Sensors and Methods for Mobile Robot Positioning

Chapter 4: Sensors for Map-Based Positioning 125
Advances in wavelength control of laser diodes now permit this radar ranging technique to be
used with lasers. The frequency or wavelength of a laser diode can be shifted by varying its
temperature. Consider an example where the wavelength of an 850-nanometer laser diode is shifted
by 0.05 nanometers in four seconds: the corresponding frequency shift is 5.17 MHz per nanosecond.
This laser beam, when reflected from a surface 1 meter away, would produce a beat frequency of
34.5 MHz. The linearity of the frequency shift controls the accuracy of the system; a frequency
linearity of one part in 1000 yards yields an accuracy of 1 millimeter.
The frequency-modulation approach has an advantage over the phase-shift-measurement
technique in that a single distance measurement is not ambiguous. (Recall phase-shift systems must
perform two or more measurements at different modulation frequencies to be unambiguous.)
However, frequency modulation has several disadvantages associated with the required linearity and
repeatability of the frequency ramp, as well as the coherence of the laser beam in optical systems.
As a consequence, most commercially available FMCW ranging systems are radar-based, while laser
devices tend to favor TOF and phase-detection methods.
4.3.1 Eaton VORAD Vehicle Detection and Driver Alert System
VORAD Technologies [VORAD-1], in joint venture with [VORAD-2], has developed a commercial
millimeter-wave FMCW Doppler radar system designed for use on board a motor vehicle [VORAD-
1]. The Vehicle Collision Warning System employs a 12.7×12.7-centimeter (5×5 in)
antenna/transmitter-receiver package mounted on the front grill of a vehicle to monitor speed of and
distance to other traffic or obstacles on the road (see Figure4.35). The flat etched-array antenna
radiates approximately 0.5 mW of power at 24.725 GHz directly down the roadway in a narrow
directional beam. A GUNN diode is used for the transmitter, while the receiver employs a balancedmixer
detector [Woll, 1993].
Figure 4.35: The forward-looking antenna/transmitter/ receiver module
is mounted on the front of the vehicle at a height between 50 and 125
cm, while an optional side antenna can be installed as shown for
blind-spot protection. (Courtesy of VORAD-2).