Method for traction control of vehicle

Abstract: A method for traction control of a vehicle, includes detecting a slipping tendency of a drive wheel in a driven state, and controlling the braking force of a drive wheel brake device (BLF,BAF) in an increasing and decreasing manner in accordance with the degree of the detected slipping tendency, wherein a rapid increase mode of rapidly increasing the braking force of the drive wheel brake device and a slow increase mode of slowly increasing the braking force can be switched from one to the other, so that at an initial stage in increasing the braking force of the drive wheel brake device on the basis of the decision that the drive wheel is in an excessive slipping tendency, the braking force is increased in the rapid increase mode.

Description: The present invention relates to a method for traction control of a vehicle. In particular, the present invention relates to detecting a slipping tendency of a drive wheel in a driven state, and controlling the braking force of a drive wheel brake device in an increasing and decreasing manner in accordance with the degree of the slipping tendency. In a conventional method, the braking force of the drive wheel brake device is increased at a given rate, when the drive wheel is in an excessive slipping tendency. An example of such a traction control method is shown in U.S. Patent No. 3,893,535. In some systems, such as WO-A-89/06615 and DE-A-3807757, slow and rapid increase modes are taught. A hydraulic pressure within a hydraulic pressure control circuit including the drive wheel brake device is equal to the atmospheric pressure at the start of the traction control, and a long time is required to increase the hydraulic pressure within the hydraulic pressure control circuit from such condition to a level in which the braking force is exhibited, resulting in inferior responsiveness. JP-A-63-222963 discloses a system in which braking pressure is increased and decreased by the control of an electromagentic valve with a pulse-shaped exciting current, wherein the valve is initially held in a pressure increasing state for a period of time longer than one pulse cycle, so that delay in the breking effect is reduced. According to the present invention, there is provided a method for traction control of a vehicle, comprising the steps of: detecting a slipping tendency of a drive wheel caused due to application of a driving force to said drive wheel; and controlling the braking force of a drive wheel brake device to be applied to said drive wheel in an increasing and decreasing manner in accordance with the degree of the detected slipping tendency, when the drive wheel shows an excessive slipping tendency, such that a rapid increase mode of rapidly increasing the braking force of the drive wheel brake device and a slow increase mode of slowly increasing the braking force are switchable from one to the other, and such that at the start of traction control, the braking force is initially increased in the rapid increase mode; the method being characterised in that the step of initially controlling the braking force in the rapid increase mode is performed only at a vehicle speed less than or equal to a predetermined reference vehicle speed. With the above method, it is possible to enhance responsiveness at the start of traction control at lower speeds by rapidly incrasing the braking force, whilst providing stability and reducing vibration at higher speeds. An embodiment of the invention will now he described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of a traction control system in accordance with an embodiment of the instant invention; Fig. 2 is a schematic diagram of the control means of Fig. 1; and Fig. 3 is a timing diagram. In the embodiment shown in the drawings the present invention is applied to a front engine and front drive vehicle. Referring first to Fig.1, a left drive wheel brake device B LF and a right drive wheel brake device B RF are mounted on left and right front wheels of the vehicle, respectively, and a left driven wheel brake device B LR and a right driven wheel brake device B RR are mounted on left and right rear wheels, respectively. A hydraulic braking pressure generating means 3 is connected to a brake pedal 1 and capable of controlling the hydraulic pressure from a hydraulic pressure supply source 2 in accordance with the amount of brake pedal 1 depressed to produce a hydraulic pressure. During normal braking, the hydraulic pressure produced by the hydraulic braking pressure generating means 3 is applied to each of the brake devices B LF , B RF , B LR and B RR . The hydraulic braking pressure of the brake devices B LF , B RF , B LR and B RR can be maintained or reduced to effect an anti-lock control through inlet solenoid valves 4 LF and 4 RF and outlet solenoid valves 5 LF and 5 RF mounted in independent association with the drive wheel brake devices B LF and B RF and an inlet solenoid valve 4 R and an outlet solenoid valve 5 R which are commonly associated with the driven wheel brake devices B LR and B RR . In addition, the hydraulic braking pressure of the drive wheel brake devices B LF and B RF can be increased to effect traction control through a traction-controlling normally-opened solenoid valve 6 and normally-closed solenoid valve 7 as well as the inlet solenoid valves 4 LF and 4 RF and the outlet solenoid valves 5 LF and 5 RF . The hydraulic pressure supply source 2 comprises a hydraulic pump P for pumping a working oil from a reservoir R, an accumulator A connected to the hydraulic pump P, and a pressure switch S for controlling the operation of the hydraulic pump P. The hydraulic braking pressure generating means 3 has an input port 3a connected to the