Vehicular Sensor Networks: General Aspects and Implementation Issues - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 216

speed sensors, etc., which measure traveled path, tilt sensing, operation time, load
pressure, load temperature, load humidity, load weight and density, and many other
parameters. This section will provide greater insight into the characteristics of the
most relevant representatives of roadside and intra-vehicle sensors, respectively.

10.5.2.1 Magnetic Sensors

Wireless magnetic sensor nodes offer a very attractive, low-cost alternative to tech-
nologies such as inductive loops, video cameras, and radar for traffic measurement
in freeways, urban street intersections, and presence detection in parking lots. They
can provide information about speed and direction of traffic, quantity of vehicles
per time on a stretch of pavement or just reliable presence or absence of a class of
vehicles. Appealing to the fact that almost all road vehicles have significant
amounts of ferrous metals in their chassis (e.g., iron, steel, nickel, cobalt, etc.),
magnetic sensors are a good candidate for detecting vehicles. They usually use the
disturbance of the magnetic field of the earth in order to determine the presence or
absence of a vehicle.
Figure 10.6 shows a good graphical example of the lines of flux from the earth
between the magnetic poles and the bending they receive as they penetrate a typical
vehicle with ferrous metals. As the lines of magnetic flux group together (concen-
trate) or spread out (deconcentrate), a magnetic sensor placed nearby will be under
the same magnetic influence the vehicle creates to the earth's field.
Most commonly used magnetic sensors are Anisotropic Magneto-Resistive sensors
or AMRs [ 30 ]. AMRs are directional sensors and provide only an amplitude response
to magnetic fields in their sensitive axis. By combining AMR sensors into two or three
axis configurations, a two- or three-dimensional measurement of the magnetic fields
passing through the sensor is possible with excellent linearity. For AMR sensors, the
sensor-resistive elements are oriented as resistive “Wheatstone bridge” that varies
resistance slightly as the magnetic field changes upon each element. There are several
methods in which AMRs can be used. The following text describes them in detail.
Vehicle detection signature . As vehicles come near the sensor, there is a shift from
the earth's magnetic field levels. Since the natural earth's magnetic field would bias the
sensors with a slight negative voltage output, increasing flux concentration would

Fig. 10.6 Earth's magnetic field through a vehicle