cascade control Loop
Tuning a cascade control scheme is similar to tuning a normal PID controller, but, because the primary controller is only connected to the process through the secondary controller, it has to be done in a fixed order:
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Connect the secondary controller to the process, but disconnect the primary from the secondary (i.e. leave the secondary so that its setpoint can be set manually).
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Tune the secondary controller as a normal PID controller. Often secondary controllers are tuned using only proportional action - the secondaries are usually connected to very fast processes allowing high gains to be used. These high gains minimise the amount of steady-state offset which appears (and any residual offset will be removed by the primary loop).
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Once the secondary is tuned, connect the primary controllers output to the secondary controller’s setpoint.
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Tune the primary as a normal PID controller. At this stage you should not touch the secondary controllers tuning constants - if you do you will change the ‘process’ the primary sees and muck up any tuning you have done!