ANTI-SKID BRAKE CONTROL SYSTEM WITH FAIL-SAFE ARRANGEMENT

Abstract: In a fail-safe arrangement for an anti-skid brake control system, several related pairs of sensor and control signals are produced only in certain possible combinations. The fail-safe arrangement checks each pair of signals and produces an error signal if a non-conventional combination of any of the pairs of signals occurs. In response to the error signal, the automatic anti-skid control system is disabled in such a way as to allow continued manual brake operation.

Description: The present invention relates to an anti-skid brake control system with a fail-safe arrangement as disclosed in US-A-3 744 851 and described below with some essential details and with reference to figure 1. In the known system fail-safe detection is achieved by one of three possibilities. The first way is to check whether control signals that should change between high and low levels are continuously present. In case a continuous potential is detected; this criterion is,utilized for disabling the automatic control. With this first approach it is not possible to detect a failure when all control signals change between high and low levels in a wrong manner. The second way is to check whether continuous potentials are provided at the valves of the non-driven wheels with non-actuated brake. With this method; it is not possible to detect a failure of the system during braking. Besides, it is necessary to use an additional "brake actuated"-signal. The third way is to check the control system continuously via a program with non-actuated brakes. Such a checking with a program is uncertain and not as reliable as the checking with a hardware arrangement. Besides, there is the disadvantage of no checking during braking. It is the object of the present invention to provide an anti-skid control system with fault detection, as generally known from US-A-3 744 851, with a fault detecting means for reliable checking of the system also during periods of braking. The invention is defined by the features of claim 1. The inventive system comprises a fault detecting means that checks combinations of signals that are provided during operation of the system and provides a fault signal in case a combination is detected which is not possible during faultless operation. The present invention will be understood more fully from the detailed description of the invention given herebelow and from the accompanying drawings of the invention, which, however, should not be taken as limitative to the invention but for elucidation and explanation only: In the drawings: Fig. 1 is a timing chart in an anti-skid control to show a variation of the rotational speed of a wheel in relation to given signal levels; Figure 2 is a circuit diagram of the preferred embodiment of the anti-skid control system with a fail-safe arrangement according to the present invention; and Figure 3 is a block diagram of a hydraulic brake system suitable for control by the anti-skid control system employing the fail-safe arrangement of the present invention. In advance of the description of the preferred embodiment of an anti-skid control system according to the present invention, general anti-skid operation will be described for better understanding of the invention, with reference to Figure 1. Such anti-skid control has been disclosed in the U.S. Patent 3,744,851 issued on July 10, 1973 to M. H. Burchardt et al. In this U.S. Patent, there is disclosed a Brake Force Regulating System for vehicles, especially for motor vehicles, for anti-skid controlling the brake force based on the rotation speed of a wheel, wheel slippage and wheel acceleration. The disclosure of this U.S. Patent is herewith incorporated by reference. In this anti-skid control method, first and second threshold values Vc₁ and Vc₂ are determined by multiplying vehicle speed Vc by two predetermined coefficients. The first and second thresholds Vc₁ and Vc₂ define wheel slippage ranges λ₀, λ₁ and λ₂. Further, a wheel acceleration is determined from a variation of the wheel rotation speed V w which wheel acceleration to detect the wheel accelerating or decelerating. The wheel rotation speed V w is distinguished with respect to the foregoing slippage ranges λ₀, λ₁ and λ₂. As will be appreciated, when the wheel is decelerating and the wheel rotation speed drops below the first threshold Vc₁, a λ₁ signal goes HIGH. Then, as the wheel rotation speed further decelerates and drops below the second threshold Vc₂ a λ₂ signal goes HIGH. On the other hand, if