Capacitive sensor, Theory, application and design


Uploaded by souppebble on 07.04.2012

Transcript:
The most amazing future of a capacitive sensor is its ability to
sense through a completely shield housing.
Capacitive sensor works by detecting the change of capacitance
due to the influence of external object. Roughly speaking,
capacitive sensor comprises of electronic circuits that measure capacitance
across electrodes
also known as an antennas.
When the capacitance changes,
the circuits and the algorithm infer the presence of the external object.
But exactly where is the capacitor, and how does the human finger change the capacitance?
A common model is that the electrode forms one plate the capacitor
and the grounded finger forms the other plate and changes to overall capacitance
of the sensor
But our experience is that user need not be electrically connected to the
circuits
or put their feet on the ground to operate the sensor.
So grounded finger as a requirement for capacitive sensor
is a misconception.
so let's look at the physics on how a ungrounded finger influence
capacitance.
when conductors are connected to the source
the electric field from the source
pushes the charges out to the conductors.
The positive charge the negative charge are attractive, so they move to each other
as close as possible.
charges on the same plate are repulsive,
so they push each other away to the edges.
As more charges join the plate
they build up electric field to oppose others from joining in.
Eventually, the nett force along the conductor is zero
and the charges stay in equilibrium.
This is when the plates have the same potential difference as the voltage source.
and the capacitance is defined as the ability to store charges on a per-volt basis
When an external conductor is put nearby, it cuts into the electric field.
The electric field polarize the conductor, and energy is transferred to
the poloarized charges.
The polarized charges get energy from the plates
and the plates now have a lower potential.
If the source is still connected, more charges will join in the plates until
the plate potential is back again.
The extenal object allows more charges to store in the plates,
and therefore it increases the capacitance.
So even when the extennal object is not physically connected to any of the plates
it is still capable of influencing the capacitance.
Any charge in the system
is subject to attactive forces from the unlike charges
and repulsive charges from the like charges. This complex tug of war happens to
every single charge in the system.
Based on this,
a mathematical model can be developed
to solve for the charge distribution
which would satisfy the equilibrium condition as mentioned previously.
The charge of distribution
shows the physics in the system
and can be used to calculate the capacitance.
For a coplanar capacitor,
numerical solution shows that the capacitance increases when a third plate is placed nearby.
and the third plate is indeed polarized by the electric field.
Two plates are required to detect a nearby object,
this dismiss the idea that cap-sense uses one plate
to detect the object's capacitance with respect to infinity.
We can also do numerical experiments
to study the effects of antenna geometry
on the sensitivity to extenal the influence. For a human touching the antenna
configuration as shown,
the capacitance increased by 0.056pF
amounts to 30% increase
If the antennas are enlarged,
they can pick up more influence. The capacitance change is more,
and the percentage change is more, too.
so larger antennas are easier for the measuring circuit.
If the distance between the antennas is shortened,
the electric field is more confined.
Although the capacitance is higher,
the delta change is actually less, and make it a less effective sensor.
In most designs, one antenna is made as the touch focus point and the other integrated
to the ground plane.
Without special care,
the ground plane can pick up the object's influence as well.
So how can we increase the sensitivity of one plate and reduce the sensitivity
of the other?
one method is to make one antenna more accessible than the other.
Another method is to control the size of the antenna and will be explained here.
In a simulation of a finger 2mm above a coplanar structure
the capacitance increased by 12%.
When the human body is included in the calculation, the capacitance
increased by 22%.
the human body itself
double the influence on the capacitance.
The body by its huge surface area and the finger by its close proximity, couple
themselves to the antennas.
If one of the plates is enlarged to provide good and constant coupling to the
human body, then the finger approaching the smaller plate
becomes the deciding factor for the change of capacitance.
Using numerical simulation for the above configuration
we see capacitance change by more than 100%.
If the finger approaches the big plates
it doesn't close the loop and the capacitance change is minimal.
So by having a large ground plane,
which is one of the antennas, the sensitivity automatically goes to the
smaller antenna,
which is meant to be the touch focus point.
Even if the ground is made smaller,
and the touch antenna made bigger, the senstivity still follows the smaller
plates.
The smaller antenna gets more sensitivity because of the higher percentage of
change compared to the bigger antenna in the same configuration.
but the smaller antenna do not necessarily get more absolute delta C
then the bigger but less sensitive antenna in another configuration.
So to enjoy higher sensitivity from a higher percentage change of a smaller
antenna
the sensing IC must have a good S/N to begin with.
If a capacitive sensor is connected to the mains wiring,
effectively one of the plate area is increased significantly.
The absolute delta C and therefore the signal level is higher. The touch
antenna because of its relatively small area
will get all the sensitivity.
The mains wire
because of its relatively big coupling area
will become insensitive.
Without the inverse relationship
the mains wire will get a lot of false triggers.
Thank you for watching and I hope you enjoy the video