The projected capacitive sensor is the underlying technology for touchscreens in the smartphones,
tablets and computers. Capacitive touchscreens are made of
transparent electrodes arranged in rows and columns, and by projecting the e-field from
the rows to the columns one by one, and measuring the signal profile, the location
of the external influence can be calculated. The technology of projected capacitive sensors
evolved from its predecessor, the self capacitive sensor,
also known as the surface capacitive ensors. In the self capacitive sensors, the touch electrode
and the fingertip form parallel plate capacitors,
and as the gap, h ,between them closes up, the capacitance increases according to 1/h.
The charges on the electrodes which are proportional to the capacitance can be measured to infer
the presence of the external object. Like the mechanical key matrix which uses
row and column scanning to locate the switch being pressed,
the capacitive sensors can use the same method for the sensing,
and this would increase the number of sensing channels with the least IO pins.
The matrix scanning requires the rows and the columns to be electrically isolated,so as
one electrode is used as the transmitter, another electrode is placed
next to it without physical contact, to sense the influence from the finger.
When the transmitter is excited with a fixed voltage, the charges induced on the receiver
will be affected by the external object. The charge quantity measured is regarded as the
mutual capacitance.
As we know, the parallel plate capacitance increases according to 1/h,
but experiments have shown that the mutual capacitance at the receiver behaves
totally differently. The mutual capacitance begin with some initial value,
decreases slowly, hits the maximum dip,
turns around and then shoots up suddenly. So now it is the dip, not the increase in
the capacitance that is used for inferring the touch.
The turnaround would infer the finger moving away, and of course that is a false detection.
The respond characteristic cannot be modelled by circuit with lumped capacitors,
and therefore the physics of electrostatics has to come in for the explanation.
In this chart, the vertical axis represents the charge transfer count which is the
inverse of the capacitance. When the finger moves towards the electrodes,
the capacitance decreases, hits it minimum and then shoot up quickly.
So why does the capacitance not increase with touch, like in the self capacitive sensor?
Just like in the self capacitive sensor, the transmitter and the ground plane together
induce the person's fingertip with negative charges, and the body with positive charges.
When the finger moves towards the receiver,
the negative charges on the fingertip repel the negative charge on the receiver,
result in the decrease in the mutual capacitance, which is the touch signal.
To a certain extent, the fingertip acts like a reflector to reflect the e-field from the
transmitter to the receiver. The signal picked up at the receiver is the
touch signal. The best location to put the reflector is
above the mid point between the transmitter and the receiver,
and therefore the gap appears to be the sensing area.
So how can we design electrodes with the maximum sensitivity?
The sensitivity depends on how much the signal is projected on the fingertip,
and the ability of the receiver to pick up the reflected signal.
The bigger the ground plane couples to the body,
the more the charges are attracted across the transmitter and the fingertip,
and therefore the stronger the projected signal. The receiver's ability to pick up the reflected signal
depends on its size and its distance from the reflector.
Very often it is placed in the centre of the transmitter ring.
A bigger area would logically pick up more signal, but it turns out that that can bring up
the turning point, increase the initial capacitance, and result in
a reduced sensitivity. The parallel plate capacitor is the cause
of the turning point. Initially, this capacitance is rather insignificant,
but when the finger moves very close to the receiver,
the parallel plate capacitance overtakes the reflected signal and the total capacitance
shoots up according to 1/h. The turnaround signal leads to the false release.
To lower the turning point caused by the parallel plates, the receiver area must be small.
So on the one hand, a bigger receiver can pick up more signal but on the other hand,
it can also bring up the turning point. As a result, it is the ratio of receiver area
to the ground plane area that matters the most.
The ground plane must be as big as possible to maximize the signal projected on the fingertip,
and the receiver can also be big as long as it remains as a small fraction to the ground plane,
and that would suppress the turning point.
In this button design, the receiver is much smaller than the ground plane,
and therefore the turning point does not occur above the overlay.
If the receiver is placed nearer to the transmitter, the initial capacitance would increase.
This simulation shows that when the coupling increases, the electrodes produce
a stronger signal, delta Cm, a higher initial capacitance, and result in
a lower percentage of change. If the sensing IC takes only delta Cm to
infer the touch, then more coupling between the
transmitter and the receiver would give a higher sensitivity. However, many sensing ICs use the
percentage of change to infer the touch, so the sensitivity turns out to be lower.
Therefore, it is necessary to maintain a reasonable gap between the transmitter and receiver,
to strike a balance between delta Cm and the percentage of change,
so that both the signal to noise ratio and sensitivity are good.
The matrix type discrete buttons can be extended to sense multi-touch,
two dimensional coordinates by means of signal processing.
During the scanning, one electrode is connected to the transmitter, another one to the receiver,
and the rest grounded to form a big ground plane.
In order to get good delta Cm and good percentage of change, it is recommended that
the gap between the electrodes be made 0.6 of the overlay thickness.
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