I am trying to read a resolver sensor attached to a stepper motor through a long cable (500 meters). More specifically with a resolver to digital converter.
However I get an error respect to the actual position like in the figure, error that is not present using a short cable instead of the long one.
In the right figure, the error in degrees is represented against turns of the motor (1 turn=360 degrees). It is clear how the error is repetitive. In fact the error is very close to this one at different rotor speeds, and with different excitation signal (7.5 kHz) peak to peak voltage values.
Assuming that in the cable there is coupling between the sine and cosine signals from the secondaries, I can reach only an expression explaining a second harmonic error with the motor position, but not the first harmonic. And the error fits quite well with a 1st + 2nd harmonic of the motor position, as shown in the following figure:
Any idea of what effect might be producing this kind of error?