The capacitive touch sensor detects changes of capacitance at the electrode occured by closing a conductive object such as a finger. There are several sensing method for the capatitance. For this project, integration method that used in capacitance meter is applied. The changes of capacitance Cx is pretty small, around 1pF to 10pF, but it will able to be detected with ease because 20fF count resolution is acheved at the capacitance meter. Also the objects to be detected must be grounded to create the Cx circuit, according to the principle. However it works well even if the human body is isolated from the ground. It can be thought as the following reason.
The conductive object with a degree of surface area has a self-capacitance. In the concrete, C = 4πε0r, where at ball shape, ε0 is vacuum permittivity, r is radius. The human body has approx. 100pF in self-capacitance according to the human model for ESD test. This is large enough compared to Cx. Therefore the human body can be thought as a grounded conductor even if not grounded. Ofcourse a ground wire or a self-capacitance equivalent to the human body is required to the equipment, too. However it is coupled with earth or human body via various stray-capacitance and creates the Cx circuit.