This invention relates to hydraulic actuation systems and in particular hydraulic actuation systems for automated transmission systems.
In automated transmission systems of, for example, the type disclosed in WO97/05410, WO97/40300 or GB 0028310.1, whose content is expressly incorporated in the disclosure content of the present application, fluid pressure actuators are used to control actuation of a clutch actuator mechanism and/or a gear engagement mechanism. WO97/05410 and WO97/40300 disclose automated transmission systems including clutch means, in which the clutch means is normally held in the engaged condition by mechanical spring means, hydraulic actuators being provided to release the clutch means. WO97/05410 discloses a hydraulic control system in which separate valves are used to control the clutch actuator mechanism and the gear engagement mechanism. WO97/40300 discloses an integrated control system in which a main control valve controls both the clutch actuation mechanism and, together with secondary valves, shift and select actuators of a gear engagement mechanism.
GB0028310.1 discloses an automated transmission system utilising an active clutch which is normally disengaged and is held in engagement by hydraulic pressure. It is advantageous with hydraulic actuators for active clutches to preload the clutch to the disengaged position, so that a predetermined threshold pressure, typically of the order of 10 bar, is required before the clutch will reach the touch point, that is the point at which the clutch begins to transmit torque. In this manner, at the touch point on disengagement of the clutch, there is sufficient pressure remaining in the system to give rapid exhaust of fluid from the clutch actuator and so ensure that there is no clutch drag.
In the hydraulic actuation system disclosed in GB 0028310.1, a pair of gear engagement actuators are used to control selection and engagement of gear ratios associated with the clutch, each gear engagement actuator requiring a separate solenoid operated control valve.
The present invention provides a hydraulic actuation system in which a pair of gear engagement actuators may be controlled by a single solenoid operated control valve, thereby reducing the number of solenoid operated control valves required in the control system and simplifying the electronic control circuitry.
According to one aspect of the present invention a hydraulic actuation system for an automated transmission system including an active clutch comprises:xe2x80x94
a hydraulic clutch actuator controlling engagement of an active clutch, the hydraulic control actuator being preloaded towards a disengaged position of the clutch, whereby pressure above a first predetermined value is required in order to move the clutch to a predetermined position;
first and second gear engagement actuators for controlling the selection and engagement of a selected gear ratio;
a source of hydraulic fluid under pressure;
a hydraulic fluid reservoir;
a clutch control valve for selectively connecting the clutch actuator to the source of hydraulic fluid under pressure or to the reservoir;
a gear engagement control valve for selectively connecting each gear engagement actuator to the source of hydraulic fluid under pressure or to the reservoir; and
a changeover valve situated between the gear engagement control valve and the first and second gear engagement actuators, said changeover valve in a first position connecting the first gear engagement actuator to the gear engagement control valve and the second gear engagement actuator to the reservoir and, in a second position, connecting the second gear engagement actuator to the gear engagement control valve and the first gear engagement actuator to the reservoir, the changeover valve being shifted from its first position to its second position when pressure in the clutch actuator rises above a second predetermined value, the second predetermined value being lower than the first predetermined value.
The predetermined position of the clutch is a position intermediate of the fully released position and the touch point at which the clutch begins to transmit torque and is preferably at or near to the touch point.
A hydraulic actuating system according to the present invention may be used to control the clutch of a single clutch transmission system. Alternatively, two similar hydraulic actuation systems may be used in parallel to control the clutches of a twin clutch transmission system.
According to a further aspect of the present invention, a proportional pressure control valve may be used to control pressure of fluid supplied from the source of hydraulic fluid under pressure to the gear engagement control valve, so that the forces exerted on the gear engagement mechanism may be controlled.