Stiffness modulation of an orthopedic robot based on large-range variable stiffness mechanisms

Abstract

Stiffness modulation is a critical challenge in orthopedic robots for deformity correction. In this work, a novel orthopedic robot with variable stiffness mechanisms is proposed for dynamic stiffness adjustment in clinical deformity correction. The new design incorporates a novel reconfigurable variable stiffness mechanism (RVSM) with linear displacement to achieve a large range of stiffness adjustments. When the spring stiffness is k = 0.5N‧mm⁻¹ and the number of proposed variable stiffness mechanisms is N = 1, the equivalent output stiffness of the RVSM is 2.84∼1.19×10⁶N‧mm⁻¹. In this paper, the principle of stiffness adjustment of the RVSM based on the slope-cam slider module is introduced, and the mathematical stiffness model of the RVSM is presented along with the characteristics analysis of the stiffness performance. With the matrix structural analysis method and the principle of virtual work, the stiffness model of the orthopedic parallel robot is established, which is used in stiffness modulation to adjust stiffness at different phases of deformity correction. The proposed stiffness modulation strategy fulfills the need for high stiffness in the early phase and low stiffness in the later phase, significantly reducing the stiffness fluctuations of the RVSMs. The efficiency of the established theoretical stiffness model is numerically validated through examples.