In an industrial plant, the operation is carried out by controlling a process, a process is the conversion of raw materials into a finished product. So in order to control the process, a certain control strategy is used so that the plant can produce good quality products and there could be safety and less maintenance due to this control. There are many types of processes that can be seen in an industry and it could be continuous batch or semi-batch. By controlling a process we can maintain proper condition in a system by varying the input variables of the system.
- What is process control?
- What is continuous process control and what are its types?
- What is computer process control?
- What are the final control elements in a control system?
- What are the types of instrumentation switches?
- What is industrial automation?
What are the types of control modes used to control the process?
There are many types of control strategy and the type of process control is selected according to the process. So a control strategy is selected that suites the process.
- ON-OFF control
- Feedback control
- Feed-forward control
- Cascade control
- Ratio control
- Inferential control
- Adaptive control
- Split range control
This is the simplest mode of control and this type of control is not widely used as the name indicates it is used for on or off control purposes. This type of control is also called a two-position control. The output of this type of control will be On or Off, this device would only switch when the process variables are beyond the limit. This can be used as a temperature control device so when the process variable crosses the temperature limit then this control would act and the temperature can be controlled. Mostly this type of process control mode is not suitable for precise control and one of the merits of this controller is that it is not expensive.
The major disadvantage of the ON/OFF controller is because of the mechanical friction or due to the arcing of the electrical contracts the ON/OFF controller could slightly go above or beyond the set point.
When should we use an on/off controller?
We should use an on/off controller by considering certain conditions like
- The reaction rate must be slow
- There shouldn’t be any dead time
- The transfer lag must be null
Feedback control is widely used in most automated systems, the major parts of a feedback control system are input, controlled process, sensing element, output, and final control element. Feedback is required to modify the control signal in case of any disturbance if the process variables deviate from the setpoint, so basically, feedback is required to get the proper output from a process. In order to get the proper output during the disturbances, we can use feedback control. There will be a feedback loop that would measure the process variable and then the controller would compare this variable to the desired value based on this certain changes will be made in the input section to get the proper output. Feedback control is used in a system if you need to control your system automatically.
There are many advantages while using feedback control.
- It can handle frictional and pressure losses
- It would help to achieve the desired process condition by changing the input as fast as possible in case of any variation
- Stabilization of the unstable process can be achieved
- This type of control system doesn’t need a detailed knowledge of the process in order to control a process
In this type of control, it would assume how a process will be affected by the disturbances and act before the disturbance could affect the process. So this type of control would require the process mathematical model in order to get a picture of how the disturbance would affect the process. The major function of this type of control is to determine the disturbance and also to take proper measures so that it won’t affect the process. A proper mathematical model of the process will be required to determine the disturbance but a clear mathematical model is not possible so the proper compensation of the disturbance won’t be possible. So due to this disadvantage, a feed-forward control is coupled with the feedback controller and because of this the process would carry out properly but the complexity and cost of the system would increase too.
Cascade control is used in a process where various disturbances could happen in a process. It is required in a process where a quick response is required and this response can’t be achieved by a single control loop. So a cascade controller would have two or more controllers. The cascade controller would have a master and a slave controller. The controlled variable deviation is sent to the master controller and instead of directly driving the final control element, the controller would generate the required setpoint of the manipulated variable and this will be provided by the second controller known as slave controller. The slave controller would drive the variable which is provided by the master controller to the setpoint by manipulating the final control element. The major advantage of cascade control is that quick recovery of the disturbance is possible. The final control element would need to control two process variables in this type of control.
This type of control is used in a process that needs a ratio of a certain amount to be mixed in a process an example of this will be the combination of the water and the acid. In this type of control, one stream will be controlled in a ratio to the other. This type of control can be used to control the feedrates so that stoichiometry can be maintained. This type of control is also used in some distillation column control, in order to set the boil-up ratios.
This type of control can be used for diluting water in acid, so this type of control system can control the ratio of the acid to the water supply. The controller would receive the input from a flow measurement device on the unregulated flow. The controller would do the ratio calculations and it would transfer the proper setpoint to another controller and this would allow the flow of another fluid.
Inferential control is a type of process control where the measurement of the primary variable is too hard and because of this, the second variable is measured with the help of a mathematical model. The primary control variable cannot be measured due to its large sampling period. This so in case of this it would estimate the controlled variable and the assumption or estimation is according to the available measurements.
It is a process control system that has auto-correction features, it can be considered as the merging of feedback control and optimal control. This type of control can be used where there will be a high variation in the system parameters and it can also be used in a process that is subjected to major external disturbances. The first step would determine the process performance and in the next step it would take an appropriate decision to improve the performance of the process after that, it would make changes in the system and this could be a physical modification.
Split range control
This type of control is used when more than one manipulated variable is required, the major advantage of this type of control is that it can handle over a wide range, and also these controls can handle more than one disturbance at a time. In this type of control, there will be one controlled output and the manipulated variable will be more than one. So there is only one controlled output and also one control signal and it would be split into two parts and each affects one of the available manipulations.