Torsion balance-based system for high-precision force measurement in horizontal plane: part I. Development concept

In this paper we present a force measurement system that is developed based on the principle of torsion balance. The system will primarily be used as a flowmeter for weakly electrically conducting fluids using the method of Lorentz force velocimetry. The paper aims to improve the technique of precision force measurements in the horizontal direction with a high dead load. The tilt dependency of the system is tested, and the tilt sensitivity is minimized to within 5.9 · 10⁻² N rad⁻¹, which is the required limit of the application. The voltage of the photoelectrical position sensor as an original output signal of the system is proportional to the introduced force. This relationship is calibrated using two different force sensing methods: (a) by generating known force values using variability aspects of the tilt dependency of the system, or (b) by applying electromagnetic forces generated with a miniature voice coil actuator. Additionally, the static and dynamic characteristics of the system are identified, from which the effective stiffness of the system is determined as 0.59 N m⁻¹, and the undamped natural frequency is 0.06 Hz with the settling time of 124 s. Finally, the force measurement system is tested extensively at the open-loop operational mode, enabling us to detect forces with 18.2 nN resolution in its linear working range of approximately  ±  30 µN. (paper)