Capacitive touch sensing on a transparent polycarbonate/acrylic 'fin' - Page 1
Forum section: electronics / projects

author: andyfug
author_rank: Newbie
timestamp: January 03, 2023, 07:26:18 pm
content: Hi,I've read a few items in several places regarding this topic, but still don't feel entirely clear on which direction I should investigate further and what might stand a chance of working.I am working on an architectural lighting project which requires touch sensing (just sensing human touch/no-touch) on a large length of transparent polycarbonate or acrylic approx. 300mm x 100mm x 4000mm(L). The polycarb/acrylic 'fin' would be mounted vertically from floor to ceiling and require touch sensing in its lower half (@ approx. 0 - 2.5m). The idea is that people would be able to walk up to and touch the fin on any of its surfaces to trigger various lighting effects. I know we can explore options like break beam sensors and cameras etc, but I would like this question to focus on capcitive sensing options or other similar 'touch sensing' technologies. I am comfortable with handling an input on a microcontroller and the onward communication etc, but it's the capacitive sensing part that I am unclear on and how viable it might be as a solution.I gather there are transparent foils available that could be stuck on the fin's surfaces, but I can't help but feel that there must be a better option; the foils often appear to come with a full matrix (which is not required) and I am concerned that a foil could degrade over time and start to delaminate/bubble. Is there another better way to approach this? Could a sensor mounted to the base of the fin feasibly detect a touch 2m away or is there no getting away from having a conductor passing up the fin some way or another?Initial research suggests ITO (Indium tin oxide) might be a good solution:https://diamondcoatings.co.uk/touchscreen-indium-tin-oxide/I imagine this would require a company like this either directly coating the fin or recessing pre-cut strips of glass/plastic.I appreciate this is a little vague, but I guess what I'm asking for are some initial pointers/ideas for what could potentially work and what is unlikely to work.I should emphasise a few key requirements:The fin should be transparentThe fin's finish should be durable and high quality as people will be viewing/touching at close proximity.The fin will be lit from each end, so ideally we'd avoid impeding the light's path through the centre of the fin wherever possible.Any help or shared experiences would be greatly appreciated and links to products/services for building a prototype/proof of concept would be terrific.Many thanks in advance.Andy

author: artag
author_rank: Super Contributor
timestamp: January 03, 2023, 08:20:10 pm
content: You can get ITO-coated polyester sheet. That might be more convenient than having the polycarbonate coated directly. It might also show up badly, I don't know. Maybe a coated sheet trapped between 2 layers of polycarbonate ?You could also look at the method used for touch-tables. Although these usually use a camera to get a 2D position, they do require actual touch rather than being purely image analysis. The reason is that they break the total internal reflection inside the plastic layer, causing a light leakage. So you might be able to measure the effect caused on light (or IR) shone into the edge of the polycarbonate.

author: andyfug
author_rank: Newbie
timestamp: January 04, 2023, 09:34:45 am
content: Thank you for your response. That's already helpful and providing food for thought.So are you suggesting an ITO-coated sheet under ~150mm of polycarb might be able to detect a touch? The fin is approx. 300mm x 100mm cross-section, so we could explore integrating a transparent ITO sheet running up the inside of the fin... that might disrupt the lighting effect, but may still end up looking ok and could even help the lighting effect in a strange sort of way (that's a creative question for elsewhere!).So looking at your idea re. touch-tables... is this 'frustrated total internal reflection'?:https://sethsandler.com/multitouch/ftir/Definitely worth considering although I'm not sure where we'd place the camera(s) because the fins will be accessible from both sides and require touch sensing on all surfaces.

author: Marco
author_rank: Super Contributor
timestamp: January 05, 2023, 12:17:39 pm
content: There's transparent paints for anti-static applications, I wonder if that wouldn't be good enough to do capacitive touch sensing with.

