| |
| | |
|
Magneto-meters/Passive magnetic systems
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)
 |
Weigh-In-Motion (WIM) systems
- Bending Plate
Bending plate WIM systems utilize plates with strain gauges bonded to the
underside.
The system records the strain measured by strain gauges and calculates the
dynamic load.
Static load is estimated using the measured dynamic load and calibration
parameters.
Calibration parameters account for factors, such as vehicle speed and pavement
or suspension dynamics that influence estimates of the static weight.
The accuracy of bending plate WIM systems can be expressed as a function of the
vehicle speed traversed over the plates, assuming the system is installed in a
sound road structure and subject to normal traffic conditions.
Advantages
Bending plate WIM systems is used for traffic data collection as well as for
weight enforcement purposes.
The accuracy of these systems is higher than piezoelectric systems and their
cost is lower than load cell systems.
Bending plate WIM systems do not require complete replacement of the sensor.
Disadvantages
Bending plate WIM systems are not as accurate as load cell systems and are
considerably more expensive than piezoelectric systems.
|
|
| | |
|
|
|
