This document discusses feedback in systems and provides some examples. It explains that feedback exists when two components of a system affect each other. It gives examples of both undesirable feedback, like audio feedback, and desirable feedback, like compound interest. The document then discusses a systems model of a bathtub with inflow and outflow that are independent of the reservoir size. It explains that a feedback loop will be added to make the drain rate dependent on the amount of water in the reservoir.

1. Systems Thinking: Systems modeling feedback
presentation

2. Feedbacks in a system
• Feedback exists between two components of a system when each affects the
other.
https://commons.wikimedia.org/wiki/File:Simple_Feedback_02.png

3. Feedbacks can cause undesirable outcomes…
• We have all probably covered our ears when experiencing audio feedback.
https://en.wikipedia.org/wiki/Audio_feedback
https://commons.wikimedia.org/wiki/File:Ru%C3%ADdo_Noise_041113GFDL.JPG

4. Feedbacks can cause desirable outcomes …
• Compound interest in finance is a feedback, and a very useful one
Cash on Deposit
Interest Earned
0
50
100
150
200
250
300
0 5 10 15 20 25
Dollars
Years
Simple vs. Compound Interest
Simple Interest
Compound Interest
Images made by K. Kreutz

5. Systemsmodelof the bathtub — review
• A STELLA model of a bathtub from Unit 3 includes a reservoir for the bathtub,
and two flows that represent the faucet and drain.
Flux Flux
Reservoir
In this case, the inflow and outflow rates are independent
of reservoir size.

6. Adding a feedback loop
• Now we will make the flow rate of the Drain dependent on the size of the
reservoir in Unit 4 Bathtub Model_Exp1
• In equation form, Drain = 0.1 * Water in Sink

7. Systems diagram of this new configuration
Faucet Bathtub Drain