A pump which drives a piston for reciprocatory motion is used as a source of braking pressure for a brake pressure circuit which controls wheel brake pressures in a number of modes including ABS(anti-lock brake pressure control), TRC(traction control), an automatic brake control inclusive of an automatic deceleration, an automatic stop, an automatic turn, a control over distribution of braking effort among wheels, for example. Such pump includes one referred to as a plunger pump.
A typical plunger pump used in a brake pressure circuit comprises a pair of pumping mechanisms, one eccentric cam and a single electric motor. A first pumping mechanism comprises a first cylinder, a first piston disposed therein, and a first return spring which urges the first piston against the cam surface of the eccentric cam. A second pumping mechanism is similarly constructed, but is disposed symmetrically to the first pumping mechanism with respect to the eccentric cam. The eccentric cam is driven for rotation by the motor, whereupon the piston in each pumping mechanism reciprocates. As a braking liquid is drawn into the cylinder of the first pumping mechanism, the braking liquid is discharged from the cylinder of the second pumping mechanism. A pump pressure from the first pumping mechanism is used for intensification/decompression of wheel brakes associated with two of four wheels of a vehicle, namely, front/rear and left/right wheels, while a pump pressure from the second pumping mechanism is used for intensification/decompression of the remaining two wheel brakes.
Japanese Laid-Open Patent Application No.74,153/89 discloses a brake controller operating with a single electric motor such that when a control over the wheel brake pressures is required without depression of a brake pedal such as during TRC, the pump is driven by the motor and a wheel brake circuit is switched into a circuit connection which applies the discharge pressure from the pump to wheel brakes.
A conventional arrangement for a motor driven pump which is used in this manner is illustrated in FIG. 4. As shown, when solenoid valves 4, 6-9 are set up such that a braking liquid pressure from a brake master cylinder 3 is fed to a wheel brake 10 associated with a front, right wheel and a wheel brake 11 associated with a rear, right wheel, as a driver depresses a brake pedal 2, a negative pressure booster intensifies the depression pressure before it is applied to the piston of the master cylinder 3, and accordingly, a braking liquid pressure at the output port of the master cylinder 3 rises. This braking liquid pressure is fed through a switching valve 4, solenoid valves 6 and 7 to be applied to the wheel cylinders of the wheel brakes 10 and 11. In the present example, a braking effort is applied to a front, right wheel Fr and a rear, right wheel Rr. While not shown, the master cylinder 3 includes another port which is connected with a similar brake pressure circuit, which similarly applies a braking effort to a front, left wheel and a rear, left wheel.
When it is desired to apply a braking liquid pressure to the wheel brakes 10, 11 automatically without depression of a brake pedal (as during TRC), the switching valve 4 is electrically energized or turned on by a controller (not shown) which essentially comprises a computer. Thereupon, a port 4a of the switching valve 4 which communicates with the output of the master cylinder 3 is disconnected from a port 4b which is connected to the solenoid valves 6 and 7, but is connected to a port 4c which communicates with a suction line Pin of a pump 1. An electric motor 1m which drives the pump 1 is energized. This allows the pump 1 to draw the braking liquid from the master cylinder 3 through the switching valve 4 and the suction line Pin, and to discharge it through a discharge line Pout. The discharge pressure is fed to the wheel brake 10 (or 11) through the solenoid valve 6 (or 7). When an intensification of the wheel brake 10 is required, the solenoid valve 7 is closed while the solenoid valve 6 is opened. When an intensification of the wheel brake 11 is required, the solenoid valve 6 is closed while the solenoid valve 7 is opened.
As the motor 1m rotates, an eccentric cam shaft 1ax is driven for rotation, and the center of rotation of a roller (eccentric cam) 1ca mounted thereon depicts a circular motion of a radius S, whereby it pushes a piston 1p which is urged by a coiled compression spring 1ps outward. In this manner, the piston 1p reciprocates with a stroke of 2S. The piston 1p is formed with a channel 1f which always opens to a suction port 1ip and which communicates with a pressure chamber A. An opening of the channel if located toward the pressure chamber A is closed by a ball 1v, acting as a check valve, which is urged by a coiled compression spring 1vs. When the piston 1p is pushed in a direction to reduce the volume of the pressure chamber A, the channel 1f is closed by the ball 1v, whereby the fluid in the pressure chamber A is pressurized by the piston 1p to feed the wheel brake 10 (or 11) through a discharge port 1ep, a check valve 18 and solenoid valve 6 (or 7). As the piston 1p is on its return stroke, the presence of the check valve 18 is effective to prevent the braking liquid from the solenoid valve 6 (or 7) from being drawn into the pressure chamber A, which therefore assumes a negative pressure, allowing the ball 1v to be opened to allow a low pressure braking liquid from the master cylinder to flow into the pressure chamber A through the switching valve 4, suction line Pin, suction port 1ip and channel 1f. A braking liquid from a reservoir 12 also flows into the pressure chamber A through a suction port 3ip. As the piston 1p reciprocates repeatedly, the discharge from the pressure chamber A to the check valve 18 and the suction into the pressure chamber A through the suction port 1ip and the channel 1f are repeated in an alternate fashion. Since the switching valve 4 provides a connection between the ports 4a/4c, a fluctuation in the suction of the pump 1 is transmitted to the brake pressure output port of the master cylinder 3 through the suction line Pin and the switching valve 4.
The plunger pump described above undergoes one suction stroke per reciprocation of the piston 1p (or per revolution of the eccentric cam 1ca), and hence the fluctuation in the suction of the pump 1 is pronounced. Accordingly, when the pump 1 is driven while the switching valve 4 is turned on, the pronounced fluctuation in the suction is transmitted to the master cylinder 3 through the suction line Pin and the valve 4, as mentioned above. As a consequence, if a brake pedal is depressed by the driver under this condition, the pedal 2 will be depressed to a greater degree. If the valve 4 connects between the ports 4a/4c and the pump 1 continues to be driven as the driver depresses the pedal 2 and increases the depression, because the braking liquid flows from the master cylinder 3 through the pump 1 to the wheel brake, there will be no further depression of the pedal 2 during the discharge stroke of the pump 1 while the depression will increase during the suction stroke, causing a stepwise depression of the pedal 2, which imparts an extraordinary or abnormal sense to the driver.