---
title: "General block diagram of a control system"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:general-bd-control-system"
source: https://www.bctengineer.com/q/semester_3/control_systems/general-bd-control-system
site: BCT Engineering
---

# General block diagram of a control system

Control Systems · Chapter 1: Control System Background

- Asked in: 2071 Chaitra, 4 marks
- Video: [General Block Diagram of Closed loop control system Explained](https://www.youtube.com/watch?v=5sU6s3gK_nE) — Vishnu Online Lectures
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/control_systems/general-bd-control-system
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

Construct a general block diagram of a control system showing the different blocks, variables and hence briefly point out their meaning.

## Short answer

A general closed-loop system has reference input, summing point for error e=r-b, controller/actuator producing manipulated variable u, plant yielding output c, and feedback sensor H. The closed-loop transfer function is C/R = G/(1+GH).

## Answer

A general (closed-loop) control system is shown below with its main blocks and signals.

```text
                                disturbance d
                                      |
 v  +-------+ r  +    e  +-------+ u  v  +-------+  c
 -->|Ref.   |--->(Σ)---->|Contr- |--->+->| Plant |--+-->
    |input  |     ^ -    |oller +|       |       |  |
    |element|     |      |actua- |       +-------+  |
    +-------+     |  b   |tor    |                  |
                  |      +-------+                  |
                  +------[Feedback element H]<------+
```

#### Blocks
- **Reference input element:** converts the command $v$ (e.g. a knob setting) into a reference signal $r$ of the same kind as the feedback.
- **Summing point (error detector):** forms $e = r - b$.
- **Controller (with actuator):** converts the error into the manipulated variable $u$ (P, PI, PID action; motor, valve, amplifier).
- **Plant / process:** the system to be controlled.
- **Feedback element (sensor):** measures $c$ and gives $b = Hc$.

#### Variables
| Symbol | Name | Meaning |
|---|---|---|
| $v$ | command input | desired value set by the user |
| $r$ | reference input | command in signal form |
| $b$ | feedback signal | measured output |
| $e$ | actuating (error) signal | $r - b$ |
| $u$ | manipulated variable | action applied to plant |
| $d$ | disturbance | unwanted input |
| $c$ | controlled variable | actual output |

The relation is $\dfrac{C(s)}{R(s)} = \dfrac{G(s)}{1+G(s)H(s)}$.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the reference input element do?
   A. Converts command v into a reference signal r of the same kind as the feedback
   B. Measures output
   C. Produces disturbance
   D. Amplifies power
   Answer: A. It translates the knob setting into a comparable signal. (Key point: Reference input element)

2. The feedback signal b is given by
   A. b = Hc
   B. b = r - e
   C. b = u/G
   D. b = d
   Answer: A. The feedback element measures c and gives b = Hc. (Key point: Feedback signal)

3. The actuating (error) signal is
   A. r + b
   B. r - b
   C. c - u
   D. d - c
   Answer: B. e = r - b. (Key point: Error signal)

4. The manipulated variable u is
   A. The unwanted input
   B. The action applied to the plant by the controller
   C. The sensor output
   D. The set point
   Answer: B. The controller converts error into u. (Key point: Manipulated variable)

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