I still don't understand why the magnetic field around the motor doesn't trip those things, though. Is it that well shielded??
I don't have any actual experience working with those inductive pickup coils, but inductance is pretty a well understood topic to EEs like myself and MrDude. I would speculate the following things:
1) The pickup is a simple coil of wire, buried just below the surface of the pavement. You can often see where the pavement was carved out and patched to install the coil, in an older intersection that was retrofitted. Of course you won't see them in a new intersection or one that's been re-paved since installation.
2) The coil of wire, which is an electric component called an inductor, is paralleled with a capacitor to create an LC "resonant circuit", which will have a particular frequency it likes to oscillate at. Smart circuitry is used to create a fairly small oscillation at the resonant frequency the LC circuit prefers.
3) The whole purpose of this exercise is to create an alternating magnetic field in and around the coil, which interacts with any conductive or magnetic objects in the field. Any conductor, but especially iron or other "ferromagnetic" metals, will interact with the magnetic field and change the frequency the LC circuit naturally oscillates at. This change in frequency is easily detected, and if it passes some threshold, an object has been detected. Since it's only the resonant frequency of the LC circuit that's sensed, external fields (like the stray fields that leak out of our motors) don't have much effect.
4) The objects having the biggest effect on the magnetic field have a high "magnetic susceptibility" -- they're naturally magnetic. Iron/steel is a great one. Neodymium is even better, but whether or not it's actually magnetized isn't relevant. It's just the fact that it's magnetic in nature, which means it can affect the field of the wire coil. More on that later, though.
5) Electrically conductive materials which are NOT magnetic can be sensed, but not nearly as well as steel. They don't directly interact with the magnetic field, but the magnetic field in them causes an electrical current to flow, which in turn DOES interact with the magnetic field. Aluminum is a perfect example of a non-ferromagnetic material that can cause a small but detectable signal. There are some magnetic materials (like ferrite) which don't conduct electricity, but would be easily detected because of their high magnetic susceptibility. Non-conductive, non-magnetic materials are essentially invisible to the coil.
6) Size, shape and orientation of the object are very important. Rare earth magnet materials would be easily sensed, but they're expensive so those magnets are usually very small, so not much use. Same goes for our electric motors, which have lots of magnetic materials in them and would be great except that they're still pretty small compared to the size of the sense coil. Keep in mind that those coils were originally designed to detect the steel floor pan of a car, which covers nearly the entire coil and is ferromagnetic, so it creates a huge change in inductance of the coil. Getting closer to the coil helps (my brother the bicyclist swears laying his bike down on top of the coil will trigger it), and location and orientation are also important....someone said driving up right on the edge of the coil, tangent to it, works well, and that makes sense. Putting the kickstand down could help too, as some people have reported, especially if you do it in the right place (I'd guess right on the coil would be best, not in the center of it). A steel-toed work boot might be effective in the right place too.
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Topic: Mounting Neodymium magnets to trigger parking gates? (Read 10933 times)