Skid control system

Claim 2

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle by controlling the fluid pressure to the brakes via a modulating valve actuated to one condition for relieving the fluid pressure from an original high pressure to a lower pressure and to another condition for applying the fluid pressure toWard the original high pressure, the invention comprising: control circuit means for actuating the modulating valve to the one condition in response to a selected occurrence which is indicative of an incipient skid condition to allow said wheel to accelerate, said control circuit means comprising acceleration means responsive to acceleration of the wheel of a first high determinable magnitude which is substantially greater than zero following actuation of the modulating valve to the one condition for actuating the valve to the other condition, and further comprising velocity means for providing a velocity signal representative of the velocity of the vehicle and for varying said first determinable magnitude in response to variations in the magnitude of said velocity signal.

Claim 3

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle by controlling the fluid pressure to the brakes via a modulating valve actuated to one condition for relieving the fluid pressure from an original high pressure to a lower pressure and to another condition for applying the fluid pressure toWard the original high pressure, the invention comprising: control circuit means for actuating the modulating valve to the one condition in response to a selected occurrence which is indicative of an incipient skid condition to allow said wheel to accelerate, said control circuit means comprising acceleration means responsive to acceleration of the wheel of a first high determinable magnitude which is substantially greater than zero following actuation of the modulating valve to the one condition for actuating the valve to the other condition, and further comprising velocity means for providing a velocity signal representative of the velocity of the vehicle and for varying said first determinable magnitude in response to variations in the magnitude of said velocity signal.

Claim 4

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle by controlling the fluid pressure to the brakes via a modulating valve actuated to one condition for relieving the fluid pressure from an original high pressure to a lower pressure and to another condition for applying the fluid pressure toWard the original high pressure, the invention comprising: control circuit means for actuating the modulating valve to the one condition in response to a selected occurrence which is indicative of an incipient skid condition to allow said wheel to accelerate, said control circuit means comprising acceleration means responsive to acceleration of the wheel of a first high determinable magnitude which is substantially greater than zero following actuation of the modulating valve to the one condition for actuating the valve to the other condition, and further comprising velocity means for providing a velocity signal representative of the velocity of the vehicle and for varying said first determinable magnitude in response to variations in the magnitude of said velocity signal.

Claim 5

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle by controlling the fluid pressure to the brakes via a modulating valve actuated to one condition for relieving the fluid pressure and to another condition for applying the fluid pressure, the invention comprising: control circuit means for actuting the modulating valve to the one condition in response to a selected occurrence which is indicative of an incipient skid condition, said control circuit means comprising acceleration means responsive to acceleration of the wheel of a first determinable magnitude following actuation of the modulating valve to the one condition for actuating the valve to the other condition, velocity means for providing a velocity signal representative of the velocity of the vehicle and varying said first determinable magnitude in response to variations in the magnitude of said velocity signal, said velocity means including means for increasing said first determinable magnitude in response to an increase in the magnitude of said velocity signal representative of an increased vehicle velocity whereby actuation of the modulating valve to said other condition will be delayed at said increased vehicle velocity.

Claim 6

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle by controlling the fluid pressure to the brakes via a modulating valve actuated to one condition for relieving the fluid pressure and to another condition for applying the fluid pressure, the invention comprising: control circuit means for actuting the modulating valve to the one condition in response to a selected occurrence which is indicative of an incipient skid condition, said control circuit means comprising acceleration means responsive to acceleration of the wheel of a first determinable magnitude following actuation of the modulating valve to the one condition for actuating the valve to the other condition, velocity means for providing a velocity signal representative of the velocity of the vehicle and varying said first determinable magnitude in response to variations in the magnitude of said velocity signal, said velocity means including means for increasing said first determinable magnitude in response to an increase in the magnitude of said velocity signal representative of an increased vehicle velocity whereby actuation of the modulating valve to said other condition will be delayed at said increased vehicle velocity.

Claim 7

The system of claim 2 with said velocity means comprising electrical storage means for storing an electrical charge with said charge being said velocity signal and having a magnitude representative of vehicle velocity with the magnitude of the charge varying in accordance with a preselected time rate.

Claim 8

The system of claim 3 comprising second acceleration means responsive to acceleration of the wheel of a second determinable magnitude following actuation of the modulating valve to the one condition for rapidly discharging said storage means whereby said charge will be reset to a new magnitude representative of vehicle velocity.

