[Ralph Lambert]

There were a number of good posts on this subject last year. You hook up a solenoid to your actuator as though you were going to just go with two positions. You then plumb the exhausts of this solenoid together and feed that combined exhaust to another solenoid. When you want to stop the actuator at a mid position position you close the second solenoid and that pressurizes the exahust side and stops the actuator in mid stroke.