PID Loop Operation
Maintenance
and Troubleshooting
8–36
PID Loop Operation
The derivative term is unique in that it has an optional gain-limiting feature. This is
provided because the derivative term reacts badly to PV signal noise or other causes
of sudden PV fluctuations. The function of the gain-limiting is shown in the diagram
below. Use bit 9 of PID Mode 1 Setting V+00 word to enable the gain limit.
Process Variable
S
Error Term
+
–
Control
Output
Setpoint
PID Mode 1 Setting V+00
013456789101112131415 2Bit
Derivative gain limit select
S
+
P
I
D
Loop Calculation
+
+
Derivative
Derivative,
gain-limited
0
1
Integral
Proportional
Loop Table
V+25 Derivative Gain Limit00XX
The derivative gain limit in location V+25 must have a value between 0 and 20, in
BCD format. This setting is operational only when the enable bit = 1.
The gain limit can be particularly useful during loop tuning. Most loops can tolerate
only a little derivative gain without going into wild oscillations.
In the widely-used position form of the PID equation, an important component of the
control output value is the bias term shown below. Its location in the loop table is in
V+04. the loop controller writes a new bias term after each loop calculation.
S
Mn = Kc * en + Ki * ei + Kr * (en – en–1) + Mo
i=1
n
Control
Output
Proportional
Term
Initial
Output
Integral
Term
Derivative
Term
Bias Term
V+04 Bias termXXXX
If we cause the error (en) to go to zero for two or more sample periods, the
proportional and derivative terms cancel. The bias term is the sum of the integral
term and the initial output
(Mo). It represents the steady, constant part of the control
output value, and is similar to the DC component of a complex signal waveform.
The bias term value establishes a “working region” for the control output. When the
error fluctuates around its zero point, the output fluctuates around the bias value.
This concept is very important, because it shows us why the integrator term must
respond more slowly to errors than either the proportional or derivative terms.
Derivative Gain
Limiting
Bias Term
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