Hydraulic positioning cylinder

A hydraulic control system comprising, in combination;

• a double acting hydraulic cylinder, a piston member located in said cylinder and including a hollow piston rod extending from one end of the cylinder and being closed at its outer end, a stationary conduit member extending through the opposite end of said cylinder and telescopingly received in said hollow piston rod, forming a variable volume fluid chamber, servo-valve means, conduit means connecting said servo-valve means to a source of hydraulic pressure and to said cylinder, a servo-cylinder having a fluid pressure chamber in communication with said stationary conduit member and a piston member therein connected to operate said servo-valve means, the piston member of the servocylinder being spring-biased in one direction and subject to the fluid pressure in said chamber in the opposite direction, means for providing a plurality of control fluid chambers in communication with said stationary conduit member, means to selectively control the volume of fluid in each of said control chambers, said stationary conduit member, control fluid chambers, and the fluid pressure chamber of the servo-cylinder being adapted to contain a fixed quantity of control fluid, the piston of said servo-cylinder having a balanced position maintained by the spring bias acting against said control fluid wherein the hydraulic pressure source is blocked from said double acting cylinder and the piston rod is in a set position, whereby said servo-cylinder may be selectively actuated by controlling given ones of said control chambers to cause movement of the piston rod of the double acting cylinder, and the system being rebalanced by movement of said rod to a set position in which the volume of said chamber is changed sufficiently to accomodate a decrease or increase in the control fluid volume produced by said given ones of said control chambers.

In a hydraulic system including a power cylinder having double acting piston means, and servo-valve means for applying fluid pressure to said power cylinder to produce movement of said piston means, the combination therewith of a servocontrol system for controlling the position of said piston means, the servo-control system comprising;

• control fluid condit means cooperating with said piston means for forming a fluid chamber adapted to contain a fixed volume of control fluid and for varying the pressure on said control fluid by varying the volume of said chamber in response to movement of said piston means, control means in communication with said chamber for selectively varying the pressure of said control fluid by varying the volume of said chamber independently of said piston means, and fluid pressure responsive sensing means connected to said control fluid conduit means and to said servo-valve means for operating the servo-valve means in response to changes in the pressure of the control fluid to produce such movement of said piston means that the associated change in volume of said chamber causes the control fluid pressure to change sufficiently to offset the change in control fluid pressure caused by operation of said control means, whereby the piston means will be moved from an original setting to a new setting spaced from the original setting a distance related to the change in volume of said chamber needed to restore the original control fluid pressure;
• first normally closed valve means for selectively supplying control fluid to said control fluid conduit means, valve operating means located in said power cylinder and connected to operate said first normally closed valve means, said valve operating means being positioned to be contacted by said piston means upon travel thereof beyond a predetermined position of travel to open said first normally closed valve means, and second normally closed valve means for selectively discharging control fluid from said control fluid conduit means to a reservoir, whereby said fixed volume of control fluid may be maintained at a constant level regardless of leakage and temperature change.