PWM generation in Simulink - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding PWM Signal Characteristics

Which parameter in a PWM signal directly controls the average voltage delivered to a load?

A Duty cycle of the PWM signal

B Amplitude of the carrier wave

C Frequency of the PWM signal

D Phase shift of the PWM signal

💡 Hint

Think about how the 'on' time affects power delivery.

The duty cycle defines the percentage of time the signal is 'on' during one period, controlling the average voltage.

❓ Predict Output

intermediate

Simulink PWM Block Output Behavior

Given a Simulink PWM Generator block configured with a 1 kHz frequency and a 40% duty cycle, what is the expected output waveform characteristic?

A A constant DC voltage at 40% of the supply voltage

B A square wave with 1 kHz frequency and 40% high time

C A sine wave with 1 kHz frequency and 40% amplitude

D A square wave with 40 kHz frequency and 1 kHz duty cycle

💡 Hint

Recall what a PWM signal looks like in time domain.

The PWM Generator outputs a square wave with specified frequency and duty cycle, so 1 kHz frequency and 40% 'on' time.

❓ data_output

advanced

Analyzing PWM Signal Data in Simulink Scope

You run a Simulink model with a PWM Generator block set at 2 kHz frequency and 75% duty cycle. The Scope block shows the output signal over 2 ms. How many complete PWM cycles appear in the Scope display?

A 4 complete cycles

B 8 complete cycles

C 1 complete cycle

D 2 complete cycles

💡 Hint

Calculate how many 0.5 ms periods fit into 2 ms.

At 2 kHz, each cycle is 0.5 ms. 2 ms / 0.5 ms = 4 cycles.

🔧 Debug

advanced

Identifying Error in PWM Duty Cycle Modulation

A Simulink model uses a PWM Generator block with a duty cycle input signal that varies from -10% to 110%. What error or unexpected behavior will most likely occur?

A The PWM signal amplitude doubles

B Simulink throws a runtime error due to invalid duty cycle values

C The PWM output frequency changes unexpectedly

D The PWM block clamps duty cycle to 0-100%, so no error but unexpected clipping

💡 Hint

Consider how duty cycle inputs outside valid range are handled.

Simulink PWM Generator clamps duty cycle inputs below 0% to 0% and above 100% to 100%, so no error but output duty cycle clips.

🚀 Application

expert

Designing a Simulink Model for Variable PWM Control

You want to design a Simulink model that adjusts PWM duty cycle based on a sensor input ranging from 0 to 5 volts, mapping 0 V to 10% duty cycle and 5 V to 90% duty cycle linearly. Which block combination correctly implements this mapping?

A Use a Gain block with gain 0.16 and a Bias block with bias 10, feeding into PWM duty cycle input

B Use a Gain block with gain 0.16 and a Bias block with bias -10, feeding into PWM duty cycle input

C Use a Gain block with gain 16 and a Bias block with bias 10, feeding into PWM duty cycle input

D Use a Gain block with gain 16 and a Bias block with bias -10, feeding into PWM duty cycle input

💡 Hint

Map 0 V to 10% and 5 V to 90% means slope = (90-10)/5 = 16, intercept = 10.

The linear equation is duty_cycle = 16 * sensor_voltage + 10. So gain=16 and bias=10.

Practice

1. What does PWM stand for in Simulink and what is its main purpose?

easy

A. Pulse Wave Measurement; it measures signal frequency.

B. Power Width Management; it manages battery power levels.

C. Pulse Width Modulation; it controls power by switching signals ON and OFF quickly.

D. Pulse Width Memory; it stores signal data.

Solution

1. Step 1: Understand PWM meaning

   PWM stands for Pulse Width Modulation, a method to control power by changing signal ON/OFF times.

2. Step 2: Identify PWM purpose in Simulink

   In Simulink, PWM controls power by switching signals ON and OFF quickly to adjust power delivery.

