Flexible Automated Depalletizing: an Unwrapping Robot to Remove Plastic from Palletized Goods

Abstract

Despite being a crucial step of the intralogistic flow, pallet unwrapping, i.e., the removal of the plastic film that protects the items stored on pallets, has not been satisfactorily automated yet. However, automated solutions may improve workplace safety and work efficiency. The automated solutions available on the market are usually bulky machines that lack flexibility. In this work, we describe an unwrapping robot mounted on a small-footprint mobile base, hence easily relocatable, that can unwrap pallets even when the knowledge of the profile is affected by some uncertainty.

Full text

Flexible Automated Depalletizing: an Unwrapping
Robot to Remove Plastic from Palletized Goods
Chiara Gabellieri†, Alessandro Palleschi†, Manuel G Catalano∗, Manolo Garabini†, Lucia Pallottino†
Abstract—Despite being a crucial step of the intralogistic
flow, pallet unwrapping, i.e., the removal of the plastic film
that protects the items stored on pallets, has not been sat-
isfactorily automated yet. However, automated solutions may
improve workplace safety and work efficiency. The automated
solutions available on the market are usually bulky machines
that lack flexibility. In this work, we describe an unwrapping
robot mounted on a small-footprint mobile base, hence easily
relocatable, that can unwrap pallets even when the knowledge of
the profile is affected by some uncertainty.
Index Terms—Logistic Automation, Depalletizing, Robotic
Physical Interactions
I. INTRODUCTION
Parcels stacked on pallets are usually tightly wrapped in
plastic to stabilize them during storing or shipping and pro-
tect them from the potentially damaging effects of adverse
environmental conditions. The removal of the plastic film,
which we refer to as unwrapping, is the preliminary and
necessary operation to make the items accessible for manip-
ulation. Despite the general trend towards the robotization
of intralogistic processes, unwrapping is particularly far from
complete automation in the current industrial scenario, due
also to the perception and control challenges that it poses.
It is emblematic that even the most advanced integrated
solutions for warehouse automation—see, e.g., TATO-20R
depalletizer by MAS PACK1—require the presence of human
operators to execute the unwrapping task.
In the literature, robotic cutting is often executed under
human-robot shared control [1]. Typical applications are sur-
gical procedures and nuclear decommissioning. Targeting the
meat industry, [2] designs a method that relies on a complex
model of the beef shoulder for cutting soft materials with
two autonomous robotic manipulators. Instead, in the case
of automatic precision-cut of metallic pieces, the accurate
knowledge of the environment allows for precise positioning
of the cutting end-effector [3]. Active force control has been
proposed for autonomous cutting tasks of uncertain profiles
†Research Center “E. Piaggio”, Department of Information
Engineering, University of Pisa, Pisa, Italy. chiara.gabellieri1@gmail.com,
alessandropalleschi94@gmail.com, manolo.garabini@gmail.com,
lucia.pallottino@unipi.it
∗Istituto Italiano di Tecnologia, Genova, Italy, Manuel.Catalano@iit.it
This work has received funding from the European Unions Horizon 2020
research and innovation program under agreement no. 732737 (ILIAD) and
by the Italian Ministry of Education, and Research (MIUR) in the framework
of the CrossLab project (Departments of Excellence). The content of this
publication is the sole responsibility of the authors. The European Commission
or its services cannot be held responsible for any use that may be made of
the information it contains.
The authors thank Elisa Stefanini and Michele Pierallini for their valuable
work during experimental validation.
1http://www.maspack.com/en/product/tato-r-400-bpm/
(a) Experimental setup: plastic
only on the lateral surface of the
pallet.
(b) Experimental setup: plastic
on both the lateral and the top
surface.
Fig. 1. Pictures of the system during unwrapping task execution. On the left,
the robot unwraps a pallet in which the plastic covers only the lateral surface;
on the right, the robot unwraps a pallet in which the plastic partially covers
also the top surface.
[4]. However, force control may, in general, have stability
issues and requires high bandwidth [5]. The injection of extra
flexibility by allowing online deviations of a pre-planned tra-
jectory based on the proximity with the environment has been
exploited in the literature in order to increase the robustness
to uncertainties of different robotic tasks [6], [7].
Besides the challenges involved, sporadic execution is an-
other reason underlying the scarce automation of the unwrap-
ping task. A human operator can be assigned to different
tasks, carried out between the two unwrapping-task executions.
However, with the recent pandemic outbreak, a higher push to-
wards complete warehouse automation has risen, to guarantee
physical distancing. This additional motivation sums up to the
improvements of safety and working conditions that would
result from the adoption of automated unwrapping solutions
—while, of course, reducing the physical strain, automated
unwrapping would avoid the need for ladders and prevent
the risk of static electric shocks while manually handling the
plastic2.
2https://www.bwintegratedsystems.com/docs/default-source/literature/
material-handling/robotic unwrapper hr.pdf?sfvrsn=d3fbe398 4

