Variant subsystems in Simulink - Mini Project: Build & Apply

Variant Subsystems in Simulink

📖 Scenario: You are designing a Simulink model for a simple control system that can switch between two different control strategies. This is common in real-world systems where different modes or configurations are needed.

🎯 Goal: Build a Simulink model using Variant Subsystems to switch between two control strategies based on a configuration variable.

📋 What You'll Learn

Create a variant control variable named controlMode with values 1 or 2

Add a Variant Subsystem block with two variants inside

Configure the first variant to be active when controlMode == 1

Configure the second variant to be active when controlMode == 2

Each variant should implement a simple gain block with different gain values

Simulate the model and observe the output switching based on controlMode

💡 Why This Matters

🌍 Real World

Variant Subsystems allow engineers to design flexible models that can switch between different configurations or control strategies without rebuilding the entire model.

💼 Career

Understanding Variant Subsystems is important for control system engineers and model-based designers working on automotive, aerospace, and industrial automation projects.

1

Create the Variant Control Variable

In the MATLAB workspace, create a variable called controlMode and set it to 1 to select the first variant initially.

Simulink

# Your code here

Hint

Use the MATLAB command window or script to define controlMode as 1.

2

Add Variant Subsystem with Two Variants

In your Simulink model, add a Variant Subsystem block. Inside it, add two subsystems named Variant1 and Variant2. Configure Variant1 to be active when controlMode == 1 and Variant2 to be active when controlMode == 2.

Simulink

controlMode = 1;
% Add Variant Subsystem block with two variants
% Configure variant conditions as specified

Hint

Use the Variant Subsystem block from Simulink library and set variant conditions in the block parameters.

3

Implement Different Gains in Each Variant

Inside Variant1, add a Gain block with gain value 2. Inside Variant2, add a Gain block with gain value 5. Connect input and output ports properly.

Simulink

controlMode = 1;
% Add Gain block with gain 2 inside Variant1
% Add Gain block with gain 5 inside Variant2

Hint

Use the Gain block from Simulink library and set the gain parameter accordingly inside each variant.

4

Simulate and Display Output

Run the simulation with controlMode = 1 and then change controlMode to 2 and run again. Observe the output changes based on the active variant. Use disp or simout to display the output values.

Simulink

controlMode = 1;
% Run simulation and display output
% Change controlMode to 2 and run simulation again
% Display output

Hint

Use the sim command to run the model and disp to show output. Change controlMode between runs.

Practice

1. What is the main purpose of a Variant Subsystem in Simulink?

easy

A. To generate code from a Simulink model

B. To switch between different subsystem implementations within one model

C. To create a new Simulink model automatically

D. To visualize simulation results in real-time

Solution

1. Step 1: Understand Variant Subsystem concept

   A Variant Subsystem allows multiple subsystem options inside one model but only runs one at a time.

2. Step 2: Identify the main purpose

   This helps switch between different designs easily without changing the whole model.

3. Final Answer:

   To switch between different subsystem implementations within one model -> Option B

4. Quick Check:

   Variant Subsystem = Switch subsystems [OK]

Hint: Variant subsystems let you pick one design option inside one model [OK]

Common Mistakes:

• Thinking it creates new models
• Confusing with code generation
• Assuming it visualizes results

2. Which of the following is the correct way to define a variant control variable in Simulink for a Variant Subsystem?

easy

A. variantControl -> 1; % Use arrow operator

B. variantControl == 1; % Use double equals for assignment

C. variantControl := 1; % Use colon equals operator

D. variantControl = 1; % Define control variable in MATLAB workspace

Solution

1. Step 1: Recall MATLAB syntax for variable assignment

   In MATLAB, variables are assigned using a single equals sign, e.g., var = value;.

2. Step 2: Identify correct syntax for variant control variable

   variantControl = 1; % Define control variable in MATLAB workspace uses = correctly. Options A, B, and C use invalid assignment operators.

3. Final Answer:

   variantControl = 1; % Define control variable in MATLAB workspace -> Option D

4. Quick Check:

   Assignment uses =, not == or := [OK]

Hint: Use single = for assignment in MATLAB workspace [OK]

Common Mistakes:

• Using == instead of = for assignment
• Using := or -> which are invalid in MATLAB
• Confusing control variable with variant condition

3. Given the following variant control variable and conditions:
variantControl = 2;
Which subsystem will run if the variant conditions are:
Subsystem A: variantControl == 1
Subsystem B: variantControl == 2
Subsystem C: variantControl == 3

medium

A. Subsystem B

B. Subsystem A

C. Subsystem C

D. No subsystem runs

Solution

1. Step 1: Check the value of variantControl

   The variable variantControl is set to 2.

2. Step 2: Match variantControl with subsystem conditions

   Subsystem B runs when variantControl == 2, which matches the current value.

3. Final Answer:

   Subsystem B -> Option A

4. Quick Check:

   variantControl 2 matches Subsystem B condition [OK]

Hint: Match variantControl value to condition to find active subsystem [OK]

Common Mistakes:

• Choosing subsystem A or C without matching condition
• Assuming multiple subsystems run simultaneously
• Ignoring exact equality in conditions

4. You have a Variant Subsystem with two variants controlled by variantControl. The control variable is set as variantControl = '1'; (a string). The variant conditions are variantControl == 1 and variantControl == 2. What error will occur?

medium

A. Error due to type mismatch between string and number

B. No error, Subsystem 2 runs

C. No error, Subsystem 1 runs

D. Error because variantControl is undefined

Solution

1. Step 1: Identify data types of variantControl and conditions

   variantControl is a string ('1'), but conditions compare to numbers (1 and 2).

2. Step 2: Understand type mismatch impact

   Comparing string to number with == causes a type mismatch error in Simulink variant evaluation.

3. Final Answer:

   Error due to type mismatch between string and number -> Option A

4. Quick Check:

   String vs number comparison causes error [OK]

Hint: Ensure variantControl type matches condition type to avoid errors [OK]

Common Mistakes:

• Assuming string '1' equals number 1
• Ignoring data type in variant conditions
• Thinking variantControl can be undefined

5. You want to compare two design options in a Simulink model using Variant Subsystems. One subsystem uses a PID controller, and the other uses a state-space controller. How should you set up the variant control variable and conditions to switch between these controllers during simulation?

hard

A. Set variantControl = true; for PID and variantControl = false; for state-space, then use conditions variantControl == true and variantControl == false

B. Use variantControl = 'PID'; and variantControl = 'StateSpace'; with conditions variantControl == 'PID' and variantControl == 'StateSpace'

C. Set variantControl = 1; for PID and variantControl = 2; for state-space, then use conditions variantControl == 1 and variantControl == 2 respectively

D. Use no variant control variable and manually enable one subsystem at a time

Solution

1. Step 1: Choose numeric variant control variable for clarity

   Using numeric values like 1 and 2 is simple and avoids type mismatch.

2. Step 2: Define variant conditions matching control variable values

   Conditions variantControl == 1 and variantControl == 2 clearly select PID or state-space subsystems.

3. Step 3: Avoid string or boolean variants for better compatibility

   Strings or booleans can cause errors or confusion in variant evaluation.

4. Final Answer:

   Set variantControl = 1 for PID and variantControl = 2 for state-space, with matching conditions -> Option C

5. Quick Check:

   Numeric control variable with matching conditions is best practice [OK]

Hint: Use numeric variantControl and matching == conditions for easy switching [OK]

Common Mistakes:

• Using strings causing type mismatch
• Using booleans which limit options
• Not using variantControl and enabling subsystems manually