Open-loop vs closed-loop systems in Simulink - Hands-On Comparison

Open-loop vs Closed-loop Systems in Simulink

📖 Scenario: Imagine you are designing a simple temperature control system for a room. You want to understand the difference between open-loop and closed-loop control systems using Simulink.

🎯 Goal: Build two Simulink models: one for an open-loop system and one for a closed-loop system. Compare how each system controls the room temperature.

📋 What You'll Learn

Create a Simulink model for an open-loop system controlling room temperature

Create a Simulink model for a closed-loop system controlling room temperature with feedback

Use a constant input for desired temperature

Add a scope to visualize the temperature output

Compare outputs of both systems

💡 Why This Matters

🌍 Real World

Temperature control systems in homes and industries use open-loop or closed-loop control to maintain desired conditions.

💼 Career

Understanding open-loop and closed-loop systems is essential for control engineers, automation specialists, and data scientists working with sensor feedback and system modeling.

1

Create the Open-loop System Model

In Simulink, create a new model. Add a Constant block named desired_temp with value 22 (degrees Celsius). Add a Gain block named heater_gain with value 1.5. Connect desired_temp to heater_gain. Add a Scope block named temp_scope and connect heater_gain output to temp_scope. This models an open-loop system where the heater output depends only on the desired temperature without feedback.

Simulink

# Create a Constant block named desired_temp with value 22
# Create a Gain block named heater_gain with value 1.5
# Connect desired_temp to heater_gain
# Connect heater_gain output to a Scope block named temp_scope

Hint

Use the Simulink Library Browser to find Constant, Gain, and Scope blocks. Set their parameters exactly as instructed.

2

Add Configuration for Closed-loop System

Create a new variable called feedback_gain and set it to 0.8. This will be used to simulate feedback strength in the closed-loop system.

Simulink

% Open-loop system setup in Simulink
% Create Constant block 'desired_temp' with value 22
% Create Gain block 'heater_gain' with gain 1.5
% Connect desired_temp to heater_gain
% Connect heater_gain output to Scope 'temp_scope'

% Create feedback_gain variable
% Your code here

Hint

Just create a variable named feedback_gain and assign it the value 0.8.

3

Build the Closed-loop System Model with Feedback

In Simulink, create a new model for the closed-loop system. Add a Sum block named error_calc to subtract feedback from desired temperature. Connect desired_temp to the positive input of error_calc. Add a Gain block named feedback_gain_block with value feedback_gain. Connect the output of the heater (use the same heater_gain block with gain 1.5) to feedback_gain_block. Connect feedback_gain_block output to the negative input of error_calc. Connect error_calc output to heater_gain. Finally, connect heater_gain output to a Scope block named closed_loop_scope. This models a closed-loop system with feedback controlling the temperature.

Simulink

% Open-loop system setup in Simulink
% Create Constant block 'desired_temp' with value 22
% Create Gain block 'heater_gain' with gain 1.5
% Connect desired_temp to heater_gain
% Connect heater_gain output to Scope 'temp_scope'

feedback_gain = 0.8; % Feedback strength for closed-loop system

% Build closed-loop system with Sum, Gain, and Scope blocks
% Your code here

Hint

Use a Sum block to calculate error by subtracting feedback. Use a Gain block with feedback_gain value to simulate feedback. Connect blocks as described.

4

Display and Compare Outputs of Both Systems

Run both Simulink models. Use the temp_scope to view the open-loop system output and closed_loop_scope to view the closed-loop system output. Print the text exactly: "Open-loop and closed-loop system outputs displayed in scopes."

Simulink

% Run both models and display outputs
# Your code here

Hint

Use print to display the exact message after running the models.

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