---
title: "Short note: Feedback control system"
subject: "Project Engineering"
chapter: 4
key: "semester_7/project_engineering:feedback-control-note"
source: https://www.bctengineer.com/q/semester_7/project_engineering/feedback-control-note
site: BCT Engineering
---

# Short note: Feedback control system

Project Engineering · Chapter 4: Project Implementation and Controlling

- Asked in: 2070 Chaitra (old course), 4 marks; 2068 Baisakh (old course), 4 marks (asked 2 times)
- Video: [Feedback Control Model](https://www.youtube.com/watch?v=0o5MG79rY-s) — Laura McHarrie
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/project_engineering/feedback-control-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/project_engineering/chapter-4

## Question

Write a short note on the feedback control system.

## Short answer

A feedback control system is a closed-loop system where actual output is measured and compared with a standard; any error triggers corrective action. Its elements are standard, process, sensor, comparator, and corrective action. It is continuous but corrects after deviation, so timely information is essential.

## Answer

A **feedback control system** is a closed-loop system in which the output (actual performance) is measured and fed back to be compared with the desired standard (plan). If there is a difference (error), corrective action is taken to bring the output back to the standard. The aim is self-regulation.

#### Elements
1. **Standard / plan (set point):** the target for time, cost and quality.
2. **Process:** the project work.
3. **Sensor / measurement:** records the actual progress, cost and quality.
4. **Comparator:** compares actual with standard and finds the variance.
5. **Decision and corrective action (effector):** the manager adjusts the inputs (resources, method) to remove the variance.

```text
            Plan / Standard (set point)
                    |
                    v
Input --> [ Process: project work ] --> Output
               ^                          |
               |                          v
        [ Corrective              [ Sensor: measure
          action ]                    actual ]
               ^                          |
               |                          v
               +------ [ Comparator: ] <--+
                         actual vs plan
```

**Example:** the budgeted cost of a building at 50 % progress is Rs 40 lakh, but the actual cost is Rs 46 lakh. The report (feedback) shows an overrun of Rs 6 lakh. The manager reduces wastage, renegotiates rates and re-plans the remaining work to bring the cost back within the budget.

**Features:** it is continuous, automatic or semi-automatic, and corrects only after a deviation has occurred (so there is some time lag); it works best with timely information.

## Check yourself

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

1. What is the essential feature of a feedback control system?
   A. The plan is fixed and never compared
   B. It has no measurement of output
   C. Only inputs are checked, not outputs
   D. Output is measured and fed back to be compared with the standard, and corrected if there is an error
   Answer: D. A closed loop feeds the measured output back to the comparator so the error can be corrected. (Key point: Feedback loop)

2. What does the comparator do in the feedback system?
   A. Carries out the project work
   B. Compares actual with the standard and finds the variance
   C. Sets the budget
   D. Measures the actual progress
   Answer: B. The comparator finds the difference between actual and standard; the sensor only measures. (Key point: Comparator)

3. Which is a limitation of feedback control?
   A. It corrects before any deviation occurs
   B. It works without any information
   C. It cannot correct anything
   D. It corrects after the deviation has occurred, so there is some lag
   Answer: D. Feedback acts after a deviation appears, so it works best with timely information. (Key point: Lag in feedback)

4. At 50% progress, budgeted cost is Rs 40 lakh and actual cost is Rs 46 lakh. What is the feedback report?
   A. Saving of Rs 6 lakh
   B. Overrun of Rs 6 lakh, so the manager reduces wastage and re-plans
   C. Overrun of Rs 46 lakh
   D. No variance
   Answer: B. 46 - 40 = 6 lakh overrun, which triggers corrective action. (Key point: Example variance)

## Related in Project Engineering

- [Elements of cost control; feedback control](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Acost-control-elements-feedback-system) — Project Engineering, chapter 4
- [Monitoring, evaluation, control must; PMIS](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Amonitoring-evaluation-control-must-pmis) — Project Engineering, chapter 4
- [Closed and open control; quality cost](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Aclosed-open-control-quality-cost) — Project Engineering, chapter 4

## Related in other subjects

- [Control system and why feedback makes response faster](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-feedback-faster) — Control Systems, chapter 1
- [Feedback effect in DC motor speed control](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Afeedback-dc-motor-speed) — Control Systems, chapter 1
- [Feedback vs non-feedback control systems](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Afeedback-vs-nonfeedback) — Control Systems, chapter 1
- [Stability of 56/[s(s⁴+7s³+6s²+42s+8)] feedback system](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Astability-56-s5) — Control Systems, chapter 4