PID Loop Operation
Maintenance
8–63
PID Loop Operation
Q. The loop Setpoint appears to be changing by itself.
A. Check the following for possible causes:
S The Ramp/Soak generator is enabled, and is generating setpoints.
S If this symptom occurs on loop Manual-to-Auto Mode changes, the loop
automatically sets the SP=PV (bumpless transfer feature).
S Check your ladder program to verify it is not writing to the SP location
(V+02 in the loop table). A quick way to do this is to temporarily place an
end coil at the beginning of your program, then go to PLC Run Mode.
Q. The SP and PV values I enter with DirectSOFT32 work okay, but these values do not work
properly when the ladder program writes the data.
A. The PID View in DirectSOFT32 lets you enter SP, PV, and Bias values in decimal,
and displays them in decimal for your convenience. For example, when the data
format is 12 bit unipolar, the values range from 0 to 4095. However, the loop table
actually requires these in hex, so DirectSOFT32 converts them for you. The values
in the table range from 0 to FFF, for 12-bit unipolar format.
Q. The loop seems unstable and impossible to tune, no matter what I gains I use.
A. Check the following for possible causes:
S The loop sample time is set too long. Refer to the section near the front
of this chapter on selecting the loop update time.
S The gains are too high. Start out by reducing the derivative gain to zero.
Then reduce the integral gain, and the proportional gain if necessary.
S There is too much transfer lag in your process. This means the PV
reacts sluggishly to control output changes. There may be too much
“distance” between actuator and PV sensor, or the actuator may be
weak in its ability to transfer energy into the process.
S There may be a process disturbance that is over-powering the loop.
Make sure the PV is relatively steady when the SP is not changing.
Bibliography
Fundamentals of Process Control Theory, Second Edition
Author: Paul W. Murrill
Publisher: Instrument Society of America
ISBN 1–55617–297–4
Application Concepts of Process Control
Author: Paul W. Murrill
Publisher: Instrument Society of America
ISBN 1–55617–080–7
PID Controllers: Theory, Design, and Tuning, 2nd Edition
Author: K. Astrom and T Hagglund
Publisher: Instrument Society of America
ISBN 1–55617–516–7
Fundamentals of Temperature, Pressure, and Flow
Measurements, Third edition
Author: Robert P. Benedict
Publisher: John Wiley and Sons
ISBN 0–471–89383–8
Process / Industrial Instruments & Controls Handbook,
Fourth Edition
Author (Editor-in-Chief): Douglas M. Considine
Publisher: McGraw-Hill, Inc.
ISBN 0–07–012445–0
pH Measurement and Control, Second Edition
Author: Gregory K. McMillan
Publisher: Instrument Society of America
ISBN 1–55617–483–7
Process Control, Third Edition
Instrument Engineer’s Handbook
Author (Editor-in-Chief): Bela G. Liptak
Publisher: Chilton
ISBN 0–8019–8242–1
Process Measurement and Analysis, Third Edition
Instrument Engineer’s Handbook
Author (Editor-in-Chief): Bela G. Liptak
Publisher: Chilton
ISBN 0–8019–8197–2
To Next Page
Loading...
