How can we have two controllers but only one FCE? Because as shown in the diagram above, the controller output signal from the outer primary controller, CO1, becomes the set point of the inner secondary controller, SP2.
The outer loop literally commands the inner loop by adjusting its set point. Functionally, the controllers are wired such that SP2 = CO1 (thus, the master and slave terminology referenced above).
Since PV2 sees the disruption first, it provides our "early warning" that a disturbance has occurred and is heading toward PV1. The inner secondary controller can begin corrective action immediately. And since PV2 responds first to final control element (e.g., valve) manipulations, disturbance rejection can be well underway even before primary variable PV1 has been substantially impacted by the disturbance.
With such a cascade architecture, the control of the outer primary process variable PV1 benefits from the corrective actions applied to the upstream early warning measurement PV2.
The advantages of cascade control are all somewhat interrelated. They include:
1. Better control of the primary variable
2. Primary variable less affected by disturbances
3. Faster recovery from disturbances
4. Increase the natural frequency of the system
5. Reduce the effective magnitude of a time-lag
6. Improve dynamic performance
7. Provide limits on the secondary variable
Cascade control is most advantageous on applications where the secondary closed loop can include the major disturbance and second order lag and the major lag is included in only the primary loop. The secondary loop should be established in an area where the major disturbance occurs
How can we have two controllers but only one FCE? Because as shown in the diagram above, the controller output signal from the outer primary controller, CO1, becomes the set point of the inner secondary controller, SP2.The outer loop literally commands the inner loop by adjusting its set point. Functionally, the controllers are wired such that SP2 = CO1 (thus, the master and slave terminology referenced above). Since PV2 sees the disruption first, it provides our "early warning" that a disturbance has occurred and is heading toward PV1. The inner secondary controller can begin corrective action immediately. And since PV2 responds first to final control element (e.g., valve) manipulations, disturbance rejection can be well underway even before primary variable PV1 has been substantially impacted by the disturbance.With such a cascade architecture, the control of the outer primary process variable PV1 benefits from the corrective actions applied to the upstream early warning measurement PV2.The advantages of cascade control are all somewhat interrelated. They include:1. Better control of the primary variable2. Primary variable less affected by disturbances3. Faster recovery from disturbances4. Increase the natural frequency of the system5. Reduce the effective magnitude of a time-lag6. Improve dynamic performance7. Provide limits on the secondary variableCascade control is most advantageous on applications where the secondary closed loop can include the major disturbance and second order lag and the major lag is included in only the primary loop. The secondary loop should be established in an area where the major disturbance occurs
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