Detection of abnormality for rotational speed sensor

Abstract: In an apparatus for detecting the abnormality of a rotational speed sensor adapted for, for example, an anti-­ skid control system, a time period within which a subsequent pulse will be generated by the rotational speed sensor in response to the rotation of a body of revolution such as wheel is estimated on the basis of the latest pulse interval of the pulses previously generated therefrom and a value corresponding to the force of inertia of the wheel developed on the braking operation thereof. The determination of abnormality of the rotational speed sensor is made when the subsequent pulse is not produced within the estimated time period. This is arranged based on the fact that the pulse interval depends upon the inertial force of the wheel. With the above arrangement, the abnormality detection of the speed sensor can be quickly made, so that a fail-safe control in a braking system can be speedily performed in response to the occurrence of abnormality of the speed sensor.

Description: The present invention relates to apparatus for detecting the abnormality occurred with regard to a rotational speed sensor, and is applicable particularly, hut not exclusively, to an anti-skid control system. Generally, anti-skid control systems for use in a vehicle are arranged to control the braking hydraulic pressure on the basis of a detection signal obtained by a wheel speed sensor with a view to maximising the braking efficiency. Since failures of the wheel speed sensor result in difficulty to keep the appropriate anti-skid control operation, the execution of fail-safe control would be required in response to the failures because of the necessity to ensure the vehicle braking operation. Therefore, various systems have hitherto been proposed for the detection of abnormality of the wheel speed sensor. One example of such systems is disclosed in Japanese Patent Provisional Publication No. 58-61052, the technique of which involves differentiating the detection signal from the wheel speed sensor and detecting the temporarily occurred abnormality by comparing the differentiated value with a reference value. However, in such prior art system a complex circuit arrangement because the detection signal from the wheel speed sensor is differentiated after being converted into an analog signal and it is difficult to detect quickly the abnormality because of the requirement of considerable processing time. It is therefore an object of the present invention to provide a new and improved system for detecting the abnormality of a rotational speed sensor which overcomes the disadvantages inherent in the above prior art system. DE-C-2 547 869 discloses apparatus for detecting an abnormality of a rotational speed sensor employed for sensing a rotational speed of a body of revolution, the apparatus comprising electronic means responsive to a signal from the rotational speed sensor for measuring an interval between successive signals therefrom, and means for determining an abnormality when the length of the interval exceeds a predetermined value. The present invention aims to provide an abnormality detection system having a simple arrangement which is capable of quickly detecting the failures of the speed sensor so that, for example, a fail-safe control in a braking system can speedily he effected with a view to providing a safer braking action. To this aim the invention provides an apparatus for detecting an abnormality of a rotational speed sensor with the features of independent claim 1 and a braking system comprising such an apparatus. Preferred embodiments of such an apparatus are defined in the dependent claims. The object and features of the present invention will become more readily apparent from the following detailed description by way of example only of the preferred embodiments taken in conjunction with the accompanying drawings in which: Figure 1 is a schematic illustration of an actuator drive system, in which control is performed on the basis of an output signal of a rotational speed sensor, incorporating an abnormality detection apparatus according to the present invention; Figure 2 is an illustration useful for describing the basis of the present invention; Figure 3 is a flow chart showing the operation of abnormality detection which is executed by the microcomputer of Figure 1; Figure 4 is an illustration of an anti-skid control system incorporating the apparatus for the detection of abnormality of a speed sensor according to the present invention; Figure 5 is an illustration of the detail of the actuators of Figure 4; Figure 6 is an illustration of the detail of the control unit of Figure 4; and Figure 7 is a graphic illustration useful for describing the functions of the present invention. Referring now to Figure 1, there is illustrated an abnormality detection system for a rotational speed sensor according to a preferred embodiment of the present invention incorporated in an apparatus for controlling an actuator on the basis of a signal produced by a rotational speed sensor. The actuator control apparatus 26 comprises a control unit 35, a waveform shaping amplifier circuit 30 and an actuator drive circuit 36, and is coupled to a rotational speed sensor 5 comprising a disc-like member 5b having, at the circumference thereof, a plurality' of projections 5a which are angularly spaced at equal intervals, and a pick-up device which is associated with the