Biomedical Engineering Reference
In-Depth Information
FIGURE 2-37
Sharp GP2Y0A21YF
distance
measurement IC.
(a) Schematic block
diagram. (b) Voltage
output as a function
of range. (Courtesy
of Sharp Electronics
http://www.
sharpusa.com/.)
A common IC that operates using this principle is the Sharp Electronics GP2Y0A21YF,
which has an operational range between 10 and 80 cm with the characteristics as illustrated
in Figure 2-37.
In more critical applications, a laser source is used because of its brightness and good
spatial coherence, and a charge-coupled device (CCD) array is often used as a receiver. On
transmit, the width of the laser beam is diffraction-limited by the size of the exit aperture.
On scattering from the target, the coherence of the laser beam is lost, so to produce the
smallest spot on the CCD array it must be placed at the focal plane of a lens (Amman,
2001).
Longer-range triangulation sensors can be used to obtain two-dimensional (2-D) or
three-dimensional (3-D) images of the terrain by scanning the beam. Such scanning mech-
anisms must be constructed to ensure that both the transmitter and the receiver can see
each point, so that as the transmitted spot is swept across the target a corresponding image
is reflected onto the CCD array (Probert-Smith, 2001).
Figure 2-38 shows a diagram of a sensor developed for 2-D scanning (Livingstone
and Rioux, 1986).
2.4.7 Time-of-Flight Ranging
The basic principles of active noncontact range-finding are similar for electromagnetic
(e.g., radar, laser) and active acoustic sensing. A signal is radiated toward an object or
target of interest, and the reflected or scattered signal is detected by a receiver and used to
determine the range.
As shown in Figure 2-39, a source of radiation is modulated and fed to a transmit
antenna, or aperture, which is usually matched to the impedance of the transmission
medium to maximize power transfer. This can take the form of a horn for acoustic or
radar sensors or an appropriately coated lens for a laser. The antenna also operates to
concentrate the radiated power into a narrow beam to maximize the operational range
and to minimize the angular ambiguity of the measurement. When the transmitted beam
strikes the target, a portion of the signal is reflected or scattered because the target has
a different impedance, or refractive index, to the medium through which the signal is
propagating. A small percentage of the reflected power travels back to the receiver (which
is often colocated with the transmitter), where it is captured by the receiver antenna and
