Antilock brake control apparatus

Abstract: Actuators includes on/off valves adjust the wheel cylinder pressure. Provided are wheel speed detectors (S₀-S₃) for detecting the speed of each wheel, a wheel speed and estimated vehicle speed calculator (CAL) for calculating the wheel and vehicle behavior, and a locking symptoms detectors (L₀-L₃) for detecting wheel locking symptoms and recovery therefrom. A target pressure setting means (PCAL₀-PCAL₃) that sets the target pressure, which is the estimated value of the wheel cylinder pressure during antilock brake control, based on the wheel cylinder pressure at the locking symptoms detection edge is also provided. A pressurization signal setting means (OUT₀-OUT₃) for setting the pressurization signal (S i ) used to drive the actuators based on the target pressure is also provided. An antilock brake control apparatus reduces the effects of pressure pulsations caused by on/off valve operation, and thus enables high precision pressure control, is provided.

Description: The present invention relates to an antilock brake control apparatus for adjusting the wheel cylinder pressure by means of actuators, said antilock brake control apparatus comprising: a wheel speed detection means for detecting the speed of each wheel; a wheel and vehicle behaviour calculation means for calculating the vehicle and wheel behaviour, including at least the estimated vehicle speed, based on the speed of each wheel detected by the wheel speed detection means; a locking symptoms detection means for detecting wheel locking symptoms and recovery from wheel locking symptoms based on the wheel and vehicle behaviour calculated by the wheel and vehicle behaviour calculation means; a target pressure setting means for setting a target pressure for the wheel cylinder pressure during each antilock brake control cycle, based on the wheel cylinder pressure at a shift from a synchronization state, in which the wheel speed is approximately equal to the estimated vehicle speed, to a locking symptoms state; and a pressurization signal setting means for setting the pressurization signal used to drive the actuator based on the target pressure. Such an antilock brake control apparatus is disclosed in EP-A-0 380 882. As will be described in detail hereinbelow, by applying open-loop control of the wheel cylinder pressure, the pulsating effects of the on/off-type valve can be reduced and the wheel cylinder pressure can thus be controlled with high precision. In an antilock brake control apparatus according to the prior art, wheel skidding is detected from the acceleration and deceleration of the wheels and the relationship between the detected wheel speed and the estimated vehicle speed. The wheel cylinder pressure is then adjusted according to the skidding of the detected wheel to maintain wheel skidding to an appropriate level, i.e., within a limited range of the peak friction coefficient of the road surface. This control process makes it possible to shorten the total braking distance and maintain vehicle and steering stability. Antilock brake control systems of this type generally use a solenoid valve or other valve as an on/off valve to regulate brake pressure. As a result, the de/pressurization characteristics of the wheel cylinder pressure relative to the operating (opening and closing) time of the on/off valve vary according to an exponential function because the increase/decrease characteristics of the wheel cylinder pressure depend upon the current wheel cylinder pressure and the pressure differential between the master cylinder pressure and the wheel cylinder pressure. It is therefore necessary to determine as precisely as possible the change in the wheel cylinder pressure in order to control the wheel cylinder pressure with high precision. However, because of the high pressure pulses accompanying operation of the on/off valve, it is extremely difficult to control the wheel cylinder pressure with high precision insofar as the on/off valve control signal is output within a closed-loop process based on sequential comparison of the actual wheel cylinder pressure and the target wheel cylinder pressure. In other words, the wheel cylinder pressure may fluctuate along a high amplitude wave due to the pulses resulting from valve operation. As a result, the wheel cylinder pressure measured by the pressure detection means mounted on the wheel cylinder at any given point during antilock brake control may not accurately express the precise actual wheel cylinder pressure at that time. It is therefore difficult to precisely control wheel cylinder pressure and braking by sequential comparison of the target and supposed "actual" wheel cylinder pressures detected by such pressure detection means. Therefore, an object of the present invention is to provide a cost effective antilock brake control apparatus for controlling the wheel cylinder pressure with high precision. This can be achieved, according to embodiments of the invention, by applying the control signal to the on/off solenoid valve in an open-loop control process, or by reducing the reading frequency of the wheel cylinder pressure by the pressure detection means, without reading the pressure measurement of the pressure detection means. According to the invention there is provided an antilock brake control apparatus as initially defined, which is characterised in that: said actuators have on/off valves; the target pressure setting means is arranged to set the pressurization command width, which width is defined as the magnitude of the increase or decrease in the wheel cylinder pressure, at each of successive control cycles based on the wheel and vehicle behaviour calculated by the wheel and vehicle behaviour calculation means; to calculate the integrated value of the pressurization command width over successive control cycles; and to define the target pressure at each control cycle as the sum of the wheel cylinder pressure at said shift from the synchronization state to the locking symptoms state and the integrated value of the pressurization command width; and the target pressure setting means is arranged to actuate said on/off valves, when the wheel cylinder pressure is to be decreased, so as to set a dump time to be related to the pressurisation command width divided by the target pressure for the control cycle. Furthermore, the pressurization signal setting means preferably sets the pressurization signal based on the target pressure and the pressurization command width. It is further preferable for the antilock brake control apparatus to comprise a fluid pressure detection means for detecting the fluid pressure of each wheel cylinder, and for the target pressure setting means to define the wheel cylinder pressure at the locking symptoms detection edge as the pressure detected by the fluid pressure detection means. Alternatively, the target pressure setting means may set the wheel cylinder pressure at the first locking symptoms detection edge to the equivalent pressure at which locking symptoms occur on a high friction coefficient road surface, and set the wheel cylinder pressure at the second and subsequent locking symptoms detection edges to the target pressure current at that time. In this case, the target pressure setting means may adjust the wheel cylinder pressure at the first locking symptoms detection edge according to the time from the first locking symptoms detection edge until the wheel speed and vehicle speed are again synchronized, or according to the time from the first locking symptoms detection edge until a time when the pressure decrease detection is acquired. Additionally in this case, the pressurization signal setting means sets the pressurization signal based on the wheel and vehicle behavior calculated by the wheel and vehicle behavior calculation means, and the target pressure setting means sets the pressurization command width from the pressurization signal set by the pressurization signal setting means, calculates the integrated value of the pressurization command widths, and sets the target pressure at each control cycle to the sum of the wheel cylinder pressure at the locking symptoms detection edge and the integrated value of the pressurization command widths for a predetermined time after the start of antilock brake control. The predetermined time used here is defined as the time from the first locking symptoms detection edge until the wheel speed and vehicle speed are again synchronized for a period, or the time from the first locking symptoms detection edge until a time when the pressure decrease detection is acquired. In this case, the target pressure setting means detects a split road surface, which is defined as a road surface where the difference between the friction coefficients on right and left sides is great. When the vehicle is thus detected to be on a split road surface, the pressurization signal setting means sets the pressurization signal of the front wheel on the high friction coefficient side to a gradual increase level for the period from the locking symptoms detection edge until the wheel speed and estimated vehicle speed are again synchronized. The target pressure setting means also sets the pressurization command width from the pressurization signal set by the pressurization signal setting means, calculates the integrated value of the pressurization command widths, and defines