Closed loop control system block diagram explained

A control system is an arrangement of physical components connected so that it commands, directs or regulates itself or another system to get a desired output. A closed-loop control system measures the actual output, compares it with the desired value and uses the difference (error) to drive the system.

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

Function of each block

1. Reference input / set point $r$: the desired value of the output (e.g. 25 °C in a room). A reference-input element may convert it into a signal of the same type as the feedback.
2. Summing point (error detector, comparator): subtracts the feedback signal from the reference, $e = r - b$. Examples: a pair of potentiometers, a differential amplifier.
3. Controller: processes the error and produces the control signal $u$ (proportional, PI or PID action). It decides how strongly and how fast to correct.
4. Actuator / final control element: converts the low-power control signal into a high-power action on the plant, e.g. a motor, valve, servo or heater contactor. (It is often shown inside the controller or plant block.)
5. Plant (process): the system being controlled, e.g. a furnace, a DC motor or a water tank. Its output is the controlled variable $c$.
6. Disturbance $d$: an unwanted input (load change, ambient temperature, noise) that affects the output.
7. Feedback element / sensor: measures the output and converts it to a signal $b$ that can be compared with $r$ (thermocouple, tachogenerator, potentiometer).

Working

If the output drifts from the set point, the error becomes non-zero, the controller and actuator act to reduce it, and the loop keeps repeating until $c \approx r$. For a negative-feedback system

$$\frac{C(s)}{R(s)} = \frac{G(s)}{1+G(s)H(s)}$$

Example: room temperature control. The thermostat setting is $r$; a temperature sensor measures the room temperature and gives $b$; the comparator finds the error; the controller switches the heater or compressor (actuator); the room is the plant.