Dalam beberapa aplikasi, 2 PID controllers digunakan dal kombinasi. Dalam konfigurasi ini, MV (manipulated variable) dari primary PID controller dihubungkan ke SV(set point) dari secondary PID controller. Ini disebut cascade control. Kelebihan dari cascade control is quicker suppression of disturbances or upsets. That is, the secondary PID loop is expected to suppress errors before their influence appears on the PV of the primary PID loop.
The figure below shows an example of cascade control.
Cascade control is effective only when the response of the secondary process is much faster than the primary process. In the above example, the primary loop is for temperature and the secondary loop is for flow rate. Normally, response of temperature control loop is far slower than the flow loop. In this example configuration, cascade control can be effective.
In cascade control, the MV tracking function in the primary PID is required. This function provides bumpless transfer when the secondary PID changes from the local auto mode to the cascade mode. Then, the MV of the primary PID tracks the local SV of the secondary PID when the secondary PID is in local mode.
NOTE: The PID3 instruction supports the cascade mode and MV tracking.
The figure below shows an example of cascade control.
Cascade control is effective only when the response of the secondary process is much faster than the primary process. In the above example, the primary loop is for temperature and the secondary loop is for flow rate. Normally, response of temperature control loop is far slower than the flow loop. In this example configuration, cascade control can be effective.
In cascade control, the MV tracking function in the primary PID is required. This function provides bumpless transfer when the secondary PID changes from the local auto mode to the cascade mode. Then, the MV of the primary PID tracks the local SV of the secondary PID when the secondary PID is in local mode.
NOTE: The PID3 instruction supports the cascade mode and MV tracking.
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