Evaluating UAS Electromagnetic Interference Influence of
Wind Vector Measurements
Bernand Floranda
University of Central Oklahoma, Edmond, OK, 73034, USA
Abstract
Meteorologists today are using Unmanned Aerial Vehicles (UAV) to measure
wind vectors. Most recent studies of wind vector were conducted by using a hexarotor
based UAV to estimate the wind vectors. That experiment used instruments such as
the 2D ultrasonic anemometer and a thermohygrometer to model the profile. Utilizing
the advance onboard computers and using a similar method that scientists Patrick P.
Neumann and Matthias Bartholmai used in their wind vector estimation, meteorologists
would not have to attach additional instruments to estimate the wind vectors.
Unfortunately, the motors used in a UAV have current that flows thorough it and will
emit electromagnetic interference that could influence the onboard computers and
potentially effect the data. Therefore, the proposed experiment that will be conducted is
to simulate and measure the amount of EMI a typical UAV produces. Once the quantity
of EMI is determined, the interference will be mitigated.
Introduction
Meteorologists today are using Unmanned Aerial Vehicles (UAV) to measure
wind vectors. The use of a tower or a weather balloon with attached radiosondes were
the general method of modeling the profiles. With UAVs becoming more easily
accessible, meteorologists have been implementing them for their research purposes.
UAVs were first proposed back in the 1970s and the most recent use of the UAVs were
conducted on 26 October and 9 November 2016 by Tomoya Shimura with the Disaster
Prevention Research Institute in Kyoto University, Kyoto, Japan. The instruments used
to measure the wind vectors were the 2D ultrasonic anemometer and a
thermohygrometer. The ideal method in using an anemometer is to have the device on
a fixed position rather than on a UAV where the downward force from the propellers will
cause a ripple in the pressure. Therefore, eliminating the anemometer from the UAV
will be the best solution by utilizing the UAV’s onboard compass and global positioning
system (GPS). The complications of using the onboard compass and GPS will be the
electromagnetic interference (EMI) that the UAV’s electronic components will produce,
because electric motors, such as the ones used for the UAV, are one of the noisiest
(EMI) components in any system. According to research by engineers at Pittman
Engineering, motor construction and operating current affect the amount of EMI an
electric motor emits. EMI could be reduced by either decreasing voltage or increasing
the motor load. Thus, influencing the suggested method’s data. A proposed
experiment will be to evaluate the EMI the UAV produces and to mitigate is as much as
possible.
Background
Tomoya Shimura and his team used a hexarotor-based UAV to measure wind
vectors that was conducted on 26 October and 9 November. The hexarotor UAVs are
excellent for hovering at a specific location in space, low altitude flights, and the ability
to take off and land in limited spaces. The 2D ultrasonic anemometer and a
thermohygrometer were the instruments used in the experiment to model the wind
profiles. A 2D ultrasonic anemometer has the ability has the ability to measure wind at
1 second intervals with an accuracy of ±2° within ±10° datum and ±4° beyond ±10°
datum for direction, ±0.5 m/s for wind speed lower than 15 m/s and ±4% for wind speed
higher than 15 m/s. Tomoya Shimura then used a meteorological observation tower,
located in the Uji-Gawa Open Laboratory in Kyoto University, to compare the wind
speed and direction.
The ideal method in using an anemometer is to have the device on a fixed position
rather than on a UAV where the downward force from the propellers will cause a ripple
in the pressure. In figure 1 is an image from NASA showing the complex flow
generated by the rotors.
Figure 1. Complex Flow
Ensuring the anemometer is not influenced by the flow from the propellers, the 2D
anemometer should not be used. Instead use a method that was proposed by
Neumann and Bartholmai that is based on the relationship between wind vectors and
the UAV’s angle of inclination using the onboard computers. Neumann and
Bartholmai’s theory is based on a wind triangle, figure 2, but instead of the UAV in flight
it will be at a fix position in space.
Figure 2. Wind Triangle
The vector 𝑣⃗ is estimated based on the UAV's roll and pitch angle as well as on the
system's orientation to the magnetic north pole, determined by the UAV’s onboard
compass. According to Neumann and Bartholmai the UAV’s inclination angle ψ is
calculated as the inverse scalar product from the cross product of the unit vectors which
is given as
0
𝑒⃗𝜙 = (cos 𝜙),
sin 𝜙
cos 𝜙
𝑒⃗𝜙 = ( 0 )
−sin 𝜙
(1)
and the 𝑛⃗⃗𝑥𝑦 . Where the 𝑛⃗⃗𝑥𝑦 = (0, 0, 1) is the normal vector of the XY – plane.
