This invention relates to a wheeled vehicle control system, and particularly to a control system for controlling a wheeled vehicle during braking or acceleration.

Several types of wheeled vehicle braking system are known, but most suffer from the disadvantage that they can introduce the phenomenon known as 'slip', where the wheels of the vehicle, during braking are caused to rotate at speeds different (slower) from the speeds corresponding to the actual speed of travel of the vehicle over the ground. If the amount of slip exceeds a certain limit the wheels of the vehicle can lock, that is cease to rotate, causing the driver to lose control of the vehicle.

A similar phenomenon can occur when a wheeled vehicle is accelerated rapidly, but in this case the wheels of the vehicle rotate at speeds faster than those corresponding to the actual speed of travel of the vehicle over the ground, this phenomenon generally being known as 'spin' .

Hereafter the word 'slip' will be used to embrace both slip as can occur during braking and spin as can occur during rapid acceleration.

An anti-lock braking system has been proposed which aims at reducing slip by sensing at each wheel of a wheeled vehicle whether there is slip. If slip is found at any wheel the

braking action to that wheel is automatically removed for a short period then re-applied. The sensing is carried out repeatedly and rapidly during braking, but the system is capable only of detecting whether slip is or is not occurring, and reacting if necessary by removing and re-applying the braking action, i.e. the system is not capable of progressively modulating braking according to the degree of slip and/or the displacement of the brake pedal affecting braking and merely operates as an on/off system.

According to the present invention there is provided a wheeled vehicle control system comprising means for sensing whether slip is occurring at any wheel of the vehicle by determining the difference between the actual speed of rotation of the wheel and the speed of rotation corresponding to the actual speed of the vehicle over the ground, and means to convert any sensed slip into control signals for controlling the braking or acceleration of the vehicle thereby to modulate the slip.

The control system can include a member operable by the driver to activate braking or acceleration of the vehicle, and means responsive to said control signals to apply a force to the member in the opposite direction to that applied by the driver thereby to modulate the slip.

Preferably, the member operable by the driver is a brake or accelerator pedal and the system includes means for sensing the displacement of the pedal, the pedal displacement being

converted, with the sensed slip, to provide said control signals.

Preferably the means to apply an opposing force is a fluid actuated ram.

Preferably said means for sensing the displacement of the pedal is an LVDT.

Preferably the system includes means to deactivate said means to apply an opposing force if the force applied by the driver to said member exceeds a predetermined value.

Preferably said deactivation means is a pressure relief valve operative to prevent operation of said ram.

Preferably the actual speed of the vehicle over the ground is optically sensed.

Preferably the speed of rotation of the wheel is magnetically sensed.

Preferably said means for converting is a microprocessor.

The invention will now be described by the way of example with reference to the drawings, of which:-

Figure 1 is a schematic diagram of a brake control system according to the invention;

Figure 2 is a longitudinal sectional view of a hydraulic actuator which can be used in the system of Figure 1;

Figure 3 is a graph illustrating variation of slip with pedal displacement; and

Figure 4 is a graph illustrating variation of slip with traction of the vehicle.

Figure 1 is a schematic diagram of a wheeled vehicle control system comprising a brake pedal 1 which is attached to a piston 9 slidable within a housing 10. The displacement of the brake pedal 1 is measured by a linear variable differential transformer (LVDT) 2 which converts the mechanical displacement of the piston 9 into an electrical signal which is then passed to a microprocessor 5. However, the displacement of the brake pedal could be measured and converted by other appropriate means.

Although the drawings depict a conventional brake pedal it is to be understood that the present invention should also be applicable to any remote control means operable by a driver of a vehicle to operate the brakes.

A spring 12 is connected to the brake pedal 1 to ensure that when pressure is removed the brake pedal will return to a non-depressed position. The brake pedal 1 is connected via a linkage 33 to a master cylinder 13. The linkage 33 acts as the interface between the brake pedal 1 and the conventional braking system for the front and rear brakes.

