  In the near future, collaborative robots (cobots) are expected to play a
vital role in the manufacturing and automation sectors. It is predicted that
workers will work side by side in collaboration with cobots to surpass fully
automated factories. In this regard, physical human-robot interaction (pHRI)
aims to develop natural communication between the partners to bring speed,
flexibility, and ergonomics to the execution of complex manufacturing tasks.
One challenge in pHRI is to design an optimal interaction controller to balance
the limitations introduced by the contradicting nature of transparency and
stability requirements. In this paper, a general methodology to design an
admittance controller for a pHRI system is developed by considering the
stability and transparency objectives. In our approach, collaborative robot
constrains the movement of human operator to help with a pHRI task while an
augmented reality (AR) interface informs the operator about its phases. To this
end, dynamical characterization of the collaborative robot (LBR IIWA 7 R800,
KUKA Inc.) is presented first. Then, the stability and transparency analyses
for our pHRI task involving collaborative drilling with this robot are
reported. A range of allowable parameters for the admittance controller is
determined by superimposing the stability and transparency graphs. Finally,
three different sets of parameters are selected from the allowable range and
the effect of admittance controllers utilizing these parameter sets on the task
performance is investigated.
