Friday, June 10, 2011

Capacitive Touch (Touch Sensing Technologies - Part 2)

Basics
It is quite likely that your interest in touch was first awakened after seeing an implementation of capacitive touch.

For changing displays and interacting with electronics and machines, touch has many great solutions, but if very precise measurements of multiple finger movements are necessary, capacitive is the technology of choice.

This article should answer some basic questions about how capacitive touch works—future articles will focus on finer details of implementation. If you have questions about implementing capacitive touch, we at Cirque would definitely enjoy discussing ideas with you.

Surface Capacitive
Surface Capacitive sensing is implemented by applying a conductive coating to one side of an insulated surface then applying a very thin insulator on top of the conductor (an insulator is a material that acts as a barrier to the flow of electricity). A current is then applied to each of the four corners of the conductive coating. When a conductor, such as a finger, contacts the surface, a capacitor is formed and draws more current or charge from the four corners. The current is measured at each corner and the ratio of the currents is used to determine where the finger is on the touchpanel.

Projected Capacitive
Projected Capacitive, instead of charging a whole surface, places an X-Y grid of conductive material between two insulator materials. The grid is often copper or gold on a printed circuit board or indium tin oxide (ITO) on glass, depending on the use of the touch product; however, there are many other options of materials to use.

An ASIC then charges and monitors the grid. As charge is pulled by a finger(s) from an area in the grid, the ASIC is made aware of the location of the finger, which then gets interpreted into a mouse cursor position or point of touch on a touchscreen.

Sensors using projected capacitive technology can be tuned to sense a finger or another capacitor that is not touching the surface. This also allows more layers on top of the sensing surface or thicker materials that can better handle vandalism and extreme conditions.

Because a grid is involved it is possible to sense the presence of more than one finger. The grid's sensing of both and X and Y coordinates also increases the accuracy of projected capacitive.

Where you've seen
You are seeing projected capacitive sensors everywhere. It is in the trackpad on your laptop and in most of the high-end touchscreen cell phones. It is in the screens of tablet computers, and as TouchAdvance readers likely know, it may be in the restaurant you're eating at this evening.

What's still missing from this report
As with most feats of electronic engineering, capacitive touch goes much deeper than can be covered in one article.

Projected capacitive can be broken down into two different methods: mutual capacitance and self-capacitance. These subjects should be explored on TouchAdvance soon.

The differences of projected and surface capacitive are highlighted fairly well, but the detailed intricacies of the underlying components will be left for another article.


* Depending on tuning, the finger does not have to actually contact the surface for projected capacitive to sense the finger's location.

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