An Open-Section Shell Designed for Customized Bending and Twisting to Ease Sitting and Rising in a Chair

This paper is concerned with a compliant-hinge mechanism in an assistive chair that does not use external power in aiding the elderly and arthritics in stand-to-sit and sit-to-stand manoeuvres. The mechanism, attached to the seat of the chair, acts like a nonlinear torsion spring that is effectively pivoted to the frame. A pair of semi-circular open-section shells that are rigidly connected to each other and fastened to the chair frame comprise the spring. A cam profile on a guide plate enables the shells to deform transversely even as they twist to provide customized torque-angle characteristic as per the weight of the occupant. The formulation of the design specification based on biomechanical considerations, kinetics of sitting and rising in a chair, kinetoelastic modelling of open-section shells, a new energy mapping method of designing the guideway for the shells, and simulation of the entire unitized compliant spring mechanism are presented