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LDC0851: Coin transit detection

Marco Bedani1
Prodigy 40 points
Part Number: LDC0851
Other Parts Discussed in Thread: , LDC1101EVM

Hi,
I am evaluating a TI inductive sensor as a soluction for detection of coin transit.

The operating principle is described in figure 1: the coin transit takes place on a flat plastic surface and the sensing elements (the spiral inductors) are underneath this plastic cover.

The main issue is that we cannot guarantee any free gap between two consecutive coins, nonetheless we must be able to distinguish the two coins.
Another important issue is that in our application the target (the coins) do not approach the inductor in front of it, but they approach sliding laterally.

We first started making some tests on the LDC1101EVM, but now we are focusing on the the LDC0851 because of its much simpler interface and I'll show you three different configurations with the LDC0851EVM.

1. Event counting
As you can see from the attached video (Mode1), the system manages to distinguish the two consecutive coins:
- the led blinks on with the first coin
- blinks off between the two coins
- then blinks on again with the second coin

The LDC0851 datasheet states (page 21) that "Two identical coils can be placed such that when one of the coils is covered by a gear tooth, the other is uncovered".
Unfortunately this is not our condition because for some moments two consecutive coins can cover, each one, both the spirals.

For this reason, I'm afraid that this working mode is not acceptable.
Moreover, to get the system working in this configurations, I must set the ADJ close to the maximum sensitivity, and I think that this is not a good starting point.

2. Event counting with stacked coils
The event counting with stacked coils is not considered in datasheet; anyway it seems that the system manages to distinguish the two consecutive coins, as well.
As you can see from the video (Mode2), I am trying to simulate a stacked coil by stacking two identical coil pcb; obviously this method should be validated by testing a more realistic couple of stacked coils on single pcb.

Here the ADJ is set to a medium sensitivity.

Is this working mode acceptable?

3. Threshold Adjust Mode for Distance Sensing Using Side by Side Coils
This configuration is similar with the one of page 13 of the datasheet with onlu one fundamental difference: the target approaches not frontally but laterally.

As you can see from the video (Mode3), the system works; but is acceptable that the target moves laterally towards the target?

I think that the best for us would be to adopt configuration 2 or 3, but I would like you if you can provide me with some considerations on this application and how appropriate and reliable are these working mode.
In other words, I have to be sure that my system is working properly.

It is understood that our main challenging task will be to design the spiral inductor optimizing it for the application.

Any help will be really appreciated.
Thank you in advance and best regards,

  • 0
    TI__Genius 15220 points

    Marco,

    Thanks for your inquiry.  It looks like you've already sorted out a significant part of the design.  It's important to keep in mind the operation of the device is looking for a variance between the two coils.  Any time the sensed inductance of the target sensor exceeds the inductance of the reference sensor the output will switch high

    As you point out, that if both coils are placed along the path of the coins, there is a case to consider where both coils may simultaneously detect a coin, resulting in a potential no read.  I would recommend avoiding this scenario.

    This leaves us with cases 2 and 3.  Stacking (Case 2) allows you to save PCB area, and works best in applications where the target is approaching perpendicular to the face of the coil.  As you have demonstrated, this worked for your setup with a very large gap.  In real application these sensors would be closer.  I would expect that you might be a little less sensitive to the coin, but if the threshold is set correctly you might be able to design a working solution.

    The most robust setup is number 3.  In this case the reference coil is not able to detect any influence from the transit of the coin.  This should be the easiest setup to get working in your application, but requires separate PCB area. 

    The direction of approach in your application is not a concern as long as the threshold is set correctly to detect the passage of the coin.  The inductance of the sensor is impacted by the eddy currents that propagate in the surface of the coin.  As more and more of the coin overlaps the sensor, the inductance of the target sensor will shift accordingly.  Once that has shifted by the threshold target the output will toggle.  

    You've rightly pointed out that the coil design is the most challenging part of the solution.  You can find a guide for that at the link below

    https://www.ti.com/lit/an/snoa930b/snoa930b.pdf

    Additionally, there are discussions of the various design challenges and helpful tools for Inductive Sensing design on the E2E forum below

    https://e2e.ti.com/support/sensors/f/1023/t/295036

    You might want to consider EMI upfront.  LDC0851 can benefit from shielding and/or careful supply/return routing

    https://www.ti.com/lit/an/snoa962/snoa962.pdf

    If you are aiming to detect specific types of coins, you may have to carefully consider your target designs and thresholds to accurately sort out which coin passes by.  Varying materials will impact the sensor coil differently.