Claim 9

The system of claim 4 with said second determinable magnitude preselected as being representative of a high spinup rate of the wheel.

Claim 10

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle by controlling the fluid pressure to the brakes via a modulating valve actuated to one condition for relieving the fluid pressure and to another condition for applying the fluid pressure, the invention comprising: control circuit means for actuating the modulating valve to the one condition in response to a selected occurrence which is indicative of an incipient skid condition, said control circuit means comprising acceleration means responsive to acceleration of the wheel of a first determinable magnitude following actuation of the modulating valve to the one condition for actuating the valve to the other condition, velocity means for providing a velocity signal representative of the velocity of the vehicle and varying said first determinable magnitude in response to variations in the magnitude of said velocity signal, second acceleration means responsive to acceleration of the wheel to a second determinable magnitude following actuation of the modulating valve to the one condition for resetting the magnitude of said velocity signal.

Claim 11

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle by controlling the fluid pressure to the brakes via a modulating valve actuated to one condition for relieving the fluid pressure and to another condition for applying the fluid pressure, the invention comprising: control circuit means for actuating the modulating valve to the one condition in response to a selected occurrence which is indicative of an incipient skid condition, said control circuit means comprising acceleration means responsive to acceleration of the wheel of a first determinable magnitude following actuation of the modulating valve to the one condition for actuating the valve to the other condition, velocity means for providing a velocity signal representative of the velocity of the vehicle and varying said first determinable magnitude in response to variations in the magnitude of said velocity signal, second acceleration means responsive to acceleration of the wheel to a second determinable magnitude following actuation of the modulating valve to the one condition for resetting the magnitude of said velocity signal.

Claim 12

THe system of claim 6 with said second determinable magnitude preselected as being representative of a high spinup rate of the wheel.

Claim 13

The system of claim 6 with said velocity means including disabling means for disabling said velocity means from providing said velocity signal for varying said determinable time when the modulating valve is in the other condition.

Claim 14

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle by controlling the fluid pressure to the brakes via a modulating valve actuated to one condition for relieving the fluid pressure and to another condition for applying the fluid pressure, the invention comprising: control circuit means for actuating the modulating valve to the one condition in response to initiation of a control signal and to the other condition in response to termination of said control signal, said control circuit means comprising output circuit means for initiating said control signal in response to a deceleration of the wheel of a first determinable magnitude and for terminating said control signal a determinable time after the deceleration decreases to a selected magnitude below said first determinable magnitude, said control circuit means further comprising velocity means for providing a velocity signal representative of the velocity of the vehicle and varying said determinable time in response to variations in the magnitude of said velocity signal.

Claim 15

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle by controlling the fluid pressure to the brakes via a modulating valve actuated to one condition for relieving the fluid pressure and to another condition for applying the fluid pressure, the invention comprising: control circuit means for actuating the modulating valve to the one condition in response to initiation of a control signal and to the other condition in response to termination of said control signal, said control circuit means comprising output circuit means for initiating said control signal in response to a deceleration of the wheel of a first determinable magnitude and for terminating said control signal a determinable time after the deceleration decreases to a selected magnitude below said first determinable magnitude, said control circuit means further comprising velocity means for providing a velocity signal representative of the velocity of the vehicle and varying said determinable time in response to variations in the magnitude of said velocity signal.

Claim 16

The system of claim 9 with said velocity means including means for increasing said determinable time in response to magnitudes of said velocity signal representative of higher vehicle velocities whereby actuation of the modulating valve to said other condition will be delayed at higher vehicle velocities.

Claim 17

The system of claim 10 with said velocity means comprising electrical storage means for storing an electrical charge with said charge being said velocity signal and having a magnitude representative of vehicle velocity with the magnitude of the charge varying in accordance with a preselected time rate.

Claim 18

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle on a surface, the method comprising the steps of: sensing the deceleration of the wheel and providing a signal for relieving the pressure for a predetermined time interval in response to a deceleration of a determinable magnitude, alternatively subsequently reapplying the pressure at the expiration of said preselected time interval whereby said preselected time interval is operative for lower coefficients of friction between the wheel and surface or subsequently reapplying the pressure in response to acceleration of the wheel of a second high determinable magnitude occurring prior to the expiration of said time interval.

