---
title: "Why closed loop suits modern complex systems"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:define-cs-cl-vs-ol-complex"
source: https://www.bctengineer.com/q/semester_3/control_systems/define-cs-cl-vs-ol-complex
site: BCT Engineering
---

# Why closed loop suits modern complex systems

Control Systems · Chapter 1: Control System Background

- Asked in: 2080 Bhadra, 2+4 marks; 2076 Chaitra, 6 marks (asked 2 times)
- Video: [Understanding Control System](https://www.youtube.com/watch?v=2BwUMk10WqI) — Sabin Civil Engineering
- 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-cl-vs-ol-complex
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

Define control system. Why closed loop control system is more pronounced in modern complex systems rather than open loop control systems? Justify.

## Short answer

A control system is a combination of components arranged to regulate, command, or direct a system to obtain a desired output. Closed-loop control dominates complex systems because feedback reduces sensitivity to parameter changes, rejects disturbances, speeds up response, eliminates steady-state error with integral action, and can stabilize unstable plants, enabling high accuracy and automation.

## Answer

#### Control system
A **control system** is a combination of components arranged so that a desired output is obtained by regulating, commanding or directing the system's input, e.g. temperature control of an oven, speed control of a motor.

#### Why closed loop dominates modern complex systems
Modern systems (aircraft, robots, power grids, process plants) must be accurate and reliable in spite of changing loads, ageing parts and noise. Open-loop systems cannot correct themselves; closed-loop systems can, because they measure the output and act on the error.

1. **Accuracy despite parameter changes:** the sensitivity of the closed-loop gain to plant variation is

$$
S_G^T = \frac{1}{1+GH} \ll 1 \quad (\text{open loop: } S_G^T = 1)
$$

2. **Disturbance rejection:** a disturbance at the plant gives output $\dfrac{G_2}{1+G_1G_2H}D$ instead of $G_2D$, i.e. it is reduced by $(1+G_1G_2H)$.
3. **Faster response:** a plant $K/(\tau s+1)$ gets time constant $\tau/(1+K)$ in closed loop.
4. **Smaller steady-state error** and the ability to track a changing reference (e.g. a missile tracking a target).
5. **Stabilizing unstable plants:** an inverted pendulum, a fighter aircraft or a magnetic levitation system is unstable open loop and can only work with feedback.
6. **Automation:** it removes the human operator from the loop, which suits large, fast or remote systems.

The disadvantages (higher cost, need for sensors, possible instability if gain is too high) are small compared with these gains, and cheap sensors and microcontrollers have made feedback easy to add. Hence closed-loop control is the normal choice in modern complex systems.

## Check yourself

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

1. What is the sensitivity of closed-loop gain to plant variation compared with open loop?
   A. 1/(1+GH) closed loop versus 1 open loop
   B. 1 closed loop versus 1/(1+GH) open loop
   C. Both equal 1
   D. (1+GH) closed loop versus 1 open loop
   Answer: A. Open-loop sensitivity is 1, while feedback reduces it to 1/(1+GH), much less than 1. (Key point: Sensitivity to plant variation)

2. A disturbance at the plant output gives G2*D in open loop. In closed loop it becomes
   A. G2 D (1+G1G2H)
   B. G2 D/(1+G1G2H)
   C. D/G2
   D. G1 G2 D
   Answer: B. The disturbance effect is divided by (1+G1G2H). (Key point: Disturbance rejection)

3. Which type of system can only work with feedback?
   A. A washing-machine timer
   B. An unstable plant such as an inverted pendulum or magnetic levitation system
   C. A simple oven with fixed heating
   D. A traffic timer
   Answer: B. Unstable plants run away in open loop; feedback is needed to stabilize them. (Key point: Stabilizing unstable plants)

4. Why can an open-loop system not keep accuracy as loads and parts age?
   A. It has too many sensors
   B. It does not measure the output, so it cannot correct itself
   C. It always has a higher gain
   D. It is too fast
   Answer: B. Without measuring the output, no correction of errors is possible. (Key point: Open-loop limitation)

## Related in Control Systems

- [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
- [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×
- [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
- [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 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
- [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

- [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
- [Complex power and why VI* rather than V*I](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acomplex-power-vi-conjugate-why) — Power System Analysis I, chapter 3
- [T/F: more circuits rather than higher voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-circuits-rather-than-voltage) — Transmission and Distribution System Design, chapter 2
- [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×