Page 484 - The Mechatronics Handbook

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between the source and the detector. Like any triangulation system, when the baseline separation
increases, the accuracy of the sensor increases, but the missing parts problem worsens.
Three-dimensional range information for an entire scene can be obtained in relatively simple fashion
through striped lighting techniques. By assembling a series of closely spaced two-dimensional contours,
a three-dimensional description of a region within the camera’s field of view can be constructed. The
third dimension is typically provided by scanning the laser plane across the scene. Compared to single-
point triangulation, striped lighting generally requires less time to digitize a surface, with fewer moving
parts because of the need to mechanically scan only in one direction. The drawback to this concept is
that range extraction is time consuming and difficult due to the necessity of storing and analyzing many
frames.
An alternative structured-light approach for three-dimensional applications involves projecting a
rectangular grid of high-contrast light points or lines onto a surface. Variations in depth cause the grid
pattern to distort, providing a means for range extraction. The extent of the distortion is ascertained by
comparing the displaced grid with the original projected patterns as follows (LeMoigue & Waxman, 1984):

• Identify the intersection points of the distorted grid image.
• Label these intersections according to the coordinate system established for the projected pattern.
• Compute the disparities between the intersection points and/or lines of the two grids.
• Convert the displacements to range information.
The comparison process requires correspondence between points on the image and the original pattern,
which can be troublesome. By correlating the image grid points to the projected grid points, this
problem can be somewhat alleviated. A critical design parameter is the thickness of the lines that make
up the grid and the spacing between these lines. Excessively thin lines will break up in busy scenes,
causing discontinuities that adversely affect the intersection points labeling process. Thicker lines will
produce less observed grid distortion resulting in reduced range accuracy (LeMoigue & Waxman, 1984).
The sensor’s intended domain of operation will determine the density of points required for adequate
scene interpretation and resolution.
Magnetic Position Measurement Systems
Magnetic tracking uses a source element radiating a magnetic field (three axes) and a small sensor (three
axes) that reports its position and orientation with respect to the source. Competing systems provide
various multi-source, multi-sensor systems that will track a number of points at up to 100 Hz in ranges
from 3 to 20 ft (Polhemus Incorporated, and Ascension Technologies). They are generally accurate to
better than 0.1 in. in position and 0.1° in rotation. Magnetic systems do not rely on line-of-sight from
source to object, as do optical and acoustic systems, but metallic objects in the environment will distort
the magnetic field, giving erroneous readings. They require cable attachment to a central device (as do
LEDs and acoustic systems). Current technology is quite robust and widely used for single or double
hand-tracking, head-mounted devices, biomechanical analysis, graphics (digitization in 3D), stereotaxic
localization, etc.
Magnetic field sources can be AC or DC. DC sources may emit pulses rather than continuous radiation
in order to minimize interference from other magnetic sources. Using pulsed systems allows measurement
of existing magnetic fields in the environment during the inactive period. Knowledge of these magnetic
fields external to the system is used to improve accuracy and to overcome sensitivity to metals.
Figure 19.85 shows a typical transmitter-drive electronics (courtesy of Ascension Technologies). It
provides DC current pulses to each antenna of the transmitter, one antenna at a time. The transmitter
consists of a core about which the X, Y, and Z antennae are wound. While a given transmitter antenna
is activated with current, readings are taken from all three antennae of the sensor. Initially the transmitter
is shut off so that the sensor can measure the x, y, and z components of the earth’s magnetic field. During
operation, the computer sends to the digital-to-analog (D/A) converter a number that represents the
amplitude of the current pulse to be sent to the selected transmitter antenna. The D/A converter converts

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