Simulation parameters (solver, step size, stop time) in Simulink - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the role of the solver in a Simulink simulation?

The solver calculates how the system changes over time by solving mathematical equations step-by-step during the simulation.

beginner

What does the step size control in a simulation?

Step size controls how often the solver calculates the system's state. Smaller step sizes give more detail but take longer to run.

beginner

Why is the stop time important in a simulation?

Stop time tells the simulation when to end. It defines how long the simulation runs in simulated time.

intermediate

What is the difference between a fixed-step and a variable-step solver?

A fixed-step solver uses the same step size throughout the simulation. A variable-step solver changes the step size to balance speed and accuracy.

intermediate

How does choosing a very small step size affect simulation?

A very small step size increases accuracy but makes the simulation slower and uses more computer resources.

What does the solver do in a Simulink simulation?

A Calculates system changes over time

B Sets the simulation stop time

C Defines the user interface

D Stores simulation results

The solver calculates how the system changes over time by solving equations step-by-step.

What happens if you increase the step size in a fixed-step solver?

A Simulation runs slower

B Simulation runs faster but less accurate

C Simulation stops earlier

D Solver changes to variable-step

Increasing step size makes simulation faster but reduces accuracy.

What does the stop time parameter control?

A When the simulation ends

B How often the solver runs

C The solver type

D The initial conditions

Stop time sets the simulation's end time.

Which solver type adjusts step size during simulation?

A Fixed-step solver

B Continuous solver

C Discrete solver

D Variable-step solver

Variable-step solvers change step size to balance speed and accuracy.

Why might you choose a smaller step size?

A To stop simulation earlier

B To reduce simulation time

C To increase simulation accuracy

D To simplify the model

Smaller step sizes increase accuracy by calculating more often.

Explain the roles of solver, step size, and stop time in a Simulink simulation.

Think about how the simulation runs and when it stops.

Describe the difference between fixed-step and variable-step solvers and when you might use each.

Consider speed versus accuracy trade-offs.

Practice

1. What does the stop time parameter control in a Simulink simulation?

easy

A. The method used to solve the model equations

B. The total duration for which the simulation runs

C. The size of each simulation step

D. The initial conditions of the model

Solution

1. Step 1: Understand the role of stop time

   The stop time sets how long the simulation will run before it stops automatically.

2. Step 2: Differentiate from other parameters

   Unlike solver or step size, stop time controls the total simulation length, not calculation method or step intervals.

3. Final Answer:

   The total duration for which the simulation runs -> Option B

4. Quick Check:

   Stop time = simulation length [OK]

Hint: Stop time means how long simulation runs [OK]

Common Mistakes:

• Confusing stop time with step size
• Thinking stop time sets solver method
• Assuming stop time changes initial conditions

2. Which of the following is the correct way to set a fixed-step solver in Simulink's simulation parameters?

easy

A. Set Solver to Fixed-step and specify Step size manually

B. Set Solver to Variable-step and Step size to 0.1

C. Set Solver to ode45 and Step size to auto

D. Set Step size to 0 and Solver to auto

Solution

1. Step 1: Identify fixed-step solver setting

   Fixed-step solver requires selecting a fixed-step solver type and manually setting the step size.

2. Step 2: Check options for correctness

   Set Solver to Fixed-step and specify Step size manually correctly sets Solver to Fixed-step and allows manual step size input, unlike others which mix variable-step or auto settings.

3. Final Answer:

   Set Solver to Fixed-step and specify Step size manually -> Option A

4. Quick Check:

   Fixed-step solver = manual step size [OK]

Hint: Fixed-step means manual step size setting [OK]

Common Mistakes:

• Choosing variable-step solver for fixed-step tasks
• Leaving step size on auto with fixed-step solver
• Setting step size to zero

3. Given a Simulink model with solver set to Fixed-step, step size 0.05, and stop time 1, how many simulation steps will be executed?

medium

A. 5

B. 50

C. 20

D. 10

Solution

1. Step 1: Calculate number of steps

   Number of steps = stop time / step size = 1 / 0.05 = 20 steps.

2. Step 2: Confirm calculation logic

   Each step advances simulation by 0.05 seconds, so 20 steps cover 1 second total.

3. Final Answer:

   20 -> Option C

4. Quick Check:

   Steps = stop time ÷ step size = 20 [OK]

Hint: Divide stop time by step size for steps count [OK]

Common Mistakes:

• Multiplying instead of dividing stop time by step size
• Ignoring fixed-step setting
• Rounding incorrectly

4. You set the solver to Variable-step but also specify a fixed step size of 0.01. What issue will this cause?

medium

A. Simulation will ignore the fixed step size and use variable steps

B. Simulation will produce an error and not run

C. Simulation will run with fixed step size ignoring variable-step solver

D. Simulation will run but results will be inaccurate

Solution

1. Step 1: Understand solver and step size interaction

   Variable-step solver automatically adjusts step size, ignoring manual fixed step size settings.

2. Step 2: Identify expected behavior

   Simulink will run simulation using variable steps despite fixed step size input, no error occurs.

3. Final Answer:

   Simulation will ignore the fixed step size and use variable steps -> Option A

4. Quick Check:

   Variable-step solver ignores manual step size [OK]

Hint: Variable-step solver ignores manual step size [OK]

Common Mistakes:

• Expecting error when mixing solver and step size
• Assuming fixed step size overrides variable-step solver
• Thinking simulation runs with fixed step size anyway

5. You want to simulate a system for exactly 2 seconds using a fixed-step solver with step size 0.3 seconds. What is the best way to set the stop time to ensure the simulation ends exactly at 2 seconds?

hard

A. Set stop time to 2.1 seconds to cover full steps

B. Set stop time to 2.4 seconds to complete full steps

C. Set stop time to 1.8 seconds to fit exact steps

D. Set stop time to 2 seconds and accept partial last step

Solution

1. Step 1: Determine required steps

   2 / 0.3 ≈ 6.67, which is not an integer number of full steps.

2. Step 2: Understand fixed-step solver behavior in Simulink

   Fixed-step solvers take full-sized steps internally but truncate the final step to a smaller size to reach exactly the stop time.

3. Final Answer:

   Set stop time to 2 seconds and accept partial last step -> Option D

4. Quick Check:

   Fixed-step truncates last step to hit stop time [OK]

Hint: Fixed-step handles partial last step for exact stop time [OK]

Common Mistakes:

• Adjusting stop time to overshoot like 2.1s unnecessarily
• Setting stop time to undershoot like 1.8s for full steps
• Assuming fixed-step cannot handle partial final steps