Quote:
Originally Posted by Al Skierkiewicz
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Don't forget that the voltage polarity on the case will reverse when the motor direction reverses.
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While this is true for the relative polarity between the two motor leads, it is not so for the can. The can will be shorted to one or other of the brushes*. The voltage on the can will then be pulled hard to Vbatt when that bit of the winding touches whatever side the Jaguar is holding to Vbatt and it will be pulled hard to Gnd when the motor turns such that that same portion of the commutator is touching the side the Jaguar is holding to Gnd.
As to noise and Jag PWMing, I think this the least of our concerns. A simple filter should make that bit go away.
As to the cRio getting upset with lower than 10K, I don't see how this comes into the picture.
Some ascii art:
Vbatt
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100 Ohm
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Motor Case -- RC filter (band pass) -- cRio Analog Input
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100 Ohm
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Gnd
The band pass should remove the DC offset of 1/2 Vbatt, allow the rail to rail swings (say 1200-18000 RPM motor speed so allow 20-300Hz or so), and remove the Jaguar PWM and other junk (1000Hz and above).
I am not a EE but it seems that there may be some room for a solution in that space.
Joe J.
*well, it will be shorted to both because the motor windings connect the two, but odds are the short will happen with the wire length to one brush being much shorter than the other so it will act like a short to that one brush.