Inductive sense keypad through metal wall

Hello Bernard,

Sensing through metal is indeed possible with LDC technology. The primary consideration is the number of skin depths of the intervening metal. As you know, the LDC sensor generates eddy currents on the surface of nearby conductors. The depth from the surface is based on the skin effect.

You can use the LDC tools spreadsheet Skin Depth calculator tab (www.ti.com/.../slyc137) to evaluate the geometries. For example, if you have aluminum walls that are 0.72mm thick, with a 13kHz sensor frequency they would be 1 skin depth thick. Any target motion behind the aluminum wall would only have at a maximum 36% of the response compared to no aluminum wall.

You also need to make sure that the target is sufficiently thick that you get a sufficient response from the target; I recommend having at least 2 skin depths for the target; in this way you would have 31% of the response compared to no aluminum wall. If the target was copper, then you would need 1.15mm thick to get 2 skin depths.

Note that 13kHz is a very low sensor frequency. Using steel targets at these frequencies is not recommended for L measurements, as the inductance can actually increase instead of decreasing with target movement.

If possible, can you decrease the thickness of the Aluminum? You may also need to increase the sensor size.

Regards,

ChrisO

Hi Chris,
Thanks a lot.
What is the minimum recommended frequency for using steel snap domes? Is 70kHz reasonable? Lower would be preferable.
We can probably go to 0.5mm aluminum. At 70kHz, the skin depth is 0.31mm and at best, 20% of the response is from the target. However, we only have to make a binary decision if the button was pressed or not, so that may be sufficient.
Regards
Bernard

Hi Bernard,

The snapdomes that I have are about 300um thick; and so at 13kHz they would only be ~14% of a skin depth. Of course, the units you are planning on using can differ.

I would also caution you in measuring L for steel at these low frequencies, as the response may not be very large compared to other materials.

You may need to use an LDC1612/LDC1614 for improved measurement resolution, as the response may be much smaller in this configuration than the LDC131x device can measure.

Regards,

ChrisO

Hi Chris

Thanks for the help.

Calculating some possible PCB inductors, it looks like the frequency is likely to be around 70-80kHz, for a skin depth of around 0.3mm. With a 0.5mm wall, about 80% of the eddy current would be in the aluminum wall. 

There is a tradeoff here, where we want to get low frequency so that it penetrates the Aluminum wall, but if it is too low the snap dome will not affect the electromagnetic field as much.  Would adding a magnetic material such as a thin nickel plate behind the snap dome increase the sensitivity? I think that even a few percent change should be sufficient to decide if the button has been pushed. Temperature drift compensation or high pass filtering would improve accuracy too.

Regards

Bernard

Hello Bernard,

You may find 70-80kHz is not optimal for sensing steel. This is a challenging application, as you need to have a low sensor frequency to obtain a good skin depth, but the thin profile of your target and the fact that it is steel complicate the sensing.

Regards,

ChrisO