Magnetic centroid

This topic has 20 replies, 11 voices, and was last updated 13 April 2016 at 22:53 by simondavies3.

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12 April 2016 at 22:54 #24492
Sam Stones
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@samstones42903
Hardly model engineering
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12 April 2016 at 22:54 #234387
Sam Stones
Participant
@samstones42903
This has hardly anything to do with model engineering, but …

As a kid (some considerable time ago now, and far less traffic) I enjoyed jumping on those wide rubber strips built into the road. Many readers will recall that the strips changed the traffic lights in favour of vehicles running over them. If you stamped hard enough, you too could `take control’ of the lights.

Then there were the cat’s-eyes, which shot a jet of water upwards if you stamped on them. If you got it wrong, the water would shoot up the leg of your shorts.

These days, we can see where inductive loops lie below the road surface. As upside down metal detectors, they are typically less sensitive to larger vehicles raised higher off the road.

My question is however, –

For best effect (sensing), where would you expect the magnetic centroid of a vehicle to be?

Regards,

Sam
12 April 2016 at 23:05 #234390
Michael Gilligan
Participant
@michaelgilligan61133
Posted by Sam Stones on 12/04/2016 22:54:59:

My question is however, –

For best effect (sensing), where would you expect the magnetic centroid of a vehicle to be?

.

It must obviously depend upon the materials used in the construction of the vehicle, but; my first approximation would be 'the same place as its centre of gyration'.

MichaelG.
13 April 2016 at 00:01 #234394
Sam Stones
Participant
@samstones42903
I like that idea Michael.

I suppose the next part of the question relates to the power relationship – i.e. Distance relative to the magnetic flux/density. Somewhere in my flimsy memory is the notion that magnetism falls off by the 7th power of the distance, but that doesn't feel right.

Sam

Edited By Sam Stones on 13/04/2016 00:02:31
13 April 2016 at 07:22 #234403
Chris Evans 6
Participant
@chrisevans6
Will the trend to build aluminium cars trigger a rethink?
13 April 2016 at 07:32 #234404
Roger Hart
Participant
@rogerhart88496
I would guess that the frequencies involved are pretty low and the presence of any metal surface thicker than about 0.1mm will screen off anything beyond – at least so far as detection is concerned. So the idea of a centroid might not apply, just the distance and area. Ferrous or not should not matter.

I like the idea of squirting cats eyes though.
13 April 2016 at 07:47 #234405
Michael Gilligan
Participant
@michaelgilligan61133
Posted by Chris Evans 6 on 13/04/2016 07:22:24:

Will the trend to build aluminium cars trigger a rethink?

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Chris,

I thought I covered that in my first response.

MichaelG.

Edited By Michael Gilligan on 13/04/2016 07:56:25
13 April 2016 at 07:55 #234406
Michael Gilligan
Participant
@michaelgilligan61133
Posted by Sam Stones on 13/04/2016 00:01:44:

I suppose the next part of the question relates to the power relationship – i.e. Distance relative to the magnetic flux/density. < etc. >

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Getting way out of my depth there, Sam … but I would suggest that you start exploring the 'HyperPhysics' site.

MichaelG.

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P.S. … Some assurance that you are not designing a Mine would bring comfort.
13 April 2016 at 08:03 #234408
john carruthers
Participant
@johncarruthers46255
The magnetic sensor on a private carpark near me will trigger if you get the engine over, or near, the sensor from the 'outside'.
It entails slowly driving your bonnet under the exit barrier until it unlocks
Rear engined vehicles are out of luck.
13 April 2016 at 08:07 #234409
Michael Gilligan
Participant
@michaelgilligan61133
Useful **LINK**

MichaelG.

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Edit: and this is rather more technical.

