prior to any filtering of the input signals from speed sensor assembly 10. A schematic illustration, in flow chart format, of the control system/method of the present invention may be seen by reference to Figure 3. As discussed above, a cycle time is selected such that if the true value of N-1 is misread as N-2 or less, then the change or first time derivative of calculated vehicle speed will exceed the predetermined reference value. Preferably, the predetermined reference value is about -20 to -30g which exceeds the maximum vehicle deceleration rate expected in vehicular applications. During each cycle the number N of tooth counts is sensed and the value of change or the first time derivative of d((N-1) K/(C N -C O ))/dt is calculated and compared to the reference value. If the change or time derivative, which is representative of vehicle acceleration/deceleration, does not indicate a tooth drop-out fault in the output signal from the sensor assembly 10, the output of the sensor assembly is filtered and then utilized to calculate the vehicular speed by formula S=(N-1)K/C N -C O . If however a toothed drop-out problem is detected, a fault is declared and the system will then default to a fail safe or fail soft logic routine. Accordingly, it may be seen that a reliable control apparatus/method has been provided for detecting "dropped tooth" type faults in the output signals from toothed rotor type rotational speed sensors. Although the present invention has been defined with a certain degree of particularity, it is understood that the preferred embodiment has been described by way of example only in the numerous modifications and rearrangements are possible without departing from the scope of the invention as hereinafter claimed.

Claims: A fault detection method for detecting "dropped pulse type" faults in the output signals from speed sensor assemblies (10) of the type comprising a rotor (12) rotatably associated with the monitored rotatable member (14) and a relatively fixed sensor member (16), said sensor assembly providing an output signal (20, 22, 24, 26) comprising a plurality of pulses having a frequency indicative of the rotational speed (S) of said rotor, the rotational speed calculated as a function of the number of said pulses over a known period of time, said method comprising the steps of calculating a value indicative of the rate of change in the rotational speed (S) wherein the rotational speed (S) is calculated in accordance with the equation: S = a function of (N-1)K/(C N -C O ) where: N = number of tooth counts or pulses per fixed duration time loop; K = a constant; C N = time of occurrence of the last N count; and C O = time of occurrence of the first N count comparing the absolute value of said value indicative of in the rate of change in the rotational speed (S) to a reference value (REF) and if the absolute value of the value indicative of the rate of change in the rotational speed (S) is greater than said reference value, declaring a fault. The fault detection method of claim 1 wherein said reference value (REF) is substantially equal to the largest expected possible deceleration rate of the monitored object. The control method of claim 2 wherein said cycle time is selected such that if the actual value of (N-1) is sensed as equalling (N-2) or less, then the calculated absolute value of the rate of change in the rotational speed (S) will be greater than said reference value, where N = number of pulse counts per fixed duration time loop. The control method of claim 3 wherein said calculated absolute value of the rate of change in the rotational speed (S) is determined by a comparison of the current calculated value for the rotational speed (S) and the last calculated value for the rotational speed (S). The control method of claim 3 wherein the calculated value for the rate of change in the rotational speed (S) is the first derivative of the value of S with respect to time. A fault detection apparatus for detecting "dropped tooth type" faults in the output signals from tooth rotor type speed sensor assemblies (10) of the type comprising a toothed rotor (12) rotatably associated with the monitored rotatable member (14) and a relatively fixed sensor member (16), said sensor assembly providing an output signal (20, 22, 24, 26) comprising a plurality of pulses having a frequency indicative of the rotational speed of said rotor, the rotational speed calculated as a function of the number of said tooth pulses sensed in a given time period, said apparatus including: means for calculating a value indicative of the rate of change in the rotational speed (S) wherein the rotational speed is calculated in accordance with the equation: S = a function of (N-1)K/(C N -C O ) where: N = number of tooth counts or pulses per fixed duration time loop; K = a constant; C N = time of occurrence of the last N count; and C O = time of occurrence of the first N count ; means for comparing the absolute value of said value indicative of the rate of change in the rotational speed (S) to a reference value (REF) and if the absolute value of the value indicative of the rate of change of the rotational speed (S) is greater than said reference value, declaring a fault. The fault detection apparatus of claim 7 wherein said reference value (REF) is substantially equal to the largest expected possible deceleration rate of the monitored object. The control apparatus of claim 8 wherein said cycle time is selected such that if the actual value of (N-1) is sensed as equalling (N-2) or less, then the calculated absolute value of the rate of change in the rotational speed (S) will be greater than said reference value, where N = number of pulse counts per fixed duration time loop. The control apparatus of claim 9 wherein the calculated value for the rate of change in the rotational speed (S) is the first derivative of the value of the rotational speed (S) with respect to time.

Inventors: MCNINCH JOSEPH HAMILTON JR, MCNINCH, JOSEPH HAMILTON, JR.
Applicants: EATON CORP, EATON CORPORATION
Application year: 1991
Main classification: G01P3
IPC codes: G01P3