[Q] How to calculate influence of metal object on dc biased coil?

[Jason Wilder]

I have a 100 mm ferrite rod on which I want to wind a coil using 0.3 mm
diameter copper wire. The goal is to detect a car passing by at three
meters. I plan to bias the coil with DC. How do I calculate the number
of windings that I have to put on the rod so that I can reliably detect
the car?

This should be easy I suppose, but I can't find anything about it. Every
book will tell you what the induced current in the car will be and that
sort of thing, but nowhere is explained how to go a bit further.

Do you have to calculate the induced current in the car and then from
there calculate the induced current in the coil? Is this back EMF? How
do you calculate that? How do you handle the ferrite rod? I am
unfortunately not very good at physics. I can think of an empirical way
to figure this out, but it will be rather time consuming.

Any help would be greatly appreciated.

Jason

 

[Jasen Betts]

I have a 100 mm ferrite rod on which I want to wind a coil using 0.3 mm
diameter copper wire. The goal is to detect a car passing by at three
meters. I plan to bias the coil with DC. How do I calculate the number
of windings that I have to put on the rod so that I can reliably detect
the car?

500000 truns should be enough

You want to get the diameter of the coil up near scale of the events
you're trying to measure. 500000 turns will be about 900mm diameter

you'll probably get better results from 200 turns on a 2m diameter
frame.

This should be easy I suppose, but I can't find anything about it. Every
book will tell you what the induced current in the car will be and that
sort of thing, but nowhere is explained how to go a bit further.

Do you have to calculate the induced current in the car and then from
there calculate the induced current in the coil? Is this back EMF? How
do you calculate that?

How do you handle the ferrite rod?

like glass it's fragile

it'll be pretty useless for this application.

 

[Jamie]

500000 truns should be enough

You want to get the diameter of the coil up near scale of the events
you're trying to measure. 500000 turns will be about 900mm diameter

you'll probably get better results from 200 turns on a 2m diameter
frame.




like glass it's fragile

it'll be pretty useless for this application.

VRS sensor technology is what he is looking for...

(Variable reluctance Sensor)..

Only good for moving objects of course. otherwise it should
be done via ultrasonic or light..

 

[Sjouke Burry]

I have a 100 mm ferrite rod on which I want to wind a coil using 0.3 mm
diameter copper wire. The goal is to detect a car passing by at three
meters. I plan to bias the coil with DC. How do I calculate the number
of windings that I have to put on the rod so that I can reliably detect
the car?

This should be easy I suppose, but I can't find anything about it. Every
book will tell you what the induced current in the car will be and that
sort of thing, but nowhere is explained how to go a bit further.

Do you have to calculate the induced current in the car and then from
there calculate the induced current in the coil? Is this back EMF? How
do you calculate that? How do you handle the ferrite rod? I am
unfortunately not very good at physics. I can think of an empirical way
to figure this out, but it will be rather time consuming.

Any help would be greatly appreciated.

Jason

If I would like to attack that problem, I would use
a bigger coil(~20 by 20 inch), air core.
I would then put it in an oscillator, and measure
changes in frequency.
Any metal in the neighbourhood will change the frequency,
just like commercial metal-detectors do.
If your oscillator is at say 50Khz or higher, mix it with a
stable 50khz or higher oscillator, and you get a nice zero-beat
tone, easy to detect by ear.
Your ferrite will make the local field of the coil smaller,
and detection difficult.
Dc measurement depends heavily on the residual magnetic
field of the car, and will be very difficult to detect,
and get mixed up with stray 60HZ fields, nearby radio
transmitters, etc.

 

[shrtrnd]

There seem to be a lot of commercial vehicle sensors in pavement in my city. Have you tried contacting any of the manufacturers of these devices? You might be able to gleen some helpful information from their contact people or literature they may send you for free on their systems. I like building my own devices, but I don't like reinventing the wheel. If you get some basic 'do's' or 'don'ts' from people who have already tried a variety of methods, it may help point you in the best direction.
I personally would worry about using a ferrite rod (I suppose there are different possible compositions), because if it's exposed to weathering, it might deteriorate.

