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.

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.