Fig. 2. Exploded view of the cutting end-effector. The cutter is attached to
the robot via a flange. The actuation is composed of an electric motor with
a planetary gearbox. The circular blade, with a diameter of 60 mm and a
thickness of 0.6 mm, is protected by a frame.
Among the examples on the market MSK3fully automatic
unwrapping machine is composed of a portal frame under
which the palletized items are placed. A large metal structure
cuts the wrapping film. Hence, a rolling cylinder winds the
sheet, which is eventually suctioned under the machine. The
BWContainer Systems automatic unwrapper4consists of a
robotic arm of considerable size that melts the plastic with
a hot air gun. A mechanism for removing the open plastic
sheet is integrated. Suitable items are bulk glass or plastic
bottles, and aluminum or steel cans. No labels, tags, tapes,
and the like are allowed in the cutting region. The VARO
unwrapping machine5works with single-item cuboid pallets,
introduced inside a metal case where a custom cutting end-
effector cuts the stretch film on the top and lateral surfaces.
Eventually, there are unwrapping systems that apply solely to
specific objects. Among these, the automatic unwrapper by
Autorema6and the CSW-Multifeeder series7are designed for
packets of end-cans.
In conclusion, the solutions available on the market are
usually bulky machines that work on very specific pallets.
Thus, they may lack flexibility.
II. CONTRIBUTION
In this paper, we describe an autonomous unwrapping robot,
shown in Fig. 1, that has been first introduced in [8]. The
robot is composed of a perception module, an impedance-
controlled robotic manipulator mounted on a mobile base, a
custom cutting end-effector, and a suitable planning strategy.
Compared to the state-of-the-art solutions, our robot is lighter
and can be mounted on a reduced-footprint mobile base.
Hence, it can be easily relocated to maximize warehouse
efficiency. Furthermore, even if at the present state we unwrap
cuboid pallets (which, anyway, characterize a large part of
all practical applications), the system is intended to handle
arbitrarily shaped pallets. The main contribution of such a
solution is twofold: on one side, the robot is equipped with a
custom cutting device, designed after extensive experimental
3https://www.msk.de/loesungen/defoliersysteme/defolierer-paletten/
4https://www.bwpackagingsystems.com/docs/librariesprovider2/
fleetwoodliterature/robotic-unwrapper-unbagger-english.pdf?sfvrsn=12
5https://www.varomachinery.com/non- food/packaging/unwrapping-system/
6https://autorema.com/en/can-making- industry/
7https://www.youtube.com/watch?v=pnLEEi9ZY5I
evaluation of existing cutting tools with different working
principles; on the other side, the robot is provided with
a Cartesian impedance and trajectory planning strategy to
effectively execute the cut of the wrapping film without
damaging the goods, accounting for uncertain knowledge of
the environment. Furthermore, in this work, the possibility of
integrating online force feedback at a planning stage in order
to trigger proper reactions is discussed.
III. SYS TE M DESCRIPTION
In this section, we provide a description of the unwrapping
system.
A. Hardware
The robot is composed of a 7-degrees-of-freedom robotic
manipulator8equipped with a custom end-effector (see Fig.
1(a) and Fig. 1(b)). The end-effector is composed of a con-
cealed and actuated circular blade (see Fig. 2). The protection
prevents any contact between the blade and the underlying
objects, facilitates the engagement of the plastic film, and is
safe to be used in a human-robot shared environment. The
actuation of the blade enables a more effective cut. The mobile
base on which the robot has been mounted uses a lidar for self-
localization and an RGB-D camera for detecting the extents
of the pallet. A suction system to remove the plastic could be
integrated as in the state-of-the-art solutions.
B. Planning and Conrol
Pure position control, making the robot stiff during the
physical interaction, is not the most suitable solution to
guarantee the integrity of the objects. Our robot is controlled
with a Cartesian impedance control law. Uncertainties, due,
e.g., to noise and estimation errors coming from the perception
module, reasonably affect our knowledge of the pallet size
and position. Cuboid pallets, being the majority of the overall
shipped pallets, are considered at this stage. The pallets are
supposed to be wrapped either on the lateral surface (see Fig.
1(a)) or both on the lateral and top surfaces (see Fig. 1(b)).
In the most complex case, the one in Fig. 1(b), the robot
end-effector moves downward to reach the top surface of the
pallet, then forward to engage the plastic film and cut it. At the
edge of the pallet, it rotates and continues the cut downward.
Suitable planning of trajectory and impedance ensures that
the plastic film is actually engaged despite the uncertainties,
e.g., that, during the first motion, the cutter touches the pallet
surface and that the forces exerted on the wrapped items stay
bounded. Sensor feedback, especially force feedback, may be
used at a planning stage to adjust the cutter trajectory on-the-
fly.
IV. EXP ER IM EN TS A ND DISCUSSION
In [8], the proposed trajectory and impedance planning has
been tested on 26 trials, 13 in which the plastic was only the
lateral surface of the parcels (see Fig. 1(a)), and 13 in which
the plastic partially covered also the top surface of the pallet
8https://www.franka.de/technology