 

[Jason Wilder]

If I would like to attack that problem, I would use
a bigger coil(~20 by 20 inch), air core.
I would then put it in an oscillator, and measure
changes in frequency.
Any metal in the neighbourhood will change the frequency,
just like commercial metal-detectors do.
If your oscillator is at say 50Khz or higher, mix it with a
stable 50khz or higher oscillator, and you get a nice zero-beat
tone, easy to detect by ear.
Your ferrite will make the local field of the coil smaller,
and detection difficult.
Dc measurement depends heavily on the residual magnetic
field of the car, and will be very difficult to detect,
and get mixed up with stray 60HZ fields, nearby radio
transmitters, etc.

I based my first thoughts on this article:
http://faculty.kfupm.edu.sa/EE/ajmal/0xx/ee445/EE445_Experiment_06.pdf
Then I started wondering how the author came to his coil design. It
looks like a copy from a book.

I want to detect moving cars that pass at a distance of about 3 meters
in front of my garden. I cannot use a loop detector in the road, because
it is public property. I do not want to use optical detection because I
want it to work in all weather conditions. Also I cannot install a
reflector on the other side of the road so the cars should reflect the
light, but they may not do that very well.

Jason

 

[Jamie]

I based my first thoughts on this article:
http://faculty.kfupm.edu.sa/EE/ajmal/0xx/ee445/EE445_Experiment_06.pdf
Then I started wondering how the author came to his coil design. It
looks like a copy from a book.

I want to detect moving cars that pass at a distance of about 3 meters
in front of my garden. I cannot use a loop detector in the road, because
it is public property. I do not want to use optical detection because I
want it to work in all weather conditions. Also I cannot install a
reflector on the other side of the road so the cars should reflect the
light, but they may not do that very well.

Jason

Utrasonic motion detection?


We have units at unit that mounts in a 1" Pipe hole that can sense up
to ~75 Feet.. Much more than what you need.. They operate on low voltage
and output a Analog signal.. You can get them also to do a logic output
with adjustable settings.. Be prepared to use some of your hard earn
money! Or Obama's free money!
http://www.senix.com/?gclid=CIXHiZuImaUCFQ915QodaH2PJg

Just an example.

 

[Sjouke Burry]

I based my first thoughts on this article:
http://faculty.kfupm.edu.sa/EE/ajmal/0xx/ee445/EE445_Experiment_06.pdf
Then I started wondering how the author came to his coil design. It
looks like a copy from a book.

I want to detect moving cars that pass at a distance of about 3 meters
in front of my garden. I cannot use a loop detector in the road, because
it is public property. I do not want to use optical detection because I
want it to work in all weather conditions. Also I cannot install a
reflector on the other side of the road so the cars should reflect the
light, but they may not do that very well.

Jason

So integrate a coil with your border decorations,
It could be hidden behind a poster/billboard in
the garden, etc.
So it does not have to be small.
The bigger it is, the greater its range.
A simple way to make such a coil, is to use a 24
or 40 wire cable, and do some fancy soldering where
the cable ends meet, to produce a 24(40) turn coil.

 

[[email protected]]

I based my first thoughts on this article:
http://faculty.kfupm.edu.sa/EE/ajmal/0xx/ee445/EE445_Experiment_06.pdf
Then I started wondering how the author came to his coil design. It
looks like a copy from a book.

I want to detect moving cars that pass at a distance of about 3 meters
in front of my garden. I cannot use a loop detector in the road, because
it is public property. I do not want to use optical detection because I
want it to work in all weather conditions. Also I cannot install a
reflector on the other side of the road so the cars should reflect the
light, but they may not do that very well.

Weather shouldn't be that much of an issue. Even during the nastiest weather
the atmosphere is still pretty transparent. Fog can be pretty dense, sure,
but you should be able to shine through it at some wavelength. A camera and a
little software could do the job (fog is an issue here). IIRC, someone here
(or in SED) had a speed detector based on image recognition, a couple of years
back.

....or you could give your neighbors a reflective driveway marker and bounce a
beam off it.