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Feedback Control System

The basic elements of a feedback control system are represented by the block diagram below. The functional relationships between these elements are also shown in the diagram.

Feedback Control System block diagram
Feedback Control System block diagram

The block diagram represents flow paths of control signals but do not represent the flow of energy through the system or process.

The plant is the system or process through which a particular quantity or condition is controlled. This is also called the controlled system.

The control elements are components needed to generate the appropriate control signal applied to the plant. These elements are also called the controller.

Related: Principles of Control Systems

The feedback elements are components needed to identify the functional relationship between the feedback signal and the controlled output.

The reference point is an external signal applied to the summing point of the control system to cause the plant to produce a specified action. This signal represents the desired value of a controlled variable and is also called the ‘’setpoint’’.

The controlled output is the quantity or condition of the plant is the quantity or condition of the plant which is controlled. This signal represents the controlled variable.

The feedback signal is a function of the output signal. It is sent to the summing point and algebraically added to the reference input signal to obtain the actuating signal.

The actuating signal represents the control action of the control loop and is equal to the algebraic sum of the reference input signal and the feedback signal. This is called the ‘’error signal’’.

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The manipulated variable is the variable of the process acted upon to maintain the plant output (controlled output) at the desired value.

The disturbance is an undesirable input signal that upsets the value of the controlled output of the plant.

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8 responses to “Feedback Control System”

  1. […] delays can cause problem for systems that rely on feedback or those that must react or adapt to their environment. By reducing the distance between the sensor […]

  2. […] time-driven sequence is open loop because there is no feedback while an event-driven task is closed loop because feedback signal is required to specify when the […]

  3. […] the voltage is varied automatically to produce the desired motion. This is where closed loop or feedback control comes in play. This requires a torque sensor to feed back the output values to continuously compare […]

  4. […] The requirement is to control the temperature ϴ in the tank. The voltage r, obtained from a potentiometer, is calibrated in terms of the desired temperature ϴc. This voltage represents the input quantity to the feedback control system. The actual temperature ϴ, the output quantity, is measured by means of a thermocouple immersed in the tank. The voltage eth produced in the thermocouple is proportional to ϴ. The voltage eth is amplified to produce the voltage b, which is the feedback quantity. […]

  5. […] to a value that is required to give the steam quality desired. However in Figure 1, it is a simple feedback control loop; the steam quality is adjusted after the spray water has been added and therefore some of the […]

  6. […] position and motion modules enable PLCs to control stepper and servo motors in feedback loops, to measure and control rotation speeds and acceleration, and to control precision tools. These […]

  7. […] or piston until the correct stem position is achieved. A positioner can be considered as a simple closed loop/feedback control system where the valve’s stem position is the process variable (PV), the command signal to the […]

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