hydraulic pressure supply source 2, an output port 3b, and a release port 3c leading to the reservoir R, and is constructed so that a hydraulic pressure according to the amount of brake pedal 1 depressed may be delivered from the output port 3b by switch-over of a communication between the output port 3b and the input port 3a and a communication between the output port 3b and the release port 3 c in response to the depression of the brake pedal 1. Each of the brake devices B LF , B RF , B LR and B RR comprises a cylinder 8 and a braking piston 9 slidably received in the cylinder 8, and is constructed to exhibit a braking force by movement of the braking piston 9 according to the hydraulic pressure applied to a hydraulic braking pressure chamber 10 defined between the cylinder 8 and the piston 9. The solenoid valves 4 LF and 5 LF are connected in parallel to hydraulic chamber 10 of drive wheel brake device B LF . Solenoid valves 4 RF and 5 RF are connected in parallel to hydraulic braking pressure chamber 10 in the drive wheel brake device B RF . Inlet solenoid valve 4 R and the outlet solenoid valve 5 R are connected in parallel to the driven wheel brake devices B LR and B RR . The inlet solenoid valves 4 LF , 4 RF and 4 R are normally-opened solenoid valves which are closed during excitation of solenoids 4 LFS , 4 RFS and 4 RS , and the outlet solenoid valves 5 LF , 5 RF and 5 R are normally-closed solenoid valves which are opened during excitation of solenoids 5 LFS , 5 RFS and 5 RS . Moreover, the inlet solenoid valves 4 LF and 4 RF are interposed between the hydraulic braking pressure chambers 10 in the drive wheel brake devices B LF and B RF and an oil passage 11, and the outlet solenoid valves 5 LF and 5 RF are interposed between the hydraulic braking pressure chambers 10 in the drive wheel brake devices B LF and B RF and the reservoir R. The inlet solenoid valve 4 R is interposed between the hydraulic braking pressure chambers 10 in the driven wheel brake devices B LR and B RR and the output port 3b of the hydraulic braking pressure generating means 3, and the outlet solenoid valve 5 R is interposed between the hydraulic braking pressure chambers 10 in the driven wheel brake devices B LR and B RR and the reservoir R. The traction-controlling normally-opened solenoid valve 6 is interposed between the oil passage 11 and the output port 3b of the hydraulic braking pressure generating means 3, and the traction-controlling normally-closed solenoid valve 7 is interposed between the oil passage 11 and the hydraulic pressure supply source 2. The energization and deenergization of the solenoids 4 LFS , 4 RFS , 4 RS , 5 LFS , 5 RFS , 5 RS , 6 S and 7 S in the solenoid valves 4 LF , 4 RF and 4 R , 5 LF , 5 RF ,5 R , 6 and 7 are controlled by a control means 12, and in a normal condition, the solenoids 4 LFS , 4 RFS , 4 RS , 5 LFS , 5 RFS , 5 RS , 6 S and 7 S are in their deenergized states. In an anti-lock brake control during braking, the inlet solenoid valve 4 LF , 4 RF , 4 R , corresponding to a wheel which is about to be locked, is brought into a closed state, thereby suppressing the increasing of the braking force to avoid the wheel becoming locked, but when the wheel is still about to be brought into its locked state, the corresponding outlet solenoid valve 5 LF , 5 RF and 5 R is brought into a communication state to provide a reduction in braking force, thereby eliminating the locking tendency of the wheel. When the drive wheel is about to slip excessively, the energization and deenergization of the solenoids 6 S and 7 S of the traction-controlling normally-opened solenoid valve 6 and normally-closed solenoid valve 7 as well as the solenoids 4 LFS and 4 RFS in the inlet solenoid valves 4 LF and 4 RF and the solenoids 5 LFS and 5 RFS in the outlet solenoid valves 5 LF and 5 RF are controlled in a switchover manner by the control means 12, thereby controlling the braking force of the left and right front wheel brake devices B LF and B RF in an increasing or decreasing manner. When the solenoids 6 S and 7 S are energized, with the solenoids 4 LFS , 4 RFS , 5 LFS and 5 RFS kept deenergized, to close the normally-opened solenoid valve 6 and to open the normally-closed solenoid valve 7, the hydraulic pressure from the hydraulic pressure supply source 2 is applied to the braking hydraulic pressure chambers 10 in the drive wheel brake devices B LF and B RF to rapidly increase the braking force. When the solenoids 4 LFS and 4 RFS are intermittently energized to intermittently open the inlet solenoid valves 4 LF and 4 RF with the solenoids 5 LFS and 5 RFS deenergized to close the outlet solenoid valves 5 LF and 5 RF in a condition of the normally-opened solenoid valve 6 closed and the normally-closed solenoid valve 7 opened, the hydraulic pressure in the hydraulic braking pressure chambers 10 are slowly increased to slowly increase the braking force from each of the brake devices B LF and B RF . When the solenoids 4 LFS and 4 RFS are energized and the solenoids 5 LFS and 5 RFS are deenergized to close the inlet solenoids valves 4 LF and 4 RF and the outlet solenoid valves 5 LF and 5 RF with the normally-opened solenoids valve 6 closed and the normally-closed solenoid valve 7 opened, the hydraulic pressure in the hydraulic braking pressure chambers 10 are maintained to keep the braking force from each of the brake devices B LF and B RF . Further, when the solenoids 4 LFS and 4 RFS are energized to close the inlet solenoid valves 4 LF and 4 RF and the solenoids 5 LFS and 5 RFS are energized to open the outlet solenoid valves 5 LF and 5 RF with the normally-opened solenoid valve 6 closed and the normally-closed solenoid valve 7 opened, the hydraulic pressure in the hydraulic braking