Aerial pruning mechanism, initial real environment test
Molina and Hirai Robot. Biomim.
Aerial pruning mechanism, initial real environment test
Javier Molina 0
0 Department of Robotics, Ritsumeikan University BKC , 1-1-1 Noji-higashi, Kusatsu-shi, Shiga 525-8577 , Japan
In this research, a pruning mechanism for aerial pruning tasks is tested in a real environment. Since the final goal of the aerial pruning robot will be to prune tree branches close to power lines, some experiments related to wireless communication and pruning performance were conducted. The experiments consisted of testing the communication between two XBee RF modules for monitoring purposes as well as testing the speed control of the circular saw used for pruning tree branches. Results show that both the monitoring and the pruning tasks were successfully done in a real environment.
Aerial robot; Skew-gripper; Grasping; Pruning
• Contact tasks
Load transportation deals with moving payload from
one place to another using an aerial vehicle. It is
commonly used a gripper to pick up the payload and move it
to the target place; this task requires to control the
stability of the aircraft which is affected by the payload [
Contact tasks using multirotor helicopters are used
to interact with the physical environment to perform
a specific task. In this case, the multirotor system is
endowed either with a tool or with a manipulator. which
allows it to execute the task. Examples of this research
are: an Asctec Pelican quadrotor endowed with a
custommade manipulator for contact inspection [
], an aerial
vehicle along with a couple of robotic arms for turning a
valve using a human–machine interface [
] and a
ductedfan aerial vehicle for ultrasonic nondestructive structural
The examples mentioned above are clearly new
applications related to multirotor helicopters. Considering that
the stability of multirotor helicopters is affected by the
payload during the process of grasping and moving and,
the flying time is in most of the cases, a crucial factor to
perform activities such as inspection by contact
operations, we propose aerial manipulation only for the initial
operation task. In other words, we propose to carry a tool
to the point of interest, fix the multirotor using a gripper,
perform the task and finally, return to the ground station.
This allows the helicopter to reduce the energy
consumption since it is only necessary to fly to the desired
position and the rest of the operation will be performed by
the tool without flying.
Particularly, we are interested in pruning tree branches
using an aerial pruning robot as it is shown in Fig. 1; this
is due to tree branches growing too close to power lines
represent a potential hazard for the security of the
residents as well as for the electricity supply. As an example,
consider the case in which a tree branch hits one of the
cables of a power line, it may cause an electrical arc
producing sparks affecting the energy supply or even fire
around the contact area [
]. In order to keep safety, tree
branches must be kept away from electric power lines.
Usually, to remove these tree branches, it is needed a
person and a crane, the latter is to access the target and the
former is to prune them with a specialized tool.
Pruning tree branches close to power lines represent
a risk; this means that there is always the possibility of
an accident caused by a high-voltage cable. Usually, the
minimum required working distance for pruning trees
close to a primary distribution line (between 750 and
150,000 V) and a transmission line must be 3 and 6 m,
respectively. For a human worker, pruning these branches
may become a difficult and hazardous task, that is, it is
necessary to find a solution to perform such activity
safety without direct human intervention in the task.
In this paper, we discuss two important tasks the aerial
pruning robot should perform, communication with the
user’s interface and pruning a real tree branch. First of all,
a description of the aerial pruning robot is explained; next,
we discuss the electronic interface and the PI control for the
pruning process. We also give a brief introduction to the
XBee modules and the interface with the microcontroller,
and finally, we give some conclusions and future work.
Aerial pruning robot workspace
Generally speaking, there are three different ways to trim
trees close to electric power lines [
]. These techniques
are, “V” pruning, “L” pruning and side pruning, as they
are shown in Fig. 2. In this project, we are focusing only
in the side pruning technique; this is due to the
mechanical characteristics of the multirotor and the grasping
technique used to fix its body to the the target branch.
Figure 3 shows a typical way to prune a tree branch close
to power lines using the side pruning technique.
The idea we propose to prune tree branches close to
power lines is to use a multirotor helicopter, a gripper
and a circular saw to perform such activity. This new
concept not only reduces the costs of using a truck with
a crane but also reduces the human risks of a potential
accident because of high voltage around the working
area. Figure 4 shows the concept of the aerial pruning
With this new idea, new challenges arise as they have to
be solved in order to perform the complete task. In order
to successfully complete the pruning task, the aerial
pruning robot should accomplish four steps:
1. First of all, the aerial pruning robot should fly to the
2. When it is reasonable close to the target, a couple
of claws-like grippers should close to grab the tree
3. Once the tree branch was grabbed by the gripper and
the complete body of the aerial pruning robot is
hanging from the tree branch, the pruning mechanism,
which is placed on the top of the multirotor, should
start pruning the tree branch.
