User manual

PID Loop Operation
(DL450 Only)
Maintenance
8--73
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
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
is in Bumpless Transfer Mode 1.
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 DirectSOFT work okay, but these values do not work
properly when the ladder program writes the data.
A. The PID View in DirectSOFT 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 DirectSOFT converts them for you. The values in
the table range from 0 to FFF, for 12-bit unipolar format. Your ladder program must
convert constant values from their BCD format (when entered as Kxxxx) to binary
with the BIN instruction or you must enter them in the constant field (Kxxxx) as the
hex equivalent of the decimal value.
Q. The loop seems unstable and impossible to tune, no matter what 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 by increasing the integral time value, 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.