High accuracy velocity control method for the french moving-coil watt balance

We describe a novel method of velocity control dedicated to the French moving-coil watt balance. In this project, a coil has to move in a magnetic field at a velocity of 2 mm s-1 with a relative uncertainty of 10-9 over 60 mm. Our method is based on the use of both a heterodyne Michelson's interferometer, a two-level translation stage, and a homemade high frequency phase-shifting electronic circuit. To quantify the stability of the velocity, the output of the interferometer is sent into a frequency counter and the Doppler frequency shift is recorded. The Allan standard deviation has been used to calculate the stability and a σy(τ) of about 2.2x10-9 over 400 s has been obtained