pressure chambers 10 can be rapidly released to rapidly decrease the braking force from each of the brake devices B LF and B RF . When the solenoids 5 LFS and 5 RFS are intermittently energized to intermittently open the outlet solenoid valves 5 LF and 5 RF with the solenoids 4 LFS and 4 RFS energized to close the inlet solenoid valves 4 LF and 4 RF in a condition of the normally-opened solenoid valve 6 being closed and the normally-closed solenoid valve 7 being opened, the hydraulic pressure in the hydraulic braking pressure chambers 10 can be slowly released to slowly decrease the braking force from each of the brake devices B LF and B RF . In other words, in this hydraulic pressure control circuit, a rapid increase mode of rapidly increasing the braking force, a slow increase mode of slowly increasing the braking force, a maintaining mode of maintaining the braking force, a rapid decrease mode of rapidly decreasing the braking force and a slow decrease mode of slowly decreasing the braking force can be switched over from one to the other to control the braking force in the increasing and decreasing manner by controlling the energization and deenergization of the individual solenoids 4 LFS , 4 RFS , 5 LFS , 5 RFS , 6 S and 7 S in a switchover manner during a traction control operation, as shown in Table 1 (below). CONTROL MODE SOLENOID 6 S 7 S 4 LFS 4 RFS 5 LFS 5 RFS Rapid inc. mode En En De De slow inc. mode En En Int. En De Ma. mode En En En De rapid dec. mode En En En En slow dec. mode En En En Int. En inc. = increase dec. = decrease Ma. = maintaining En = Energization De = Deenergization Int. En = Intermittent Energization Referring to Fig.2 illustrating an arrangement of that section of the control means 12 which corresponds to the traction control, wheel speed detectors 13 LF and 13 RF for detecting the speeds of the left and right drive wheels, (i.e., the left and right front wheels), and wheel speed detectors 13 LR and 13 RR for detecting the speeds of the left and right driven wheels, (i.e., the left and right rear wheels) are connected to the control means 12. The control means 12 controls, in a switchover manner, the energization and deenergization of the solenoids 4 LFS and 4 RFS of the inlet solenoid valves 4 LF and 4 RF and the solenoids 5 LFS and 5 RFS of the outlet solenoid valves 5 LF and 5 RF associated with the left and right drive wheel brake devices B LF and B RF as well as the solenoid 6 S of the traction-controlling normally-opened solenoid valve 6 and the solenoid 7 S of the traction-controlling normally-closed solenoid valve 7 by detected values provided in their wheel speed detectors 13 LF , 13 RF , 13 LR and 13 RR . The control means 12 comprises filters 14 LF , 14 RF , 14 LR and 14 RR independently connected to the wheel speed detectors 13 LF , 13 RF , 13 LR and 13 RR , respectively, a high select circuit 15 connected to the filters 14 LR and 14 RR , a lower-side reference speed determining circuit 16 and a higher-side reference speed determining circuit 17 which are connected to the high select circuit 15, a left front wheel braking-force control section 20 L connected to the filter 14 LF and the lower-side and higher-side reference speed determining circuits 16 and 17 incorrespondence to the solenoids 4 LFS and 5 LFS , a right front wheel braking-force control section 20 R connected to the filter 14 RF and the lower-side and higher-side reference speed determining circuits 16 and 17 in correspondence to the solenoids 4 RFS and 5 RFS , and a common control section 21 commonly connected to the left and right front wheel braking force control sections 20 L and 20 R and connected to the solenoids 6 S and 7 S . Left and right driven wheel speeds V WLR and V WRR detected in the left and right driven wheel speed detectors 13 LR and 13 RR are cleared of noise in the filters 14 LR and 14 RR and inputted to the high select circuit 15. The high select circuit 15 selects the higher one of the left and right driven wheel speeds V WLR and V WRR as a vehicle speed V R . The vehicle speed V R obtained in the high select circuit 15 is delivered to the lower-side and higher-side reference speed determining circuits 16 and 17. The lower-side reference speed determining circuit 16 determines a lower-side reference speed V RL as shown in Fig.3(a) from the vehicle speed V R in accordance with an acceptable slippage of the drive wheels, and the higher-side reference velocity determining circuit 17 determines a higher-side reference speed V RH as shown in Fig.3(a) from the vehicle speed V R on the basis of the decision that the drive wheels are excessively slipping. Left and right drive wheel velocities V WLF and V WRF detected in the left and right drive wheel speed detectors 13 LF and 13 RF are cleared of noise in the filters 14 LF and 14 RF and inputted to the left and right front wheel braking-force control sections 20 L and 20 R . The left and right front wheel braking-force control sections 20 L and 20 R have the same construction and hence, components of the left front wheel braking force control section 20 L will be described below using reference characters with a suffix L attached thereto, and components of the right front wheel braking force control section 20 R corresponding to those of the left front wheel braking force control section 20 L are only shown using reference characters with a suffix R attached thereto. The left front wheel braking force control section 20 L comprises a differentiating circuit 22 L for differentiating the left drive wheel velocity V WLF filtered of noise in filter 14 LF and inputted thereto to calculate a drive wheel acceleration or deceleration α WL , four comparators 23 L , 24 L , 25 L , and 26 L , AND circuits 27 L , 