Woodward VariStroke-I (VS-I) Electro-hydraulic Actuator

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 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.

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. Two, V25

and V45 servo sizes are available. Smaller servo – V25 (flow 68-169 l/min) works

with 4 and 6 inch actuators while servo V45 (flow 212-530l/min) works with 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. Both servos are available 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.