WESTLOCK CONTROLS CORPORATION
280 N. MIDLAND AVE., STE. 258, SADDLE BROOK, NJ 07663 TEL: 201-794-7650 FAX: 201-794-0913
www.westlockcontrols.com
10/5/2013
TECH-477/D.W.O. 21424
Page 37 of 206
1.
Auto PID: this option triggers the Auto PID procedure. It will override the
positioner control and will modulate the valve in order to automatically find
out the best P, I and D parameters.
2. Manual Tuning: the Fine Tune Proportional (PCAL), Derivative (DCAL) and
Integral (ICAL) gain settings can be varied incrementally on a scale from 1-255.
The fine tuning values are directly related to the PID mathematical formula
time constant values (Ti or Td) and the Proportional gain value (Kc). The
proportional gain (Kc) has a direct effect on the system response time. As Kc
is increased the valve movement response time becomes faster and the error
between setpoint and actual position becomes less. The drawback of
increasing the value of the Kc is that the overshoot and settling time will
increase. If Kc is increased too much the valve’s position control becomes
unstable, oscillating forever around the set point. The integrative term (Ti)
affects the dynamic response of the position control by damping the
positioner response according to the Ti value. So a lower value of Ti will have
less damping of the dynamic response. This means the lower the Ti the more
oscillations are observed before the stabilization of the position and vice-
versa. However, the higher the value of Ti the longer will be the settling time.
Regarding the Derivative term (Td); the greater its value the greater will be the
derivative effect. Different from the proportional gain and integrative term, the
derivative term is not proportional to the error but to the rate of change of the
position. The effect of increasing Td is to decrease the overshoot and the
control reaction to the rate of change of the position. The drawback of
increasing the Td too much is that the control system can become more
unstable.
Snsr
The sensor calibration is a self-adjustment that sets the positioner’s Hall-
Effect circuitry. This is automatically done during the
ACAL (Automatic
Calibration)
routine. The sensor calibration also shows up under the
MCAL
menu. This calibration only needs to be performed under the
MCAL
routine
when the positioner is set-up on a new application and only if the
ACAL
routine is not performed.
Trnd
The purpose of this function is to calibrate the positioner’s pressure
transducer. It should be performed only after a manual sensor calibration.
Flow
This function allows for the setting of the flow
characteristic of the positioner. This allows you to
adjust the “positioner-setpoint” to “actual-valve-
position” ratio to compensate for non-linear valve
flow characteristics. This can be used so the
“positioner-setpoint” will represent a closer
approximation to the actual process flow. The
VALVE POSITION % OF TRAVEL
QUICK OPENING
80
60
40
20
0
0
20
40
60
80
100
100
EQUAL PERCENTAGE
LINEAR
VALVE FLOW CHARACTERISTICS