A magnetic sensor 3 (or other suitable means, for example an optical or Hall effect device) detects the speed of rotation of each of the wheels of the vehicle (only one shown) and an optical sensor 4 (or other suitable means, for example a radar sensor or a microwave sensor) which is not shown in Figure 1 detects the actual speed of the vehicle over the ground. The optical sensor 4 could be, for example, a retro

reflective sensor attached to the underside of the vehicle. The measured wheel rotation speeds and the vehicle speed are passed to the microprocessor 5 which calculates the degree of slip (if any) occurring at each of the wheels of the vehicle by determining the difference between the actual speeds of rotation of the wheels and the speeds of rotation corresponding to the actual speed of the vehicle over the ground.

If slip is detected at one or more of the wheels the microprocessor 5 will provide control signals derived from the amount of slip and the displacement of the brake pedal 1, which control signals control a hydraulic system comprising solenoid operated valves 7 and 8, a pressure relief valve 11 and a pressure regulator 6. Fluid flows from a pressure supply P via the pressure regulator 6 which adjusts any variations in the supplied fluid pressure to maintain the pressure in the system substantially constant. Under normal braking conditions when no slip is detected, the control signals from the microprocessor 5 will maintain valves 7 and 8 in closed positions (as shown in Figure 1) and the pressure relief valve 11 will be biased into a closed position (as shown in Figure 1) by a spring 44. In such a situation, the displacement of the brake pedal 1 is allowed to proceed unopposed by the control system provided that the wheels do not slip, and any fluid in housing 10 is able to exhaust to a reservoir R through valve 8.

If a situation occurs in which slip is detected at any of the wheels of the vehicle then the control signals from the

microprocessor 5 will energise valves 7 and 8 which then move into open positions.

The pressure relief valve 11 taps off the pressure in the system at points A and B so that as long as the pressure at B together with the bias of the spring 44 is greater than the pressure at A, the valve 11 will remain closed as shown. The fluid supplied to the housing 10 via the valves 7 and 8 will then provide a resistive force opposing the displacement of the brake pedal 1.

The control system thus operates to detect the amount of slip occurring at any wheel and determines the opposing force (provided by the fluid in the system) necessary to oppose the force applied to the brake pedal to modulate braking so that the slip is reduced to an optimum level for the vehicle.

If a panic situation occurs and a force above a certain limit is applied to the brake pedal by the driver, then the pressure at A will overcome the force of the. spring 44 and the pressure at B, and the valve 11 will move into an open position whereby fluid in the housing 10 is exhausted to the reservoir R via valve 11. Thus, the pressure relief valve 11 becomes operative when a panic force is applied to the brake pedal 1, and the control system becomes inoperative.

Thus, it can be seen that when slip is detected the control system of the invention provides progressive modulation of braking as a continuous operation by offering resistance to

operation of the brake pedal (except in panic situations) , rather than simple on/off braking which is typical of prior art non-slip systems.

Figure 2 depicts a hydraulic actuator which can be used in the system shown in Figure 1. However, the hydraulic actuator shown in Figure 2 is operated such that a pushing force on the brake pedal (not shown) will result in a pulling force on the piston 9 to withdraw it from housing 10.

The piston 9 is connected via an actuator rod 35 to a bearing 14 of the brake pedal. When depressed the brake pedal will pull the piston 9 from left to right as shown in Figure 2. A fluid system of the type shown in Figure 1 supplies fluid to cavity 15 in the housing 10. The piston 9 slides within a sleeve 22 in the cavity 15, but fluid is prevented from passing from the right hand side of the piston to the left hand side (as shown in Figure 2) by a suitable sliding seal 17.

The piston is connected to a slug 18 of the linear variable differential transformer (LVDT) 2. The slug 18 is connected to the piston 9 and is slidable within the LVDT housing. LVDT converts the mechanical displacement of the slug 18, and thus the displacement of the brake pedal, into an electrical signal which is passed to the microprocessor. The microprocessor then determines the required position for the brake pedal to reduce any slip to an optimum level. (A discussion of optimum slip level will be given later with

reference to Figures 3 and 4) . If slip is detected fluid will enter cavity 15 via ports (not shown) and provide resistance to the movement of piston 9. It will be understood that other means equivalent to an LVDT could be used to determine the displacement of the brake pedal and convert the displacement into an electrical signal.

