Basics of Feedback Control – Elements of Feedback control

Feedback Control

Feedback control is the collection and comparison of data on the controlled variable to the set level ; a choice dependent on that contrast ; and intervention to eliminate any distinctions between the controlled variable and the set point

There is a steady stream of data within a closed loop in feedback control. All the tools in the circuit to regulate a system are called the feedback control system.

Elements of the Feedback control loop

In the feedback control circuit, instruments are used to: track and evaluate system parameters, transmit data, determine the intervention required, trigger intervention to occur, or take action. In a feedback control loop, the tools used are mentioned below:

The Sensor or Transducer	Monitors and measures the process parameters you want to control, which is the controlled variable For example, a thermocouple used in a heating process to sense the temperature in a vessel
The Transmitter	Relays information about the controlled variable (sensed by the sensing instruments) to a controlling instrument or other instruments in the loop Using the temperature example, the thermocouple output is picked up by the transmitter which sends a signal to the controller. Before the signal can be sent, the transmitter first transforms the sensed value into  a signal which can be transforms this into a 4-20 milliampere electric signal which the controller can receive.
The Controller	Determines needed action by comparing information received from the transmitter to the set point (the desired value of the controlled variable which is entered in the controller). If there is a difference between the set point and the actual value of the controlled variable (the deviation), the controller sends a signal to the final control element. For example, if the controller determines that the temperature in a vessel is too low. The controller sends a signal to the final control element to increase heat to the vessel.
Final Control element	Takes action to adjust the manipulated variable in the process. This adjustment drives the value of the controlled variable back toward the set point The final control element in our temperature example could be a gas valve which opens to allow more gas (manipulated variable) in the dryer to increase the temperature (controlled variable)

Other Instruments that support a control loop

There are other tools that help the feedback control loop in relation to detectors, transmitters and controllers and ultimate control components. These are tools that provide the user with data or help in process control. Typical instances of these tools are:

Recorders	Recorders keep a continuous record of the value of the controlled variable so that operators, instrument  technicians, and managers can evaluate the process over a period of time
Indicators	These are lights and displays which graphically show a value of the controlled variable so that  operators and instrument technicians can see at a glance what is occurring in the process.
Alarms	Alarms tell the operators and instrument technicians when the value of the controlled variable is out of limits. They alert operators and technicians to problems or potential problems.
Interlocks	Interlocks are devices which will not allow one part to function unless another part is functioning. Interlocks are frequently used as safety measures or to protect equipment