And what is induction, anyway?
In: Engineering
When a wire experiences changing magnetic pull, voltage is generated in that wire. This is called induction.
When electricity runs through a wire, it becomes a magnet, temporarily.
So when you try to pass electricity through a wire, especially a coiled one where each loop magnetically interacts with the other loops because they’re right next to eachother, it’s kind of difficult. The electricity takes time to build up in the loop, as the voltage from the magnetism fights against the voltage you’re using to push electricity through the wire. Then, when you stop applying a voltage, it takes a while for the electricity to stop as the voltage from the magnetism is now keeping it going
Another way to put this: Inductance is a ‘force’ that fights to keep the same amount of electricity running through a wire. If you want more or less electricity to flow, you need to use a voltage. A coil of wire has more inductance than a single wire, and a coil of the same size with more loops has more inductance.
You can measure inductance pretty easily. You try to pass electricity through a wire and see how long it takes to get going.
If you bring a magnetic material like iron near the loop, it amplifies the magnetic strength of the wire and so increases the inductance. If you bring a non-magnetic conductive material like copper near the loop, it allows electricity to freely flow which reduces the inductance.
Traffic signals put a big loop of wire in the ground and repeatedly test its inductance, and when this inductance changes enough they know that a large metal object is nearby.
Ever made an electromagnet? You can wrap wire around an iron nail and connect a battery to it. Current passing through the coil of wire induces a magnetic field in the nail. It even works in reverse- if you pass a coil of wire through a magnetic field a voltage will be induced in the wire. Electric guitar pickups consist of a coil of wire around a permanent magnet. The vibrating string distorts the magnetic field which induces the signal that the amplifier picks up. Metal detectors work by pulsing current through a coil. The brief pulse of current induces a magnetic field around the coil, which quickly collapses as the pulse ends. As the field collapses it induces its own signal in the wire which can be read by onboard processing equipment. The presence of materials which interact with a magnetic field (typically metals) slightly alters the shape of the collapsing magnetic field which tells the metal detector there’s metal in range. A traffic signal induction loop is essentially a massive metal detector embedded in the ground pointing upwards.
You probably know, since you aren’t literally 5 years old, that something called electricity exists. Electricity can easily flow through some materials, which are called conductive. One interesting thing about electricity is that it can interact with itself. There are certain kinds of devices, called inductors and capacitors, which you can put in an otherwise conductive wire to change how it responds to electricity.
One important aspect of electricity flowing through conductors, which is unintuitive based on your ordinary experience, is that how the the electrical current flows through the conductor can be changed by objects outside of it, especially objects that are themselves conductive. Similarly, changing the current in a conductor can also cause electricity to flow in other conductors that are nearby, but not touching.
Induction is a feature of some objects we can build. Because the electricity flowing in a wire can affect nearby objects, even including the same wire that’s nearby, we can build a device which resists changes in electric current. What that means is, if you flip on a switch and you want to turn a circuit on, the inductor reduces the amount of current that flows right after you flip the switch. Eventually, it allows the full current to flow through, but then when you close the switch again to turn off the current, the inductor actually keeps the current flowing for a little while.
So how does an inductive loop work to detect traffic? Well, first they set up a loop of wire with inductors in it. This is put in the ground by a traffic light. They have some electronic circuits that cause current to run through the wire at a particular rate. You can think of this as flicking the switch on and off at a specific rate, like one time a second or two times a second or, in reality for this kind of device, thousands of times a second. Because the people who installed the equipment no exactly what they installed, they can install sensors to tell them if something strange is happening to the current flowing through the wire, or the voltage across it. (Voltage is kind of like pressure in water. If one place has a higher voltage relative to another place, then current will want to flow between them from higher voltage to lower voltage.)
You might ask what could possibly cause the current in the voltage to change from what the designers knew it would be when they built the circuit. The answer is that motor vehicles have a lot of metal in them, and as I previously mentioned, electrical current flowing through a circuit can both affect things that aren’t directly touching the circuit, and *be* affected by things that aren’t directly touching the circuit. When a big lump of metal sits close enough to the induction loop, current starts flowing in that lump of metal, but that sucks some energy out of the main loop. For really small lumps of metal, this doesn’t really make any difference, but for something like the engine for the body frame of a car, the change in what’s happening in the main induction loop is noticeable. So, after enough change happens, some circuitry automatically knows that there’s a car there and changes how the lights are turned on and off accordingly.
Induction loops can have a problem with smaller vehicles like motorcycles or especially bicycles, where the loop is designed not to respond to small changes. The reason people do this, is that you don’t want your induction loop to think a vehicle is waiting at the light just because of pedestrian with a big keychain and some coins in his pocket walks by. Sometimes people with motorcycles or bicycles will apply strong magnets to parts of their vehicle, because electricity and magnetism are basically the same thing, and a strong magnet moving near the induction loop looks like a big piece of metal. Another solution, which is most common in big cities, is that there will be special induction loops for cyclists / motorcyclists. These induction loops are designed to be more sensitive to smaller chunks of metal, but they’re also small, so false positives are less likely. That’s why you might see a sign that says something like “cyclists, for green wait on line” (meaning the white line that is where you’re supposed to stop if you are a car).
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