A critical look at human´s bimanual lifting of objects with a power assist robot and its applications to improve the power-assist control

A one degree-of-freedom (vertical up-down) power assist robot was developed for lifting objects. The robot was simulated and six subjects independently lifted objects of three different sizes with the robot. In each trial, the subject used his two hands synchronously to lift the object with the robot and estimated the perceived weight of the lifted object by comparing it to a reference weight. We then critically analyzed subjects´ bimanual weight perception, load forces, object´s displacement and acceleration features. The results show that the perceived weight was 30% of the actual weight and the subjects applied 6.3 times larger than the actually required load forces. Again, the time trajectories of load force and acceleration were synchronized, but the trajectory of displacement was different from that of load force and acceleration etc. We used these findings to model a novel control strategy to modify the power-assist control. The novel control was such that a virtual mass exponentially declined from a large value to a small value when the subject lifted the object with the robot and the commanded velocity exceeded a threshold. The control modification reduced the excessive load forces and thus enhanced maneuverability, safety etc. of the system. Finally, we proposed to use the findings to develop power assist devices to manipulate heavy objects in industries.