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[DPRG] Motor Filter Capacitors
Subject: [DPRG] Motor Filter Capacitors
From: Chuck McManis
cmcmanis at mcmanis.com
Date: Thu Mar 22 16:58:17 CDT 2007
Eric Sumner wrote:
> In the robot I am building for SMU, the servo controller crashes
> whenever the motor has an instantaneous load (hard start, hits a bump,
> etc). I don't have any sort of filter capacitor across the motor
> leads. I have several to choose from, how can I determine what the
> proper value is?
There have been lots of good answers to your question here, allow me to put
a plug in for understanding what is going on.
Ask yourself this question "What happens when the motor has an instantaneous
load?"
Generally you will find the characteristics of your circuit change in a
fundamental way (in this case probably in the total current flowing through
it).
Then ask yourself, for a rapid change in current demand (and potentially
supply), what will be the responses of various parts in my circuit?
Places to think about are your source of current (battery, powersupply, etc)
if it has a moderate internal resistance what will the resulting voltage be?
Remember that the voltage across an inductor is Ldi/dt which is inductance
times the change in current over the change in time. So a "sudden" (like
.001 second change) in current of 100 mA is a .1/.001 or 100*L change in
voltage. That change can be -100*L if the current is going one way and
positive if it is going the other way.
Put an oscilloscope on your power supply and watch what happens when your
motor stalls or has different loads if you can. You may find its output goes
to zero briefly, many microprocessors will detect that as a brown out and
reset themselves.
Look at the whole problem and the solution will emerge.
--Chuck
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