Control system importance and closed loop components

A control system is a set of components connected together so that the output of a system (temperature, speed, voltage, position) is kept at, or made to follow, a desired value.

Importance

• Automation of industries, homes and transport (lifts, washing machines, CNC).
• Accuracy and quality of products and processes.
• Disturbance rejection and steady operation under changing loads, e.g. a hydropower governor holds 50 Hz.
• Safety (boiler pressure control, aircraft autopilot) and energy saving.
• Makes unstable systems usable (rockets, fighter aircraft).

Components of a closed-loop system

                       disturbance d
                            |
 r   +----+  e   +------+ u +---+---+   c
---->| Σ  |----->|Contr-|-->| Plant |----+---->
     +----+      | oller|   +-------+    |
      - ^        +------+                |
        |  b    +-----------------+      |
        +-------| Feedback/sensor |<-----+
                +-----------------+

1. Reference input (set point): desired value of output.
2. Error detector (comparator): forms error $e = r - b$, e.g. differential amplifier, potentiometer pair.
3. Controller: decides the corrective action from the error (P, PI, PID, microcontroller).
4. Actuator: converts controller output to power/motion, e.g. motor, valve, relay.
5. Plant/process: the system controlled, giving output $c$.
6. Feedback element/sensor: measures output, e.g. thermocouple, tachogenerator, encoder.
7. Disturbance: unwanted input acting on the plant.

Example: in a room heater, the thermostat setting is the reference, the bimetal strip is both sensor and comparator, the contacts act as controller, the heating coil is the actuator, and the room is the plant.