dadman
Posts: 1
Joined: Mon Nov 18, 2019 9:53 am

magnetometer for detecting objects

Mon Nov 18, 2019 9:57 am

Hello

I am interested in using a magnetometer to detect objects near my house. Like cars or even lol flying air planes. Can any one suggest a magnetometer that would be sensitive enough to do that?

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karrika
Posts: 1088
Joined: Mon Oct 19, 2015 6:21 am
Location: Finland

Re: magnetometer for detecting objects

Mon Nov 18, 2019 8:02 pm

The magnetic field gets weaker by the inverse square of the distance. Which means that you need a very sensitive device to detect a car from far away. I used to work with MRI equipment and it was a problem if the hospital was next to a garage as every car moving around twisted the magnetic field so that you ruined the MRI image.

The key point is to get the distance as small as possible between the car you want to detect and the coil you use for detection. Perhaps embedding the coil in the road would work.

Optics is definitely better for long distance detection.

simonmsh
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Joined: Sat Jun 01, 2019 6:07 pm

Re: magnetometer for detecting objects

Wed Nov 20, 2019 6:21 pm

I tried to do a bit of car-detection with the magnetometer on a Sense Hat. It seemed to be able to detect a stationary car (1 tonne, mostly steel) at 3 metres, but not moving cars at 8 metres. It also has a fairly sensitive accelerometer, but I wasn't able to detect the ground vibrations of passing cars at 8 metres with that either. I didn't put much effort into it, so take those results with a pinch of salt. Although the Sense Hat is great for learning about sensors in general, there might be better dedicated magnetometers available at a similar price. My next attempt will involve a camera and OpenCV.

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karrika
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Location: Finland

Re: magnetometer for detecting objects

Thu Nov 21, 2019 9:51 am

simonmsh wrote:
Wed Nov 20, 2019 6:21 pm
I tried to do a bit of car-detection with the magnetometer on a Sense Hat. It seemed to be able to detect a stationary car (1 tonne, mostly steel) at 3 metres, but not moving cars at 8 metres. It also has a fairly sensitive accelerometer, but I wasn't able to detect the ground vibrations of passing cars at 8 metres with that either. I didn't put much effort into it, so take those results with a pinch of salt. Although the Sense Hat is great for learning about sensors in general, there might be better dedicated magnetometers available at a similar price. My next attempt will involve a camera and OpenCV.
Sounds logical. The RPi_Cam_Web_Interface github contains motion detection of objects out of the box.

You can also use the GPU to track moving objects. An object can be defined as a bunch of corners moving statistically at the same speed to some direction. So you don't really need to recognize what the object is. This makes the task easier to implement.

PiGraham
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Location: Waterlooville

Re: magnetometer for detecting objects

Thu Nov 21, 2019 10:34 am

simonmsh wrote:
Wed Nov 20, 2019 6:21 pm
It also has a fairly sensitive accelerometer, but I wasn't able to detect the ground vibrations of passing cars at 8 metres with that either.
It seems MEMS accelerometers could detect traffic if they were close enough, but maybe not suited to remote sensing over many metres.

https://www.kickstarter.com/projects/11 ... d-invisibl
1G earthquakes!

https://ieeexplore.ieee.org/abstract/document/7744466

Traffic detected with sensors embedded in the road surface.

Magnetometers probably have similar range limitations. In-road magnetic loops work well but I don't think you would easily detect even very strong magnets from several metres.

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