Why use Cascade control loop?
In the process industry, many process requirements cannot be solved by simple loops, which rely solely on feedback control or feedforward control. Several types of processes that have very large time constants. Although the derivative unit has been installed in the controller, due to the slow process, the process reaction is still slow. In terms of operations, this delayed process is very detrimental. To overcome this, two or more measurement elements are used in one control loop, known as the complicated control loop, as we will discuss CASCADE CONTROL.
Cascade control loop working:
The cascade control loop is simply a cascade of two unique control loops. Cascade control is used for processes with slow dynamics such as temperature, level, humidity. Cascade control can be applied in a useful way to any process in which a measurable secondary variable directly influences the main controlled variable. In a single control loop, the operator sets the controller set point.
A Cascade control system uses two PID controllers Master and the slave controller.
The general objective of a cascade control circuit is to closely control ONE variable at its set point. This is the Process Variable of the Master Loop. The slave loop also has a PV. But the only reason we care about the slave PV is that having an individual control of this slave PV gives us a better (faster) control of the master PV. For example:
In the picture above the manipulated variable from the temperature controller TIC (called primary or master) becomes the set point for the pre-controller (called secondary or slave). The application of cascade control can be detrimental if the processing element in the primary loop is faster than the processing element in the secondary loop because the system will tend to oscillate (variation) due to the emergence of interactions between the primary loop and the secondary loop. So the cascade control system can only be applied to processes with primary elements that are much slower than the secondary elements.
Process in the above example:
The temperature controller (master) compares the desired temperature of the supply water with the actual temperature. If the water needs to be heated, there will be a positive error in the master controller and the master reverse action controller will reduce its output.
This will indicate to the slave controller that the level should fall. The slave controller will open the valve to lower the level; The temperature of the feed water will increase.
If the water is not yet warm enough, the master controller will ask for an even lower level. This entire cycle is repeated until we reach the temperature set point. Eventually, there will be a zero temperature error and a zero level error and everything will be nice and stable.
Application of Cascade control:
Lots of cascade control applications usually found at:
- Arranging fuel (fuel oil) in the furnace.
- Control the flow rate of hot and cold water on the Heat Exchanger. If excessive heat or hot fluid output temperature is too low, then the cascade control will prevent it.
- Boiler temperature setting steam flow, and boiler drum level setting feedwater flow.
Advantages of Cascade control:
- Isolation of load disturbances,
- Improved speed of response,
- Precise manipulation of mass or energy,
- Compensation for nonlinearities
