WHEEL SLIPPING CONTROL APPARATUS

Abstract: A wheel slipping control apparatus for controlling a braking hydraulic pressure to be applied to a wheel so as to perform an effective braking operation. The wheel slipping control apparatus comprises a control unit for generating a control signal in accordance with the wheel slipping state so as to perform a braking pressure control cycle so that the braking hydraulic pressure is set to a pressure decreasing mode and then to a pressure increasing mode. In the control unit, one of the braking pressure control cycles to be performed is determined as a learning control cycle to obtain a learning pressure-decreased value decremented in the learning control cycle and to obtain a learning pressure-increased value incremented in the learning control cycle. A present pressure-decreased value decremented in a control cycle after the learning control cycle is obtained to calculate a target pressure-increased value on the basis of the obtained learning pressure-decreased value and learning pressure-increased value whereby the braking hydraulic pressure is first increased to the vicinity of a wheel locking limit pressure after decreased in the pressure-decreasing mode.

Description: The present invention relates generally to a wheel slipping control apparatus according to the preamble of claim 1. Anti-skid control systems are known as systems for principally controlling the braking hydraulic pressure in a brake control system to prevent the motor vehicle from being slipped laterally due to locking of a vehicle wheel on a rapid braking operation. For effectively performing the braking operation, it is desirable that the braking hydraulic pressure is increased in advance to a pressure value immediately before occurrance of the locking of the vehicle wheel and is kept for a long time as it is. However, since the locking pressure is varied in accordance with the running road surface state, initially taking a long time to increase the hydraulic pressure may result in excessive increase of the hydraulic pressure and locking of the vehicle wheel. Therefore, it is not allowed to increase the hydraulic pressure at once and it may be required to gradually increase the hydraulic pressure in accordance with the slipping state of the vehicle wheel. This does not result in an effective braking control. An attempt to avoid this problem is disclosed in JP-A-60-47751, where, as shown in Fig. 13 thereof, the locking prevention apparatus is arranged such that in the present pressure increasing control cycle, a time ta taken for increasing the pressure is determined as ta = f(n) where f is a constant function and n (=4) is the number of times that the hydraulic pressure has been increased in the previous pressure increasing control cycle. However, in such a locking prevention apparatus there arises problems because of no consideration of the previously decreased hydraulic pressure P1 and the presently decreased hydraulic pressure P2. More specifically, if there is the difference in the braking sensitivity between two types of a motor vehicles, the difference results in variation of the present decremented hydraulic pressure (P2, P3). That is, the decrement of hydraulic pressure is varied, for example, between the first type in which the releasing of the braking force is highly responsive to the decrease of the hydraulic pressure and the slipping of the vehicle wheel is settled rapidly, and the second type in which the sensitivity is slightly inferior to the first type and the wheel slipping is settled gradually. Furthermore, the inertia force of the driven wheel is varied in accordance with the gear position of the transmission and the settlement of the wheel slipping is made under different conditions. For example, the slipping is rapidly settled under the condition of the neutral gear position and the slipping is gradually settled due to the engine braking action under the condition of the low-speed gear position. Therefore, when applied to a motor vehicle, the conventional apparatus requires to appropriately and accurately obtain the function f(n) in consideration of braking sensitivity and so on. If the function f(n) is not appropriate, there exists a problem in that, when, as indicated by P2 in Fig. 13, the presently decreased amount of the hydraulic pressure is small, the next pressure increasing time period ta results in the fact that the braking hydraulic presure exceeds the lock limit pressure, and when, as indicated by P2 in the figure, the present decrement is great, the pressure increasing time period ta results in the fact that the braking hydraulic pressure is separated insufficiently from the lock limit pressure. It is therefore the object of the present invention to improve a wheel slipping control apparatus according to the preamble of claim 1 in such a way that it can be easily applied for motor vehicles irrespective of variations of the braking sensitivity and that it is capable of initially increasing the hydraulic pressure smoothly to the vicinity of the locking limit pressure of the wheel. According to the present invention, this object is solved by the advantageous features indicated in the characterizing part of claim 1. The object and features of the present invention will become more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: Fig. 1 is a schematic diagram of a basic arrangement of the present invention; Fig. 2 is an illustration of a wheel slipping control apparatus according to a first embodiment of the present invention which is incorporated into a motor vehicle; Fig. 3 is an illustration for describing the functions of actuators; Fig. 4 is a block diagram showing a detailed arrangement of an electronic control unit which is used for the present invention; Fig. 5A is a flow chart for describing the principal processes executed by a microcomputer; Fig. 5B is a flow chart showing a timer interrupt routine executed for each of several wheels; Fig. 5C is a flow chart showing an interrupt routine executed in response to a pulse signal from each of several wheel speed sensors; Fig. 6 is a flow chart for describing a stepwise pressure increasing control in the first embodiment; Fig. 7 is a flow chart for describing a pressure increasing time learning control in the first embodiment; Fig. 8 is a timing chart showing the relationship between the wheel speed and the braking hydraulic pressure for a better understanding of the first embodiment; Fig. 9 is a graphic illustration for obtaining the target hydraulic pressure coefficient K; Fig. 10 is a flow chart for describing the operation executed in a second embodiment; Fig. 11 is a flow chart for describing the pressure increasing time learning control in the second embodiment; Fig. 12 is a timing chart which is intended for a better understanding of the second embodiment; and Fig. 13 is a timing chart for describing a prior art control apparatus. Prior to describing the embodiments of the present invention in detail, a brief description of the basic arrangement of the invention will be first made with reference to Fig. 1 for a better understanding of the invention. The wheel slipping