Measurement performance and experimental study of a stiffness-adjustable over-constrained parallel six-axis force sensor

• Influence mechanism of stiffness on the measurement error of the over-constrained SAFS is revealed. • Integrated parallel SAFS having stiffness-adjustment device with measurement branch. • Errors fell from 7.96%FS to 3.26%FS stiffness-adjustment device with measurement branch. The stiffness differences among the elements of an over-constrained structure can affect the internal force distribution among its internal elements. Due to this phenomenon, the application of an over-constrained configuration to a parallel six-axis force sensor (SAFS) has an impact on its measurement error. Based on this, research on reducing the measurement error of over-constrained parallel SAFS has been carried out. In this paper, the force method is used to reveal the influence mechanism of stiffness on the measurement error of over-constrained SAFSs, and the principle and conceptual design of applying a stiffness-adjustment mechanism in SAFSs are proposed. Firstly, a measurement model of the sensor considering displacement compatibility conditions is established. Then, the influence of the stiffness differences of measurement branches on the measurement error is analyzed. The proposed SAFS prototype is designed and fabricated. Finally, the prototype is evaluated by conducting load calibration experiments. The type-I errors of the proposed parallel SAFS with a stiffness-adjustment mechanism decreased from 5.85%FS, 5.24%FS, 7.96%FS, 4.73%FS, 4.96%FS, and 5.72%FS to 2.85%FS, 3.26%FS, 5.91%FS, 2.54%FS, 3.21%FS, and 2.92%FS, respectively, and the maximum type-II error decreased from 4.13%FS to 3.06%FS. These results indicate that stiffness adjustment is practically feasible for reducing the measurement error of over-constrained parallel SAFSs.