Design a vehicle inductive loop detector circuit

Hi,

I want to design a dual channel inductive-loop vehicle detector, I haven't found enough circuit diagrams online.

And the products on Aliexpress are either connected to relays or have factory fabrication where I can't get signal output to read by Arduino board.


I learned the basic principle of designing inductive-loop circuit, which is:
1. Use 18 AWG wire.
2. Do the turns until I get inductivity by the LCR from 50-220uH or similar values.
3. Run the circuit by a certain voltage.
4. The frequency should be 120kHz.
5. Then I can measure the difference in the voltage, by using a current sensor.

But is the circuit that easy ?

Hi,

I'm working on the initial draft of a project for parking counter system using Arduino.

The project main design features:
1. There are 3 sections now to cover which are away of each other by several hundred meters. So there are 3 nodes.
2. There is one display panel at the entrance of the parking lot with the main control box.
3. The communication between the nodes and the control box should be done with nrf24L01 modules with the antenna.
4. I've decided to use vehicle inductive loop detection system which is the design problem now. Where I thought of using ultrasonic sensors but because it may cause miss counting with walking people by the sensors, so I thought they aren't suitable for the design.

I've selected most the parts. Which are:
1. P10 display panels x12.
2. Power supply for the whole display panel @5V - 300W with data and power cables
3. x5 Arduino nano boards.
4. x5 5V power adapters.
5. Vehicle inductive loop detector system.



My problem now is how to design the circuit to read the coil signals to the Arduino board ?

I learned the basic parts of vehicle inductive loop detector:
1. 18 AWG wire with certain number of turns to get the required inductance.
2. Voltage source with frequency from 30k-150kHz.

Any ideas of a good circuit to read the signals to the Arduino ?

The principle is that a mass of iron changes inductance of a wire coil below it. You send a waveform of fixed frequency around the loop at fixed amplitude. When amplitude (or waveform) changes, it tells you a vehicle is over the wire coil.

It would help your progress if you can get a look at existing systems. Youtube videos are popular for experimenters to show what they've done.

After some thought I'm picturing a magnet suspended in oil inside a sealed capsule buried in the road. It moves toward an iron mass passing over it. A sensor (optical or reed switches) lets the magnet detect whether the vehicle is stationary or moving, and its direction.

BradtheRad said:

It would help your progress if you can get a look at existing systems. Youtube videos are popular for experimenters to show what they've done.

Click to expand...

I've checked many videos.

There's one that sells the kit with the circuit diagram and firmware. But the problem the code is in C# and very long. But that shouldn't stop me, I can work on the code for some time until I get it to work.

dual-channel-inductive-loop-vehicle-detector/

After some thought I'm picturing a magnet suspended in oil inside a sealed capsule buried in the road. It moves toward an iron mass passing over it. A sensor (optical or reed switches) lets the magnet detect whether the vehicle is stationary or moving, and its direction.

Click to expand...

Is there reed switch for parking projects ?

eagle1109 said:

Is there reed switch for parking projects ?

Click to expand...

The reed switch is activated only when near a magnet. Plain iron or steel is ineffective. You don't need any particular type of reed switch for your project.

The magnet should be small and powerful. In my experiments a small neodymium magnet can be made to float on water by resting it on a tab of dry paper. It drifts toward magnets held a few inches away. You'll need to do your own experiments, holding large iron objects nearby, to see if it drifts toward them.

The effect needs to be stronger than the earth's magnetic field. The magnet automatically acts like a compass needle, turning to orient itself North-South, when placed to sit on the center of its own N-S poles.

Furthermore this idea won't work if the magnet ends up sitting in one place close to the reed switch even if there's no vehicle overhead. Then it sends a false detection. Or else it might park itself to one side of the dish as a vehicle departs in the right direction.

My first thought involves a pair of inductors both part of independent LC oscillators. The one is the roadbed coil. The other is selected such that its L is greater than "unloaded" road coil L but less than "minimum detectable iron mass" loaded L. That way the road-coil oscillator frequency will swing from above (unloaded) to below (loaded). The reference coil would want a closed core so it's insensitive to proximate iron.

Then feed those two frequencies to a PFD or the front end of a cheap little PLL chip and use Vtune or the up/down outputs as your over/under logic signal.

BradtheRad said:

The reed switch is activated only when near a magnet. Plain iron or steel is ineffective. You don't need any particular type of reed switch for your project.

Click to expand...

The magnet should be small and powerful. In my experiments a small neodymium magnet can be made to float on water by resting it on a tab of dry paper. It drifts toward magnets held a few inches away. You'll need to do your own experiments, holding large iron objects nearby, to see if it drifts toward them.
The effect needs to be stronger than the earth's magnetic field. The magnet automatically acts like a compass needle, turning to orient itself North-South, when placed to sit on the center of its own N-S poles.

Furthermore this idea won't work if the magnet ends up sitting in one place close to the reed switch even if there's no vehicle overhead. Then it sends a false detection. Or else it might park itself to one side of the dish as a vehicle departs in the right direction.

Click to expand...

Yes, the reed switch to my knowledge is for very short range application like the proximity inductive sensor.

I now excluded the reed switch, proximity inductive sensor and the ultrasonic sensor, all of them aren't suitable.

The best solution is like what is know since ever, the inductive loop detection.

I found another website uses an improved microwave sensor which is easier to install but I think it would be expensive to purchase. Which is this one:

vehicle-detection-sensor-for-barriers-and-gates

There are other solutions; like, using cameras and image processing with OpenCV which might not be a bad idea and I'm taking it right now into consideration.

dick_freebird said:

My first thought involves a pair of inductors both part of independent LC oscillators. The one is the roadbed coil. The other is selected such that its L is greater than "unloaded" road coil L but less than "minimum detectable iron mass" loaded L. That way the road-coil oscillator frequency will swing from above (unloaded) to below (loaded). The reference coil would want a closed core so it's insensitive to proximate iron.

Then feed those two frequencies to a PFD or the front end of a cheap little PLL chip and use Vtune or the up/down outputs as your over/under logic signal.

Click to expand...

Can you provide me a link to this project ?