Simulation parameters (solver, step size, stop time) in Simulink - Step-by-Step Execution

Concept Flow - Simulation parameters (solver, step size, stop time)

Set Solver Type

↓

Set Step Size

↓

Set Stop Time

↓

Start Simulation

↓

Solver computes each step

↓

Check if current time >= Stop Time?

Yes→Stop Simulation

↩Back to Solver computes each step

Simulation parameters control how Simulink runs a model: the solver decides calculation method, step size sets time increments, and stop time defines when simulation ends.

Execution Sample

Simulink

solver = 'ode45';
step_size = 0.1;
stop_time = 2.0
for t in range(0, int(stop_time / step_size)):
    current_time = t * step_size
    simulate_step(current_time, solver)

This code runs simulation steps from 0 to stop time using a solver and fixed step size.

Execution Table

Step	Current Time (t)	Condition (t < stop_time)	Action	Output
1	0.0	True	Simulate step at t=0.0	Step done at 0.0
2	0.1	True	Simulate step at t=0.1	Step done at 0.1
3	0.2	True	Simulate step at t=0.2	Step done at 0.2
4	0.3	True	Simulate step at t=0.3	Step done at 0.3
5	0.4	True	Simulate step at t=0.4	Step done at 0.4
6	0.5	True	Simulate step at t=0.5	Step done at 0.5
7	0.6	True	Simulate step at t=0.6	Step done at 0.6
8	0.7	True	Simulate step at t=0.7	Step done at 0.7
9	0.8	True	Simulate step at t=0.8	Step done at 0.8
10	0.9	True	Simulate step at t=0.9	Step done at 0.9
11	1.0	True	Simulate step at t=1.0	Step done at 1.0
12	1.1	True	Simulate step at t=1.1	Step done at 1.1
13	1.2	True	Simulate step at t=1.2	Step done at 1.2
14	1.3	True	Simulate step at t=1.3	Step done at 1.3
15	1.4	True	Simulate step at t=1.4	Step done at 1.4
16	1.5	True	Simulate step at t=1.5	Step done at 1.5
17	1.6	True	Simulate step at t=1.6	Step done at 1.6
18	1.7	True	Simulate step at t=1.7	Step done at 1.7
19	1.8	True	Simulate step at t=1.8	Step done at 1.8
20	1.9	True	Simulate step at t=1.9	Step done at 1.9
21	2.0	False	Stop simulation	Simulation ends at t=2.0

💡 Current time reached stop time 2.0, condition t < stop_time is False, simulation stops.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	After 9	After 10	After 11	After 12	After 13	After 14	After 15	After 16	After 17	After 18	After 19	After 20	Final
t	0.0	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1.0	1.1	1.2	1.3	1.4	1.5	1.6	1.7	1.8	1.9	2.0	2.0

Key Moments - 3 Insights

Why does the simulation stop exactly at the stop time and not go beyond it?

The execution_table row 21 shows the condition t < stop_time becomes False when t reaches 2.0, so the simulation stops to avoid going past the set stop time.

What happens if the step size does not evenly divide the stop time?

If step size does not fit evenly, the last step may be smaller or the simulation stops before exceeding stop time. This is because the condition checks t < stop_time each step as shown in the execution_table.

How does the solver choice affect the simulation steps?

Solver type (like 'ode45') determines how each step is calculated internally but the step size and stop time control when and how many steps run, as seen in the variable_tracker and execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at step 10. What is the current time t?

A 0.9

B 1.0

C 1.1

D 0.8

💡 Hint

Check the 'Current Time (t)' column in execution_table row 10.

At which step does the condition t < stop_time become False?

A Step 20

B Step 19

C Step 21

D Step 22

💡 Hint

Look at the 'Condition (t < stop_time)' column in execution_table; find where it changes to False.

If the step size was changed to 0.5, how many simulation steps would run before stopping?

A 4 steps

B 5 steps

C 3 steps

D 6 steps

💡 Hint

Use variable_tracker to calculate steps: stop_time 2.0 divided by step size 0.5 equals 4 full steps plus start at 0, total 5 steps.

Concept Snapshot

Simulation parameters control how Simulink runs models:
- Solver: method to compute steps (e.g., ode45)
- Step size: time increment per step (fixed or variable)
- Stop time: simulation end time
Simulation runs steps until current time reaches stop time.

Full Transcript

Simulation parameters in Simulink include solver type, step size, and stop time. The solver decides how calculations are done each step. Step size sets how big each time step is. Stop time tells when to end the simulation. The simulation starts at time zero and runs steps increasing time by step size. At each step, the solver computes the model state. When current time reaches or passes stop time, the simulation stops. This process ensures simulations run correctly and finish on time.

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