Game Development Reference
In-Depth Information
disturbed, causing a change in the capacitance. To understand how capacitive screens
work, let's quickly review capacitance in general.
A capacitor in its simplest form is two conductors, usually thin plates, separated by an
insulator. If you apply a voltage across the two conductors, a current will flow and charge
will build up. Once the voltage across the plates is equal to the supply voltage, the current
will stop. The amount of charge built up in the plates is what we measure as the capac‐
itance. Previously, we noted that one issue with resistive screens is that one part must
always flex to close a insulating gap and complete a circuit. This repetitive action even‐
tually leads to mechanical failure. A capacitor can be dynamically formed by any two
conductors separated by an insulator. Noting that glass is a good insulator, it is easy to
see that a finger separated from a conductor by glass can change the capacitance of a
system. In this way, the finger or stylus doesn't have to cause any mechanical action, yet
it can still effect changes to the sensors, which are then used to measure the location of
the touch.
The methods of determining location based on capacitance on mobile devices are self-
capacitance and mutual capacitance .
Self-capacitance
Anyone who has lived in a dry winter has felt the shock of a static electricity discharge.
This zap is possible because the human body is a pretty good capacitor with a capacitance
of about 22 pico-farads. This property is known as body capacitance . Self-capacitance
screens take advantage of a physical property defined by the amount of electrical charge
that must be added to an isolated conductor to raise its potential by one volt. When the
fingers act as a conductor of the body's inherent capacitance, the sensors on the other
side of the glass experience a rise in electric potential. Given that the sensors are on the
other side of a good insulator, glass, there won't actually be any discharge of energy,
unlike when you touch your metal car door and get “zapped.” Self-capacitance in this
manner produces a very strong signal but lacks the ability to accurately resolve multiple
touches. Therefore, it is often used in conjunction with the next type of touch screen
we'll discuss, mutual capacitance.
Mutual capacitance
The other form of capacitance-sensing screen, mutual capacitance, is formed by a grid
of independent capacitors. A probing charge is sent over the rows or columns. As the
capacitors charge and discharge, the system can sense the capacitance of each individual
capacitor. As just discussed, the body is a good capacitor, and bringing part of it close
to the capacitor grid changes the local electric field. Those capacitors that are under a
finger or other conductor will read lower values than normal. Each capacitor can be
scanned independently, enabling high resolution of where the touch event is occurring.
Additionally, because they act independently of one another, it is possible to accurately
register multiple touches. Think of this system as taking a picture of the capacitance on