GB2078887A

GB2078887A - Antiskid brake control

Info

Publication number: GB2078887A
Application number: GB8119871A
Authority: GB
Original assignee: Akebono Brake Industry Co Ltd
Current assignee: Akebono Brake Industry Co Ltd
Priority date: 1980-07-01
Filing date: 1981-06-26
Publication date: 1982-01-13
Also published as: JPS5715050A; AU7856381A

Abstract

In an antiskid control system, brake pressure is reduced when wheel lock is detected and the brake pressure is raised again when wheel velocity VW is related to the minimum wheel speed VWL by a predetermined value K1 so that K1VW = VWL. The predetermined value K1 is changed to a second predetermined value K1' when the minimum speed VwL is lower than a value K2Vwo, where Vwo is the wheel speed at a start of a reduction of the brake pressure, and K2 is a constant. The second value K1' is smaller than the first value K1. <IMAGE>

Description

SPECIFICATION Antiskid brake control method and apparatus This invention relates to an antiskid control method for preventing the wheels of a vehicle becoming locked during a brake application. The invention also extends to apparatus arranged to perform such a method.

There has been provided an antiskid control method for maintaining a suitable degree of slippage between the wheels of a vehicle and a road surface, so as to allow directional stability to be maintained and to preventthe braking distance becoming too great, by lowering an excessive braking force but by raising again the braking force as the occasion allows.

The basic principle of such control is there is generated a wheel velocity signal which is proportional to the instantaneous rotational rate of the wheel and which is used as a reference signal, and if a sudden decrease in the wheel velocity signal takes place during brake application then this is considered to represent a wheel locking condition. If such sudden decrease is detected, the brake pressure is reduced to have the excessive braking force accordingly reduced. On the other hand, when the wheel velocity signal begins to recover because of the reduction in the brake pressure, the brake pressure is again increased to prevent the braking distance becoming too long. These actions are repeated in order to carry out the antiskid control.

The minimum value of the wheel velocity signal, which occurs when the signal begins to recover from its dropped state on the locked condition being released, varies with the running and braking conditions of the vehicle, such as the braking force, the coefficient of friction of the road suface, and so on.

Therefore, this has presented the problem that, if the brake pressure is to be re-applied by compring the instantaneous wheel velocity signal with a tracking signal arranged to decrease at a relatively great predetermined rate from the instantaneous wheel velocity signal at the commencement of a reduction in the brake pressure, the recovery of the wheel velocity would be insufficient and thus the wheel would be caused to enter the locked condition again.

Conversely, if the above-stated predetermined rate is set at a relatively low value, the timing for the re-application of the brake pressure would be delayed and this would result in an unnecessarily elongated braking distance.

The present invention attempts at least to reduce the above-stated problem and it is therefore a general object of the invention to provide an antiskid method and apparatus for brake control in which the starting point at which brake pressure is increased again is inter-related to the extent by which the wheel velocity signal dropped so thatthe wheel is prevented from being locked again on re-appliation of the brake pressure. The time allowed to lapse before the pressure is re-applied is arranged to be relatively large when the wheel velocity signal drops greatly, but the brake pressure is promptly reapplied when the wheel velocity signal drops only to a small extent.

According to one aspect of this invention, there is therefore provided an antiskid control method of the type wherein brake pressure is reduced when occurrence of a wheel locking condition is detected and the brake pressure is raised again when cessation of the wheel locking condition is detected on the basis of a given degree of recovery of a wheel velocity signal Vw from a low peak value VwL thereof, the wheel velocity signal being arranged to recover as the Ibrake pressure is reduced, in which method the detection of the cessation of the wheel locking condition is performed by comparing the low peak value VwL with a divided value K1Vw obtained by dividing the wheel velocity signal Vw by a first predetermined value K1 and the first predetermined value K1 is changed to a second predetermined value K1'when the low peak value VwL is lower than a divided value K2Vw obtained by dividing the wheel velocity signal Vw at the starting point of the reduction of the brake pressure by a predetermined value K2, the second predetermined value K1' and the first predetermined value K1 being arranged to satisfy the relation K1' < K1.

