Arduino Forum

Using Arduino => Sensors => Topic started by: valerio_sperati on Sep 15, 2020, 02:41 pm

Title: Capacitive sensor: how to modify its reading with a passive component
Post by: valerio_sperati on Sep 15, 2020, 02:41 pm
Greetings,
In a research project I'm happily using the "Adafruit MPR121 12-Key Capacitive Touch Sensor Breakout", to detect touch from users.

In the project, I would like to be able to distinguish the touch by two different users (let's say user A and user B), i.e. to understand who is touching the sensor.

In my current working solution, I put an Hall sensor beneath the capacitive sensor, then I put a magnet on the palm of user A. In this way I can distinguish the identity of the user touch, just combining the readings from Hall and the capacitive sensor.     

Now, I would like to get rid of the Hall sensor,  and I'm wondering if there is some way to modify the reading of the capacitive sensor, I mean something that can "disturb" or alter the reading.

Limitations: whatever active or passive component should be put on the palm of the user (maybe enveloped in a glove), i.e. it must be tiny and light.

Thanks a lot,
Valerio
Title: Re: Capacitive sensor: how to modify its reading with a passive component
Post by: Klaus_K on Sep 16, 2020, 02:10 am
What is the point of detecting user A vs user B? Are they voluntarily identifying themselves or do they need to be treated as potential foe?

In the first case you could ask them to tap a pattern or touch a certain amount of time.

In the second case I believe you need to look for another solution. If the capacitance could be used to identify rightful owners of phones, smartphones would not use complicated touch sensors.
Title: Re: Capacitive sensor: how to modify its reading with a passive component
Post by: hammy on Sep 16, 2020, 09:25 am
RFID chip carried by each person that can identify them , or a swipe card ?

Google access control systems
Title: Re: Capacitive sensor: how to modify its reading with a passive component
Post by: valerio_sperati on Sep 16, 2020, 11:13 am
Thanks a lot for your suggestions, I clarify here the purpose of my request, I hope this help:



Rationale: this type of device can be potentially used for example during social games, i.e. activities where it is important that the two users have to take turns.

If the device could autonomously detect the user, the toy could emit different sensorial outputs (the coloure dlights) and facilitate the therapist work.

RFID proposed solution is not good for this type of device (the child cannot use the swipe card). For this reason I look for a very simple solution, something that can be hosted in the palm glove worn by the tharapist.

I hope this help you,
please do not esitate to ask additional information,
thanks a lot,
Valerio
Title: Re: Capacitive sensor: how to modify its reading with a passive component
Post by: Klaus_K on Sep 17, 2020, 03:57 am
Maybe the simplest solution would be a separate touch button that the therapist touches with the other hand.
Title: Re: Capacitive sensor: how to modify its reading with a passive component
Post by: valerio_sperati on Sep 17, 2020, 11:12 am
Thanks for the suggestion of the touch button.
Actually this solution was previously explored and was not successful for the following reason:
generally a therapist playing with a child  is really focused on the activity, and can easily forget to handle other devices (as a touch button); in other words, it can be difficult to tell the therapist something like "remember that every time you touch the sensor you have even to touch the button!".

I realised in my reasearch experience that the behaviour of such type of devices should be automated as much as possible: this is why I'm looking for a solution where the device recognises in an autonomous manner the identity of the users.

As I previously said in the first post, the solution I found (i.e. using an Hall sensor detecting the presence of a magnet enveloped in a glove worn by the therapist), seems to work quite well.

Now I'm looking for a possible simpler solution (from the electronic point of view), as I read somewhere that a capacitive sensor can be someway "disturbed", i guess through some active or passive component (always enveloped in the glove worn by the therapist).
A "disturbed" reading of the touch sensor could be enough for me to distinguish the touch of the therapist form the touch of the child.

Thanks again for your help,
Valerio