control apparatus according to the present invention is shown in Fig. 1 as basically comprising wheel speed sensing means M6, control means M0 and braking pressure adjusting means M7. The control means M0 generates a control signal in accordance with the slipping state of the wheel obtained on the basis of the rotational speed signal from the wheel speed sensing means M6 so that the braking hydraulic pressure is set to either a pressure-increasing mode or a pressure-decreasing mode. The braking pressure adjusting means M7 is responsive to the control signal in such a way that a control cycle in which the braking pressure is once decreased from a value corresponding to the locking limit pressure and again increased to the value corresponding to the locking limit pressure is repeatedly performed several times, resulting in the adjustment of the braking hydraulic pressure and the basic execution of the wheel slipping control. The control means M0 includes learning condition decision means M1 for determining a learning control cycle to be learned, learning pressure decrement detection means M2 for obtaining the learning pressure-decreased value corresponding to the magnitude decreased in the learning control cycle, learning pressure increment decision means M3 for obtaining the learning pressure-increased value corresponding to the magnitude increased in the learning control cycle, present pressure decrement detection means M4 for obtaining the current pressure-decreased value corresponding to the magnitude decreased in a control cycle after the learning control cycle, and target pressure-increase command value calculating means M5 for generating a command signal indicative of a first target pressure-increased value in the control cycle in which the present pressure decrement is obtained, the first target pressure-increased value being obtained in accordance with the following function: learning pressure-increased value/learning pressure-decreased value x the present pressure-decreased value. The learning condition decision means M1 performs a decision on the basis of the presence or absence of the learning value, that is, determines, under the condition of the absence of the learning value, that the control cycle is a learning control cycle. In this learning control cycle, the learning pressure decrement detection means M2 and the learning pressure increment detection means M3 respectively obtain the learning pressure-decreased value and the learning pressure-increased value corresponding to the decreased and increased magnitudes. In a control cycle thereafter, the present pressure decrement detection means M4 obtains the present pressure-decreased value corresponding to the magnitude decreased in the cycle. The target pressure-increase command value calculating means M5 calculates the first target pressure-increased value in the cycle in which the present pressure-decreased value has been obtained, in accordance with the above-mentioned function. A command signal indicative of the first target pressure-increased value is supplied from the control means M0 to the braking pressure adjusting means M7 in such a way that the braking pressure is controlled appropriately. That is, the locking limit pressure, which is the highest value in the range in which the wheel locking does not occur, can be obtained on the basis of the learning pressure-decreased value and the learning pressure-increased value obtained in accordance with the road surface condition and the present pressure-decreased value obtained in the control cycle thereafter. This will be understood from the following: In response to the start of the wheel slipping control, repeatedly performed is a control cycle in which the braking pressure is once decreased from the value corresponding to the locking limit pressure and again increased thereto. At this time, since the learning control cycle and the current control cycle are in short time relation to each other, the variation of the road surface state, or locking limit pressure, does not occur substantially therebetween and, in other words, the road surface states or locking limit pressures in those cycles are substantially equal to each other. Therefore, the ratio of the learning pressure-decreased value and the learning pressure-increased value in the learning control cycle is equal to that of the pressure-decreased value and the pressure-increased value in the current control cycle, i.e., learning pressure-decreased value:learning pressure-increased value = present pressure-decreased value:target pressure-increased value. Because the above-mentioned function is obtained from this equation, it is possible to accurately calculate the first target pressure-increased value. Thus, after the learning control cycle, the braking pressure can be increased rapidly by the first target pressure-increased value which has been obtained on the basis of the locking limit pressure, resulting in shortening the braking distance and performing an effective braking operation. If the road surface state is varied after the learning control cycle, the learning pressure-decreased value and the learning pressure-increased value are cleared and a new learning will be effected to allow a more practical braking operation. Referring now to Fig. 2, there is schematically illustrated an arrangement of a wheel slip control apparatus according to a first embodiment of the present invention, which is incorporated into a four-wheel motor vehicle wherein the front wheels are steered and the rear wheels are driven. In Fig. 2, rotational speed sensors 9, 11, 13 and 15 which are of electromagnetic pickup type or of photoelectric conversion type are respectively provided for the front-right wheel 1, front-left wheel 3, rear-right wheel 3 and rear-left wheel of the motor vehicle and generate pulse signals in accordance with rotational speeds of the respective wheels 1, 3, 5 and 7. Further for the respective wheels 1, 3, 5 and 7 are provided hydraulic pressure braking devices 17, 19, 21 and 23. The hydraulic pressures generated due to depression of a brake pedal 25 or operation of pressure control actuators 27, 29, 31 and 33 are supplied through hydraulic pressure passages 35, 37, 39 and 41 to the respective hydraulic pressure braking devices 17, 19, 21 and 23. Therefore, the braking forces for the wheels 1, 3, 5 and 7 can be adjusted by means of the actuators 27, 29, 31 and 33 or the brake pedal 25. The degree of depression of the brake pedal 25 is detected by a stopswitch 43 which generates an ON signal during braking and generates an OFF signal during non-braking. Normally, the hydraulic pressure is produced in a hydraulic pressure cylinder 45 in response to the depression of the brake pedal 25, thereby allowing the braking of the respective wheels 1, 3, 5 and 7. On the other hand, as a hydraulic pressure source for wheel slippage control, a hydraulic pressure pump 47 is further provided which generates a hydraulic pressure in response to the driving of the engine or electric motor. An electronic control unit (ECU) 49 controls the respective actuators 27, 29, 31 and 33 such that the hydraulic pressure from