Anti-lock control system for motor vehicles

Abstract: In an anti-lock control system for motor vehicle, the design is made such that the braking distance of the motor vehicle is prevented from being increased by excessive reduction of brake hydraulic pressure for a medium- or high-µ road surface, while at the same time occurrence of vibration in the motor vehicle is avoided which tends to be caused due to the excessive reduction of the brake hydraulic pressure.

Description: BACKGROUND OF THE INVENTION Field of the Invention: This invention relates to a novel and improved anti-lock control system for motor vehicles, which is operative to prevent the wheels of the motor vehicle from being locked during braking operation of the motor vehicle. Description of the Prior Art: Generally, with an anti-lock control system for motor vehicles, anti-lock control is effected by means of microcomputers such that hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake hydraulic pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance. Figure 1 of the accompanying drawings illustrates, by way of example, manners in which wheel speed Vw, wheel acceleration and deceleration dVw/dt and brake hydraulic pressure Pw are varied during the operation of the conventional anti-lock control system, together with hold signal HS and decay signal DS for opening and closing hold valves and decay valves as , disclosed in U. S. Patent No. 4,741,580. When the brake equipment of the motor vehicle is not being operated while the motor vehicle is running, the hold valves remain open while the decay valves remain closed, the brake hydraulic pressure Pw is not increased; and when the brake equipment is operated, the brake hydraulic pressure Pw is rapidly increased at time t0 so that the wheel speed Vw is decreased (normal mode). A reference wheel speed Vr is set up which is lower by a predetermined amount ΔV than the wheel speed Vw and follows the latter with such a speed difference. More specifically, the reference wheel speed Vr is set up so that when the deceleration (negative acceleration) dVw/dt of the wheel reaches a predetermined threshold level, say - 1.1G at time t1, anti-lock control is started, and the reference wheel speed Vr is thereafter made to linearly decrease with a deceleration gradient ϑ (= -1.1G). At time t2 when the deceleration dVw/dt of the wheel reaches a predetermined maximum value -Gmax, the hold signal HS is generated so that the hold valves are closed, thus holding the brake hydraulic pressure Pw. With the brake hydraulic pressure Pw being held, the wheel speed Vw is further decreased. At time t3, the wheel speed Vw and the reference wheel speed Vr become equal to each other, and a first cycle of anti-lock control is started; and the decay signal DS is generated, by which the decay valves are opened so that reduction of the brake hydraulic pressure Pw is started. As a result of this reduction of the brake hydraulic pressure Pw, the wheel speed Vw is changed from increase to decrease at time t4 when a low peak VL of the wheel speed Vw occurs. At the time t4, the decay signal DS is interrupted as shown in Figure 1, and as a result the decay valves are closed so that the reduction of the brake hydraulic pressure Pw is stopped and thus the brake hydraulic pressure is held. The wheel speed Vw is further increased up to the level of a speed Vc that is higher than the low peak speed VL by 80% of the difference Y between the wheel speed Va occurring at the time t3 when the reduction of the brake hydraulic pressure Pw was started and the low peak speed VL, i.e., Vc = VL + 0.8Y . Subsequently, at time t7, a high peak VH of the wheel speed Vw is reached; thereupon, the brake hydraulic pressure Pw is again increased. In this case, the buildup of the brake hydraulic pressure Pw is effected in such a manner that the brake hydraulic pressure Pw is alternately increased and held in succession by the fact that the hold signal HS is turned on and off mincingly, i. e., with relatively short intervals so that the brake hydraulic pressure Pw is caused to build up gradually. In this way, the wheel speed Vw is decreased, and at time t8 (corresponding to the time t3), a second cycle of the mode for reduction of the brake hydraulic pressure occurs. The time period Tx of the initial brake hydraulic pressure buildup occurring at the time t7 is determined on the basis of calculation of the average acceleration (Vc - Vb )/ΔT over the time interval ΔT between the time t5 and the time t6 (the average acceleration depends on the friction coefficient µ of the road surface), and the time period of the subsequent pressure holding or pressure buildup is determined on the basis of the acceleration or deceleration of the wheel which is detected immediately prior to the pressure holding or pressure buildup. The brake