Technical Specifications

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Progress Dialog
SVI II AP Monitor Screen
Parameter Definitions
P Proportional gain in %. Common values for the positioner are 50
for small valves up to 4000 for large valves.
I Integral time or reset time in 1/10th sec, is the time constant of
integral control. Higher values of I cause less integral action, how-
ever a value of 0 gives no integral action. Common values are 10 to
200.
D Derivative time or rate time (msec) is the time constant of deriva-
tive control. Common values are 10 to 100
PAdjust Valves often have significantly different response when filling
verses exhausting. The proportional gain is adjusted by adding
Padj (%) to P when the valve is exhausting.
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. Typi-
cal beta value for a valve position controller is 7 or 8.
Position Compensation
Coefficient
The response of the valve is different when the valve is nearly
closed than when the valve is nearly open. The position compensa-
tion coefficient, which is a number between 0 and 20. Make adjust-
ments to try to equalize the valve response. The normal value is 6.
For springless actuators the value is 15.
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%. Range: 0 to 5%
Boost The boost field in the PID adjustments controls a supplemental
pressure, or boost, to speed up initial valve response. This compen-
sates for pneumatic deadband. Range: 0 to 20.
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 AP.
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.