[HN Gopher] How cheap digital verniers work (2017)
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How cheap digital verniers work (2017)
Author : jstanley
Score : 127 points
Date : 2023-02-07 12:32 UTC (10 hours ago)
(HTM) web link (www.mjt.me.uk)
(TXT) w3m dump (www.mjt.me.uk)
| Animats wrote:
| The key insight is in the patent: _" The transmitting electrodes
| are powered with A.C. voltage in such a way that an electrical
| rotary field forms which generates A.C. voltages on the receiving
| electrodes, and the phase position from the transmitting signal
| to the receiving signal is proportional to the displacement
| between the stationary part and the displaceable part of the
| transducer"_ It's not necessary to measure capacitance directly.
| Just phase angle. Which has the nice property that, as you move
| from one section of the comb to another, you're not going to get
| a jump. It's inherently monotonic.
| krisoft wrote:
| > It's not necessary to measure capacitance directly. Just
| phase angle.
|
| Can that be done with analog circuitry or does the signal need
| to be digitised?
| KaiserPro wrote:
| you can turn capacitance into a waveform pretty easily, thats
| what the 555 timer does effectively, measure the time it
| takes to fill a capacitor.
| (https://en.wikipedia.org/wiki/555_timer_IC its analogue.)
| michaelt wrote:
| Article author here - apologies if you can't see the charts, it
| seems in the time since 2017 some subresource-integrity-related
| issue has arisen. I'll try to look into that, but in the meantime
| I can assure you that you're not missing too much :)
| toss1 wrote:
| Loved the "Hello NoScript Users! A graph will appear here if
| you enable javascript for ...". Accurate.
|
| Very cool bit of work and writeup, I use these all the time but
| never did a teardown -- thanks!
| [deleted]
| criddell wrote:
| Why do so many cheap digital calipers have an issue with the zero
| point jumping to 0.2" or 5.08 mm?
| dTal wrote:
| It's right in the article:
|
| >The comb on the vernier shaft has a tooth pitch of 5mm. In
| other words, if the slider instantly moved 5mm, it would be
| undetectable, or 'ambiguous'.
| [deleted]
| ly3xqhl8g9 wrote:
| It's such a constant bug across cheap vendors that there are
| only two possibilities: (i) a global conspiracy from Big
| Vernier, (ii) all the vendors in cause are using the same buggy
| firmware.
| jstanley wrote:
| I've never observed this behaviour, but presumably it's because
| 5.08mm is about the width of one of the pads on the "T"-shaped
| section, and it falsely detected a movement when there was
| none?
| ly3xqhl8g9 wrote:
| It's much weirder than that: you can close and open the
| vernier in a particular fashion (very slow and then very
| fast, it's an art) and keep adding 5.08mm in quantum steps:
| 5.08, 10.16, 15.24. The largest value I got on a boring day
| was around 200mm, on a 150mm vernier.
| londons_explore wrote:
| I'm kinda surprised it doesn't self-reset when closed and
| then opened fully. That would be a very easy way to
| automatically 'fix' any location ambiguity with software-
| only changes.
| mschuster91 wrote:
| It would need some form of precise detection for that
| event, which would be expensive.
| ly3xqhl8g9 wrote:
| Yes, as noted, it probably is some shared firmware bug,
| it's just too consistent across vendors.
| adolph wrote:
| Big Clive also covered this a couple years later. Has more of an
| emphasis on the teardown and schematic. The comments are also
| great for links to other people's findings. This article is great
| for more details about how the electrons move.
|
| https://www.youtube.com/watch?v=fKSSY1gzCEs
|
| I'd like to adapt the concept using flexible pcbs to make a
| largish Nest-style rotary encoder.
|
| To understand Vernier measurement conceptually, the Wikipedia
| article is actually not bad:
| https://en.wikipedia.org/wiki/Vernier_scale
| jstanley wrote:
| I had always assumed that they worked like optical mice (i.e.
| shine a light at a slightly-textured surface and watch the
| surface move underneath), but it's much simpler than that.
| Cthulhu_ wrote:
| I think you wouldn't get very good sub-millimeter precision
| with optical; I'm not very well versed in the subject, but
| infrared has a wavelength of between 780 nm and 1 mm, these
| things have to be more accurate than that.
| amluto wrote:
| The little PCB pads are something like 500000nm across, so
| 780nm is nothing. You don't need a sharp picture of the thing
| you're looking at to locate it.
| pjc50 wrote:
| You can get very high precision with light using a similar
| technique: cause interference patterns. This lets you do sub-
| wavelength measurement.
