---
title: "Closed loop control system block diagram explained"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:what-is-cs-bd-closed-loop"
source: https://www.bctengineer.com/q/semester_3/control_systems/what-is-cs-bd-closed-loop
site: BCT Engineering
---

# Closed loop control system block diagram explained

Control Systems · Chapter 1: Control System Background

- Asked in: 2076 Asoj, 6 marks; 2074 Chaitra, 5 marks; 2080 Baisakh, 5 marks (asked 3 times)
- Video: [Closed Loop System Explained: Block Diagram, Applications, Pros, and Cons](https://www.youtube.com/watch?v=RKajJKFEooE) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/control_systems/what-is-cs-bd-closed-loop
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

What is control system? Draw the block diagram of a closed loop control system and briefly explain the function of each block.

## Short answer

A control system is an arrangement of components that commands or regulates a system to obtain a desired output. In a closed-loop system, the reference input is compared with the measured output via a summing junction; the error signal drives a controller and actuator to adjust the plant, and a feedback sensor provides the measurement. Essential blocks include reference input, summing point, controller, actuator, plant, and feedback element.

## Answer

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.

```text
                       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.

## Check yourself

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

1. What makes a control system closed loop?
   A. It uses a timer to set the output
   B. The output is measured and compared with the desired value, and the error drives the system
   C. It has no controller
   D. It does not respond to disturbances
   Answer: B. A closed-loop system measures the actual output, compares it with the reference and uses the error to act. (Key point: Closed-loop principle)

2. What does the summing point (error detector) compute in the closed-loop block diagram?
   A. e = r + b
   B. e = r - b
   C. e = r x b
   D. e = c - d
   Answer: B. With negative feedback the comparator subtracts the feedback signal b from the reference r. (Key point: Summing point)

3. What is the function of the actuator (final control element)?
   A. Measure the output with a sensor
   B. Compute the error
   C. Convert the low-power control signal into a high-power action on the plant
   D. Generate the disturbance
   Answer: C. Actuators such as motors and valves turn the controller's small signal into a powerful action on the plant. (Key point: Role of actuator)

4. For a negative-feedback system the overall transfer function C(s)/R(s) is
   A. G(s)H(s)/(1+G(s))
   B. G(s)/(1-G(s)H(s))
   C. G(s)/(1+G(s)H(s))
   D. 1/(1+G(s)H(s))
   Answer: C. Negative feedback gives C/R = G/(1+GH). (Key point: Closed-loop transfer function)

## Close variants set in other exams

- [Define control system and closed loop block roles](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adefine-cs-bd-role) — Control Systems, chapter 1, asked 5×

## Related in Control Systems

- [Control system block diagram and closed loop advantages](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-bd-cl-advantages) — Control Systems, chapter 1
- [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×
- [Control system importance and closed loop components](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adefine-cs-importance-components) — Control Systems, chapter 1
- [General block diagram of a control system](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Ageneral-bd-control-system) — Control Systems, chapter 1
- [Block diagram reduction: G1–G4, H1, H2, unity loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Abdr-g1-g4-h1h2-2075a) — Control Systems, chapter 3, asked 4×
- [Block diagram reduction with positive H2 loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Abdr-g1-g4-h1-h2pos-2079b) — Control Systems, chapter 3, asked 3×

## Related in other subjects

- [Steps to design closed loop control system](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-design-steps) — Embedded System, chapter 7
- [Closed loop speed control design conditions](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-speed-control-conditions) — Embedded System, chapter 7
- [Closed loop diagram; P, PI, PD, PID](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-block-diagram-pid-types) — Embedded System, chapter 7
- [Digital control system block diagram and advantages](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adcs-block-diagram-advantages) — Digital Control System, chapter 1, asked 3×