---
title: "Open and closed loop systems with block diagram"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:ol-cl-bd-role"
source: https://www.bctengineer.com/q/semester_3/control_systems/ol-cl-bd-role
site: BCT Engineering
---

# Open and closed loop systems with block diagram

Control Systems · Chapter 1: Control System Background

- Asked in: 2066 Bhadra (old course), 8 marks
- Video: [What is Control System.Control System Engineering.Open Loop and Closed Loop Control System.Explained](https://www.youtube.com/watch?v=DtV0ASunhqU) — Instrumentation Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/control_systems/ol-cl-bd-role
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

What are open loop and closed loop control systems? Draw the block diagram of closed loop control system and explain the role of each block.

## Short answer

Open-loop systems have no feedback; control action is independent of output. Closed-loop systems measure output, compare with reference, and use the error to drive the plant. Components: reference input, error detector, controller, actuator, plant, feedback sensor, and disturbance input. Feedback gives accuracy and disturbance rejection.

## Answer

#### Open-loop control system
A system in which the **control action does not depend on the output**. The output is neither measured nor fed back.

```text
 r   +------------+  u   +-------+   c
---->| Controller |----->| Plant |------>
     +------------+      +-------+
```

Examples: electric toaster with timer, washing machine on timer, fixed-time traffic lights. They are simple and cheap but cannot correct errors caused by disturbances or parameter changes.

#### Closed-loop control system
A system in which the **output is measured and fed back** to the input, and the control action depends on the error between the desired and actual output. Examples: room air conditioner with thermostat, motor speed control with tachometer, voltage regulator.

```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 $r$.

The overall transfer function is $\dfrac{C(s)}{R(s)} = \dfrac{G(s)}{1+G(s)H(s)}$, where $G$ is the forward path (controller, actuator, plant) and $H$ the feedback element.

#### Comparison
| Point | Open loop | Closed loop |
|---|---|---|
| Feedback | absent | present |
| Accuracy | depends on calibration | high |
| Disturbance effect | fully at output | reduced by $1+GH$ |
| Stability | generally stable | may become unstable |
| Cost, complexity | low | higher |
| Example | toaster | air conditioner |

## Check yourself

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

1. In an open-loop control system, the control action
   A. does not depend on the output
   B. depends on the error between reference and output
   C. is based on a sensor measuring the output
   D. is always zero
   Answer: A. Open-loop output is neither measured nor fed back. (Key point: Open-loop definition)

2. Which pair is correctly classified?
   A. Toaster with timer is closed loop; air conditioner with thermostat is open loop
   B. Toaster with timer is open loop; air conditioner with thermostat is closed loop
   C. Both are open loop
   D. Both are closed loop
   Answer: B. A timer ignores the actual output, whereas a thermostat senses temperature and feeds it back. (Key point: Examples)

3. The role of the feedback element/sensor is to
   A. apply power to the plant
   B. produce the disturbance
   C. measure the output c and convert it to b, comparable with r
   D. generate the reference input
   Answer: C. The sensor gives the feedback signal b used at the summing point. (Key point: Role of sensor)

4. Compared with open loop, closed loop has
   A. lower cost and no chance of instability
   B. accuracy that depends on calibration only
   C. full disturbance effect at the output
   D. higher accuracy and disturbance reduced by 1+GH, but may become unstable
   Answer: D. Feedback improves accuracy and disturbance rejection but costs more and can destabilize. (Key point: Comparison)

## 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
- [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×
- [Traffic light system of Kathmandu: open or closed loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atraffic-light-justify) — Control Systems, chapter 1
- [LTI system and closed loop in modern systems](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Alti-define-cl-pronounced) — Control Systems, chapter 1
- [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
- [Why closed loop response is faster than open loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Acl-faster-than-ol) — Control Systems, chapter 1, asked 4×

## Related in other subjects

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- [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
- [Open loop and closed loop V/f inverter-fed drive](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aclosed-loop-vf-inverter-drive) — Utilization of Electrical Energy, chapter 3
- [Open loop vs closed loop instrumentation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aopen-closed-loop) — Instrumentation II, chapter 1, asked 2×