Disturbance rejection in Simulink - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a disturbance input block in Simulink.

Simulink

add_block('simulink/Sources/[1]', 'model/Disturbance')

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

ASine Wave

BStep

CConstant

DPulse Generator

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a 'Constant' block which does not simulate a disturbance change.

Using 'Sine Wave' which is periodic, not a step disturbance.

The 'Step' block is commonly used to simulate a disturbance input in Simulink models.

2fill in blank

medium

Complete the code to connect the disturbance block to the plant input.

Simulink

add_line('model', 'Disturbance/1', '[1]/1')

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

AController

BSensor

CPlant

DReference

Attempts:

3 left

💡 Hint

Common Mistakes

Connecting disturbance to the controller input instead of the plant.

Connecting disturbance to the sensor which measures output.

The disturbance affects the plant input, so the line connects to the 'Plant' block.

3fill in blank

hard

Fix the error in the code to correctly set the disturbance amplitude parameter.

Simulink

set_param('model/Disturbance', '[1]', '5')

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

AAmplitude

BFrequency

CPhase

DSampleTime

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Frequency' which does not exist for Step block.

Using 'SampleTime' which controls timing, not amplitude.

The 'Amplitude' parameter sets the disturbance strength for the Step block.

4fill in blank

hard

Fill both blanks to create a feedback loop that rejects disturbance.

Simulink

add_line('model', '[1]/1', '[2]/1', 'autorouting', 'on')

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

APlant/1

BController/1

CSum/1

DOutput/1

Attempts:

3 left

💡 Hint

Common Mistakes

Connecting plant output directly to summation input.

Connecting output block instead of controller.

The controller output connects to the summation block input to form feedback for disturbance rejection.

5fill in blank

hard

Fill all three blanks to define a disturbance rejection test scenario.

Simulink

set_param('model/Disturbance', '[1]', '10'); set_param('model/Controller', '[2]', '1'); add_line('model', '[3]/1', 'Plant/1')

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

AAmplitude

BGain

CDisturbance/1

DFrequency

Attempts:

3 left

💡 Hint

Common Mistakes

Using Frequency instead of Amplitude for disturbance.

Connecting wrong blocks in add_line function.

Set disturbance amplitude to 10, controller gain to 1, and connect disturbance output to plant input.

Practice

1. What is the main goal of disturbance rejection in a Simulink control system?

easy

A. To keep the system output stable despite external changes

B. To increase the system's input signal strength

C. To remove all feedback loops from the system

D. To make the system run faster without delays

Solution

1. Step 1: Understand disturbance rejection purpose

   Disturbance rejection aims to keep the system output steady even when outside changes happen.

2. Step 2: Compare options to this goal

   Only To keep the system output stable despite external changes matches this goal by focusing on output stability despite disturbances.

3. Final Answer:

   To keep the system output stable despite external changes -> Option A

4. Quick Check:

   Disturbance rejection = output stability [OK]

Hint: Disturbance rejection means steady output despite changes [OK]

Common Mistakes:

• Confusing disturbance rejection with increasing input
• Thinking disturbance rejection removes feedback loops
• Assuming it speeds up the system only

2. Which Simulink block is commonly used to reduce the effect of disturbances in a control system?

easy

A. PID Controller block

B. Gain block

C. Scope block

D. Mux block

Solution

1. Step 1: Identify blocks that affect disturbance

   PID Controller blocks adjust system response to reduce disturbance effects.

2. Step 2: Eliminate unrelated blocks

   Gain changes signal strength, Scope shows signals, Mux combines signals but doesn't reduce disturbances.

3. Final Answer:

   PID Controller block -> Option A

4. Quick Check:

   PID Controller reduces disturbances [OK]

Hint: PID blocks help control disturbances in Simulink [OK]

Common Mistakes:

• Choosing Scope block which only displays signals
• Confusing Gain block with disturbance rejection
• Selecting Mux block which combines signals

3. Given this Simulink feedback loop with a disturbance input, what will happen if the PID controller gains are set too low?

Disturbance --> + --> System --> Output
               ^                 |
               |-----------------|
               PID Controller

medium

A. The system output will oscillate rapidly

B. The system output will closely follow the disturbance

C. The system output will reject the disturbance completely

D. The system output will be zero regardless of disturbance

Solution

1. Step 1: Understand low PID gains effect

   Low PID gains mean weak correction to disturbances, so system output follows disturbance closely.

2. Step 2: Analyze other options

   Complete rejection needs strong gains, oscillation usually comes from too high gains, zero output is unrealistic here.

3. Final Answer:

   The system output will closely follow the disturbance -> Option B

4. Quick Check:

   Low gains = poor disturbance rejection [OK]

Hint: Low PID gains mean weak disturbance correction [OK]

Common Mistakes:

• Assuming low gains cause oscillations
• Thinking output will be zero regardless
• Believing disturbance is fully rejected with low gains

4. You have a Simulink model with a disturbance rejection loop, but the output still shows large fluctuations. Which of these is the most likely error?

medium

A. The feedback loop is missing the disturbance input

B. The system has no input signal

C. The Scope block is not connected properly

D. The PID controller gains are set too high causing instability

Solution

1. Step 1: Identify cause of large output fluctuations

   High PID gains can cause the system to become unstable and oscillate, causing large fluctuations.

2. Step 2: Check other options

   Missing disturbance input means no disturbance effect, Scope connection affects display only, no input signal means no output but not fluctuations.

3. Final Answer:

   The PID controller gains are set too high causing instability -> Option D

4. Quick Check:

   High gains cause instability and fluctuations [OK]

Hint: Too high PID gains cause unstable output [OK]

Common Mistakes:

• Blaming Scope block for output fluctuations
• Ignoring PID gain settings
• Assuming missing disturbance input causes fluctuations

5. You want to design a Simulink model that rejects a sudden step disturbance affecting the system output. Which combination of blocks and settings is best to achieve fast disturbance rejection with minimal overshoot?

hard

A. Use a Scope block to monitor output and manually adjust input

B. Use only a Gain block with high gain and no feedback loop

C. Use a PID Controller block with tuned proportional and derivative gains, plus a feedback loop including the disturbance input

D. Use a Mux block to combine disturbance and input signals without control

Solution

1. Step 1: Identify blocks for disturbance rejection

   PID Controller with tuned gains helps react quickly and reduce overshoot; feedback loop is needed to correct disturbances.

2. Step 2: Evaluate other options

   Gain block alone cannot reject disturbances well; Scope only monitors signals; Mux combines signals but does not control.

3. Final Answer:

   Use a PID Controller block with tuned proportional and derivative gains, plus a feedback loop including the disturbance input -> Option C

4. Quick Check:

   PID + feedback = fast, minimal overshoot rejection [OK]

Hint: Combine PID with feedback loop for best disturbance rejection [OK]

Common Mistakes:

• Relying on Gain block without feedback
• Using Scope as control instead of monitoring
• Ignoring feedback loop importance