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I have read that the inductors are imbedded in asphalts in small cities and this is how the traffic light detects if a car is approaching.

I have two question.

1- So if they are embedded would the magnetic field be strong enough to pass the asphalt without attenuation?

2- The cores of the inductors remain the same and what is changing is not the magnetic filed inside, but outside. From the basic physics we know that the change in inductance depends on the permeability of the core, area of the core and the length of the wire. So nothing in core changes when a car stops on top of an inductor. So the question is how would the voltage changes in this case?

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  • \$\begingroup\$ These inductive loop sensors are also used in big cities - In Vancouver BC they are frequently used in left turn lanes so the left turn signal is only activated if there are left-turners waiting. \$\endgroup\$ – Peter Bennett Mar 26 '16 at 18:33
  • \$\begingroup\$ (1) Asphalt is not ferro-magnetic. It won't affect the EM field. (2) There is no core until a vehicle sits on it. The inductive loops are air-cored. \$\endgroup\$ – Transistor Mar 26 '16 at 18:41
  • \$\begingroup\$ @transistor , but the core is the section inside the loop and the loop is embedded (with its core). \$\endgroup\$ – Jack Mar 26 '16 at 18:44
  • \$\begingroup\$ the "core" doesnt have to necessarily be the section inside the loop. Of course the area inside the loop is more effective but elements outside do affect its inductance. i.e.: one inductor close to another may affect each other, even though one is not "inside" the other: forum.polkaudio.com/discussion/111744/… \$\endgroup\$ – Wesley Lee Mar 26 '16 at 18:52
  • \$\begingroup\$ I guess permeabilty part of inductance accounts for both inside the loop and outside. That is the only reasonable explanation that I could think of. \$\endgroup\$ – Jack Mar 26 '16 at 19:19
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Inductive traffic loops work by embedding one or more series-connected loops of wire in a slot cut in the road surface. (Typically one to three turns are used.) The two ends are connected back to a control box. The loop forms and inductive coil which is used as part of an oscillator circuit in the control box.

Loop inductance is not affected by asphalt or concrete as they typically don't contain any metal. The presence of a vehicle, however, will affect the inductance of the loop and a change in frequency of the oscillator will result. When the frequency change is large enough the controller will switch its output to signal vehicle present.

To answer your specific questions:

1- So if they are embedded would the magnetic field be strong enough to pass the asphalt without attenuation?

The asphalt (or concrete) does not affect the inductance of the loop. I can't find any figures but I suspect its permeability is going to be very much closer to that of air than that of iron, for example.

2- The cores of the inductors remain the same and what is changing is not the magnetic filed inside, but outside. From the basic physics we know that the change in inductance depends on the permeability of the core, area of the core and the length of the wire. So nothing in core changes when a car stops on top of an inductor.

The inductor is, effectively, air-cored. There is no metal involved until a vehicle arrives and it becomes the core. The sensing height will depend on the length of the shortest side of the loop - see below. This makes intuitive sense. Think of an inductor wound on a cardboard tube with a steel rod sliding in and out: It will have to get close to the coil before it starts to make a difference. An interesting though on this is that we could make much more sensitive drive-through loops - much like airport metal detectors - but this has not been considered a good idea for obvious reasons.

So the question is how would the voltage changes in this case?

The voltage might not change but the oscillator frequency will. The frequency of an LC (inductor-capacitor) oscillator is given by

$$f = \frac {1}{2 \pi \sqrt{LC}}$$

so we can see that if L increases with the presence of a vehicle the frequency will drop.


There is an interesting presentation PDF from Eberle Design Inc. which addresses the installation of inductive traffic loops. Extracts below.

a. The typical sensing height is 2/3 of the shortest leg of a loop (in feet). Therefore a 4’ x 8’ loop typically has a detection height of 2.66 feet.

b. The inductance of a conventional four-sided loop can be estimated using the formula: \$L = P \times (T^2 + T) / 4\$ Where L = Loop Inductance in micro Henries, P = Loop Perimeter in feet and T = Number of turns of wires in saw slot.

Therefore a 4’ x 8’ loop with 4 turns would be: \$L = (4 + 8 + 4 + 8) (4^2+ 4) / 4 = 24 (16 + 4) / 4 = 24 \cdot 20 / 4 = 24 \cdot 5 = 120~\mu H\$ Note: Loop feeder cable typically adds 0.22 micro Henries of inductance per foot of cable.

The article also covers series connection of loops and how to install loops on both sides of a sliding gate while avoiding detection of the gate.

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"The cores of the inductors remain the same" - not really:

The car acts as an iron core to the inductor, thus increasing inductance if compared to an "air core". So you know there is something there.

enter image description here

This pdf has some nice info on them.

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  • \$\begingroup\$ How can the car acts as a core when the core is embedded inside the ground? The core should be inside the loop, that is the definition of the core. \$\endgroup\$ – Jack Mar 27 '16 at 2:15
  • \$\begingroup\$ me and transistor have already told you (and sent you urls to documentation) explaining that objects around a loop(not necessarily inside) can affect its inductace... i dont think repeating this over and over will be any more productive, so unless you have a new question I'll stop answering.. \$\endgroup\$ – Wesley Lee Mar 27 '16 at 6:48
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    \$\begingroup\$ @MaryannEthan: THERE IS NO CORE embedded in the ground. See Wesley's diagram. It's just a loop of wire that's pushed down through a rectangular slot cut in the asphalt with a concrete saw. They don't dig the road to do it. The resultant coil / inductor is air-cored. \$\endgroup\$ – Transistor Mar 27 '16 at 12:18
  • \$\begingroup\$ @transistor, I think I get it now. I was imagining an inductor being embedded in ground horizontally. So basically it is an inductor perpendicular to the ground, and that is why its core's permeability will change. Thanks. \$\endgroup\$ – Jack Mar 27 '16 at 19:52
  • \$\begingroup\$ The inductor coil is horizontal. If you like, the inductor's axis is vertical. It's just several turns of wire pushed into a rectangular slot. See my separate answer. \$\endgroup\$ – Transistor Mar 27 '16 at 20:35

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