Saturday, April 14, 2007

Static Friction and Loop Detectors

The May 2007 issue of Motorcycle Consumer News has great coverage on loop detectors. What's a loop detector you ask? Ah! Newer drivers won't have noticed, but those of us who have been driving for a decade or two will recall a time when lights became much "smarter" -- they just seem to know when you've arrived at the intersection!

So, you've probably noticed the cuts in the pavement, in large box shapes, situated near the stop line in each lane at intersections, right? The physics geeks among us know (or at least, suspect) that these lines in the pavement are where wires are buried in order to use magnetic fields to detect the presence of cars. This is the loop detector. Or actually, this is the loop, the detector is housed in a control box on the side of the road.

The basic principle is that a current runs through the wire loop under the ground, and when a car approaches, the metal in the car, and the car's alternator disrupt that magnetic field in a way that can be measured by the equipment in the box on the side of the street (the detector).

MC News has the details from the City of Irvine, "Loop detectors are three loops of 12 or 14-gauge copper wire, buried 3" under the pavement. An electrical current is sent through the loop, making it an electromagnet. Once a car or motorcycle drives over the loop, the metal on the vehicles underside causes a disturbance in the magnetic field, tripping the detector and sending a signal to the traffic light to turn green." Lovely.

Of course, those of us who ride motorcycles know differently. The detector does a fine job of detecting the mass of cars. Not so much, with motorcycles. The rule of thumb is that revving the motor of your motorcycle will generate enough of a field to have an effect -- you may have seen the guy at the light doing this before; no, it's not just so that he sounds cool. Because those of us who ride motorcycles also know... that doesn't always work. Then, we get to sit through another cycle, or wait for a car to trip the light.

A suggested solution for motorcyclists is to attach a strong magnet to the underside of the bike to disrupt the field. No more revving, no more missed lights. But another answer is to brush up on physics, or read MC News, because it turns out that proper positioning of the motorcycle is the key to successfully disrupting the field.

I had a hell of a time in my Electromagnetic Field Theory class, so it's no wonder that I'd forgotten -- instead of sitting within a few inches of the line in the ground, I have tended to place the bike in the center of the loop. For a car, that's just fine. But, the true physicists among us (and those who have read up lately) will know that the electromagnetic field is strongest immediately adjacent to the conductor, and falls of as a function of the distance from the wire. So, the middle of the loop is just about the worst possible place to sit with a motorcycle. Near the wire, better.

Now if the crazy April weather around here would just cooperate, I'd go test the theory!

Ah! And a word of warning to all, especially those in CA: "Emergency vehicles (fire trucks, ambulances and police) are equipped with a device called an Emergency Vehicle Pre-empt Emitter, which uses either infrared signals or a strobe light to turn a signal green when rushing to a call. In Irvine, as in other cities, a cylindrical receiver sits on top of the traffic signal arm to receive these signals. ... Remember that fact when you hear a siren approaching, as the traffic light that just turned green for you may immediately turn red to let an emergency vehicle pass. Be prepared."

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