Development of Garbage Collecting Robot for Marine Microplastics
Mitsuki Uno1 and Ryo Kurazume2

Abstract— Marine microplastics originate from plastic products and are crushed as they drift through the ocean, posing
a serious threat to marine ecosystems. Nonetheless, collecting
these scattered and small microplastics by hand from washedup beaches is challenging. Consequently, we are working on
designing a cleaning robot to automatically gather marine
microplastics on beaches. This paper outlines the primarymechanism of the cleaning robot and explores the necessaryfunctions for effective microplastic collection. Two kinds ofexperiments exploring the behavior of sand in stationary anddynamically states, ”Pseudo-Angle of Repose Measurement
Experiment” and ”Claw Excavation Experiment”, are carriedout using an experimental model of a cleaning robot.

I. INTRODUCTION
Marine microplastics consist of small plastic products likebottles and straws, discarded into the ocean, crushed asthey drift, and eventually washed up on beaches and otherlocations. This garbage poses a significant threat as marineorganisms might mistake it for food, disrupting the marine
ecosystem. Presently, local residents manually clean up thelarger plastic garbage. However, as depicted in Fig.1, dealing
with crushed plastic garbage is extremely challenging due toits integration with sand and widespread scattering.

Fig. 1.

Marine microplastics on beaches

Presently, we are in the process of developing a cleaning
robot capable of automatically collecting marine microplastics on beaches. As a primary step, this paper introduces thefundamental mechanism of the cleaning robot and examines
the necessary functions for efficient microplastic collection
on beaches.
II. PREVIOUS RESEARCH
Numerous proposals have been made regarding the useof robots as a solution to marine pollution, leading to
1 Mitsuki
Uno is with Graduate School of Information
Science and Electrical Engineering, Kyushu University, Japan.

uno@irvs.ait.kyushu-u.ac.jp
2 Ryo Kurazume is with Faculty of Information Science andElectrical Engineering, Kyushu University, Fukuoka 819-0395, Japan.

kurazume@ait.kyushu-u.ac.jp

the development of various garbage collection robots inprevious studies. For example, the small clean robot utilizes
tires suitable for rough terrain and employs rotating shovelshaped claws for garbage collection [1], while the cleaning
robot mimics a broom and dustpan mechanism for garbage
collection [2]. Additionally, there is a proposed robot thatutilizes a conveyor belt for collecting discarded bottles [3].
However, these robots primarily focus on gathering larger
garbage items such as plastic bottles, and an efficient cleaning robot specifically designed for collecting small marine
microplastics (less than 5 mm) scattered on sandy beaches
has not been proposed. As an alternative approach, we havepreviously developed a robot that collects garbage using avacuum [4] as shown in Fig.2. This particular robot uses
suction force to collect fine garbage, but it faces challenges
concerning power efficiency and the separation performanceof garbage and sand.

Fig. 2.

Vacuum-type marine microplastics cleaning robot [4]III. REQUIRED SPECIFICATIONS
The functions essential for collecting garbage on beaches
involve ”excavating” sand and garbage, as well as ”separating” sand and garbage. To effectively address these functions,
we propose a robot equipped with a conveyor belt featuring
a mesh net and claws to efficiently collect small garbage
from the beach, as depicted in Fig.3. The claws ensure stable
excavation, while the belt conveyor with mesh effectively
separates sand and garbage. Furthermore, the garbage isclassified by size by squeegees on the conveyor belt. Thesemechanisms can efficiently collect sandy beach garbage asshown in Fig.4.
This paper primarily focuses on the ”excavating” functionof retrieving sand and garbage. To achieve this, the study
adopts ”claws” for excavation. By affixing large claws to therobot’s front, it can simply move forward to dig out sandand garbage. This method was chosen due to its simplicity
and reliable performance in achieving the objective.

In this research, the mechanism under study involves
considering the claw as an inclined plane rather than the
traditional horizontal plane associated with the angle of repose. To examine this, the experiment modifies the horizontal
surface into a slope or step, as shown in Fig.5, to obtain apseudo-angle of repose. The objective of this experiment is
to explore the impact of the angle of repose on the claws
used for sand o excavation and determine the optimal angle
and shape for the claws.

Fig. 5.

Fig. 3.

