What is control loop?
A Control loop is the fundamental building block of the industrial control system or industrial automation. It is a group of components working together as a system to achieve and maintain the desired value of a system variable by manipulating the value of another variable in the control loop.
An instrumentation control loop consists of a controller that can adjust the process variable equal to setpoint by measuring the current process variable using sensors.
Components of a Control loop:
There are different types of control loop components combinedly work for the common desire of the system or to attain the setpoint. They are:
- Primary element/sensor
- Correcting element/final control element
Sensors are the first element in the control loop which measures the change in the process and reporting the process variable so they are also called as the primary element. Sensors are devices which cause change when affected by a change in the process variable. There are different types of sensors for measuring variables like Pressure, Temperature, Flow, Level, pH, Vibration etc.,
There are different types of sensors available for various process variable:
- Thermocouples, RTD for Temperature measurement
- Strain gauge, Pressure sensing diaphragm, capacitive cells for pressure measurement
- Orifice plate, Pitot tube, Magnetic flow tube etc., for flow measurement
There are so many other sensors used to measure different variables like vibration, pH, force, weight etc..,
A transducer converts any form of energy into another form. In electrical instrumentation field Transducers are devices which converts a physical variable into electrical signals. Another name transducers are Pick-ups.
In process control, a converter used to convert a 4–20 mA current signal into a 3–15 psi pneumatic signal is called a current-to-pressure converter. There are different types of transducer classified based on their working principle.
Transmitters are devices that convert the signal into a standard signal that can be transmittable through the control loop and the parameters can be monitored remotely.
- Pressure transmitters
- Flow transmitters
- Temperature transmitters
- Level transmitters
- Analytic transmitters
Signals are used to transmit process variable from transmitter to the controller and sent back the feedback signal from the controller to the final control elements. There are three types of signal in industrial automation:
- Pneumatic signal: Air pressure in the pneumatic pipeline change according to the change in the process variable. The standard pneumatic pressure in the signal pipe in industries are in a range of 3-15psi.
- Analog signal: Analog signals are mostly used control signals, the transmitter sends the signal through a set of electrical wire. The standard signal range is within 4-20mA, for LRV valve a 4mA signal is produced and for URV it is 20mA.Other common standard electrical signals include the 1–5 V (volts)
signal and the pulse output.
- Digital signal: Digital signals are special protocols used for communication in industries. All protocol are owned by specific companies, they include Fieldbus foundation, Modbus from Modicon, Profibus, DeviceNet from Rockwell automation.
An indicator is human readable devices that display the process variable. There are analog indicators such as used in pressure, temperature gauges and there are digital indicators that display process variables as the digits. Even though the process varible is connected to the controller, the indicators are used industries for different purposes.
Recorders are used in industries to provide history on the process and to be submitted to regulatory agencies for verification. By recording the readings of critical measurement points and comparing those readings over time with the results of the process, the process can be improved.
Controllers are the centre of process control, which receives process variable then compare with setpoint stored in the controller and sends a feedback as the controller output to control the final control element. There are pneumatic and electronic or programmable such as DCS, PLC uses a complex mathematical algorithm to perform the control action.
PLC (Programmable Logic Controller): PLCs are usually computers connected to a set of input/output (I/O) devices. The computers are programmed to respond to inputs by sending outputs to maintain all processes at setpoint.
DCS (Distributed Control Signal): DCSs are controllers that, in addition to performing control functions, provide readings of the status of the process, maintain databases and advanced man-machine-interface.
Final control element:
Final control elements are the correcting elements that receives signal from the controller and make a change in process to adjust the process variable at the desired parameter. In any control loop, the speed with which a final control element reacts to correct a variable that is out of setpoint is very important. Many of the technological improvements in final control elements are related to improving their response time.
Pumbs and Control valves – Final control devices of Flow control system
Heaters and Boilers – Final control devices of temperature control system
Compressor and Valves: Final control devices of pressure control system
An actuator is the most important part of the final control element, a device that causes physical change in the final control element. For a valve actuator is the valve stem actuator and for a heater, it is the heating coil. An actuator can be controlled by pneumatically, Hydraulically, Electrically.