  The accumulation of litter is increasing in many places and is consequently
becoming a problem that must be dealt with. In this paper, we present a
manipulator robotic system to collect litter in outdoor environments. This
system has three functionalities. Firstly, it uses colour images to detect and
recognise litter comprising different materials. Secondly, depth data are
combined with pixels of waste objects to compute a 3D location and segment
three-dimensional point clouds of the litter items in the scene. The grasp in 3
Degrees of Freedom (DoFs) is then estimated for a robot arm with a gripper for
the segmented cloud of each instance of waste. Finally, two tactile-based
algorithms are implemented and then employed in order to provide the gripper
with a sense of touch. This work uses two low-cost visual-based tactile sensors
at the fingertips. One of them addresses the detection of contact (which is
obtained from tactile images) between the gripper and solid waste, while
another has been designed to detect slippage in order to prevent the objects
grasped from falling. Our proposal was successfully tested by carrying out
extensive experimentation with different objects varying in size, texture,
geometry and materials in different outdoor environments (a tiled pavement, a
surface of stone/soil, and grass). Our system achieved an average score of 94%
for the detection and Collection Success Rate (CSR) as regards its overall
performance, and of 80% for the collection of items of litter at the first
attempt.