hydraulic pressure increasing, holding and reducing modes are effected in combination as mentioned above, and thus the wheel speed Vw can be controlled so that the vehicle speed can be decrease, while the wheels of the motor vehicle are prevented from being locked. When the above-mentioned anti-lock control is effected, wheel speed Vw which is used as a reference speed is computed on the basis of a frequency signal derived from a speed sensor provided in association with the wheel and is subjected to filtering in the range from +30G to -30G for example to prevent faulty operation due to noise. To compute the wheel speed Vw, a predetermined period of time is required so that the computed speed Vw shown by a solid line in Figure 2 tends to be delayed by a certain time period say 10 to 16 ms with respect to the real wheel speed Vw′ shown by a one-dotted chain line. Thus, ideally, the pressure reduction should be interrupted at a time point P′ when a low peak of the real wheel speed Vw′ occurs, and thereafter the brake hydraulic pressure should be held as shown by a one-dotted chain line Pw′ in Figure 2; actually, however, it is at a time point P later by a certain time period than the time point P′ that the pressure reduction is interrupted so that the brake hydraulic pressure Pw is correspondingly further reduced. In general, the decreasing gradient of the brake hydraulic pressure as the anti-lock control is being effected is higher than the increasing gradient; thus, the brake hydraulic pressure is excessively reduced because of the delayed interruption of the pressure reduction. For a medium- or high-µ road surface representing a high friction coefficient µ, difficulties are encountered in that due to the excessive reduction of the brake hydraulic pressure, the braking distance is increased, and also the variation range of the brake hydraulic pressure is so widened that vibration is caused to occur in the motor vehicle. In the EP-A-0 198 691 an antiskid control system is disclosed wherein a brake pressure reduction period is variable depending on wheel deceleration so that there is a tendency that erroneous operation is caused by a noise signal included in the wheel deceleration signal. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an anti-lock control system for a motor vehicle, which is so designed as to prevent the braking distance of the motor vehicle from being increased by excessive reduction of the brake hydraulic pressure for a medium- or high-µ road surface, while at the same time avoiding occurence of vibration in the motor vehicle which tends to be caused due to the excessive reduction of the brake hydraulic pressure. According to the invention an anti-lock control system for motor vehicles is provided according to claim 1. According to a first aspect of the present invention, there is provided an anti-lock control system, wherein a predetermined pressure reduction period beginning at start of pressure reduction is set up; and for a medium- or high -µ road surface, when the predetermined pressure reduction period elapses at a time point that is later than a time point when pressure reduction is started but earlier than a time point when a low peak wheel speed is reached, the pressure reduction is interrupted at the end of the predetermined reduction period. According to a second aspect of the present invention, there is provided an anti-lock control system, wherein a predetermined pressure reduction period is set up; a pressure reduction resuming threshold speed is set up which is lower than and follows, in a predetermined relation, a computed vehicle speed which is computed on the basis of the highest one of four wheel speeds; for a medium- or high-µ road surface, when the predetermined pressure reduction period elapses at a time point that is later than a time point when reduction of brake hydraulic pressure is started but earlier than a time point when the wheel speed becomes equal to or lower than the pressure reduction resuming threshold speed, the pressure reduction is interrupted at the end of the predetermined pressure reduction periods. Furthermore, when the wheel speed becomes equal to or lower than the pressure reduction resuming threshold speed at a time point that is later than time point when the pressure reduction is interrupted but earlier than a low peak of the wheel speed is reached, pressure reduction is resumed at a time point when the wheel speed becomes equal to the pressure reduction resuming threshold speed, and continued until the time point when the low peak of the wheel speed is reached. As will be appreciated from what has been described above, according to the present invention, a predetermined pressure reduction period for a a medium- or high-µ road surface is set up; for such a road surface, pressure reduction of brake hydraulic