DC motor modeling in Simulink - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the armature resistance in the DC motor model.

Simulink

R_a = [1]; % Armature resistance in ohms

Drag options to blanks, or click blank then click option'

A2.0

B0.5

C10

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a very high resistance value which is unrealistic.

The armature resistance is typically a small value; here, 0.5 ohms is correct.

2fill in blank

medium

Complete the code to calculate the back EMF constant K_b in the DC motor model.

Simulink

K_b = [1]; % Back EMF constant in V/(rad/s)

Drag options to blanks, or click blank then click option'

A0.1

B0.01

C1.0

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a value too large which would not match typical motor specs.

The back EMF constant is often a small decimal; 0.1 V/(rad/s) is typical.

3fill in blank

hard

Fix the error in the code to correctly compute the motor torque T using torque constant K_t and armature current I_a.

Simulink

T = [1] * I_a; % Torque in Nm

Drag options to blanks, or click blank then click option'

AK_t

BI_a

CR_a

DK_b

Attempts:

3 left

💡 Hint

Common Mistakes

Using current I_a alone or resistance instead of torque constant.

Torque is calculated as torque constant K_t times armature current I_a.

4fill in blank

hard

Fill both blanks to create a transfer function for the motor speed with input voltage V and output angular velocity omega.

Simulink

motor_tf = tf([[1]], [[2] 1]);

Drag options to blanks, or click blank then click option'

AK_t

BJ

CR_a

DL_a

Attempts:

3 left

💡 Hint

Common Mistakes

Using resistance or inductance in numerator incorrectly.

The numerator is the torque constant K_t, and the denominator includes the moment of inertia J for the motor speed transfer function.

5fill in blank

hard

Fill all three blanks to define the state-space matrices A, B, and C for the DC motor model.

Simulink

A = [[-[1]/[2], -[3]/[2]], [[3]/[2], 0]];

Drag options to blanks, or click blank then click option'

AR_a

BJ

CK_b

DK_t

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing torque constant K_t with back EMF constant K_b.

Matrix A uses resistance R_a and inertia J in denominators, and back EMF constant K_b in off-diagonal terms.

Practice

1. What is the main purpose of using Simulink for DC motor modeling?

easy

A. To write code for motor control algorithms

B. To manufacture DC motors automatically

C. To simulate motor behavior before building physical hardware

D. To measure motor speed with physical sensors

Solution

1. Step 1: Understand Simulink's role

   Simulink is a tool for creating visual models and running simulations, not for manufacturing or direct measurement.

2. Step 2: Identify the correct purpose

   Simulink helps simulate motor behavior to predict performance before hardware is built.

3. Final Answer:

   To simulate motor behavior before building physical hardware -> Option C

4. Quick Check:

   Simulate motor behavior = To simulate motor behavior before building physical hardware [OK]

Hint: Simulink simulates, it does not build or measure [OK]

Common Mistakes:

• Confusing simulation with physical measurement
• Thinking Simulink writes hardware code
• Assuming Simulink manufactures motors

2. Which Simulink block is typically used to represent the electrical input voltage to a DC motor?

easy

A. Step block

B. Integrator block

C. Constant block

D. Gain block

Solution

1. Step 1: Identify input voltage representation

   Step block is commonly used to simulate a voltage that changes at a specific time, like turning the motor on.

2. Step 2: Compare other blocks

   Gain scales signals, Constant holds fixed value, Integrator accumulates signals, so Step fits best for input voltage changes.

3. Final Answer:

   Step block -> Option A

4. Quick Check:

   Voltage input changes = Step block [OK]

Hint: Use Step block for voltage changes over time [OK]

Common Mistakes:

• Using Constant block for changing voltage
• Confusing Gain with input source
• Thinking Integrator models voltage input

3. Given this Simulink model snippet for a DC motor, what will the scope display if the input voltage is a step from 0 to 12V at 1 second?

Electrical input: Step block (0 to 12V at t=1s)
Motor speed output connected to Scope

medium

A. Motor speed decreases after 1s

B. Motor speed jumps instantly to max at 1s

C. Motor speed remains zero throughout

D. Motor speed gradually increases after 1s

Solution

1. Step 1: Understand motor response to voltage step

   When voltage steps up, motor speed does not jump instantly due to inertia; it rises gradually.

2. Step 2: Analyze scope output

   Scope shows motor speed increasing smoothly after 1 second, reflecting physical motor acceleration.

3. Final Answer:

   Motor speed gradually increases after 1s -> Option D

4. Quick Check:

   Motor inertia causes gradual speed rise [OK]

Hint: Motor speed changes smoothly, not instantly [OK]

Common Mistakes:

• Expecting instant speed jump
• Thinking speed stays zero
• Assuming speed decreases after voltage step

4. You built a DC motor model in Simulink but the motor speed output stays zero even after applying voltage. What is the most likely error?

medium

A. Step block voltage set to zero

B. Integrator block missing in the mechanical subsystem

C. Scope block not connected to output

D. Gain block value set to negative

Solution

1. Step 1: Check mechanical subsystem setup

   Integrator block models motor speed from acceleration; missing it means speed never changes.

2. Step 2: Evaluate other options

   Voltage zero means no input but question says voltage applied; scope connection affects display but not speed; negative gain affects direction but not zero speed.

3. Final Answer:

   Integrator block missing in the mechanical subsystem -> Option B

4. Quick Check:

   Missing integrator means no speed change [OK]

Hint: Mechanical integrator needed to get speed from acceleration [OK]

Common Mistakes:

• Ignoring missing integrator effect
• Assuming scope connection causes zero speed
• Confusing gain sign with zero output

5. You want to model a DC motor in Simulink that includes both electrical and mechanical dynamics with load torque changing over time. Which approach is best?

hard

A. Use separate electrical and mechanical subsystems connected by torque and speed signals, with a time-varying load torque input

B. Model only electrical subsystem and ignore mechanical load

C. Use a single Gain block to represent entire motor behavior

D. Apply a Constant block for load torque and ignore time variation

Solution

1. Step 1: Understand modeling requirements

   Electrical and mechanical parts affect motor behavior; load torque changes over time must be included.

2. Step 2: Choose modeling approach

   Separate subsystems allow clear representation and connection via torque and speed signals; time-varying load torque input models real conditions.

3. Step 3: Evaluate other options

   Ignoring mechanical or load dynamics oversimplifies; Gain or Constant blocks cannot capture dynamics properly.

4. Final Answer:

   Use separate electrical and mechanical subsystems connected by torque and speed signals, with a time-varying load torque input -> Option A

5. Quick Check:

   Separate subsystems + time-varying load = Use separate electrical and mechanical subsystems connected by torque and speed signals, with a time-varying load torque input [OK]

Hint: Model electrical + mechanical parts separately with load input [OK]

Common Mistakes:

• Ignoring mechanical subsystem
• Using constant load torque only
• Oversimplifying motor as single Gain block