4. Finally, when the pruning task is done, the aerial
pruning robot should come back to the home position.
Mechanical description of the prototype
The robotic gripper that we are using in this research is
composed by a couple of claws with teeth for grasping the tree
branch firmly; since each claw is placed in different planes,
we call this configuration “skew-gripper.” The pruning
system is composed by a DC motor, a gear box, and a circular
saw; Fig. 6 shows the complete CAD model of the
mechanism, which is placed on the top of a multirotor helicopter.
By choosing a multirotor helicopter as a carrier of the
pruning mechanism, specifically an hexarotor
configuration, the complete mechanism is mounted on the top of
it using a 3-mm aluminum base-plate. Figure 7 shows the
CAD model of the prototype hanging from a tree branch
using the skew-gripper as well as a circular saw ready to
Figure 8 shows how the skew-gripper works. For grasping
a tree branch, the couple of claws should open in an
opposite direction, and when the tree branch is firmly grasped,
there is no way for opening and closing anymore because
of the shark-like teeth, which are well inserted in the tree
branch. As the couple of shafts of the servo motors of the
skew-gripper are aligned on the same rotational axis, the
body of the helicopter with the circular saw is able to rotate
along such axis creating a circular motion which is used to
prune tree branches by means of a circular saw.
Characteristics of the circular saw
The skew-gripper can grasp a tree branch with a
maximum diameter of 40 mm without any problem; however,
the final goal is pruning; therefore, the diameter range, db
is set as:
where a and b are the smallest and the largest diameters
of the branch, respectively; this is due to the diameter of
the circular saw introduce a restriction in the pruning
task. In other words, the effectiveness diameter db of the
branch to be pruned using a circular saw is given by:
where rs is the radius of the circular saw to be used and
ra is the radius of the outer washer used to lock the saw
to the actuator. Hence, the maximum diameter, b, of the
branch to be pruned is determined by two factors, the
db = rs − ra
circular saw and the outer washer. Figure 9 shows this
relationship, and Table 1 shows the diameter of the
circular saw to be used depending on the diameter of the
branch the operator wants to prune.
On the other hand, the total weight of the aerial
pruning robot is another factor which restricts the diameter
db of the branch to be pruned. That means, if the branch
is too small, the total weight of the aerial pruning robot
may bend it resulting in an inappropriate grasping or
even the tree branch cannot support the weight of the
helicopter and it may fall down. In order to prevent this
possible issue, the minimum diameter, a, was
determined experimentally in several branches resulting in
a = 17 mm, which can hold the helicopter safely.
Controlling the rotational motion
In order to test the pruning system in a real
environment, a hardware for controlling the swinging motion
for pruning purposes was designed. A couple of
hightorque servo motors from HITEC [
] and a Futaba 14SG
radio transmitter and its respective receiver, the Futaba
], were chosen. These HITEC servo motors
are used to move the skew-gripper for grasping as well
as for creating the rotational motion for pruning tasks. In
addition, for powering the servomotors and the receiver,
a 7.4-V LiPo battery was used. Note that this battery is
exclusively used for feeding the servomotors and the
receiver; Fig. 10 shows these connections.
Controlling the circular saw
The hardware designed for controlling the speed of the
circular saw is composed by a power electronic
module, which is used for commuting the circular saw’s DC
motor, a back-electromotive force (back-EMF)
module for sensing the motor’s speed, a PI controller
programmed in an Arduino Uno board for regulating the
speed of the circular saw and a RF XBee wireless module
Fig. 11 power_module. Power electronics module
for transmitting the information of the pruning process
to the operator. Figure 11 shows the schematic of the
power system module, PIN 1 and PIN 2 of J1
connector are connected to ground and to 16.5V, respectively,
PIN 1 and PIN 2 of J2 are connected to the terminals of
the circular saw’s DC motor. From the connector J3, PIN
1 is the back-EMF signal, which is used for sensing the
DC motor’s speed, PIN 2 was used as a back-EMF with
a low-pass filter, but it was not worked well because of
the noisy, and we decide to use the raw signal and design
the low-pass filter separated as it will be explained later.
Finally, PIN 4 is the PWM signal from the Arduino Uno
board for the speed control the DC motor. The power
electronic module is composed by a power MOSFET
transistor together with a photo-transistor to protect
it in case of over-current since the gate of this type of
device is quite sensitive.
Figure 12 shows the schematic of the back-EMF
sensing module. This sensing module is for reading the
back-EMF and is composed by a resistor divider to
reduce the voltage from 16.5 to 5.5 V (J1 PIN 3), which
comes from the drain pin of the MOSFET transistor; in
addition, a low-pass filter is used to remove the noise
from the DC motor. An operational amplifier
configured as a voltage follower is used to reinforce the signal
from the low-pass filter which also set the output
voltage from 0 to 5 V (J1 PIN 5), suitable for an Arduino
The wireless communication module, which is used to
communicate the aerial pruning robot with the ground
station, is composed by two XBee series 1 (S1) from DIGI
]. Some of the most important
characteristics are summarized in Table 2.