Claim 19

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle on a surface, the method comprising the steps of: sensing the deceleration of the wheel and providing a signal for relieving the pressure for a predetermined time interval in response to a deceleration of a determinable magnitude, alternatively subsequently reapplying the pressure at the expiration of said preselected time interval whereby said preselected time interval is operative for lower coefficients of friction between the wheel and surface or subsequently reapplying the pressure in response to acceleration of the wheel of a second high determinable magnitude occurring prior to the expiration of said time interval.

Claim 20

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle on a surface, the method comprising the steps of: sensing the deceleration of the wheel and providing a signal for relieving the pressure for a predetermined time interval in response to a deceleration of a determinable magnitude, alternatively subsequently reapplying the pressure at the expiration of said preselected time interval whereby said preselected time interval is operative for lower coefficients of friction between the wheel and surface or subsequently reapplying the pressure in response to acceleration of the wheel of a second high determinable magnitude occurring prior to the expiration of said time interval.

Claim 21

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle on a surface, the method comprising the steps of: sensing the deceleration of the wheel and providing a signal for relieving the pressure for a predetermined time interval in response to a deceleration of a determinable magnitude, alternatively subsequently reapplying the pressure at the expiration of said preselected time interval whereby said preselected time interval is operative for lower coefficients of friction between the wheel and surface or subsequently reapplying the pressure in response to acceleration of the wheel of a second high determinable magnitude occurring prior to the expiration of said time interval.

Claim 22

The method of claim 12 further comprising the steps of: increasing the effective magnitude of said preselected time interval for higher, as compared to lower, vehicle velocities.

Claim 23

The method of claim 13 further comprising the step of: increasing said second determinable magnitude for higher, as compared to lower, vehicle velocities.

Claim 24

The method of claim 12 further comprising the step of: increasing said second determinable magnitude for higher, as compared to lower, vehicle velocities.

Claim 25

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle on a surface, the method comprising: relieving the pressure in response to a selected occurrence which is indicative of an incipient skid subsequently reapplying pressure with the time of pressure reapplication being varied such as to be shorter for higher coefficients of friction between the wheel and the surface and longer for lower coefficients of friction between the wheel and the surface.

Claim 26

In a skid control system for controlling fluid pressure actuated brakes of at least one wheel of a wheeled vehicle on a surface, the method comprising: relieving the pressure in response to a selected occurrence which is indicative of an incipient skid subsequently reapplying pressure with the time of pressure reapplication being varied such as to be shorter for higher coefficients of friction between the wheel and the surface and longer for lower coefficients of friction between the wheel and the surface.

Claim 27

The method of claim 16 further comprising the step of: increasing the time of pressure reapplication for higher, as compared to lower, vehicle velocities.

Claim 28

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for detecting an incipient skid condition and for providing a skid signal representative thereof to said brake relieving means for relieving the brake of said wheel; means for providing a vehicle velocity signal which is at least approximately indicative of vehicle velocity; and means responsive to said wheel speed signal for detecting a predetermined magnitude of acceleration of said wheel after said relief of the brakes of said wheel by said brake relieving means for terminating the relief of said brakes by said brake relieving means, said means including means responsive to said vehicle velocity signal for varying said predetermined magnitude of acceleration of said wheel in accordance with said vehicle velocity signal.

Claim 29

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for detecting an incipient skid condition and for providing a skid signal representative thereof to said brake relieving means for relieving the brake of said wheel; means for providing a vehicle velocity signal which is at least approximately indicative of vehicle velocity; and means responsive to said wheel speed signal for detecting a predetermined magnitude of acceleration of said wheel after said relief of the brakes of said wheel by said brake relieving means for terminating the relief of said brakes by said brake relieving means, said means including means responsive to said vehicle velocity signal for varying said predetermined magnitude of acceleration of said wheel in accordance with said vehicle velocity signal.

Claim 30

A skid control system according to claim 18 wherein said means for varying said predetermined magnitude of acceleration of said wheel increases said predetermined magnitude in accordance with increases in said vehicle velocity signal.

Claim 31

A skid control system according to claim 18 wherein said means for providing said vehicle velocity signal is responsive to said wheel speed signal for providing an approximation of said vehicle velocity.