the target pressure at each control cycle as the sum of the wheel cylinder pressure at the locking symptoms detection edge and the integrated value of the pressurization command width. An antilock brake control apparatus according to the present invention as described above does not require a closed-loop process to control the actuator, and is not affected by fluid pressure pulsations caused by on/off valve operation while comparing the target and actual wheel cylinder pressures at each control cycle because of the target pressure setting means (which sets the target pressure (which is the estimated wheel cylinder pressure during antilock brake control) based on the wheel cylinder pressure at the locking symptoms detection edge) and the pressurization signal setting means (which sets the pressurization signal used to drive the actuator based on the target pressure). In addition, the fluid pressure detection means, which is used to detect the wheel cylinder pressure, is not necessary when the wheel cylinder pressure at the first locking symptoms detection edge after antilock brake control starts is set to the pressure corresponding to the pressure when locking symptoms develop on a high friction coefficient road surface, and the wheel cylinder pressure at the second and subsequent locking symptoms detection edges is set to the target pressure current at that time. In this case, the target pressure rapidly converges to the actual pressure when the wheel cylinder pressure at the first locking symptoms detection edge is adjusted according to the time from the first locking symptoms detection edge to the time when the wheel speed is again synchronized with the estimated vehicle speed, or according to the time from the first locking symptoms detection edge until a time when the pressure decrease detection is acquired. Furthermore, when the pressurization signal is set according to the wheel and vehicle behavior during the period from the first locking symptoms detection edge after antilock brake control starts to the time when the wheel speed is again synchronized with the estimated vehicle speed (or from the first locking symptoms detection edge until a time when the pressure decrease detection is acquired), the target pressure rapidly converges to the actual pressure even when the difference between the actual pressure and the target pressure, (which is the pressure corresponding to the pressure when locking symptoms develop on a high friction coefficient road surface and is set as the wheel cylinder pressure at the first locking symptoms detection edge after antilock brake control starts) is great. In addition, if the pressurization signal for the front wheel on the high friction coefficient side is set to a gradual increase level when the vehicle is detected to be on a split road surface, which is defined as a surface with a great difference between the road surface friction coefficients on right and left sides of the vehicle, sufficient braking control and driving stability can be maintained even on such split road surfaces. The present invention will become more fully understood from the detailed description given by way of example and with reference to the accompanying diagrams wherein: Fig. 1 is a block diagram of the connections and components of an antilock brake control apparatus according to the first embodiment of the invention, Fig. 2 is a block diagram of the signal processor in the first embodiment, Fig. 3 is a graph of the pressure characteristics of an on/off valve, Fig. 4 is a graph of the relationship between the wheel cylinder pressure and depressurization width to depressurization time, Fig. 5 is a flow chart of the operation of the first embodiment, Fig. 6 is a flow chart of step #4 in Fig. 5, Fig. 7 is a flow chart of step #4 in Fig. 5, Fig. 8 is a flow chart of step #5 in Fig. 5, Fig. 9 is a flow chart of step #7 in Fig. 5, Fig. 10 is a graph of the control process according to the first embodiment of the invention, Fig. 11 is a flow chart used to describe the operation of the second embodiment, Fig. 12 is a graph of the relationship between the target pressure and real pressure in the second embodiment, Fig. 13 (A), (B), and (C) are flow charts used to describe the operation of the third embodiment, Fig. 14 is a flow chart used to describe the operation of the fourth embodiment, Fig. 15 is a flow chart of step #71 in Fig. 14, Fig. 16 is a graph of the control process according to the fourth embodiment of the invention, Fig. 17 is a flow chart used to describe the operation of the fifth embodiment, Fig. 18 is a flow chart of step #91 in Fig. 17, Fig. 19 is a graph of the control process according to the fifth embodiment of the invention, and Fig. 20 is a block diagram of an alternative embodiment of the invention. Preferred embodiments of the present invention are described hereinbelow with reference to the accompanying figures. The antilock brake control apparatus according to the first embodiment of the invention as shown in Figs. 1 and 2 is for use on a four wheel motor vehicle. In this antilock brake control apparatus, an inlet valve 3A, 3B, 3C, 3D is positioned between each wheel cylinder 2A, 2B, 2C, 2D (at left and right front, and left and right rear wheels, respectively) and the master cylinder 1. Each of the inlet valves is an on/off-type solenoid valve. The brake fluid is returned from the wheel cylinders 2A - 2D through the corresponding outlet valves 4A - 4D (which are also on/off-type solenoid valves) and pump motor 6 to the master cylinder 1 via the return line 7. Note the buffer chamber 8 placed in the return line 7 between the outlet valves 4A - 4D and the pump motor 6. The wheel speed detection means S 0 , S 1 , S 2 , S 3 detect the speed of the left and right front, and left and right rear wheels, respectively, and output the detected speed as the wheel speed signal to the signal processor 10, which is described below. The fluid pressure detection means PS 0 , PS 1 , PS 2 , PS 3 detect the fluid pressure in each of the wheel cylinders 2A - 2D, respectively, and output the detected fluid pressure as fluid pressure signals PSNS i (where i is the value 0, 1, 2, or 3 corresponding to and indicating the left and right front, and left and right rear wheels, respectively) to the signal processor 10. The signal processor 10 is a microcomputer comprising, as shown in Fig. 2, the wheel and vehicle behavior calculation means CAL, locking symptoms detection means L 0 , L 1 , L 2 , and L 3 , target pressure setting means PCAL 0 , PCAL 1 , PCAL 2 , and PCAL 3 , and the pressurization signal setting means OUT 0 , OUT 1 , OUT 2 , and OUT 3 . The signal processor 10 processes the wheel speed signals and the fluid pressure signals PSNS i to output the pressurization signals S i to the actuators ACT 0 , ACT 1 , ACT 2 , and ACT 3 . The actuators ACT 0 - ACT 3 are each equipped with one inlet valve 3A - 3D and outlet valve 4A - 4D. The wheel and vehicle behavior calculation means CAL calculates the wheel speed SPEED i of each wheel as an expression of both wheel and vehicle behavior, the wheel acceleration d/dt (SPEED i ), the second order derivative d 2 /dt 2 ( SPEED i ) of the wheel speed SPEED i , and the estimated vehicle speed VREF according to methods of common knowledge based on the wheel speed signals input from the wheel speed detection means S 0 - S 3 . The wheel and vehicle behavior calculation means CAL also compares the wheel speed SPEED i with the estimated vehicle speed VREF to determine a synchronization state when the wheel speed SPEED i is approximately equal to (synchronized with) the estimated vehicle speed VREF, and an asynchronization state when there is a significant difference between these compared values. In this first embodiment of the invention, the synchronization state is specifically defined by equation [1] below. VREF - SPEED i ≦ VREF/64 + 1 When equation [1] above is true, a synchronization state is determined to exist, and the synchronization flag SYFLG i is set to "0." When the synchronization flag SYFLG i is "0" in the preceding control cycle, the locking symptoms detection means L 0 - L 3 add "1" to the synchronization timer SYTMR i . The synchronization timer SYTMR i is used to track how long the synchronization state continues. If, however, the synchronization flag SYFLG i is "1" in the preceding control cycle and equation [1] above is true in the current control cycle, the locking symptoms detection means L 0 - L 3 clear the synchronization timer SYTMR i . It is to be noted that the maximum value of the synchronization timer SYTMR i is 255. When either equation [2] or [3] below is true, the locking symptoms detection means L 0 - L 3 determine detection of locking symptoms. VREF - SPEED i ≧ VREF/16 + 3 and d/dt(SPEED i ) ≦ -1.5 g where g is the acceleration of gravity; or VREF - SPEED i ≧ VREF/2 + 5 and d/dt(SPEED i ) ≦ 0 In other words, when the difference between the wheel speed SPEED i and estimated vehicle speed VREF is not significantly great but the wheel deceleration d/dt(SPEED i ) is high, or when wheel deceleration d/dt(SPEED i ) is not significantly great but the difference between the wheel speed SPEED i and estimated vehicle speed VREF is great, the locking symptoms detection means L 0 - L 3 return a "locking symptoms detected" result. When the presence of locking symptoms is thus detected, the locking