---
title: "Open loop vs closed loop instrumentation"
subject: "Instrumentation II"
chapter: 1
key: "semester_5/instrumentation_ii:open-closed-loop"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/open-closed-loop
site: BCT Engineering
---

# Open loop vs closed loop instrumentation

Instrumentation II · Chapter 1: Microprocessor Based Instrumentation System

- Asked in: 2073 Shrawan, 5 marks; 2070 Chaitra, 4 marks (asked 2 times)
- Video: [Open loop and Closed loop instrumentation system | Instrumentation II | IOE PU TU](https://www.youtube.com/watch?v=kuRXSXl_G88) — Biplav Paudel
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/open-closed-loop
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-1

## Question

Differentiate between open loop and closed loop instrumentation system along with block diagrams.

## Short answer

Open-loop systems operate without feedback, so accuracy depends on component calibration and disturbances are not corrected. Closed-loop systems measure output with a sensor, feed it back to compare with setpoint, and the microprocessor corrects error for high accuracy, but require sensor, ADC, and control algorithm, increasing complexity.

## Answer

An **open loop** system produces its output from the input command only, without checking the result. A **closed loop** system measures the output with a sensor and feeds it back to the microprocessor, which corrects the control action to reduce the error.

#### Open loop MBI
```text
 Set value -> [ Microprocessor ] -> [DAC/driver] -> [Actuator] -> Process
```

#### Closed loop MBI
```text
 Set value ->(+)-> [Microprocessor] -> [DAC/driver] -> [Actuator] -> Process
              ^ -                                                    |
              |                                                      |
              +------ [ADC] <- [Signal conditioner] <- [Sensor] <----+
```

| Point | Open loop | Closed loop |
|---|---|---|
| Feedback | None | Output is measured and fed back |
| Accuracy | Depends on calibration of components | High, error is corrected |
| Effect of disturbance | Not corrected | Corrected automatically |
| Complexity and cost | Simple and cheap | Needs sensor, ADC, algorithm; costly |
| Stability | No stability problem | May oscillate if badly tuned |
| Example | Washing machine timer, traffic light | Oven temperature control, motor speed control |

## Check yourself

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

1. What is the basic difference between open and closed loop systems?
   A. Open loop uses a sensor, closed loop does not
   B. Closed loop measures the output and feeds it back to correct error; open loop does not
   C. Closed loop has no microprocessor
   D. Open loop corrects disturbances automatically
   Answer: B. Only the closed loop has feedback from a sensor to reduce error. (Key point: Feedback)

2. Which blocks does a closed loop MBI need beyond the open loop?
   A. Sensor, signal conditioner and ADC in the feedback path
   B. Only a bigger actuator
   C. A second DAC and no sensor
   D. Only a keypad
   Answer: A. Feedback requires sensor, conditioner and ADC to bring the output back. (Key point: Extra blocks in closed loop)

3. Which statement about stability is correct?
   A. Closed loop is always stable
   B. Open loop may oscillate due to feedback
   C. Open loop has no stability problem, while closed loop may oscillate if badly tuned
   D. Both always oscillate
   Answer: C. Without feedback there is no loop to oscillate; closed loop needs tuning. (Key point: Stability)

4. Which pair of examples is correct?
   A. Open: oven temperature control; closed: traffic light
   B. Open: motor speed control; closed: washing machine timer
   C. Both are open loop
   D. Open: washing machine timer; closed: oven temperature control
   Answer: D. Timer-based traffic light or washing machine are open loop; oven control uses feedback. (Key point: Examples)

## Related in Instrumentation II

- [MBI features; open vs closed loop compared](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-features-open-closed) — Instrumentation II, chapter 1
- [Rationale for microprocessor; open-loop MBI; DMA](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Arationale-open-loop-dma) — Instrumentation II, chapter 1
- [Closed loop MBI system](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aclosed-loop-mbi-define) — Instrumentation II, chapter 1, asked 2×
- [Case study: existing vs proposed system](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-2074asoj) — Instrumentation II, chapter 10
- [Microprocessors in instrumentation systems (case study)](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-casestudy-statement) — Instrumentation II, chapter 1
- [Case study: new system vs existing (cost, manpower)](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-cost-manpower-automation) — Instrumentation II, chapter 10, asked 4×

## 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×
- [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
- [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
- [Closed vs open loop step response analytically](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Acl-vs-ol-step-response) — Control Systems, chapter 1, asked 2×