Configuration Synthesis, Modeling, and Analysis of a Spatial Compliant Constant-Force Mechanism for Suppressing Parasitic Displacements

Abstract Compliant constant-force mechanisms are critical for precision operations, but traditional planar configurations suffer from undesirable parasitic displacements under external disturbances due to inadequate out-of-plane constraints, leading to significant performance degradation. To address this limitation, this article proposes a spatial compliant constant-force mechanism synthesized via degrees-of-freedom (DOF) analysis integrated with the freedom and constrained topology method. Composed of parallel flexible units, the mechanism achieves a single translational DOF and inherently exhibits enhanced constraint stability, effectively suppressing parasitic displacements. A systematic design approach is implemented, including theoretical modeling, parameter sensitivity analysis, and multi-objective structural optimization, to realize ideal constant-force characteristics. Experimental validation of the fabricated prototype demonstrates that the output force remains constant at 8.93 N within a stroke of 1.91 mm. Comparative tests under lateral disturbance demonstrate that the spatial configuration reduces out-of-plane deviations by approximately 80% compared to traditional planar designs, owing to its significantly enhanced out-of-plane stiffness. These results validate the proposed spatial design paradigm as a robust solution for applications requiring high precision and stable constant-force output.