28 L , 32 L , 36 L and 40 L , OR circuits 29 L , 31 L , 35 L and 37 L , a timer 30 L , and oscillator circuits 33 L and 34 L . In the comparator 23 L , the left drive wheel speed V WLF is delivered from the filter 14 LF to a non-inverted input terminal of the comparator 23 L , and the higher-side reference speed V RH is delivered from the higher-side reference speed determining circuit 17 to an inverted input terminal of the comparator 23 L . Thus, if V WLF > V RH , then a higher level signal is produced from the comparator 23 L . In the comparator 24 L , the lower-side reference speed V LR is delivered from the lower-side reference determining circuit 16 to a non-inverted input terminal of the comparator 24 L , and the left drive wheel velocity V WLF is delivered from the filter 14 LF to an inverted input terminal of the comparator 24 L . Thus, a higher level signal is produced from the comparator 24 L when V RL > V WLF . In the comparator 25 L a left drive wheel acceleration or deceleration α WL is delivered from the differentiating circuit 22 L to a non-inverted input terminal of the comparator 25 L , and a given reference acceleration α H is delivered from a reference terminal 38 L to an inverted input terminal of the comparator 25 L . Thus, a higher level signal is produced from the comparator 25 L when α WL > α H . Further, in the comparator 26 L , a given reference deceleration α L is delivered from a reference terminal 39 L to a non-inverted input terminal of the comparator 26 L , and a left drive wheel acceleration or deceleration α WL is delivered from the differentiating circuit 22 L to an inverted input terminal of the comparator 26 L . Thus, when α L > α WL a high level signal is produced from the comparator 26 L . Output signals from the comparators 23 L and 25 L are applied to the AND circuit 27 L . Thus, the AND circuit 27 L produces a high level signal when V WLF > V RH and α WL > α H . This signal indicates that the left drive wheel is in an excessive slipping tendency. Output signals from the comparators 23 L and 26 L and applied to the AND circuit 28 L . Thus, the AND circuit 28 L produces a high level signal when V WLF > V RH and α L > α WL . Further, output signals from the comparators 24 L and 26 L are input to the OR circuit 29 L which produces a high level signal when V RL > V WLF or α L > α WL . The timer 30 L produces a high level signal which is sustained for a given time T₁ from a time point when the high level signal is input thereto. An output signal from the comparator 23 L is supplied in an inverted manner to the timer 30 L . Further, the output signal from the timer 30 L is supplied to one of input terminals of the OR circuit 31 L , and the output signal from the comparator 23 L is supplied to the other input terminal of the OR circuit 31 L . Thus, when the output from the comparator 23 L is of a higher level, i.e., V WLF > V RH , or after expiry of the given time T₁ elapsed from a time point when the output from the comparator 23 L becomes a low level, i.e., the relation V WLF ≦ V RH has been established, the OR circuit 31 L produces a higher level signal. The output signals from the OR circuits 29 L and 31 L are supplied to the AND circuit 32 L . Thus, when both of the outputs from the OR circuits 29 L and 31 L are of a higher level, i.e., when V RL > V WLF or α L > α WL , and V WLF > V RH or time T₁ has not elapsed since V WLF ≦ V RH , the AND circuit 32 L produces a higher level signal. The output signal from the AND circuit 32 L is applied to the oscillator circuit 33 L . The oscillator circuit 33 L produces an oscillation signal alternately repeating high and low level states, in response to the high level signal inputted thereto from the AND circuit 32 L . The signal from the oscillator circuit 33 L is supplied to one of input terminals of the OR circuit 37 L . The output signal from the AND circuit 28 L is supplied to the other input terminal of the OR circuit 37 L . Thus, the OR circuit 37 L produces a signal which becomes a high level in response to the high level signal produced from the AND circuit 28 L when V WLF > V RH and α L > α WL , and which is of a high level in a high level state of an oscillation signal produced from the oscillator circuit 33 L , when the output from the AND circuit 28 L is of a low level, i.e., when V WLF ≦ V RH or α L ≦ α WL . An output terminal of the OR circuit 37 L is connected to the solenoid 5 LFS . The output signal from the AND circuit 27 L is supplied to the oscillator circuit 34 L . When V WLF > V RH and α WL > α H , the oscillator circuit 34 L produces an oscillation signal alternately repeating higher and lower level states, in response to reception of the higher level signal from the AND circuit 27 L . This signal from the oscillator circuit 34 L is applied to one of input terminals of the OR circuit 35 L . An output signal from the OR circuit 35 L is invertedly supplied to one of input terminals of the AND circuit 36 L whose output terminal is connected to the solenoid 4 LFS . The output signal from the AND circuit 27 L is supplied to one of input terminals of the AND circuit 40 L , and an output signal from the AND circuit 40 L is supplied to the other input terminal of the OR circuit 35 L . The common control section 21 comprises OR circuits 41, 43 and 44, timers 42 and 45, an AND circuit 46 and a comparator 47. Output signals from the AND circuits 27 L and 27 R in the left and right wheel braking-force control sections 20 L and 20 R are supplied to the OR circuit 41. When at least one of the AND circuits 27 L and 27 R produces a higher level signal, i.e., in at least one of two conditions: a condition where the left drive wheel is in an excessive slipping tendency and relations V WLF > V RH and α WL > α H are established, and a condition where the right drive wheel is in an excessive slipping tendency and relations V WRF > V RH and α WR > α H are established, the OR circuit 41 produces a high level signal. The output signal from the OR circuit 41 is invertedly supplied to the timer 42 and to one of input terminals of the OR circuit 43. The timer 42 produces a high level signal which is sustained for a given time T₂, when a high level signal is inputted thereto, i.e., in response to the falling of the output signal from the OR circuit 41. The output signal from the timer 42 is applied to the other input terminal of the OR circuit 43. Thus, as long as at least one of the left and right drive wheels has an excessive slipping tendency and until expiry of the given time T2 elapsed from the elimination of such an excessive slipping tendency, the OR circuit 43 produces a high level signal. An output terminal of the OR circuit 43 is connected to the solenoids 6 S and 7 S . The output signals from the OR circuits 31 L and 31 R in the left and right front wheel braking-force control sections 20 L and 20 R are inputted to the OR circuit 44, and an output signal from the OR circuit 44 is invertedly supplied to a reset input terminal of the timer 42. Thus, the timer 42 is reset when both of the output signals from the OR circuits 31 L and 31 R are brought into a lower level. It should be noted that the output signal from the OR circuit 43 is also inputted to the other input terminals of the AND circuits 36 L and 36 R in the left and right front wheel braking-force control sections 20 L and 20 R . When the output signals from the OR circuits 35 L and 35 R are of a low level and the output signal from the OR circuit 43 is of a high level, the AND circuits 36 L and 36 R produce a high level signal, thereby exciting the solenoids 4 LFS and 4 RFS . In addition, the output signal from the OR circuit 43 is also inputted to the timer 45 which produces a high level signal which is sustained for a given time T₃ from a time point when a high level signal is inputted thereto. The output signal from the timer 45 is applied to one of input terminals of the AND circuit 46. An output signal from the comparator 47 is applied to the other input terminal of the AND circuit 46. A reference signal corresponding to a reference vehicle speed V RO , e.g., 15 to 20 km/hr is inputted to a non-inverted input terminal of the comparator 47, and a vehicle speed V R is inputted from the high select circuit 15 to an inverted input terminal of the comparator 47. When the vehicle speed V R is equal to or less than the reference vehicle speed V RO , the comparator 47 produces a high level signal. Thus, when the vehicle speed V R is equal to or less than the reference vehicle speed V RO , the AND circuit 46 produces a higher level signal until expiry of the given time T₃ which starts from a time point when the output from the OR circuit 41 is brought into a high level, i.e., when at least one of the left and right drive wheels enters an excessive slipping tendency. The output signal from the AND circuit 46 is applied to the other input terminals of the AND circuits 40 L and 40 R in the left and right front wheel braking-force control sections 20 L and 20 R . Thus, when the vehicle speed V R is equal to or less than the reference vehicle speed V RO and a given time T₃ has not elapsed from a time point when at least one of the left and right drive wheels entered the excessive slipping tendency, each of the AND circuits 40 L and 40 R produces a high level signal. In the above-described control means 12, the given time T₁ in the timers 30 L and 30 R , the given time T₂ in the timer 42 and the given time T₃ in the timer 45 are set as follows: (a) The given time T₂ is set in correspondence to a normal time within which the excessive slipping of the drive wheel can be eliminated. For example, the given time T₂ is 1,000 m sec. The given time T₂ is a time for keeping the traction-controlling normally-opened and normally-closed solenoid valves 6 and 7 energized to permit a traction control swiftly responsive to occurrence of a subsequent excessive slipping after the elimination of the excessive slipping tendency of the drive wheel. (b) The given time T₁ is shorter than the given time T₂ and is set, for example, at 500 m sec. If the solenoids 6 S and 7 S are kept energized for a relatively long time T₂ (1,000 m sec.) to accommodate to the subsequent excessive slipping as described above, the accommodation is impossible when an accelerator is rapidly returned within such time T₂ to rapidly reduce the drive wheel speeds V WLF and V WRF to the vehicle velocity V R or when a braking is conducted. The period of time is constant from a point when the left and right drive wheel speeds V WLF and V WRF become equal to or less than the higher-side reference velocity V RH (V WLF ≦ V RH and V WRF ≦ V RH ) to a point when the drive wheel enters the subsequent excessive slipping tendency and hence, the given time T₁ is set at a level enough to cover such period of time. Moreover, the OR circuits 31 L and 31 R have their outputs each brought into a low level after a lapse of the time T₁ from a point when V WLF ≦ V RH and V WRF ≦ V RH . When the outputs from the OR circuits 31 L and 31 R become the lower level, the timer 42 is reset. When at least one of the left and right drive wheels is in a relation V WLF > V RH or V WRF > V RH , the timer 42 is not reset, and the solenoids 6 S and 7 S are kept energized. Therefore, the given time T₁ can be set to a relatively short period of time. Taking them into consideration, the given time T₁ is sets, for example, at 500 m sec. (c) The given time T₃ is set, for example, at 60 m sec. The