How to Create Feedback Loop in Simulink: Step-by-Step Guide

To create a feedback loop in Simulink, connect the output of a block back to its input using a Sum block and a Unit Delay block to avoid algebraic loops. This setup allows signals to flow in a loop while maintaining simulation stability.

Syntax

In Simulink, a feedback loop is created by connecting blocks in a circular path. The key blocks used are:

• Sum: Combines input signals, often used to subtract feedback from input.
• Unit Delay: Delays the signal by one time step to break algebraic loops.
• Other functional blocks: Represent system components.

The typical connection pattern is:

Input --> Sum --> System Block --> Output --> Unit Delay --> Sum (feedback input)

plaintext

Sum --> System Block --> Unit Delay --> Sum (feedback input)

Example

This example shows a simple feedback loop where the output is fed back to the input through a delay. It models a system where the next output depends on the previous output.

plaintext

1. Open Simulink and create a new model.
2. Add the following blocks from the Simulink Library Browser:
   - <code>Step</code> (under Sources)
   - <code>Sum</code> (under Math Operations)
   - <code>Gain</code> (under Math Operations)
   - <code>Unit Delay</code> (under Discrete)
   - <code>Scope</code> (under Sinks)
3. Connect the blocks as follows:
   - Connect <code>Step</code> output to the positive input of <code>Sum</code>.
   - Connect <code>Unit Delay</code> output to the negative input of <code>Sum</code>.
   - Connect <code>Sum</code> output to <code>Gain</code> input.
   - Connect <code>Gain</code> output to <code>Unit Delay</code> input.
   - Connect <code>Gain</code> output to <code>Scope</code> input.
4. Set <code>Gain</code> block gain to 0.9.
5. Run the simulation and observe the output on <code>Scope</code>.

Output

The Scope shows a signal starting at 1 (Step input) and gradually stabilizing as the feedback loop influences the output, demonstrating a decaying feedback effect.

Common Pitfalls

Common mistakes when creating feedback loops in Simulink include:

• Not using a Unit Delay or similar block to break the algebraic loop, causing simulation errors.
• Incorrect sign in the Sum block inputs, which can invert feedback effect.
• Forgetting to connect all feedback signals properly, leading to disconnected loops.

Always ensure the feedback path includes a delay to allow the solver to compute values step-by-step.

plaintext

Wrong way:
Sum block inputs both positive without delay:
Step --> Sum (+)
Unit Delay output --> Sum (+)

Right way:
Sum block with one positive and one negative input and Unit Delay in feedback:
Step --> Sum (+)
Unit Delay output --> Sum (-)

Quick Reference

Tips for creating feedback loops in Simulink:

• Use Sum block to combine input and feedback signals.
• Insert a Unit Delay block in the feedback path to avoid algebraic loops.
• Check the sign of feedback in the Sum block to control positive or negative feedback.
• Use Scope to visualize the feedback effect during simulation.

Key Takeaways

Always include a Unit Delay block in the feedback path to prevent algebraic loops.

Use the Sum block to combine input and feedback signals with correct signs.

Connect feedback output back to the Sum block input to form the loop.

Visualize feedback effects using Scope during simulation.

Check connections carefully to avoid disconnected or incorrect feedback paths.