Claim 32

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: detecting means responsive to said wheel speed signal for detecting an incipient skid condition to provide a skid signal representative thereof to said brake relieving means for relieving the brake of said wheel and for detecting the recovery of said wheel from said incipient skid condition for terminating the relief of said brakes by said brake relieving means, said detecting means being responsive at least at times to a vehicle velocity signal; and vehicle velocity means responsive to said wheel speed signal for providing said vehicle velocity signal, said means causing said vehicle velocity signal to have a magnitude which is correlative to the magnitude of said wheel speed signal during decreases in the magnitude of said wheel speed signal below a predetermined rate of decrease, and to have a magnitude which decreases at a lesser rate than said wheel speed signal during decreases in the magnitude of said wheel speed signal which are greater than said predetermined rate of decrease, and to have a magnitude which increases with increasing wheel speed when the magnitude of said vehicle velocity signal is again correlative to the magnitude of said wheel speed signal, said vehicle velocity means being responsive to an acceleration of said wheel of a predetermined magnitude after said brake relief for resetting said vehicle velocity signal so that said vehicle velocity signal is then correlative to said wheel speed signal.

Claim 33

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: detecting means responsive to said wheel speed signal for detecting an incipient skid condition to provide a skid signal representative thereof to said brake relieving means for relieving the brake of said wheel and for detecting the recovery of said wheel from said incipient skid condition for terminating the relief of said brakes by said brake relieving means, said detecting means being responsive at least at times to a vehicle velocity signal; and vehicle velocity means responsive to said wheel speed signal for providing said vehicle velocity signal, said means causing said vehicle velocity signal to have a magnitude which is correlative to the magnitude of said wheel speed signal during decreases in the magnitude of said wheel speed signal below a predetermined rate of decrease, and to have a magnitude which decreases at a lesser rate than said wheel speed signal during decreases in the magnitude of said wheel speed signal which are greater than said predetermined rate of decrease, and to have a magnitude which increases with increasing wheel speed when the magnitude of said vehicle velocity signal is again correlative to the magnitude of said wheel speed signal, said vehicle velocity means being responsive to an acceleration of said wheel of a predetermined magnitude after said brake relief for resetting said vehicle velocity signal so that said vehicle velocity signal is then correlative to said wheel speed signal.

Claim 34

A skid control system according to claim 21 in which said predetermined magnitude of wheel acceleration is representative of a high magnitude of wheel acceleration.

Claim 35

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for providing a skid signal to said brake relieving means for relieving the brake of said wheel when said wheel speed signal is indicative of a predetermined magnitude of wheel deceleration; means for providing a vehicle velocity signal which is at least approximately indicative of vehicle velocity; and means responsive to said vehicle velocity signal for terminating said skId control signal for reapplying the brake of said wheel after a determinable period of time and for varying said determinable period of time with said vehicle velocity signal.

Claim 36

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for providing a skid signal to said brake relieving means for relieving the brake of said wheel when said wheel speed signal is indicative of a predetermined magnitude of wheel deceleration; means for providing a vehicle velocity signal which is at least approximately indicative of vehicle velocity; and means responsive to said vehicle velocity signal for terminating said skId control signal for reapplying the brake of said wheel after a determinable period of time and for varying said determinable period of time with said vehicle velocity signal.

Claim 37

A system according to claim 23 wherein said last means increases said determinable period of time in response to magnitudes of said vehicle velocity signal representative of higher vehicle velocities whereby the reapplication of said brake will be delayed for longer periods of time at higher vehicle velocities.

Claim 38

A skid control system according to claim 23 wherein said means for providing said vehicle velocity signal is responsive to said wheel speed signal for providing an approximation of said vehicle velocity.

Claim 39

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for providing a skid signal to said brake relieving means for relieving th brake of said wheel when said wheel speed signal is indicative of a first predetermined magnitude of wheel deceleration and for providing an indication of when the deceleration of said wheel reaches a second predetermined magnitude of wheel deceleration which is less than said first predetermined magnitude of wheel deceleration; and means responsive to said indication for terminating said skid control signal for reapplying the brake of said wheel after the expiration of a determinable period of time subsequent to the provision of said indication.

Claim 40

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for providing a skid signal to said brake relieving means for relieving th brake of said wheel when said wheel speed signal is indicative of a first predetermined magnitude of wheel deceleration and for providing an indication of when the deceleration of said wheel reaches a second predetermined magnitude of wheel deceleration which is less than said first predetermined magnitude of wheel deceleration; and means responsive to said indication for terminating said skid control signal for reapplying the brake of said wheel after the expiration of a determinable period of time subsequent to the provision of said indication.

