1. Field of the Invention
The present invention relates generally to a source of hydraulic fluid under pressure via which hydraulically operated devices such as active suspensions and the like are operated and more specifically to such an arrangement which permits the size of the switching valve associated with a dual pump system to be reduced in size.
2. Description of the Prior Art
JP-A-2-123284 discloses a hydraulic fluid supply arrangement which includes two separate pumps, one having a larger displacement capacity than the other. Each of the pumps is arranged to discharge fluid under pressure into separate discharge passages in which respective first and second check valves are disposed. The two discharge passages merge into a single supply passage or conduit at a point downstream of the check valves.
In order to control the amount of fluid which is discharged into the supply passage, a drain control valve arrangement is operatively disposed in two drain passages which respectively lead from the first and second discharge passages at locations upstream of the check valves, and is arranged to selectively determine the amount of fluid which is permitted to drain back to a reservoir from which the pumps induct.
The drain control valve arrangement can be conditioned to assume a first state (mode 1) wherein the drain passage which is associated with the larger of the two pumps is open and the drain passage which is associated with the smaller of the two, is closed; a second state (mode 2) wherein the drain which is associated with the smaller is open and that associated with the larger pump is closed; and a third state (mode 3) wherein both of the drain passages are closed. Accordingly, in mode 1 only the discharge of the small pump is output while in mode 2 the output of the small pump is drained and replaced with that of the larger one. In mode 3 the combined discharge of both pumps is output (both drains being closed).
The drain control valve arrangement can consist of two serially arranged solenoid operated valves. The first and upstream valve is fluidly communicated with the two drain conduits and is such as to select the drain conduit of the larger pump when de-energized and to switch to the drain conduit of the smaller pump when energized. On the other hand, the second and downstream valve is arranged to determine if the drain passage selected by the first upstream valve is to be permitted to communicate with the reservoir or not. In this instance, this second valve is arranged to establish communication between the first valve and the reservoir when energized. In other words, the first valve is used to determine which of modes 1 and 2 is employed while the second valve induces mode 3 when de-energized.
However, this arrangement is such that, when the spool of the second valve arrangement is moved from its communication cut-off position to the one wherein draining is permitted, the flow of hydraulic fluid through the valve produces a force which tends to move the valve spool back toward its communication cut-off position. Accordingly, as the spring is used to bias the valve spool toward its cut-off position, the solenoid must be able to produce a sufficient force to overcome both the spring and the force produced by the flow of hydraulic fluid through the valve in order to be able to move the spool.
With the prior art arrangement, depending on the state of the first valve arrangement and the conditions to which the system is subjected, upon the second valve arrangement being switched to a position wherein drainage is permitted, it is possible for the spool of the second valve arrangement can be exposed to the flow produced by the larger of the two pumps. As this relatively large flow produces a force larger than produced that by the output of the smaller of the two pumps, it is necessary to provide a solenoid which can definitely produce a sufficient force to move the spool against the spring force and the relatively large force produced by the large flow to the position wherein the valve is open and drainage is permitted.
This of course induces the drawback that this relatively large solenoid, which is capable of overcoming the maximum resistance to spool movement which is apt to be encountered during system operation, increases the overall bulk, weight and cost of the drain control valve.
A further drawback which has been encountered with the above type of arrangement is that the temperature of the hydraulic fluid is not taken into consideration. As the temperature of the hydraulic fluid varies, the discharge characteristics of the pumps vary along with the control characteristics of the device which is motivated by the supply of hydraulic fluid under pressure.
Another problem which has been encountered with the above type of arrangement is that when the pumps are driven by a belt or similar type of connection with the engine crankshaft at high engine speeds, upon the drain control arrangement being conditioned to stop the draining of both pumps, the resistance to rotation which is exhibited by the pumps sometimes leads to a situation wherein driven connection (e.g. belt) slippage occurs.