---
title: "Control type for constant-speed actuator"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:constant-speed-actuator"
source: https://www.bctengineer.com/q/semester_3/control_systems/constant-speed-actuator
site: BCT Engineering
---

# Control type for constant-speed actuator

Control Systems · Chapter 1: Control System Background

- Asked in: 2078 Bhadra, 5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/control_systems/constant-speed-actuator
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

As a control engineer, you are required to control the actuator of a rotating system to get constant speed. Which type of control system will you suggest and why?

## Short answer

Use a closed-loop speed control system with a PI controller. It rejects load disturbances (steady-state error becomes zero due to integral action), reduces sensitivity to parameter variations, and speeds up response by reducing the motor time constant.

## Answer

I would use a **closed-loop (feedback) speed control system** with a speed sensor (tachogenerator or encoder) and a **PI controller**.

```text
 ωref +    e  +----+  +-----+ Va +-------+  ω
 ---->(Σ)---->| PI |->|Amp/ |--->| Motor |-+-->
      ^ -     +----+  |drive|    | +load | |
      |               +-----+    +-------+ |
      |    +------------+            ^ T_L |
      +----| Tacho  Kg  |<-----------------+
           +------------+
```

#### Why closed loop
1. **Load disturbance rejection:** the load torque $T_L$ changes the speed. In closed loop the speed change is divided by $(1+K)$, where $K$ is the loop gain; with the integral term, the steady-state speed error for a step change in load becomes **zero**.
2. **Insensitive to parameter changes:** armature resistance rises with heating and the supply voltage varies; the sensitivity of speed to these is $1/(1+GH)$ instead of 1.
3. **Faster response:** the motor time constant $\tau_m$ becomes $\tau_m/(1+K)$, so the speed recovers quickly after a disturbance.
4. **Accurate tracking** of the set speed, needed in conveyors, paper mills, fans and pumps.

#### Why not open loop
An open-loop drive applies a fixed voltage. Its speed is $\omega = \dfrac{V - I_aR_a}{K_b}$, so it falls whenever the load (and hence $I_a$) increases, and it cannot correct the drop.

#### Why PI
Proportional control alone leaves a steady-state error $1/(1+K_p)$; the integral action removes it. A derivative term is usually not needed for speed loops and it amplifies tacho noise. The gains are chosen to give a fast response with small overshoot.

## Check yourself

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

1. For constant speed under varying load you should choose
   A. an open-loop fixed-voltage drive
   B. a timer-based controller
   C. no controller
   D. a closed-loop speed control with tachogenerator feedback
   Answer: D. Feedback senses the speed and corrects load-induced drops. (Key point: Choice of system)

2. An open-loop drive's speed w = (V - Ia Ra)/Kb falls when load increases because
   A. armature current Ia rises and nothing corrects the drop
   B. Kb increases with load
   C. V automatically decreases
   D. Ra becomes zero
   Answer: A. Higher load raises Ia, so Ia Ra rises and speed falls. (Key point: Why not open loop)

3. Why is the integral term used?
   A. It increases tacho noise
   B. It removes the steady-state error that proportional control alone leaves (1/(1+Kp))
   C. It makes the system open loop
   D. It reduces the loop gain to zero
   Answer: B. Integral action drives steady-state error to zero for a step load. (Key point: Why PI)

4. A derivative term is usually not used in speed loops because
   A. it removes the integral action
   B. it makes steady-state error infinite
   C. it amplifies tacho noise and is not needed
   D. it cannot be implemented
   Answer: C. Derivative action boosts high-frequency noise. (Key point: Derivative term)

## Related in Control Systems

- [Characterizing control system by speed, accuracy, stability](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Aspeed-accuracy-stability) — 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
- [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 and its significance in engineering](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-significance) — Control Systems, chapter 1
- [General block diagram of a control system](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Ageneral-bd-control-system) — Control Systems, chapter 1

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- [Synchronous speed; rotor follows rotating field](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Asynchronous-speed-rotor-direction) — Electrical Machines, chapter 5
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- [Constant V/f control of induction motor](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aconstant-v-f-control-induction-motor) — Utilization of Electrical Energy, chapter 3, asked 2×