Power assist device for control system

A power assist device adapted to amplify force of a control signal to actuate an apparatus, the power assist device including a directional valve means having a valve body with a valve bore, and a valve spool movable within the bore along a valve axis in response to the control signal, the valve body having a fluid inlet means to receive pressurized fluid and a fluid outlet means to discharge fluid, the valve body also having first and second fluid conduits which can communicate through the valve bore with either the fluid inlet means or fluid outlet means as determined by relative position of the valve spool so as to transmit fluid relative to the valve body, the device also including a fluid power cylinder assembly having a cylinder body, a piston and a piston rod, the piston rod and piston being fixed so that the cylinder body is reciprocable relative to the piston, the cylinder body being connected to the valve body for mutual movement, and also cooperating with the apparatus to apply an amplified control signal thereto, the piston defining in part first and second chambers on opposite sides thereof, the first and second chambers communicating with the first and second fluid conduits of the directional valve means, the power assist device being further characterized by:

• (a) a guide rod having a longitudinal guide axis disposed parallel to the valve axis, the rod being connected essentially rigidly to the valve spool for equal concurrent movement therewith along the guide axis in response to the control signal, the rod also having guide indexing means representing particular control settings,
• (b) a fixed guide holder cooperating with the guide rod to permit the guide rod to move axially relative to the guide holder, the guide holder having a holder indexing means complementary to the guide indexing means to serve as a datum and to register with the guide indexing means at a particular control setting,
• so that when the valve spool is shifted to direct fluid into one chamber and discharge fluid from the opposite chamber, the indexing means are disengaged and engaged as required to indicate attainment of a particular control setting, and the cylinder body shifts to actuate the apparatus in response to the control signal.