Question

How can a magnetic sensor be employed to measure the movement of vehicles?

Answer 1

magnetic sensor be employed to measure the movement of vehicles

The presence, direction, and speed of a vehicle can be determined by employing magnetic
sensors. The technique requires a magnetic field of known strength and direction. A moving
vehicle can disturb the distribution of the magnetic field either by producing its own magnetic field or simply by cutting across it. As the magnitude and direction of the disturbance
depends on the speed, size, density, and permeability of the vehicle, it is possible to use
magnetic sensors to quantify the disturbance.
Magnetic sensors can be classified into low-field, medium-field, and high-field sensors,
according to the range of the strength of the magnetic field they measure . Low-field sensors measure magnetic field strength below 1 µG(micro Gauss); medium-field sensors measure between 1 µG and 10 G; high-field sensors
can measure above 10 G. The Earth’s magnetic field is found in the medium field.
Magnetic fields are set up by the motion of electrical charges. For instance, the magnetic field of a bar magnet is created by the motion of negatively charged electrons within
iron atoms. The cause of the Earth’s magnetic field is not completely understood, but it is
believed to be associated with electrical currents produced by the coupling of convective
effects and rotations in the spinning liquid metallic outer core of iron and nickel. It has a
uniform distribution over a wide area (several kilometers). It was first measured by Carl
Friedrich Gauss in 1835 and has been repeatedly measured since then, showing a relative
decay of about 5% over the last 150 years.

Sensors that can measure the Earth’s magnetic field comprise an alloy of nickel and iron.
Typical examples are anisotropic magnetoresistive (AMR) sensors whose resistive property
changes according to the Earth’s magnetic field strength. AMR sensors can measure both
linear and angular positions and displacement in the Earth’s magnetic field.
Almost all road vehicles, including those with polymer body panels, contain a large mass
of steel. Since the magnetic permeability of steel is much higher than the surrounding air, it
has the capacity to concentrate the flux lines of the Earth’s magnetic field. The concentration
of magnetic flux (disturbance) at a particular location varies as the vehicle moves and can
be detected from a distance of up to 15 m . how an AMR sensor can be used to measure the disturbance in the Earth’s magnetic field caused by a moving vehicle.

It is possible to distinguish between different types of vehicles (car, bus, minibus, truck,
etc.) by modeling a vehicle as a composition of many dipole magnets . These dipoles have north–south orientations that cause distortions in the Earth’s magnetic field. The extent of the distortions of the dipoles depends on, among other things, the permeability of the dipoles. For example, the engine and wheel areas exert stronger distortions than the other parts of a vehicle, and for each vehicle class of interest, it is possible to produce a unique model. When a vehicle passes close to a magnetic sensor, or drives over it, the sensor can detect the different dipole moments of the various parts of
the vehicle. The field variation reveals a detailed magnetic signature.