According to a second aspect of this invention, there is provided antiskid control apparatus arranged to perform the aforesaid method, in which apparatus there is provided a low peak value hold circuit, a sample-and-hold circuit, a first dividing circuit able to divide its input by either one of two predetermined values and a second dividing circuit able to divide its input by another predetermined value, all of said circuits being connected to a wheel velocity signal generator for operating thereon, and there is a first comparator arranged to compare the outputs of the low peak value hold circuit and the first dividing circuit and to issue a signal indicative of cessation of the wheel locking condition on detecting co-incidence, and a second comparator arranged to compare the outputs of the sample-and-hold circuit and the second dividing circuit and to issue a signal which changes the dividing value of the first dividing circuit on detecting coincidence, the sample-and-hold circuit being arranged to sample the wheel velocity signal at the commencement of the detection of a wheel locking condition.

Preferably, there is provided a deceleration detecting means which provides a control output when the detected deceleration is less than a predetermined value. This control output can then be used to cause the first dividing circuit to divide its input by a third predetermined value less than either of the two predetermined dividing values aforesaid. Moreover, the control output also can cause the second dividing circuit to divide its input by a second predetermined value greater than the first-mentioned predetermined value.

Though the basic detection of the wheel locking condition may be performed in any one of a variety of manners, it is preferred for there to be means which comprise a subtractorto subtract a constant pre-set value from the wheel velocity signal, a deceleration generator to limit the maximum gradient of the subtracted signal and a comparator to compare the thus-processed signal with the instan taneous wheel velocity signal, the comparator being arranged to provide a brake-pressure reducing signal whenever coincidence is detected between its two inputs.

In orderthat the invention may better be understood, it will now be described in greater detail and a specific example thereof given, reference being made to the accompanying drawings, in which: Figure lisa block diagram of a circuit configuration able to perform the antiskid control method of the present invention; Figure 2 is a graphical representation of the characteristic curves showing the wheel velocity V when underthecontrol of the circuit shown in Figure 1;and Figure 3 is a circuit diagram showing the practical embodiment of part of the circuit shown in Figure 1.

Figure 1 is a block diagram showing a control circuit suitable for use in carrying out the control method of the present invention. The drawing shows a speed sensor 1 coupled to a frequency-to-voltage (F-V) converter 2, the output of which is a wheel velocity signal Vw of a voltage proportional to the speed of wheel rotation; the signal Vw is supplied to a subtractor 3 which subtracts voltage -AV; and the output from subtractor 3 drives a predetermined deceleration generator 4. These components constitute a circuit arrangement to produce a tracking velocity signal VTwhich varies with a predetermined difference with respect to variations in the wheel velocity signal Vw, which signal decreases during the application of the brakes of a vehicle.In other words, the tracking velocity signal VT follows the wheel velocity signal Vw with a fixed velocity difference AV. However, when the wheel velocity signal Vw decreases at a rate exceeding a pre-set gradient G, the tracking velocity signal VT is unable to follow the velocity signal Vw maintaining the difference AV and instead drops at the pre-set gradient G. When a comparator 5 detects the existence of a reltion Vt > Vw, this is considered to be representative of an abnormal drop of wheel velocity. Accordingly, a brake pressure reducing signal S1 is produced by the comparatorto set a flip-flop circuit 6.The output of the flip-flop circuit, when set, drives the coil of a solenoid valve 9 through an AND-gate 7 and an amplifier 8, the valve 9 forming a part of a pressure reducing device (not shown) arranged to perform brake pressure reducing control. Another input to the AND-gate 7 has an operation signal Br, Sw supplied thereto from a brake switch, and the valve operating signal S can be produced by the AND-gate only when signals S1 and Br, Sw are present.

The arrangement described in the foregoing serves to determine the point at which brake pressure starts to be reduced, and is not directly related to the present invention. Accordingly, the described arrangement which detects a wheel locking condition on the basis of the decrease of the wheel velocity signal Vw at a rate in excess of a pre-set value may be replaced by some other known method.

Referring further to the drawing, there are provided a hold circuit 10 which holds the low peak value VwL (minimum value) of the wheel velocity signal, a dividing circuit 11 which produces a divided signal K1Vw obtained by dividing the wheel velocity.

signal Vw by a predetermined dividing value K1 (0 < K1 < 1), and a comparator 12 which compares the low peak value VwL with the divided signal K1Vw and is arranged to produce a signal S2 serving to raise the brake pressure again when the result of the comparison shows the relation of K1Vw > VwL. The signal S2 is supplied to the re-set input of the flip-flop circuit 6, so as to stop the circuit 6 from causing the AND-gate producing signal S.