the hydraulic pressure cylinder 45 or the hydraulic pressure pump 47 is regulated before being supplied to the respective hydraulic pressure braking devices 17, 19, 21 and 23, and the braking forces for the wheels 1, 3, 5 and 7 are independently adjustable. It is also appropriate that the hydraulic pressure braking devices 21 and 23 for the rear wheels 5 and 7 are controlled by only one actuator. In this case, three actuators are used for hydraulic pressure regulation. A main relay 51 is arranged so as to switch the connection between the electromagnetic solenoids of the actuators 27, 29, 31 and 33 and a power supply source, and an indicator lamp 53 is provided for informing a system abnormality to the vehicle driver and is energized by a signal from the electronic control unit 49 in response to failure of the slippage control apparatus such as any disconnection of the wheel rotational speed sensors 9, 11, 13 and 15, a breaking of the electromagnetic solenoids of the actuators 27, 29, 31 and 33, or a disconnection of the stopswitch 43. The electronic control unit 49 is powered in response to the turning-on of an ignition switch 50 and is responsive to signals from the wheel rotational speed sensors 9, 11, 13 and 15 and a signal from the stopswitch 43 in order to perform processes for the slippage control, and it generates output signals for the control of the main relay 51, the indicator lamp 53 and the actuators 27, 29, 31 and 33. Each of the actuators 27, 29, 31 and 33, as shown in Fig. 3, comprises a regulator section 55 and a control valve section 57 with an electromagnetic solenoid for performing the switching between the braking hydraulic pressure increasing, decreasing and maintaining modes, the regulator section 55 being provided for selecting either the hydraulic pressure from the hydraulic pressure cylinder 45 or the hydraulic pressure from the hydraulic pressure pump 47,and for regulating the hydraulic pressure to a predetermined value. The hydraulic pressures outputted from the respective actuators 27, 29, 31 and 33 are transferred through the hydraulic pressure passages 35, 37, 39 and 41 to wheel braking cylinders of the respective hydraulic pressure braking devices 17, 19, 21 and 23 whereby the respective wheels 1, 3, 5 and 7 are braked independently in accordance with the transferred hydraulic pressures. The braking hydraulic pressure control electromagnetic solenoid of the above-mentioned control valve section 57 is arranged so that the hydraulic pressure is increased or decreased during energization or maintained in accordance with the current level thereof. The magnitudes of the hydraulic pressures are controlled in accordance with the time period of the hydraulic pressure supply to the respective hydraulic pressure braking devices 7, 19, 21 and 23 (pressure-increasing time period) or the time period of the hydraulic pressure release therefrom (pressure-decreasing time period). That is, as the pressure-increasing time is longer, the hydraulic presure becomes higher, and, as the pressure-decreasing time is longer, the hydraulic pressure becomes lower. The electronic control unit 49 is arranged as shown in Fig. 4. Waveform shaping and amplifying circuits 60, 62, 64, and 66 respectively convert the signals from the respective wheel speed sensors 9, 11, 13 and 15 into pulse signals which are adaptable to a microcomputer 68, and a buffer circuit 70 temporarily holds the signal from the stopswitch 43. A power supply circuit 72 supplies a constant voltage to the microcomputer 68 and so on in response to the turning-on of the ignition switch 50. The microcomputer 68 is provided with a central processing unit (CPU) 76, a read only memory (ROM) 78, a random access memory (RAM) 80, an input/output circuit (I/O) 82 and so on, and produces control signals on the basis of the input data and supplies the control signals to drive circuits 84, 86, 88, 90, 92 and 94 which in turn generate output signals in accordance with the control signals from the microcomputer 68. Of these drive circuits, the actuator drive circuits 84, 86, 88, 90 drive the electromagnetic solenoids of the respective actuators 27, 29, 31 and 33, and the main relay drive circuit 92 energizes a coil 98 of the main relay 51 so that a normally opened contact 96 is closed. The indicator lamp drive circuit 94 turns on the indicator lamp 53. Operation and Process of an anti-skid control apparatus thus arranged will be described hereinbelow. In response to the turning-on of the ignition switch 50 of the motor vehicle, a constant voltage from the power supply circuit 72 is applied to the microcomputer 68 and so on, and the CPU 76 of the microcomputer 68 starts the arithmetic processes in accordance with a program stored in advance in the ROM 78. Fig. 5A is a flow chart schematically showing the basic process in this arithmetic operation. The basic process starts with a step 100 for initialization in which, for example, various counters which will be described hereinafter are cleared. Thereafter, in accordance with the result of the decision of a step 150, a series of steps 110 through 150 or a series of steps 110 through 170 are repeatedly performed until the ignition switch 50 is turned off. The step 110 is provided for the execution of a control allowance decision process. That is, performed is a process for determining whether there exists a need that an anti-skid control, which will be described hereinafter, is effected in response to the depression of the brake pedal by the vehicle driver, a process for determining the execution of the operation in which, when a system abnormality is present in the step 150, the anti-skid control is inhibited and control is switched therefrom to the normal braking operation or the like. In the step 120, an estimated vehicle speed calculating process is performed to calculate an estimated vehicle speed. That is, The estimated vehicle speed is determined by selecting the highest speed of the wheels speeds of the four wheels and then by assuming a value between the upper limit speed and the lower limit speed obtained on the basis of the highest wheel speed and the previously determined estimated vehicle speed in consideration of the upper limit of the vehicle acceleration and the upper limit of the vehicle deceleration which can be taken in the actual vehicle travelling state, including the braked state. The step 130 is executed to perform a reference speed calculating process to obtain a reference speed for starting the pressure decrease, a reference speed for preventing any malfunction resulting from a vehicle vibration, i.e., a road surface noise, and so on, on the basis of the estimated vehicle speed determined in the step 120. Control then goes to the step 140 to check the system abnormality. Data corresponding to operating states of the system elements on system normal operation which are stored in advance in the ROM 78 are compared with data representing operating states of the system elements read in the process. If the decision made in accordance with the comparison is the system abnormality, an abnormality flag is set in order to indicate that the system operating