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User Manual
GFK-1503F
The ISA (PID_ISA) algorithm has different coefficients for the terms:
PID Output = Kc * (Error + Error * dt/Ti + Td * Derivative) + CV Bias
where Kc is the controller gain, Ti is the Integral time and Td is the Derivative time. The
advantage of PID_ISA is that adjusting Kc changes the contribution for the integral and
derivative terms as well as the proportional term, which can simplify loop tuning.
If you have the PID_ISA Kc, Ti and Td values, use the following equations to convert them to
use as:PID_IND parameters:
Kp = Kc, Ki = Kc/Ti, and Kd = Kc * Td
The following diagram shows how the PID_IND algorithm works:
Figure 214
The ISA Algorithm (PID_ISA) is similar except that its Kc gain coefficient is applied after the
three terms are summed, so that the integral gain is Kc / Ti and the derivative gain is Kc *
Td.
Bits 0, 1 and 2 in the Config Word set the Error sign, Output Polarity and Derivative Action,
respectively.
14.4.1
Error Term
Both PID algorithms calculate the Error term as
which can be changed to Reverse Acting mode:
by setting the Error Term mode (bit 0) in the Config Word – (word 13 of the reference array)
to 1.
Reverse -Acting mode is used if you want the CV output to move in the opposite direction
from PV input changes (CV down for PV up) instead of the normal CV up for PV up.
Bit 0 of Config Word
(Word 13)
0
1
PID Built-in Function Block
Error = (SP – PV),
Error = (PV – SP)
Error Term Mode
Normal
Reverse Acting
Error Term
SP – PV
PV – SP
Chapter 14
Jan 2020
262
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