Edited By Michael Gilligan on 13/04/2016 08:20:05
13 April 2016 at 08:46 #234412
Howi
Participant
@howi
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Edited By Howi on 13/04/2016 08:47:40
13 April 2016 at 09:05 #234416
Chris Evans 6
Participant
@chrisevans6
Michael, I should read the posts a little better. My excuse is it was early and the coffee had not yet wakened me. Chris.
13 April 2016 at 09:17 #234422
Mike
Participant
@mike89748
Unless things have changed in the last 20 or so years, it doesn't take much steel to trigger the light-changing mechanism. A pal who was a traffic lights engineer for the local council used to test lights by walking over the mechanism carrying a dustbin lid. I don't really understand the technicalities of this discussion, but I suggest there is enough steel in an aluminium-bodied car to do the business.
13 April 2016 at 09:18 #234423
Michael Gilligan
Participant
@michaelgilligan61133
No problem, Chris

It's turning into an interesting topic.

MichaelG.
13 April 2016 at 09:29 #234427
Ajohnw
Participant
@ajohnw51620
Posted by Michael Gilligan on 13/04/2016 08:07:10:

Useful **LINK**

MichaelG.

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Edit: and this is rather more technical.

Edited By Michael Gilligan on 13/04/2016 08:20:05

The 2nd one is interesting Michael. A friend of mine was convinced that they were being used to detect speeding in some areas of the country.

John

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13 April 2016 at 09:41 #234428
Jon Gibbs
Participant
@jongibbs59756
There is no need for the vehicle to be ferrous-based to trigger such an upside down metal detector in principle since any metal object will have eddy currents induced in it by the fluctuating magnetic field around the coil which either changes it's impedance or the magnetic effect of the eddies can be sensed by a receiving coil.

[Edit: Sorry I just saw Michael's link and it is confirmed… "The ferromagnetic effect increases loop inductance. However, vehicle-induced eddy currents decrease loop inductance even more. Therefore, net effect is decreased loop inductance when a vehicle passes through the detection zone of an inductive loop."]

My old Dad used to have a very crude metal detector in the late 70's when I was a kid and we spent all of our time digging up old drinks can tabs, which were aluminium of course and before they had to be captive.

It'd also be a pretty pointless hobby if all you could find were ferrous rusty blobs and no prospect of gold or silver coins although I don't remember us finding anything other than the drinks can tabs

Jon

Edited By Jon Gibbs on 13/04/2016 09:47:52
13 April 2016 at 14:22 #234489
mark costello 1
Participant
@markcostello1
Jumping up and down on traffic control and getting a splash? Never heard of them. You Guys have all the cool toys!
13 April 2016 at 14:26 #234490
Ajohnw
Participant
@ajohnw51620
They are usually called cat's eyes Mark Small square things that mark traffic lanes etc. The idea is that from time to time traffic drives over them which forces the top part down and in doing so wipes the small round reflector clean. I'm sure they are all over the world.

Water collects in them so jump hard enough and it squirts out.

John

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13 April 2016 at 16:18 #234501
Jon Gibbs
Participant
@jongibbs59756
Cat's eye description here… **LINK**
13 April 2016 at 22:45 #234541
Sam Stones
Participant
@samstones42903
Many thanks to M.E. contributors.

A special` thank you’ to Percy Shaw, Edward Current, et al for steering me through some of this labyrinth.

From the little I can find out, it appears that the (attraction/repulsion) force exerted between two magnets or between a magnet and another object being attracted can vary between the distance cubed and the distance to the seventh power. Something to do with shape again.

Knowing how to approach and position my car over the loops will never be the same again.

Regards to all,

Sam
13 April 2016 at 22:53 #234542
simondavies3
Participant
@simondavies3
Posted by Mike on 13/04/2016 09:17:10:

Unless things have changed in the last 20 or so years, it doesn't take much steel to trigger the light-changing mechanism. A pal who was a traffic lights engineer for the local council used to test lights by walking over the mechanism carrying a dustbin lid. I don't really understand the technicalities of this discussion, but I suggest there is enough steel in an aluminium-bodied car to do the business.

Having run a number of fibreglass cars over the years, I can recall several occasions where I had to wait at the traffic lights for somebody else to trigger the sensors – rather frustrating.

On the up side, the old radar traps that the Gendarmarie used to use were insufficently sensitive to detect me at speed.
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