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

[Gordonjcp]

On Tue, Mar 23, 2021 at 01:54:49PM +0000, Richie Burnett wrote:
> 
> 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.

Not only that but speed of response affects stability, and if you've got a good fast wideband control loop you'll get fast correction at the expense of short-term stability (jitter), with a lowpass loop filter providing greater long-term stability at the risk of introducing low-frequency oscillations.

I improved the short-term stability of my old Range Rover by improving the loop filter with one with better low-pass characteristics - I replaced the worn-out steering damper with a nice new Terrafirma gas-filled one, and it's absolutely transformed the handling.

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Gordonjcp