Conveyor belt-type marine microplastics cleaning robot

Fig. 4.

Application of angle of repose

1) Procedures: Fig.6 presents the 3D models of the angleof repose measuring instrument and the conical base used inthis experiment. The cone is placed inside the circular cavity
at the instrument’s center, and sand is poured through the top
funnel of the instrument to create a sand pile atop the cone.
The pseudo-angle of repose, shown by the red line in Fig.7
is then measured from the formed sand pile using a digital
protractor.

Garbage collection process

IV. DESIGN OF CLAW
The excavating claw’s parameters encompass the angle
and shape of the claw. To elucidate these characteristics,
this study conducted three experiments: the ”Pseudo-Angle
of Repose Measurement Experiment,” ”Claw Excavation
Experiment,” and ”Sandy Beach Experiment.” The ”PseudoAngle of Repose Measurement Experiment” aims to determine the appropriate angle and shape of the claws under
static conditions, where only gravity influences the sanddeposited on the claws. The ”Claw Excavation Experiment”
seeks to identify claws that efficiently collect garbage by
utilizing sand piles generated during excavation. Lastly, the”Sandy Beach Experiment” evaluates the garbage collection
performance on-site against the actual garbage present.

Fig. 6. A : Pseudo-angle of repose measuring instrument, B : Circular
truncated coneA. Pseudo-Angle of Repose Measurement Experiment
The angle of repose represents the angle between the slope
and horizontal plane of a granular deposit during its formation. Various methods, such as the tilting box method, fixed
funnel method, and drum method, are used to measure it [5].
While the general angle of repose for sand is approximately
34°, it can vary based on the shape and size of the grains[6]. Reference paper [7] indicates that sand piles depositedalong walls exhibit deformations corresponding to their angleof repose.

Fig. 7.

Pseudo-angle of repose measurement experiment

The measurement items consisted of two types of cones: a
50 mm diameter cone with 0, 1, 2, and 3 steps and a 100 mm
diameter cone with 0, 2, 4, and 6 steps as shown in Fig.8.
Each cone was made with a slope ranging from 0 to 35° atintervals of 5°. The study examined the impact of changing
the slope and surface geometry on sand deposition, as wellas the behavior of sand piles at different scales.

Fig. 8.

Shapes of cones with or without steps

2) Results: Fig.9 illustrates the measurement results at 50mm, and Fig.10 at 100 mm. Both graphs present the pseudoangle of repose on the vertical axis and the slope angle on thehorizontal axis, with each line representing the variation insteps. Regions with a pseudo-angle of repose of 0° indicate
areas where no sand pile was formed. For cones with a 50
mm diameter, the pseudo-angle of repose remained stable atapproximately 30-31°, and sand piles could not form when
the slope exceeded 25°. In the case of the 100 mm diameter
cone, the pseudo-angle of repose remained stable at about
32°, and sand piles could not form when the slope exceeded
30°. In both instances , the angle of repose showed little
influence from the number of steps.

Fig. 9.

Results of experiments with a 50 mm diameter coneFig. 10.

Results of experiments with a 100 mm diameter cone

3) Discussion: Based on the experiment’s findings, thepseudo-angle of repose for the sand used in this study isestimated to be approximately 30 to 32°. However, sand piles
could not form at slope angles of 25° or higher for the 50 mm
diameter cone and 30° or higher for the 100 mm cone. Thisoccurrence is likely attributed to the claws mostly replacing
the sand pile, resulting in an insufficient of sand to forma stable pile. The discrepancy in the slope angle limit forforming sand piles between the 50 mm and 100 mm diametercones can be attributed to the larger slope surface area of the
100 mm diameter cone, enabling it to hold more sand. Given
that sand piles cannot form beyond slope angles of 25-30°,
it can be assumed that sand can be deposited at a minimum
angle of approximately 20° when this slope angle is applied
as a claw angle. Nevertheless, it is essential to note that thisexperiment solely involved sand deposition from above anddid not encompass actual sand excavation. Thus, considering
lateral forces during excavation, it is conceivable that sandpiles could be formed at steeper angles.

