IEEE Robotics & Automation Magazine - March 2012 - 53

Economics and Technology [Bundesministerium f€
ur Wirtschaft und Technologie (BMWi)] [21]. One result of the
project is a gas-sensitive sensor module (approximately
200 g) for the Airrobot AR100-B microdrone (Figure 1).
Robotic Platform
The Airrobot AR100-B microdrone (mostly referred to as
microdrone in this article) has a diameter of 1 m and is driven
by four brushless electric motors. The microdrone can precisely
navigate to a certain region of interest for remote sensing without endangering persons in critical areas. The maximum payload mass amounts to 200 g with a total flight mass of about
1:3 kg. The maximum flight time is about 20-30 min. The
microdrone can withstand a maximum wind speed of 8 msÀ1 .
The flight control relies on an onboard inertial measurement
unit (IMU), which also provides the basis for the wind vector
estimation presented in the "Estimation of the Wind Vector"
section. It consists of a three-axis accelerometer and a three-axis
rotation rate sensor. Magnetic field sensor (compass) and global
positioning system (GPS) improve the accuracy of the IMU and
are used to compensate for the sensor drift. A barometric pressure sensor is used to control the altitude of the microdrone.
Communication with the ground station is established by a
wireless radio link. Data packets can include control instructions
or data coming from the microdrone's onboard sensors. The
operating distance of the remote control and communication
link is 1 km. The microdrone can be operated manually or in
GPS mode, e.g., by autonomous waypoint following.

the lower limit of detection. We showed in [21] that
measurements of gas concentrations in a large volume are
feasible for the gas-sensitive microdrone.
To improve the measurement capabilities for small
plumes, three different design approaches that lead to less
diluted gas-air mixture at the gas sensors were implemented
and analyzed with respect to their functional performance:
the passive, semiactive, and active gas transport approach.
When applying the passive gas transport approach, no artificial airflow is used to bring the gas to the sensors. When
applying the semiactive gas transport approach, the gas-air
mixture is conveyed through a carbon fiber tube using the
suction effect of one rotor. The tube of the active gas transport approach protrudes from the radius of the microdrone
by nearly 0:3 m. Here, an axial fan was mounted inside the
tube to draw in the gas-air mixture.
Sensor response and decay can be sped up to some
degree by an artificially generated airflow to achieve faster
and more accurate gas concentration measurements with
shorter residence time of the microdrone as shown in [22].
Experimental Setup
Reproducible environmental conditions are needed to compare the different gas transport approaches. We used a gas
source with a constant release rate and stable airflow conditions. An experimental setup to create such a controlled environment was build up at BAM. A CO2 gas bottle was used as
the emission source. A fan generated the stable airflow conditions and dispersed the gas additionally. A defined measuring

Integration of Gas Sensors
A commercially available gas detector (Dr€ager X-am 5600),
which is originally designed as a handheld device for personal
safety, is the base unit of the gas-sensitive payload. It features
low mass and compact design. The modular concept allows
application and ad hoc exchange of the four gas sensors,
which enables users to customize the device for specific scenarios. The Dr€ager gas detector can measure many combustible gases and vapors with a catalytic sensor as well as different
(toxic) gases with electrochemical and infrared sensors.
An additional electronic board with the dimension of an
AA battery controls the communication between the gas
detector and the microdrone via an integrated microcontroller and appropriate device interfaces (IrDA, RS232, and I2 C).
A temperature and humidity sensor (SHT15, Sensirion AG)
were also integrated as both factors may affect the measurement data (however, no compensation for varying temperature or humidity was applied in the experiments presented in
this work). The casing of the gas detector is protected against
water and dust and therefore capable of working outdoors.
Gas Transport to the Sensors
Gas transport to the sensors is a critical process due to the
induced disturbance by the rotors of the microdrone,
which basically dilutes and disperses the surrounded gas-
air mixture. This could be problematic for scenarios where
punctual gas sources are present or the gas sensors work at

(a)

(b)
Figure 1. (a) The Airrobot AR100-B microdrone and (b) the
gas-sensitive payload. (Photos courtesy of Oldenbourg
Wissenschaftsverlag GmbH, www.oldenbourg.de.)

MARCH 2012

IEEE ROBOTICS & AUTOMATION MAGAZINE

http://www.oldenbourg.de

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - March 2012