TM 5-3895-374-24-2
955-257
TUNING PROCEDURE WHEN NO OSCILLATIONS ARE OBSERVED
1)
Measure the steady-state deviation, or droop, between setpoint and actual temperature with minimum PB setting.
2)
Increase the PB setting until the temperature deviation (droop) increases 65%. Nomogram I in previous step 8
provides a convenient method for calculating the desired final temperature deviation.
3)
Set the RESET 1 to a high value (10 R/M). Set the RATE 1 to a corresponding value (.02 MIN). At this point, the
measurement should stabilize at the setpoint temperature due to reset action.
4)
Since we were not able to determine a "CRITICAL" oscillations time, the optimum settings of the RESET and
RATE adjustments must be determined by trial and error. After the temperature has stabilized at setpoint,
increase the setpoint temperature setting by 10 degrees. Observe the overshoot associated with the rise in actual
temperature. Then return the setpoint setting to its original value and again observe the undershoot associated
with the actual temperature change.
4.8 TUNING THE PRIMARY FOR COOLING CONTROL
The same procedure is used as defined for heating. The process should be run at a setpoint that requires cooling
control before the temperature will stabilize. Note that the Self Tune feature (Models 2003 and 2004) does not work
for cooling outputs.
4.9 SIMPLIFIED MANUAL TUNING PROCEDURE FOR PID CONTROLLERS
The following procedure is a graphical technique of analyzing a process response curve to a step input. It is much
easier with a strip chart recorder reading the process variable (PV). Refer to the diagram below.
1.
Starting from a cold start (PV at ambient), apply full power to the process without the controller in the loop i.e.
open loop controller in MANUAL mode. Record this starting time.
2.
After some delay (for heat to reach the sensor), the PV will start to rise. After more of a delay, the PV will reach a
maximum rate of change (slope). Record the time that this maximum slope occurs, and the PV at which it occurs.
Record the maximum slope in degrees per minute. Turn OFF system power.
3.
Draw a line from the point of maximum slope back to the ambient temperature axis to obtain the lumped system
time delay Td (see example below). The time delay may also be obtained by the equation:
Td = time to max. slope-(PV at max. slope-ambient)/max. slope
4.
Apply the following equations to yield the PID parameters:
Pr. Band = Td X max. slope X 100/Span = % of Span
Reset = 0.4/Td = resets/minute
Rate = 0.4 X Td = minutes
ECLIPSE INSTRUMENTATION DIVISION
page 3-1102
