Establishment of KRISS watt balance system to have high uniformity performance

Watt balance system is the promising equipment to determine Planck constant without accurate measurement of magnetic flux and coil length. To obtain high measurement uncertainty, uniform magnetic flux density and constant velocity control are required during the coil movement. For that KRISS watt balance system incorporates a closed-type cylindrical permanent magnet, vertical linear motor and piston guide. In this paper, the vertical actuating motor including piston guide is controlled to have constant speed and magnet assembly was fabricated to have high uniform magnetic flux density during the vertical coil movement. The velocity of moving coil was controlled within 4 × 10-3 (1σ) at the velocity of 2 mm s-1 using a linear motor and linear encoder feedback. Using 3-vertical homodyne laser interferometer the velocity ripple of coil located inside magnet is measured within the level of 7.35 × 10-3 (1σ) at the same speed. A magnet assembly, which consists of two permanent magnet rings and closed yokes, was fabricated. A flux shunt was designed for the magnet for minimizing the temperature effect. The yokes were divided into three parts: top, center, and bottom yokes. By re-machining of center outer yoke, magnetic flux density uniformity was improved. The uniformity of the magnetic field was within 3.9 × 10-4 over a vertical distance of 20 mm.