Technical Specifications

146 | =GE Oil & Gas

Position Compensation

Coefficient (Pos Comp)

The response of the valve is different when the valve is nearly closed

than when the valve is nearly open. The position compensation coef-

ficient, which is a number between 0 and 20, make adjustments to

try to equalize the valve response. The normal value is 6. For spring-

less actuators the value is 15.

Beta Beta is a nonlinear gain factor, ranging from -9 to 9. When beta is 0,

the controller gain is linear. Otherwise the gain is the function of

error. The larger the beta, the smaller the gain for small error. Typical

beta value for a valve position controller is 7 or 8.

Dead Zone When the valve position is within the setpoint +/- the dead zone, no

additional position control is performed. This value is normally 0%,

however for high friction valves (e.g. valves with graphite packing) a

higher dead zone (%) helps avoid limit cycling due to the stick/slip

action of the valve. In these cases the dead zone chosen might be

0.5% to 1%.

The values determined above provide sufficient response for most applications. However

these values are determined by a target set of performance criteria built into the SVI II ESD.

You may want performance different than this set and may therefore want to set the PID

values differently than determined by Autotune. You can individually set tuning values from

the Calibrate screen.