Multi-touch Systems
To allow people to use touch commands that require multiple
fingers, the iPhone uses a new arrangement of existing technology. Its
touch-sensitive screen includes a layer ofcapacitive material, just like many other touch screens.
However, the iPhone's capacitors are arranged according to a coordinate system.
Its circuitry can sense changes at each point along the grid. In other words,
every point on the grid generates its own signal when touched and relays that
signal to the iPhone's processor. This allows the phone to determine the
location and movement of simultaneous touches in multiple locations. Because of
its reliance on this capacitive material, the iPhone works only if you touch it
with your fingertip -- it won't work if you use a stylus or wear non-conductive
gloves.
Unlike the mutual capacitance circuitry, a
self capacitance screen contains a transparent electrode layer.
The iPhone's screen detects touch through one of two methods:
Mutual capacitance or self capacitance. In mutual capacitance, the capacitive circuitry requires two
distinct layers of material. One houses driving lines, which carry current, and
the other houses sensing lines, which detect the current at nodes. Self
capacitance uses one layer of
individual electrodes connected with capacitance-sensing circuitry.
Both of these possible setups send touch data as electricalimpulses. In the next section, we'll
take a look at exactly what happens.
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