Variant subsystems in Simulink

Introduction

Variant subsystems let you choose different parts of a model to run without changing the whole setup. This helps test or use different designs easily.

You want to test different control strategies in one model without rebuilding it.

You need to switch between different algorithms based on conditions or settings.

You want to compare multiple designs quickly in the same simulation.

You want to keep your model clean by grouping alternatives in one place.

You want to deploy different versions of a system from the same model.

Syntax

Simulink

1. Add a Variant Subsystem block from the Simulink library.
2. Inside it, add multiple variant choices as subsystems.
3. Set the 'Variant control' variable to select which choice runs.
4. Use conditions like Variant == 1, Variant == 2, etc., for each choice.

The 'Variant control' variable controls which subsystem is active during simulation.

Only the active variant runs, so simulation is faster and clearer.

Examples

This activates the first variant choice.

Simulink

Variant == 1
% Runs the first variant subsystem

This activates the second variant choice.

Simulink

Variant == 2
% Runs the second variant subsystem

This activates the third variant choice.

Simulink

Variant == 3
% Runs the third variant subsystem

Sample Program

This example shows how to switch between two variant subsystems that output different constants. Changing the 'Variant' variable selects which output you get.

Simulink

% Create a simple model with a Variant Subsystem
% 1. Open Simulink and create a new model.
% 2. Add a Variant Subsystem block.
% 3. Inside it, add two subsystems:
%    - Subsystem1: outputs constant 5
%    - Subsystem2: outputs constant 10
% 4. Set Variant control variable 'Variant' in base workspace.
% 5. Set Variant conditions:
%    - Subsystem1: Variant == 1
%    - Subsystem2: Variant == 2

% MATLAB commands to set up and simulate:
Variant = 1;
simOut1 = sim('your_model_name');
disp('Output when Variant = 1:');
disp(simOut1.get('yout').signals.values(end));

Variant = 2;
simOut2 = sim('your_model_name');
disp('Output when Variant = 2:');
disp(simOut2.get('yout').signals.values(end));

Important Notes

Make sure the 'Variant control' variable is defined before simulation.

Only one variant can be active at a time during simulation.

Variant subsystems help keep models organized and flexible.

Summary

Variant subsystems let you pick one of many subsystem options easily.

Use a control variable to switch between variants during simulation.

This helps test and compare different designs without changing the whole model.

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