Cascade control loop is simply a cascade of two single control loop. Cascade control is used for process with slow dynamics like tempreture, level, humidity.
Cascade control can be usefully applied to any process where a measurable secondary variable directly influences the primary controlled variable.
In a single control loop, the controller's set point is set by an operator. For example, consider the heat exchange process here.
The water in the tank is heated by the steam flowing through the fire tube inside the tank. Temperature of the water flowing out of the tank is measured, controller compares the temperature with the set point. And controller operates the inlet valve of the steam flow channel.
However due to the change in upstream steam pressure, the steam flow rate may change, though the control valve is at the same position. This will affect the amount of heat exchanged and the temperature at the water outlet. It will take time to detect the temperature change due to the upstream pressure. And thus for the control action.
The above problem can overcome by controlling the flow rates, by using the cascade control loop.
In cascade control system an additional controller is used to the check the effect of disturbance the inlet flow. This additional controller work as the inner loop of the control system.
The primary controller, which is the temperature controller gives the control action as the set point of the secondary or flow controller. Flow controller gives the final control action to the valve, comparing the Flow meter measurement and the temperature controller output.
In this example the disturbance arise in the inner loop or in flow is controlled by the inner loop itself, before effecting the outer temperature loop. Outer loop control other disturbance from tank.
Advantages of the cascade control loop
Isolating the slow outer loop from valve problems and some disturbances. Fast inner loop take care of these
Even if, the process shows non-linearity in it's characteristics, the process can be stabilized using cascade loop.
We can eliminate the effect of disturbance.
Cascade control is useful in process with relatively slow process dynamics.