20-08-2013, 04:56 PM
Sensor Performance Characterization for Use on a Micro Aerial Vehicle
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ABSTRACT
This paper details the characterization of the Hokuyo UTM-30LX and the Xbox Kinect for use
on a Micro Aerial Vehicle (MAV). The MAV is to be used for the exploration of unknown
territories, and thus must be able to localize itself in various different conditions. To accomplish
this, both the indoors and outdoors performance of the sensors are examined with different
objects in their immediate surroundings.
The characterization begins with tests using objects with different surface properties and in
different indoor lighting conditions. Furthermore, tests are conducted indoors to see how the data
received by the sensors is distributed. Hence, the results show how the data can be interpreted
using a probability distribution in software to aid exploration algorithms.
The same characterization is carried out outdoors to compare the data. The effects of sunlight on
the sensors is noted. Then, further experiments allow us to determine the conditions in which the
data from each sensor are not likely to be trustworthy. We can make the quadrotor ignore these
values when exploring.
Finally, we show the initial part of an experiment conducted to test how information gained from
the characterization can be used to optimize motion estimation. We continue to work on the the
rest of the experiment beyond the submission of the paper.
INTRODUCTION
Autonomous aerial vehicles require much more control than autonomous ground robots. The
movement of ground robots is generally only in the x-y plane and has three degrees of freedom
{ , , }. On the other hand aerial vehicles travel in three-dimensional space. Due to the extra
dimension, aerial robots have six degrees of freedom to contend with. The extra degrees of
freedom are problematic and no one sensor is able to determine 3D movement with high degree
of accuracy. This means that the robot needs to fuse data from multiple sensors in order to get a
view of its pose and surroundings.
In order to get a sense of the pose of the robot, we use six variables. The variables are x-
displacement, y-displacement, z-displacement, roll, pitch and yaw { , , , , , }. The
quadrotor we are using contains three sensors: a laser range finder, a Kinect sensor and an
inertial measurement unit (IMU). As each sensor has a different mode of operation, each gets a
reading of a different subset of the six variables. Since no sensor can be 100% reliable in all the
measurements it makes, there is an overlap between the values measured by each sensor.
The Xbox Kinect
The Xbox Kinect works on the principle of binocular disparity in order to find depth data. The
Kinect has an infrared(IR) projector that projects a dot pattern of IR light on objects in its field of
view. An IR camera positioned a slight distance away is separate from the projector and views
the position of the dots. Depending on where the dots are formed, the disparity (d) is detected.
PREVIOUS WORK
Effect of Surface Properties of an Object
The surface properties of an object have a greater effect on the laser range finder readings than
the Kinect readings. The following experiments have been conducted by others for laser
scanners. While their experiments have not been conducted using the same Hokuyo sensor that
we are using, they still describe the nature of the interaction of the laser beam with different
surfaces.
Roughness
Rough objects cause reflections in all directions, while smoother objects cause only direct
reflections. Direct reflections do not work well for the laser range finder. When the laser beam
impacts the object at an angle, very little light would reflect back to the object. We deal with a
very smooth surface (glass) in Section 6 and see that the laser range finder only has a limited
field of view when pointed towards the glass.
Reflectance
The reflectance of an object has an effect on the intensity of the reflected laser beam. Luo et
Al.[5] explain that when the laser hits a highly reflective object, specular reflections are caused.
This saturates the photo diode which causes the object to appear to be closer than it is. On the
other hand when an object is not reflective, it slows down the return beam and causes the object
to appear to be further away.
Sensor Drift
Previous authors icluding Lee[10] have stated that drift in the measurement initially occurs
when the laser range finder is turned on. After that, it settles down. Figure 7 shows the drift
measured by Lee. Experiments we carried out were unable to categorically prove or disprove this
with the Hokuyo scanner. The result can be seen in Figure 8. If there is any drift, the magnitude
is very small and can be ignored.
Failed Measurements
Failed measurements can cause a severe problem when operating outdoors due to the effect of
the sun on the sensors. The Kinect is extremely sensitive to sun rays. When the sun is present, all
the distance values in the point cloud go to zero. This, coupled with the small range of the
Kinect, makes it almost impossible to use to Kinect outdoors. Instead, the other sensors on the
quadrotor must be relied on.
Results
As mentioned before, the laser scanner sees the window perfectly within its line of sight.
However, due to the smoothness of glass, at larger angles the reflection of the laser beam off the
glass does not return to the photo diode in the laser scanner. As distance from the glass increases,
the intensity of the laser beam reduces. That also reduces the field of view of the laser scanner.
The experiment was performed from inside as well as from outside to see how the readings
differ. The results are plotted in Figure 17. The field of view fits well as an exponential function
of distance. Another significant finding is that the field of view from outside is significantly
larger than the field of view from inside.
CONCLUSION
We have investigated the limits of the Hokuyo laser range finder and the Xbox Kinect in
different indoor and outdoor environments.
With the data we have determined that indoors, in areas of average reflectance, the behavior of
both sensors is predictable. They both follow models derived from their modes of operation. We
specified models for our sensors so that the information the quadrotor has is as complete as the
information provided by the sensors.
We also performed numerous experiments outdoors to find that there is only a limited area
visible to the sensors in direct sunlight. This enables us to specify that the quadrotor must be
selective in its use of sensors in these conditions. From the two sensors we found that the Kinect
does not work at all in direct sunlight, while the Hokuyo laser range finder works within a range
of 9-10 meters.