B. Claw Excavation Experiment
As shown in Fig.11, for the waste to be conveyed tothe separation mechanism situated behind the claw, it must
be displaced by the sand flow created during the formationof sand piles during excavation. However, the configurationof this sand pile varies based on factors such as the claw
angle, excavation speed, and sand’s water content. To explore claws that enable efficient excavation, this experiment(Fig.12) measured the amount of sand required to form asand pile (amount of sand formation [cm3 ]), the quantity ofsand that displaces the garbage (amount of outflow [cm3 ]),and the associated garbage collection rate [%] (number ofgarbage collected per all garbage prepared). To obtain theamount of formation, the difference in volume between theclaws and the sand pile was measured using point clouddata acquired through the Azure Kinect depth camera. The
amount of outflow and the collection rate were determinedfrom the mass of sand collected in the box (Fig.13) and thenumber of garbage pieces retrieved. In evaluating the index,
superior excavation performance was determined by lower
the amount of formation and higher the amount of outflow,
as this suggests more efficient garbage excavation throughthe formation of smaller sand piles and increased sand flow
when excavating over the same distance.

Fig. 11.

Sand and garbage flow from excavation

The experiment is depicted in Fig.15. The excavating claw
was inserted into the sand by 2 to 2.5 cm, and then therail-mounted claw was pulled using a motor and wire for
excavation. The excavation distance covered approximately
1 m. The claw slope angle was set to four patterns: 20, 40,
60, and 80°, while the excavation speed had six patterns:
0.05, 0.1, 0.15, 0.2, 0.25, and 0.3 m/s. Additionally, the sandmoisture content was tested in three patterns: approximately
0, 8, and 16%.
Fig. 12.

Evaluation index

1) Procedures: The 3D model of the claw used in thisexperiment is shown in Fig.13. It was fabricated using a 3D
printer and a polycarbonate plate. The claw has a height of 5
cm, while the plate has a height of 14 cm and a width of 12cm. At the rear of the claw, a collection box gathers the sandand garbage that flows out during the process. The claws are
efficiently held in place by the polycarbonate plates, ensuring
secure retention of the sand for waste removal and facilitating
observation of the formed sand pile’s condition.

Fig. 15.

Fig. 13.

Claw

The equipment used for continuous excavation in theexperiment is presented in Fig.14. It consists of a frame madefrom polycarbonate sheets and an aluminum frame, designed
to collect sand. Two 1-meter-long rails are positioned on
the frame, and the claws are mounted between these rails.
Additionally, the motor winds the wire shown by the yellow
line in the diagram, enabling the claws to be pulled for
excavation.

Fig.16 illustrates the arrangement of garbage used in theexperiment. Plastic plates of approximately 3 mm in size,
resembling the secondary microplastics commonly foundscattered on beaches, were fabricated and employed. Tenplates were positioned at equal intervals in front of the claws,
as indicated by the red circle in the diagram. Although theposition of these plates will affect the garbage collection rate,they were placed along the center line in order to compare
the basic performance of the claws. For each pattern, fiveexcavations were conducted, and measurements were taken
for the amount of formation, the amount of outflow, and thecollection rate.

Fig. 16.

Fig. 14.

Excavation test equipment

Claw excavation experiment

10 pieces of plastic garbage

2) Results: Each evaluation indicator presents, the amountof formation in Fig.17, the amount of outflow in Fig.18, andthe collection rate in Fig.19.

Fig. 17.

Amount of formation

the collection rate tended to decline. Additionally, as theangle increased, the amount of formation tended to decrease,
while the amount of outflow and the collection rate tended
to increase.
3) Discussion: First, we will discuss each of the evaluation indices. The increase in the amount of formation
with rising water content can be attributed to the sandbecoming lumpy due to increased moisture. Conversely, thedecrease in the amount of formation with a higher claw
angle is likely due to the reduction in claw size, leading todecreased support force for the sand pile. The reduction in
the amount of outflow and the collection rate with increased
water content can be attributed to the sand becoming lumpy
and less flowable due to higher moisture content. On theother hand, the increase in the amount of outflow and thecollection rate with an increased claw angle is likely due tothe smaller claws providing reduced support force for thesand pile, thereby enhancing flowability.
In this experiment, the most favorable results were observed when the claw angle was set to 80° for all threeevaluation indices. Based on this result, it is considered that aclaw angle of 80° is a practical choice for garbage collection.
C. Sandy Beach ExperimentFig. 18.

