For reading the touch sensors, i simply use the "touchRead()" operator that you get with Teensyduino (Teensy 3.0 and higher). To be honest, i don't know precisely what the Teensy is doing here. It's some advanced capacitive sensing technology that is implemented with the new Teensy boards. I read something about a reference capacitor equalling out changing conditions. It's said to be faster and giving more reproducible results than when using the old technique with 2 pins, a resistor and the CapSense library (Arduino CapSense article).

Now i may have made a mistake in the design as i was just reading some articles about capacitive touch sensing and it seems that you have improved readings using a grounding plate surrounding the sensor. It seems that the better touch sensing is with an electrical field between two capacitor plates that you change while bringing your finger closer to this system. This would mean there is a metal sensor object that is connected to the touch pins of the Teensy and a grounding plate (connected to ground), surrounding this object. In my design, i just soldered cables to the nails and connect the cables to Teensy's touch pins, making use of touchRead().

This is working quite good, but I have to tell you, that the pin readings change drastically when i touch, for example, the housing of my laptop computer. I will try to make some changes and improve my design.

See The art of capacitive touch sensing for more information on the topic, though this may not be the same technology as the one that touchRead() is working with in the new Teensy boards. Also have a look at the PRJC (Teensy) forum. There are some posts on this topic.