Page 80 - Dynamic Vision for Perception and Control of Motion

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3.3.1 Sensors for Ground Vehicle Guidance

In ground vehicles, speed sensors (tachometers) and odometers (distance traveled)
are the most common sensors for vehicle guidance. Formerly, these signals were
derived from sensing at just one wheel. After the advent of antilock braking sys-
tems (ABS), the rotational speed of each wheel is sensed separately. Because of the
availability of a good velocity signal, this state variable does not need to be deter-
mined from vision but can be used for motion prediction over one video cycle.
Measuring oil or water temperature and oil pressure, rotational engine speeds
(revolutions per minute) and fuel remaining mainly serves engine monitoring. In
connection with one or more inertial rotational rate sensors and the steering angle
measured, an “electronic stability program” (ESP or similar acronym) can help
avoid dangerous situations in curve steering. A few top-range models may be or-
dered with range measurement devices to objects in front for distance keeping (ei-
ther by radar or laser range finders). Ultrasound sensors for (near-range) parking
assistance are available, too. Video sensors for lane departure warning just entered
the car market after being available for trucks since 2000.
Since the U.S. Global Positioning System (GPS) is up and open to the general
public, the absolute position on the globe can be determined to a few meters accu-
racy (depending on parameters set by the military provider). The future European
Galileo system will make global navigation more reliable and precise for the gen-
eral public.
The angular orientations of the vehicle body are not measured conventionally, in
general, so that these state variables have to be determined from visual motion
analysis. This is also true for the slip (drift) angle in the horizontal plane stating the
difference in azimuth as angle between the vehicle body and the trajectory tangent
at the location of the center of gravity (cg).
Though ground vehicles did not have any inertial sensors till lately, modern cars
have some linear accelerometers and angular rate sensors for their active safety
systems like airbags and electronic stability programs (ESP); this includes meas-
urement of the steering angle. Since full sets of inertial sensors have become rather
inexpensive with the advent of microelectronic devices, it will be assumed that in
the future at least coarse acceleration and rotational rate sensors will be available in
any car having a vision system. This allows the equivalent of vestibular – ocular
data communication in vertebrates. As discussed in previous chapters, this consid-
erably alleviates the vision task under stronger perturbations, since a subject’s body
orientation can be derived with sufficient accuracy before visual perception starts
analyzing data on the object level. Slow inertial drifts may be compensated for by
visual feedback. External thermometers yield data on the outside temperature
which may have an important effect on visual appearance of the environment
around the freezing point. This may sometimes help in disambiguating image data
not easily interpretable.
For a human driver guiding a ground vehicle, the sense of vision is the most im-
portant source of information, especially in environments with good look-ahead
ranges and sudden surprising events. Over the last two decades, the research com-
munity worldwide has started developing the sense of vision for road vehicles, too.
[Bertozzi et al. 2000] and [Dickmanns 2002 a, b] give a review on the development.