the wheel rotation speed increases beyond the second threshold Vc₂, the λ₂ signal level goes LOW and as the wheel rotation speed further increases beyond the first threshold Vc₁, the λ₁ signal level goes LOW. Here, as will be appreciated from Figure 1, there is no wheel rotation speed at which the λ₁ signal will be LOW and the λ₂ signal will be HIGH. Therefore, if such a signal condition occurs, it shows that the anti-skid control system is malfunctioning. The fail-safe arrangement according to the present invention is intended to detect such errors in the anti-skid control system and perform the fail-safe operation in response thereto. Referring now to Figure 2, there is illustrated the preferred embodiment of an anti-skid control system with the fail-safe arrangement of the present invention. A wheel rotation speed sensor 1 is adapted to produce a pulse train with a frequency proportional to the wheel rotation speed. The pulse train is converted into an analog signal by a frequency-to-voltage converter (F/V converter) 2, which analog signal V w has a voltage proportional to the wheel rotation speed. Based on the variation of the analog signal value, a differentiation circuit 3 produces a signal α w at a voltage representative of instantaneous wheel acceleration. The signal α w is fed to a wheel acceleration comparator 4 which comprises first to fourth differential amplifiers 4a to 4d. The signal α w is inputted to the positive input terminals (+) of the first and second differential amplifiers 4a and 4b and to the negative input terminals (-) of the differential amplifiers 4c and 4d. The negative input terminals of the differential amplifiers 4a and 4b respectively receive reference voltages α₁ and α₂ corresponding to acceleration levels of 1.0G and 0.5G. On the other hand, the positive input terminals of the differential amplifiers 4c and 4d receive reference voltages α₃ and α₄ respectively corresponding to acceleration levels of -1.0G and -1.5G. Therefore, when the signal value of the signal α w is greater than the reference voltages α₁, and α₂, the outputs e₁ and e₂ of the differential amplifiers 4a and 4b will be HIGH and outputs e₃ and e₄ of the remaining differential amplifiers 4c and 4d will be maintained at LOW level. When the wheel acceleration drops to less than 1.0G but is still greater than 0.5G, then the output e₁ of the differential amplifier 4a turns to LOW level and the output e₂ is maintained at HIGH level. On the other hand, when the wheel acceleration α w is in the range between 0.5G and -1.0G, the outputs of all the differential amplifiers 4a to 4d are LOW level. As the wheel decelerates and thus, the signal value α w of the signal α w drops to a value less than the reference voltage α₃, the output e₃ of the differential amplifier 4c goes HIGH and the other output signals remain LOW. As the wheel decelerates further to become lower than -1.5G and thus the voltage of the signal α w drops below the reference voltage α₄, then the outputs e₃ and e₄ both go HIGH. The outputs of the differential amplifiers 4a to 4d in relation to the wheel acceleration α w are summarized in the following table: As will be appreciated, all possible combination of the outputs e₁ to e₄ are shown hereabove. If a combination different from the shown five combinations occurs, it means that the anti-skid control is malfunctioning. Therefore, by detecting combinations different from the foregoing five combinations, errors in the anti-skid control can be detected. On the other hand, a vehicle speed sensor 5c produces a vehicle speed signal indicative of the vehicle speed. The vehicle speed signal is fed to a vehicle speed signal generator 5 which is adapted to produce a reference signal Vc at a voltage related to vehicle speed and which changes at a rate related to vehicle acceleration. The reference signal Vc is fed to reference voltage generators 5a and 5b. The reference voltage generator 5a produces a reference voltage Vc₁ having a value corresponding to 85% of the signal value of the reference signal Vc. On the other hand, the reference voltage generator 5b produces a reference voltage Vc₂ having a value corresponding to 70% of the signal value of the reference signal Vc. The reference voltage generators 5a and 5b are respectively connected to positive input terminals of differential amplifiers 6a and 6b. The analog signal V w ; representative of the wheel rotation speed is also inputted to the negative input terminals of the differential amplifiers 6a and 6b. The differential amplifiers 6a and 