outdoor mapping. Yi used a much bigger UAV with a total weight of 330kg to do big scale terrain mapping.
A few groups already published work on 3D-measurement of forest surface shape. Nagai et al. in their paper reported tree
height estimation and terrain mapping with a UAV and several sensors. Wallace et al., 2012 deliver a very in depth analysis of
the precision of their 3D-laserscan data for vegetation measurement.
The sensor-system described in this paper is distinguished from the mentioned work by its light-weight, modularity and
flexibility. The realised application has not been described in this form up to now to the best knowledge of the authors.
3.
SENSOR PACKAGE 3.1 Concept
The sensor package was developed by keeping in mind the experiences from several projects with different sensor
configurations. In Airshield we acknowledge the funding by the BMBF several gas-sensors have been tested in a drone
swarm. In an other project, mainly different cameras have been involved to equip the several drones of a swarm. For automatic
landing experiments the drones have been used with laser scanners, a stereo camera, ultrasonic sensor, and optical flow
sensors.
During these experiments it became obvious that it is not trivial to operate different sensors on even only one drone. Handling a
swarm with heterogeneous drones and different sensors improves the complexity. Thus, there was a substantial need to
treat the data of the different sensors in a unified way to allow an easy and modular way of utilisation. Central design criteria
have been small weight and low power consumption.
Also the package was intended to be independent of the type of the drone. Thus, it was decided to use an embedded computer to
make it completely modular. In the beginning only one processor was involved but due to increasing requirements in
processing power now several processors have been combined.
3.2 System
The actual system is shown in figure 1. It consists of a 400MHz ARM9 System from ICnova blue module at the middle of the
image. On this part runs the flight control. Thus, the sensor package can also be used as an intelligent high-level controller
for autonomous take-off and automatic way-point navigation. The drone can communicate via WLAN with other drones or
the ground station. Also if operated in a swarm a decentralized collision avoidance is implemented.
In several real-time application it is advantageous to execute complex sensor evaluation tasks directly on the drone. This way
the communication with the ground station can be reduced significantly; minimizing bandwidth requirements of the
communication channel and relieving the operator from information overload. To allow these complex computations the
computing power has been increased by utilizing an additional Tegra2 Board from Toradex green module at the left side. This
processor contain 21Ghz cores, 512MByte RAM and a NVIDIA GeForce GPU.
Earlier experiments showed that straying radiation from computing equipment may heavily disturb the operation of the
drone. Thus, both processor modules are fixed to a self- developed basis board which was designed by taking carefully
into account electromagnetic compatibility.
The basis board provides the necessary electrical power conversion Fig.1 right side when feed from the accumulator of
the drone. Of course, also a completely drone independent operation of the sensor package is possible by adding a
dedicated accumulator to it. The basis board links the processors and a bunch of interfaces upper side. Overall
power consumption is about 2W.
Figure 1. Embedded System of the sensor package The laser scanning experiments started with the light-weight,
small-size scanner from Hokuyo 30LX shown in figure 2. Originally it is intended for wide angle obstacle surveillance in
ground vehicles but due to its operating characteristics it seemed to be suited for aerial laser scanning.
It was connected to the basis board via the network interface. Its operating rage is about 30m for compact, not too dark objects.
Figure 2. First used laser scanner Hokuyo 30LX
This contribution has been peer-reviewed. 236
During the experiments it was discovered that for very small and fragmented objects this range was not always reached.
Because the system was intended to measure conductor-ropes and vegetation a significantly smaller measurement distance
became necessary. To ensure the safety of the measurements, on the other hand, the drone has to be at least 10m distant from the
target. Some promising first result, which will be presented in the next chapter, showed the feasibility of the approach but
flight planning was a critical issue. Thus, this system is mainly used for scanning buildings and landscape where range is not of
ultimate interest but low weight is needed.
Figure 3. Sensor package with laser scanner, camera, and computing system
In figure. 3 our second generation system is shown. A laser scanner Sick LD-MRS-400001 constitutes the main sensor. It
has an operating range of up to 200m and an angle resolution of 0.25 degree. Thus, also small objects can be reliably measured
at quite high distances. A video camera is added to the package for control in real-time and also for documentation purposes.
The resolution of the camera is 13001300 pixel which is already a bit too high for online transmission -and thus
converted down for the video channel- but a good compromise if medium quality imagery is needed. Overall power
consumption is about 15W. This amount of energy does not significantly reduce the operation-time of micro UAVs that can
carry a payload of 1.5kg.
4.
AN APPLICATION: POWER LINE MONITORING 4.1 Task
In the sequel an application shall be presented which has been realised with the system described in the preceding chapter. This
work has been done in cooperation with Cenalo GmbH.
The basic problem is that vegetation has to be regularly supervised in the region under and near high-voltage
transmission lines because it tends to grow into the line and may cause a short time overload and a fire. Especially at windy
weather conditions this may lead to complete breakdown of the line. Figure 4. shows a typical configuration.
The situation is actually complicated by the fact that not seldom the ground beneath the lines is not owned by the operator of the
line. So, the status of the vegetation structure has to be well measured and documented to be usable in negotiations with the
owners about the appropriate actions. Figure 4. Typical flight configuration
The most-often used, actual approach is to fly over the lines by a helicopter with quite high speed with one pilot and one person
visually checking the state of the line and manually documenting it. This method is expensive and due to the speed
error prone. It delivers a first hint were problems may occur but no solid measurements. Even imagery often is not available.
In praxis the information should be sufficient to find metrical borders of critical areas and map these. Further, the
measurement should be precise enough to determine single trees which have to be cut down.
The critical quantity that has to be measured is the distance between the highest trees and the conductor-ropes. It has been
tried to measure it using ground views but this method is only applicable in a few very special landscape configurations.
Under typical conditions the line has to be switched off and workers climb onto it to do a manual measuring. Obviously, this
method is very expensive.
Here laser scanning as a precise method of distance measurement comes into play. Some experiments with
helicopters have been performed. In this approach standard industrial scanners can be applied because their weight is easily
bearable.
To catch even small branches the helicopter must fly rather slow and close to the transmission line and the trees. It must fly so
close that the downwash of the helicopter moves the trees and pushes them down. Thus, the measurement set-up changes the
measurement values and produces unreliable data.
4.2 Approach
