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. 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. 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. 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. Greets, Kiste -- 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.
