  When a human and a humanoid robot collaborate physically, ergonomics is a key
factor to consider. Assuming a given humanoid robot, several control
architectures exist nowadays to address ergonomic physical human-robot
collaboration. This paper takes one step further by considering robot hardware
parameters as optimization variables in the problem of collaborative payload
lifting. The variables that parametrize robot's kinematics and dynamics ensure
their physical consistency, and the human model is considered in the
optimization problem. By leveraging the proposed modelling framework, the
ergonomy of the interaction is maximized, here given by the agents' energy
expenditure. Robot kinematic, dynamics, hardware constraints and human
geometries are considered when solving the associated optimization problem. The
proposed methodology is used to identify optimum hardware parameters for the
design of the ergoCub robot, a humanoid possessing a degree of embodied
intelligence for ergonomic interaction with humans. For the optimization
problem, the starting point is the iCub humanoid robot. The obtained robot
design reaches loads at heights in the range of 0.8-1.5 m with respect to the
iCub robot whose range is limited to 0.8-1.2 m. The robot energy expenditure is
decreased by about 33%, meanwhile, the human ergonomy is preserved, leading
overall to an improved interaction.