disc-like member 5b to produce a signal corresponding to the rotational speed of the disc-like member 5b. The disc-like member 5b is attached to a body of revolution 1 so that it is rotated together in accordance with the revolution of the body 1. The control unit 35 comprises a microcomputer 35 including a central processing unit (CPU) 35a, a read-only memory (ROM) 35b, a random access memory (RAM) 35c and an input/output port (I/O) 35d. An actuator 17 is controlled through the actuator drive circuit 36 by the control unit 35. The waveform shaping amplifier 30 is responsive to a detection signal from the rotational speed sensor 5 and produces a pulse signal as illustrated in Fig. 2. In Fig. 2, pulses of the pulse signal are represented by P1, P2,... Pn-3, Pn-2, Pn-1, Pn, and the pulse intervals are indicated by Δt₁, Δt n-2 , Δt n-1 , Δt n . A speed variation will be occurred in response to the braking operation of the body of revolution 1 and the rate of speed variation G can be calculated on the basis of the pulse intervals. For example, the rate of speed variation G with respect to Δt n-1 and Δt n is given by the following equation. The above equation is written with respect to Δt n as follows. where: Vn-1 = wheel speed obtained on the basis of Δtn-1 Vn = wheel speed obtained on the basis of Δtn K = constant determined on the basis of the interval of circumferentially spaced projections 5a of the member 5b of the wheel sensor 5, the diameter of the body of revolution 1 and so on. The rate of speed variation G generally depends upon the force of inertia of the body of revolution 1 developed on braking operation thereof and therefore the rate of speed variation G represents the magnitude of the inertial force. Therefore, it will be understood that the following pulse generating point can be estimated in accordance with the above-described equation, i.e., on the basis of the previous pulse interval and the inertial force of the wheel indicated by the rate of speed variation. When a maximum rate of speed variation which can be obtained with regard to the body of revolution rapidly braked is Gmax (for example, -60G to -70G) and Δtn obtained at the time is Δt max , it will be seen from the above equation 1 that Δt max can be determined as a function of Gmax and the previous pulse interval Δt n-1 . Δt max represents a longest time period within which a subsequent pulse will be generated as long as the rotational speed sensor 5 is in the normal condition. Therefore, if the subsequent signal is not produced up to the time point elapsed by Δt max from the time point at which the previous pulse generated, the failure of the sensor 5, such as disconnection or the like, is estimated and the determination of abnormality can be made. Furthermore, according to this abnormality detection method, it is possible to quickly detect the abnormality in accordance with the driving conditions of the body of revolution because Δt max is shortened in response to the increase of wheel speed as obvious from the equation 1. One example of the operation for the determination of abnormality made by the control unit 35 based on the above-noted principle will be described with reference to Fig. 3. Program shown in Fig. 3, in accordance with instructions stored in the ROM 35b, starts with a block 100 where the CPU 35a checks the presence of the output signal of the rotational speed sensor 5. In response to the presence control advances to a block 102 to initiate the operation of a timer. A subsequent block 104 is successively executed to derive a pulse interval between successive pulses, i.e., the previous pulse and the present pulse and a block 106 follows to calculate the speed V of the body of revolution 1 on the basis of the pulse interval derived in the block 104. Control proceeds to a decision block 108 in which the speed V calculated in the step 106 is compared with a reference value (for example, 30 km/hr) to detect if the rotational speed V is greater than the reference value. If smaller, this routine is terminated because of the prevention of the error of determination, that is, a low speed results in a significantly long pulse interval and causes the determination error. If the speed V is greater than the reference, the block 108 is followed by a block 110 where Δt max is derived on the basis of the above-noted equation 1. It is also appropriate that Δt max is given by adding a value ΔT to the previous pulse interval t n-1 , i.e., by the equation Δt max = Δt n-1 + ΔT . The value ΔT is a constant or is derived as a function of the wheel speed. Exit from the operations block 110 is to a decision block 112, so that Δt max is compared with the time T elapsed from the start of the timer. If T > Δt max , a block 114 follows to determine the abnormality of the rotational speed sensor 5. If T ≦ Δt max , this routine is terminated. In the next execution of this routine, the absence of the pulse causes an exit from the decision block 100 to the block 112. Fig. 4 is an illustration of an anti-skid control system incorporating the system for the detection of abnormality of a speed sensor according to the present invention. Front wheel sensors 5 and 6, which are of the electromagnetic pick-up type or the photoelectric type, are respectively provided on front-right and front-left wheels 1 and 2, each producing a signal indicative of a wheel speed and the produced signal being supplied to a control unit 26, and a rear wheel speed sensor 7 is