Analysis: as engineers and scientists know, a badly designed system may collapse due to positive feedback

By Harun SiljakTCD

Every once in a while, I have an Inigo Montoya moment. I exclaim "You keep using that word, I do not think it means what you think it means." It is triggered by the most terrifying term I could imagine, one I would dress up as for Halloween and scare every control scientist and engineer: positive feedback.

We hear it everywhere, from Alice renting a room on Airbnb to Bob organising a workshop. They have all received positive feedback and they are extremely happy. So why am I terrified?

The feedback mechanism lies in the foundation of control engineering and cybernetics. While control engineering may sound artificial and make us think of a control system in a nuclear power plant or in a spaceship, the core principles of control lie in nature, human organisation and behaviour.

This morning, you probably employed it successfully in the shower:

Ouch! Too hot! (turn it to the left fast!)
Aaargh! Too cold! (turn it to the right fast!)
Nn..not bad, but could be a bit colder (a gentle turn to the left)
Okkk… now it’s just a bit chilly (a gentle turn to the right)
Excellent (don’t touch it)

From Today With Sean O'Rourke show, Paul Mooney from Tandem Consulting on giving and receiving feedback in the workplace

It is simple: you have a vision of the desired output (the temperature you like), and you will make changes on the system until you reach it. You use sensory information (the current water temperature) and compare it with the desired output. That is also how you adjust your car speed with respect to other cars or the balance between your studying and social life. And that is the negative feedback loop.

It is called negative because the control is performed using the difference between desired output and the currently achieved one. The action control is proportional to this difference, as we have seen in the shower example. If the real temperature is far off, we turn the knob more than in the case of a small difference. As the consequence of our control actions, the difference decreases. When we hit the sweet spot and the difference is (close to) zero, we keep it that way.

If subtracting the desired output from the current state of the system is the mechanism of negative feedback, positive feedback is about addition. In our shower example, it ends badly, as it would mean that any difference between the desired and actual temperature would be quickly increased by our actions. The effect would be similar to a bad prank where someone switches the cold and hot water inlets in our shower. 

From RTÉ Radio One's The Business, managing director of QT Comments Ronan Morris and journalist Edel Coffey talk about online feedback

We say that negative feedback is self-regulating, while positive feedback is a self-reinforcing mechanism. The negative feedback mechanism drives a system to an equilibrium, while positive feedback drives it to an extreme. In biblical terms, positive feedback is introduced with "whoever has will be given more, and they will have an abundance". While that does sound financially attractive, positive feedback goes both ways: small debts start an avalanche and lead to complete ruin.

This is why negative feedback controls most of our life, both within our body (that is how blood sugar is regulated) and in the natural and artificial systems that surround us (schools are full of negative feedback loops). Extraordinary events call for positive feedback such as childbirth (the contractions are in a positive feedback loop with a hormone called oxytocin), blood clotting and viral tweets. If a system is badly designed, it may collapse due to positive feedback. Engineers rarely put it in the system on purpose, but the fear of inadvertent positive feedback loop always lurks.

Alice and Bob from earlier both use negative feedback loop to manage their business. They use the input from the clients to make their rooms and workshops as close as possible to the clients’ expectations. So when they speak of positive feedback, they in fact mean that the actual state surpassed the clients’ expectations (a hypothetical quantity reality minus expectations is positive), resulting in positive emotions. Negative feedback for them is the one where the reality doesn’t meet the expectations, resulting in negative emotions. In cybernetical/control-theoretic terms, Alice and Bob actually speak of the sign of the error (as in the difference between the desired output and the achieved one).

Negative feedback controls much of our life, both within our body and in the natural and artificial systems that surround us

Now, here’s a funny twist. If the robots of the future accept the misnomers that Alice and Bob use and call positive response positive feedback, the terms will mean completely the opposite. Namely, maths and automation engineering running in the background of control processes define error as desired output minus the achieved one.

To put this into perspective, imagine having an LCD display in your shower which shows the difference between the desired 35 degrees and achieved 40 degrees, -5 degrees. In an attempt to bring the error to zero, you would turn the knob to make the water warmer, and that would be the wrong move (the water is already too hot). This is why the instruments for human use, adjusting to our psychological reaction, show the opposite of the automation error; they show the achieved output minus the desired one.

Hence, if Bob tells his future robot colleague how he received a positive feedback, Carol the robot will respond "oh, you mean negative feedback. Silly humans". That’s the dystopian version, though. In the utopian version, Alice, Bob, and Carol will all stop using the positive/negative feedback misnomer.

Dr Harun Siljak is an EDGE MSCA Research Fellow at CONNECT at Trinity College Dublin


The views expressed here are those of the author and do not represent or reflect the views of RTÉ