Sample time configuration in Simulink - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is sample time in Simulink?

Sample time is the interval at which a block in Simulink executes or updates its output. It controls how often the block processes data.

beginner

What does a -1 sample time mean in Simulink?

A sample time of -1 means the block inherits its sample time from the driving block or the context it is in.

beginner

How do you specify a fixed-step discrete sample time of 0.1 seconds in Simulink?

You set the sample time parameter of the block to 0.1, which means the block updates every 0.1 seconds.

intermediate

What is the difference between continuous and discrete sample times?

Continuous sample time means the block runs continuously without fixed intervals. Discrete sample time means the block runs at fixed time steps.

intermediate

Why is configuring sample time important in Simulink models?

Proper sample time configuration ensures accurate simulation results and efficient computation by controlling when blocks update.

What does a sample time of 0 indicate in Simulink?

A Sample time disabled

B Discrete sample time of zero seconds

C Inherited sample time

D Continuous sample time

A sample time of 0 means the block runs continuously without discrete steps.

If a block has sample time set to -1, what happens?

A It runs continuously

B It inherits sample time from upstream blocks

C It runs at fixed 1 second intervals

D It does not run

Sample time -1 means the block inherits sample time from its driving block.

Which sample time value sets a block to run every 0.05 seconds?

A 0

B -1

C 0.05

D 1

Setting sample time to 0.05 means the block updates every 0.05 seconds.

Why might you choose a discrete sample time over continuous?

A To reduce computation by updating less often

B To make the model run slower

C To ignore input signals

D To disable the block

Discrete sample times reduce computation by updating at fixed intervals.

What happens if sample times are not configured properly?

A Simulation may be inaccurate or inefficient

B Model runs faster always

C Blocks ignore inputs

D No effect on simulation

Incorrect sample times can cause wrong results or slow simulation.

Explain what sample time means in Simulink and why it matters.

Think about how often a block updates its output.

Describe the difference between continuous and discrete sample times with examples.

Consider how often the block runs in each case.

Practice

1. What does the sample time in Simulink control?

easy

A. The color of the block in the model

B. How often a block executes during simulation

C. The size of the block on the screen

D. The type of data the block processes

Solution

1. Step 1: Understand sample time meaning

   Sample time defines the frequency at which a block runs during simulation.

2. Step 2: Match sample time to options

   Only How often a block executes during simulation describes execution frequency, others describe unrelated features.

3. Final Answer:

   How often a block executes during simulation -> Option B

4. Quick Check:

   Sample time = execution frequency [OK]

Hint: Sample time means block run frequency [OK]

Common Mistakes:

• Confusing sample time with block appearance
• Thinking sample time controls data type
• Assuming sample time affects block size

2. Which of the following is the correct way to set a fixed sample time of 0.1 seconds in a Simulink block?

easy

A. Set Sample time to 'inf'

B. Set Sample time to '-1'

C. Set Sample time to '0.1'

D. Set Sample time to 'triggered'

Solution

1. Step 1: Identify fixed sample time syntax

   Fixed sample time is set by a positive number representing seconds, e.g., 0.1.

2. Step 2: Evaluate options

   '-1' means inherited, 'inf' means continuous, 'triggered' is not a numeric value.

3. Final Answer:

   Set Sample time to '0.1' -> Option C

4. Quick Check:

   Fixed sample time = positive number [OK]

Hint: Fixed sample time is a positive number [OK]

Common Mistakes:

• Using '-1' for fixed sample time
• Confusing 'inf' with fixed sample time
• Entering non-numeric values for sample time

3. Consider a Simulink model with two blocks: Block A has a sample time of 0.05 seconds, and Block B has a sample time of 0.1 seconds. If the simulation runs for 0.2 seconds, how many times will Block A execute?

medium

A. 4 times

B. 3 times

C. 5 times

D. 2 times

Solution

1. Step 1: Calculate executions for Block A

   Block A runs every 0.05 seconds. Total simulation time is 0.2 seconds. Executions occur at t=0, 0.05, 0.1, 0.15 (not at 0.20 since simulation ends at 0.2).

2. Step 2: Divide total time by sample time

   Number of executions = (0.2 / 0.05) = 4 (executions at 0, 0.05, 0.1, 0.15).

3. Final Answer:

   4 times -> Option A

4. Quick Check:

   Executions at t=0, 0.05, 0.1, 0.15 = 4 times [OK]

Hint: Number of executions = floor(total time / sample time) + 1 [OK]

Common Mistakes:

• Forgetting the initial execution at t=0
• Confusing Block A's sample time with Block B's
• Counting execution at simulation end time incorrectly

4. You set a block's sample time to '-1' but the block does not inherit the sample time as expected. What is the most likely cause?

medium

A. The block has a fixed sample time set elsewhere

B. The sample time value '-1' is invalid

C. The block is set to continuous sample time

D. The block is inside a triggered subsystem

Solution

1. Step 1: Understand '-1' sample time meaning

   Sample time '-1' means inherit from parent or driving block.

2. Step 2: Identify why inheritance fails

   If block is inside a triggered subsystem, inheritance may not work because triggered subsystems have their own timing.

3. Final Answer:

   The block is inside a triggered subsystem -> Option D

4. Quick Check:

   Triggered subsystem blocks may block inheritance [OK]

Hint: Triggered subsystems block sample time inheritance [OK]

Common Mistakes:

• Thinking '-1' is invalid
• Ignoring subsystem context
• Assuming continuous sample time causes issue

5. You want to speed up your simulation by reducing the number of block executions but keep accuracy high. Which sample time configuration strategy is best?

hard

A. Use a larger fixed sample time for less critical blocks and smaller for critical ones

B. Set all blocks to continuous sample time

C. Set all blocks to inherited sample time '-1'

D. Use triggered sample time for all blocks

Solution

1. Step 1: Understand trade-off between speed and accuracy

   Larger sample times reduce executions but may reduce accuracy; smaller sample times increase accuracy but slow simulation.

2. Step 2: Apply selective sample time setting

   Assign larger sample times to less critical blocks and smaller to critical blocks to balance speed and accuracy.

3. Final Answer:

   Use a larger fixed sample time for less critical blocks and smaller for critical ones -> Option A

4. Quick Check:

   Selective sample time balances speed and accuracy [OK]

Hint: Adjust sample times by block importance [OK]

Common Mistakes:

• Setting all blocks to continuous slows simulation
• Using inherited sample time everywhere loses control
• Triggering all blocks may cause unnecessary executions