[sdiy] How to explain how negative feedback lowers noise?

Richie Burnett rburnett at richieburnett.co.uk
Tue Mar 23 14:54:49 CET 2021


All of these are good examples.  Automatic cruise control is another one that is often used in control theory class and textbooks, but very similar to the lawn mower throttle control. 

They also lead on well to the subject of stability... If you're drifting ever so slightly to the left in a car you don't instantly slam the steering wheel to the right limit to correct it! You'd probably move it slightly to the right and then see if the situation starts to improve. If you're still heading for the left verge you might turn it a bit further to the right. If instead you're heading quickly towards the left verge you might choose to apply more drastic correction, apply it quicker, or even do both. That is what PID control is all about.

-Richie, 

---- Ian Fritz wrote ----

>Or driving a car. If it veers to the right, you steer towards the left to correct it.
>
>> On Mar 23, 2021, at 6:33 AM, thresholdpeople via Synth-diy <synth-diy at synth-diy.org> wrote:
>> 
>> Negative feedback is something that's applied to everyone's actions all the time. It also tends to bring systems/attempts to into equilibrium.
>> 
>> Something as 'simple' as correcting your walking trajectory: if you see someone else walking toward you perpendicularly and you anticipate that you will both collide, so you adjust.
>> 
>> Thermal regulation: you overheat, so your body generates sweat to cool your body. If you cool down too much, your body begins to convulse, shivering to warm the body up through movement.
>> 
>> It's also at the core of cybernetics, through which it's applied to many systems- human, natural, machine- and as a consequence through parallels of regulation and understanding of those systems in that way comes the tendency to bundle all those up into one.
>> 
>> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
>> 
>>> On Tuesday, March 23rd, 2021 at 7:55 AM, cheater cheater <cheater00social at gmail.com> wrote:
>>> 
>>> Ok but if feedback increases linearity then:
>>> 
>>> Without feedback we have less linearity, therefore the original signal
>>> 
>>> comes with a bunch of distortion products. Meanwhile the noise can
>>> 
>>> keep raising in power longer before it starts clipping. So in that
>>> 
>>> case, you're actually increasing noise, while the signal doesn't
>>> 
>>> increase, only its distortion products.
>>> 
>>> Is this logic correct?
>>> 
>>>> On Mon, Mar 22, 2021 at 9:17 PM Gordonjcp gordonjcp at gjcp.net wrote:
>>>> 
>>>>> On Mon, Mar 22, 2021 at 06:40:52PM +0100, cheater cheater wrote:
>>>> 
>>>>> feedback and not some form of other regulation. What's a simple
>>>>> 
>>>>> /physical/ negative feedback?
>>>> 
>>>> The throttle on your lawnmower. The throttle cable has a spring that pulls the throttle butterfly in the carb open, and a wire link to a flappy thing beside the cooling fan. The faster the engine goes, the more the cooling fan blows on the flappy thing, which pulls the throttle closed against the spring.
>>>> 
>>>> When you hit a big divot of couch grass with the blades the engine revs drop, the cooling fan doesn't blow as hard, and the flappy thing gets pulled in by the spring opening the throttle. As the engine speeds up it blows the flappy thing back again and pulls the throttle shut until the revs settle, with the engine making more power.
>>>> 
>>>> Once the blades have chewed their way through the couch grass the engine will rev up freely, the flappy thing will get blown out further, the throttle will close, and the revs will stabilise again.
>>>> 
>>>> When you're done you push the throttle to idle relaxing the tension on the spring (your control input) and the flappy thing will blow further out again because the spring isn't holding it, and close the throttle.
>>>> 
>>>> --
>>>> 
>>>> Gordonjcp
>>>> 
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