Open-loop vs closed-loop systems in Simulink

Introduction

Open-loop and closed-loop systems help us control machines or processes. They show how we use feedback to make systems work better or not.

When you want to control a heater without checking the temperature (open-loop).

When you want to keep a room at a set temperature by measuring and adjusting (closed-loop).

When designing a washing machine cycle that runs the same way every time (open-loop).

When controlling a drone that adjusts its position based on sensors (closed-loop).

When testing simple systems where feedback is not needed (open-loop).

Syntax

Simulink

Open-loop system:
Input --> System --> Output

Closed-loop system:
Input --> System --> Output
          ^               |
          |--------------- Feedback

Open-loop systems do not use feedback to adjust their output.

Closed-loop systems use feedback to compare output with desired input and adjust accordingly.

Examples

The fan runs at a fixed speed regardless of the room temperature.

Simulink

Open-loop example:
Turn on a fan at full speed without checking room temperature.

The heater changes power to keep the room at the desired temperature.

Simulink

Closed-loop example:
Thermostat controls heater by measuring room temperature and adjusting heat.

Sample Program

This code shows a simple open-loop system where output is just twice the input.

Then it shows a closed-loop system where output moves closer to the set point using feedback.

Simulink

/* Simulink model description in MATLAB code form */

% Open-loop system example: Constant input to a system
open_loop_input = 5;
open_loop_output = open_loop_input * 2; % System doubles input
fprintf('Open-loop output: %d\n', open_loop_output);

% Closed-loop system example: Simple feedback control
set_point = 10; % Desired output
output = 0;
for i = 1:5
    error = set_point - output;
    control_signal = error * 0.5; % Controller gain
    output = output + control_signal; % System updates output
    fprintf('Step %d, Output: %.2f\n', i, output);
end

Important Notes

Open-loop systems are simpler but can be less accurate because they don't adjust based on output.

Closed-loop systems are more complex but can correct errors and maintain desired output.

Summary

Open-loop systems work without feedback and do not adjust automatically.

Closed-loop systems use feedback to compare output with desired input and adjust accordingly.

Closed-loop systems are common in everyday devices like thermostats and cruise control.

Practice

1. Which of the following best describes an open-loop system in Simulink?

easy

A. A system that uses sensors to monitor output and correct errors.

B. A system that continuously adjusts output based on feedback.

C. A system that operates without using feedback to adjust its output.

D. A system that always maintains a constant output regardless of input.

Solution

1. Step 1: Understand open-loop system definition

   An open-loop system works without feedback; it does not adjust based on output.

2. Step 2: Compare options to definition

   A system that operates without using feedback to adjust its output. matches this definition exactly, while others describe feedback or constant output.

3. Final Answer:

   A system that operates without using feedback to adjust its output. -> Option C

4. Quick Check:

   Open-loop = no feedback [OK]

Hint: Open-loop means no feedback used to adjust output [OK]

Common Mistakes:

• Confusing open-loop with closed-loop systems
• Thinking open-loop uses feedback
• Assuming open-loop always maintains constant output

2. Which Simulink block is typically used to create a closed-loop system?

easy

A. Gain block

B. Sum block with feedback connection

C. Constant block

D. Scope block

Solution

1. Step 1: Identify closed-loop system structure

   Closed-loop systems require feedback, often created by feeding output back to input using a Sum block.

2. Step 2: Match blocks to feedback role

   Sum block with feedback connection allows error calculation, enabling closed-loop control.

3. Final Answer:

   Sum block with feedback connection -> Option B

4. Quick Check:

   Feedback needs Sum block [OK]

Hint: Closed-loop needs feedback; Sum block enables feedback loop [OK]

Common Mistakes:

• Choosing Gain block which only scales signals
• Selecting Constant or Scope blocks which don't create feedback
• Ignoring the feedback connection requirement

3. Consider this Simulink setup: a Gain block with gain 2 connected directly to input, no feedback. What is the output if input is 3?

medium

A. 3

B. 1.5

C. 0

D. 6

Solution

1. Step 1: Identify system type and operation

   The system is open-loop (no feedback). Output = input * gain.

2. Step 2: Calculate output value

   Output = 3 * 2 = 6.

3. Final Answer:

   6 -> Option D

4. Quick Check:

   Output = input x gain = 6 [OK]

Hint: Multiply input by gain for open-loop output [OK]

Common Mistakes:

• Assuming feedback changes output
• Using input value directly without gain
• Confusing gain with division

4. You have a closed-loop system in Simulink but the output does not stabilize and keeps oscillating. What is the most likely cause?

medium

A. Gain value is too high causing instability

B. Feedback loop is missing

C. Input signal is zero

D. Sum block is not connected

Solution

1. Step 1: Understand oscillation cause in closed-loop

   Oscillations often occur when gain is too high, causing the system to overreact and become unstable.

2. Step 2: Eliminate other options

   Feedback loop missing or Sum block not connected would stop feedback, not cause oscillations. Zero input does not cause oscillations.

3. Final Answer:

   Gain value is too high causing instability -> Option A

4. Quick Check:

   High gain causes oscillation [OK]

Hint: High gain in feedback causes oscillations [OK]

Common Mistakes:

• Thinking missing feedback causes oscillations
• Assuming zero input causes oscillations
• Ignoring gain effect on stability

5. You want to design a Simulink model to maintain room temperature at 22°C despite outside temperature changes. Which system type and feature should you use?

hard

A. Closed-loop system with temperature sensor feedback

B. Open-loop system with timer-based heater control

C. Open-loop system with fixed heater power

D. Closed-loop system without feedback

Solution

1. Step 1: Identify control needs for stable temperature

   Maintaining temperature despite changes requires feedback to adjust heater power.

2. Step 2: Choose system type with feedback

   Closed-loop system with temperature sensor feedback adjusts output based on actual room temperature.

3. Final Answer:

   Closed-loop system with temperature sensor feedback -> Option A

4. Quick Check:

   Stable control needs feedback loop [OK]

Hint: Use feedback to adjust for changing conditions [OK]

Common Mistakes:

• Choosing open-loop which can't adjust to changes
• Ignoring need for sensor feedback
• Selecting closed-loop without feedback