Control System

What is cascade control?

The cascade control can be described as a method of feedback combining. In this method of control, the inner loop controller would get its set-point from an outer-loop control. So in this control system, there would be two controllers and the output of one controller is the input for the other one. So in this controller,, the secondary controller is used in conjunction with the primary controller to improve the control of the primary process variable.

The cascade loop would have a second measurement and also secondary feedback. In a cascade control system, there would be two controllers and in which the output of one controller would drive the setpoint of the other one. This is a multiloop structure in which the output of the controller in the outer loop which is called primary, is the set point of the controller in the inner loop. So two controllers are used in the cascade control because the process is divided into two and one process variable will be manipulated.

When should we use a cascade control?

  • We should use a cascade if there could be a lot of disturbance in a single loop
  • We can also use a cascade control if we would need a quick response
  • If there is no quick response from the primary variable during the disturbances
  • In case of process changes that would affect the control variable
  • It can be used if another variable is affected by the disturbances and this variable is related to the control variable
  • It can be used if the secondary variable, is controllable and also it would provide a quick response to the primary controller.

What are the requirements of a cascade control?

  • The primary loop must not be faster than the secondary loop
  • The secondary loop should experience the maximum disturbance
  • The secondary loop should have disturbances that are not expected
  • The secondary loop must have a direct influence on the primary loop
  • The secondary loop must be controllable and it should be measured too
  • The disturbance must be quickly recognized and corrected so that the primary control loop won’t be affected. So the secondary loop must operate faster than the primary loop.
  • The secondary loop must be 5 times faster than the primary loop

Where should we use a cascade control?

In a certain process, two or more process variables would affect equally the behavior of each other where one can be controlled with the control combination of another so in this process we can use the cascade control. This is a type of process control in which one process variable is controlled by the control combination of the two process variable. In this type of control, two process variables are controlled with a single final control element.

How does a cascade controller work?

This is the major multi-loop that is widely used, in this system, there are two controllers, two sensors, and also an actuator. The primary controller’s output is used to manipulate the set-point of the secondary controller both of these controllers are cascaded with each other but each has its own process variable input. The controller would use the actuator to help with the control effort to the secondary phase. There is an inner loop in the secondary controller and the primary controller has an outer loop. The inner loop is the same as the conventional feedback system in which it is composed of a setpoint, process variable, and a controller this controller would act on the process with the help of an actuator. The outer loop is similar to the inner loop and the only difference is that it uses the entire inner loop as the actuator. The primary controller has an independent setpoint and its output goes to the secondary controller. The secondary controller would receive the primary controller’s output as setpoint input and it also has its control output as the output to the process. The secondary controller is a closed loop in a primary loop. So if the cascade control is done then disturbance from the quick change of the secondary controller wouldn’t affect the primary controller.

How to do the start-up of a cascade system?

If the cascade system needs to function properly then it must be set be properly. The inner loop of the system must be tuned before the outer loop.

Cascade control starting procedure

  • The primary controller must be placed in the manual mode so that it would break the cascade and isolate the secondary controller and it can be tuned.
  • The secondary controller must be tuned in a way, by considering it as the only control loop.
  • The secondary controller must be returned to the remote setpoint and we should place the primary controller in automatic mode this would isolate the primary controller and it can be tuned.
  • We must manipulate the set point of the secondary controller in order to tune the primary control loop.
  • If the system is not stable while the primary controller is automatic then reduce the gain of the primary controller.

What are the failures that could happen in a cascade control system?

The cascade controller is not a fail-safe system, the purpose of the cascade control is to make the system responsive. If the master control in the cascade system fails then the whole system would fail. In order to keep the cascade controller safe, we can use multiple controllers as the master controllers, and then we can use selectors to choose which reading must be used. In case if the cascade control fails, we can do certain things to run the system during the repair of the controller. The controller can be manually operated by a person during the failure, or we can use previous readings to send a constant signal to the system.

How to do the cascade control tuning?

Due to the complexity of the cascade controller, the tuning would be much high. The cascade system should be tuned starting with the innermost loop and if it is tuned then it is placed in cascade control and also the loop driving its setpoint is tuned. We must not use the quarter amplitude damper tuning rules to tune the control loops in a cascade structure because it would make the system unstable.

The secondary controller should be tuned first and after that, the primary controller and both these tuning would be done by the same tuning tool. It should be tuned in a way that the inner loop must be faster than the outer loop, if not then the controllers will be will compete and this would make the closed-loop system unstable. The loop must be tuned faster and tighter than the loop around it. The flow controller in the cascade system must be tuned tight.

What are the types of cascade control mode?

In many applications, the control loop won’t be able to function as a cascade loop all the time. The operator would be able to change the modes, there are three modes and they are manual mode, auto mode, and cascade mode. In manual mode, the set point of the flow controller tracks the actual flow. In the auto mode, the output of the temperature controller tracks the set point of the flow controller. In the cascade mode, the temperature controller manipulates the set-point of the flow controller.

Why should we need a cascade control?

The simple control like a closed loop, the controller is controlling a single variable at a point and this can be considered as single point closed-loop control. So in this type of control, the process has only one stage of information for control and is less effective during the change in set point. So by using a cascade control we are controlling a single variable in two stages. So there will be effective control of the process in case of any disturbance or the change in set point. By using a cascade control we can do proper control of a system. The load changes in the process can be maintained and controlled by a cascade control  

What are the advantages of cascade control?

  • It would provide good control of the primary variable
  • The primary variable is less affected by the disturbance
  • The recovery from the disturbance is quick
  • The natural frequency of the system would increase
  • The magnitude of the time lag would be reduced
  • The dynamic performance would be better
  • It would limit the secondary variable
  • It can be combined with feedforward control or other forms of control

What are the disadvantages of a cascade control?

  • Complexity
  • The measurement of the second variable is costly
  • The tuning of the cascade controller is really difficult
  • If the inner loop is not faster than the outer loop then it would affect the whole system.
  • It requires additional measurement to work

What are the applications of cascade control?

  • Reboiler temperature setting steam flow
  • Boiler drum level setting feed-water flow
  • Reactor temperature control

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