3. Final Answer:

   Pulse Width Modulation; it controls power by switching signals ON and OFF quickly. -> Option C

4. Quick Check:

   PWM = Pulse Width Modulation controlling power [OK]

Hint: PWM controls power by fast ON/OFF switching [OK]

Common Mistakes:

• Confusing PWM with measuring signals
• Thinking PWM stores data
• Mixing PWM with battery management

2. Which Simulink block is commonly used to generate PWM signals?

easy

A. Pulse Generator block

B. Gain block

C. Integrator block

D. Scope block

Solution

1. Step 1: Recall Simulink blocks for PWM

   The Pulse Generator block is designed to create pulses with adjustable width and frequency.

2. Step 2: Match block to PWM function

   Since PWM requires pulses with specific widths, the Pulse Generator block is the correct choice.

3. Final Answer:

   Pulse Generator block -> Option A

4. Quick Check:

   Pulse Generator block creates PWM pulses [OK]

Hint: Pulse Generator creates PWM signals in Simulink [OK]

Common Mistakes:

• Choosing Gain block which scales signals
• Selecting Integrator block which sums signals
• Picking Scope block which only displays signals

3. Given a Pulse Generator block with Amplitude=5, Period=10, Pulse Width=30%, what is the ON time duration of the PWM signal?

medium

A. 3 seconds

B. 5 seconds

C. 7 seconds

D. 10 seconds

Solution

1. Step 1: Calculate ON time from pulse width and period

   ON time = Pulse Width (%) x Period = 30% x 10 = 0.3 x 10 = 3 seconds.

2. Step 2: Confirm units and values

   Period is in seconds, so ON time is 3 seconds.

3. Final Answer:

   3 seconds -> Option A

4. Quick Check:

   ON time = 30% x 10 = 3 seconds [OK]

Hint: ON time = pulse width % x period [OK]

Common Mistakes:

• Using amplitude instead of period for calculation
• Confusing pulse width percentage with time
• Calculating OFF time instead of ON time

4. You set the Pulse Generator block with Pulse Width = 110%. What will happen during simulation?

medium

A. The block will generate an error and stop simulation.

B. The block will generate a continuous ON signal (always high).

C. The block will generate a continuous OFF signal (always low).

D. The block will ignore the pulse width and use default 50%.

Solution

1. Step 1: Understand pulse width limits

   Pulse Width should be between 0% and 100%. Values above 100% mean the signal is ON all the time.

2. Step 2: Predict simulation behavior

   With 110%, the block treats it as always ON, so the output stays high continuously.

3. Final Answer:

   The block will generate a continuous ON signal (always high). -> Option B

4. Quick Check:

   Pulse Width > 100% means always ON [OK]

Hint: Pulse Width >100% means signal always ON [OK]

Common Mistakes:

• Expecting simulation error for >100% pulse width
• Thinking signal stays OFF
• Assuming default pulse width is used

5. You want to simulate a PWM signal that controls a motor speed with a frequency of 50 Hz and a duty cycle of 40%. How do you set the Pulse Generator block parameters in Simulink?

hard

A. Set Period = 0.02 seconds, Pulse Width = 50%, Amplitude = motor voltage level

B. Set Period = 50 seconds, Pulse Width = 40%, Amplitude = motor voltage level

C. Set Period = 0.04 seconds, Pulse Width = 40%, Amplitude = motor voltage level

D. Set Period = 0.02 seconds, Pulse Width = 40%, Amplitude = motor voltage level

Solution

1. Step 1: Calculate period from frequency

   Period = 1 / Frequency = 1 / 50 = 0.02 seconds.

2. Step 2: Set pulse width and amplitude

   Duty cycle is 40%, so Pulse Width = 40%. Amplitude should match motor voltage level to control speed.

3. Final Answer:

   Set Period = 0.02 seconds, Pulse Width = 40%, Amplitude = motor voltage level -> Option D

4. Quick Check:

   Period = 1/50 = 0.02s, Pulse Width = 40% [OK]

Hint: Period = 1/frequency; set pulse width as duty cycle % [OK]

Common Mistakes:

• Using period equal to frequency value
• Setting wrong pulse width different from duty cycle
• Confusing period units (seconds vs milliseconds)