given time T₃ is set as a time required to increase the hydraulic pressure in the hydraulic braking pressure chambers 10 in the left and right drive wheel brake devices B LF and B RF from atmospheric pressure to a level at which the braking force acts. Here, the timer 45 is operated after an excessive slipping tendency occurs in either one of the left or right drive wheels. For the given time T3 for example, 60 m sec is set as a time sufficient to increase the hydraulic pressure in the hydraulic braking pressure chamber 10 in each brake device B LF , B RF from atmospheric pressure to a level at which the braking force acts, even when the left and right drive wheels concurrently produce the excessive slipping tendency as well as when the left and right drive wheels produce the excessive slipping tendency out of phase. The operation of this embodiment will be described below with reference to Fig.3. When the drive wheel speed V WLF or V WRF varies as shown in Fig.3(a), and the drive wheel acceleration or deceleration α WL or α WR varies as shown in Fig.3(b), the output from the comparator 23 L or 23 R is as shown in Fig.3(c); the output from the comparator 24 L or 24 R is as shown in Fig.3(d); the output from the comparator 25 L or 25 R is as shown in Fig.3(e); and the output from the comparator 26 L or 26 R is as shown in Fig.3(f). The control condition of the hydraulic braking pressure to be controlled in accordance with the foregoing is as shown in Fig.3(g). The energization and deenergization of the solenoids 6 S and 7 S in accordance with such control condition of the hydraulic braking pressure are as shown in Figs.3(h) and 3(i)respectively. The energization and deenergization of the solenoid 4 LFS or 4 RFS and the energization and deenerigzation of the solenoid 5 LFS or 5 RFS are as shown in Figs. 3(j) and 3(k), respectively, and the braking force of the drive wheel brake device B LF or B RF is as shown in Fig.3(1). In Figs 3(h) to 3(k), a high level indicates the energization, and a low level indicates the deenergization. In Figs.3(j) and 3(k), an intermediate level indicates a condition where the energization and deenergization are intermittently switched over. In Fig.3(h), a high level of the solenoid 6 S indicates a closed state of the normally-opened solenoid valve 6, and in Fig.3(i), a high level of the solenoid 7 S indicates an opened state of the normally-closed solenoid valve 7. In Fig.3(j), a high level of the solenoid 4 LFS or 4 RFS indicates a closed state of the inlet solenoid valve 4 LF or 4 RF , and an intermediate level of the solenoid 4 LFS or 4 RFS indicates a condition where the inlet solenoid valve 4 LF or 4 RF is intermittently opened and closed. In Fig.3(k), a high level of the solenoid 5 LFS or 5 RFS indicates an open state of the outlet solenoid valve 5 LF or 5 RF , and an intermediate level of the solenoid 5 LFS or 5 RFS indicates a condition where the outlet solenoid valve 5 LF or 5 RF is intermittently opened and closed. When the left drive wheel velocity V WLF exceeds the higher-side reference velocity V RH and the left drive wheel acceleration or deceleration α WL exceeds the reference acceleration α H , the outputs from the comparators 23 L and 25 L become a high level, and on the assumption that the left drive wheel enters an excessive slipping tendency, a high level signal is produced from the AND circuit 27 L . This causes the solenoids 6 S and 7 S to be energized, thereby closing the traction-controlling normally-opened solenoid valve 6 and opening the traction-controlling normally-closed solenoid valve 7. In this case, when the vehicle speed V R is equal to or less than the reference vehicle speed V RO the output from the AND circuit 46 is of a high level for the given time T₃ from a point when the above-described excessive slipping tendency occurs. When the excessive slipping tendency continues while the output from the AND circuit 46 is of a high level, the output from the AND circuit 40 L is of a high level. Hence, the output from the AND circuit 36 L is of a low level and the solenoid 4 LFS remains deenergized. In addition, during this time, each of the outputs from the comparators 24 L and 26 L is of a low level, and the output from the OR circuit 37 L is also of a low level and hence, the solenoid 5 LFS is also in its deenergized state. This results in the rapid increase mode, so that the hydraulic pressure from the hydraulic pressure supply source 2 can be rapidly applied to the hydraulic braking pressure chamber 10 in the drive wheel brake device B LF to rapidly increase the braking force, thereby providing an improvement in responsiveness of the traction control. Moreover, the output from the timer 45 becomes a low level after a lapse of the given time T₃ (e.g., 60 m sec.) from a point when the excessive slipping tendency occurs. In response thereto, the output from the AND circuit 40 L also becomes a low level, so that the control of the braking force in the rapid increase mode is carried out only for the given time T₃ from the point when the drive wheel enters an excessive slipping tendency. The control of the braking force in the rapid increase mode is performed at an initial stage of the traction control only when the vehicle speed is equal to or less than the reference vehicle speed V RO (e.g., 15 to 20 km/hr). This is for the purpose of providing a slow increase in braking force to provide a moderation of a vibration of the vehicle body, taking great account of stability in a higher velocity region exceeding the reference vehicle velocity V RO , because when the left and right drive wheels are independently controlled, there is a possibility that the braking forces of the left and right wheel brake devices B LF and