Eric Sumner wrote:<br>> In the robot I am building for SMU, the servo controller crashes
<br>> whenever the motor has an instantaneous load (hard start, hits a bump,
<br>> etc). I don't have any sort of filter capacitor across the motor
<br>> leads. I have several to choose from, how can I determine what the
<br>> proper value is?
<br><br>There have been lots of good answers to your question here, allow me to put a plug in for understanding what is going on. <br><br>Ask yourself this question "What happens when the motor has an instantaneous load?"
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<br>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).<br><br>Then ask yourself, for a rapid change in current demand (and potentially supply), what will be the responses of various parts in my circuit?
<br><br>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? <br><br>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.
<br><br>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.
<br><br>Look at the whole problem and the solution will emerge.<br>--Chuck<br><br>