pressure is interrupted at a time point when the predetermined pressure reduction period elapses, so that excessive reduction of the brake hydraulic pressure is prevented, thereby making it possible to shorten the braking distance. It is also possible to avoid occurrence of vibration in the motor vehicle which tends to be caused by such excessive reduction of the brake hydraulic pressure. Further, according to the present invention, a pressure reduction resuming threshold speed is set up , and control is effected in such a manner that pressure reduction is always effected when the wheel speed is lower than the pressure reduction resuming threshold speed so that even when rapidly falling, the wheel speed can be recovered from the falling state, thereby making it possible to prevent the wheel from being locked. Other objects, features and advantages of the present invention will become apparent from the dependant claims and from the ensuing description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 and 2 are views useful for explaining how anti-lock control is effected with the prior-art anti-lock system for motor vehicles. Figure 3 is a block diagram showing the four-sensor, three-channel type anti-lock control system for motor vehicles according to an embodiment of the present invention. Figures 4 to 7 are views useful for explaining the relationship between wheel speed and brake hydraulic pressure that occurs in the system of the present invention. Figure 8 is a flow chart of pressure reduction period judging routine according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The anti-lock system for motor vehicles according to an embodiment of the present invention will be described with reference to the accompanying drawings. Referring to Figure 3, there are shown, in a block diagram, a three-channel type anti-lock control system embodying the present invention, wherein outputs of wheel speed sensors 1 to 4 are passed to and computed in computing circuits 5 to 8 , from which signals representing wheel speeds Vw1 to Vw4 are derived. Lefthand front wheel speed Vw1 and righthand front wheel speed Vw2 are respectively provided directly to first and second control logic circuits 9 and 10 as first and second channel speeds Vs1 and Vs2. Lower one of lefthand rear wheel speed Vw3 and righthand rear wheel speed Vw4 is selected by a select-low circuit 11 and is provided to a third control logic circuit 12 as a third channel speed Vs3. With the respective control logic circuits 9, 10 and 12, the opening/closing operation of hold valves HV and decay valves DV is controlled by using each of the channel speeds Vs1 to Vs3 as a control object wheel speed Vw which will be referred to as wheel speed Vw hereinafter. The respective signals representing the wheel speeds Vw1 to Vw4 are passed to a computing circuit 13 which in turn provided a computed vehicle speed. In the computing circuit 13, the highest one of the four wheel speeds Vw1 to Vw4 is selected and subjected to filtering of its acceleration and deceleration in a range from +1G to -1G so that the computed vehicle speed Vv is provided which in turn is passed to the control logic circuits 9, 10 and 12. The computed vehicle speed Vv is also provided to a declaration gradient judging circuit 14. A signal representing the deceleration gradient judged by the circuit 14 is transmitted to the control logic circuits 9, 10 and 12. The deceleration gradient of the computed vehicle speed Vv represents the friction coefficient µ of the road surface. In this embodiment, the deceleration gradient value of the computed vehicle speed Vv corresponding to a lower limit value µ1 for the friction coefficient of a medium-µ road surface is set at 0.2G; and when the deceleration gradient is steeper than 0.2G, the road surface is judged as medium- or high-µ road surface. For a first cycle of the anti-lock control process, judgment of the deceleration gradient of the computed vehicle speed Vv is made on the basis of the deceleration gradient which occurs during the time period from a time point when the anti-lock control is commenced to a time point when the subsequent buildup of the brake hydraulic pressure is started; and for second and succeeding cycles of the anti-lock control process, judgment of the deceleration gradient is made on the basis of the deceleration which occurs during the time period from a time point when buildup of the brake hydraulic pressure is started to a time point when the next buildup of the brake hydraulic pressure is started. The control logic circuits 9, 10 and 12 are respectively provided with pressure reduction period judging sections 15, 16 and 17 including switches SW1 to SW3 which are turned on, so that the sections 15 to 17 are operated, during the time period from a time point when pressure reduction is started