It is a feature of this invention that the predetermined value K1 which is used for dividing the wheel velocity signal is arranged to be variable according to the rate at which the wheel velocity signal Vw drops during brake application. With this arrangement, the wheel velocity can be sufficiently recovered when the drop is great, and can be promptly recovered when the drop is small. Again referring to Figure 1,there is provided a sample-and-hold circuit 13 which holds the wheel velocity signal value Vwo obtained by sampling the signal Vw at the starting point to of the antiskid control. The signal value Vwo is divided by a predetermined dividing value K2 by a dividing circuit 14to obtain a divided value K2Vwo.

This divided value K2Vwo is compared with the instantaneous wheel velocity signal Vw by means of a comparator 15. Then, when the result of the comparison performed by the comparator 15 shows K2Vwo < Vw, the predetermined dividing value used by the dividing circuit 11 is arranged to be K1 (0 < K1 < 1) but, when the result of comparison is K2Vwo > Vw, the dividing value of the circuit 11 is reduced to K1 ' which satisfies the relation 0 < K1' < K1 < 1.

The relation described in the foregoing is as shown in Figure 2. When occurrence of a wheel locking condition during brake application is detected, the brake pressure reducing signal S1 is produced on the basis of the relation of Vw < VT at the point of time to. The signal S1 causes the solenoid valve 9 of the reducing device to be driven to effect a reduction in the brake pressure. Concurrently with this, the sample-and-hold circuit 13 holds the wheel velocity signal value Vwo at time to and the signal K2Vwo obtained by dividing signal Vwo by the predetermined value K2 is supplied to the comparator 15. The comparator 15 is turned on when the wheel velocity signal Vw, which is representative of the dropped instantaneous wheel velocity, becomes lower than the divided value K2Vwo.

With the comparator 15 turned on, the predetermined dividing value K1 of the dividing circuit 11 is changed to value K1'.

In the antiskid control action during the first stage shown in Figure 2, the divided signal for determinilCg the time pointt2 at which the brake pressure is raised again is at the value K1 'Vw because there has been detected the relation of K2Vwo > Vw. Compared with a case where the divided signal is greater than the value K1 'Vw (indicated by K1Vw in the drawing), the time point at which the signal K1 'Vw comes to exceed the low peak value VwL is delayed.

It will be understood, therefore, that the wheel velocity signal Vw can be allowed time to recover sufficiently.

In the second stage of the antiskid control action, the wheel velocity drop is small. Therefore, in this case, it is the value Ki Vw that is compared with the low peak value VwL. Accordingly the time t2' at which the brake pressure is raised once again is not delayed and the brake pressure can promptly be raised again, so that the braking distance can be prevented from becoming unnecessarily large.

Figure 3 shows by way of example an arrangement for the circuit encompassed within broken lines in Figure 1. This circuit arrangement includes an operational amplifier OP1, diodes D1 and D2, transistors Q1 and Q2 and resistors R1 to R3 which constitute a sample-and-hold circuit arranged to hold the wheel velocity signal value Vwo prevailing when a signal Vsig produced by the flip-flop circuit 6 is supplied thereto. An operational amplifier OP3 forms a comparator which compares the wheel velocity signal Vw with a predetermined value K2Vwo obtained through division by resistors R4 and R5, and which thus detects the relation of K2Vwo > Vw. The output signal of this comparator causes a thyristor SCR to turn on.The dividing circuit 11 then has the predetermined dividing value K1 changed to K1' until the signal Vsig (which also drives a transistor Q4) is no longer produced, thereby turning off transistor Q4.

In this circuit, the predetermined dividing values of the dividers 11 and 14 may be arranged to be switched over from one to the other in the case where the road surface condition is one which has a low coefficient of friction, which would tend to cause skidding. In Figure 1, there is shown a vehicle deceleration detecting switch 16 which is provided for this purpose. In Figure 3, resistors R6 and R7 and a transistor 03 show a predetermined dividing value varying circuit, also provided for this purpose and operated as shown by switch 16. Further, if neces sary, the dividing signal value K1 ,which is to be compared with the low peak value VwL, may be arranged to be shifted in two or more steps.

