Cascaded controller is a Multi-variable controller where two controllers are cascaded to control the process variable. One controller is primary (Master) controller and another controller is secondary (slave) controller. The secondary loop is setup to control a process variable of major source which causes disturbance for the primary control variable. The set-point of secondary controller is derived from the primary controller. i.e the output of primary controller is given as a summing set point to secondary control loop.
The cascading of controllers concept is widely used in temperature control because of the high span of time for a small change. On this reason the important temperature controllers are placed with cascaded controllers in most of the industries. In this case if a single controller is placed with the set point as temperature, the performance of the controller may not be satisfactory for the dynamic process changes at different conditions.
How cascaded controller works
Consider a shell and tube type heat exchanger with primary fluid is in shell side and secondary fluid is in tube side. The secondary fluid flow should be varied to control the temperature of the shell side. The dynamic changing of flow control valve cannot be possible with temperature control alone because the change in temperature can be rapid or very small based on four factors such as primary fluid flow & temperature and secondary fluid flow & temperature.
So the temperature of the fluid in shell side is measured and compared with the temperature set point and hence applied to control algorithm ( PI or PD) which provides the output according to deviation error. Hence the output of primary temperature controller is given as summing set-point to secondary flow controller i.e the out put is added with the existing flow value from the flow transmitter. This summation result will be the new set-point for the flow control which then processed by the flow control algorithm (PI) and hence adjusts the actuator of control valve according to the output.
Practical example of cascaded controller: Consider temperature controller shown in the above figure
Assume the temperature set-point (S.P) = 100
Current temperature is (P.V) = 90
Error % = (P.V – S.V)/100 = (90-100 )/100 = -10 %
Temperature controller(Assumed as Proportional Controller with Gain of 2 ) output
= 2 * (-10 %) = -20 %
Assume the the Present flow value = X%
Then Summing set-point for
secondary controller = (X-20)%
Error % in flow = (X – 20) – X = -20%
The Flow controller output will be adjusted to reduce the error to zero. Here it has assumed as PI Controller with previous output 60 %
Then the output will reduce from 60% to some less value according to negative error up to 50 % according to Kp and Ti constants.
Tuning of cascaded controller
Cascaded controller tuning a bit tough task because of dependency of the slave on master controller. The first part in the tuning of cascaded control is to tune the secondary slave control for various flow set-points with the available tuning techniques. Tuning the primary controller is not advisable because the secondary controller output is the major source which creates the disturbance to the primary controller.
The primary controller is only set-point deciding device for secondary controller so tuning tuning primary controller first may cause drastic change in set-point for secondary controller also. Because of this the secondary controller should be tuned for various set-point change intervals and then tune the primary according to the changes in temperature dynamics.
Algorithm of cascaded controller
In this algorithm the primary controller is taken as Proportional differential (PD) controller and secondary controller is taken as Proportional controller (PI).
‘C’ Code for cascaded controller
- Provide better control for temperature
- Fine and dynamic control is possible
- Difficult in realization and tuning.
- Cost is high.