The present invention relates to an anti-lock hydraulic control device, and more particularly to providing more suitable hydraulic distribution to the front-wheel and rear-wheel brakes, and improving braking efficiency.
Anti-lock hydraulic devices are conventionally provided with a proportioning valve (hereinafter abbreviated to "P-valve") for reducing output fluid pressure in proportion to inlet fluid pressure.
The P-valve has the function of appropriately setting distribution between rear-wheel brake fluid pressure and front-wheel brake fluid pressure, so as to prevent the car losing stability due to the rear wheels locking before the front wheels. The P-valve is therefore essentially unnecessary if an anti-lock hydraulic control device is installed.
However, the imbalance in braking power between the front wheels and rear wheels means that the anti-lock hydraulic control device is actuated to lock at the earliest stage even if the coefficient is relatively high, i.e., the condition of deaccelerating speed being small. Thus to prevent discomfort associated with noise and vibrations generated when the anti-lock hydraulic control device functions at an early stage, notwithstanding road-surface conditions which make locking less likely to occur in the first place, and to prevent the car losing stability when the anti-locking device is faulty and the rear wheels lock before the front wheels, P-valves are often provided in spite of the provision of anti-lock hydraulic control devices.
FIG. 6 shows an example of an anti-lock hydraulic control device with P-valves as described above.
The anti-lock hydraulic device is constructed of X-circuit. The front-wheel brakes 1A, 1B and the rear-wheel brakes 2A, 2B on the left and right sides have anti-locking devices (ABS devices) 3A, 3B, 3C, 3D respectively, and P-valves 4A, 4B are disposed between the rear-wheel brakes 2A, B and the master cylinder 5.
The pressure detection holes 6a, 6b in the respective P-valves 4A, 4B are connected with the outlet side of the master cylinder 5 of the other circuit. It is not always necessary to provide the holes 6a, 6b, and it may be employed a P valve having no-function of failure detection. To prevent the rear wheels from locking before the front wheels, the P-valves 4A, 4B are constructed so as to reduce the fluid pressure of the rear-wheel brakes 2A, 2B in proportion to the fluid pressure of the front-wheel brakes 1A, 1B, in accordance with the outlet side pressure of the master cylinder 2 detected by means of the pressure detection holes 6a, 6b; and in addition, should a defect arise, to cancel the proportional decompression of the P-valves 2A, 2B of the other circuit and increase the fluid pressure of the rear-wheel brakes 2A, 2B.
As in an anti-lock hydraulic control device such as that described above the rear wheels are generally most likely to lock when, apart from the driver, the car is unladen (i.e. when empty), the characteristics of the P-valves are set accordingly.
However, the ideal distribution of fluid pressure to the front-wheel and rear-wheel brakes when the car is carrying goods and passengers (i.e. when laden) calls for greater distribution to the rear-wheel side than when the car is empty. Thus with hydraulic distribution set for when the car is empty, braking force distributed to the rear wheels is unduly low compared with the ideal braking force.
In such circumstances, the subsequently overloaded front wheels have a tendency to lock before the deceleration of the car, which could be achieved if the ideal distribution was originally provided can be reached, and the anti-locking device goes into operation. If the driver then presses the brake pedal further, the rear-wheel brakes can immediately be increased. However, as it is generally believed that the operation of the anti-locking device is a warning that the brakes have been applied too hard, drivers often do not press the brake pedal any harder.
With the anti-lock hydraulic control device shown in FIG. 6, anti-locking control is actuated only in the front-wheel brakes, as rear-wheel brake fluid pressure is controlled by the P-valve. There is thus insufficient braking power in the rear-wheel brakes, which in turn results in insufficient braking power for the whole car; and so despite the installation of an anti-lock hydraulic control device, there is the problem that the braking distance increases.
An example of a means of solving the above problem is the anti-lock brake system described in Japanese Utility Model Application No. 3-47185. In this example, the brake pressure of the other brake circuit is detected by means of pressure detection holes. When said pressure is low, a P-valve with the function of suppressing the ordinal proportional decompression function of the P-valve (i.e. a defect-countering P-valve) is used to suppress the proportional decompression function which detected anti-lock control being actuated in the front-wheel brakes of the other circuit, and the brake pressure of the rear-wheel brakes is increased.
Use of a defect-countering P-valve in the anti-lock brake system described above leads inevitably to increases in size and cost, as compared with use of a normal P-valve. In addition, the defect-countering P-valve is constructed so that rear-wheel brake pressure rises suddenly when anti-lock is actuated on the front-wheel side. There is thus the possibility that the rear wheels may lock suddenly and stability be lost.