On these cheap Digital calipers.. - Page 1
Forum section: electronics / projects

author: ImABeginner
author_rank: Contributor
timestamp: March 05, 2023, 05:54:37 am
content: I have a project that requires encoding positions to about .1mm, and with a very strict power and space requirement.Luckily, these digital caliper thingies seem to have exactly what I need:They seem to be just right for my needs (slow response, medium accuracy, low profile). Trouble is that these are still a bit too clunky, even after taking off the jaws and LCD. So I was hoping that members here may know more than I. I'll share what I have found so far.There's some good docs on these, including this article on the protocol that they seem to use:https://www.electroschematics.com/digital-caliper/Another quick read on how the protocol differs based on the measurement mode:https://www.instructables.com/Hacked-Digital-Vernier-Caliper-Using-Arduino/There's this wonderful project on re-engineering the stator portion of the caliper, but has nothing on the capacitive sensor:https://www.instructables.com/Reengineering-the-Stator-of-a-Digital-Caliper/This video explains the very basics on the capacitive encoder inside:https://www.youtube.com/watch?v=XLa5PICnxpg,but don't go into any sort of detail on the engineering or principles.Finally, this short article seems to explain the digital communication protocol with an emphasis on the methodology:http://www.shumatech.com/support/chinese_scales.htmWith these things in mind, here are my questions:- What sort of sensor may one select to recreate this on one's own? It seems that presently the chip is something with 8 capacitive pads used for sensing, likely giving analog readings. This isn't something that we normally see on every day MCUs, and even specialized chips never seem to have 8 of these sensors.- Which principles are these designed on? Specifically, how do they maintain such precision in their measurements with such low cost?- Capacitive encoding isn't a new technology, why don't we see these systems sold on places such as digikey? I suspect that I may be searching in the wrong places, so please correct me if I'm wrong.Thanks in advance!

author: ImABeginner
author_rank: Contributor
timestamp: March 05, 2023, 06:32:52 pm
content: Hello, I have found more info, but the mystery continues.https://www.ien.eu/article/linear-encoder-kit/I found this kit online for a simple AB type encoder. Strangely enough it uses the same snot that calipers use to cover the chip... So what sort of chip could be under there?https://www.semanticscholar.org/paper/Prototyping-of-flexible-capacitive-encoder-with-Kimura-Gondo/a7879da5bacdac2d835aaa91829e5372b2fdc223+This paper is interesting, as it uses a pressure-sensitive pad to capacitively encode position, a really neat concept. Not what a caliper does, but there are some interesting results.https://www.semanticscholar.org/paper/Capacitive-Type-Flexible-Linear-Encoder-With-Slider-Kimura-Gondo/e452964477592e0b4c5d51b620d7d2e6c475db49/figure/0This paper seems to be the most interesting, as it explains a LOT of the theory behind this. It seems that there are actually three capacitive plates. Two are to transmit AC power at 180 degrees opposed, and finally an interlaced 3rd set of detection plates.I'm guessing that there's some sort of voltage waveform that shows up on the detector portion of the interlaced plates, and that it changes ever so slightly with motion. I have yet to find a chip will facilitate this, but I think that I have a good idea of what sort of analog circuitry may help detect these capacitance changes.

author: thm_w
author_rank: Super Contributor
timestamp: March 07, 2023, 12:16:41 am
content: You can buy the rails and encoder, for use in a CNC machine or similar:https://www.amazon.com/iGaging-35-706-P-Digital-Readout-Display/dp/B01G4FQI64/Can also get the magnetic version, the read head is a bit taller:https://www.aliexpress.us/item/3256805079907432.htmlWhat specifically is your use? What measurement distance?Another option would be optical linear encoders as used on inkjet printers.

author: Martinn
author_rank: Frequent Contributor
timestamp: March 08, 2023, 07:42:11 pm
content: You'll find most background in the relevant patents.The base technology was invented by Sylvac co-founder Hans-Ulrich Meyerhttps://www.linkedin.com/in/hans-ulrich-meyer-104a068a?originalSubdomain=chSee alsohttps://microtech.ua/img/cms/Downloads/enghistory.pdfIt was licensed to Mitutoyo and expired in 2001 (that's why we get so many of those cheap clones recently).Best information from patent search - I prefer "espacenet" by the european patent agencyhttps://worldwide.espacenet.com/patent/searchResults:https://worldwide.espacenet.com/patent/search?q=ia%20%3D%20%22meyer%20hans%20ulrich%22&queryLang=enOver time Hans-Ulrich Meyer published a handful of caliper patents, probably this onehttps://worldwide.espacenet.com/patent/search/family/004342580/publication/EP0053091A1?q=pn%3DEP0053091A1is most relevant. Note that each patent family has patents in different languages - US patents will be english language.The only significant improvement of that original principle I am aware of is the "Absolute" scale by Mitutoyo; they added two extra data tracks in order to get the absolute position after power on. All others are incremental only and would lose position when not powered. For that reason they either have no "off" switch or just switch off the display, draining the batteries constantly with 40 uA or so.The "Absolute" patent might be this one:https://worldwide.espacenet.com/patent/search/family/027524757/publication/US4879508A?q=pn%3DUS4879508A

author: ImABeginner
author_rank: Contributor
timestamp: March 10, 2023, 12:56:33 am
content: I wish that I could say exactly, but the travel length is about 200mm, and follows a non-linear, arbitrary path. The whole thing will be about the size of the first Harry Potter book, or a small first aid kit. small for the field of encoding.

author: ImABeginner
author_rank: Contributor
timestamp: March 10, 2023, 12:59:53 am
content: Wow! these patents are very detailed. I imagine the modern day ones simply measure the response, instead of using the capacitor itself. If I can't find any systems, this is what I'll be basing a lot of my prototyping off of.

author: Martinn
author_rank: Frequent Contributor
timestamp: March 10, 2023, 07:25:48 pm
content: This one is probably closest to what you are looking for:https://www.posic.com/EN/linear-encoder-id4501l.htmlMany DIYers use a AS5311https://ams.com/as5311