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- [Jill] Hi, I'm Jill Butler, and this is Universal Principles of Design. In this movie, feedback loops, or the importance of staying in the loop. The Tacoma Narrows Bridge was a suspension bridge built in 1940 in Washington state that spanned the Tacoma Narrows strait of Puget Sound. The bridge became famous for its tendency to undulate wildly in windy conditions, leading construction workers to name it, Galloping Gertie.
So, what made Galloping Gertie gallop? The Tacoma Narrows employed a novel girder design for that time to support its roadbed. Previous bridge designs allowed air to pass through their support structures, but the I-beams used in Galloping Gertie were solid and redirected air flow above and below the roadbed. This caused the bridge to flex and sway in high winds. Now, it's important to note that all bridges flex and sway under loads like this, but they're designed to do so within strict limits, and then to flex back and quickly stabilize.
Galloping Gertie had always eventually stabilized as well, until one fateful day in November 1940. On this day, just five months after it opened, Galloping Gertie twisted and flexed in 40 mile per hour winds until it ultimately collapsed. And the key to understanding why it collapsed is feedback loops. A feedback loop is a cause-and-effect chain that forms a loop. This loop produces output, which then feeds back into the system as input, changing its subsequent output.
And there are two types of feedback loops, positive and negative. Everything in the universe can be modeled and understood using just these two types of feedback loops in various combinations. They are like the atoms of systems thinking and design. Positive feedback loops amplify output, resulting in accelerated growth. And negative feedback loops dampen output, resulting in equilibrium around a point. So, to better understand what all this means, let's again look at what happened to Galloping Gertie.
Gertie had always flexed in the wind, flexed a lot in fact, but on the day the bridge collapsed, the winds were at just the right speed and coming from just the right direction to create a positive feedback loop, which caused her to flex a little too far. You know the escalating shriek that happens when a microphone gets too close to the speaker? This is the result of positive feedback. The speaker produces sound, which the microphone picks up as input, which is then amplified and emitted by the speaker, which is then picked up by the microphone and so on, until something breaks the cycle.
So, here's how this same cycle affected Gertie. When the wind hit the I-beams, the bridge deflected in one direction. Then, when the bridge flexed back, it was pushed by the wind, which caused it to flex back in the other direction even higher, which was again pushed by the wind, which caused it to flex back in the other direction even higher and so on, a phenomenon known as aeroelastic flutter. The effect is similar to pushing someone in a swing, where you give the rider an extra push on the backside of each swing, launching them higher and higher with each cycle.
Stuck in this positive feedback loop, the escalating bending and twisting eventually broke the bridge. What Gertie needed to gallop a little less violently was more negative feedback in the design, something that could stiffen the bridge, dampen the oscillations, and let the bridge stabilize. In Gertie's short five month lifespan, engineers tried various solutions to build in more negative feedback, with reinforced girders, additional tension cables, and reduced traffic loads, but unfortunately, none of these were enough to counteract the forces of the angry winds of November.
So, what can designers do with feedback loops? Most importantly, feedback loops can help you deeply understand system behaviors, which then enables you to identify distant and root causes to a problem, both of which are essential to troubleshooting and fixing bugs. For example, take fishing policy. Plentiful fishing waters mean more money for the boat owners, which means they can buy more fishing boats, which means more fish are being caught, which means more money, which means they buy more fishing boats, and so on.
Competition between fishing boats can accelerate this positive feedback loop even more, as everyone is trying to beat everyone else to get the best catch. Left alone, this results in fishing stocks depleting faster than they can replenish, which results in an overshoot and collapse of both the fishing population and all of the fishing businesses. This is why there are usually restrictions on hunting and fishing. Quota limits act as a negative feedback loop, ensuring stable fish and wildlife populations.
So, if you are designing a system, a product to start up or whatever, and you're looking to increase output and growth, you should think in terms of positive feedback loops. Whether the herding behaviors caused by crazy sales like you see on Black Friday, the advertising mean that everyone views and shares, or a reward system to build brand loyalty, all rely on positive feedback loops to grow. Just beware of unrestrained growth, which will inevitably result in overshoot and collapse.
If you're designing a system that needs to moderate, self-manage, self-balance, or stabilize around a set point, you should think in terms of negative feedback loops. Whether the homeostasis of a cell, the self-balancing mechanism of a segway, or a stock exchange circuit breaker that halts panic selling, all rely on negative feedback loops to maintain stability. Just beware of resisting change too much. Sometimes systems need to change to survive. And finally, if you're designing instruction or games, you'll need to employ a combination of positive and negative feedback loops to make game play engaging.
For example, positive feedback can be used such that achieving goals levels up the player, making subsequent achievements easier. And negative feedback can be used to moderate their progress, to keep the experience challenging. Too much positive feedback, and the user progresses too quickly and gets bored. Too much negative feedback, and the player stalls out and gets frustrated. So, whether you used your knowledge of feedback loops to increase gameplay in video games, to create a self-balancing skateboard control system, or to prevent the next Galloping Gertie from galloping itself into collapse, remember, great designers know the importance of staying in the loop.
Q: Why can't I earn a Certificate of Completion for this course?
A: We publish a new tutorial or tutorials for this course on a regular basis. We are unable to offer a Certificate of Completion because it is an ever-evolving course that is not designed to be completed. Check back often for new movies.