What makes industrial capacitive sensors so good?

Question

I am using a proximity sensor on a battery powered wearable sensor and maybe facing some detection issues because of a bad grounding.

I am seeing detection differences between the sensor system powered through USB (which I undestand,) the battery powered sensor in a hand, and the battery powered sensor on a desk.

I have spent a lot of time searching about that topic but I didn't really find answers to fully understand.

Do you have links/documents to explanations giving equations, drawings and numerical examples?

Additionally:

1. I have seen that self-capacitance is really dependant on good grounding (system to ground only, as human body to ground is generally high enough to not be considered) but that for mutual-capacitance, the sensor sensitivity is not, and really low impacted by it. Why is that?
2. Is it the reason why smartphones or other commercialized capacitive sensors are so performant, or is it due to hardware specifity or even different algorithm used?

EDIT: 10/11/2020 14h33 I am using a microchip MTCH101 IC as a proximity detector. I know this is not exactly the same technology, as proximity detector is based on a self-capacitance, increasing the range of detection but with maybe some unwanted coupling, and capacive screen are mainly using mutual-capacitance technology, increasing precision thanks to a radiated field vertically lower and more directionnal which avoids unwanted coupling with others adjacent sensors.

Is this the mutual-capacitance architecture or the algorithm behind that makes smartphone more robust to grounding issue ? or the Hardware architecture maybe?

I have read many times that mutual-capacitance is less dependant from grounding issue than self-capacitance, is it true? why ? (but never seen equation to prove it) ?

Is any fundamental detecting methods (CTMU, CVD, CSM,...) better than an other for grounding issue?

For self-capacitance, the grounding issue is comming from the fact the wearable battery power supplied device has a unfix and sometime low capacitance between its local ground and the earth. There is 2 cases: when it's handled by the second hand of the user and when the device is laid on a table. When there is a good coupling (high capacitance), its negligible and only the sensing electrode is measured. When it's not, the ground to earth coupling is taking part to the measurement as parasitic. I have found that the following way would help to improve this coupling :

Big ground plane at the device back, to increase surface which will be seen by the second hand or the table.

connexion from the device local ground to the device casing to let the human hand which hold the device have a contact with local ground and a better coupling.

Heating pipe (which i do not know)

"Use human finger to modify an electrical field between the electrode and the system ground, working as a dielectric constant modified". I interpret that as mutual-capacitance, is that correct ?

Finally, why the mutual capacitance would have less that kind of issue since that the charge exchanged with the finger are still flowing throught the earth and the device ground ?

Answer 1

A few things make capacitive sensors in commercial products like smartphones and induction cookers so performant. It looks like they never fail. It's amazing. Yet when you do it yourself it never seems to be stable.

1. Commercial appliance and smartphone producers had teams of hundreds of engineers working on this for 20 years on a very few, very specific applications. While you try to do the same alone in a few days.
2. A smartphone is quite specific. If you use the touch screen of a smartphone outside a smartphone environment it will probably not work better than your DIY experiment. It's performant both because they developed the hardware and an intelligent algorithm specifically for smartphones.
   The algorithm is very important because it regulates the sensitivity, the speed reaction and error tolerance according to the user activity. It's many lines of advanced software codes just to detect if a finger swipes the screen intentionally or not.
3. You won't find the same hardware for custom projects. You can find smartphones parts but then, it will be difficult for you to write the software that works properly with it. At the end of the day you would have re-invented the smartphone.
   Appliance manufacturers have chips and hardware specially designed. Chips that you can buy on online electronic stores are multipurpose. They have no idea what you are doing with them. And would be probably very surprised if they did.
4. EMI and RF shielding is extremely important. You need to shield as best as possible. Maybe you underestimated the importance of it. Shielding the enclosure, the processor etc...
5. The back side of a touch sensor usually has a trace net used to shield the sensor pads. Full copper areas reduce sensitivity, while too few copper increase the risk of false triggering. This is very subtle.

Search for "Touch Sensing Design" or "Secrets for Successful Touch Sensing Design". "Touch Sensor Guide". "Touche Sensor techniques" and other keywords of that effect.