Control apparatus for pneumatically operated extensible member and method

The method of positioning an external load moved by a pneumatic linear actuator having an air operated cylinder and piston powering an extensible member for moving the external load comprising the steps of:

• applying sufficient force by supplying air under pressure to the cylinder and piston to match the external load for movement responsive thereto;
• utilizing threadable frictional engagement between threads carried by the piston and threads carried by a lead screw which is alone sufficient to prevent movement of the load responsive to supplying air under pressure in the absence of an external torque rotating the lead screw;
• controlling the output of an electric motor to provide sufficient external torque to overcome the frictional engagement thereby rotating the lead screw permitting the force applied by the air under pressure to move the external load;
• and
• discontinuing the application of the external torque by the electric motor when the desired movement of the external load has been accomplished by the force applied by the air under pressure thereby returning the lead screw to frictional engagement in respect to the piston to discontinue movement of the load responsive to the force applied by air under pressure;
• whereby a heavy load can be moved and then positioned in a predetermined location by exerting sufficient external torque to overcome the frictional engagement thereby imparting rotation to the lead screw during movement of the load and then discontinuing the application of external torque by the electric motor.