Open-loop vs closed-loop systems in Simulink - Visual Side-by-Side Comparison

Concept Flow - Open-loop vs closed-loop systems

Start

↓

Input Signal

↓

Open-loop System

→Output

↓

No Feedback

↓

Start

↓

Input Signal

↓

Closed-loop System

↓

Output

↓

Feedback: Output -> Comparator

↩Back to System Input

Shows two paths: open-loop with no feedback and closed-loop with feedback adjusting the system input.

Execution Sample

Simulink

input = 5
open_output = input * 2
error = input - open_output
closed_output = open_output + error * 0.5
print(open_output, closed_output)

Calculates output for open-loop and closed-loop systems with simple feedback correction.

Execution Table

Step	Variable	Value	Action	Explanation
1	input	5	Assign input	Input signal set to 5
2	open_output	10	Calculate open-loop output	Output is input multiplied by 2
3	error	-5	Calculate error	Error is input minus open_output (5 - 10)
4	closed_output	7.5	Calculate closed-loop output	Output corrected by adding half the error
5	print	10 7.5	Display outputs	Shows open-loop and closed-loop outputs

💡 All steps executed; outputs calculated for both systems

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
input	undefined	5	5	5	5	5
open_output	undefined	undefined	10	10	10	10
error	undefined	undefined	undefined	-5	-5	-5
closed_output	undefined	undefined	undefined	undefined	7.5	7.5

Key Moments - 2 Insights

Why is the error negative in step 3?

Because error = input - open_output = 5 - 10 = -5, showing the open-loop output overshoots the input.

How does feedback affect the closed-loop output?

Feedback uses the error to adjust output, reducing the difference between input and output, as seen in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'closed_output' at step 4?

A -5

B 10

C 7.5

D 5

💡 Hint

Check the 'Value' column for 'closed_output' at step 4 in the execution_table.

At which step is the error calculated?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look for the step where 'error' variable gets its value in the execution_table.

If the feedback gain changed from 0.5 to 1, how would 'closed_output' at step 4 change?

A It would be 10

B It would be 5

C It would be 0

D It would be -5

💡 Hint

Calculate closed_output = open_output + error * gain with gain=1 using values from variable_tracker.

Concept Snapshot

Open-loop system: output depends only on input, no feedback.
Closed-loop system: output is adjusted using feedback from output.
Feedback reduces error between desired and actual output.
Simple formula: closed_output = open_output + feedback_gain * error.
Open-loop is simpler but less accurate; closed-loop is more stable and precise.

Full Transcript

This visual execution compares open-loop and closed-loop systems. We start with an input signal of 5. The open-loop output is calculated by multiplying input by 2, resulting in 10. The error is the difference between input and open-loop output, which is -5, indicating the output overshoots the input. The closed-loop output adjusts this by adding half the error, resulting in 7.5. This shows how feedback in closed-loop systems helps correct the output closer to the input. The variable tracker shows how each variable changes step-by-step. Key moments clarify why error is negative and how feedback improves output. The quiz tests understanding of these steps and effects of changing feedback gain.

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