to a time point when the pressure reduction is interrupted ( when a low peak of the wheel speed Vw occurs), i.e., from time t3 to time t4 in Figure 1. In the pressure reduction period judging sections 15 to 17, normal pressure reduction time T1 say 16 to 24 ms for the brake hydraulic pressure is set up; in the case where it is judged by the deceleration gradient judging circuit 14 that the deceleration gradient of the computed vehicle speed Vv is steeper than 0.2G, i.e., in the case where the road surface is judged as a medium- or high-µ road surface, the brake hydraulic pressure reduction is interrupted when the predetermined pressure reduction time T1 has elapsed at a time point B later than a time point A when the pressure reduction is started but earlier than a time point C when a low peak of the wheel speed Vw occurs, as shown in Figure 4. With the conventional system, the pressure reduction is interrupted at the time point C as shown by one-dotted chain line in Figure 4, whereas with the system of the present invention, as mentioned just above, the pressure reduction is interrupted at the time point B so that the brake hydraulic pressure is prevented from being excessively reduced for a medium- or high-µ road surface. Further, in the pressure reduction period judging sections 15 to 17, a threshold speed VT1 for resuming pressure reduction is set up which is lower than and follows the computed vehicle speed Vv in a predetermined relationship therewith, in accordance with the following expression: VT1 = a x Vv - b where a = 0.75 to 0.95, and b = 10 to 20 km/hour. As shown in Figure 5, when the wheel speed Vw has become lower than the pressure reduction resuming threshold VT1 before the predetermined pressure reduction time T1 elapses, the brake hydraulic pressure reduction is not interrupted at a time point B when the predetermined pressure reduction time T1 elapses from a time point A when the pressure reduction is started, and is continued a low peak point is reached, thereby permitting the wheel speed Vw to be rapidly recovered from the falling state. As shown in Figure 6, even when the pressure reduction is interrupted at a time point B when the predetermined pressure reduction time T1 elapses from a pressure reduction starting time point A, the brake hydraulic pressure reduction is started at a time point D when the wheel speed Vw has become lower than the pressure reduction resuming threshold VT1 before the wheel speed Vw reaches a low peak point C, and is continued until the low peak point C is reached, thereby permitting the wheel speed Vw to be recovered from the falling state. Computation is made of the average deceleration AVG of the wheel speed Vw which occurs during the time period from a time point A when pressure reduction is started to a time point B when the predetermined pressure reduction period T1 elapses. In the case where the average deceleration AVG is equal to or higher than 10G so that the wheel speed Vw is rapidly decreased as shown in Figure 7, it is judged that the friction coefficient of the road surface is rapidly changed from medium- or high-µ to low-µ, so that even when a condition for interruption of the pressure reduction is satisfied at the time point B, the brake hydraulic pressure reduction is not interrupted at the time point B but continued until and interrupted at a time point when a low peak of the wheel speed Vw occurs, thereby preventing the wheel from moving toward being locked. Figure 8 is a flow chart showing a control routine executed by the pressure reduction period judging sections 15 to 17. At step S1, judgment is made as to whether or not the predetermined pressure reduction period T1 elapses after pressure reduction is started. The result of the judgment at the step S1 is "NO" until the pressure reduction period T1 elapse, and thus the process proceeds to step S2 where judgment is made as to whether or not the wheel speed Vw is lower than the the pressure reduction resuming threshold VT1. The result of the judgment at the step S2 is "NO" until the wheel speed Vw reaches the pressure reduction resuming threshold VT1, and thus the process proceeds to step S3 where judgment is made as to whether or not a low peak of the wheel speed Vw is reached. The process returns to the step S1 until a low peak of the wheel speed Vw occurs. When such a low peak occurs, the result of the judgment at the step S3 is "YES", and a hold mode is established at step S2. At the next step S5, the decay valves are turned off (closed) while the hold valves remain turned on (closed), so that the pressure reduction is interrupted. When the result of the judgment at the step S3 is "NO", the process returns to the step S1. Description will now be made of the case where no low peak of the wheel speed Vw occurs until the predetermined pressure reduction