located on a drive shaft 8 that drives rear-right and rear-left wheels 3 and 4, the rear wheel speed sensor 7 producing a signal indicative of an average rear wheel speed and the produced signal being also supplied to the control unit 26. Pressure control signals are supplied from the control unit 26 to solenoid-operated actuators 17, 18 and 19, which respectively supply the hydraulic pressure from a hydraulic pressure generating cylinder 15 through passages 20, 21, 22 and 23 to hydraulic brake devices 9, 10, 11 and 12 under the control of the control unit 26, so that the braking force for each of the wheels is controlled. A stop switch 14 is located in proximity to a braking pedal 13 and generate braking pedal ON or OFF signal indicative of the presence or absence of the braking operation, and the signal from the stop switch 14 is supplied to the control unit 26. Illustrated at numeral 24 is a main relay which causes a power supply (not shown) to switch into connection to the solenoid of each the actuators 17, 18, 19 in accordance with a control signal from the control unit 26. An indicator lamp 25 is provided to alert vehicle passengers when abnormality such as disconnection have occurred in the circuits to the actuator's solenoids and the stop switch 14. The control unit 26 is responsive to the signals from the wheel speed sensors 5, 6, 7 and the stop switch 14, and performs the process for anti-skid control and the control of the main relay 24 and the indicator lamp 25. As illustrated in Fig. 5, each of the solenoid-operated actuators 17―19 comprises a control valve unit 27 for switching the braking hydraulic pressure into pressure-increasing mode, pressure-decreasing mode or pressure-maintaining mode, and a pressure-regulating unit 28 including a reservoir and a pump whereby the braking liquid is returned to the side of the hydraulic pressure generating cylinder 15 after temporarily stored on decrease of the hydraulic pressure. The hydraulic pressure supplied from the each the actuators is delivered through the passages 20, 21, 22, 23 to the wheel braking cylinder of each of the hydraulic brake devices 9―12. Fig. 6 is an illustration of the detail of the control unit 26 of Fig. 4, in which parts corresponding to those of Fig. 1 are marked with the same numerals and characters. Wheel speed signals from the sensors 5, 6 and 7 are shaped to rectangular pulses by waveshaping amplifier circuits 30, 31 and 32, and a stop signal from the stop switch 14 is fed to a buffer circuit 33. A microcomputer 35, which is powered by the constant voltage of a power circuit 34 in response to an ignition switch 41, includes a central processing unit (CPU) 35a that receives these signals through an input/output port (I/O) 35d to perform operations according to programmed instructions stored in a read-only memory (ROM) 35b. A random access memory (RAM) 35c is used for storing temporary data during the process of input data. Brake control signals from the microcomputer 35 are fed through drive circuits 36, 37, 38 to the solenoid-operated actuators 17, 18, 19 respectively, which are connected in circuit with the relay contact 24a of a main relay 24 which is normally opened. The relay contact 24a is closed in response to the energisation of a coil 24b of the main relay 24 performed by a main relay drive circuit 39 under control of the microcomputer 35. An indicator lamp drive circuit 40 energises an indicator lamp 25 under control of the microcomputer 35. Figure 7 is a graphic illustration useful for describing the functions of the above embodiment of the present invention. Let it be assumed that a vehicle is driven with a constant speed and then the running speed is decreased in response to the braking pedal 13 being depressed at the time t1. The speed-decreasing condition is detected by the wheel speed sensors 5―7 and the control unit 26 computes a speed-decreasing ratio or a slip ratio on the basis of the wheel speed signals. When the speed-decreasing ratio or slip ratio exceeds a predetermined level, the control unit 26 performs anti-skid control to obtain the optimal slip ratio by controlling the braking hydraulic pressure applied through the actuators 17―19 to the hydraulic brake devices 9―12. Furthermore, let it be assumed that the wheel speed sensors 5―7 have become abnormal at the time t2 as a result of disconnection or the like. Due to this abnormality the wheel speed signal becomes zero level and the wheel acceleration value is rapidly decreased and assumes a negative value. In response to the speed and acceleration value thus obtained, the control unit 26 switches the actuators 17―19 into the pressure-decreasing mode, and therefore the hydraulic pressures in the hydraulic brake devices 9―12 are respectively reduced as indicated by a dashed line. However, as illustrated in Figure 3, in this embodiment of the invention the abnormality of the wheel speed sensor can be quickly detected based on the absence of wheel speed signal within a longest estimation time period Δt max . Therefore, in response to the detection of abnormality it is possible to speedily perform a fail-safe control, whereby, for example, the anti-skid control is stopped and the hydraulic pressure is transmitted directly from the hydraulic pressure generating cylinder 15 to the hydraulic brake devices 9―12, resulting in the prevention of rapid decrease of the hydraulic pressure in each the hydraulic brake device and increase in safety of the vehicle.