The LVDT 2 has at each end mountings 19 and 20 and a plug 21 located on mounting 20 which allows the LVDT 2 to be removed from the housing 10 if necessary.

Seals 23 and 24 are provided between the sleeve 22 and housing 10, and piston 9 and housing 10 respectively.

A rubber gaiter 25 covers the end of piston 9 to prevent dust or the like entering the hydraulic actuator and affecting its performance.

The housing 10 also contains a pressure relief valve 26 which corresponds to valve 11 in Figure 1. Valve 26 comprises a piston 27 which is normally biased into a closed position by a tension spring 28, in which position a port 34 is blocked and fluid present in the valve 26 is not allowed to exhaust to the reservoir R.

The valve 26 is connected to the fluid supply (not shown) by a port 37 which corresponds to point B in Figure 1, and by a port 36 which corresponds to point A in Figure 1. A duct 16 leads from the cavity 15 to the interior of valve 26.

The piston 27 will remain in its closed position until

the pressure supplied at port 36 is greater than the pressure supplied at port 37 and the bias of the spring 28. The piston 27 will then move into the open position shown in Figure 2 and fluid will be allowed to exhaust from cavity 15 via passage 16 and port 34. Accordingly, in a panic situation the pressure relief valve 26 allows any fluid which resists movement of the brake pedal to be exhausted to the reservoir R.

The valve 26 has several seals 29, 30, 31 which retain the valve within the housing 10.

The hydraulic actuator is secured at one end via bearing 14 to the brake pedal and at its other end via a bearing 32 to any rigid part of the vehicle body which lies in front of the brake pedal.

The hydraulic actuator could with modification be operated by a pushing force.

Although the control system shown in Figures 1 and 2 is for the braking system of a vehicle and the progressive modulation of braking, the control system of the invention could also be applied to the accelerator pedal to provide progressive modulation of acceleration to reduce spinning of the wheels of the vehicle. When a wheel of a vehicle spins it is because the actual speed of rotation of the wheel is greater than that corresponding to the actual speed of the vehicle over the ground, whereas when the wheel of a vehicle slips as in braking it is because the speed of rotation of the wheel is less than that corresponding to the actual speed of the vehicle

over the ground. Thus, it is clear that the control system of the invention is equally applicable to either the brake pedal or the accelerator pedal of a vehicle.

The control system of the invention works on the principle of keeping slip (as herein defined) of the wheels within acceptable limits.

Figure 3 is a graph illustrating a typical variation of vehicle slip ratio with pedal displacement. The slip ratio is positive in a braking situation and negative in an accelerating situation, and is calculated as follows:-

Vehicle slip ratio = 1 - wheel speed vehicle speed

The maxium displacement of the pedal in either situation for a given slip ratio can be determined from the graph. The slip ratio is given as a percentage value on the x-axis of the graph. Various experiments have revealed that the optimum slip ratio lies between 17% and 20%, but this value depends to a large extent on the ground characteristics and vehicle tyres used. The microprocessor 5 in Figure 1 will be programmed to control the system so that slip is kept within acceptable limits. Points X and Y on the graph can be altered according to the limits of slip which are acceptable in a particular situation by making appropriate adjustments to the programming of the microprocessor.

Figure 4 is a graph illustrating a typical variation of

tyre slip ratio with longitudinal force on the vehicle (braking or traction) . In this case

Tyre slip ratio = 1 - vehicle speed wheel speed

Thus, it can be seen that there is an optimum tyre slip ratio which provides maxium longitudinal force (braking or traction) which is important during braking or acceleration.

Clearly, it would be possible to incorporate the means for sensing slip described above with a standard ABS braking system as already present on many vehicles. The vehicle speed sensor and the sensors for wheel speed would replace the sensors presently used which can only sense whether or not a wheel has locked. The microprocessor could then be employed to activate the brakes at the wheels of the vehicle to alleviate locking.