state is abnormal. If the normality is detected the abnormality flag is kept in the reset state. The step 150 checks the abnormality in accordance with the state of the flag. If the system state is normal, control goes to the control allowance decision process step 110. If the system state is abnormal, the steps 160 and 170 are successively executed and then control goes to the step 110. The step 160 is provided for informing the vehicle driver that the system state is abnormality and that the anti-skid control is not effective. In the step 160, only when the system state abnormality is detected for the first time in the decision step 150, a control signal is supplied to the indicator lamp drive circuit 94 to energize the indicator lamp 53. The indicator lamp drive circuit 94 latches the control signal so as to successively light the indicator lamp 53. In the step 160, it is also appropriate that, if the system is returned to the normal state after the output of the control signal, a control sisgnal is supplied to the indicator lamp drive circuit 94 to turn off the indicator lamp 53. The step 170 is provided for performing a fail-safe process, where a control signal is generated to deenergize the coil 98 of the main relay 51 so that the braking hydraulic pressure control is set to the non-anti-skid control, that is, is switched to the normal mode in which the braking is performed by the hydraulic pressure in response to the depression of the brake pedal 25, irrespective of the states of the pressure increasing, maintaining and decreasing electromagnetic solenoids of the four acuators 27, 29, 31 and 33. In response to the deenergization of the coil 98, the contact 96 is switched to the opening state whereby the power supply for the electromagnetic solenoid of each of the actuators 27, 29, 31 and 33 is cut off so that the normal braking operation is effected until released from the system state abnormality. In this system fail-safe process step 170, it is also appropriate that, in order to further improve the safety a control signal is additionally supplied to each of the actuator drive circuits 84, 86, 88 and 90 to deenergize the solenoid. Fig. 5B is a flow chart showing a timer interrupt routine at a predetermined interval on execution of the Fig. 5A control processes. This timer interrupt routine starts with a step 200 in which the speed of each of the wheels is calculated. In the wheel speed calculating step 200, the wheel speed is obtained using the difference between the pulse count value in the current process and the pulse count value in the previous process, the time interval, and a constant in accordance with a given equation. If required, a filter process for averaging the obtained wheel speeds is performed. After execution of the step 200, a step 210 is executed to calculate the wheel acceleration of each of the wheels. In this wheel acceleration calculating step 210, the wheel acceleration is obtained on the basis of the difference between the wheel speed in the current execution and the wheel speed in the previous execution of the step 200, the time interval and a constant in accordance with a given equation. If required, a process substantially corresponding to the filter process is aditionally performed for averaging. A subsequent step 220 checks whether the decision in the step 110 of Fig. 5A represents the control allowance state. If not allowed, that is, in the case that the stop switch 43 is not turned on or in the other cases, the process advances to a step 230. On the other hand, if allowed, that is, in the case that the control allowance flag is set, steps 240 through 270 are successively executed. In the step 230, a control signal is supplied to each of the actuators drive circuits 84, 86, 88 and 90 so that all of the acutators 27, 29, 31 and 33 are respectively returned to the non-operated state. In response to receipt of the control signals, each one of the actuator drive circuits 84, 86, 88 and 90 is kept to the state corresponding to each of the control signals and the solenoid of each of the actuators 27, 29, 31 and 33 is deenergized, and the braking hydraulic pressure control is performed in the normal mode. On the other hand, in the step 240 which is executed when the control is allowed, the respective wheel speeds and the respective wheel accelerations calculated in the above-mentioned wheel speed calculating step 200 and the wheel acceleration calculating step 210 are respectively compared with the respective reference speeds calculated in the reference speed calculating step 130 of Fig. 5A and the respective accelerations set in advance. A step 250 is then executed to perform a process to select a drive pattern of each of the pressure increasing, maintaining and decreasing electromagnetic solenoids in accordance with the results of the comparison in the step 240. The drive patterns for the solenoids are stored in advance in the ROM 78. This process includes a pressure decreasing mode taken in accordance with the increase of wheel slip rate or the increase of the wheel deceleration and a stepwise pressure increasing mode comprising pressure increasing and pressure maintaining taken in accordance with the wheel restoration. A step 260 is provided for performing a learning control process of the pressure increasing time period in which a target hydraulic pressure coefficient is calculated on the basis of the pressure decreasing time period in the pressure decreasing mode and the added value of the pressure increasing time periods in the stepwise pressure increasing mode comprising the pressure increasing and pressure maintaining so that the first pressure increasing amount in the next stepwise pressure increasing mode and the ratios of the pressure increasing time periods and the pressure maintaining time periods taken thereafter are varied in order to make small the average hydraulic pressure inclination per time after the learning control is performed. This will be described in detail hereinafter. In a step 270, a control signal representing the final pattern is supplied to the corresponding one of the actuator drive circuits 84, 86, 88 and 90 which in turn generates the drive outputs to determine the drive states of the actuators 27, 29, 31 and 33. After the step 270, the process of Fig. 5A which is now interrupted is subsequently executed. Fig. 5C is a flow chart showing an interrupt routine executed in connection with each of the rotational speed sensors 9, 11, 13 and 15. The interrupt routine is executed each time that the pulse signal from the rotational sensor and the waveform shaping and amplifying circuit is inputted into the microcomputer 68. During the execution of this interrupt routine, the above-mentioned processes are interrupted. A step 280 is first executed to count the pulses therefrom, the count value being used for the wheel speed calculating step 220 in the above-mentioned timer interrupt routine. The stepwise pressure increasing control executed in the step 260 of Fig. 5B will be described in detail with reference to the flow chart of Fig. 6. In response to the start of the control, a step 310 is executed to check whether a