---
title: "Closed loop configuration, advantages and disadvantages"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:cl-config-adv-disadv"
source: https://www.bctengineer.com/q/semester_3/control_systems/cl-config-adv-disadv
site: BCT Engineering
---

# Closed loop configuration, advantages and disadvantages

Control Systems · Chapter 1: Control System Background

- Asked in: 2065 Shrawan (old course), 8 marks
- Video: [Open vs Closed Loop Systems in Control Engineering: A Detailed Comparison](https://www.youtube.com/watch?v=JGyk957Oyys) — 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/cl-config-adv-disadv
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

Draw the closed loop control system configuration showing major components. Discuss the advantages and disadvantages of closed loop systems.

## Short answer

Closed-loop configuration includes reference, error detector, controller, actuator, plant, sensor, and disturbance. Advantages: high accuracy, disturbance rejection, lower sensitivity to parameter changes, faster response. Disadvantages: more complexity, reduced overall gain, possible instability, reliance on sensor accuracy, and careful design needed.

## Answer

A closed-loop control system measures the output with a sensor, compares it with the desired value and uses the error to drive the plant.

```text
                       disturbance d
                            |
 r   +----+  e   +------+ u +---+---+   c
---->| Σ  |----->|Contr-|-->| Plant |----+---->
     +----+      | oller|   +-------+    |
      - ^        +------+                |
        |  b    +-----------------+      |
        +-------| Feedback/sensor |<-----+
                +-----------------+
```

#### Major components
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 output relation is $C(s) = \dfrac{G(s)}{1+G(s)H(s)}R(s)$.

#### Advantages
1. **High accuracy:** output follows the reference; with integral action the steady-state error to a step is zero.
2. **Low sensitivity to parameter changes:** $S_G^T = 1/(1+GH)$.
3. **Disturbance rejection:** load or noise effects are cut by $(1+GH)$.
4. **Faster response, wider bandwidth:** time constant $\tau/(1+K)$.
5. **Stabilizes unstable plants** and reduces the effect of non-linearities.
6. **Automatic operation** without human supervision.

#### Disadvantages
1. **More complex and costly:** sensors, comparators and controllers are needed.
2. **Reduced overall gain:** gain falls from $G$ to $G/(1+GH)$; extra amplification is needed.
3. **Possible instability:** high loop gain can cause oscillation or instability (e.g. $K/[s(s+1)(s+2)]$ is unstable for $K > 6$).
4. **Sensor noise and errors** enter the loop directly; output accuracy depends on the accuracy of the feedback element ($C/R \approx 1/H$).
5. More careful **design and maintenance** are required.

**Example:** the automatic voltage regulator of a generator senses terminal voltage and adjusts field current, holding voltage constant despite load changes, but needs careful tuning to avoid voltage oscillation.

## Check yourself

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

1. In the closed-loop configuration, the signal e from the summing point goes next to the
   A. controller
   B. sensor
   C. plant directly
   D. reference input
   Answer: A. Error e -> controller -> u -> actuator/plant -> output. (Key point: Block order)

2. Why does closed loop have lower overall gain than open loop?
   A. Gain rises to G(1+GH)
   B. Gain falls from G to G/(1+GH)
   C. Gain becomes GH
   D. Gain is unchanged but noise rises
   Answer: B. The factor 1+GH in the denominator reduces overall gain. (Key point: Disadvantage: gain)

3. A disadvantage of closed loop shown by K/[s(s+1)(s+2)] is that
   A. it is stable only for K > 6
   B. it is unstable for K < 6
   C. it becomes unstable for K > 6
   D. it can never oscillate
   Answer: C. High loop gain can cause oscillation; this system is unstable for K > 6. (Key point: Disadvantage: instability)

4. Output accuracy of a closed-loop system depends on the feedback element because
   A. C/R is approximately H
   B. C/R is approximately G
   C. the sensor does not affect C/R
   D. C/R is approximately 1/H for large loop gain
   Answer: D. With large GH, C/R = G/(1+GH) ~ 1/H, so errors in H appear directly in the output. (Key point: Sensor accuracy)

## Related in Control Systems

- [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
- [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
- [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×
- [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×
- [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
- [Why closed loop suits modern complex systems](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adefine-cs-cl-vs-ol-complex) — Control Systems, chapter 1, asked 2×

## 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
- [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×
- [Closed-loop control of multi-motor drive](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aclosed-loop-multi-motor-drive) — Utilization of Electrical Energy, chapter 2, asked 2×