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Fundamentals of electrostatics & capacitance for capacitive touch sensors

Capacitive sensing can be overwhelming for a newcomer. The nature of the underlying electromagnetic phenomena is complex. That’s why many newcomers tend to cut corners. They tend to skip the part of understanding the physics involved and jump head-first to projects. Naturally, they soon feel lost and resort to trial and error.

This chapter contains all the knowledge we wish we had when we first started working on capacitive sensing. It will teach you the basics of electrostatics, conductors, and capacitance tailored to the need of a capacitive touch sensor designer.

Then you’ll familiarize yourself with one of the most useful concepts in capacitive sensing: the fundamental case of the parallel plate capacitor. Understanding this case can help you unlock the mysteries of capacitive sensing!

Finally, you’ll get an intuitive understanding of how certain parameters, like electrode geometry, dielectrics, and traces affects the performance of a capacitive touch sensor.

Thanks to Benjamin Franklin’s pioneering experiments, that took place almost three centuries ago, almost everyone knows today that electric charges can travel through materials causing flow of electric current.

The tendency of a material to allow the flow of electric current through its body is described by conductivity, σ. On the contrary, the difficulty that a material poses to electric charge conduction can be quantified by resistivity, ρ.

conductivity sensitivity relationship

We must keep in mind that conductivity, therefore also resistivity, are inherent properties of the material, so they are independent of the size and shape of the sample. Based on its electrical conductivity, almost every material found on earth can be classified as a conductor or insulator. Conductors, like metals, have very high conductivity values (σ > 105 S/m) and consequently very low resistivity (ρ < 10-5 Ohm⋅m). So the electric charges can travel through them very easily, with a high speed and in large amounts. On the other hand, insulators (or dielectrics), like plastics and polymers, have low conductivity values (σ < 10-8 S/m), as they inhibit the motion of electric charges, allowing just a low current flow in the presence of high electric potential. Materials with intermediate conductivity values are called semiconductors, and generally bridge the conductivity gap between conductors and dielectrics.

material classification based on conductivity
Considering the materials that are most commonly used in capacitive touch sensors, PET, FR-4 and glass are insulators, whereas silver, copper, and ITO (Indium Tin Oxide), are all metals, that is, conductors, so they have very high conductivity values.
You can read and learn more about the fundamentals of electrostatics & capacitance for capacitive touch sensors here.

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