Calculating for the direction 𝜃𝑣 , first the angle λ between the viewing direction of the
UAV, considered as the negative normal vector −𝑛⃗⃗𝑌𝑧 = (−1, 0, 0) and the projection of
the vector 𝑒⃗𝜃 × 𝑒⃗𝜙 on the XY-plane is to be calculated using the equation,
𝜆 = cos−1 (
⃗⃗⃗⃗⃗⃗⃗𝑦𝑧 ⋅(𝑒⃗𝜃 ×𝑒⃗𝜙 )
−𝑛
𝑥𝑦
|−𝑛
⃗⃗𝑥𝑦 |⋅|(𝑒⃗𝜃 ×𝑒⃗𝜙 )
𝑥𝑦
|
)
(2)
To determine whether the vector 𝑒⃗𝜃 × 𝑒⃗𝜙 is to the left or right of the UAV with respect to
the viewing direction, the following equation is used,
𝑛⃗⃗𝑥𝑧 ⋅ (𝑒⃗𝜃 × 𝑒⃗𝜙 )
𝑥𝑦
< 0, 𝑖𝑓 𝑒⃗𝜃 × 𝑒⃗𝜙 𝑖𝑠 𝑙𝑒𝑓𝑡
= {> 0, 𝑖𝑓 𝑒⃗𝜃 × 𝑒⃗𝜙 𝑖𝑠 𝑟𝑖𝑔ℎ𝑡
0,
𝑜𝑡ℎ𝑒𝑟𝑤𝑖𝑠𝑒
Equation (3) is needed to be solved as the solution from equation (2) will give a result
between the intervals of 0 and 180°. The direction, 𝜃𝑣 is calculated by using the
angle λ and the onboard compass angle of the viewing direction of the micro UAV.
The wind speed will be determine using the UAV’s onboard computers. As the
crosswinds move the UAV in space, the computer will send signals to the rotors to
counter the effects. The amount needed to counter will be used to calculate the wind
speeds.
Figure 3. UAV’s Coordinate System.
(3)
Therefore, eliminating the anemometer from the UAV will be the best solution by
utilizing the UAV’s onboard compass and GPS to determine the UAV’s angle of
inclination and obtaining the wind speed and direction. Unfortunately, the UAV’s
electronic components will produce a certain amount of EMI and influencing the
onboard computers. For these reasons, an experiment on a UAV would be conducted
to measure how much EMI influences the onboard computers and strive to mitigate the
EMI.
Experiment
The experiment will take place at the National Severe Storm Laboratory,
Oklahoma University Research Campus, Norman, Oklahoma. An experimental UAV
will be built by Douglas Kennedy of National Oceanic and Atmospheric Administration
(NOAA). In order to fly the UAV during the experiment, permission must be granted
from NOAA’s Office of Marine and Aviation Operations (OMAO) in Silver Spring, MD.
Fortunately, there is a loop hole. By grounding the UAV, the experiment could be
conducted without seeking permission from OMAO. The first step in the experiment is
to develop a data logger that will measure the magnetometer of the UAV. Next, with the
vehicle off the background field will be measured. Now with the vehicle grounded, the
UAV will simulate flight by powering the motors individually, simultaneously, and/or a
various of combinations. The motors will then give off noise or EMI and thus give a
value from the magnetometer. During the simulation the motors will give off current and
with that the magnetometer could be observed and determine the changes it
experiences from the various currents of the rotors during the simulation. With the
results from the experiment, a mitigation strategy will be the next phase. Such strategy
are either to move the magnetometer, run multiple magnetometer at different positions
on the platform, or to implement an EMI shield.
Conclusion
In conclusion, the UAV is an astounding piece of equipment that will greatly
impact current and future atmospheric research for meteorologists. With the ability to
take data at various altitudes and precise position it is more practical to use a UAV than
the traditional weather balloons. Unfortunately, the electronics (i.e. motor and battery)
that require the UAV to function properly would also impede in achieving accurate data
a meteorologist is studying due to any electromagnetic interference it emits. Therefore,
the proposed experiment would measure how much EMI such a UAV would produce
and determine the amount of influences on any data that is recorded during
observations. Mitigating any EMI it produces would significantly decrease any error the
data the instruments record. Upon completion of the experiment, it will advance the
knowledge of UAV usage in the meteorology world.
References
[1]
Suppressing noise in DC motors. (1997). Machine Design, 69(8), 104-106. Retrieved from
http://vortex3.uco.edu/login?url=https://search-proquestcom.vortex3.uco.edu/docview/217165050?accountid=14516
[2]
Shimura, T., M. Inoue, H. Tsujimoto, K. Sasaki, and M. Iguchi, 2018: Estimation of Wind
Vector Profile Using a Hexarotor Unmanned Aerial Vehicle and Its Application to
Meteorological Observation up to 1000 m above Surface. J. Atmos. Oceanic
Technol., 35, 1621–1631, https://doi.org/10.1175/JTECH-D-17-0186
[3]
Neumann, P. P., and M. Bartholmai, 2015: Real-time wind estimation on a micro
unmanned aerial vehicle using its inertial measurement unit. Sens.
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Shinde, S., Pommerenke, David, Beetner, Daryl, Fan, Jun, Khilkevic, Victor, & Stutts,
Daniel. (2017). EMI Measurement and Modeling Techniques for Complex Electronic
Circuits and Modules, ProQuest Dissertations and Theses.