Can you 'fool' a proximity sensor?

B. Elliott

I own a small one that that can detect metallic objects. Automotive use. I believe most (if not all) are based off of an inductive loop design. Anyway, I was playing around with it and a some coins and the question popped into my head....

Is it possible to 'trick' an inductive loop into not being able to detect or see a metal object?


Brett

Integral

Not clear, what you mean by fool. Proximity sensors of all sorts can fail, but for the most part they are extremely reliable. They only fail if physically abused or become mechanially mis aligned.

B. Elliott

Sorry for not being very clear. What I mean by 'fool' would be to make the the metal object invisible to the proximity sensor. Since they operate by detecting a change in the electromagnetic field that they produce, could they possibly be 'fooled' if you were to somehow emit a stronger field? Or maybe cancel out the field in some way?

TechSpec

Probably not, sensors which are based on earths magnetic field changes like traffic sensors, are hard to pass, anything that affects into a field may trigger the sensor. Even if trying to jam the field with an another field would be very difficult as you dont know the reference level. In theory, you might have a chance to create a field which is negative to that field that is affecting to a sensor, creating a +- zero situation to a sensor "sight". But as i said, that is nearly impossible, sensors are very accurate and sensitive.
But if you are not affecting into a sensor you may pass it. In traffic, someone told that a motorbike or a sports cars done with a carbon fibres could pass those sensors.

berkeman

That's not true, TechSpec. That's not how inductive loop traffic sensors work. They use an AC signal, not the Earth's magnetic field.

And to the original question, yes, you can make a circuit that would actively pick up the detector signal and emit a signal that compensated for the metal object. There are some limitations in terms of sizes and geometries, naturally.

TechSpec

Berkeman, i agree, AC loops wont work with earths magnetic field. However, there are different types of sensors available like Honeywell. They have sensors to detect vehicles with a different principle of detection. They are based on earths magnetic field variations caused by a vehicle near to it.
You may read about them from here:
http://www.ssec.honeywell.com/magnet...eets/an218.pdf

As the sensor technology has inproved the last few years rapidly, and making them easier and cheaper than the "old technology", i assume that AC loops may be a history in a while... who knows.

berkeman

I'll be darned! I hadn't heard about this technology, TechSpec. Thanks for the info -- I learn something new every day!