---
title: "Closed loop better than open loop (cruise)"
subject: "Embedded System"
chapter: 7
key: "semester_6/embedded_system:closed-loop-better-cruise-integral"
source: https://www.bctengineer.com/q/semester_6/embedded_system/closed-loop-better-cruise-integral
site: BCT Engineering
---

# Closed loop better than open loop (cruise)

Embedded System · Chapter 7: Control System

- Asked in: 2079 Asoj, 6 marks
- Video: [Open Loop Systems | Closed Loop System | Automotive | Difference | Embedded World](https://www.youtube.com/watch?v=g_vbIQNMREI) — Embedded World Automotive
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/embedded_system/closed-loop-better-cruise-integral
- Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-7

## Question

How is a closed loop control system better than an open loop controller? Explain with the example of a cruise control system.

## Short answer

Closed-loop cruise measures speed and computes u=Kp·(r-v), reducing slope error from d/b (open-loop) to d/(b+Kp). Adding integral term u=Kp·e+Ki∫e dt forces steady-state error to zero regardless of disturbance or parameter changes. Trade-off: requires speed sensor and gain tuning to avoid oscillation.

## Answer

A closed-loop controller measures the actual output and corrects the control action, so it can compensate for disturbances and model errors that an open-loop controller never sees.

#### Cruise control example

Plant: m·dv/dt = -b·v + u + d (u = engine force, d = slope force, v = speed, r = set speed).

```text
 Open loop:
  r --> [K=b] --u--> (+)--> [Car] --> v
                      ^ d
 Closed loop:
  r -->(+)--e--> [Kp] --u--> (+)--> [Car] --+--> v
        ^ -                   ^ d           |
        +------ [speed sensor] <------------+
```

- **Open loop:** u = b·r. In steady state v = r + d/b. On an uphill (d negative) the car slows down and the controller does nothing. If the mass or drag changes, v also drifts.
- **Closed loop (P):** u = Kp·(r - v). Steady state: v = (Kp·r + d)/(b + Kp). The error from the slope becomes d/(b + Kp), which is smaller than d/b, and shrinks as Kp grows.
- **With an integral term:** u = Kp·e + Ki·∫e dt. In steady state e must be zero, so v = r regardless of slope.

#### Why closed loop is better

| Point | Open loop | Closed loop |
|---|---|---|
| Road disturbance | Speed error d/b | Reduced to d/(b+Kp), or zero with I |
| Parameter change | Speed drifts | Compensated |
| Needs accurate model | Yes | Less so |
| Needs speed sensor | No | Yes |

Cost: a sensor, and the gain must be limited to avoid overshoot and oscillation.

## Check yourself

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

1. With P control u = Kp(r - v), what is the slope-caused error?
   A. d/b
   B. d/(b + Kp)
   C. d*Kp
   D. Zero
   Answer: B. It is smaller than the open-loop error d/b and shrinks as Kp grows. (Key point: P-control error)

2. Why does adding an integral term give v = r regardless of slope?
   A. In steady state the error must be zero for the integral to stop changing
   B. It makes Kp infinite
   C. It removes the plant's drag
   D. It ignores the disturbance sensor
   Answer: A. The integrator keeps changing until e = 0, so v = r. (Key point: Integral term)

3. What does the open-loop controller do when the car slows uphill?
   A. It increases engine force
   B. It switches to closed loop
   C. It reduces the set speed
   D. Nothing, since it does not measure speed
   Answer: D. u = b*r stays fixed, so speed drifts. (Key point: Open-loop uphill)

4. What is the price of closed-loop control?
   A. Inability to reject disturbances
   B. A more accurate model is required
   C. A speed sensor and limited gain to avoid oscillation
   D. No steady-state error is possible
   Answer: C. Needs sensor and gain limits. (Key point: Cost of feedback)

## Related in Embedded System

- [Open-loop automobile cruise controller design](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-loop-cruise-controller) — Embedded System, chapter 7, asked 3×
- [Open-loop cruise control using P control](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-loop-cruise-p-control) — 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×
- [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
- [Closed loop diagram; P, PI, PD, PID](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-block-diagram-pid-types) — Embedded System, chapter 7

## Related in other subjects

- [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×
- [Why closed loop rejects disturbances better](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adefine-cs-disturbance-rejection) — 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
- [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