author: Nominal Animal
author_rank: Super Contributor
timestamp: January 05, 2023, 02:48:23 pm
content: Conductive coating (indium-tin oxide) would make the entire surface a capacitive sensor, with human touch rather easily detected by the change in capacitance for the surface. Thing is, this is quite a large surface, so the capacitance change could be outside what capacitive touch controllers (essentially capacitance-to-digital converters) can handle, and you'll probably need to implement your own capacitance measuring circuit anyway.There can be a protective coating on top of the conducting layer, and the capacitance change will still be measurable.(So, ITO-coated polycarbonate sheets should probably be on theoutsidesurfaces.)I wonder if ultrasonic acoustic waves (technically surface acoustic waves) would be useful here.The idea is that you place one or more ultrasonic transducers and two or more receivers within the polycarbonate.Physical touch on the surface will change the propagation of the ultrasonic wave pulse, and thus be detectable.Problem is, the acoustic properties do drift with temperature, relative moisture in air, moisture on surface etc., so calibration is an issue: you'd have to have a scheme to regularly recalibrate the sensors. If you incorporate a microwave radar, so that if there is no motion near the 'fin' for say 15 seconds the acoustic sensors are recalibrated, I'm pretty sure you could make it work.(But I'm just a hobbyist, and really know nothing.)If I were you, I would have to make a full-size prototype sculpture first, and then experiment to find what works best.

author: Ian.M
author_rank: Super Contributor
timestamp: January 05, 2023, 03:39:04 pm
content: Many many years ago, some early touchscreen monitors were implemented by putting load cells under each corner of the monitor base and reading the change in load when the user touched the screen (somewhat firmly) and calculating the touch position by how much the load at each corner changed, proportional to the others.You could do something similar with load cells at the top and bottom corners of your fin sensing lateral pressure on it.

author: artag
author_rank: Super Contributor
timestamp: January 05, 2023, 04:01:57 pm
content: 150mm could well be too thick - I was thinking half of 100mm.However if you use a proximity rather than a touch sensor, or build your own, you should be able to get theremin-level sensitivity which is out near there. It's important to look for a sudden change rather than an absolute level, which will drift a lot.For the FTIR I was thinking you'd illuminate the edge from the bottom of the panel and examine the light returned, hoping it would change as more or less was allowed to leak from the flat surface.

author: andyfug
author_rank: Newbie
timestamp: January 06, 2023, 02:01:41 pm
content: Thank you all for your input. There are some great ideas to consider here. Equally it sounds like we may have a bit of a challenge to implement it.I did find this, which is pretty similar to what we want to achieve (touch wise):https://awards.mediaarchitecture.org/mab/project/58https://iart.ch/en/work/british-pavilion-expo-2017They mention embedding a capacitive sensor. I wonder how they achieved it.

author: Marco
author_rank: Super Contributor
timestamp: January 06, 2023, 03:56:35 pm
content: I'm going to guess it was antistatic coated.10^7 range resistance is still doable, you don't absolutely need ITO.

author: andyfug
author_rank: Newbie
timestamp: January 07, 2023, 12:17:06 pm
content: Understood re. anti-static coatings. Excuse my electronics ignorance, but looking into it briefly just now, does an anti-static coating effectively create a conductive surface? It sounds like you're suggesting it would be quite high resistance, but still potentially suitable for capacitive sensing?What advantages do you think it might offer over an ITO coating? More cost effective/readily available? More durable?So for testing the theory, would one of the acrylic/polycarb products from here potentially give me something to play with (albeit in sheet form):https://www.aiplastics.com/products/anti-static-plastics.htmlPlus do we think something like this hooked into a microcontroller may allow me to detect the touch?:https://learn.adafruit.com/adafruit-cap1188-breakout/downloadsI picked the above breakout because it looked like it might be easier for me to implement that without getting too involved in the capacitive sensing electronics, but would still give me some parameters to tweak re. sensitivity etc... but is it likely that I would need to devise my own sensor circuit?Thanks again for the help.

author: Marco
author_rank: Super Contributor
timestamp: January 07, 2023, 02:34:09 pm
content: ITO needs to be sputtered, they can make transparent coatings from a lacquer with carbon nanoparticles/fibers. It's cheaper and more available.You could order a sample but unless they come out and say it, its hard to guess what kind of sheet resistance they have.Don't think a generic touch detection chip would take kindly to being attached to a surface with huge resistance and it's own huge parasitic capacitance, it's going to confuse it. I think you're going to have to DIY that even if you could find an acrylic sheet with ITO.PS. with a high resistance surface you probably want to measure at a lower frequency than usual and you really want to use Earth ground as a reference (not generally possible in mobile applications, but it is for you and simplifies measurement).