Open-loop vs closed-loop systems in Simulink - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the main difference between open-loop and closed-loop systems

Which statement best describes the key difference between an open-loop and a closed-loop system?

A A closed-loop system operates without any input, but an open-loop system requires continuous input.

B An open-loop system always has a sensor, but a closed-loop system never uses sensors.

C An open-loop system does not use feedback to adjust its output, while a closed-loop system uses feedback to control its output.

D Open-loop systems are always digital, while closed-loop systems are always analog.

💡 Hint

Think about whether the system checks its output to make adjustments.

Open-loop systems do not monitor their output and cannot correct errors automatically. Closed-loop systems use feedback from the output to adjust inputs and maintain desired performance.

❓ Predict Output

intermediate

Simulink block output behavior in open-loop system

Consider a Simulink model where a constant input signal passes through a gain block with gain 3, without any feedback. What is the output if the input is 5?

Simulink

Input = 5;
Gain = 3;
Output = Gain * Input;

A 15

B 5

C 0

D 8

💡 Hint

Multiply the input by the gain value.

The output is the input multiplied by the gain. Since there is no feedback, output = 3 * 5 = 15.

❓ data_output

advanced

Effect of feedback on system output in closed-loop control

A closed-loop Simulink system uses a proportional controller with gain 2 and a feedback sensor measuring output. If the reference input is 10 and the system output initially is 6, what is the controller output after one iteration?

Simulink

Reference = 10;
Output = 6;
Error = Reference - Output;
ControllerGain = 2;
ControllerOutput = ControllerGain * Error;

A 16

B 12

C 4

D 8

💡 Hint

Calculate the error, then multiply by the controller gain.

Error = 10 - 6 = 4. Controller output = 2 * 4 = 8.

❓ visualization

advanced

Identify system type from Simulink block diagram

Given a Simulink block diagram with a summation block subtracting feedback from output and feeding into a controller block, which system type does this represent?

A Closed-loop system

B Open-loop system

C Feedforward system

D Hybrid system

💡 Hint

Look for feedback paths in the diagram.

The presence of a feedback loop where output is compared to input indicates a closed-loop system.

🚀 Application

expert

Predict system behavior with delayed feedback in closed-loop system

In a closed-loop Simulink system, a delay block is added in the feedback path. How does this delay affect the system's stability and response?

A The delay has no effect on system behavior in closed-loop control.

B The delay can cause oscillations or instability by slowing feedback response.

C The delay improves system stability by smoothing output changes.

D The delay converts the system into an open-loop system.

💡 Hint

Consider how feedback timing affects control accuracy.

Delays in feedback slow the system's ability to correct errors quickly, which can cause oscillations or instability in the control loop.

Practice

1. Which of the following best describes an open-loop system in Simulink?

easy

A. A system that uses sensors to monitor output and correct errors.

B. A system that continuously adjusts output based on feedback.

C. A system that operates without using feedback to adjust its output.

D. A system that always maintains a constant output regardless of input.

Solution

1. Step 1: Understand open-loop system definition

   An open-loop system works without feedback; it does not adjust based on output.

2. Step 2: Compare options to definition

   A system that operates without using feedback to adjust its output. matches this definition exactly, while others describe feedback or constant output.

3. Final Answer:

   A system that operates without using feedback to adjust its output. -> Option C

4. Quick Check:

   Open-loop = no feedback [OK]

Hint: Open-loop means no feedback used to adjust output [OK]

Common Mistakes:

• Confusing open-loop with closed-loop systems
• Thinking open-loop uses feedback
• Assuming open-loop always maintains constant output

2. Which Simulink block is typically used to create a closed-loop system?

easy

A. Gain block

B. Sum block with feedback connection

C. Constant block

D. Scope block

Solution

1. Step 1: Identify closed-loop system structure

   Closed-loop systems require feedback, often created by feeding output back to input using a Sum block.

2. Step 2: Match blocks to feedback role

   Sum block with feedback connection allows error calculation, enabling closed-loop control.

3. Final Answer:

   Sum block with feedback connection -> Option B

4. Quick Check:

   Feedback needs Sum block [OK]

Hint: Closed-loop needs feedback; Sum block enables feedback loop [OK]

Common Mistakes:

• Choosing Gain block which only scales signals
• Selecting Constant or Scope blocks which don't create feedback
• Ignoring the feedback connection requirement

3. Consider this Simulink setup: a Gain block with gain 2 connected directly to input, no feedback. What is the output if input is 3?

medium

A. 3

B. 1.5

C. 0

D. 6

Solution

1. Step 1: Identify system type and operation

   The system is open-loop (no feedback). Output = input * gain.

2. Step 2: Calculate output value

   Output = 3 * 2 = 6.

3. Final Answer:

   6 -> Option D

4. Quick Check:

   Output = input x gain = 6 [OK]

Hint: Multiply input by gain for open-loop output [OK]

Common Mistakes:

• Assuming feedback changes output
• Using input value directly without gain
• Confusing gain with division

4. You have a closed-loop system in Simulink but the output does not stabilize and keeps oscillating. What is the most likely cause?

medium

A. Gain value is too high causing instability

B. Feedback loop is missing

C. Input signal is zero

D. Sum block is not connected

Solution

1. Step 1: Understand oscillation cause in closed-loop

   Oscillations often occur when gain is too high, causing the system to overreact and become unstable.

2. Step 2: Eliminate other options

   Feedback loop missing or Sum block not connected would stop feedback, not cause oscillations. Zero input does not cause oscillations.

3. Final Answer:

   Gain value is too high causing instability -> Option A

4. Quick Check:

   High gain causes oscillation [OK]

Hint: High gain in feedback causes oscillations [OK]

Common Mistakes:

• Thinking missing feedback causes oscillations
• Assuming zero input causes oscillations
• Ignoring gain effect on stability

5. You want to design a Simulink model to maintain room temperature at 22°C despite outside temperature changes. Which system type and feature should you use?

hard

A. Closed-loop system with temperature sensor feedback

B. Open-loop system with timer-based heater control

C. Open-loop system with fixed heater power

D. Closed-loop system without feedback

Solution

1. Step 1: Identify control needs for stable temperature

   Maintaining temperature despite changes requires feedback to adjust heater power.

2. Step 2: Choose system type with feedback

   Closed-loop system with temperature sensor feedback adjusts output based on actual room temperature.

3. Final Answer:

   Closed-loop system with temperature sensor feedback -> Option A

4. Quick Check:

   Stable control needs feedback loop [OK]

Hint: Use feedback to adjust for changing conditions [OK]

Common Mistakes:

• Choosing open-loop which can't adjust to changes
• Ignoring need for sensor feedback
• Selecting closed-loop without feedback