One of the two XBee modules is connected to an
XBee shield from Sparkfun [
], and this in turn is
connected to an Arduino Uno board and hence sends
the data to the computer placed in the ground
station. The other XBee module, which plays the role of
receiver, was connected to the USB port of the
computer’s ground station for receiving the data from the
aerial pruning robot. Unlike the XBee module used
in the aerial pruning robot, the XBee module is
connected to the computer’s ground station needless
an interface to connect to it. This module is called
“XStick,” and its shape is similar to an USB memory;
however, it is used for wireless communication
purposes. Figure 13 shows the wireless interface between
the Arduino Uno and the computer for data
transmission, and Fig. 14 shows the complete block diagram
used for controlling and also for sending data to the
computer in the ground station.
In order to validate the performance of the pruning
mechanism in a real environment, several experiments
were performed. The aim of these experiments was to
prove the effectiveness of the wireless communication
between the aerial pruning robot and the ground station
for monitoring the speed of the circular saw; in addition,
the swinging motion for the pruning process produced
by the couple of servomotors was also tested. For this
experiment, a professional tipped-saw was selected as it
will be described later.
In order to prune a tree branch ranging from 12mm to
40 mm, the pruning mechanism should start swinging to
go through the tree branch progressively. In these initial
tests, the operator decides when the pruning mechanism
should turn back and goes again through the tree branch
based on a visual inspection of the graphic of the
circular saw’s speed provided by the computer placed in the
ground station. For the sake of clarity, Fig. 15 shows the
complete pruning task in four single steps,
understanding that in a real situation, this swinging process should
be repeated several times until the tree branch has been
completely pruned. Figure 16 shows the sequence of
pruning of the real prototype pruning a 17-mm-diameter
tree branch. The circular saw used in these experiments
has the characteristics mentioned as follows:
• Outer diameter: 100 mm
• Blade thickness: 1.3 mm
• Number of blades: 36
• Inside diameter (for attaching to the gear box):
Wireless communication and PI control performance
Figure 17 shows the graphics of the performance of
the pruning process; as it can be observed, the circular
saw’s speed is constant with some fluctuation as a result
of the contact force at the moment of pruning. The
communication between the XBee modules was around
10 m without loosing the connection during the
Fig. 17 pruning2. PI speed control
The tree branch used in this experiments as well as the
performance of the pruning mechanism is mentioned
in Table 3. From this table, one may appreciate that the
pruning time is related to two important factors: the
stiffness of the tree branch and the diameter. Figure 18 shows
the tree branch pruned at approximately 90% of the total
Regarding the energy consumption of the whole system,
the main source of energy consumption is the multirotor
helicopter, and it consumes around 25 A during flying.
On the other hand, the circular saw consumes only 4 A
during the pruning process which takes around 8 min
or less, depending of the diameter of the tree branch. In
these experiments, a 5100-mAh 4S LiPo battery was used
for powering the multirotor and the circular saw as well.
This battery at full charge gives 16.8 V and should not
go down less than 12.8 V at full discharge. For practical
applications, we establish a boundary in 14.5 V to have
enough time for landing in case the battery has achieved
the minimal boundary and thus prevent a permanent
damage. This range allows the operator to fly the
multirotor around 7 min which is enough time for grasping and
pruning at least, one tree branch.
In this paper, the hardware description and some
experimental results regarding pruning tree branches in a real
environment using an aerial pruning robot were shown.
Results obtained show that the PI control implemented
to control speed of the circular saw was helpful for
pruning a real tree branch. In addition, the wireless
communication between the aerial pruning robot and a ground
station has shown that it is possible to follow the
pruning task monitoring the performance of the speed of the
circular saw to avoid possible accidents. In the future,
there are some necessary improvements to increase
the performance of the pruning task such as
monitoring the process using either a smart phone or a tablet
instead of a PC along with a more powerful wireless
communication to cover a large working area. Moreover,
a quadrotor helicopter in a coaxial configuration is also
being considered to increase the payload capacity and
thus allowing the operator to place an extra battery to
increase the flying time, which is crucial to accomplish
the pruning task.
Both authors contributed in the same proportion. Both authors read and
approved the final manuscript.
This work was sponsored by the Strategic Research Foundation Grant-aided
Project for Private Universities “Research on infrastructural technologies for
information-driven mechanical system that propels growth of the next
generation ‘satoyama satoumi’”.
The authors declare that they have no competing interests.
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Research on infrastructural technologies for information-driven mechanical
system that propels growth of the next generation “satoyama satoumi”.
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