The actuator’s output shaft position is controlled by a digital controller with an

internal rotary servo valve that ports supply oil to and from its power cylinder

piston. This actuator’s digital controller architecture allows it to perform stable

position control during normal conditions, and also respond quickly to desired

valve step changes during system or plant transients. The actuator output force

is generated only by oil pressure for double-acting power cylinder. For the spring

assist actuator output force is a combination of force from hydraulic pressure and

spring. Spring assist cylinder is still working as double acting actuator but it has a

spring installed inside the cylinder. Spring can be mounted either on the piston or

rod side and it generates force toward the fail safe position. There are 3 different

spring force categories for each cylinder diameter, except for 8” and 10” cylinder

bores which need 4 spring force categories to cover application needs. The

springs are rated at about ~1.5%, ~2.5%, ~5.5% and ~10.5% of stall force at 500

psi supply pressure.