symptoms detection means L 0 - L 3 set the control request REQ i to "decrease," and set the synchronization flag SYFLG i to "1." When locking symptoms are not detected, however, the control request REQ i is set to "increase" or "increase" depending on how long the synchronization state has continued. When the locking symptoms detection means L 0 - L 3 detect a shift from the synchronization state to a locking symptoms state, i.e., the locking symptoms detection edge, the fluid pressure signals PSNS i for the wheel cylinders 2A - 2D detected by the fluid pressure detection means PS 0 - PS 3 are read as the wheel cylinder pressure PL i at the locking symptoms detection edge. The target pressure setting means PCAL 0 - PCAL 3 sets the pressurization command width DP i , which is the amount of increase/decrease of the wheel cylinder pressure at each control cycle, based on the wheel and vehicle behavior calculated by the wheel and vehicle behavior calculation means CAL at each control cycle, and sets the target pressure PT i (which is the estimated wheel cylinder pressure) based on the pressurization command width DP i and the wheel cylinder pressure PL i at the locking symptoms detection edge. When the control request REQ i is "decrease," the target pressure setting means PCAL 0 - PCAL 3 sets the pressurization command width DP i to "rapid decrease" if either equation [4] or [5] below is true. If neither equation is true, the pressurization command width DP i is set to "gradual decrease." d 2 /dt 2 (SPEED i ) ≦ -2 g and d/dt(SPEED i ) ≦ 0 or d/dt(SPEED i ) ≦ -6 g and d 2 /dt 2 (SPEED i ) ≦ 0 In other words, the target pressure setting means PCAL 0 - PCAL 3 determine whether gradual or rapid decrease is appropriate based on the first and second order differentials of the wheel speed SPEED i . In addition, the target pressure setting means PCAL 0 - PCAL 3 set the pressurization command width DP i to DP i = -10 (bar) when the control request REQ i is "rapid decrease," but set the pressurization command width DP i by calculating equation [6] when the control request REQ i is "gradual decrease." DP i = d/dt(SPEED i ) + d 2 /dt 2 (SPEED i ) When the control request REQ i is "increase," the target pressure setting means PCAL 0 - PCAL 3 sets the pressurisation command width DP i to "rapid increase" if equation [7] below is true. If equation [7] below is not true, the pressurization command width DP i is set to a "gradual increase" rate based on the synchronization timer SYTMR i . IDP i < K 1 · IDPMN i and SYFLG i = 0 where IDP i is the integrated value of the pressurization command width DP i from the locking symptoms detection edge, IDPMN i is the minimum integrated value IDP i , and K 1 is a constant determined by the vehicle type and other conditions. In the first embodiment of the invention, the target pressure setting means PCAL 0 - PCAL 3 sets the pressurization command width DP i to DP i = 6 (bar) when "rapid increase" is required. When "gradual increase" is required, the pressurization command width DP i is set to a "first gradual increase" by calculating equation [8] if a synchronization state exists. If a synchronization state does not exist, the pressurization command width DP i is set to a "second gradual increase" of DP i = 0.5 (bar). DP i = 0.5 + SYTMR i /128 The target pressure setting means PCAL 0 - PCAL 3 also provide a minimum pressurization command width DP i for when the control request REQ i is "decrease," and resets the pressurization command width DP i to this minimum value when the above operation would otherwise set the pressurization command width DP i below the minimum. This is because of the on/off solenoid valves used for the inlet valves 3A - 3D and the outlet valves 4A - 4D. Specifically, as shown in Fig. 3, the relationship between the target pressure PT i and the valve operating time t is defined by an exponential function, and when decrease in any given target pressure PT i is completed during control cycle time ΔT, decrease width greater than the pressure drop DP' during this control cycle time ΔT is not possible. More specifically, if the equation DP i < K 2 · (-PT) is true when the pressurization command width DP i is a decrease signal in this first embodiment, then the pressurization command width DP i is set to DP i = K 2 · (-PT) where K 2 is a constant value determined by the characteristics of the brake system. The target pressure setting means PCAL 0 - PCAL 3 also calculate the integrated value of the pressurization command width DP i from the locking symptoms detection edge, specifically the integrated value IDP i defined by equation [11], and the minimum IDPMN i integrated value IDP i each control cycle. IDP i = ∫DP i . When IDP i < -PL, IDP i is forcibly set to -PL. The target pressure setting means PCAL 0 - PCAL 3 also calculate the target pressure PT i , which is the estimated wheel cylinder pressure, each control cycle using equation [12]. PT i = PL i + IDP i where PL i is the wheel cylinder pressure at the locking symptoms detection edge as described above, and the target pressure PT i is the sum of the wheel cylinder pressure PL i at the locking symptoms detection edge and the integrated value IDP i of the pressurization command width DP i . In the first embodiment of the invention, the fluid pressure signals PSNS i , which represent the actual wheel cylinder pressure detected by the fluid pressure detection means PS 0 - PS 3 , are defined as the wheel cylinder pressure PL i values. The pressurization signal setting means OUT 0 - OUT 3 set the pressurization signal S i according to the control request REQ i . When the control request REQ i is "decrease," the pressurization signal setting means OUT 0 - OUT 3 sets the dump time t dump , which is the time during which the outlet valves 4A - 4D of the actuators ACT 0 - ACT 3 are open and the inlet valves 3A - 3D are closed, according to equation [13]. t dump = -DP i · L/PT i The dump time t dump is set by equation [13] using the relationship shown in Fig. 4, i.e., if the relationship between the decrease width ΔP and the target pressure PT for a given dump time t dump is obtained, the decrease width ΔP is proportional to the target pressure PT i for each dump time t dump . The hold time t hold for holding the outlet valves 4A - 4D and inlet valves 3A - 3D of the actuators ACT 0 - ACT 3 closed when the control request REQ i is "decrease" is obtained from equation [14]. t hold = ΔT - t dump The apply time t apply for opening the inlet valves 3A - 3D and closing the outlet valves 4A - 4D of the actuators ACT 0 - ACT 3 when the control request REQ i is "increase" is set equivalent to the control cycle time ΔT, and the hold time t hold is obtained from equation [15]. t hold = ΔT · (ΔP/DP i -1) In other words, when the wheel cylinder pressure is to be decreased, pressure increase is maintained for a period equivalent to the control cycle time ΔT at every nth control cycle in this first embodiment. Also in the first embodiment, the pressurization command width DP i is set to 127 (bar), the pressurization signal S i sets the hold time t hold to 0, and the apply time t apply to the maximum value (255) of the synchronization timer SYTMR i when the control request REQ i is "end increase." Thus, the pressurization signal setting means OUT 0 - OUT 3 set the pressurization signal S i based on the target pressure PT i and the pressurization command width DP i , and the pressurization signal S i is expressed as a function of the target pressure PT i and the pressurization command width DP i as shown by equation [16] below. S i = S i (PT i ,DP i ) The pressurization signal S i whereby the apply time t apply , dump time t dump , and hold time t hold are determined, is output to the actuators ACT 0 - ACT 3 . In a increase state, the inlet valves 3A - 3D are opened and closed according to the pressurize time t apply and hold time t hold , and the outlet valves 4A - 4D are closed. In a decrease state, the outlet valves 4A - 4D are opened and closed according to the dump time t dump and hold time t hold , and the inlet valves 3A - 3D are closed. In this first embodiment, the actual wheel cylinder pressure detected by the fluid pressure detection means PS 0 - PS 3 is read and set as the target pressure PT i at each control cycle only at the locking symptoms detection edge, the pressurization signal is output to the actuators ACT 0 - ACT 3 in an open-loop control so the actual wheel cylinder pressure (actual pressure) tracks the target pressure PT i , and the brake pressure at each of the wheel cylinders 2A - 2D is thus controlled. The operation of the signal processor 10 is described next with reference to the flow charts in Figs. 5 - 9. The first steps (steps #1, #2, and #3 in Fig. 5) are to calculate each wheel speed SPEED i by processing the wheel speed signals from each of the four wheel speed detection means S 0 - S 3 (step #1), calculate the estimated vehicle speed VREF (step #2), and read the fluid pressure signals PSNS i from the fluid pressure detection means PS 0 - PS 3 . Whether a synchronization state exists is then evaluated, and any wheel locking symptoms or recovery therefrom are detected (step #4). These processes are described in detail in Figs. 6 and 7. The synchronization flag SYFLG i in the preceding control cycle is read first (step #11). If the synchronization flag SYFLG i is "0" (i.e., a