  Mobile Manipulation (MM) systems are ideal candidates for taking up the role
of a personal assistant in unstructured real-world environments. Among other
challenges, MM requires effective coordination of the robot's embodiments for
executing tasks that require both mobility and manipulation. Reinforcement
Learning (RL) holds the promise of endowing robots with adaptive behaviors, but
most methods require prohibitively large amounts of data for learning a useful
control policy. In this work, we study the integration of robotic reachability
priors in actor-critic RL methods for accelerating the learning of MM for
reaching and fetching tasks. Namely, we consider the problem of optimal base
placement and the subsequent decision of whether to activate the arm for
reaching a 6D target. For this, we devise a novel Hybrid RL method that handles
discrete and continuous actions jointly, resorting to the Gumbel-Softmax
reparameterization. Next, we train a reachability prior using data from the
operational robot workspace, inspired by classical methods. Subsequently, we
derive Boosted Hybrid RL (BHyRL), a novel algorithm for learning Q-functions by
modeling them as a sum of residual approximators. Every time a new task needs
to be learned, we can transfer our learned residuals and learn the component of
the Q-function that is task-specific, hence, maintaining the task structure
from prior behaviors. Moreover, we find that regularizing the target policy
with a prior policy yields more expressive behaviors. We evaluate our method in
simulation in reaching and fetching tasks of increasing difficulty, and we show
the superior performance of BHyRL against baseline methods. Finally, we
zero-transfer our learned 6D fetching policy with BHyRL to our MM robot
TIAGo++. For more details and code release, please refer to our project site:
irosalab.com/rlmmbp
