Woodward VS-I Integrated and Remote Construction

The VariStroke-I Remote Servo Kit contains the same primary

components as Integrated version, This kit allows the Hydraulic Power Cylinder

to be mounted separately from the servo in applications where space is

constrained.

Hydraulic Power Cylinder

The simple and robust design of VS-I hydraulic cylinder (Figure 1-3) is capable of

consistent performance for extended periods in challenging environments.

Hydraulic cylinder is designed to operate in wide range of hydraulic pressures

and with high oil contamination. The actuation stroke range can be adjusted

precisely using PC service tool allowing the same actuator to accommodate a

variety of strokes.

The hydraulic power cylinder is designed to be field replaceable in turbine shut

down condition.

Rotary Servo Valve

The servo valve has four ports: Supply, two Control Ports, and Drain/Tank. With

the hydraulic valve in its middle position, all ports are blocked. As the valve

rotates, the supply is connected to a control port while simultaneously connecting

the drain to the other control port. The combined action of the servo position

controller and cylinder position controller modulate the power cylinder position as

necessary to match the input demand.

Additionally, the Remote Servo has an OVBD (Over Board Drain) port which is

permanently connected to drain. This port can (optionally) be connected to the

OVBD connection on Hydraulic Power Cylinder to drain any leakage pass the

primary rod seal.

A unique function of the software is a periodic, symmetrically opposed impulse

which flushes silt and debris from the servo valve without causing undue wear

called “Silt Buster”. At the interval and amplitude selected by the user, this

function provides a very rapid motion of the hydraulic valve allowing any silt to be

flushed to the drain passage. This motion is followed immediately by a step of

equal amplitude in the opposite direction. The opposing symmetry of the impulse

results in no net change in fluid volume to the controlled servo valve, and thus

does not interrupt the control of the turbine. This unique function provides a

higher degree of stability, reliability, and silt resistance.

If the unit detects any diagnostic shut down condition, or if the detected

diagnostic condition prevents reliable control, or if a loss of power occurs, the

servo valve return spring forces the valve to connect the appropriate control

pressure to drain causing the cylinder to move to the fail-safe position.

Servo Valve Actuator

The VS-I uses a rotary limited angle torque (LAT) actuator. The permanent

magnet rotor is directly coupled to the servo valve.

The position of the rotor is measured by a solid state integrated circuit on the

PCB which detects the orientation of the sensing magnet on the shaft. The

H-bridge drive is regulated by the microprocessor to precisely control the servo

valve position and maintain the cylinder stroke position demand.