Manual 26727
VariStroke-I (VS-I) Electro-hydraulic Actuator
Woodward
12
Chapter 1.
General Information
Introduction
The VariStroke-I is a linear electro-hydraulic actuator that utilizes a double-acting or spring-assist power
cylinder with integrated electronic driver module, servo valve, and redundant MLDTs (Magnetostrictive
Linear Displacement Transducer) – based position feedback sensors to precisely control steam turbine
valves. The actuator’s driver module accepts one or two (redundant) 4–20 mA demand setpoints and
compares these setpoints to the sensed actuator shaft position to accurately control output shaft position.
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 responds 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 failsafe position. There are three different spring force categories for each
cylinder diameter, except for 8” and 10” cylinder bores which need four 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.
As a means of protecting the turbine, an internal servo valve-return spring forces the actuator to a failsafe
position to safely close turbine control valves upon any internal unit failure (electrical input power failure,
position sensor failure, processor failure, etc.). Additionally for the spring assist, power cylinder the
actuator spring assists in closing the valve in the event of the loss of oil pressure and helps to maintain
fail safe position.
The VariStroke-I actuator is a product family with many different models available for purchase depending
on the force, stroke, and redundancy required. Servo V45 (flow 212-530l/min) works with 4, 5, 6, 8 and 10
inch actuators. Cylinders are available with standard bore diameters and standard stroke ranges. The
VariStroke’s unique “variable stroke” capability also allows users to customize/set the actuator’s exact
maximum stop position in the field to meet their requirement. The VariStroke-I is available as an
integrated unit, or as a Remote Servo kit where the cylinder can be mounted up to 3 meters (approx.
10 feet) away from the servo. V45 servo is available also as Servo Only option for users who wish to use
their own hydraulic cylinder.
The VariStroke-I 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-I also includes a 4–20 mA output channel to
indicate output shaft (control valve) position, and unit alarm and shut down relay outputs for use as unit
health and status indications.
The total installed cost for this fully integrated actuator is low because it has been completely assembled
and tested at the factory. This greatly reduces OEM and end-user fabrication time, testing time, and site
assembly time.
The VariStroke Actuator offers the following benefits to the user in comparison to other electro-hydraulic
actuators:
Released
