The improvement of an elastic hinge-type torque standard machine in NIMT

Abstract

The National Institute of Metrology of Thailand’s (NIMT) strain-controlled elastic hinge-type torque standard machine was designed to cover a measuring range of 1 N·m to 1 kN·m. The elastic hinge was used both at the fulcrum and the hanger of the lever arms. The designed elastic hinge’s thickness, 0.50 mm, caused a higher stiffness than a sheet metal plate of other types of torque machines. The bending moment of all elastic hinges affected the sum of the torque signal on the lever arm that was used to observe the balancing of the lever. The residual torque sensitivity, which was no better than 0.20 mN·m, significantly affected the uncertainty of the low-range torque realisation.

The calibration and measurement capabilities of the machine were 0.010 % (k = 2) in the measurement range of 10 N·m to 1 kN·m and 0.030 % (k = 2) in the measurement range of 1 N·m to 10 N·m. In the transducer calibration, the influence of the random bending moment of the elastic hinge affected the repeatability, reproducibility, and linearity of the low torque measurements. The cause of the bending moment of the elastic hinges was a result of the deviation of the centre of gravity (CG) of the weight on the pan from the reference line. To improve CMCs, separate signal calibrations were selected for this experiment i.e. the left hinge, the right hinge, and the fulcrum. The torque in each signal calibration was combined by software and was used to correct the calibration value of the torque.