learning value is present, the learning value being a value determined in the pressure increasing time learning control which will be described hereinafter. The learning value is set after the first (learning) control cycle, that is, it is absent at the start time of the slip control. Thus, the decision in the step 310 is negative, thereby followed by a step 320 to check the number of times for which the pressure increasing is performed after the start of the step 200 control. If first time, in a step 330, the pressure increasing is performed for the pressure increasing time period Tup11 which is calculated on the basis of the wheel acceleration Vw and the pressure decreasing time Tdw1 in the first control cycle. This pressure increasing time period Tup11 is set to be longer as the pressure decreasing time period Tdw1 becomes longer and is set to be longer as the wheel acceleration Vw becomes greater. If the number of times is 2 to KNUP1 in the step 320, a step 340 is executed where a relatively sight pressure increasing is performed, that is, the pressure increasing is performed only for a predetermined pressure increasing time period Tc1. If the number of times is not less than KNUP1 + 1 in the step 320, a step 350 is executed wherein a relatively large pressure increasing is performed, that is, the pressure increasing is performed for a predetermined pressure increasing time period Tc2 (> Tc1). After the pressure increasing, the braking pressure is maintained for a predetermined time period in a subsequent step 360. In the case that the stepwise pressure increasing control corresponds to the stepwise pressure increasing in a control cycle after the learning control cycle, the learning value is obtained from the pressure decreasing time period Tdw1 and pressure increasing time periods Tup11, Tup12, Tup13, Tup14 in the first control cycle. Thus, after the next control cycle, the decision in the step 310 is positive, followed by a step 370 in which the number of times of the pressure increase is checked. If the pressure increase is the first one increase in the control cycle after the learning control cycle, a step 380 is executed where the hydraulic pressure is increased for a first target pressure increase time period Tupn (Tup21). The first target pressure increasing time period Tupn is obtained in accordance with a calculation which will be described hereinafter. If the number of times is 2 to KNUP2 in the step 370, a step 390 is executed where a relatively slight pressure increase is performed for a predetermined time period Tc3 (Tc3 < Tc1) as compared to that of the step 340. If the number of times is not less than KNUP2 + 1, a step 400 is executed where a relatively large pressure increase is performed for a predetermined time period Tc4 (Tc4 > Tc3). After the execution of pressure increasing in the step 380 or 390, a step 410 is executed where the braking pressure is maintained constant for a predetermined time period Th2 (Th2 > Th1) which is longer than that of the learning control cycle (step 360). After the execution of the step 400, in a step 420, the braking pressure is kept constant for a relatively short time period Th3 (Th3 < Th2). According to this stepwise pressure increasing control, a proper and rapid pressure increasing can be performed with the learning value in the first time pressure increasing in the stepwise pressure increasing control after the learning control cycle. Furthermore, since, after the pressure increasing based on the learning value, the hydraulic pressure can be maintained for a long time with it being close to the locking limit pressure by a longer maintaining time period, an effective braking is achieved and the braking distance becomes shorter. A pressure increasing time learning control for obtaining the above-mentioned larning value and the target pressure increasing time period will be described hereinbelow with reference to Fig. 7. Learning Control Cycle Initially, in a step 510 it is checked whether the learning value is set. In the state that there is no learning value, for example, in the case of a first control cycle or after a rapid variation of the road surface conditions, the decision in the step 510 is negative, and therefore the process goes to the learning control cycle and a step 520 is executed in order to check whether the pressure decreasing mode is terminated or not. That is, if the pressure decreasing mode is not completed, the decision is negative and the laerning pressure decreasing time period Tdw1 in the learning control cycle is counted. The above-mentioned learning value is cleared each time the hydraulic pressure control is started for the excessive slip prevention, and therefore the learning value is always absent at the time of the control start. In response to termination of the pressure decrease, the process goes to a step 540 to check whether the pressure increase of the learning control cycle is terminated or not. Here, while the stepwise pressure increasing control in Fig. 6 is effected, the decision is negative and the learning pressure increasing time period Tup1 is obtained by adding calculation, that is, Tup1 is obtained by counting the sum of the pressure increasing time periods Tup11, Tup12, Tup13, Tup 14. In response to termination of the pressure increasing mode, the decision in the step 540 becomes positive and a step 560 is executed to correct the obtained learning pressure increasing time period Tup1 and the pressure decreasing time period Tdw1 to obtain T'up1 and T'dw1. The correction of the learning pressure increasing time period Tup1 and of the pressure decreasing time period Tdw1 is performed in order to obtain the actual pressure increasing and decreasing time periods in consideration of the responses of the respective actuators 27, 29, 31, 33 and other valves. In a step 570, the ratio of the learning correction pressure increasing time period T'up1 and the correction pressure decreasing time period T'dw1 (i.e., T'up1/T'dw1) is calculated in order to obtain a learning value. A step 580 is then executed so that a target hydraulic pressure coefficient K used for the calculation of the target pressure increasing time period is set to 0.8 as the initial value. Thereafter, the process is terminated once. After Learning Control Cycle In the control cycle after the learning control cycle, since the learning value has been already obtained in the step 570, the decision in the step 510 is positive and a step 590 is then executed to check whether the pressure decreasing mode in the present control cycle has been terminated or not. If the decision in the step 590 is "NO", a step 600 is executed subsequently in order to count the present pressure decreasing time period Tdwn, that is, to measure the time period Tdw2. If the pressure decreasing is terminated, the decision in the step 590 is "YES" and a step 610 is executed in order to correct the pressure decreasing time period to obtain the present correction pressure decreasing time period T'dwn, as similarly performed in the step 560. A step 620 follows to check whether the present correction pressure decreasing time period T'dwn exceeds the