6b vary the level of the outputs E₁ and E₂ depending on the slippage of the wheel determined on the basis of the reference voltages Vc₁ and Vc₂. The variation of the output level of the differential amplifers 6a and 6b with respect to the variation of the analog signal V w in relation to the reference voltages Vc₁ and Vc₂ is illustrated in the following table: The outputs e₁ to e₄ and E₁ and E₂ are fed to a logic circuit 15. The logic circuit 15 determines signal level of an inlet signal E v and an outlet signal A v according to the following chart: The function or distinction in the logic circuit has been illustrated in detail in the above-referred U.S. Patent 3,744,851. Referring to Figure 3, when the inlet signal E v and the outlet signal A v are both LOW, the brake pressure to be applied to wheel cylinders 29 provided for each vehicle wheel 30 is increased. When the inlet signal E v goes HIGH and the outlet signal A v remains LOW, the brake pressure is held constant. When the inlet signal E v and the outlet signal A v are both HIGH, the brake pressure is reduced. To carry out the foregoing anti-skid brake control operation, the brake system is provided with an inlet valve 22 and an outlet valve 23 in the hydraulic circuit. The inlet valve 22 includes an inlet valve actuator 16 and the outlet valve 23 includes an outlet valve actuator 17, as shown in Figure 3. The inlet valve 22 is connected to a brake master cylinder 25 to receive a working fluid through a passage 26 and is also connected to the wheel cylinder 29 to apply the working fluid thereto. The master cylinder 25 responds to depression of an associated brake pedal 24 to increase the fluid pressure within passage 26. The working fluid is thus fed to the wheel cylinder 29 when a brake pedal 24 is depressed and the inlet valve 22 is opened. The inlet valve actuator 16 is responsive to the HIGH-level inlet signal E v to be energized and close the inlet valve. On the other hand, the outlet valve 23 is a normally closed valve and is responsive to energization of the outlet valve actuator 17 to open in response to the outlet signal A v . When the inlet valve 22 and the outlet valve 23 are both closed, the fluid pressure applied to the wheel cylinder 29 is maintained at a constant level. The outlet valve 23 is connected to the brake master cylinder 25 via a fluid pump 27 and a one-way valve 28. When the outlet valve 23 is open the working fluid in the passage between inlet valve 22 and wheel cylinder 29 is fed back to the master cylinder 25 through the fluid pump 27 and the one-way valve 28. Thus, the fluid pressure applied to the wheel cylinder 29 is decreased to release the brake pressure. The inlet and outlet valve actuators 16 and 17 are respectively operated by switches 20 and 21 which are turned on in response to the inlet and outlet signals E v and A v . Returning to Figure 2, the fail-safe operation is carried out after detection of errors in the anti-skid brake control. In order to detect errors in the anti-skid brake control, AND gates 7 and 8 are provided. The AND gate 7 has an input terminal connected to the differential amplifier 4d and an inverting input terminal connected to the differential amplifier 4c. As will be appreciated from the foregoing table, there are no allowable output conditions in which the output e₃ of the differential amplifier 4c is LOW and the output e₄ of the differential amplifier 4d is HIGH, therefore, if the AND gate 7 outputs a HIGH signal, it means the anti-skid brake control system is malfunctioning. Likewise, by connecting the differential amplifiers 4a and 4b to the AND gate 8 via an input terminal and an inverting input terminal respectively, errors in the control system can be detected. The AND gates 7 and 8 produce HIGH level signals only when the acceleration comparator is malfunctioning. The output terminals of the AND gates 7 and 8 are connected to the SET input terminal S of an R-S-type flip-flop 12 via an OR gate 11. When the SET terminal receives a HIGH-level input, the flip-flop 12 produces a SET signal at its Q-output terminal to turn ON a transistor Tr1 to illuminate a fault indicator lamp 13 and stops producing a RESET signal at its Q -output terminal to turn OFF another transistor Tr₂ to de-energize a relay coil 18 which opens a relay switch 18a and thereby disables the anti-skid control operation of the inlet and outlet valve actuators 16 and 17. Likewise, an AND gate 9 is connected to the differential amplifiers 6a and 6b to receive the outputs E₁ and E₂. One of the input terminals