  In this paper, we introduce the concept of using passive arm structures with
intrinsic impedance for robot-robot and human-robot collaborative carrying with
quadruped robots. The concept is meant for a leader-follower task and takes a
minimalist approach that focuses on exploiting the robots' payload capabilities
and reducing energy consumption, without compromising the robot locomotion
capabilities. We introduce a preliminary arm mechanical design and describe how
to use its joint displacements to guide the robot's motion. To control the
robot's locomotion, we propose a decentralized Model Predictive Controller that
incorporates an approximation of the arm dynamics and the estimation of the
external forces from the collaborative carrying. We validate the overall system
experimentally by performing both robot-robot and human-robot collaborative
carrying on a stair-like obstacle and on rough terrain.
