Hi Kiste, 2010/10/31 Oliver Seitz <[email protected]>
> Yahoo lets me only occasionally post to the list... They say I'm a spammer > ;-) > > At last I did connect a voltage source, a motor and a scope. The voltage at > the motor indeed is just noisy. Yes noisy, yet usable > But if you're connecting a little resistor in series to look at the > current, it's quite interesting. > > When you switch the scope to "AC" (that is, you've got a capacitor in > series) you see an oddly shaped alternating voltage. This is in fact the > magnetic poles of the rotor passing the permanent magnets. The frequency of > this voltage is straight proportional to the rotational speed. The > proportionality factor is the number of magnetic poles on the rotor. (or > twice the number, haven't looked that exactly yet.) > > So, by looking at the current, with a schmitt-trigger perhaps, you're not > only able to estimate the speed, but you can count fractions of a rotation. > Do you think it can be "visible" for any DC motors ? The most dirty motors may give the better results ? (lots of friction, maybe ?) > > Though there are motors with only 2 poles, they usually have an odd number > of poles, 3 and 5 is common for little DC motors, but there are racing > motors with 7 or 9 poles. > And this can be configurable in firmware ;) > > A big advantage is, if you're just counting zero-crossings, you don't have > to care about the actual voltage at all. 3V or 48V, the operating voltage > itself is subtracted by the capacitor, and a zero-crossing stays a > zero-crossing. > > A big disadvantage is, you'll get problems counting while directly powering > the motor by PWM. You might need to filter out the PWM frequency with a > low-pass filter, or switch the motor to full-on or full-off periodically to > measure the speed. > While experimenting back-emf, I switched off motors while acquiring/reading. What do you mean by "switch full-on/full-off", isn't just turning off while reading enough ? Also, I didn't power the motors with PWM but rather controlling pre-existing h-bridges, bypassing PWM signals from a RC tank control board itself. Would this still be a problem and would require to filter PWM frequency ? And, just my curiosity, do you have any pictures about what you've seen on the scope ? Cheers, Seb -- You received this message because you are subscribed to the Google Groups "jallib" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/jallib?hl=en.
