GB2072282A

GB2072282A - A Motor Vehicle Hydraulic Braking System

Info

Publication number: GB2072282A
Application number: GB8009241A
Authority: GB
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1980-03-19
Filing date: 1980-03-19
Publication date: 1981-09-30
Also published as: JPS56160257A; GB2072282B

Abstract

The invention relates to valves for modulating the hydraulic pressure of a vehicle braking system in response to a change in load on the vehicle. The valve is controlled by two vibration frequency sensors 21 and 22, which are attached to sprung and unsprung portions of the vehicle respectively. The frequency signals from the two sensors are analysed and compared to produce a resultant signal when the two respective vibrational amplitudes of the signals are equal. The natural frequency at which said amplitudes are equal is related to the load on the vehicle and the value of the natural frequency can be utilised for controlling the modulating valve. <IMAGE>

Description

SPECIFICATION A Motor Vehicle Hydraulic Braking System This invention relates to vehicle hydraulic brake systems having pressure modulating valves and in particular, but not exclusively, to pressure reducing valves.

Pressure reducing valves are commonly fitted to the hydraulic brake system of motor vehicles and are located between the hydraulic master cylinder and the rear brakes. These reducing valve operate to ensure that on application of the brakes, once a predetermined master cylinder pressure has been reached, the rate of increase in pressure to the rear brakes is less than the rate of increase in pressure in the master cylinder and hence the front brakes. The master cylinder pressure at which the valve begins to operate to modulate the pressure in the rear brakes will be called the operating point.

Pressure reducing valves can be made sensitive to the vehicle load so that the operating point varies according to the load on the vehicle, for example, if the load on the vehicle is increased then the predetermined pressure at which the valve begins to operate is also increased, The normal method for making a reducing valve sensitive to vehicle load is to follow the collapse of the vehicle suspension through a system of springs and levers, for example, as shown in our British Patent No. 1,555,667. The valve typically has a piston that is acted upon by a loaded control spring and it is the load in the spring that determines the operating point. Hence, altering the spring load by means of movement of the said system of levers and springs alters the operating point.A problem associated with this type of load sensitive reducing valve is that the various springs have to be set up correctly in the factory and that subsequent tampering with a spring setting can make the valve ope; ;te at an incorrect operating point.

It has also been proposed in British Patent Application No. 2,016,103 A to vary the load on the control spring without the use of the levers and springs. As an alternative the above application proposes the use of a frequency vibration sensor on the spring part of the vehicle.

From the theory of the period of vibration of a spring, it is possible to relate the frequency of vibration to the means acting on the spring.

Hence, in theory the load on the vehicle suspension can be related to the frequency of oscillation of the vehicle suspension and this is measured by measuring the frequency of vibration of the spring portion of the vehicle.

However, the proposal will not in fact work because the vibrations in the spring portion of the vehicle are not only due to the load on the lorry but also to the surface condition of the road on which the vehicle is moving and this is a varying quantity. Therefore the frequency response measured by the vibration sensor does give a clue as to the actual natural frequency of the suspension oscillations.

The present invention seeks to overcome the disadvantage of the braking system described in British Patent Application No.2,016,103 A.

Accordingly there is provided a motor vehicle hydraulic braking system having a hydraulic pressure modulating valve with means responsive to vehicle load to modulate the hydraulic pressure downstream of the valve, wherein the modulating valve is controlled by two vibration frequency sensing devices, one sensing vibrations in an unsprung portion of the vehicle and the other sensing vibrations in a sprung portion of the vehicle, the sensing devices each producing a primary electrical signal indicative of the frequency and amplitude of its sensed vibrations, and means for analysing and comparing the primary signals to produce a resultant signal when the respective vibrational amplitudes, within a predetermined frequency range, are equal, said resultant signal controlling the modulating valve.

It can be shown that the amplitude ratio A for the respective amplitudes at any frequency ratio for the sprung and unsprung sensors is:

where f= frequency ratio z= damping ratio of the suspension system Now when the respective amplitudes of the two sensors are equal A=1 and therefore:-

Hence, f2=2 or O

It can be seen that when the amplitude ratio = 1 the frequency ratio is aand this is independent of the damping ratio z of the suspension.

Therefore, by determining the frequency at which the amplitudes of vibration of the sprung and unsprung portions of the vehicle are equal i.e.

A=1, it is possible to determine the natural frequency of the suspension system fn.

f=natural frequency fd=frequency at which the vibration amplitudes are equal And hence relate the accurately determined natural frequency to the load on the vehicle suspension for controlling the operating point of the modulating valve.