Claims: Apparatus for detecting an abnormality of a rotational speed sensor (5, 6, 7) employed for sensing a rotational speed of a body of revolution, the apparatus (26) comprising electrical means (35) responsive to a signal from the rotational speed sensor for measuring an interval (Δt₁ ... Δt n ) between successive signals (P1, P2, ... P n ) therefrom, characterised by means (35a, 35b) for estimating a time period (Δt max ) within which a subsequent signal is generated by the rotational speed sensor (5, 6, 7) from the previously measured interval (Δt₁ ... Δt n ), the time period (Δt max ) being shortened in response to an increase of rotational speed; and means for determining an abnormality of said rotational speed sensor when the signal is not generated within the estimated time period. Apparatus (26) according to claim 1, wherein said estimating means (35a, 35b) is arranged to estimate said time period (Δt max ) by adding a value (ΔT) to the previously measured interval (Δt n-1 ), said value being derived as a function of a rotational speed (V n-1 ) of the body of revolution on the basis of the measured interval (Δt n-1 ). Apparatus (26) according to claim 1 wherein the estimating means (35a, 35b) is arranged to estimate the time period (Δt max ) on the basis of the previously measured interval (Δt n-1 ) and a value indicative of an inertial force of the body of revolution developed on the braking operation thereof. Apparatus (26) according to claim 3, which is arranged to derive the inertial force indicating value obtained on the basis of the rate of speed variation of the body of revolution. Apparatus (26) according to claim 3, wherein the inertial force indicating value corresponds to a maximum inertial force which can be obtained on the braking operations. Apparatus (26) according to claim 1 comprising means (35) for deriving a rotational speed (V n-1 ) of the body of revolution as a function of the measured interval (t n-1 ); and means (35) for deriving a value indicative of the force of inertia of the body of revolution developed on the braking operation thereof, on the basis of the variations of the derived rotational speed, and wherein the estimating means is arranged to estimate said time period (Δt max ) on the basis of the previously measured interval (Δt n-1 ) and the derived inertial force indicating value. Apparatus (26) according to claim 1 which is arranged to calculate the speed of (V n-1 ) of the body of revolution on the basis of the internal (Δt₁ ... Δt n ) between successive signal (P1, P2 ... P n ) from the rotational speed sensor (5, 6, 7,) and the determining means (35) is arranged to determine an abnormality when the calculated speed is greater than a reference value. A braking system comprising apparatus (26) according to any preceding claim and further comprising braking means (9, 10, 11, 12) responsive to a pressure from a pressure-generating device (15) so as to brake the body of revolution (1, 2, 3, 4) through a directional control valve (27) and a hydraulic pressure control device (28) to said hydraulic pressure generating device (15) and control means (35) which is responsive to a signal indicative of an abnormality of a speed sensor (5, 6, 7) to perform fail-safe control by controlling the directional control valve (27). A braking system according to claim 8, wherein the estimating means (35a, 35b) is arranged to estimate said time period (Δt max ) on the basis of the previous measured interval (Δt n-1 ) and a predetermined value (ΔT) corresponding to the maximum inertial force of the wheel (1, 2, 3, 4) which can be obtained on the braking operation thereof.

Inventors: ASAMI KEN, HIRANO SATOSHI, MAEHATA HIROMI, MASAKI SYOUICHI, OKUDA YUTAKA, SAKAI KAZUNORI, ASAMI, KEN, HIRANO, SATOSHI, MAEHATA, HIROMI, MASAKI, SYOUICHI, OKUDA, YUTAKA, SAKAI, KAZUNORI
Applicants: NIPPON DENSO CO, TOYOTA MOTOR CO LTD, NIPPONDENSO CO., LTD., TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1986
Main classification: B60T8
IPC codes: G01P21, B60T8, G01P3