Abstract: Closed center valves include a first member movable toward and away from a vent opening in the valve for respectively closing and opening the vent opening, and a second member movable toward and away from the first member for respectively closing and opening a high pressure passage through the first member. A biasing force is applied to the first member for biasing the first member toward the vent opening. Different magnitudes of force may be applied to the second member to cause the second member to move toward the first member first to close the high pressure passage through the first member while the vent opening remains closed by the first member, and then to cause the first member to move away from the vent opening to open the vent opening while the high pressure passage in the first member remains closed by the second member.

Abstract: A method of adaptive control for use in a controlled system having an adjustable forward loop gain and a control system apparatus for carrying out the method are disclosed. The control system measures the value of a parameter which is to be controlled and generates an error signal by comparing the measured valve of the parameter with a desired valve. The error signal is squared and the time derivative of the squared error signal is obtained. The control system compares the length of time that the derivative so obtained has a positive value to the time it has a negative value to obtain a gain control signal, and applies the gain control signal to the forward loop gain. The gain control signal reduces the adjustable forward loop gain until a preselected margin of stability in the controlled system is obtained.

Abstract: A servoactuator for stopping a ram in its existing position or retracting the ram at a fixed rate to a locked position.

Abstract: A servoactuator for stopping a ram in its existing position or retracting the ram at a fixed rate to a locked position.