|
| See also https://en.wikipedia.org/wiki/Very-long-
| baseline_interferome..., where the wavelengths are typically
| larger than light.
| mattkrause wrote:
| A millimeter is 1,000,000 nm, so even if you were wavelength-
| limited (and interferential techniques let you side-step
| that), there's plenty of wiggle room. Heck, you could go down
| to visible blue light and free up another octave too.
| londons_explore wrote:
| Optical mice actually aren't good at precise relative
| positioning because the way the lens is setup, if they are
| moved slightly closer to the surface they 'see' more movement.
|
| You would imagine that you could use some kind of registration
| pattern(s) to solve this, but sadly nearly all mouse sensors
| are made by Avago, and Avago runs their own algorithm on the
| raw sensor data, and won't let you have the full data stream
| (you can only receive it via a debug API which only lets you
| get the data out super slowly). The avago ROM's are built into
| the sensor, and run a little 8080 CPU with a neat DSP for
| processing the image data, but sadly the system has too little
| RAM for any exploit to run your own code on it.
| bloopernova wrote:
| Oh lord that would be an amazing hack in a story. A super
| secure facility gets compromised despite using custom
| firmwares on everything, but the mouse sensor slipped through
| the cracks.
| mk_stjames wrote:
| Very large digital scales for digital readouts (DROs) on
| machines like manual lathes and mills actually do use
| quadrature optical encoding via etched glass scales inside
| aluminum housings, similar to the light gate on a scroll wheel
| of a mouse, but instead detecting fringe pattern light/dark
| from the etching in the glass scale. The etching is typically 5
| microns in spacing, so a resolution of 0.005mm, or sometimes
| smaller. Because the glass is very stable this can be pretty
| repeatable over large distances.
| jbay808 wrote:
| Just to expand on that, usually the etched scale pitch is 20
| microns, but digital quadrature detection gives you 4x the
| resolution of the scale pitch, resulting in a 5 um distance
| between successive rising / falling edges of the quadrature
| signal. Analog measurement can be used to interpolate to much
| finer resolution, even 1 nm for good quality optical scales
| with good electronics.
| schneems wrote:
| What do the expensive ones like mitutoyo do differently? I've got
| a pair and the are definetly more consistent than $30 neikos.
| eternityforest wrote:
| My iGauging pair uses induction rather than capacitance, and it
| has absolute positioning rather than dead reckoning, which is
| really important because the cheap ones run the battery down
| all the time, even when it's off it's still keeping track of
| movement as far as I know.
| michaelt wrote:
| Article author here. Mitutoyo verniers are too expensive for me
| to take apart for a lark, but as I understand it:
|
| Cheap verniers are 'absolute modulo 10mm' as the T-shaped bar
| pattern repeats itself. They produce 'absolute' measurements by
| reading fast enough to keep track of how many whole bars
| they've moved.
|
| Fancier verniers instead have two tracks in parallel, with
| slightly different pitches. Checking the offset between the two
| patterns at the current location gives an absolute measurement.
| This of course requires them to be factory calibrated etc.
|
| They might also have slightly better electronics (some of these
| cheap verniers need a new battery every 3 months or so) or
| other differences.
| Blackthorn wrote:
| The electronics are absolutely a killer difference. The mitus
| can actually turn off rather than just turn the screen off.
| TheSpiceIsLife wrote:
| I was just having a laugh with a near-retirement fitter only
| yesterday about this: digital calipers _are not vernier_
| calipers.
|
| From Wikipedia:
|
| _A vernier scale, named after Pierre Vernier, is a visual
| aid to take an accurate measurement reading between two
| graduation markings on a linear scale by using mechanical
| interpolation, thereby increasing resolution and reducing
| measurement uncertainty by using vernier acuity to reduce
| human estimation error._
| dheera wrote:
| > Mitutoyo verniers are too expensive for me to take apart
|
| https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m.
| ..
| schneems wrote:
| Thanks! It reminds me of those black and white wheels you use
| to digitally encode rotation but stretched out linearly.
| genpfault wrote:
| > those black and white wheels you use to digitally encode
| rotation
|
| Gray codes[1]?
|
| [1]:
| https://en.wikipedia.org/wiki/Gray_code#Position_encoders
| kevin_thibedeau wrote:
| Mitsutoyo absolutes have a pattern that doesn't repeat. You
| can pull the battery and reinstall at any setting and all you
| lose is the zero offset correction.
| regularfry wrote:
| Oddly enough I've got a cheap pair that is quite easy to
| accidentally trick into having a 5mm offset one way or the
| other. I was wondering what the mechanism was. But that
| doesn't match up with 10mm bars. Different pattern, or
| different logic?
