Closed-loop control – Feedback, Feedforward, Cascade, Split Range, and Ratio control

Feedback control:

Feedback control is a control system in which the control action depending on process output. This type of control system measures process variables on the process output.

Every time there is a measurable change in the process outlet, due to the effects of disturbances (load) the feedback control system reacts to provide corrective action to eliminate errors.

So the feedback control system will react after the effects of disturbances perceived in the process output. The above figure is an example of the type of feedback control system in its application, namely furnaces in the industry process.

Feedforward control

Unlike feedback configuration, feedforward control does not wait for effect disturbances perceived by the process, otherwise, it will act before disturbances affect the system to anticipate the effects that will be caused by it.

In feedforward control, as shown in the above figure, every time there is a change on the feed inlet, it will move the controller to regulate fuel oil so that the feed flow will be proportional to the fuel oil flow (creating energy balances). Thus the effect caused by the change in feed is not felt at the process output (output temperature).

The weakness of feedforward control in the above application is when there is a disturbance, on the fuel oil, the controller cannot feel the change so that an error occurred in the process output (output temperature).

Cascade Control

In the above figure, Closed-loop feedback control illustrates the control of feed outlet temperature in a furnace. Load-on loop is depicted as a change in feed flow at the inlet.

When the feed suddenly increases, then the heat energy of the fuel oil working in the furnace will not be sufficient. As a result, the feed outlet temperature will drop and the new controller read the error, then use it as the basis for calculating increase fuel oil flow.

However, this system does not consider the load or other disturbance on the system, namely a decrease in fuel oil pressure. In this system, the temperature control will not immediately notice a change in fuel oil pressure before the feed temperature of the outlet really went down. To overcome these problems, the control system is perfected by adding a pressure controller between the temperature controller and a control valve as shown in the figure.

As shown in the figure above, the manipulated variable (MV) of the temperature controller TIC (which is called primary or master) becomes the set point for the pressure controller (which is called secondary or slave).

The application of cascade control can be destructive if process elements are the primary loop is faster than the processing elements in the secondary loop, because the system will tend to oscillate due to the interaction between the primary loop and secondary loops.

Therefore, the cascade control system can only be applied to a process with a primary element that is much slower than its secondary element.

Split Range Control

In the split-range control configuration, it has only one measurement (PV) and more than one manipulated variable (MV). Control of one process variable is done by coordinating several manipulated variables which all have the same effect on the process variables.

The above figure illustrates the application of split-range control in industrial processes. This configuration can provide additional security and optimization operational if needed.

For example, a feed will be heated in a furnace using fuel. Feed temperature at vessel outlet maintained at a certain temperature. There are two types of fuel available that is, fuel oil is the main fuel, and fuel gas is used as the main fuel balance. A split-range temperature controller will maintain outlet temperature by manipulating valve openings on both fuels.

By configuration this control action can be set as follows:

As the TIC controller output increases from 0 – 50 %, so control valve V1 (for fuel oil) will open continuously until the opening is full, while the control valve V2 (for fuel gas) remains closed.

If the TIC controller output still increases, from 50 – 100%, then control valve V2 will open continuously until it is fully opened, while valve V1 is still fully open.

Thus fuel gas is used as a balance if fuel oil is still not enough to increase output temperature.

The following table illustrates how the Split Range Control controller works.

Output Controller (TIC) Control Valve V1 Control Valve V2  
0 – 50% (50 – 0%)   Open (close) continuously until maximum (minimum)   Closed  
50 – 100% (100 – 50%)   Open   Open (close) continuously until maximum (minimum)  

Ratio Control

Ratio control is a control system used in a process that requires a mixture composed of two or more components with a certain comparison.

Ratio control is also a special type of feedforward control with two disturbances (loads) are measured and kept at a constant ratio of one to each other.

Usually, this control configuration is used to control the comparison. The flow rate of the two streams. One of the fast streams uncontrolled streams is usually referred to as a wild stream. The following is wrong an example of a process control system that uses a ratio .configuration control.

The two flow rates are measured and through the divider, the ratio of the two are counted. The results of this comparison are then compared with the comparison desired (desired ratio as a set point), and the error between comparisons which is measured by the setpoint produces an actuation signal as a ratio controller.

Author : PSS Bapu Rao

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