Where the co-efficient of friction of the road surface is low and the deceleration of the vehicle which takes place during brake application is less than a predetermined value, the predetermined value K1' may be arranged to be changed again to a still smaller value K1" (0 < K1" < K1') in response to operation of the switch 16 in such a manner that the wheel velocity can be recovered to an even greater extent thereby. Also, with the switch 16 providing a detection signal to the dividing circuit 14, it is possible to have the wheel velocity recovered to a sufficient degree by changing the above stated predetermined dividing value K2 to a greater value K2' (0 < K2 < K2' < 1).

As will be appreciated from the foregoing, the antiskid control method of the present invention effectively prevents the occurrence of a wheel locking condition during brake application and also prevents the braking distance from becoming un necessarily long. The method of this invention thus has very great practical advantages.

Claims

1. An antiskid control method of the type wherein brake pressure is reduced when occurrence of a wheel locking condition is detected and the brake pressure is raised again when cessation of the wheel locking condition is detected on the basis of a given degree of recovery of a wheel velocity signal Vw from a low peak value VwL thereof, the wheel velocity signal being arranged to recover as the brake pressure is reduced, in which method the detection of the cessation of the wheel locking condition is performed by comparing the low peak value VwL with a divided value K1Vw obtained by dividing the wheel velocity signal Vw by a first predetermined value K1, and the first predetermined value K1 is changed to a second predetermined value K1' when the low peak value VwL is lower than a divided value K2Vw obtained by dividing the wheel velocity signal Vw at the starting point of the reduction of the brake pressure by a predetermined value K2, the second predetermined value K1' and the first predetermined value K1 being arranged to satisfy the relation K1' < K1.

2. A method according to claim 1, in which the predetermined value K1 is changed to a third predetermined value K1" which is smaller than the second predetermined value K1' when the deceleration of a vehicle fitted with apparatus arranged to perform the method is less than a predetermined value.

3. A method according to claim 1 or claim 2, in which the predetermined value K2 which is divided into the instantaneous wheel velocity signal at the starting point of the reduction of the brake pressure is arranged to be changed to another predetermined value greaterthan the predetermined value K2 when the decleration of a vehicle fitted with apparatus arranged to perform the method is less than a predetermined value.

4. A method according to any of the preceding claims, in which the wheel locking condition is detected by comparing the instantaneous wheel velocity signal with a processed signal derived from the wheel velocity signal and when coincidence therebetween is detected an output indicative of the wheel locking condition is given, the processed signal being obtained by subtracting a constant pre-set value from the wheel velocity signal and then limiting the maximum gradient thereof.

5. A method according to claim 1 and substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

6. Antiskid brake control apparatus arranged to perform a method according to any of claims 1 to 5, wherein there is provided a low peak value hold circuit, a sample-and-hold circuit, a first dividing circuit able to divide its input by either one of two predetermined values and a second dividing circuit able to divide its input by another predetermined value, all of said circuits being connected to a wheel velocity signal generator for operating thereon, and there is a first comparator arranged to compare the outputs of the low peak value hold circuit and the first dividing circuit and to issue a signal indicative of cessation of the wheel locking condition on detecting coincidence, and a second comparator arranged to compare the outputs of the sample-and-hold circuit and the second dividing circuit and to issue a signal which changes the dividing value of the first dividing circuit on detecting coincidence, the sample-and-hold circuit being arranged to sample the wheel velocity signal at the commencement of the detection of a wheel locking condition.

7. Antiskid brake control apparatus according to claim 6, wherein there is provided a deceleration detecting means which is connected to the first dividing circuitto cause that circuit to divide its input by a third predetermind value less than either of the two predetermined dividing values aforesaid, when the detected deceleration is less than a predetermined value.

8. Antiskid brake control apparatus according to claim 7, wherein the deceleration detecting means also is connected to the second dividing circuit to cause that circuit to divide its input by a second predetermined value, greater than the firstmentioned predetermined value, when the detected deceleration is less than a predetermined value.

9. Antiskid brake control apparatus according to any of claims 6 to 8, wherein means are provided to detect a wheel locking condition, which means comprises a subtractorto subtract a constant pre-set value from the wheel velocity signal, a deceleration generator to limit the maximum gradient of the subtracted signal and a comparatorto compare the thus-processed signal with the instantaneous wheel velocity signal, the comparator being arranged to provide a brake-pressure reducing signal whenever coincidence is detected between its two inputs.

10. Antiskid brake control apparatus substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 3 of the accompanying drawings.