synchronization state existed during the preceding control cycle), the synchronization timer SYTMR i is incremented by one at the next step (#12). If, however, the synchronization flag SYFLG i is "1" (i.e., a synchronization state did not exist during the preceding control cycle), equation [1] is evaluated at step #13 to determine whether a synchronization state now exists. If a synchronization state is determined to now exist in step #13, the synchronization flag SYFLG i and synchronization timer SYTMR i are cleared in steps #14 and #15, respectively. At the first step (step #16, Fig. 7) in the locking symptoms detection process, the presence of any locking symptoms is evaluated based on equations [2] and [3]. If locking symptoms are detected, the control request REQ i is set to "decrease" in step #17. The synchronization flag SYFLG i is then checked at step #18. If at step #18 the synchronization flag SYFLG i is "0," then the locking symptoms detection edge is detected because a synchronization state existed until the preceding control cycle and locking symptoms have been detected in the current control cycle. As a result, the fluid pressure signal PSNS i read in step #3 is defined as the wheel cylinder pressure PL i in step #19, and the integrated value IDP i of the pressurization command width DP i and the minimum integrated value IDPMN i are both cleared at step #20. The synchronization flag SYFLG i is then set to "1" at step #21. If at step #18 the synchronization flag SYFLG i is "1," locking symptoms have not been first detected in this control cycle (it is not a locking symptoms detection edge), and the process skips directly to step #21. If at step #16 locking symptoms are not detected, the duration of the current synchronization state is checked in step #22, and the control request REQ i is set accordingly. Specifically, if the synchronization flag SYFLG i is "0" and the synchronization timer SYTMR i is ≧128, the synchronization state is determined to have continued long enough for the control request REQ i to be set to the "end increase" at step #23. If the conditions of step #22 are not satisfied, however, the control request REQ i is set to "increase" at step #24. Returning to Fig. 5, the pressurization command width DP i is then set at step #5. The procedure used to set the pressurization command width DP i in step #5 is shown in Fig. 8. The pressurization command width DP i is initialized to 127 (bar) at step #25 (Fig. 8) corresponding to the "end increase" control request REQ i . The pressurization command width DP i is then set according to the control request REQ i . Specifically, if the control request REQ i is "decrease," the conditions requiring a "rapid decrease" are evaluated at step #27 based on equations [4], [5], and [6]. If "rapid decrease" is required, the pressurization command width DP i is set to rapid decrease at step #28, otherwise the pressurization command width DP i is set to gradual decrease at step #29. If the control request REQ i is increase and at step #30 the rapid increase conditions are not met, the synchronisation flag SYFLG i is checked at step #32. If the synchronization flag SYFLG i is not "1" (i.e., a synchronisation state exists), the pressurization command width DP i is set to the first gradual increase level based on equation [8] at step #33. If the synchronization flag SYFLG i is "1" because an asynchronization state exists, the pressurization command width DP i is set to the second gradual increase level at step #34. The value of the set pressurization command width DP i is then checked at step #35 based on equation [9]. If step #35 is true, i.e., the pressurization command width DP i is less than the minimum threshold value, then the pressurization command width DP i is reset to (-K · PT) at step #36. Returning again to Fig. 5, the integrated value IDP i of the pressurization command width DP i is next calculated using equation [11] at step #6, and the minimum integrated value IDPMN i is also obtained. The target pressure PT i is also calculated from equation [12] in step #6. The pressurization signal S i is then set in step #7. This process is shown in Fig. 9. The control request REQ i is polled at step #37 (Fig. 9). If the control request REQ i is "decrease," the dump time t dump is set by applying equation [13] and the hold time t hold is set by applying equation [14] in steps #38 and #39, respectively. If, however, step #37 returns NO and the control request REQ i is "increase" (step #40 returns YES), the apply time t apply is set equal to the control cycle time ΔT (step #41) and the hold time t hold is set by applying equation [15] (step #42). If both step #37 and step #40 return NO because the control request REQ i is "end increase" (step #43 returns YES), the apply time t apply is set equal to 255 (step #44) and the hold time t hold is set to "0" (step #45). After thus setting the dump time t dump , apply time t apply , and hold time t hold of the pressurization signals S i , the pressurization signals S i are output to the actuators ACT 0 - ACT 3 (step #8, Fig. 5). A graph of the various signal values during actual antilock brake control operation of an antilock brake control apparatus according to the first embodiment is shown in Fig. 10. Referring to Fig. 10, when time t is at a (t=a), corresponding to the first locking symptoms detection edge after the start of antilock brake control, the synchronization flag SYFLG i is set to "1," and the fluid pressure signals PSNS i from the pressure detection means PS i are read as the wheel cylinder pressure PL i values. When t is between a and b ( t=a-b ), the control request REQ i is "decrease" and the pressurization command width DP i is set to "rapid decrease." The pressurization signals S i are thus set by applying equations [13] and [14] according to the pressurization command width DP i and the target pressure PT i . When t is between b and c ( t=b-c ), the control request REQ i is "decrease" and the pressurization command width DP i is set to "gradual decrease." The pressurization signals S i are thus set by applying equations [13] and [14] according to the current pressurization command width DP i and the target pressure PT i . When t is between c and d ( t=c-d ), the control request REQ i is "increase" but a synchronization state does not exist. The pressurization command width DP i is therefore set to the "second gradual increase" value, and the pressurization signals S i are set by applying equation [15] according to the current pressurization command width DP i and the target pressure PT i . When time t is at d ( t=d ), a synchronization state is restored. When t is between d and e ( t=d-e ), the pressurization command width DP i is therefore set to rapid increase, and the synchronization flag SYFLG i to 0. When t is between e and f ( t=e-f ), the pressurization command width DP i is set to the first gradual pressurization value. When t is at f ( t=f ), the locking symptoms detection edge is again detected. The fluid pressure signals PSNS i indicating the current wheel cylinder pressure output from the fluid pressure detection means PS 0 - PS 3 are thus read as the wheel cylinder pressure PL i values, and the same control process is repeated. As described above, the fluid pressure signals PSNS i are read as the wheel cylinder pressure PL i from the fluid pressure detection means PS 0 - PS 3 only when locking symptoms are detected in this first embodiment, and the target pressure PT i is set based on these values. In other words, the frequency at which the pressure values output from the fluid pressure detection means PS 0 - PS 3 are read is reduced to achieve an open-loop control process in this first embodiment of an antilock brake control apparatus. As a result, this antilock brake control apparatus is not affected by the pressure pulsations accompanying on/off solenoid valve operation, and the wheel cylinder pressure can be controlled as desired. Fig. 11 is a flow chart of the alternative control steps characterizing the second embodiment of the invention. The connections and configuration of the second embodiment are identical to those of the first embodiment described above except for the elimination of the fluid pressure detection means PS 0 - PS 3 shown in Fig. 1. As a result, the operation of this second embodiment is also similar to that of the first embodiment except for the elimination of step #3 in the Fig. 5 flow chart, and the substitution of the process shown in Fig. 11 for the steps indicated by the double-dot dash line (1) in Fig. 7. When the locking symptoms detection means L 0 - L 3 detect locking symptoms and the control request REQ i is set to "decrease" in this second embodiment, the synchronization flag SYFLG i is polled at step #51. If the synchronization flag SYFLG i is "1", i.e., an asynchronization state existed also during the preceding control cycle, control flows directly to step #21 in Fig. 7. If, however, the synchronization flag SYFLG i is "0" because of the locking symptoms detection edge, it is determined in step #52 whether this is the first locking symptoms detection edge. Specifically, if the synchronization timer SYTMR i test SYTMR i ≧ 128 is true, it is determined that the duration of the previous synchronization state continued for a sufficiently long period, and that the present locking symptoms detection edge is the