Fig. 19.

Amount of outflow

Collection rate

The vertical axis of each graph represents the amount offormation, the amount of outflow, and the collection rate,
while the horizontal axis indicates the velocity. In the graphs,
the three blue lines represent claws with slope angles of 20°,
red for 40°, green for 60°, and purple for 80°. Each of thethree lines indicates an increase in moisture content from 0,
8 and 16% as the colour becomes darker.
Across the graphs, there was minimal variation with speed.
However, as the moisture content increased, the amount offormation tended to rise, while the amount of outflow andBased on the excavation experiments, it was concluded
that the most effective excavation was achieved using 80°
claws. In this particular experiment, the performance of excavating claws on an actual sandy beach was evaluated using
the crawler robot shown in Fig.20. To make a comparison,claws with a slope angle of 40° were also fabricated using
the same method, and their performance was assessed.

Fig. 20.

Crawler robot

1) Procedures: The experiments took place on Terayama
beach in Fukuoka City, Japan. The sand at the site had
a moisture content of 2 to 3% and was interspersed withwood pieces, plastic, seaweed, and other materials. Fig.21
showcases four types of claws used in the experiment: twotypes with 40° claws and two types with 80° claws. These
claws were fabricated from rust-resistant stainless steel plates
and measured approximately 200 mm in height and 500 mm
in width. The key distinctions among each claw were theslope angle and the length of the side wall: for 40° (1) and80° (1), the length of the side wall was unified and longer
than the horizontal length of the claws; for 40° (2) and 80°(2), the length of the side wall matched the horizontal lengthof each claw. This design aimed to reduce the amount of
excavated sand deposited and enhance excavation efficiency.
Unlike in the Claw Excavation Experiment, we didn’t mounta large side walls against the claws, because the actual claws
are so wide and large that we thought it would be sufficient tojust mount a side walls about the same size as the claws. The
claws were mounted on a crawler robot (OREC CO., LTD.),
which conducted the excavations by moving forward.

3) Discussion: In 40° (1) and 80° (1), the long side wall
resulted in a substantial accumulation of sand in front of
the claws, hindering forward movement despite successful
excavation due to the driving power limitation of the crawler
robot. In addition, in 80° (2), sand did not accumulate in frontof the claw because it flowed outward from the short side
wall, and thus preventing excavation. On the other hand, the40° claw with the short side wall was more effective than the
80° claw for excavating sand while reducing sand retention.
As a result, under conditions of limited drive force, the 40°
claw with shorter side wall proved superior to the 80° claw
in this experiment.
V. CONCLUSIONS

Fig. 21.

4 types of claws

This paper focuses on defining the required specifications
for a robot capable of collecting marine shredded plastic
garbage, which is challenging for humans to collect. It
conducts experiments to investigate the necessary functions
and trends, primarily focusing on the ”excavation” aspect.
The ”Pseudo-Angle of Repose Measurement Experiment”
explored the behavior of sand in a stationary state, confirmingthat the pseudo-angle of repose remained consistent acrossdifferent shapes and sizes of slopes, with sand being deposited minimally at an approximate slope of 20°. Buildingon this, the ”Claw Excavation Experiment ” set the claw
angle at 20° as the baseline and investigated claws with
angles of 40, 60, and 80° to determine the optimal claw
angle. In the ”Sandy Beach Experiment,” actual excavatingclaws were manufactured, and it was demonstrated that claws
with a 40° angle outperformed those with an 80° angle.
Going forward, the aim is to develop a more efficient garbage
collection robot based on the improvement proposals derived
from these experiments.
ACKNOWLEDGMENT

2) Results: Excavation was successfully carried out using
three types of claws, with the exception of 80° (2). However,in both 40° (1) and 80° (1), the robot encountered difficulty
moving forward due to the weight of sand accumulated in
front of the claw. Only with 40° (2) was continuous operationachieved through digging and moving forward. Fig.22 showsthe excavation carried out using 40° (2). On the left is therobot, and on the right is the view of the claw from above.
The areas circled in red indicate the formation of a sand pile
within the claw as a result of the excavation.

Fig. 22.

Sand piles by excavation

This work was partially supported by JSPS Grant-in-Aid
for Scientific Research JP20H00230.
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