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:
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).
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).
Once the secondary is tuned, connect the primary controllers output to the secondary controller’s setpoint.
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!