Capacitive sensors work by detecting the change of capacitance
introduced by the finger touching near the electrode.
Such change is so small that detecting it unambiguously is challenging,
and has become the primary task for the application design.
The capacitance that the sensor IC is measuring is from a sensor port
with respect to the circuit ground.
Two conductive plates are required,
one connected to the sensor port and the other one to the circuit ground
to form a capacitive transducer.
So the ground plane is not just like a shield in the standard PCB layout design,
but it is also an indispensable part of a 2 plate capacitor.
Being the front end of the sensor,
the electrode’s ability pick up the signal from the finger
has a direct and significant impact on the overall performance.
For the surface capacitive transducer, the reason an external object would
increase the capacitance, is that
the electric field projected from the transducer
polarizes the object,
and the polarized charges in return
attract more charges from the source to join the transducer.
The increase in the charge storage on the electrodes
means the increase in the capacitance.
The capacitance is more sensitive to the external object,
if the electric field is projected more into the air to reach the object.
The electric-field projection or distribution depend very much on the
transducer’s layout design,
and therefore
the layout design has a direct effect on the transducer’s sensitivity.
In the classic parallel plate capacitor,
the electric field is so confined in between the plates that
the capacitance could hardly be influenced by the external object.
This means that a touchpad directly above the ground plane
is a poor capacitive transducer.
When the plates are shifted apart from overlapping,
more electric field will be projected into the air,
and the capacitance will be more sensitive to the finger’s influence.
A pad with a ground ring projects some electric field into the air
and therefore it is a good electrode for a touch button application.
But to further increase the sensitivity for proximity sensing,
the ground plane got to be further away.
In this simulation,
the finger influences the capacitance from a further distance
and the touch signal is stronger,
But when the finger moves to the ground plane, the same amount of signal
can also be read.
With similar signal levels, the sensor IC cannot differentiate
the touch on the touchpad,
from the touch on the ground plane.
As the objective is to make the touchpad the only sensing port,
methods to sensitize the touchpad, and to desensitize the ground plane
is very much needed.
One common method is by putting the touchpad closer to the user,
and hiding the ground plane
deeper inside the housing.
Another method which is the centrepiece of this video, is by having
a bigger ground plane compared to the touchpad.
Automatically, the sensitivity will always shift to the smaller plate
which is the touchpad.
Since the total charges on both plates are the same, the plate with the smaller area
will have a higher charge density, more concentrated flux lines,
stronger electric field,
stronger polarization force,
and therefore more sensitive to the influence from the external object.
In this simulation,
a touch on the touchpad gives a 62% change in capacitance
compared to a 7% change for a touch on the ground plane.
With such distinctive signal levels,
an appropriate threshold level can be used to differentiate the touch on the touchpad,
from the touch on the ground plane.
A rigorous explanation for the shifted sensitivity is by means of
mutual capacitance between the human and touchpad, C_TF,
and mutual capacitance between the human and the ground plane, C_GF.
As these two mutual capacitances vary with the human movement,
they disturb the overall capacitance.
The disturbance pathway is in series,
so C_TF and C_GF, each imposes a upper limit on the disturbance.
When the finger closes the gap at the touchpad and provides a good link,
the bottleneck of the disturbance rests on the remaining link, C_GF.
By having a big ground plane constantly coupled to the human,
the bottleneck is eased,
and the signal will be strong.
When the finger moves towards the ground plane,
the loop is not close properly, so the signal will be weak.
This is the mechanism behind the inverse property
which gives the higher sensitivity to the smaller plate.
In summary, a big ground plane,
non-overlapping with the touchpad,
will give two positive effects;
the increase in the overall sensitivity,
and the shift of the sensitivity towards the touchpad.
Such advantages can be fully exploited by connecting the circuit ground
to the mains wiring,
which is the biggest ground plane readily available.
Naturally, the signal captured will be the strongest,
and the sensitivity will occur at the touchpad because of its
relatively smaller size.
The grounded chassis and the mains wiring,
though a bigger part of the transducer,
will produce minimum signal even when touched
because of the inverse property.
I find it amazing that the inverse property happened to be compatible with
the standard design rules;
big ground plane and earthed circuit.
Had the inverse property not there,
we will be getting false triggers from touching the ground plane and the earth line.
I hope you enjoy the video
and in the mean time, I am working on the projected capacitive sensor.