I think the digital potentiometer will do the trick. Seems simple enough and I'm not really worried about efficiency so don't necessarily want to go down the PWM route. I guess the question with the digital potentiometer is can I vary the resistance (ie. voltage) and syncronise it with the speed movement of the XY table? So, if the sewing machine sews slower, the XY table moves slower and if the machine sews faster, the XY table sews faster.