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motor drift

Hi,

The motor voltage would be defined as the voltage across the motor coil, or:

Vmotor=Imotor*Rcoil, neglecting inductance of the coil.

So here is what you can do, measure across the coil of the motor on an oscilloscope. If you are commanding the motor with a sinewave, you should see a sinusoidal motor voltage. Look on the scope and see if there is a DC offset.

Offset = (Vpositivepeak-Vnegativepeak)/2

If there is an offset it will be integrated over time to produce a net velocity in the direction of the offset.

If there is no offset what Kevin said above would probably be the case, where the motor has a "preferred direction". Even if the motor wasn't designed to move in one direction or the other, it could have slightly more torque in one direction that the other which would produce the same effect. If what you said about the velocity being higher for a larger sinewave is true this may very well be the case.

T=Kt*I, where I is current, T is actual torque, and Kt is the torque constant, except the torque constant is never actually constant. Like Kevin said it may vary. In fact it will definitely vary with rotor angle somewhat, which means it could be different at one end of the sinewave than the other.

Rather than try to counteract the ending velocity by multiplying your waveform by K, you should offset it slightly in the direction opposite the ending velocity. So if a positive DAC voltage makes the motor turn clockwise, and your ending velocity is clockwise, output a sinewave with a small negative DC offset and see what happens. Keep increasing the negative offset (more and more negative) until the ending velocity is negative. Then try to hone in on exactly where the ending velocity is zero. You should see that by adjusting the offset you will linearly affect the final velocity.
-Devin
I got 99 problems but 8.6 ain't one.
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