Cascade control system
The basic principle of cascade control is that if the secondary variable responds to the disturbance sooner than the primary variable, then there is a possibility to capture and nullify the effect of the disturbance before it propagates into the primary variable.
In cascade control configuration, we have one manipulated variable and more than one measurement. It is an alternative to consider if direct feedback control using the primary variable is not satisfactory and a secondary variable measurement is available. Cascade control uses the output of the primary controller to manipulate the set point of the secondary controller.
The two measurements are taken from the system and used in their respective control loops. In the outer loop, the controller output is the set point of the inner loop. Thus, if the outer loop dynamic variable changes the error signal affects a change in set point of the inner loop. Even though the measured value of the inner loop has not changed, the inner loop experiences an error signal and produces a new output by virtue of the set point change. Cascade control generally provides better control of the outer loop variable than is accomplished through a single variable system. The primary objective of cascade control is to divide an otherwise difficult control process into two portions; whereby a secondary control loop is formed around major disturbances, leaving only minor disturbances to be controlled by the primary controller.
The outer controller is also known as primary or master controller and inner controller is also known as secondary or slave controller.
Features of cascade control
The main features of cascade control are:
● The output of the master controller serves as the set point for the slave controller.
● More than one measurement, but one manipulated variable.
● Two feedback loops are nested together.
● Inner loop has the effect of reducing time lag in the outer loop, so that cascade control responds very quickly and improves dynamic performance.
● Decrease variation in primary variable.
● Enhances stability characteristics.
● Insensitive to modeling errors.
● The secondary controller corrects the disturbances arising within the secondary loop before they affect the primary variable.
Rules for designing and tuning cascade control
- Let the secondary loop be the input point of the most serious disturbance. Secondary loop should reduce the effect of one or more disturbances.
- Make the secondary loop fast by including only minor lags of complete control system. Secondary loop must be at least 3 times faster than the primary loop.
- Choose a secondary variable which will provide stable performance with narrow proportional band.
The correct sequence of operation while tuning the controller that work in cascade is as follows:
- Set the primary controller to manual
- Tune the secondary controller
- Put the secondary controller in automatic mode
- Tune the primary controller
Advantages of cascade control
● Better control of the primary variable
● Primary variable is less affected by disturbances
● Faster recovery from disturbances
● Increase the natural frequency of the system
● Reduce the effective magnitude of a time-lag
● Improve dynamic performance
● Provides limits on the secondary variable
Disadvantages of cascade control
● It requires an additional measurement to work
● The control strategy is more complex
● Required an additional controller and it has to be tuned.
Examples of cascade control system
Cascade control of a jacketed CSTR
Consider the CSTR shown in the figure. The reaction is exothermic and the heat generated is removed by the coolant, which flows in the jacket around the tank. The control objective is to keep the temperature of the reacting mixture constant at the desired value. Possible disturbances to the reactor include the feed temperature and the coolant temperature.
Now the next figure illustrates how the cascade control is implemented on jacketed CSTR. The response of simple feedback control to the change in coolant temperature can be improved by measuring the change in coolant temperature, and taking control action before its effect has been felt by the reacting mixture. If the coolant temperature goes up, the control action increases the flow rate of the coolant.