Introduction

The VariStroke-II is a linear electro-hydraulic actuator that utilizes a double-acting power cylinder with

integrated electronic driver module, servo valve, and redundant LVDT (Linear Variable Differential

Transformer) position feedback sensors to precisely control steam turbine valves. The actuator’s driver

module accepts one 4–20 mA position demand and compares it to the sensed actuator shaft position to

accurately control output shaft position.

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

rotary

servo valve that ports supply oil to and from its power cylinder. The actuator 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. As a means of protecting the turbine, an internal

servo valve return spring forces the actuator to a failsafe position upon any internal unit failure (electrical

input power failure, position sensor failure, processor failure, etc.).