Feedback loop in Simulink - Mini Project: Build & Apply

Feedback loop in Simulink

📖 Scenario: You are designing a simple control system in Simulink that uses a feedback loop to maintain a desired output level. Feedback loops are common in real-world systems like thermostats, cruise control in cars, and automatic lighting.

🎯 Goal: Build a Simulink model with a feedback loop that takes an input signal, processes it through a gain block, and feeds the output back to the input to stabilize the system.

📋 What You'll Learn

Create a Simulink model with a Step input block

Add a Gain block with gain value 2

Add a Sum block configured to subtract feedback

Connect blocks to form a feedback loop

Add a Scope block to visualize the output

💡 Why This Matters

🌍 Real World

Feedback loops are used in many control systems to maintain desired outputs, such as temperature control, speed regulation, and signal processing.

💼 Career

Understanding feedback loops in Simulink is essential for engineers and data scientists working on control systems, automation, and system modeling.

1

Create the initial Simulink model with input and gain

Open Simulink and create a new model. Add a Step block named step_input and a Gain block named gain_block with gain set to 2. Connect the output of step_input to the input of gain_block.

Simulink

% Add a Step block named step_input
% Add a Gain block named gain_block with gain 2
% Connect step_input output to gain_block input
% Your code here

Hint

Use add_block to add blocks and set_param to set the gain value. Use add_line to connect blocks.

2

Add the Sum block for feedback subtraction

Add a Sum block named sum_block configured with the list of signs '+ -' to subtract the feedback signal. Connect the output of sum_block to the input of gain_block. Connect the output of step_input to the positive input of sum_block.

Simulink

% Add Sum block named sum_block with signs '+ -'
% Connect step_input output to sum_block positive input
% Connect sum_block output to gain_block input
% Your code here

Hint

Configure the Sum block to subtract the feedback by setting Inputs to '+ -'.

3

Create the feedback loop by connecting Gain output to Sum negative input

Connect the output of gain_block to the negative input of sum_block to complete the feedback loop.

Simulink

% Connect gain_block output to sum_block negative input
% Your code here

Hint

Use add_line to connect the output port 1 of gain_block to input port 2 of sum_block.

4

Add Scope block and run the simulation to visualize output

Add a Scope block named scope_block. Connect the output of gain_block to the input of scope_block. Run the simulation for 10 seconds and display the scope window.

Simulink

% Add Scope block named scope_block
% Connect gain_block output to scope_block input
% Set simulation stop time to 10
% Run simulation
% Your code here

Hint

Use add_block to add the Scope block and sim to run the simulation. Use open_system to open the scope window.

Practice

1. What is the main purpose of a feedback loop in Simulink?

easy

A. To use the output to control the input and stabilize the system

B. To create random signals for testing

C. To display the output only without affecting input

D. To disconnect blocks automatically

Solution

1. Step 1: Understand feedback loop concept

   A feedback loop uses the output signal to influence the input, helping control system behavior.

2. Step 2: Identify the purpose in Simulink

   In Simulink, feedback loops stabilize or regulate system outputs by adjusting inputs based on output values.

3. Final Answer:

   To use the output to control the input and stabilize the system -> Option A

4. Quick Check:

   Feedback loop = control input from output [OK]

Hint: Feedback loops control input using output signals [OK]

Common Mistakes:

• Thinking feedback loops only display output
• Confusing feedback with feedforward
• Assuming feedback loops generate random signals

2. Which block is commonly used in Simulink to combine signals for creating a feedback path?

easy

A. Sum block

B. Gain block

C. Scope block

D. Constant block

Solution

1. Step 1: Identify blocks that combine signals

   The Sum block adds or subtracts multiple input signals, combining them into one output.

2. Step 2: Understand feedback path creation

   Feedback loops require combining signals, which the Sum block does, unlike Gain or Scope blocks.

3. Final Answer:

   Sum block -> Option A

4. Quick Check:

   Sum block combines signals [OK]

Hint: Sum block adds signals for feedback paths [OK]

Common Mistakes:

• Choosing Gain block which scales but does not combine
• Selecting Scope block which only displays signals
• Confusing Constant block with signal combiner

3. Consider a feedback loop in Simulink with a Gain block of 2 and an Integrator block. If the input is a constant 1, what is the expected behavior of the output over time?

medium

A. Output will oscillate without settling

B. Output will remain constant at 1

C. Output will increase linearly over time

D. Output will decrease exponentially

Solution

1. Step 1: Analyze Gain and Integrator effect

   The Gain block multiplies input by 2, and the Integrator accumulates this value over time.

2. Step 2: Predict output behavior

   Since input is constant 1, output = integral of 2*1 = 2*t, which increases linearly over time.

3. Final Answer:

   Output will increase linearly over time -> Option C

4. Quick Check:

   Integrator accumulates scaled input linearly [OK]

Hint: Integrator sums input; gain scales it, causing linear growth [OK]

Common Mistakes:

• Assuming output stays constant with integrator
• Thinking output oscillates without a feedback oscillator
• Confusing exponential decrease with integration

4. You created a feedback loop in Simulink but the model simulation fails with an algebraic loop error. What is the most likely cause?

medium

A. Using a Sum block instead of a Gain block

B. Missing delay or memory block in the feedback path

C. Incorrect block parameter values in Gain block

D. Not connecting the output to a Scope block

Solution

1. Step 1: Understand algebraic loop error

   An algebraic loop occurs when output depends directly on itself without delay, causing simulation failure.

2. Step 2: Identify cause in feedback loops

   Feedback loops need a delay or memory block to break direct dependency; missing this causes algebraic loops.

3. Final Answer:

   Missing delay or memory block in the feedback path -> Option B

4. Quick Check:

   Algebraic loop = no delay in feedback [OK]

Hint: Add delay block to break algebraic loops [OK]

Common Mistakes:

• Blaming Sum block for algebraic loop
• Ignoring need for delay in feedback
• Thinking Scope block affects algebraic loops

5. You want to design a feedback loop in Simulink that stabilizes a system output at a target value. Which combination of blocks should you use?

hard

A. Gain block for input scaling, Integrator block for output, and Constant block for feedback

B. Constant block for input, Scope block for output, and Gain block for scaling

C. Sum block to add signals, Constant block for feedback, and Scope block to display

D. Sum block to compare output and target, Gain block for feedback strength, and Integrator block to accumulate error

Solution

1. Step 1: Identify blocks for feedback control

   Sum block compares output with target to find error; Gain block adjusts feedback strength; Integrator accumulates error over time.

2. Step 2: Understand how these stabilize output

   Accumulated error helps correct system output gradually, stabilizing it at the target value.

3. Final Answer:

   Sum block to compare output and target, Gain block for feedback strength, and Integrator block to accumulate error -> Option D

4. Quick Check:

   Sum + Gain + Integrator = stable feedback control [OK]

Hint: Use Sum, Gain, and Integrator for stable feedback loops [OK]

Common Mistakes:

• Using Scope or Constant blocks instead of error calculation
• Missing Integrator to accumulate error
• Confusing feedback with feedforward blocks