Allen-Bradley LOGIX 5000 Reference Manual page 268

Chapter 2 Drives
Examples
In many applications an integral gain component is included in the closed loop
regulator design in order to eliminate or minimize error in the system being
regulated. A straight proportional-only regulator will not tend to drive error in the
system to zero. A regulator that uses proportional and integral gain, however,
tends to drive the error signal to zero over a period of time. The INTG instruction
uses the following equation to calculate its output.
In the chart below, the input to the block moves from 0 to +200 units. During this
period, the output of the block integrates to 2800 units. As In changes from +200
units to 0 units, Out maintains at 2800 units. When In transitions from 0 to -300
units, Out slowly integrates down to -1400 units until In transitions back to 0.
Finally, as In moves from 0 to +100, Out integrates back to 0 where In is set to 0
coincidentally with Out reaching 0.
This characteristic of the integrator - continually driving in a specific direction
while any input to the function is present or holding at any level during the point
where the input is at zero - is what causes a regulator using integral gain to drive
toward zero error over a period of time.
The following example shows how the INTG instruction can be used. In many
instances, the HighLimit and LowLimit inputs limit the total percentage of
control that the integral gain element might have as a function of the regulator's
total output. The HoldHigh and HoldLow inputs, on the other hand, can be used
to prevent the output from moving further in either the positive or negative
direction. The HoldHigh and HoldLow inputs prevent the INTG instruction
268 Rockwell Automation Publication 1756-RM006K-EN-P - November 2018