Woodward VariStroke-II (VS-II) Electro-Hydraulic Actuator

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.).

The VariStroke-II actuator is part of a product family with many different models available for purchase

depending on the force, stroke, and redundancy needs. This actuator is available with standard bore

diameters and standard stroke ranges. The VariStroke-II’s unique “variable stroke” capability also allows

users to customize/set the actuator’s exact stroke length in the field to meet their requirement.

The VariStroke-II is factory and/or field configurable via a computer-based service tool. The actuator’s

PCI Service Tool uses a simple, user-friendly format to allow users to easily configure, calibrate, and

adjust all internal functions and response settings. The VariStroke-II also includes a 4–20 mA output

channel to indicate output shaft position, and unit alarm and shutdown relay outputs for use as unit health

and status indications.

The total installed cost for the VariStroke-II is low because it is a fully integrated actuator that has been

completely assembled and tested at the factory. This greatly reduces OEM and end-user fabrication time,

testing time, and site assembly time.