Cascade Control

Cascade Control

Cascade Control diagram

Cascade control involves the use of two controllers with the output of the first controller providing a set point for the second controller, a feedback loop for one controller for the other controller.
Cascade control is used when there is more than one measurement, but only one control variable is available.
In single-loop control, the set point of the controller is set by the operator and its output drives the final control element. Cascade control is basically a control algorithm in which the output of one control loop provides the target for another loop. The ultimate goal of a cascaded loop is to control the final process.
Cascade loops are always installed to prevent external disturbances from entering and exiting the primary controlled variable.

What is Cascade Control?

In single-loop control, the set point of the controller is set by the operator and its output drives the final control element. For example: A level controller operates a control valve to keep the level at its set point. In a cascade control system, there are two (or more) controllers in which the output of one controller drives the set point of the other controller. For example: A level controller operating the set point of a flow controller to keep the level at its set point. The flow controller, in turn, operates the control valve to match the flow to the set point requested by the level controller.
The controller that drives the set point (the level controller in the example above) is called the primary, external or master controller. A controller that receives the set point (for example a flow controller) is called a secondary, internal or slave controller.
Single loop cascade control

Advantages of Cascade Control

Cascade control is used on advanced thermal products. The purpose of cascade control is to improve process efficiency by reducing disturbances through the use of thermocouple controls. Following advantages are achieved by cascade control
Less overall variability: A fast inner loop can respond to perturbations faster than an outer loop. Therefore, it reduces the severity of the disturbance and limits the variability affecting the heating process.
Effective Response to Disruption : In loops where cascade architecture is well deployed, there is effective response to disturbances. The inner loop is faster and closer to the source of the disturbance than the outer loop. This procedure allows for faster correction of discomfort.
More conservative outer loop tuning : Design engineers can tune the outer loop for stable control. The inner loop makes coarse adjustments, the outer loop is the only option for fine-tuning. It has high amperage power control and enhanced control options that improve the reliability of cascade control.

Disadvantages of Cascade Control

  • A additional measurement (usually flow rate) is required to perform the function.
  • There is an additional controller that needs to be tuned. 
  • basically cascade control strategy is more complex 
  • Measuring another variable in cascade control is very expensive
  • Cascade control is very difficult in tuning

Application  of Cascade Control

Cascade control regulates the temperature
A cascade control controls the flow of steam
A cascade control controls the flow of feed water

When Should Cascade Control be Used?

Cascade control is often used if you have a relatively slow process (such as level, temperature, composition, humidity) and need to manipulate liquid or gas flow or some other relatively-fast process to control the slow process. For example: changing the cooling water flow rate to control the condenser pressure (vacuum) or changing the steam flow rate to control the heat exchanger outlet temperature. In both cases, the flow control loop is used as the inner loop in the cascade arrangement.

When Should Cascade Control Not be Used?

Cascade control is beneficial only if the dynamics of the inner loop is faster than the outer loop. Cascade control should not generally be used unless the inner loop is at least three times faster than the outer loop, as the improved performance may not justify the added complexity.
In addition to the lesser benefits of cascade control when the inner loop is not significantly faster than the outer loop, there is also the risk of interaction between the two loops which can lead to instability especially if the inner loop is tuned too aggressively.

How Should Cascade Controls be Tuned?

A cascade arrangement should be tuned starting from the innermost loop. Once it is tuned, it is placed in cascade control or external set point mode and then the loop driving its set point is tuned. Do not use quarter-amplitude-damping tuning rules (such as the unmodified Ziegler-Nichols and Cohen-Koon rules) to tune control loops in a cascade structure because the process dynamics of the inner and outer loops can cause instability. The same.

How Does Cascade Control Work?

Basically this cascade control consists of two controllers and this first controller output drives the set point of the second controller.

What are the types of cascade control mode?

There are basically three modes of cascade control and they are manual mode, auto mode and cascade mode. In auto mode, the output of the temperature controller tracks the set point of the flow controller. In cascade mode, the temperature controller manipulates the set-point of the flow controller.

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