In cascade, two controllers are connected in a series. The primary or master controller is one of them, and the secondary or slave controller is the other. The final control element is operated by the secondary controller’s output.
The temperature transmitter’s output is sent as a measurement signal to the master controller, the TIC (Temperature indicating controller). Similar to this, the secondary controller, the PlC (Pressure indicating controller), receives the output of the pressure transmitter as a measurement signal. The valve is operated by the PlC, which receives the output of the TIC (Temperature indicating control) at the specified set point to the PIC. The TIC’s required temperature is set.
Use of cascade system:
- In order to keep external disturbances from affecting the operation, cascade loops are always installed.
- The standard single controller, as depicted in the diagram, is unable to react to a change in fuel gas pressure until the process temperature sensor notices it.
- In other words, corrective action cannot be conducted unless there is a temperature detection error.
- whereas the cascade loop reacts instantly, adjusting for the impact of a change in pressure before it may affect the process temperature.
- The effect of relative speeds and immediate lags on cascading-induced improvements in control quality.
- For this kind of control, a sluggish primary variable (the Master) and a fast-reacting secondary variable (the Slave) are perfect combinations.
- In order to prevent the primary (master) variable’s control from being disrupted, the slave must be able to react quickly to fast disturbances.
- On heat transfer equipment, it can be claimed that using cascade control enables quick recovery from load fluctuations or other disruptions.
- It provides improved disturbance rejection and control accuracy by using the output of one control loop as the setpoint for another.
- It allows for better control of both slow and fast dynamics, resulting in faster response times.
- Cascade control enhances the robustness of the system by compensating for uncertainties and variations in process dynamics.
- It also offers flexibility in system design and allows for modular control implementation.
- cascade control improves control system performance, stability, and efficiency in various industrial applications.
Disadvantages of cascade control
- The disadvantages of cascade control include increased system complexity, which can make troubleshooting and maintenance challenging.
- Tuning cascade control is more difficult than single-loop control due to the interaction between primary and secondary loops.
- Cascade control is sensitive to inaccuracies in process models, which can lead to reduced performance.
- Measurement noise can propagate and affect the secondary loop, potentially causing instability.
- Implementing cascade control requires additional hardware, leading to increased costs.