Cascade Magnetic Suspension Balance System Accommodating Vertical Coaxial Errors

The concept of magnetic suspension balance (MSB) represents a novel approach to the non-contact sensing of trace sample mass. Nevertheless, the actual processing and assembly are challenging to guarantee that the centreline of the magnetically coupled structure is entirely coaxial with gravity. This results in periodic rotation and jittering of the kinematic levitation body, and the mass measurement exhibits a similar pattern of error. This paper presents a cascade MSB (CMSB) system designed based on Hall sensors and electronic analytical balances. The system employs the magnetic coupling force as the internal force, thereby mitigating the adverse effects of rotation instability on measurement results. The parameters of the coils and permanent magnets in the magnetic coupling unit are optimised to ensure that the magnetic coefficients are not sensitive to height. Furthermore, the optimal arrangement strategy of the Hall sensors is obtained based on this optimised structure in combination with electromagnetic simulation. The present CMSB system is capable of isolated measurement of sample mass while retaining the high measurement performance of the electronic analytical balance. The experiments show that the proposed CMSB has a mean absolute error of 1.03 mg and a root mean square error of 0.37 mg in the measurement of milligram samples compared to the pre-optimisation period, which is a 10.3% improvement in accuracy and a 27.3% improvement in stability.