The null balance method is used for precision force measurement in the precision industry in order to overcome the limitations of the load cell. The performance of any micro- weighing device is determined by the design of the precision mechanism. Such a device should be designed for high repeatability, large measurement range with high resolution and fast response characteristics. The device consists of a displacement amplification mechanism and a parallel spring. The mechanism proposed here is analyzed against the design conditions. Four design specifications were proposed for the improvement of force measurement performances. After each mechanical component was designed and analyzed, optimal design was applied.
A sensitive null position sensor was constructed by a laser diode and 2-element photo diode and electromagnetic part was designed by the permeance method and verified by an analysis program. PID control was applied to the complete system and output signal was filtered for the improvement of readability.
Double force compensation concept was proposed and applied in order to enhance the force measurement performance.
A lot of error source was analyzed. Measurement error by the temperature variation was compensated through the measurement of temperature by the Pb thermometer.
The system performances, such as resolution of 0.1mgf, the measurement range of over 300gf and settling time below 5 sec were obtained from experiment.
