---
title: "Open loop versus closed loop control"
subject: "Embedded System"
chapter: 7
key: "semester_6/embedded_system:open-vs-closed-loop"
source: https://www.bctengineer.com/q/semester_6/embedded_system/open-vs-closed-loop
site: BCT Engineering
---

# Open loop versus closed loop control

Embedded System · Chapter 7: Control System

- Asked in: 2072 Asoj, 3 marks; 2075 Bhadra, 3 marks; 2076 Baisakh, 4 marks; 2078 Baisakh, 2 marks; 2078 Poush, 4 marks; 2078 Chaitra (asked 6 times)
- Video: [Open Loop Systems | Closed Loop System | Automotive | Difference | Embedded World](https://www.youtube.com/watch?v=g_vbIQNMREI) — Embedded World Automotive
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/embedded_system/open-vs-closed-loop
- Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-7

## Question

Differentiate between open-loop and closed-loop control systems. Which system is better for control system?

## Short answer

Open-loop lacks feedback; accuracy relies on model calibration and disturbance rejection is poor. Closed-loop measures output and corrects errors via feedback, offering high accuracy and disturbance rejection. Closed-loop is superior for most control applications.

## Answer

An **open-loop** system applies a control action computed only from the input (reference); it does not measure the output. A **closed-loop** (feedback) system measures the output, compares it with the reference and uses the error to correct the control action.

#### Block diagrams

```text
Open loop:
 r --> [Controller] --u--> [Plant] --> y
                              ^
                              d (disturbance)

Closed loop:
 r -->(+)-e->[Controller]-u->[Plant]--+--> y
       ^ -                      ^ d   |
       |                              |
       +------[Sensor / feedback]<----+
```

#### Differences

| Point | Open loop | Closed loop |
|---|---|---|
| Feedback | None | Output is fed back |
| Accuracy | Depends on a good model and calibration | High; error is corrected |
| Disturbance | Not rejected | Reduced by feedback |
| Parameter change | Output drifts | Largely compensated |
| Stability | Stable if the plant is stable | May oscillate or become unstable if gain is too high |
| Sensor | Not needed | Required |
| Cost, complexity | Low, simple | Higher |
| Examples | Toaster, washing-machine timer | Cruise control, thermostat, servo |

#### Which is better?

Closed loop is better for control. Real plants have disturbances (road slope, wind) and uncertain parameters, so an open-loop controller cannot guarantee the output. Feedback gives small steady-state error and good disturbance rejection, at the price of a sensor and careful design for stability. Open loop is acceptable only when the plant is predictable and disturbances are negligible.

## Check yourself

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

1. What is the defining difference between open-loop and closed-loop control?
   A. Closed loop has no controller
   B. Closed loop measures the output and uses the error to correct the control action
   C. Open loop uses a sensor but closed loop does not
   D. Open loop compares output with reference, closed loop does not
   Answer: B. Only the closed-loop system feeds the measured output back and corrects using the error. (Key point: Feedback)

2. Why can an open-loop cruise controller not hold speed on a hill?
   A. It never measures the output, so disturbances are not rejected
   B. Its gain is always too high
   C. It always oscillates on slopes
   D. It needs a larger engine
   Answer: A. With no feedback the controller cannot see the speed drop caused by the disturbance. (Key point: Disturbance)

3. Which risk is specific to closed-loop systems?
   A. They drift when parameters change
   B. They need no sensor
   C. They cannot reject disturbances
   D. They may oscillate or become unstable if the gain is too high
   Answer: D. Feedback with high gain can cause oscillation or instability; open loop is stable if the plant is stable. (Key point: Stability)

4. When is an open-loop controller acceptable?
   A. When high accuracy is required under varying load
   B. When the plant parameters change often
   C. When the plant is predictable and disturbances are negligible
   D. When disturbance rejection is essential
   Answer: C. Open loop is only acceptable for predictable plants with negligible disturbance; otherwise closed loop is better. (Key point: Which is better)

## Related in Embedded System

- [Closed loop better than open loop (cruise)](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-better-cruise-integral) — Embedded System, chapter 7
- [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 cruise control using P control](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-loop-cruise-p-control) — 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
- [Open-loop automobile cruise controller design](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-loop-cruise-controller) — Embedded System, chapter 7, asked 3×

## Related in other subjects

- [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×
- [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
- [Why closed loop rejects disturbances better](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adefine-cs-disturbance-rejection) — Control Systems, chapter 1
- [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