Control System

What are the final control elements in a control system?

The final control element would make the required changes in the manipulated variables and this variable will satisfy the requirements. In a control system the selection of the proper control element and also the selection of control element, location is an important factor. We can describe the final control element as a component in a control system that would regulate the flow of energy to the process. The final control element would directly determine the manipulated variable value. Mostly the final control element would be a valve and its main purpose is to open and close as per the requirement of the controller. There are other final control elements that can be seen in control systems rather than the control valve and they are pump motors, solenoids, etc.

What does a final control element do in a control system?

The final control element is also known as the correcting part of the control system. This element would receive the input from the controller and this input will be converted into a proportional operation which will be performed on the process. In any type of control system, the final control element would correct a variable that is not close to the set point. As we discussed above mostly the final control element would be a valve or motor. The final control element can vary the controlled variable to the required value. This part in the control system implements the decision which is made by the controller.

What are the elements of the final control operation?

Signal conversion is done to the control signal, the control signal will be modified so that it can be transmitted to the next part which is the actuator. In order to do the signal, a conversion transducer is used because it can convert control signals from one form to the other. The signals could be current, pressure, temperature, voltage, etc. The final control element would operate according to the control signal and we must take care of the final control element properly because it does the final part in a control system. So if there is any problem with the final control element then it must be rectified quickly. The final control element would be operated by means of pneumatic, electric, hydraulic, or mechanical methods.

What are the types of final control elements which is used in a control system?

Control valve

A control valve would be the most common final control element in a process control industry. The control valve can control the flowing fluid and it could be gas, water, or chemicals. So by doing this the control valve could handle the load disturbance and also the process variable can be kept close to the set point. The control valve can control the elements in a process, the elements which are to be controlled by a control valve would be the capacity, range, and pressure of the process. So the control valve must be able to provide the proper quantity of the process fluid otherwise it could affect the process.  The above image shows the parts of a control valve in that we can see an actuator and the actuator would locate the valve plug to a position as per the control signal. There are certain types of actuators such as pneumatic, hydraulic, or electric. The positioner is another part of the control valve and it would position the actuator of the control valve as per the control signal. A positioner would get the input signal and it could be either electrically or pneumatically and it gives the required power to the actuator.

Solenoid valve is a type of control valve which can be seen in many process control applications. The flow can be turned on or off if it is a two-port valve, if it is a three-port valve then the outflow can be switched between the two outlet ports. The control valve is used in industries for many purposes like it can be used to control the process of water for batching, mixing, washing, blending, or also for on/off type purpose. 

SENSOR

We can see certain sensors as the final control element in a control system and they are pressure sensor, flow sensor, and temperature sensor. Mostly used pressure sensor would be the bourdon tube and its pressure range measurement would be according to the thickness of the valve and also according to the material of the tubing. Bellows and diaphragm sensors are other types of pressure sensors that would be used as a final control element. The major type of flow sensor which is used as the final control element is the orifice meter. The magnetic flow meter is also used to do the flow control. There are certain level sensors such as differential pressure, float, and air bubble sensor. By using a differential pressure sensor we can detect the pressure at the bottom of the liquid and also that above the liquid.

Many types of temperature sensors can also be seen in the final part of the control system and they are thermometers, thermistors, and RTD.

Metering pump

A metering pump can displace a certain amount of liquid at a proper time period so that it can do the accurate flow of the liquid. Sometimes metering can also be explained as providing fluids at the required flow rate. Most of the metering pumps can pump into high discharge pressure. These devices are produced to meter at flow rates which would be constant for a wide range of discharge pressure. The metering pump can be considered as the positive displacement chemical dosing device and it can vary the capacity manually or automatically as per the requirement of the process condition. This device has good repetitive accuracy and they are capable to pump certain chemicals including acids, bases, corrosive or viscous liquids, and also slurries. A reciprocating piston would do the pumping action and it could be in direct contact with the process fluid or a diaphragm would be used to shield it from the process fluid.  Mostly the metering pumps are used for the process that needs low flow rates, it is used in the process that has high system pressure. The metering pumps are also used in a process where a good flow rate is required and it is also used in the process which deals with corrosive or hazardous fluid. It is also used in the process where viscous fluid or slurries needs to be pumped.

There are different types of metering pump and each of them would have different temperature and pressure ratings so we must choose a pump which will be suitable for our applications. So a metering pump must be selected according to the pump material and also according to its operating temperature and pressure. A metering pump consists of a pump head and a motor, the liquid which is pumped would travel through the pump head. The motor which is used is an electric motor and it would drive the pump head. We can use a metering pump for various applications such as chemical processing, chemical processing, medical applications, oil, and gas, textiles, and a lot more.

Variable speed drive (VSD)

A variable speed drive is a device that can be used to control the speed of machinery. It is used in many industrial applications so that we can control the operating speed of certain equipment and this can be achieved by using a variable speed drive. We can use a variable speed drive to control the speed of the motor or compressor etc.

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