- The most amazing future of a capacitive sensor is its ability to sense through a complete shield housing. Capacitive sensor works by detecting the change of capacitance due to the influence of external object.
- Capacitive sensor comprises electronic circuits that measure capacitance across electrodes also known as antennas. When the capacitance changes, the circuits and the algorithm infer the presence of the external object. But 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 needs 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 the underground finger influence capacitance is when conductor is connected to the source the electric field from the source pushes the charges out to the conductors. The positive charge and 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 net 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 nearly, it cuts into the electric field. The electric field polarizes the conductor, and energy is transferred to the polarized 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 external object allows more charges to store in the plate, and therefore it increases the capacitance. So even when the external object is not physically connected to any of the plates it is too capable of influencing the capacitance.
- Any charge in the system is subject to attractive forces from the unlike charges and repulsive charges from the like charges. This complex tug of war happens to every system.
- Based on this, a mathematical model can be developed to solve for the charge distribution which would satisfy the equilibrium condition. The charge of distribution in the system and can be used to calculate the capacitance. For a coplanar capacitor, numerical solution shows that the capacitance increase when a third plate is placed nearby and the third plate are indeed polarized by the electric field.
- Two plates are required to detect a nearby object, this dismisses 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 external the influence. For a human touching the antenna configuration. The capacitance increased by 0.056pF amount to 30% increase. If the antenna are enlarged, they can pick up more influence. The capacitance change is more, and the percentage change is more, 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 2 mm 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 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 sensitivity 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 than 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 main wire because of its relatively big coupling area will become insensitive. Without the inverse relationship the main wire will get a lot of false triggers.