period T1 elapses from a time point when the pressure reduction is started. When the result of the judgment at the step S1 is "YES", judgment is made at the next step S6 as to whether or not it is in the second or later cycle of the anti-lock control that the current pressure reduction is being effected. This is because the deceleration of the computed vehicle speed Vv cannot be judged until after the second or later cycle of the anti-lock control. When the result of the judgment at the step S6 is "YES" the process proceeds to the next step S7 where judgment is made as to whether or not the friction coefficient of the road surface is equal to or higher than a lower limit value µ1 for the friction coefficient of a medium-µ road surface. This judgment is made through the judgment with respect to the deceleration of the computed vehicle speed Vv as described above. More specifically, the road surface is judged as medium- or high-µ road surface when the deceleration gradient of the computed vehicle speed Vv is steeper than 0.2G. At the next step S8, judgment is made as to whether or not the average deceleration AVG of the wheel speed Vw occurring during the time period from the pressure reduction starting point to the time point when predetermined pressure reduction period T1 elapses, is equal to or higher than a predetermined value G1, say 10G. When the result of the judgment is "NO", the process proceeds to the next step S9 where while the hold valves remain turned on (closed), the decay valves are turned off (closed) to interrupt the pressure reduction; and thereafter the process proceeds to the step S2. When it is judged at the step S2 that the wheel speed Vw is lower than the pressure reduction resuming threshold VT1, the process proceeds to step S10 where while the hold valves remain turned on (closed), the decay valves are turned on (opened), so that the brake hydraulic pressure is reduced again. When it is judged at the next step S3 that a low peak of the wheel speed Vw occurs, a hold mode is established at step S4, and the pressure reduction is interrupted at step S5. When it is judged at the step S2 that the wheel speed Vw is not lower than the pressure reduction resuming threshold VT1, the process proceeds to step S3 while the pressure reduction remains interrupted. Further, when the result of the judgment at the step S6 or S7 is"NO", or when the result of the judgment at the step S8 is "YES", the pressure reduction is not interrupted, and the process proceeds to the step S2; when the result of the judgment at the step S2 indicates that Vw is equal to or lower than VT1, the process proceeds to step S10 so that the pressure reduction is continued until and interrupted at a time point when a low peak of the wheel speed Vw is reached. In summary, in the anti-lock control system according to the above-described embodiment of the present invention, when the deceleration gradient of the computed vehicle speed Vv is steeper than 0.2G, the following modes of control are effected: (1) When a low peak point C of the wheel speed Vw occurs before a time point B when the predetermined pressure reduction period T1 elapses, the pressure reduction is interrupted at the low peak point C. (2) When the time point when the predetermined pressure reduction period T1 elapses occurs before the low peak point C, the pressure reduction is interrupted at the time point B (see Figure 4). However, the pressure reduction is not interrupted at the time point B but continued until the low peak point C is reached, in each of the cases where: (a) the wheel speed Vw becomes lower than the pressure reduction resuming threshold VT1 before the wheel speed Vw reaches the point B (see Figure 5); and (b) the average deceleration of the wheel which occurs during the time period from the time point A to the time point B, becomes equal to or higher than 10G (see Figure 7). (3) When the wheel speed Vw becomes lower than the pressure reduction resuming threshold VT1 after the pressure reduction is interrupted at the time point B, the pressure reduction is resumed at a time point D when the wheel speed Vw reaches the pressure reduction resuming threshold VT1, and the pressure reduction is continued until the low peak point C is reached (see Figure 6). As will be appreciated from the above discussion, by effecting the above-described anti-lock control in accordance with the present invention, it is possible to prevent the braking distance of the motor vehicle from being increased by an excessive reduction of the brake hydraulic pressure for a medium- or high-µ road surface. It is also possible to avoid occurrence of vibration in the motor vehicle which tends to be caused by the fact that the amplitude of the pressure variation is increased as a result of the excessive reduction of the brake hydraulic pressure.

