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Magneto-meters monitor for fluctuations in the relative strength of the Earth’s
magnetic field, which is changed by the presence of a moving metal object i.e.,
a vehicle.
A single passive magnetic system collects flow and occupancy.
Two magneto-meter systems collect flow, occupancy, vehicle length, and speed.
Two types of magnetic field sensors are used for traffic flow parameter
measurement.
The first type, the two-axis flux-gate magneto-meter, detects changes in vertical
and horizontal components of the Earth s magnetic field produced by a ferrous
metal vehicle.
The two-axis flux-gate magneto-meter contains a primary winding and two secondary
sense winding on a coil surrounding high permeability soft magnetic material
core.
The second type of magnetic field sensor is the magnetic detector, more properly
referred to as an induction or search coil magneto-meter shown in
Fig. 1.
It detects the vehicle signature by measuring the change in the magnetic lines
of flux caused by the change in field values produced by a moving ferrous metal
vehicle.
These devices contain a single coil winding around a permeable magnetic material
rod core.
However, most magnetic detectors cannot detect stopped vehicles, since they
require a vehicle to be moving or otherwise changing its signature
characteristics with respect to time.
Advantages
- More usually mounted in a small hole in road surface and hardwired to the
processing unit.
Suitable for deployment on bridges.
Disadvantages
- Possibly damaged by utility maintenance activities, as with IDLs.
Figure 1:
Weigh-In-Motion Detector system (Source: FHWA vehicle detection
manual)
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