Allen-Bradley LOGIX 5000 Reference Manual page 198

Controllers advanced process control and drives and equipment phase and sequence instructions

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Process Control Instructions

The following illustration shows the IMC function block configuration.

At each execution, the IMC function block compares the actual PV measurement

with PV prediction. The result is called disturbance estimate, which is the effect of

unmeasured process disturbances combined with the modeling imprecision. The

disturbance estimate is used as a bias for the setpoint of the control variable. In the

ideal case of no disturbances and perfect modeling, the disturbance estimate (the

feedback signal) becomes equal to zero.

First Order Model

Inverse of Model

First Order Filter

PV prediction = exp(-D*s)/(e*s+1) * (SP - Dist. estimate)

Model time constant

Model deadtime

Response time constant

Laplace variable

The function block then calculates the CV value (CVHLimit, CVLLimit, and rate

of change limits are imposed) and the PV prediction.

The IMC function block can be used in place of a PID function block with the

advantage over the PID control variable when controlling processes with large

Rockwell Automation Publication 1756-RM006K-EN-P - November 2018

M = K/(T*s+1)*exp(-D*s)

Inv = (T*s+1)/K

F = 1/(e*s+1)

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