ELECTRONIC CONTROL OF PRECISION MASS BALANCE

摘要

Electronic balance control of a high precision two-pan flexure-strip mass comparator for double weighing at the UK National Physical Laboratory has been implemented. Critical damping of the freely oscillating loaded beam has been achieved. With a 10 mg differential in 1 kg mass comparisons, steady-state compensation was held for 10 minutes alternatively on either pan, with a stability in mass reading of (standard deviation) 3 parts in 10~(-10) of a kilogram. Noise attenuation resulted in signal-to-noise ratio of 80 dB on a recovered mass difference signal magnitude of 6 volts dc equating to 1μg in 1 kilogram. The design enables a two-pan balance to be used in single and double weighing mode. The paper briefly outlines previous work on high-precision mass comparators, then outlines the basic system features of the NPL high precision mass comparator and describes the design and performance of the automatic balance control. The work undertaken confirms that the flexure strip balance, with correctly designed and constructed electronic control, forms a superior instrument for accurate measurement of mass (< μg in 1 kg).