Design and analysis of an active stabilisation mechanism for mobile patient hoists using actuated counterweights

Abstract

Mobile patient hoists are mechanical devices designed to ease the physical demands on healthcare staff by providing a safer and more controlled method of lifting and lowering patients, reducing the need for manual handling. However, instability in mobile patient hoists poses a safety risk, particularly when patient loading shifts the centre of gravity and potentially destabilises the hoist. Similar destabilising effects may occur when the hoist is operated on inclined surfaces or encounters obstacles. This article proposes a solution to enhance the stability of mobile patient hoists by introducing three actuated counterweights that dynamically redistribute mass along orthogonal axes while preserving the original support footprint. The positions of the counterweights are controlled in real time based on a continuous estimate of the combined centre of gravity derived from measured reaction forces at the four wheels. This approach regulates the combined centre of gravity of the hoist, patient, and counterweights to remain within the support polygon defined by the wheel contact points. A 3D model of a typical mobile patient hoist was adapted to include three motorised counterweights, two along the legs and one perpendicular to them. The stability margin during patient loading, the maximum tilt angle on inclined surfaces, and the safe travel speed during collisions were evaluated for all counterweight mass combinations between zero and the maximum allowable mass defined by geometric constraints. Although maximum stability is achieved by maximising counterweight mass, such configurations significantly increase system weight and reduce manoeuvrability. The presented results, therefore, enable the selection of optimised counterweight combinations, providing the corresponding stability margin, tilt angle, and safe travel speed for each configuration.