A simplified brake control system comprises a source of pressurized fluid, a spring-set brake with a hydraulic release mechanism and a control valve connected between the brake and the source. The control valve has a valve body and a flow control mechanism in the body and mounted for movement between a first (solenoid-energized) position and a second (solenoid-de-energized) position. A one-way check valve is built into the valve body. How the system operates depends upon two factors, namely, (a) whether the valve flow control mechanism is in its first or its second position, and (b) the relationship of the pressure of the source, i.e., the first pressure, and the second pressure in the release mechanism. When the flow control mechanism is in the first position, the release mechanism may be pressurized for brake release. And when such mechanism is in the second position, the release mechanism is vented to the reservoir through the brake port, the second passage and the drain port. A new method for controlling a brake is also disclosed.
RELATED APPLICATION
This application is a continuation-in-part of application Ser. No. 08/694,559 filed on Aug. 9, 1996 now U.S. Pat. No. 5,738,142, titled "PRESSURE HOLDING DIRECTIONAL CONTROL VALVE" and assigned to the same assignee as this application.
A control device for the parking brake of a motor vehicle has a hydraulic cylinder with a piston chamber that can be pressurized by means of a valve arrangement, and has a hydraulic piston connected mechanically to engagement elements of the parking brake. The parking brake is forced into an engaged or disengaged position depending on the hydraulic pressure in the piston chamber. The piston chamber connects to a pressure sensor providing pressure signals that represent the state of the parking brake and that are evaluated by an electrical control unit. The control unit defines a first predetermined pressure threshold corresponding to the engaged state of the parking brake, and a second predetermined pressure threshold corresponding to the disengaged state of the parking brake.
The hydraulic brake system comprises a piston/cylinder unit (14) for activating and releasing a brake (12), comprising a volume (16) adapted to have a pressurized working fluid supplied thereto for moving the piston (24) from a braking position into a release position for releasing the brake. Further, the hydraulic brake system comprises a supply line (18) for the supply of working fluid from a reservoir (22), a discharge line (30) for the discharge of working fluid from the volume (16) of the piston/ cyliinder unit (14), a first valve (34) and an element (36) for determining the flow resistance, a bypass line (32) provided with a second ON/OFF valve (38) and arranged for a flow in parallel to the discharge line (30). At the start of a braking process, among the two ON/OFF valves (34,38) which in the released condition of the brake are in the closed condition, the first ON/OFF valve (34) is opened and, upon lapse of a presettable first time period (T.sub.delay1) from the opening of the first ON/OFF valve (34), the second ON/OFF valve (38) is opened.
A driveline vehicle parking brake actuator including a reciprocable piston, a spring guide having a tubular distal portion telescopically received on the piston and a radial proximal portion which is telescopically supported within the housing by an annular wear guide, and nested cylindrical coil springs including an outer cylindrical coil spring biased between the housing and the radial portion of the spring guide and a nested inner coil spring having a greater spring rate biased between the spring guide and the piston functioning in series. The distal end of the piston is attached to a cable and the springs are compressed by hydraulic or pneumatic pressure acting against the piston.