of the AND gate 9 is an inverting input terminal connected to the differential amplifier 6a. Thus, the AND gate outputs a HIGH-level signal when the output E₁ of the differential amplifier 6a is LOW and the output E₂ of the differential amplifier 6b is HIGH, which is not possible under normal circumstances. The AND gate is also connected to the SET terminal of the flip-flop 12 via the OR gate 11 in order to disconnect the anti-skid control system when such an erroneous combination of control signals is detected. Another AND gate 10 is connected to the flip-flop 12 via the OR gate 11 for fail-safe purposes. The AND gate 10 is connected to the logic circuit 15 to receive the outlet signal A v at one input terminal and the inlet signal E v at an inverting input terminal. As will be appreciated from the foregoing table, it is not possible to have the inlet signal E v LOW and the outlet signal A v HIGH. Therefore, by detectinng this condition, malfunction of the control system can be detected and by deactivating the relay coil 18, the fail-safe operation can be performed. It should be noted that when the relay switch 18a is opened and thus both of the inlet and outlet valve actuators 16 and 17 are disabled, the inlet valve 22 remains open and the outlet valve 23 remains closed in order to enable manual braking. It should be additionally noted that the flip-flop 12 should be RESET initially in order to enable automatic anti-skid control. This can be realized in any of a number of ways which will occur to those skilled in the art. As set forth, according to the present invention, errors in the anti-skid control system can be satisfactorily detected and fail-safe operation can be performed by disabling the control system.

Claims: An anti-skid control system with fault detection, said system comprising: a wheel rotation speed sensor (1) coordinated to each wheel; an acceleration calculating circuit providing a signal indicating wheel acceleration, or deceleration, respectively; a wheel acceleration comparator (4) providing at least the following signals: a first acceleration signal(e1) when wheel acceleration is above a first threshold, a second acceleration signal (e2) when wheel acceleration is above a second threshold lower than said first threshold; a first deceleration signal (e3) when wheel deceleration is below a third threshold, and a second deceleration signal (e4) when wheel deceleration is below a fourth threshold lower than said third threshold; a vehicle speed sensor (5c); a vehicle speed comparator (6) receiving signals from said vehicle speed sensor and said wheel rotation speed sensor (1) for providing at least the following signals: a first slippage signal (E1) when wheel speed is below a first partial value of vehicle speed, and a second slippage signal (E2) when wheel speed is below a second partial value of vehicle speed, lower than said first partial value; and a logic circuit (15) providing at least an inlet signal (Ev) to an inlet valve and an outlet signal (Av) to an inlet valve; and a fault detecting means said fault detecting means, characterised in that comprises AND gates (7, 8, 9, 10) each with an inverting and a non-inverting input terminal for detecting combinations of at least two of said signals and for providing a fault signal in case that at least one of the following combination is detected each by means of one of said AND gates which combinations are not possible during a faultless operation of the system; a first fault condition whether said first deceleration signal (e3) is not present while said second deceleration signal (e4) is present; a second fault condition whether said first acceleration signal (e1) is present while said second acceleration signal (e2) is not present; a third fault condition whether said first slippage signal (E1) is not present while said second slippage signal (E2) is present and a fourth fault condition whether said inlet signal (Ev) is not present while said outlet signal (Av) is present. The system as set forth in claim 1, characterized by means for disabling anti-skin control in response to said fault signal. The system as set forth in claim 1, characterized by disabling means for disabling a control valve when receiving said fault signal. The system as set forth in claim 1, characterized by display means for indicating in response to the fault signal that the anti-skid control is malfunctioning.

Inventors: MATSUDA, TOSHIRO
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1982
Main classification: B60T17
IPC codes: B60T8