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- Super Mod
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However, these days, I reckon the airforce would scramble a fighter to shoot him down when he flies around those sky-scrapers. Life's not easy when you're the man of steel!
(* nor ride a bike, come to think of it. )
Music was better when ugly people were allowed to make it ....
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- Joined: Sat May 26, 2007 7:25 pm
- Location: NW Sydney
If I have stopped at lights, and no other cars at the time, I stop on the centre of the fig 8 loops. If a vehicle pulls up behind, I move forward, point to them, and then point to the detection loop on the road. The driver generally gets the message and moves forward onto the loop.
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Perhaps I misunderstood my instructors when I did my electronics apprenticeship back in the 60s, but the "induced electric field" you speak of is an "electro-magnetic field" so by introducing a magnetic disturbance into this field, this alters the field, which is the signal the loop is designed to detect. I can't see how dragging non-magnetic aluminium through the induced field will be any more effective than taking a "magnet" through it.
(I have some background in this stuff).
The key thing is it doesn't have to be magnetic material. A magnet as such will have virtually no effect, it's got more to do with the material the magnet is made of than the fact it is magnetic.
When a field passes through a loop it acts like a short circuited transformer. The shorted current that flows in the shorted turn acts against the field that is emitted from road sensor and it is possible to detect this.
There' two ways induction loops can detect metal. The first is based changes in the field when something magnetic like iron is put in the field. In this case the best mode of detection is in the centre. The other is eddy currents being induced in the conductive material - like a shorted turn on a transformer. As the article states this is more often the mode of detection. In this mode you want the field from the sensor to pass through as much crossectional area as possible. A vertically bike has a very low crossection when looking up from the road. However a car has quite a large crosssection, being wider then it is high.
The field from the road loops shoots vertically upward at the centre. So if you are at the center of the loop the best way for detection is to tilt the bike over so for vertical field can past through the some of the "loops" on the bike - if the bike is vertical there is not loop. In effect you are making the area of the bike larger as seen from the ground. (A steel bike may be detected in the centre through the first mode of detection.)
At the sides of the road loop the field bends outwards in all directions (just like the pattern of iron filings at the end of a magnet). This means that at the edges of road loop the bent field can actually pass through the side of the bike. Looking from the side the bike has a wider cross-section and therefore it can be detected. If you want to improve detection at the sides you can lean the bike over towards the centre of the road loop.
The side by side figure 8-pattern road loops are more complicated to explain. Just accept they are design to detect at the centre and should pickup a vertical bicycle.
This site shows the spots:
http://www.geocities.com/siliconvalley/ ... ensors.htm
(As a side point the square loop can detect the bike at the leading and trailing edges as opposed to the sides as shown, however, you would have to rotate the bike so the large area of the biek was along the side - which not practical.)
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RobS wrote:If you want to improve detection at the sides you can lean the bike over towards the centre of the road loop.
Very good explanation RobS. Here's a another interesting exposition, by Stephen Goodridge (cyclist and MIEEE with PhD) "Detection of Bicycles by Quadrupole Loops at Demand-Actuated Traffic Signals". Goodridge generally agrees with you but one point of difference is that he argues that getting your wheel rim above the edge of the loop (in the direction of travel) is sufficient - and mentions in his US context that traffic light technicians often use just a wheel rim to adjust the loop sensitivity to pick up a bicycle.
Goodridge's idea that getting the rim over the left edge lines rather than through the center of the loop lines up with my experience of what seems to work. Have you done any experimentation with your idea that leaning the frame (or wheels) towards the center improves the detection chances ?
Somebody has to do something, and it's just incredibly pathetic that it has to be us -Jerry Garcia
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That was "the article" I was referring to, joeblake posted the link above.
In the field of metal detection all that stuff is common knowledge. Metal detectors, coin detectors and various other detectors use the same ideas.
> Goodridge generally agrees with you but one point of difference is that he argues that getting your wheel rim above the edge of the loop
I suspect in practice that is sufficient. The road loops don't go very deep into the road which makes the field drop-off quickly as you go further from the road surface. So the main contribution of detection will be near the edge and close to the road. However if the it's a marginal case tilting the bike over may just give it that last nudge.
> Have you done any experimentation with your idea that leaning the frame (or wheels) towards the center improves the detection chances ?
It's very hard to know cause and effect. You get to the lights, you wait ..., then you move the bike around, tilt it etc and it trips. Due to the traffic light delay, there's no instant feedback about what did it. In some cases the traffic lights just take time and they may well have detected you the first time through!
Sometimes I wonder if it's worth going through the motions trying to force the lights to change on an empty street when it's just as safe to go through the red light.
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