OXTS Survey+ v3, User Manual, Page: 73 / 103

Revision: 190902
73
RT-Range User Manual
The distances from the measurement point on the to the measurement point of
the wheel speed encoder in the vehicle coordinate frame should be input. The directions
can be selected from the dropdown lists. If the wheel speed is from a prop shaft then the
distance should be measured half way between the two wheels. The illustrations in the
image will change depending on the settings you choose, to help visualise the position of
the in relation the wheel speed sensor.
Measurements by default are made to an accuracy of 10 cm. Using higher precision for
the measurement does not improve the results. Using an accuracy figure worse than 20
cm will increase the drift of the . The accuracy can be specified exactly in the
Accuracies tab in the Advanced Tools section along with other measurements such as
antenna position.
Enter the pulses per metre of the wheel speed. A value that is accurate to 10% is sufficient
unless you know the figure more accurately. The will improve this scaling factor itself
when GNSS is available as it is a state monitored by the Kalman Filter. The
Improved settings
utility (see page 84 for details) can be used to apply a more accurate value calculated by the
from a calibration run. If this option is used, then the should be allowed to
recalibrate the scaling value occasionally to account for tyre wear.
The wheel speed corrections will not be as effective in reducing the drift of the
if the wheel speed is measuring two wheels (i.e. after a differential), since the actual
position of the wheel is required for accurate navigation. If a post-differential encoder
must be used then the accuracy cannot be guaranteed.
For best results, a front wheel drive vehicle should be used with the wheel speed on a rear
wheel. The wheel speed pulses from driven wheels are less accurate.
Output smoothing tab
When the Kalman filter in the determines that there is some error to correct, this
error is applied smoothly rather than as a jump. The output smoothing controls how fast
the correction is applied to the outputs. This option is particularly useful for autonomous
vehicles or path-following robots as a rapid change in position can lead to a large change
in the steering angle.
Figure 29 shows the Output smoothing tab in the Advanced Tools section within
NAVconfig. Click the checkbox to enable output smoothing and unlock the properties for
editing.