first edge. The wheel cylinder pressure PL i is therefore set to the pressure corresponding to the locking symptoms start pressure on a high friction coefficient road surface (step #53). These values may be, for example, 100 (bar) for PL 0 and PL 1 (the left and right front wheels), and 60 (bar) for PL 2 and PL 3 (the left and right rear wheels). However, if equation [17] is not true, the current locking symptoms detection edge is determined to not be the first in the current control operation, and the target pressure PT i of the preceding control cycle is set to the wheel cylinder pressure PL i (step #54). If at step #51 a synchronization state is detected (NO is returned), the integrated value IDP i of the pressurization command width DP i and the minimum integrated value IDPMN i are both cleared (step #55). In other words, the target pressure setting means PCAL 0 - PCAL 3 in the second embodiment set the wheel cylinder pressure at the first locking symptoms detection edge after antilock brake control start to the pressure corresponding to the locking symptoms start pressure on a high friction coefficient road surface, but use the current target pressure PT i as the wheel cylinder pressure PL i at the second and subsequent locking symptoms detection edges. The relationship between the target pressure PT i and the actual wheel cylinder pressure (actual pressure) in this second embodiment is shown in Fig. 12. As shown in this graph, the wheel cylinder pressure can be controlled as desired because the target pressure PT i converges quickly with time on the actual pressure PRi. The reasons for this rapid convergence between the target pressure PT i and the actual pressure PRi follow. First, if the pressurization command width DP i is set to decrease width ΔP1 at time t = t1 in Fig. 12, the dump time from the decrease characteristics of the target pressure PT i will be Δt1. However, when the actual pressure PRi is less than the target pressure PT i , the actual pressure PRi will only drop by ΔP2, which is less than ΔP1, even though the pressure drop continues for Δt1. The target pressure PT i thus effectively converges on the actual pressure PRi because the decrease amount of the actual pressure PRi is only deficient by an amount equal to the difference between the target pressure PT i and the actual pressure PRi. Second, when the drop in the actual pressure PRi is less than the drop in the target pressure PT i as above, locking symptoms continue to be detected. As a result, the pressurization command width DP i is steadily increased (as absolute value) for even greater decrease, and the convergence is accelerated. When the second embodiment comprises on/off solenoid valves as above, the pressure detection means can be eliminated while maintaining control because of the exponential function defining the decrease characteristics of the wheel cylinder pressure, and this elimination of parts reduces the cost of the system. The third embodiment of the invention is described below with reference to Fig. 13. In this third embodiment, step #56 (Fig. 13 (A)) is inserted for part 2 ○ in Fig. 6, step #57 (Fig. 13 (B)) is inserted for part 3 ○ in Fig. 6, and steps #58 - #64 (Fig. 13 (C)) are inserted for part 4 ○ in Fig. 7. In this third embodiment the signal processor 10 further comprises asynchronization timers DMPTMR i to correct the wheel cylinder pressure PL i for each of the left and right front wheels P0 and P1 at the locking symptoms detection edge according to the time from the first locking symptoms detection edge after the start of antilock brake control for that wheel until the wheel speed SPEED i and vehicle speed VREF are again synchronized. Thus, when the asynchronization time is long during initial skidding by one of the front wheels, the friction coefficient of the road surface corresponding to the skidding wheel is determined to be low, resulting in an adjustment of that front wheel and the wheel cylinder pressure on the same side. If the synchronization flag SYFLG i is "1" (i.e., a synchronization state did not exist during the preceding control cycle) at step #11 in Fig. 6, the asynchronization timer DMPTMR i is incremented by one in step #56 (Fig. 13). If, however, the synchronization flag SYFLG i is "0" (i.e., a synchronization state did exist) at step #11 in Fig. 6, the asynchronization timer DMPTMR i is cleared in step #57 (Fig. 13). Then at step #59 it is determined whether a front or rear wheel is being processed by evaluating the value of i . If i = 2 or 3, a rear wheel is being processed and the procedure advances directly to step #21. If i = 0 or 1, however, a front wheel is being evaluated. Then, in step #60, the detection whether it is in the initial skidding, or not, is checked. When this test is true, the procedure advances to step #61. The asynchronization timer DMPTMR i is checked at step #61 if a value is greater than 50 (equal to 0.4 sec.). If DMPTMR i ≧ 50, the road is determined to be an icy road (or the equivalent friction coefficient) at step #62, the front wheel cylinder pressure PL i (i = 0 or 1) is set to 20 (bar), and the wheel cylinder pressure PL i +2 of the same-side rear wheel is set to 10 (bar). If DMPTMR i < 50 (step #61), the asynchronization timer DMPTMR i is checked at step #63 for a value ≧25 (equal to 0.2 sec.). If DMPTMR i ≧ 25, the road is determined to be a snowy road (or the equivalent friction coefficient) at step #64, the front wheel cylinder pressure PL i (i = 0 or 1) is set to 40 (bar), and the wheel cylinder pressure PL i +2 of the same-side rear wheel is set to 30 (bar). In this third embodiment, therefore, the target pressure setting means PCAL 0 - PCAL 3 set the wheel cylinder pressure PL i at the first locking symptoms detection edge after the start of antilock brake control to a pressure equivalent to the pressure when locking symptoms start on a high friction coefficient road surface, and set the wheel cylinder pressure at all following locking symptoms detection edges to the current target pressure PT i , thereby adjusting the wheel cylinder pressure according to the duration of the asynchronization time. In this third embodiment, for example, the wheel cylinder pressure PL i at t = a (Fig. 10) is set to 100 (bar) for PL 0 and PL 1 , and to 60 (bar) for PL 2 and PL 3 . If the asynchronization time as measured by the asynchronization timer DMPTMR i (the period from t = a - d ) is greater than 0.2 sec, the front wheel cylinder pressure PL i (i = 0 or 1) is reduced to 40 (bar) and the same-side rear wheel cylinder pressure PL i +2 is reduced to 30 (bar) as indicated by the dotted line at the moment the initial skid time lasts 0.2 sec. When skidding continues and the asynchronization timer DMPTMR i is greater than 0.4 sec. (shown in Fig. 10), the front wheel cylinder pressure PL i and the same-side rear wheel cylinder pressure PL i +2 are then reset to 20 and 10 (bar), respectively. In this third embodiment, therefore, the wheel cylinder pressure at the first locking symptoms detection edge is set to a value corresponding to a high friction coefficient road surface without using any pressure detection means, and the wheel cylinder pressure PL i at the first locking symptoms detection edge is dynamically adjusted according to the time between the first locking symptoms detection edge and restoration of a asynchronization state between the corresponding wheel speed SPEED i and the estimated vehicle speed VREF. As a result, the target pressure PT i can be controlled to converge more quickly to the actual wheel cylinder pressure. It is to be noted that the wheel cylinder pressure PL i can be dynamically adjusted according to the time between the first locking symptoms detection edge and the time when the control request REQ i is set to "decrease". The fourth embodiment of the invention is described below with reference to Fig. 14. The lines and configuration of the fourth embodiment are identical to those of the first embodiment described above except for the elimination of the fluid pressure detection means PS 0 - PS 3 shown in Fig. 1. With this fourth embodiment as with the first through third embodiments described above, the first steps are to calculate each wheel speed SPEED i (step #1) and the estimated vehicle speed VREF (step #2). The third step in the fourth embodiment, however, is to evaluate the synchronization state and check for any wheel locking symptoms or recovery therefrom. When it is detected that the detected locking symptoms detection edge to be the first locking symptoms detection edge, the wheel cylinder pressure PL i at the locking symptoms detection edge is therefore set to the pressure corresponding to the locking symptoms start pressure on a high friction coefficient road surface. The next step (#70) is to determine whether the locking symptoms detected in step #3 are the first locking symptoms detection edge. Specifically, if SYTMR i ≧ 128, the duration of the synchronization state is determined to be long enough. This is the same test used in the second embodiment. During the first skid period, the pressurization signal setting means OUT 0 - OUT 3 set the pressurization signal S i from the wheel and vehicle behavior calculated by the wheel and vehicle behavior calculation means CAL (step #71). The procedure executed in Fig. 14 step #71 is shown in Fig. 15. The control request REQ i is checked at