Passive compliant mechanism based knee exoskeleton to alleviate knee joint moment of workers during blueberry harvesting

Abstract

The blueberry industry is a key contributor to the New Zealand economy but is currently facing labour shortages that threaten yield security. Blueberry harvesting is physically demanding, limiting the available workforce. Fully autonomous solutions are financially unfeasible and compromise the berry marketability. As an intermediate solution, this paper develops a passive robotic knee exoskeleton designed to reduce the knee moment experienced by workers while harvesting. The device utilises compliant mechanisms to store elastic energy and assist users in rising from a deep squat. Presented in this paper is the optimisation of a linear compliant mechanism to deliver 117.85 N, based on the knee moment of a 43.46 kg human during squatting. FEA was used to optimise the mechanism shape and dimensions to achieve a uniform stress distribution across the material. Simulation with nylon-6-PLA achieved 100% of the target force with a stress safety factor of 1.56. A CNC-machined prototype was compression tested, revealing that 67% of the knee moment could be alleviated. Despite the deviation from FEA, results suggest compliant mechanisms can be effectively integrated into a passive knee exoskeleton to reduce the knee moment of blueberry harvesters.