Red Lion TCU - Manual (Page 89)

APPENDIX “D” - MANUAL TUNING
Open Loop Step Response Method
The Open Loop Step Response Method is a tuning procedure that does not
induce process oscillations. This method involves making a step change to
the process and observing the process reaction. A strip paper recorder or other
high resolution data logging equipment is required for this procedure. This
procedure requires that all disturbances to the process are minimized because
the data is influenced by these disturbances.
1) Connect a chart recorder to log temperature and set the paper speed
appropriate for the process.
2) Set the controller to manual (user) control mode.
3) Allow the process to stabilize (line out).
4) Make a step change of 10% or more in the controller output. It may be
necessary to increase the size of the step to yield a sufficient process
reaction curve.
5) Record the response of the process. Use the information from the table to
calculate the controller tuning values. The PID tuning parameters are
determined graphically from Figure 43, Process Reaction Curve. Draw a
vertical line at the moment the step change was made. Draw a line (labeled
tangent) through the process reaction curve at its
maximum upward slope.
Extend this line to intersect the vertical line.
Example : From the Process reaction Curve
a = 30°, t = 300 sec, step = 10%, thermocouple range = 1700°F.
For fast response:
Prop = 35.3%
Intt = 900 sec
dert = 120 sec
OPdP = 15
Parameter
Fast
Response
Damped
Response
Slow
Response
Proportional Band (%) 20000a
Range Step% ´
40000a
Range Step% ´
60000a
Range Step% ´
Integral Time (Sec) 3t 4t 5t
Derivative Time (Sec) 0.4t 0.4t 0.4t
Output Power
Dampening (Sec) t/20 t/15 t/10
81
Figure 43, Process Reaction Curve