A torsional sub-milli-Newton thrust balance based on a spring leaf strain gauge sensor

The development of a torsional thrust balance with a spring leaf sensor based on strain gauges is described in the present work. The balance is capable of measuring thrust forces up to 225 mN with micro-Newton resolution. Steady state thrust force is hereby measured via displacement of the balance arm, which in return leads to a displacement of the sensor. The strain induced by the force on the flexure member of the sensor is measured using strain gauges. The development of the sensor and the dynamic behavior of the balance are based on analytical analysis. The oscillatory motion of the balance arm is damped by a passive eddy current damper. An automated calibration mechanism with a pulley and calibration weights is used to calibrate the balance prior to measurement. Atmospheric as well as in situ calibration, single and multiple calibration runs with automatic averaging of calibration coefficients, is possible at all operational stages. The calibration of the balance exhibits excellent linearity and repeatability, while the dynamic behavior shows some deviations from the analytical prediction. The sensor exhibits low noise, with an estimated resolution within 15 μN. A state-of-the-art ratio of resolution to measurement range is achieved. As a conclusion, a series of thrust measurements in the range of 29 μN-37.04 mN are presented and briefly discussed.