Magnetic sensors can measure distances between vehicles

Oct 18, 2011 by Lisa Zyga feature
By determining a clear inverse relationship between a vehicle’s magnetic field and its distance (obtained from a sonar sensor), scientists can estimate the distance between two vehicles at close spacing with a simple, inexpensive sensor. Image credit: S. Taghvaeeyan and R. Rajamani. ©2011 American Institute of Physics

(PhysOrg.com) -- Every vehicle has a magnetic field, and researchers have now found that a vehicle’s magnetic field has an inverse relationship with distance at small distances. The relationship provides a way to estimate a vehicle’s position using its magnetic field when the vehicle is less than a few meters away, which could be useful for detecting imminent collisions just before they occur.

S. Taghvaeeyan and R. Rajamani of the University of Minnesota have published their study on using vehicles’ magnetic signatures for position estimation in a recent issue of .

“The main sources of magnetic fields are the magnetized steel belts in the tires, the ignition, the alternator, air conditioning system, speakers, etc.,” Rajamani told PhysOrg.com. “The significant metal in the engine block, transmission, driveline, etc., can also be magnetized.”

To measure a vehicle’s magnetic field, the researchers used anisotropic magnetoresistive (AMR) on the vehicle that’s doing the measuring, while the other vehicles do not need to be equipped with any kind of device. The AMR sensors, which contain silicon chips with a thick coating of piezoresistive nickel-iron, can detect a change in the ambient magnetic field induced by a passing vehicle. The change in magnetic field causes a change in the resistance of the nickel-iron layer in the AMR sensors. While measuring a vehicle’s magnetism has previously been used to measure traffic flow rates on a road, it’s never been applied to estimating the distance between two vehicles.

In their study, the researchers performed a theoretical analysis and experimental measurements with different vehicle types to determine exactly how the magnetism relates to distance. They found a clear non-linear relationship between the measured magnetic field and distance below about 6 meters.

“In general, the magnetic field for a magnetized body varies with the powers of the inverse of the distance,” Rajamani said. “In the case of cars, we were able to show in this paper that cars have a magnetic field and that the variation could be described by a first order inverse relationship with distance.”

However, the researchers also found that this relationship depends on the type and size of a vehicle, and also changes from one location to another. To estimate a vehicle’s position without knowing these variables, the researchers found they could use two AMR sensors separated by a certain distance along with an adaptive estimation algorithm. As long as an approaching vehicle is close enough to affect both AMR sensors, the two sensors can accurately estimate its position, regardless of the vehicle type, vehicle size, or general location.

The magnetic field technique is not the first attempt at measuring inter-vehicle distances. Currently, some luxury vehicles use radar or laser sensors to measure distances to other vehicles. However, these sensors have two drawbacks: they cannot measure distances of less than 1 meter, and a typical radar distance measuring unit can cost over $1000. In comparison, the AMR sensors can measure distances of less than a meter and cost less than $10.

By further improving these sensors and tailoring them for commercial use, the researchers envision that the sensors could be applied all around a vehicle’s body, where they could detect nearby vehicles in all directions.

“The sensors will be useful for detecting an imminent collision,” Rajamani said. “The sensors will provide information on the relative velocity and position of the impending crash on the vehicle. This could be used to pre-tighten seat belts, inflate airbags and deploy other active structural enhancement measures that can protect the occupants in the car during the crash.”

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More information: S. Taghvaeeyan and R. Rajamani. “Use of vehicle magnetic signatures for position estimation.” Applied Physics Letters 99, 134101 (2011). DOI:10.1063/1.3639274

4.6 /5 (18 votes)

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Eikka
1 / 5 (2) Oct 18, 2011
So the sensor sees the steel parts of the car, but is completely oblivious to what else is there.

So you can happily rear-end a modern car, because much of it is made of plastic or aluminium which is paramagnetic, and you don't know how far the bumper is from the parts that you can sense.

Which kinda makes the system useless at a range of less than a meter.
wealthychef
not rated yet Oct 18, 2011
So the sensor sees the steel parts of the car, but is completely oblivious to what else is there.

So you can happily rear-end a modern car, because much of it is made of plastic or aluminium which is paramagnetic, and you don't know how far the bumper is from the parts that you can sense.

Which kinda makes the system useless at a range of less than a meter.


If you read the article, it says it senses the engine, tires, etc. Even a car with a plastic bumper has tires and an engine. It will still be magnetized. But this does seem like a weak method compared to simply mounting cameras on your car and using computer vision, which is probably doable nowadays.
Eikka
4 / 5 (1) Oct 18, 2011

If you read the article, it says it senses the engine, tires, etc. Even a car with a plastic bumper has tires and an engine. It will still be magnetized.


Yes, yes, but knowing where the tires or the steel gas tank is doesn't help you to know the extents of the plastic bodywork and the bumpers.

It would make a really lousy parking radar, because a the sub-meter range you could have someone's head in between there and it wouldn't see it.

CHollman82
1 / 5 (1) Oct 18, 2011
So the sensor sees the steel parts of the car, but is completely oblivious to what else is there.

So you can happily rear-end a modern car, because much of it is made of plastic or aluminium which is paramagnetic, and you don't know how far the bumper is from the parts that you can sense.

Which kinda makes the system useless at a range of less than a meter.


Yeah, engineers never account for margins of error... I'm sure they would let you get right up to within a millimeter of the detected position of the other vehicle...

Oh wait, no, the opposite of that is true.... OBVIOUSLY.
Eikka
4 / 5 (1) Oct 18, 2011

Yeah, engineers never account for margins of error... I'm sure they would let you get right up to within a millimeter of the detected position of the other vehicle...

Oh wait, no, the opposite of that is true.... OBVIOUSLY.


Indeed, but the article claims that this device would be actually useful at under three feet of another vehicle - except it won't be because you have to take into account that you can have three feet of non-magnetic materials between you and the part that the sensor actually senses.

The necessary margins of error render the invention just as useless as the others.
CHollman82
1 / 5 (2) Oct 18, 2011
What is the maximum range though (I apologize if it is mentioned in the article I only skimmed it). If the maximum range is decent this could be used in accident avoidance systems.
Parsec
1 / 5 (1) Oct 18, 2011
Obviously this sensor would be combined with other sensors for a any kind of complete approach. However, a very inexpensive sensor that could detect magnetic fields at such close distances could be a deal changer in terms of making fender benders less likely. The plastic rear end problem could be fixed by requirements that bumpers of all new cars had to transmit the magnetic field of the car (trivial and VERY inexpensive from a manufacturing standpoint). We already require bumpers to stand up to a certain amount of punishment.
Graeme
not rated yet Oct 20, 2011
with enough sensors it could detect a more complex field and then use that to identify the kind of vehicle and its exact orientation and position. Then using a model of the vehicle type it could be aware of where those plastic parts were. It could also detect electrostatic field or capacitance to notice those non ferrous components.