Analytical Design of an Adjustable Constant-Force Mechanism for Body Weight Support Systems

Abstract Body weight support systems (BWSSs) are widely used in rehabilitation to assist patients by completely or partially unloading body weight, thereby reducing joint loading and enhancing stability during gait training. To address the limitations in adaptability and force tuning found in conventional systems, this article presents an analytical design for BWSSs that incorporates an adjustable constant-force mechanism (ACFM). The ACFM integrates a gear-lever-spring mechanism to generate a constant supporting force and a six-bar linkage to constrain motion to the vertical axis. This architecture enables stable and controllable support across a wide range of displacements, allowing for the automated adjustment of force output to match varying patient weights and rehabilitation tasks. The key advantages of the proposed design include the achievement of a large constant-force stroke and a highly adjustable force output, making the system suitable for diverse training motions. In this work, detailed kinematic modeling and force analysis are conducted to derive the design conditions for constant force. The influence of gear friction is also examined to assess its impact on system performance. Numerical and simulation studies are used to validate the effectiveness of the proposed design, demonstrating a constant-force stroke of up to 1 meter and force adjustability ranging from 0 to 1000 N. These results validate the feasibility and versatility of the ACFM-based BWSS as a mechanically efficient and adaptable solution for personalized rehabilitation applications.