Parameters influencing the accuracy of the point of force application determined with piezoelectric force plates

Errors up to +/- 30 mm in determining the point of force application with piezoelectric force plates have been reported in the literature (Kistler, 1984. Multicomponent Measuring Force Plate for Biomechanics and Industry. Kistler, Switzerland; Bobbert and Schamhardt, 1990. Journal of Biomechanics 23, 705-710; Sommer et al., 1997. Proceedings of the XVI th I.S.B. Congress). To explain the main factors influencing the systematic errors the force plate system is modeled as a two-dimensional beam structure. By this model it is strongly indicated that the cause for the errors in determining the point of force application are bending moments in the measurement posts. The main parameters influencing the shape and magnitude of the error function are the ratios between the bending stiffness of the plate and the bending and compressive stiffnesses of the measurement posts. In the current design it is therefore not possible to eliminate the cause for the errors by changing the constructive parameters. By comparing the error functions derived with the beam model to the correction formulas given in the literature an improved algorithm is proposed. This paper shall help biomechanists in understanding the basic concepts of determining the point of force application with force plates and in constructing custom-made force plates for specific applications.