1. Introduction to
    control engineering

1.3 Principle of closed loop control

The basic principle of closed loop control is that the controlled variable x has to be kept at a certain value regardless of any disturbing influences. To do this, the controlled variable is measured and compared with the reference variable w (setpoint) in a control system.

If a control difference e occurs, the controller tries to influence the controlled system using the manipulated variable y in accordance with the transfer function so that the controlled variable again approaches the reference variable.

Related to the case of level control:

  1. The level sensor (2) measures the level in the tank (1) (controlled variable x).

  2. The current level (controlled variable x) is compared with the required reference variable (w).

  3. If the level is too low, the controller (3) uses the control valve (4) to increase the inflow (y).

  4. When the level (x) in the tank (1) reaches the required value, the inflow is reduced again.

  5. If the level is too high, the controller (3) uses a lower manipulating variable to reduce the inflow (y) until the desired level is reached.

  6. Faults can be generated by opening the drain valve (5), for example.

This control process runs continuously.

Measured variables over time

This video uses a car to show how you can simulate robustness to system variations.

The video models and simulates the car with variations such as different number of passengers. The goal is to maintain the speed of the car at a certain value. The video shows that system variations affect open-loop system behavior and open-loop control needs calibration each time system parameters vary. You will see how feedback control deals with system variations such as different number of passengers.