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[DPRG] Re: Motor stuff
Subject: [DPRG] Re: Motor stuff
From: Chris Jang
cjang at ix.netcom.com
Date: Sun Apr 1 21:59:43 CDT 2007
>I (wrongly) suspected that my SMPS were not able to supply enough
>regulation/power/power sag was taking place...
>
>A scope across the microcontroller power terminals told the story...
>
>The moment the motors started a lot of NOISE appeared in the power supply...
>
>I already had the diode/capacitor shunt in place and expected this to
>attenuate noise.. but that was not to be.
>
>Finally, did a complete optoisolated board and now my bot's as merry
>as I wanted :)
I've done it both ways.
1. one battery pack for motors and power/microcontroller electronics
one battery pack for a digital computer
opto-isolators connecting between the two separate circuits
digital computer is inside a metal enclosure
2. one battery pack for everything: power and digital
no opto-isolators
no metal enclosures, everything inside a plastic shell
One difference is the size of the motors. For 1., the motors may
draw tens of amps. So I went with full isolation out of caution.
Around the time I was designing it, I read on Carmack's Armadillo
Aerospace blog about computer reset problems with only partial
isolation between big servos and the flight computer. Spikes
were traveling back along the control lines into the computer.
For 2., the motors are small as they are from a RC toy car. At
most, only a few amps are required. So I just guessed that
isolation was not required. It seems to work ok and is stable.
I've never seen a reset.
Another factor is that I use single board computers with onboard
switching regulators. These often have a wide input voltage range
from around 7 volts up to 20 to 30 volts. They may provide a lot
of power stability automatically.
This is a good question though for system design - when should
power and logic be isolated? When is partial isolation sufficient?
I will venture that there is no simple answer to this question.
That's why systems engineers speak of "sneak circuit analysis".
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