Claims: Anti-lock control system for motor vehicles, wherein a) a brake hydraulic pressure (Pw) is built up decreasing wheel speed (Vw), b) reduction of said pressure (Pw) is started, c) the decreasing of the wheel speed (Vw) changes, at a time point (C) when low peak of wheel speed (Vw) is detected, to increasing as a result of pressure reduction of the brake hydraulic pressure (Pw), d) said reduction is interrupted so that there occurs a change of mode from pressure reduction to pressure hold, characterized by e) means (1,2,3,4; 9,10,12; 13; 14; 15,16,17) arranged to carry out said interruption of pressure reduction at the end (B) of a predetermined reduction period (T1) beginning at start of pressure reduction which is set up for the brake hydraulic pressure (Pw) when the conditions are established that : -e1) the friction coefficient (µ) of a road surface on which the motor vehicle runs is detected to be equal or higher than a predetermined value and -e2) said predetermined pressure reduction period (T1) elapses, before said low peak occurs, and f) said means (1,2,3,4; 9,10,12; 13; 14; 15,16,17) being also arranged to carry out said interruption of pressure reduction at the time point (C) of low peak when the conditions that -f1) the friction coefficient (µ) of a road surface on which the motor vehicle runs is detected to be equal to or higher than a predetermined value and -f2) said predetermined pressure reduction period (T1) elapses, before said low peak occurs, are not established. Anti-lock control system according to claim 1, characterized in that e) the said means (1,2,3,4; 9,10,12; 13; 14; 15,16,17) is arranged to carry out said interruption of pressure reduction at the end of the predetermined reduction period (T1) which is set up for the brake hydraulic pressure (Pw), when the further condition is established that : -e3) an average deceleration of said wheel speed (Vw) which occurs during said predetermined pressure reduction period (T1) is not lower than a predetermined value (G1), and f) the said means (1,2,3,4; 9,10,12; 13; 14; 15,16,17) is also arranged to carry out said interruption of pressure reduction at the time point (C) of low peak when the further condition : - f3) that an average, deceleration of said wheel speed (Vw) which occurs during said predetermined pressure reduction period (T1) is not lower than a predetermined value (G1) is not established. Anti-lock control system according to claim 2, characterized in that said predetermined value (G1) for the average deceleration is set to be 10G. Anti-lock system according to claim 1, characterized in that said predetermined pressure reduction period (T1) is set to be 16 to 24 ms. Anti-lock system according to claim 1, characterized in that it comprises means for computing a computed vehicle speed (Vv) by selecting the highest one of four wheel speeds (Vw1 to Vw4) and limiting the deceleration/acceleration of the selected highest wheel speed to be in a predetermined range. Anti-lock system according to claim 5, characterized in that said predetermined range is from +1G to -1G. Anti-lock system according to claim 5, characterized in that it comprises means for computing a pressure reduction resuming threshold speed (VT1) which is lower than and follows said computed vehicle speed (Vv) in predetermined relationship therewith. Anti-lock system according to claim 7, characterized in that said pressure reduction resuming threshold speed VT1 is computed in accordance with the following equation: VT1 = a x Vv - b where a = 0.75 to 0.95, and b = 10 to 20 km/hour. Anti-lock system according to claim 7, characterized in that when said predetermined pressure reduction period (T1) elapses at a time point that is later than a time point when the pressure reduction is started but earlier than a time point the wheel speed (Vw) becomes lower than said pressure reduction resuming threshold speed.(VT1), the interruption of the pressure reduction is effected at the end of said predetermined pressure reduction period (T1); and when the wheel speed (Vw) becomes lower than said pressure reduction resuming threshold speed (VT1) after the pressure reduction has been interrupted, reduction of the brake hydraulic pressure (Pw) is resumed at a time point when the wheel speed (Vw) reaches said pressure reduction resuming threshold, and the pressure reduction is continued until a low peak of the wheel speed (Vw) occurs. Anti-lock control system according to claim 1, characterized in that said means for judging the friction coefficient (µ) of the road surface comprises means for judging a deceleration gradient of said computed vehicle speed (Vv). Anti-lock system according to claim 10, characterized in that for a first cycle of the anti-lock control, the deceleration gradient of the computed vehicle speed judged by said deceleration gradient judging means is that that occurs during the time period from a time point when the anti-lock control is started to a time point when subsequent pressure reduction is started; and for a second or later cycle of the anti-lock control, said deceleration gradient is that that occurs during the time period from pressure reduction is started to a time point when subsequent pressure reduction is started. Anti-lock control system according to claim 11, characterized in that said deceleration gradient judging means is arranged to judge that the friction coefficient (µ) of the road surface is equal to or higher than said predetermined value (µ1) when the deceleration gradient of said computed vehicle speed (Vv) is steeper than -0.2G.

Inventors: SHIMANUKI HARUKI, SHIMANUKI, HARUKI
Applicants: AKEBONO BRAKE IND, AKEBONO BRAKE INDUSTRY CO., LTD.
Application year: 1989
Main classification: B60T8
IPC codes: B60T8