Optimized design and experimental verification of an adjustable compliant constant-force mechanism

Abstract

The capability to provide an adjustable constant-force output is highly valuable for applications such as micromanipulation and precision assembly. The output force of conventional constant-force mechanisms is difficult to adjust. Their constant-force performance is also highly dependent on structural parameters. To address these limitations, this study proposes an adjustable constant-force mechanism that combines positive and negative stiffness. The mechanism consists of trapezoidal and bistable beams, and its constant-force output can be adjusted by varying the preload displacement between the positive- and negative-stiffness modules. First, analytical models of the trapezoidal and bistable beams were established separately, and the force-displacement relationship of the constant-force mechanism under the zero-preload condition was derived. Subsequently, the key design variables were identified through a sensitivity analysis of the structural parameters. The design was optimized using the non-dominated sorting genetic algorithm II, yielding a set of Pareto-optimal solutions, from which a representative compromise solution was selected. Finally, a prototype was fabricated and experimentally tested. Under the zero-preload condition, optimization increased the constant-force stroke from 0.7 to 2.1 mm and reduced the relative output-force fluctuation within the constant-force region from 10% to 5.5%. Moreover, varying the preload displacement enabled stable adjustment of the constant-force output across different force levels. The experimental results confirm the effectiveness and feasibility of the proposed mechanism and optimization method.