Several different mechanisms rely on a series of hydraulically operated actuators for accomplishing a desired result. For example, a multi-speed transmission typically includes plural torque delivery paths between driving and driven members. A plurality of hydraulically operated clutch assemblies control relative motion of various gear elements forming the torque delivery paths in the transmission. Actuation of a particular clutch assembly controls which path is conditioned to deliver torque and power between the drive and driven members.
As will be appreciated, simultaneous engagement of more than one clutch assembly at any given time causes severe problems within the transmission often resulting in extensive damage to the clutch assemblies and related components. Therefore, each transmission clutch assembly is typically controlled by its own solenoid valve. The function of the solenoid valve is to provide actuating fluid to a respective clutch when the valve is energized and allow the clutch to be connected to exhaust upon de-energization of the valve.
Although each clutch assembly is provided with its own solenoid valve, problems still persist. Because each clutch assembly has its own solenoid valve, the possibility exists that more than one clutch assembly can be pressurized at one time as a result of a mechanical or electrical failure with the valve. To reduce this problem from occurring, individual pressure switches are provided to sense when the clutches are pressurized. The function of the pressure switches are such that if more than one pressure switch detects hydraulic pressure in a clutch assembly, a control module will disconnect the power supply to all of the solenoids. Disconnecting the power to all of the solenoids automatically conditions the transmission in a "Neutral" mode of operation.
Providing separate solenoid valves for each clutch assembly adds substantially to the cost of the transmission. Moreover, the addition of separate pressure switches for detecting which clutch assembly is activated adds still further costs to the transmission. Transmissions used on off-highway equipment are often subject to brutal field conditions. Such field conditions can and often do cause one or more of the solenoid valves, one or more of the pressure switches, or any combination thereof to malfunction and/or fail thereby conditioning the transmission in "Neutral". Of course, with the transmission in "Neutral" the equipment or apparatus is essentially useless. Thus, repair time is required to fix or repair the malfunctioning parts.
Thus, there is a need and a desire for a control system which eliminates redundant parts while assuring that only one hydraulic actuator will be operated at any given time. When installed in combination with a transmission, the control system should be designed such that a mechanical or electrical failure will nevertheless allow the transfer of power through the transmission to enable the equipment driven thereby to be moved to a suitable location for repair.