B RF may be alternately increased to produce a vibration of the vehicle body, if the phases of the left and right wheels are displaced. When the control of the engine power output and the control of braking force are used in combination, and if the control of the braking force is prohibited in the rapid increase mode in the higher speed region as described above, it is envisaged that in such a high speed region where the engine power output is stable at a level more than a predetermined value and the output is controllable, the contribution of the control of the braking force can be lowered to prevent the occurrence of any excessive control and any abnormal behavior of the vehicle body. When the left drive wheel acceleration or deceleration α WL becomes a relation α H ≧ α WL > α L as a result of the control of the braking force in the rapid increase mode in a condition where the left drive wheel speed V WLF exceeds the higher-side reference speed V RH , the output from the AND circuit 27 L becomes a low level. When the right drive wheel is not in the excessive slipping tendency, the output from the OR circuit 41 becomes a lower level, but the output from the OR circuit 43 is of a high level until the given time T₂ is elapsed by the operation of the timer 42, so that the energization of the solenoids 6 S and 7 S is sustained. Then, the output from the AND circuit 40 L becomes a low level and in response thereto, the output from the AND circuit 36 L becomes a high level, thereby energizing the solenoid 4 LFS to close the inlet solenoid valve 4 LF . This results in the maintaining mode in which the solenoids 4 LFS and 4 RFS are energized and the solenoids 5 LFS and 5 RFS are deenergized to close the inlet solenoid valves 4 LF and 4 RF and the outlet solenoid valves 5 LF and 5 RF , so that the hydraulic pressure in the hydraulic braking pressure chambers 10 are maintained to keep the braking force of the brake device B LF . When the left drive wheel speed V WLF exceeds the higher-side reference speed V RH , but α WL < α L as a result of the control of the braking force in the maintaining mode, the control of the braking force is carried out in the rapid decrease mode. More specifically, the condition in which the left drive wheel speed V WL exceeds the higher-side reference speed V RH is basically a condition in which a substantial slipping occurs in the drive wheel and intrinsically, the braking force should be increased, but such condition can be predicted as being a condition in which an excessive braking force overcoming a great driving force acts from the fact that α WL < α L . In such condition, the output from the AND circuit 28 L becomes a high level in response to the output from the comparator 26 L becoming a high level, and the output from the OR circuit 37 L becomes a high level, thereby energizing the solenoid 5 LFS to open the outlet solenoid valve 5 LF . This permits the hydraulic pressure in the hydraulic braking pressure chamber 10 in the brake device B LF to be rapidly released, so that the braking force is rapidly reduced. When the left drive wheel speed V WLF becomes equal to or less than the higher-side reference speed V RH as a result of the control of the braking force in the rapid decrease mode, the control of the braking force in the slow decrease mode is carried out in a region in which the left drive wheel acceleration or deceleration α WL is less than the reference deceleration α L . More specifically, in the region of V RL < V WLF ≦V RH , in principle, the wheel is in an ideal slipping state and in this case, intrinsically, the braking force should be maintained, but it can be predicted that the establishment of the relation, α WL < α L is because a slightly excessive braking force acts, as compared to in the rapid decrease mode. In such region, in response to the output from the comparator 26 L becoming a high level, the high level signal from the AND circuit 32 L is supplied to the oscillator circuit 33 L and in response thereto, high and low level signals from the OR circuit 37 L are alternately outputted in a given cycle, thereby intermittently energizing the solenoid 5 LFS . This causes the outlet solenoid valve 5 to be intermittently opened and closed, so that the hydraulic pressure in the hydraulic braking pressure chamber 10 is slowly released, thereby slowly decreasing the braking force of the left drive wheel brake device B LF . Then, when the left drive wheel velocity V WL becomes equal to or less than the lower-side reference speed velocity V RL , the left drive wheel is brought into an ideal slipping state, i.e., a condition displaced from a target region of the traction control intended to achieve the relation V RL < V WLF < V RH . As apparent, it is surmised that an excessive braking force acts in this condition. In this case, the output from the comparator 24 L becomes a high level and hence, the solenoid 5 LFS is intermittently energized in the same manner as described above, whereby intermittent opening and closing of the outlet solenoid valve 5 LF are continued to sustain a condition in which the braking force of the left drive wheel brake device is slowly decreased. When V WLF > V RL as a result of the above-described slow decrease of the braking force, the output from the comparator 24 L becomes a low level and hence, the output from the AND circuit 32 L also becomes a low level, so that the solenoid 5 LFS is deenergized to close the outlet sdlenoid valve 5 LF , and the control of the braking force is carried out in the maintaining mode. Moreover, the control of the braking force in the slow decrease mode and the maintaining mode is ensured by the operation of the timer 30 L . When the left drive wheel enters the excessive