sum of the learning correction pressure decreasing time period T'dwn and a predetermined time period x1. If not, that is, in the case that there is no rapid variation of the road surface state, a step 630 is executed to calculate the present target pressure increasing time period Tupo, in accordance with the following equation: the first target pressure increasing time period Tupo = K · the present correction pressure decreasing time period (T'dwn) · learning value (T'up1/T'dw1) . The above-mentioned first target pressure increasing time period Tupo is used for the first pressure increasing in the control cycle after the learning control cycle, i.e., in the step 380 of the stepwise pressure increasing control shown in Fig. 6. A step 640 is then executed to check whether the pressure increasing mode is present or not. If so, a step 650 follows to count the added pressure increasing time period Tupn. This corresponds to the stepwise pressure increasing state between time Tup21 and Tup24 in the second control cycle. That is, the added pressure increasing time Tupn, which corresponds to the sum of the pressure increasing time periods Tup21, Tup22, Tup23, Tup24, is counted. In response to the termination of the pressure increasing mode, the decision in the step 640 is negative, and a step 660 is executed to correct the counted added pressure increasing time period Tupn to obtain the corrected added pressure increasing time period T'upn similar to the process of the step 560. Thereafter, in a step 670 it is checked whether the corrected added pressure increasing time period T'upn exceeds the sum of the product of the present correction pressure decreasing time T'dwn of the step 610 and the learning value (T'up1/T'dw1) and a predetermined time period x2. If not exceeding, the road surface state is not rapidly varied and the decision in the step 670 is negative so that a step 680 follows to correct the target hydraulic pressure coefficient K. The correction of the target hydraulic pressure coefficient K is effected in accordance with the graph shown in Fig. 9 using a value A obtained by the following equation. This correction is performed for the purpose of correcting the slippage of pressure increasing and decreasing with the passage of time. Here, the target hydraulic presure coefficient K is used in the step 630 for establishing the target pressure increasing time period and the target pressure increasing time period is used in the step 380 of the stepwise pressure increasing control for the first pressure increasing. Thus, the first pressure increasing in the control cycle after the learning control cycle is always set to the vicinity of the locking limit pressure. Therefore, the pressure increasing thereafter assumes a smaller value (steps 390, 400) as compared with the case of no learning value (steps 340, 350), resulting in maintaining a high braking force immediately before the excessive slipping for a long time. In the case that the decision in the step 620 or 670 is positive, the pressure decreasing time and the pressure increasing time are greatly varied because of rapid variation of the road surface state. In this case, since it is not proper to use the learning value, the learning value is cleared. In the n th control cycle after calculation of the learning value in the learning control cycle (n = 2, 3, ....), if the road surface state is not greatly varied, the present target pressure increasing time Tupn is calculated in a series steps of 510, 590, 600, 610, 620, 630, 640, 650, 660, 670, and 680. On the other hand, before reaching the n th cycle, if the road surface state is greatly varied, the learning value is cleared in the step 620 or 670, and a new learning value for the varied road surface state is obtained in the steps 520 to 580. In this embodiment, the hydraulic pressure sensor is not used, and the braking hydraulic pressure can be easily increased at a time to the vicinity of the locking limit pressure at the first increasing time in the control cycle after the learning control cycle irrespective of difference of inertia due to the rapid braking, soft braking and speed change ratio, resulting in effective braking operation. In addition, since, after increased to the vicinity of the locking limit pressure at the first pressure increasing time, the hydraulic pressure is gradially increased in steps, it is possible to provide an effective braking for a long time. Furthermore, since the target hydraulic pressure coefficient K is always corrected by the pressure increasing time period and the pressure decreasing time period newly obtained, the hydraulic pressure can be controlled to obtain a desirable braking state irrespective of the slippage of the target hydraulic pressure coefficient K with the passage of time. This results in a shortening of the braking distance, makes stable the braking force and further improves the braking feeling. In the above-mentioned first embodiment, the learning value is obtained by (learning correction pressure increasing time period T'up1)/(learning correction pressure decreasing time period T'dw1) in the learning control cycle, and the first target pressure increasing time period Tupo is determined by performing mulitiplication on the learning value, the present correction pressure decreasing time period T'dwn in the control cycle therafter and the coefficient K. However, it is also appropriate that the time periods are used before their correction. Furthermore, it is appropriate, without previously obtaining T'up1/T'dwn as the learning value, to use the following equation for a direct calculation. Tupo = K (T'up1/T'dw1) x T'dwn (1). In the equation (1), a change of the calculation order is allowed. Furthermore, in the case that control is performed in order to provide a temporary pressure maintaining mode during the pressure decreasing, a time period other than the maintaining mode may be counted as the pressure decreasing time period. A second embodiment will be decribed hereinbelow with reference to Figs. 10 through 12. In the first embodiment, it is assumed that the pressure decreasing time period is proportional to the decreased value of the hydraulic pressure, and the pressure decreasing time epriods Tdw1 and Tdwn are used therefore. However, in the actual hydraulic pressure pipe system, the actual hydraulic pressure decrease value is in non-linear relation to the pressure decreasing time period. The second embodiment is made in consideration of this point. In the second embodiment, as the learning pressure decreasing value in the learning control cycle and the pressure decrease value in the n th control cycle therafter, {1 - exp (- T'dw1/T1)} and {1 - exp (- T'dwn/T1)} are used. The corresponding values may be calculated in advance and stored as a map in a memory. T'dw1 represents the corrected pressure decreasing time period in the learning control cycle, T'dwn is the corrected pressure decreasing time period in the n th control cycle, and T1 is a constant previously determined in accordance with the hydraulic pressure system. Although the basic hydraulic pressure control in the second embodiment is similar to the process