---
title: "Why closed loop preferred despite cost"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:cl-preferred-despite-cost"
source: https://www.bctengineer.com/q/semester_3/control_systems/cl-preferred-despite-cost
site: BCT Engineering
---

# Why closed loop preferred despite cost

Control Systems · Chapter 1: Control System Background

- Asked in: 2075 Asoj, 4 marks
- Video: [Feedback Control Systems | Understanding Control Systems, Part 2](https://www.youtube.com/watch?v=5NVjIIi9fkY) — MATLAB
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/control_systems/cl-preferred-despite-cost
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

In spite of cost and complicated design, closed loop control systems are widely preferred over open loop control system. Justify the statement with some examples.

## Short answer

Closed-loop systems are preferred because they provide accuracy, disturbance rejection, faster response, and can use cheaper components. Feedback compensates for plant variations and stabilizes unstable systems. Examples include hydropower governors, air conditioners, and autopilots, where open-loop would fail under changing conditions.

## Answer

Closed-loop systems need sensors, comparators and more careful design, so they cost more and can become unstable. Still they are widely preferred because their performance advantages are much larger than these costs.

#### Justification
1. **Accuracy:** the output is forced to follow the reference; sensitivity to plant changes is $1/(1+GH)$ instead of 1.
2. **Disturbance rejection:** load changes and noise are corrected; their effect is reduced by $(1+GH)$.
3. **Faster response:** time constant $\tau \to \tau/(1+K)$.
4. **Can use cheaper, less precise plant components:** feedback compensates for their tolerance and ageing, which often saves overall cost.
5. **Stabilizes unstable systems** and allows full automation with less human labour.
6. **Cheap electronics today:** sensors and microcontrollers are inexpensive, so the extra cost is small.

#### Examples
| System | Why feedback is needed |
|---|---|
| Hydropower governor | holds 50 Hz under changing load |
| Air conditioner / refrigerator | holds temperature despite weather and door opening |
| Automatic voltage regulator | keeps voltage constant with load |
| Aircraft autopilot, missile | must track path despite wind |
| Cruise control in cars | holds speed on slopes |
| Human body temperature | biological feedback |

In each case open-loop control would give large errors whenever load or environment changes, so the extra cost of feedback is justified.

## Check yourself

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

1. What are the drawbacks of closed-loop systems?
   A. Sensors needed, higher cost, possible instability
   B. Lower accuracy
   C. Slower response
   D. No disturbance rejection
   Answer: A. They need sensors and careful design and can become unstable. (Key point: Costs of closed loop)

2. How can feedback reduce overall cost?
   A. Allowing cheaper, less precise plant components because feedback compensates for tolerance and ageing
   B. Removing the plant
   C. Using only expensive sensors
   D. Avoiding controllers
   Answer: A. Feedback masks component tolerance and ageing. (Key point: Cheaper components)

3. Which is a closed-loop example from the answer?
   A. Cruise control in cars
   B. Fixed timer for traffic lights
   C. A fan without thermostat
   D. Washing machine timer
   Answer: A. Cruise control measures speed and holds it on slopes. (Key point: Example)

4. Sensitivity of closed-loop output to plant changes is
   A. 1
   B. 1/(1+GH)
   C. 1+GH
   D. 0 always
   Answer: B. Feedback reduces it from 1 to 1/(1+GH). (Key point: Sensitivity benefit)

## Related in Control Systems

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- [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×
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- [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×
- [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 other subjects

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- [Open loop versus closed loop control](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-vs-closed-loop) — Embedded System, chapter 7, asked 6×
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- [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