Claim 41

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for providing a skid signal to said brake relieving means for relieving the brake of said wheel when said wheel speed signal is indicative of a first predetermined magnitude of wheel deceleration and for providing an indication of when the deceleration of said wheel reaches a second predetermined magnitude of wheel deceleration which is less than said first predetermined magnitude of wheel deceleration; means for providing a vehicle velocity signal which is at least approximately indicative of wheel velocity; and means responsive to said indication for terminating said skid control signal for reapplying the brake of said wheel after the expiration of a determinable period of time subsequent to the provision of said indication, said means responsive to said indication being also responsive to said vehicle velocity signal for varying said determinable period of time with said vehicle velocity signal.

Claim 42

A skid control system for a wheeled vehicle having at least one wheel, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for providing a skid signal to said brake relieving means for relieving the brake of said wheel when said wheel speed signal is indicative of a first predetermined magnitude of wheel deceleration and for providing an indication of when the deceleration of said wheel reaches a second predetermined magnitude of wheel deceleration which is less than said first predetermined magnitude of wheel deceleration; means for providing a vehicle velocity signal which is at least approximately indicative of wheel velocity; and means responsive to said indication for terminating said skid control signal for reapplying the brake of said wheel after the expiration of a determinable period of time subsequent to the provision of said indication, said means responsive to said indication being also responsive to said vehicle velocity signal for varying said determinable period of time with said vehicle velocity signal.

Claim 43

A system according to claim 27 wherein said last means increases said determinable period of time in response to magnitudes of said vehicle velocity signal representative of higher vehicle velocities whereby the reapplication of said brake will be delayed for longer periods of time at higher vehicle velocities.

Claim 44

A skid control system for a wheeled vehicle having at least one wheel on a surface, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for detecting an incipient skid condition and for providing a skid signal representative thereof to said brake relieving means for relieving the brake of said wheel; means for reapplying the brake of said wheel either in response to a predetermined magnitude of acceleration of said wheel or alternately in response to the expiration of a preselected period of time if the predetermined magnitude of acceleration of said wheel is not attained so that said preselected period of time is operative for lower coefficients of fraction between the wheel and the surface and sAid determinable magnitude of acceleration is operative for higher coefficients of friction between the wheel and the surface.

Claim 45

A skid control system for a wheeled vehicle having at least one wheel on a surface, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for detecting an incipient skid condition and for providing a skid signal representative thereof to said brake relieving means for relieving the brake of said wheel; means for reapplying the brake of said wheel either in response to a predetermined magnitude of acceleration of said wheel or alternately in response to the expiration of a preselected period of time if the predetermined magnitude of acceleration of said wheel is not attained so that said preselected period of time is operative for lower coefficients of fraction between the wheel and the surface and sAid determinable magnitude of acceleration is operative for higher coefficients of friction between the wheel and the surface.

Claim 46

A skid control system according to claim 29 wherein said means for detecting an incipient skid condition provides said skid signal in response to a deceleration of said wheel of a determinable magnitude.

Claim 47

A skid control system for a wheeled vehicle having at least one wheel on a surface, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for detecting an incipient skid condition and for providing a skid signal representative thereof to said brake relieving means for relieving the brake of said wheel; means for providing a vehicle velocity signal which is at least approximately indicative of vehicle velocity; and means responsive to the acceleration of said wheel for reapplying the brake of said wheel either in response to a predetermined magnitude of acceleration of said wheel or alternately in response to the expiration of a preselected period of time so that said preselected period of time is operative for lower coefficients of friction between the wheel and the surface and said determinable magnitude of acceleration is operative for higher coefficients of friction between the wheel and the surface, said means responsive to the acceleration of the wheel varying said preselected period of time with said vehicle velocity signal.

Claim 48

A skid control system for a wheeled vehicle having at least one wheel on a surface, a brake for said wheel, means for providing a wheel speed signal representative of the speed of said wheel, and means for relieving the brake of said wheel comprising: means responsive to said wheel speed signal for detecting an incipient skid condition and for providing a skid signal representative thereof to said brake relieving means for relieving the brake of said wheel; means for providing a vehicle velocity signal which is at least approximately indicative of vehicle velocity; and means responsive to the acceleration of said wheel for reapplying the brake of said wheel either in response to a predetermined magnitude of acceleration of said wheel or alternately in response to the expiration of a preselected period of time so that said preselected period of time is operative for lower coefficients of friction between the wheel and the surface and said determinable magnitude of acceleration is operative for higher coefficients of friction between the wheel and the surface, said means responsive to the acceleration of the wheel varying said preselected period of time with said vehicle velocity signal.