of the first embodiment, the stepwise pressure increasing control and the pressure increasing time learning control are slightly differed thereform and thus will be described with reference to Figs. 10 and 11. In the stepwise pressure increasing control shown in Fig. 10, a step 375 is further provided for checking whether a flag is set to indicate that the pressure increasing inclination is great, and in accordance with the decision, the step 400 is executed if the decision is "YES" and the step 390 is executed if the decision is "NO". With the exception that in the step 370 the number of decisions is changed for convenience, the other steps are equal to those in the first embodiment. In the pressure increasing time period learning control in Fig. 11, steps 515, 575, 633 and 637 are provided instead of the steps 510, 570, 630 in the first embodiment, and steps 653 and 657 are further provided therein. The step 515 is executed to check whether learning values 1 and 2, which will be described hereinafter, are present. In the step 575, the learning correction pressure increasing time period T'up1 and the learning correction pressure decreasing time period T'dw1 in the learning control cycle are respectively stored as the learning values 1 and 2. In the step 633, the decreased pressure ratio D is calculated on the basis of the learning correction pressure decreasing time period T'dw1 in the learning control cycle and the present correction pressure decreasing time period T'dwn in the present control cycle, i.e., as a function of F(T'dw1, T'dwn), in accordance with the following equation. D = {1 - exp (- T'dwn/T1)}/{1 - exp (- T'dw1/T1)} This decreased pressure ratio D is calculated in advance and may be obtained by using a map. The step 637 is executed to calculate the first target pressure increasing time period Tupo in accordance with the following equation. Tupo = K · learning correction pressure increasing time T'up1 · decreased pressure ratio D , where K is the target hydraulic pressure coefficient as described above and initially set to 0.8. here, T'up1 · D corresponds to the locking vicinity pressure increasing time period A taken for reaching the locking limit hydraulic pressure; this time period A is also calculated in the step 637. The step 653 is executed to check whether the added pressure increasing time period Tupn exceeds the sum of the lock vicinity pressure increasing time period A and a predetermined time period x3. If not, the process is terminated at once. If the decision is "YES", the step 657 is executed to set the flag indicating that the pressure increasing inclination is great. Therefore, it is estimated that the added pressure increasing time period Tupn becomes more than a predetermined value, that is, the road surface µ state is varied to a high µ. The features of the control according to the second embodiment will be described hereinbelow with reference to Fig. 12. If the road surface is not varied, the second enbodiment is operated as in the first embodiment. That is, the first control cycle is the learning control cycle and the learning pressure decreasing time period (Tdw1) and the learning pressure increasing time (Tup11 + Tup 12) can be obtained, and then the target pressure increasing time period (Tupo) at the time of the first pressure increasing in the second control cycle can be obtained on the basis of the learning pressure decreasing time period and the learning pressure increasing time period and, further, the pressure decreasing time period (Tdw2) whereby the hydraulic pressure is controlled. Here, for describing the difference between the first embodiment and the second embodiment, it is assumed that during the second control cycle the road surface state is varied to the high µ side and the locking limit hydraulic pressure has become higher. When the added pressure increasing time period. Tupn exceeds the sum of the lock vicinity pressure increasing time period A and the predetermined time period x3, the decision in the step 653 in Fig. 11 becomes positive, and the flag indicating that the pressure increasing inclination is great is set in the step 657. When the number of pressure increasing becomes KNUP1, the step 375 is executed. The decision therein is positive because the flag is set, followed by the step 400 in which a slightly great pressure increasing is performed (Tc4), and further followed by the step 420 in which the pressure maintaining is performed for a shorter time period (Th3). If the number of pressure increasing is not less than KNUP1 and the flag indicating that the pressure increasing inclination is great is set, the stepwise pressure increasing control is performed in the steps 370, 375, 400 and 420. This corresponds to the rapid stepwise pressure increasing after the time 24 of Fig. 12. Therefore, even if the road surface state is changed to a high µ state during the slip control on the braking, it is possible that the braking hydraulic pressure is rapidly brought to the vicinity of the lock limit pressure, resulting in an effective braking operation. In the above-mentioned first and second embodiments, in the case that the road surface state is greatly changed, the learning value is cleared for the first time and a new learning for the road surface state is obtained. It is also appropriate that each of the control cycles is set to the learning control cycle to update the learning value at every cycle and the target pressure increasing time period in the control cycle immediately thereafter is obtained on the basis of the updated learning value. This is easily achieved by performing the calculation for updating the learning value, instead of the calculation for correcting the target hydraulic pressure coefficient K in the step 680 in the first and second embodiments. For example, in the first embodiment, T'upn (corrected added pressure increasing time period)/T'dwn (present correction pressure decreasing time period) may be updated as the learning value, and in the second embodiment, T'upn may be updated as the learning value 1 and T'dwn may be updated as the learning value 2. In this case, in the second control cycle or the control cycle after the second control cycle, the learning value in the control cycle immediately therebefore is used, and therefore an effective braking operation can be performed in response to variations of the road surface state. In the present invention, the learning pressure decreasing and learning pressure increasing are obtained in the learning control cycle and the present pressure decreasing is obtained in the next control cycle, the lock limit pressure is estimated on the basis of the obtained values and the first target pressure increasing value is calculated. Therefore, in the control cycle after the learning control cycle, on the basis of the target pressure increasing value, the hydraulic pressure can be brought into the vicinity of the lock limit pressure at the initial time. This allows to effectively perform the braking operation, and allows easy application even if there is any difference in the braking sensitivity between motor vehicles because the learning value is used.

