Agriculture Reference
In-Depth Information
left side and the other one on the right side of the driving direction - the roll can be
indicated via their relative height. And by simple trigonometric calculations con-
cerning the target point, the path can be corrected. The
yaw
of the tractor can be
obtained via the relative forward positions of these two antennas. Finally, if three
antennas are used - with the third antenna mounted in front of the other two - it is
possible to indicate
pitch
. Adapting the steering control to the positional data
requires suitable algorithms (Heraud and Lange
2009
). Yet correcting the guidance
on slopes on the basis of several antennas is state of the art.
The use of GNSS guidance sometimes is supplemented by signals from
inertial
sensors
. These inertial sensors measure either linear- or rotational accelerations and
hence can also indicate changes in positions of vehicles (Yazdi et al.
1998
). They are
very tiny devices and used in huge quantities for a variety of applications,
e.g.
in
automobiles for airbag employment and for dynamic stability control. When used
adequately, these inertial sensors can measure translations as well as rotations in
three spatial dimensions, hence with six degrees of freedom along and around the
axes of roll, pitch and yaw.
Despite these abilities and their low cost, normally inertial sensors are not
used to completely replace guidance by GNSS. Instead there are commercial
solutions that rely on several GPS antennas plus inertial sensors for terrain
compensation.
Because inherently, inertial sensors are not able to steer precisely for a long time
period along fi xed lines, let alone to guide along paths that were used months or
years ago. This inability of inertial sensors results from the operating principle: they
rely totally on
motion memory
. Their basis is that when a vehicle starts from a
known position and then moves in a known direction for a known time, the fi nal
position is known. But this motion memory inherently includes the accumulation of
errors along its path, often denoted as drift. Whereas with GNSS guidance, the
errors that arise are corrected - constantly and automatically - by new signals that
are received from satellites and base stations, with inertial sensors this does not
occur. The present recommendation therefore is to use the signals from inertial sen-
sors not primarily for controlling the
heading
or the
yaw
of vehicles, for which
properly used GPS signals are superior.
The situation is different when it comes to controlling the effects of the
attitude
that are defi ned by roll and pitch. These attitude errors hardly can accumulate,
since they do not occur in the direction of the path or of the heading of the tractor.
Hence in controlling the effects of roll and pitch there is a choice: this could be on
the basis of either additional GPS antennas or of inertial sensors. Li et al. (
2009
) as
well as Kellar et al. (
2008
) got about the same accuracies in guidance of vehicles
when either using GPS with multiple antennas or - as an alternative - employing
inertial sensors for the control of roll and pitch in combination with a single antenna
GPS for the control of yaw. They mention that the expenses for the latter alterna-
tive are lower.
However - even for the same positional dimension - there are reasons for com-
bining GPS receivers and inertial sensors. GPS indications that are received have a
rather long-term stability, but its signals can be unavailable or blocked by trees,