slipping tendency again, so that the left drive wheel velocity V WL exceeds the higher-side reference speed V RH and at the same time, the left drive acceleration or deceleration α WL exceeds the reference acceleration α H , the control of the braking force is turned to the slow increase mode. More specifically, after a lapse of the given time T₃ from the point of initial occurrence of the excessive slipping tendency, the AND circuit 46 produces a low level output and hence the OR circuit 35 L repeatedly produces high and low level signals in a given cycle, so that the solenoid 4 LFS is intermittently energized to intermittently open and close the inlet solenoid valve 4 LF , thereby permitting the hydraulic pressure in the hydraulic braking pressure chamber 10 in the left drive wheel brake device B LF to be slowly increased to slowly increase the braking force. When the given time T₁ (e.g., 500 m sec.) is elapsed after the left and right drive wheel speeds V WLF and V WRF become equal to or less than the higher-side reference velocity V RH , the timer 42 is reset and hence, when the excessive slipping tendency is eliminated before the given time T₂ is elapsed after occurrence of the excessive slipping tendency, the solenoids 6 S and 7 S are deenergized to open the traction-controlling normally-opened solenoid valve 6 and close the traction-controlling normally-closed solenoid valve 7, ensuring that the hydraulic pressure of both drive wheel brake devices B LF and B RF can be properly controlled in response to the operation of rapidly returning the accelerator or the conducting of braking. Such a series of controls of the braking force is repeated in accordance with the left drive wheel speed V WLF and the left drive wheel acceleration or deceleration α WL , thereby eliminating the excessive slipping tendency of the left drive wheel. At an initial stage of the traction control, the control of the braking force is conducted in the rapid increase mode and when the braking force is subsequently increased, the control of the braking force is conducted in the slow increase mode. Therefore, it is possible to avoid a reactive consumption of the hydraulic braking pressure to provide a good responsive traction control. Further, when an excessive braking force acts during the traction control, it is necessary to reduce the braking force, wherein the control of the braking force is carried out in the rapid and slow reduction modes switched over one to another in accordance with the degree of excess. Thus, it is possible to provide an appropriate traction control according to the situation. The above-described control of the braking force is also likewise performed with regard to the right drive wheel. In spite of the same phase or different phases of the left and right wheels, the controls of the braking force for the left and right drive wheels are independently carried out.

Claims: A method for traction control of a vehicle, comprising the steps of: detecting a slipping tendency of a drive wheel caused due to application of a driving force to said drive wheel; and controlling the braking force of a drive wheel brake device (B LF , B RF ) to be applied to said drive wheel in an increasing and decreasing manner in accordance with the degree of the detected slipping tendency, when the drive wheel shows an excessive slipping tendency, such that a rapid increase mode of rapidly increasing the braking force of the drive wheel brake device and a slow increase mode of slowly increasing the braking force are switchable from one to the other, and such that at the start of traction control, the braking force is initially increased in the rapid increase mode; the method being characterised in that the step of initially controlling the braking force in the rapid increase mode is performed only at a vehicle speed (V R ) less than or equal to a predetermined reference vehicle speed (V RO ). The method of claim 1, wherein the initial rapid increase mode is performed for a maximum predetermined period of time (T₃), but is interrupted prior to expiration of said predetermined period of time (T₃) either of the following two conditions is established: (a) when the wheel speed (V WRF , V WLF ) of said drive wheel falls below a predetermined reference speed (V RH ) which is set higher than said vehicle speed (V R ); or (b) when an acceleration/deceleration (α WR , α WL ) of said drive wheel falls below a predetermined reference acceleration (α H ). The method of claim 1 or 2, wherein the reference vehicle speed (V RO ) is from 15-20 kph. The method of any preceding claim, wherein said controlling step also includes maintaining the braking force of the drive wheel brake device in a maintaining mode in order to maintain pressure in the brake device (B LF , B RF ). The method of any preceding claim, wherein the braking force is controlled in the slow increase mode when it is detected that the vehicle speed (V R ) exceeds the reference speed (V RO ) and/or the predetermined period of time (T₃) has expired, and it is detected that the drive wheel shows an excessive slipping tendency in which its wheel speed (V WRF , V WLF ) exceeds a predetermined reference speed (V RH ) and the drive wheel acceleration or deceleration (α WR , α WL ) exceeds a predetermined reference acceleration (α H ).

Inventors: KOJIMA TAKASHI C O KABUSHIKI K, MATSUDA SHOHEI C O KABUSHIKI K, SUZUKI JIRO C O KABUSHIKI KAIS, TASHIMA KAZUTOSHI C O KABUSHIK, KOJIMA, TAKASHI, C/O KABUSHIKI KAISHA HONDA, MATSUDA, SHOHEI, C/O KABUSHIKI KAISHA HONDA, SUZUKI, JIRO, C/O KABUSHIKI KAISHA HONDA, TASHIMA, KAZUTOSHI, C/O KABUSHIKI KAISHA HONDA
Applicants: HONDA MOTOR CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: B60T8
IPC codes: B60T8, B60W10