step #75 (Fig. 15). If REQ i = "decrease," step #76 is executed; otherwise, step #77 is evaluated. At step #76 the conditions requiring "rapid decrease" are evaluated based on equations [4] and [5]. If rapid decrease is required, step #78 is executed; otherwise, step #79 is executed. At step #78 the pressurization signal S i is set to "rapid decrease." When rapid decrease is required, the pressurization signal S i is set to continuously reduce the pressure at each control cycle. Specifically, the pressurization signal S i causes the increase of brake pressure where the dump time t dump is equal to the control cycle time ΔT, and the apply time t apply and hold time t hold are both 0. At step #79, the pressurization signal S i is set to "gradual decrease." In this case the pressurization signal S i causes, for example, dump and hold to be alternately applied at each control cycle. Specifically, the pressurization signal S i is set so that decrease where the dump time t dump is equal to the control cycle time ΔT and the hold where the hold time t apply is equal to the control cycle time ΔT are alternated each control cycle. If step #75 returns NO, the control request REQ i is checked at step #77 for REQ i = "increase." If REQ i is increase, step #80 is evaluated. At step #80 the pressurization signal S i is set to the "second gradual increase." In this fourth embodiment, therefore, the pressurization signal S i is set to a rapid decrease, gradual decrease, or a second gradual increase level, and in each case the wheel cylinder pressure is increased, decreased, or held constant for a period equal to the control cycle time ΔT at each control cycle. Once the pressurization signal S i is thus set, the target pressure setting means PCAL 0 - PCAL 3 calculate the pressurization command width DP i from the pressurization signal S i at each control cycle (step #72). When the current control cycle is a dump cycle, the pressurization command width DP i is calculated from the target pressure PT i using equation [18]. DP i = -PT i · a where a is a variable determined according to the vehicle and brake characteristics for any given target pressure PT i . When the current control cycle is a apply cycle, the pressurization command width DP i is set to a constant A (bar) irrespective of the target pressure PT i . When the current control cycle is a hold cycle, the pressurization command width DP i is set to 0. After thus setting the pressurization command width DP i from the pressurization signal S i and target pressure PT i , the target pressure setting means PCAL 0 - PCAL 3 calculate the integrated value IDP i of the pressurization command width DP i and the target pressure PT i using equations [11] and [12] (step #73). The pressurization signal S i set in step #71 is then output to the actuators ACT 0 - ACT 3 in step #8. If, however, it is not determined in step #70 that this is the first skid, the pressurization command width DP i, target pressure PT i , and pressurization signal S i are set in steps #5, #6, and #7, respectively, as in the third embodiment, and the pressurization signal S i is then output to the actuators ACT 0 - ACT 3 in step #8. In this fourth embodiment, therefore, the pressurization signal S i is set according to the wheel and vehicle behavior, and the pressurization command width DP i and target pressure PT i are set according to the pressurization signal S i , during the period from the first locking symptoms detection edge after the start of antilock brake control until synchronization of the wheel speed SPEED i and estimated vehicle speed VREF is restored. In the embodiment shown in Fig. 14, steps #71-#73 may be carried out only during the pressure decrease period in the first skid period. At the second and subsequent locking symptoms detection edges, the current target pressure PT i is used as the wheel cylinder pressure PL i at locking symptoms detection, and the pressurization command width DP i , target pressure PT i , and pressurization signal S i are sequentially set as in the first to third embodiments above. It is to be noted that the pressurization signal S i can be directly obtained from the wheel and vehicle behavior during the time between the first locking symptoms detection edge and the time when the control request REQ i is set to "decrease". Fig. 16 is a graph of the antilock brake control sequence according to the fourth embodiment. Referring to Fig. 16, time t = a corresponds to the first locking symptoms detection edge after the start of antilock brake control. The wheel cylinder pressure PL i at this time is set to the pressure when locking symptoms start on a high friction coefficient road surface. The synchronization flag SYFLG i is also set to "1" . Between t = a - b , the control request REQ i is "decrease" and the pressurization signal S i is set to rapid decrease. Between t = b - c , the control request REQ i is "decrease" and the pressurization signal S i is set to gradual decrease. Between t = c - d , the control request REQ i is "increase" but a synchronization state does not exist. The pressurization signal S i is therefore set to the second gradual increase level. At t = d , synchronization of wheel speed SPEED i and estimated vehicle speed VREF is restored. Between t = d - e , the pressurization command width DP i is set to the rapid increase. Between t = e - f , the pressurization command width DP i is set to the first gradual increase level. At t = f if locking symptoms are again detected, the pressurization command width DP i , target pressure PT i , and pressurization signal S i are sequentially set as in the first embodiment above. During initial skidding with the fourth embodiment, therefore, the pressurization signal setting means OUT 0 - OUT 3 define the pressurization signals based on the wheel and vehicle behavior, drive the actuators ACT 0 - ACT 3 based on the set pressurization signals, and the target pressure setting means PCAL 0 - PCAL 3 also calculate the pressurization command width DP i from the pressurization signal S i to set the target pressure PT i . As a result, even when the pressure that is set as the wheel cylinder pressure PL i at the first locking symptoms detection edge after the start of antilock brake control (i.e., the pressure when locking symptoms start on a high friction coefficient road surface) is significantly different from the wheel cylinder pressure when locking symptoms occur on the real road surface, the target pressure PT i can be made to quickly converge on the actual pressure PRi. In addition, the pressurization command width DP i is set from the wheel and vehicle behavior, and the actuators ACT 0 - ACT 3 are controlled by the pressurization signal S i set according to the pressurization command width DP i at the second and subsequent skidding occurrences in the fourth embodiment. As a result, the effects of pulsation can be prevented, and high precision control of the wheel cylinder pressure can be achieved. The fifth embodiment of the invention is described below with reference to Fig. 17. The lines and configuration of the fifth embodiment are identical to those of the first embodiment described above except for the elimination of the fluid pressure detection means PS 0 - PS 3 shown in Fig. 1. This fifth embodiment is characterized by controlling gradual increase of only the front wheel on the high friction coefficient road surface side of the vehicle when the vehicle is travelling with the right and left sides on surfaces having different coefficients of friction. Such a road surface is referred to below as a "split road surface," and this control method as "split control." As shown in Fig. 17, the first steps in the control process of the fifth embodiment are to calculate each wheel speed SPEED i (step #1), the estimated vehicle speed VREF (step #2), and in step #3 to evaluate the synchronization state and check for any wheel locking symptoms or recovery therefrom based on the wheel and vehicle behavior expressed by the SPEED i and VREF values. Whether split control is necessary is then determined at step #90. As described above, split control is necessary when the friction coefficients of the road surfaces on the right and left sides of the vehicle are significantly different. This evaluation is made in this embodiment by measuring the amount of slipping by the right and left side wheels. If one side is slipping significantly more than the other, then the friction coefficients are determined to be significantly different and split control to be necessary. Specifically, for example, the left and right front wheel speeds SPEED0 and SPEED1 are applied in equation [19] to determine the split control need. |SPEED0 - SPEED1| ≧ 10 It is to be noted that the determination of split control need is not limited to the test in equation [19]. It is also possible to provide sensors to measure the friction coefficient of the road surface on right and left sides, obtain the difference between the measured friction coefficients, and apply split control when the difference between the measured friction coefficients becomes very high. Alternatively, the split control can be applied when the decrease time between the right and left wheels becomes very large. If split control is determined necessary at step #90, step #91 is executed to set the pressurization signal S i of the front wheel on the high friction