Hi,
In most touch sensing applications the human finger is considered as one of the capacitor plates, while the conductive electrode is the complementary plate. This assumption is valid only if the human and the touch sensing device use the same potential reference, that is, only when the touch sensing device is physically grounded. An example of this user scenario is when a human is standing on the floor of their house touching the panel of an appliance that is physically grounded.
Make sure that the metalized piece that will be doing the touch is grounded, otherwise, you will need to perform your own tests, since we cannot warranty the correct behavior of the touch measurement, it’s true that it will alter the capacitance, but also there could be other factors (like environment) that could affect the performance of the sensor if the metal piece is not grounded.
However, also take in mind that our baseline algorithms will adjust over capacitive variations, include temperature, voltage, humidity (as long as it’s only humidity and not water droplets, which are a whole other thing), etc.
So, it would be very difficult for the MPR121 to distinguish between a human touch and a metal touch detections, but you can probably test with different metal sizes and a human finger touch, the best approach is to use grounded metal pins with a flat surface with different areas (for example, different sizes of screw heads) to try to identify the small difference between different metal touch and human finger touch and set the thresholds according to these differences. I cannot warranty that it can work since we have never tested before.
You can probably find further information in the following application note AN3863: http://cache.freescale.com/files/sensors/doc/app_note/AN3863.pdf
Regards,
Jose
