Pressure-sensitive capacitive touch: how is it done? - Page 1
Forum section: electronics / projects

author: L1L1
author_rank: Regular Contributor
timestamp: December 11, 2020, 08:00:56 am
content: Hi all,I came across this interesting synthesizer that uses a PCB as a keyboard:https://www.arturia.com/products/hardware-synths/microfreak/overviewOne of the big claims of this PCB keyboard is that it offerspressure-sensitive capacitive touch. It seems to work really well. I was wondering how it's done.While I have played with capacitive touch ICs in the past, they all seemed to provide a rather binary response (either "on" or "off"). Some capacitive touch ICs do provide an analog reading but in my experience, the response curve is very steep, nothing that could be exploited for a musical instrument. But my knowledge is limited here.So does anyone know how to build a "pressure-sensitive capacitive touch" sensor on a PCB like that? Any ICs or technique to recommend?Alain

author: Berni
author_rank: Super Contributor
timestamp: December 11, 2020, 08:18:31 am
content: Sometimes you can cover the capacitive button with a soft pad. The pressure squishes the pad down to bring the finger closer and this can help give it more linear range.Tho this device does not appear to have that, perhaps they are just linearizing the response the best they can with with a high resolution capacitive measurement and a lookup table. Another method is a load cell like device under it to record global force

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2020, 10:25:44 am
content: They use multiple capacitance sensors per key. If someone presses lightly, only one, or two will be activated. The harder one pushes, the more will be actuated. This would mean if someone has fat fingers, they would appear to press harder, so it could account for that by looking at the rate of change, of the number of sensors activated, or a fat fingered user just might have to learn to use less force.

author: L1L1
author_rank: Regular Contributor
timestamp: December 11, 2020, 09:48:46 pm
content: Quote from: Zero999 on December 11, 2020, 10:25:44 amInteresting.I guess it's a valid possibility. But what makes you so sure? Have you tried such a design?I mean let's assume that we have 5 capacitance sensors per key. There are 25 keys. So that's 125 capacitive touch sensors in total.

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2020, 09:51:26 pm
content: Quote from: L1L1 on December 11, 2020, 09:48:46 pmIt's how mouse pads on laptops work. I took apart an old laptop from the mid 90s, with a pressure sensitive mouse pad. It was just an array of PCB capacitive sensors.

author: Marco
author_rank: Super Contributor
timestamp: December 12, 2020, 10:29:17 pm
content: There's lots of ways. Some mousepads can pivot a bit to measure pressure, on iPhone screens the glass has become so thin they can simply put an elastic material underneath and capacitively measure the depression of small sections instead of measuring the capacitive disturbance of the finger.

author: nctnico
author_rank: Super Contributor
timestamp: December 12, 2020, 10:54:43 pm
content: Likely it is using rate of change. That is a very common way in electronic keyboards but done by measuring the time between breaking and making a contact operated by the key. I don't think something like that is hard to implement in a capacitive touch sensor; it just takes scanning the sensors fast enough to detect a finger approaching and landing on the sensors.