The invention will be described by way of example and with reference to the following drawings in which: Fig. 1 is a sketch of a lorry having a brake system according to this invention; and Fig. 2 is a block diagram of a vibration sensor system as utilised in Fig. 1.

With reference to Fig. 1 and Fig. 2, a motor vehicle, iliustrated by a lorry 11 in Fig. 1 has a driver operable hydraulic master cylinder 12 which is connected by a dual circuit braking system to the front brake wheel cylinders 13 and the rear brake wheel cylinders 14.

A pressure modulating valve 1 5 is a pressure reducing valve and is interposed between the master cylinder 12 and the rear brakes 14 so as to modulate the hydraulic pressure in the rear brakes once the reducing valve 1 5 begins to operate. The operating point of the valve 1 5 is determined by the load exerted by a control spring 1 6 on a piston (not shown) in the valve 1 5. The reader is directed to our British Patent No.

1,555,667 to ascertain exactly how the load in the control spring determines the onset of pressure modulation.

The load in the control spring 1 6 is controlled by, for example, an electrical servomotor 17 which can compress or expand the spring 1 6 in response to signals from a logic circuit 1 8. An increase in weight on the vehicle suspension will result in the spring 16 being compressed to exert an increased load on the valve piston, and a reduction in weight will result in the spring being allowed to expand to reduce the load on the valve piston.

Two vibration frequency sensors 21 and 22, in the form of accelerometers, are attached respectively one to a sprung portion of the vehicle e.g. the chassis, and the other to an unsprung portion of the vehicle e.g. the suspension member. Assuming that the vehicle is in motion and generating vibrations in its chassis and suspension members, then each vibration sensor 21 and 22 senses the amplitude and frequency of the vibrations in that part of the vehicle to which it is attached and gives out a random signal, 23 and 24 respectively. Each random signal 23 or 24 is passed through a series of band filters 25 and 26 respectively, over the range 0.4 to 1 OHz which split the signal into a number of frequency bands and also giving the average amplitude of the signal within each band.

Each series of band filters 25 and 26 is scanned by a multiplex scan 27 and 28 respectively, which registers the frequency and amplitude in each frequency band. A comparator 29 compares the amplitude of the signal from the two sensors 21 and 22 for each frequency band and when the amplitudes for a given frequency band are equal the comparator generates a resultant signal 30 indicative of that frequency band. The band filter and scanner are shown as the analyser 20 in Fig. 1.

The signal 30 is then fed into the logic circuit 1 8 which works out the natural frequency of the vibrations, as previously indicated, and then relates the natural frequency to the load on the vehicle. The logic circuit 1 8 can then generate a signal 35 proportional to the load on the vehicle and this signal can be passed through a comparator 32 and compared with a signal 36 from a travel transducer 31 across the control spring 1 6 so that the servomotor 17 can be made to increase or decrease the length of the spring 16, and hence the spring load, until the signals 35 and 36 are equal.

The aforementioned sensing system only operates when the vehicle is in motion and generating vibrations between the frequencies 0.4-10Hz. Hence when a vehicle is in motion and the valve 1 5 is set according to the vehicle load it is necessary for analyser 20 and logic circuit 1 8 to make constant checks on the accelerometer signals. These checks should be made for an average level over a period of 1030 seconds. This time interval being greater than the random intervals between the transient signals generated as the vehicle moves along the road.

Therefore in use when a lorry having a valve load setting of X arrives in a yard and is loaded up, the valve load setting will not change to Y until after the vehicle has been driven for at least one check interval, say 10 seconds, and the new natural frequency analysed. There is no provision for altering the valve setting whilst the vehicle is static.

This type of pressure reducing valve control system could be particularly suitable for use with a reducing valve as described in our co-pending Application No. 79.43237.

Claims

1. A motor vehicle hydraulic braking system having a hydraulic pressure modulating valve with means responsive to vehicle load to modulate the hydraulic pressure downstream of the valve, wherein the modulating valve is controlled by two vibration frequency sensing devices, one sensing vibration in an unsprung portion of the vehicle and the other sensing vibration in a sprung portion of the vehicle, the sensing devices each producing a primary electrical signal indicative of the frequency and amplitude of its sensed vibrations, and means for analysing and comparing the primary signals to produce a resultant signal when the respective vibrational amplitudes, within a predetermined frequency range, are equal, said resultant signal controlling the modulating valve.

2. A braking system as claimed in Claim 1, wherein the primary signals are each passed through frequency filters over the range 0.41 OHz, the frequency bands obtained from the filters are scanned by a multiplex scanner and their amplitudes are compared to find the frequency band in which the amplitudes are equal.

3. A braking system having a means, substantially as described herein, of controlling a hydraulic pressure modulating valve.