(a) (b) (c)
(d) (e) (f)
Fig. 3. Photo sequence of the unwrapping of a cuboid pallet using reactive planning. The cutter starts above a region free of plastic in 3(a). Hence, it moves
downwards until it senses a contact with the surface in 3(b). The contact triggers the stop of the descending motion and the beginning of a forward motion.
The cutter slips on the surface of the objects until it senses the force exerted by the engaged plastic film, in 3(c). Hence, the blade is actuated and the plastic
cut. When the plastic on the edge of the pallet exerts a force on the cutter, in 3(d), the cutter rotates of 90 degrees, in 3(e). After completing the rotation, the
cutter moves downward finishing the cut of the plastic along the lateral surface of the pallet. (3(f)).
(see Fig. 1(b)). The results showed 100% of success in the
scenario of Fig. 1(a) and 76.92% in that of Fig. 1(b).
From experience acquired in [8], it seems beneficial to
confer some awareness of the current task execution status
to the robot planner. Specifically, preliminary experiments
suggest that the percentage of success may be increased by
using online force feedback in the planning. For instance,
because it could be possible to rotate the cutter at the right
moment in the case of Fig. 1(b). Bad timing of the cutter
rotation due to inaccurate estimation of the pallet edge was
the main cause of failure in the previous results. Figure 3
shows a preliminary experiment in which a reactive planning
strategy for cuboid pallets has been adopted.
Moreover, if an unexpected collision is detected, a reactive
planner might trigger an emergency policy, the simplest one
being stopping the robot and calling for human assistance.
Finally, it could also be possible to detect the loss of the
engagement with the plastic film by sensing a reduction of
the external forces on the cutter under a certain threshold.
This work described an unwrapping robot composed of a
robotic arm mounted on a mobile base and equipped with a
cutting custom end-effector, and a perception system to acquire
information of the pallet to unwrap. Impedance law allows
controlled physical interactions with the environment, while
proper planning ensures effective task execution and bounded
forces on the underlying items. In the future, extensive tests
using force feedback at a planning stage will be carried out
in realistic environments. Irregularly shaped pallets will be
tackled, and the recognition of plastic-free regions of the
parcels will be automated. A prismatic joint may be added
at the base of the arm to augment its vertical workspace.
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