Publication 1763-RM001C-EN-P - October 2009
280 Process Control Instruction
error, the smaller the output signal. An additional value (feed forward or
bias) can be added to the control output as an offset. The PID result
(control variable) drives the process variable toward the set point.
The PID Equation
The PID instruction uses the following algorithm:
Standard equation with dependent gains:
Standard Gains constants are:
The derivative term (rate) provides smoothing by means of a low-pass
filter. The cut-off frequency of the filter is 16 times greater than the corner
frequency of the derivative term.
Term Range (Low to High) Reference
Controller Gain K
C
0.01 to 327.67 (dimensionless)
(1)
(1) Applies to MicroLogix 1100 PID range when Reset and Gain Range (RG) bit is set to 1. For more information on reset
and gain, see PLC 5 Gain Range (RG) on page 294.
Proportional
Reset Term 1/T
I
327.67 to 0.01 (minutes per repeat)
(1)
Integral
Rate Term T
D
0.01 to 327.67 (minutes)
(1)
Derivative
Output K
C
E()
1
T
I
---- -
E()td
∫
T
D
dPV()
dt
---------------
⋅++ bias+=
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