coefficient road surface side from the wheel and vehicle behavior and the pressurization command width DP i of the same wheel based on the selected pressurization signal S i , and then set the pressurization command width DP i of the other wheels (i.e., the front wheel on the low friction coefficient road surface and the two rear wheels) based on the wheel and vehicle behavior. This process is shown in Fig. 18. The control request REQ i is checked at step #95 (Fig. 18). If REQ i = "decrease," step #97 is executed; otherwise, step #96 is evaluated. At step #97 the conditions requiring "rapid decrease" are evaluated based on equations [4] and [5]. If rapid decrease is required, the pressurization command width DP i is set to rapid decrease at step #98; otherwise, the pressurization command width DP i is set to gradual decrease at step #99. Note that the specific processes used in steps #97, #98, and #99 are the same as those executed in steps #27, #28, and #29 in the first embodiment above. If step #95 returns NO, the control request REQ i is checked for REQ i = "increase" in step #96. If REQ i = "increase," step #100 is evaluated. In step #100, it is determined whether the current wheel being processed is a front wheel on a high friction coefficient road surface. If it is, step #101 is executed; otherwise, step #102 is evaluated. At step #101 the pressurization signal S i is set to "split gradual increase." In "split gradual increase," the pressurization signal S i holds the hold time t hold equal to the control cycle time ΔT for, e.g., 19 consecutive control cycles, and sets the apply time t apply equal to the control cycle time ΔT for increase of brake pressure during the one following (the twentieth) control cycle. In other words, the pressurization signal S i is set so that the wheel cylinder pressure is applied for only one of every twenty control cycles during the split gradual increase, and is held constant for the other 19 control cycles. The pressurization command width DP i is set-in step #103 at each control cycle. Similarly to the fourth embodiment, when the pressurization signal S i is set to "split gradual increase" in the fifth embodiment, either pressurization or pressure hold is maintained throughout the control cycle time ΔT for each control cycle. Thus, the pressurization command width DP i is set to a constant value A (bar) irrespective of the target pressure PT i during a pressurization control cycle, and to 0 during a hold control cycle. The conditions requiring rapid increase are evaluated at step #102. If the conditions are met, the pressurization command width DP i is set to rapid increase at step #104, otherwise the synchronization flag SYFLG i (= 1) is checked at step #105. The pressurization command width DP i is set for "first gradual increase" at step #106 if the synchronization flag SYFLG i does not equal "1" (a synchronization state exists) in step #105. It is otherwise set to for "second gradual increase" at step #107. It is to be noted that steps #102 to #107 above are the same as the corresponding steps (#30 - #34) in the first embodiment. Steps #108 and #109 also correspond to steps #35 and #36 in the first embodiment, i.e., if the pressurization command width DP i is less than a predetermined value, the pressurization command width DP i is reset to equal that predetermined value. The target pressure PT i is then calculated from equation [12] at step #92. At step #93 the pressurization signal S i is set as in the first embodiment for all wheels other than the one front wheel on the high friction coefficient road surface for which the pressurization signal S i was already set in step #91. The pressurization signals S i are then output to the actuators ACT 0 - ACT 3 in step #8. If, however, it is determined in step #90 that split control is not necessary, the pressurization command width DP i , target pressure PT i , and pressurization signal S i values are set in steps #5 - #7 as in the first embodiment. As thus described, the fifth embodiment evaluates whether the front wheel is on a high friction coefficient road surface (step #90) only when the control request REQ i is set to "increase." For example, when the left side of the vehicle is on a low friction coefficient road surface (e.g., there is ice on the road on that side of the vehicle) and the right side of the vehicle is on a high friction coefficient road surface (e.g., the asphalt is dry on that side), locking symptoms will first be detected in the left side wheels (i = 0, 2) where the friction coefficient is low. As a result, the wheel speeds SPEED1 and SPEED3 for the right side (high friction coefficient side) will be synchronized with the estimated vehicle speed VREF, and the control request REQ1 for the right front wheel will always be set to "increase." A specific example of this operation is described below with reference to Fig. 19. It is assumed in this graph that the left side of the vehicle is on a low friction coefficient road surface, e.g., ice or snow, and the right side is on a high friction coefficient surface, e.g., dry asphalt. Time t = a corresponds to the first locking symptoms detection edge for the left front wheel (i = 0). Between t = a - b , the control request REQ0 for the left front wheel is "decrease," and the pressurization signal S 0 is "rapid decrease." At the same time, the high friction coefficient side (right) front wheel (i = 1) does not indicate any locking symptoms, and the pressurization signal S 1 is thus set to "split gradual increase." At t = b - c , the control request REQ0 for the left front wheel is "decrease," and the pressurization signal S 0 is "gradual decrease." Split gradual increase of the right front wheel continues. At t = c - d , the control request REQ0 for the left front wheel is "increase," and the pressurization signal S 0 is "gradual increase." Split gradual increase of the right front wheel still continues. At t = d when recovery from the locking symptoms is detected in the left front wheel, the pressurization signal S 1 of the right front wheel P1 is set to gradual increase. Between t = d - e , the pressurization signal S 0 for the left front wheel is set to rapid increase, and is set to first gradual increase after t = e . Thus, when there is a significant difference between the road surface friction coefficients on right and left sides of the vehicle, split control in which the front wheel on the high friction coefficient side is regulated to gradual increase. As a result, the effects of pressure pulsations are reduced and good control of the wheel cylinder pressure is maintained. In addition, good braking performance can be obtained and both vehicle stability and steering control can be sustained on split road surfaces because the drive wheel on the high friction coefficient side for which locking symptoms are not detected is regulated to gradual pressurization. It is to be noted that the present invention shall not be limited to the embodiments described above, and other variations are possible. For example, in the first through fifth embodiments, the actuators ACT 0 - ACT 3 use on/off solenoid valves as the inlet valves 3A - 3D and outlet valves 4A - 4D to regulate the fluid pressure in the wheel cylinders 2A - 2D by means of three position control: pressurize, depressurize, and hold. It is possible, however, to use mechanical valves for the inlet valves 3A', 3B', 3C', and 3D' (Fig. 20) while using on/off solenoid valves as the outlet valves 4A - 4D. When mechanical inlet valves 3A' - 3D' are used, there are two operating fluid pressures, the inlet pressure PV1 and the outlet pressure PV2, with a spring 12 working in the same direction as the inlet pressure PV1. These mechanical inlet valves 3A' - 3D' are normally at position X in Fig. 20, but when the inlet pressure PV1 exceeds the outlet pressure PV2, and the differential pressure exceeds the spring 12 strength, the valve moves to position Y and the fluid pressure in wheel cylinder 1 is transmitted through the orifice to the wheel cylinder 2 side. When these mechanical inlet valves 3A' - 3D' are used, two position control results. Specifically, when the on/off solenoid outlet valves 4A - 4D are opened, a dump state results. An apply state results when the valves are closed. There is no hold state. As a result, the speed of the decrease and increase cycles can only be regulated by controlling the opening and closing time of the outlet valves 4A - 4D in the device shown in Fig. 20. Furthermore, the invention shall not be limited to the equations [1] - [20] used for the operation of the first through fifth embodiments above, and the equations may be modified to accommodate other various conditions. As described hereinabove, the antilock brake control apparatus according to the present invention does not use a closed-loop control process due to the fluid pressure behavior and characteristics of on/off valves. By using an open-loop control process, the effects of pressure pulsations accompanying on/off valve operation can be reduced, and the wheel cylinder pressure can be controlled with high precision. Furthermore, when the wheel cylinder pressure is set to a pressure level corresponding to a high friction coefficient road surface at the first detected skid without using a pressure detection means in the antilock brake control apparatus of the invention, the cost of