Aerial pruning mechanism, initial real environment test
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.
KeywordsAerial robot Skew-gripper Grasping Pruning
In recent years, the popularity of multirotor helicopters has drastically increased to the point that almost everyone has heard the word drone. Low prices and a very competitive market have made possible the access to this technology either for fun or for research activities. Nowadays, the most popular application of multirotor helicopters is aerial video and photography; however, this activity is only for the purpose of capturing video and pictures from the environment without contact.
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 [1, 2, 3, 4, 5].
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 custom-made 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 ducted-fan aerial vehicle for ultrasonic nondestructive structural inspection .
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.
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
First of all, the aerial pruning robot should fly to the target branch.
When it is reasonable close to the target, a couple of claws-like grippers should close to grab the tree branch.
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.
Finally, when the pruning task is done, the aerial pruning robot should come back to the home position.
Mechanical description of the prototype
Characteristics of the circular saw
Relationship between the branch and the circular saw to be used
Diameter of the branch and circular saw
Diameter of branch (mm)
Circular saw (mm)
Controlling the rotational motion
Controlling the circular saw
Specifications of the XBee S1 module
XBee S1 module
Up to 100 ft (30 m)
Outdoor RF line-of-sight range
Up to 300 ft (90 m)
RF data rate
Serial interface data rate
1200 bps–250 kbps
Transmit current (typical)
45 mA (@ 3.3 V)
6 10-bit ADC input pins
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.
Outer diameter: 100 mm
Blade thickness: 1.3 mm
Number of blades: 36
Inside diameter (for attaching to the gear box): 20 mm
Wireless communication and PI control performance
Main characteristics of the tree branch pruned in this experiment
Length from the pruning area to the tip (m)
Time for pruning
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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