Cascade Controller Integral Windup

Integral windup has already been [discussed]. When commercial PID controllers are directly connected to the control valve, integral windup should be handled automatically provided the output limits have been properly set.

When a controller is used as a primary in a cascade loop, however, this is not the case. The problem is that the primary controllers output is the setpoint for the secondary controller and not the control valve. The secondary controller adjusts the control valve.

Now, if something happens in the secondary loop that causes the control valve to bump against a limit, the primary controller has no way of knowing this has happened and will continue with integral action (changing the secondary setpoint).

The solution to this problem is to provide some means of communication between the secondary and primary controllers that allows the secondary to inform the primary when the output has saturated.

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