the antilock brake control apparatus can be reduced because the pressure detection means can be eliminated. Moreover, the target pressure can be quickly converged to the real wheel cylinder pressure by setting the wheel cylinder pressure at the first locking symptoms detection edge to the pressure when locking symptoms start on a high friction coefficient road surface, and setting the wheel cylinder pressure at the first locking symptoms detection edges according to the time from detection of the first locking symptoms until the wheel speed and estimated vehicle speed are again synchronized, or according to the time from the first locking symptoms detection edge until a time when the pressure decrease detection is acquired. In addition, when the wheel cylinder pressure at the first locking symptoms detection edge is set to the pressure when locking symptoms start on a high friction coefficient road surface, and the pressurization signal is set based on the wheel and vehicle behavior, the target pressure can be made to quickly converge on the actual pressure even when the difference between the real pressure and the pressure that is set as the wheel cylinder pressure at the first locking symptoms detection edge is great. As a result, the time from detection of the first locking symptoms until the wheel speed and estimated vehicle speed are again synchronized, or the time from the first locking symptoms detection edge until a time when the pressure decrease detection is acquired, is shortened. When a split road surface is detected, it is still possible to reduce the effects of wheel cylinder pressure pulsations. When locking symptoms are detected on a split road surface, it is also possible to maintain good braking performance, vehicle stability, and good steering control by using gradual pressurization for the wheels on the high friction coefficient side. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: An antilock brake control apparatus for adjusting the wheel cylinder pressure by means of actuators (ACT 0 -ACT 3 ), said antilock brake control apparatus comprising: a wheel speed detection means (S 0 -S 3 ) for detecting the speed of each wheel; a wheel and vehicle behaviour calculation means (CAL) for calculating the vehicle and wheel behaviour, including at least the estimated vehicle speed (VREF), based on the speed of each wheel detected by the wheel speed detection means; a locking symptoms detection means (L 0 -L 3 ) for detecting wheel locking symptoms and recovery from wheel locking symptoms based on the wheel and vehicle behaviour calculated by the wheel and vehicle behaviour calculation means; a target pressure setting means (PCAL 0 -PCAL 3 ) for setting a target pressure (PT i ) for the wheel cylinder pressure during each antilock brake control cycle, based on the wheel cylinder pressure (PL i ) at a shift from a synchronization state, in which the wheel speed is approximately equal to the estimated vehicle speed (VREF), to a locking symptoms state; and a pressurization signal setting means (OUT 0 -OUT 3 ) for setting the pressurization signal (S i ) used to drive the actuator based on the target pressure, characterised in that: said actuators (ACT 0 -ACT 3 ) have on/off valves; the target pressure setting means (PCAL 0 -PCAL 3 ) is arranged to set the pressurization command width (DP i ), which width is defined as the magnitude of the increase or decrease in the wheel cylinder pressure, at each of successive control cycles based on the wheel and vehicle behaviour calculated by the wheel and vehicle behaviour calculation means; to calculate the integrated value (IDP i ) of the pressurization command width over successive control cycles; and to define the target pressure (PT i ) at each control cycle as the sum of the wheel cylinder pressure (PL i ) at said shift from the synchronization state to the locking symptoms state and the integrated value (IDP i ) of the pressurization command width; and the target pressure setting means (PCAL 0 -PCAL 3 ) is arranged to actuate said on/off valves, when the wheel cylinder pressure is to be decreased, so as to set a dump time t dump to be related to the pressurisation command width (DP i ) divided by the target pressure (PT i ) for the control cycle. An antilock brake control apparatus according to claim 1, wherein the pressurization signal setting means (OUT 0 -OUT 3 ) is arranged to set the pressurization signal (S i ) based on the target pressure (PT i ) and the pressurization command width (DP i ). An antilock brake control apparatus according to claim 1, further comprising a fluid pressure detection means (PS 0 -PS 3 ) for detecting the fluid pressure of each wheel cylinder, and wherein the target pressure setting means (PCAL 0 -PCAL 3 ) is arranged to set the wheel cylinder pressure (PL i ) at said shift from the synchronization state to the locking symptoms state as the pressure (PSNS i ) detected by the fluid pressure detection means (PS 0 -PS 3 ). An antilock brake control apparatus according to claim 1, wherein the target pressure setting means (PCAL 0 -PCAL 3 ) is arranged to set the wheel cylinder pressure (PL i ) at the first shift from the synchronization state to the locking symptoms state to the equivalent pressure at which locking symptoms occur on a high friction coefficient road surface; and to set the wheel cylinder pressure (PL i ) at the second and any subsequent shifts from the synchronization state to the locking symptoms state to the target pressure current at that time. An antilock brake control apparatus according to claim 4, wherein the target pressure setting means (PCAL 0 -PCAL 3 ) is arranged to adjust the wheel cylinder pressure (PL i ) at the first shift from the synchronization state to the locking symptoms state according to the time from the occurrence of that shift until the wheel speed and vehicle speed are again synchronized. An antilock brake control apparatus according to claim 4, wherein the target pressure setting means (PCAL 0 -PCAL 3 ) is arranged to adjust the wheel cylinder pressure (PL i ) at the first shift from the synchronization state to the locking symptoms state according to the time from the occurrence of that shift until a time when a pressure decrease request from the pressurisation signal setting means (OUT 0 -OUT 3 ) ends. An antilock brake control apparatus according to claim 4, wherein the pressurization signal setting means (OUT 0 -OUT 3 ) is arranged to set the pressurization signal (S i ) based on the wheel and vehicle behaviour calculated by the wheel and vehicle behaviour calculation means (CAL) for a predetermined time after the start of antilock brake control. An antilock brake control apparatus according to claim 7, wherein the target pressure setting means is arranged to set (PCAL 0 -PCAL 3 ) the pressurization command width (DP i ) at each control cycle from the pressurization signal set by the pressurization signal setting means (OUT 0 -OUT 3 ) for a predetermined time after the start of antilock brake control, to calculate the time-integrated value (IDP i ) of the pressurization command width; and to define the target pressure (PT i ) at each control cycle as the sum of the wheel cylinder pressure (PL i ) at said shift from the synchronization state to the locking symptoms state and the time-integrated value (IDP i ) of the pressurization command width. An antilock brake control apparatus according to claim 7, wherein the predetermined time is defined as the time from the first shift from the synchronization state to the locking symptoms state until the wheel speed and vehicle speed are again synchronized. An antilock brake control apparatus according to claim 7, wherein the predetermined time is defined as the time from the first shift from the synchronization state to the locking symptoms state until a time when a pressure decrease request from the pressurisation signal setting means (OUT 0 -OUT 3 ) ends. An antilock brake control apparatus according to claim 4, wherein the target pressure setting means (PCAL 0 -PCAL 3 ) is arranged to detect a split road surface, which is defined as a road surface where the difference between the friction coefficients on right and left sides is great, and when the vehicle is thus detected to be on a split road surface; the pressurization signal setting means (OUT 0 -OUT 3 ) is arranged to set the pressurization signal (S i ) of the drive wheel on the high friction coefficient side to a slow pressurization level for the period from said shift from the synchronization state to the locking symptoms state until the wheel speed and estimated vehicle speed are again synchronized; and the target pressure setting means (PCAL 0 -PCAL 3 ) is arranged to set the pressurization command width (DP i ) from the pressurization signal (S i ) set by the pressurization signal setting means (OUT 0 -OUT 3 ); to calculate the time-integrated value (IDP i ) of the pressurization command width (DP i ); and to define the target pressure (PT i ) at each control cycle as the sum of the wheel cylinder pressure (PL i ) at said shift from the synchronization state to the locking symptoms state and the integrated value (IDP i ) of the pressurization command width.

Inventors: FUJIOKA HIDEAKI, FUJIOKA, HIDEAKI, FUJIOKA, HIDEAKI, C/O ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1992
Main classification: B60T8
IPC codes: B60T15, B60T8