A Parasitic Displacement Measurement System for Suspended Coil in Joule Balance

Alignment is very important for Kibble balances and the joule balance at the measurement uncertainty region of several parts in 108. In the joule balance, if the suspended coil is in a misalignment state, parasitic displacements including the horizontal displacements along the x- and y-axes and the angular displacements around the x-, y-, and z-axes, will be produced and introduce alignment errors during the measurement of the joule balance. It is key for the joule balance to measure these parasitic displacements of the suspended coil during the alignment stages. So this paper describes the parasitic displacements measurement method based on our laser interferometer system. By studying and discussing the decoupling methods, we can calculate the five degrees of freedom parasitic displacements without complicating the structures of the suspended coil. In addition, several uncertainty sources have been theoretically analyzed to simplify the adjustment procedure during the parasitic displacements measurement. The experimental results show that the decoupling methods are effective for the parasitic displacements of the suspended coil, and this paper can contribute to a precise alignment in the joule balance.

I. Introduction

It is key for the joule balance, or Kibble balance to perform a satisfactory alignment procedure [1]–[4]. In the joule balance, the ideal alignment state means that the vectors of gravity of the standard mass, the electrical–magnetic force, the moving direction of the exciting magnet, and the laser beam propagation must coincide with each other [5]–[7]. However, it is hard to achieve a perfect aligned state in the joule balance, there are always some parasitic vector components during the measurement, especially for the suspended coil hang under the mass comparator. This coil is employed to produce an electrical–magnetic force to balance the gravity of the measured mass. When the coil is in a misalignment state, the force produced by the coil current produces a displacement of the coil, along five degrees of freedom, relative to its zero-force, free-hanging position, namely, the horizontal displacements along the x- and y- axes and the angular displacements around the x-, y-, and z- axes, and then induce some extra work and alignment errors during the measurement of the joule balance [8]. Thus, it is key for the joule balance to measure these parasitic displacements during the alignment stages.