---
title: "Define control system and closed loop block roles"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:define-cs-bd-role"
source: https://www.bctengineer.com/q/semester_3/control_systems/define-cs-bd-role
site: BCT Engineering
---

# Define control system and closed loop block roles

Control Systems · Chapter 1: Control System Background

- Asked in: 2079 Bhadra, 1+4 marks
- Video: [Feedback Control Loop Block Diagram](https://www.youtube.com/watch?v=KD58uQcroIE) — LearnChemE
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/control_systems/define-cs-bd-role
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

Define control system. Draw the block diagram of closed loop control system and explain the role of each block briefly.

## Short answer

A control system regulates output to a desired value. In a closed-loop system, the reference input sets the target; the error detector compares it with feedback; the controller generates a control signal; the actuator drives the plant; the sensor measures output for feedback; disturbances are unwanted inputs.

## Answer

A **control system** is an arrangement of physical components connected so as to regulate or direct a system to obtain a desired output.

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

#### Role of each block
1. **Reference input $r$ (set point):** desired value of the output.
2. **Error detector / summing point:** compares $r$ with feedback $b$ and gives error $e = r - b$.
3. **Controller:** acts on the error (P, PI, PID) and produces control signal $u$.
4. **Actuator:** power element (motor, valve, amplifier) that applies $u$ to the plant.
5. **Plant / process:** the system being controlled; gives the **controlled output $c$**.
6. **Disturbance $d$:** unwanted input acting on the plant (load change, noise).
7. **Feedback element / sensor:** measures $c$ and converts it to $b$, comparable with $r$.

Overall: $\dfrac{C(s)}{R(s)} = \dfrac{G(s)}{1+G(s)H(s)}$.

## Check yourself

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

1. A control system is
   A. an arrangement of components that regulates or directs a system to obtain a desired output
   B. a device that stores energy
   C. a circuit that always has feedback
   D. an amplifier of the reference signal
   Answer: A. Its purpose is to obtain the desired output. (Key point: Definition)

2. The disturbance d in the block diagram is
   A. the desired output value
   B. an unwanted input acting on the plant, such as load change or noise
   C. the signal from the sensor
   D. the control signal u
   Answer: B. Disturbances are unwanted inputs that disturb the output. (Key point: Disturbance)

3. Overall transfer function C/R with forward path G and feedback H:
   A. G/(1-GH)
   B. 1/(1+GH)
   C. G/(1+GH)
   D. GH/(1+GH)
   Answer: C. Negative feedback closed-loop transfer function. (Key point: Overall TF)

## Close variants set in other exams

- [Open and closed loop systems with block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Aol-cl-bd-role) — Control Systems, chapter 1, asked 2×
- [Closed loop control system block diagram explained](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-bd-closed-loop) — Control Systems, chapter 1, asked 4×

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