Claims: Wheel slipping control apparatus for controlling the hydraulic braking pressure applied to a wheel, comprising: [a] sensor means (9, 11, 13, 15, M6) for detecting a rotational speed of said wheel (1, 3, 5, 7) ; [b] detection means for detecting the slipping state of said wheel (1, 3, 5, 7) on the basis of the detected rotational speed thereof; [c] control means (M0) for generating a control signal in accordance with the detected slipping state in order to perform a braking pressure control cycle in which the hydraulic braking pressure is repeatedly set to a pressure-decreasing mode and then to a pressure-increasing mode; and [d] pressure adjusting means (27, 29, 31, 33; M7 ) which controls the hydraulic braking pressure in accordance with said control signal; characterized in that said control means comprises: [e] decision means (M1) for selecting one of said braking pressure control cycles as a learning control cycle; [f] means (M2) for obtaining a learning pressure-decreased value (Tdw1) which corresponds to the magnitude of pressure decremented during the whole learning control cycle; [g] means (M3) for obtaining a learning pressure-increased value (Tup1) which corresponds to the magnitude of pressure incremented during the whole learning control cycle; [h] means (M4) for obtaining a present pressure-decreased value (Tdwn) which corresponds to the magnitude of pressure decremented during a control cycle which begins after said learning control cycle; and [i] calculating means (M5) which [i1] calculates a target pressure-increased value (Tup0) on the basis of said learning pressure-decreased value (Tdw1), said learning pressure-increased value (Tup1) and of said present pressure-decreased value (Tdwn) , and which [i2] generates a control signal for said pressure adjusting means (27, 29, 31, 33; M7) which is indicative of said target pressure-increased value (Tup0) and which, during each subsequent control cycle, increases the hydraulic braking pressure after the pressure-decreasing mode so that it nearly reaches a wheel locking limit pressure. Wheel slipping control apparatus according to claim 1, characterized in that said calculating means (M5) calculates said target pressure-increased value (Tup0) in accordance with the following equation: Tup0 = K * (Tup1 / Tdw1) * Tdwn wherein K is a constant. Wheel slipping control apparatus according to claim 1 or 2, characterized in that said decision means (M1) determines that said learning control cycle is to be selected when neither of the learning values (Tdw1 or (Tup1 ) is present. Wheel slipping control apparatus according to one of claims 1 to 3, characterized in that said control means (M0) clears said learning pressure-increased value (Tup1) and said learning pressure-decreased value (Tdw1 ) when the present pressure-decreased value (Tdwn) increases above the sum of said learning pressure-decreased value (Tdw1) and a first predetermined value (x₂) . Wheel slipping control apparatus according to one of claims 1 to 4, characterized in that said control means (M0) clears said learning pressure-increased value (Tup1) and said learning pressure-decreased value (Tdw1) when the present pressure-decreased value (Tdwn) , after said increasing of the hydraulic braking pressure to nearly reach said wheel locking limit pressure according to feature [i2] , increases above the sum of said target pressure-increased value (Tup0) and a second predetermined value (y₂) . Wheel slipping control apparatus according to claim 4 and 5, characterized in that said control means (M0) corrects said constant K when the present pressure-decreased value (Tdwn) is smaller than said sum defined in claim 4 and when the present pressure-increased value (Tupn) is smaller than said sum defined in claim 5. Wheel slipping control apparatus according to one of claims 1 to 6, characterized in that said control means (M0) , in said learning control cycle and after a pressure-decreasing mode, increases the hydraulic braking pressure for a time period which is calculated on the basis of a detected wheel acceleration (Vw) and increased in steps of a first predetermined time period (TH1) , and in the pressure-increasing mode of the subsequent control cycle increases the hydraulic braking pressure by the target pressure-increased value and then in steps of a second predetermined time period (Th2) , wherein said second predetermined time period (Th2) is longer than said first predetermined time period (Th1) .

Inventors: ASAMI, KEN, KISHIMOTO, MASASHI, SAKAI, KAZUNORI 1377-1,AZA HASUIKE
Applicants: NIPPONDENSO CO., LTD., TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1987
Main classification: B60T8
IPC codes: B60T8