Biomedical Engineering Reference
In-Depth Information
on the order of 5 × 10 −15 W. These devices have a wide dynamic range and
good linearity properties. Inexpensive units have typical operational biases
from 2 to 100 V, with active areas from a few millimeters to 1 cm. They gen-
erally have uniform responsivity over the entire active area, and are quite
stable with time and temperature.
The PINs are typically packaged in modified versions of standard TO-5
and TO-18 cans, with either flat glass or convex lenses. Several special pack-
age designs are available for fiber-optic applications where the detector is
mounted in a connector shell with a pigtailed fiber. Since the detector active
area is not typically well matched with the size of the fiber, coupling losses
as well as volume requirements can be reduced significantly through the use
of a specially packaged detector, so that such an approach is well suited for
fiber sensor systems.
4.12 FiberMagnetometerApplications
To sense a vehicle with a fiber magnetometer, the vehicle must be the source
of a magnetic field. Such a field may exist or may develop through the inter-
actions of the vehicle's ferromagnetic material with the earth's magnetic
field. The vehicle-generated field is usually small or nonexistent. For the lat-
ter field, there is direct correlation between the local earth's field strength
and direction and that of the field generated. The total local field is then
the vector sum of the two fields and can be considered to be the perturbed
earth's field.
The magnetic sensor in the near vicinity must adequately detect the per-
turbation above the background earth's field. Adequate detection leads
to the determination of both vehicle type and location. The model of a
vehicle in the earth's field is that of a magnetic moment, the magnitude
of which is determined by the vehicle volume and geometry as well as
the earth's field. The moment direction can be expected to follow closely
the field direction of the earth, although the vehicle geometry may dictate
that they are not exactly parallel. The field of a dipole moment is well
known and can be expressed in polar coordinate form in terms of R and
θ, where R is the distance or range from the dipole moment and θ is the
angle between the R direction and the moment direction. Two indepen-
dent measurements are required to evaluate either R or θ with additional
information required to evaluate vehicle type. If two directional sensors
are arranged orthogonally, R and θ can be determined. The easiest situa-
tion to analyze is a common plane formed between the sensor direction
and the vehicle moment. If this is not so, only the field component for the
moment in the plane of the sensors is detected. Knowledge of the moment
angle with the sensor plane is necessary if evaluation of R and θ is to be
 
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