If this were to happen, we know we would need the control valve to close off in order to counter-act this change: sending less steam to a reactor that is getting too hot. Applying another “thought experiment” to this system, we may imagine the reactor temperature suddenly increasing. Now that we know the slave controller must be reverse-acting, we may choose the action of the master controller. We shall denote this with a “−” label next to the process variable (PV) input, and a “+” label next to the remote setpoint (RSP) input: Since the valve is still air-to-open, this requires a decrease in the output signal from the FIC. If we imagine the steam flow rate suddenly increasing, we know we need the control valve to close off in order to counter-act this change. Once again we may apply a “thought experiment” to this system in order to choose the appropriate slave controller action.

Just as we would first tune the slave controller in a cascade system prior to tuning the master controller, we should first determine the correct action for the slave controller prior to determining the correct action for the master controller. In order to determine the proper actions for each controller in this system, it is wise to begin with the slave controller (FIC), since the master controller (TIC) depends on the slave controller being properly configured in order to do its job properly. Let us discuss a steam-heated reactor control system with a cascade strategy, where the temperature controller drives a setpoint signal to a “slave” steam flow controller: Cascade Control Loop

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