  For a successful deployment of physical Human-Robot Cooperation (pHRC),
humans need to be able to teach robots new motor skills quickly. Probabilistic
movement primitives (ProMPs) are a promising method to encode a robot's motor
skills learned from human demonstrations in pHRC settings. However, most
algorithms to learn ProMPs from human demonstrations operate in batch mode,
which is not ideal in pHRC. In this paper we propose a new learning algorithm
to learn ProMPs incrementally in pHRC settings. Our algorithm incorporates new
demonstrations sequentially as they arrive, allowing humans to observe the
robot's learning progress and incrementally shape the robot's motor skill. A
built in forgetting factor allows for corrective demonstrations resulting from
the human's learning curve or changes in task constraints. We compare the
performance of our algorithm to existing batch ProMP algorithms on reference
data generated from a pick-and-place task at our lab. Furthermore, we show in a
proof of concept study on a Franka Emika Panda how the forgetting factor allows
us to adopt changes in the task. The incremental learning algorithm presented
in this paper has the potential to lead to a more intuitive learning progress
and to establish a successful cooperation between human and robot faster than
training in batch mode.