| DoingIsLearning wrote:
| > Fancier verniers instead have two tracks in parallel, with
| slightly different pitches. Checking the offset between the
| two patterns at the current location gives an absolute
| measurement.
|
| In your article you link to a patent on the cheaper T shaped
| coatings, do you also have a reference to a patent
| specifically using this system of parallel + different pitch
| tracks?
|
| Great write up by the way!
| jseutter wrote:
| I have one of the cheap calipers that needs new batteries
| every 3 months. It both sucks and is surprisingly useful for
| stuff around the house. I ended up buying a 20 pack of
| batteries and leave them sitting in the same drawer as the
| calipers so they're at hand when the current batteries
| inevitably die. The Mitutoyo's are definitely a better tool.
| Steltek wrote:
| I bought a set of analog ("actual vernier") calipers for
| daily use. I don't really need more than 0.1mm accuracy
| anyway.
|
| At the same time, I soldered a tiny power switch into my
| cheap calipers to stop the battery drain issue.
| aidenn0 wrote:
| Totally OT, but is your username a WC Privateer
| reference?
| Steltek wrote:
| It is. I am uncreative and it seemed suitably obscure to
| be available on most sites.
| joshvm wrote:
| Ditto, you can get an analog Mitutoyo for not too much -
| I think my 530 series was about 40-50 bucks, +-0.05mm.
| It's nice because it never runs out of batteries,
| accuracy is fine for everything I need it for (plus it's
| traceable), and the slide action is very smooth. The
| warranty is a bit lousy, only up to 3 years.
| GuB-42 wrote:
| The problem I have with actual verniers is not accuracy,
| my vernier is actually more accurate than the cheap
| plastic digital calipers I use most often. It is that
| digital calipers are much easier to read, especially in
| awkward positions.
| jhallenworld wrote:
| The built-in metric/imperial/fraction conversion is also
| nice. I tried dial calipers that do this (they have two
| dials), but they are even harder to read. It's better to
| buy one imperial and one metric.
|
| https://www.amazon.com/Imperial-Stainless-Accurate-
| Shockproo...
| phs2501 wrote:
| They (digital calipers) are also a lot more convenient in
| mixed-unit environments (e.g. PCB layout, which is why I
| purchased mine) than analog ones. Granted since I live in
| the US everything is mixed unit whether I like it or not.
| :)
| NegativeLatency wrote:
| I just take the battery out, been using the same one for
| like 3 years
| kurthr wrote:
| Hahahaha... OMG I worry that calipers (that aren't in a
| machinists shop) are non-functional, if there isn't a
| battery taped to the side!
| martyvis wrote:
| it's why in the case for mine there are two holes in the
| foam for spare batteries
| serf wrote:
| I was OK with doing that for years and years until I bit
| the bullet and bought a pair of (current day) Mitutoyos.
| Now battery replacement is more on the order of 2 or 3
| years under constant use, even with the occasional 'woops
| I left it on' slip-up -- almost on-par with my 90s solar
| Mitus'.
|
| I feel like the cash spent on replacing batteries in my
| 'crappy environment' calipers (usually harbor freight or
| equivalent brands) could've easily paid for keeping
| Mitutoyos in the bad environments, anyway.
| CamperBob2 wrote:
| I used to turn my Mitutoyo off after using it, but
| eventually realized that it doesn't make enough
| difference to bother. If I leave it on, the battery is
| good for 2-3 years. If I turn it off, it will last for 5
| or more years... but who cares, it's less trouble to
| change the battery every couple of years than it is to
| turn the caliper on and off every time I use it.
|
| It's one of those from-my-cold-dead-hands kinds of tools.
| TheSpiceIsLife wrote:
| There was a period there a few years back where it was
| difficult to find digital calipers that wouldn't stay off,
| were so sensitive they'd automatically turn back on at the
| slightest movement in the room, and didn't have auto-off
| functionality.
|
| Fortunately that phase seems to have passed and it's now
| possible to buy very good digital calipers that
| consistently match my $300 Mitatoyos, stay off when turned
| off, and automatically turn off after a few minutes of
| inactivity.
|
| I was so impressed I bought another two and gave them away,
| saves pulling out the Mitatoyos when someone at work wants
| to borrow 'em.
| YeahNO wrote:
| I bought an AA battery case, bored a small hole in the
| caliper's battery cover, fed wires through, secured them to
| the contacts and used velcro to hold the AA battery case to
| the back of the caliper. I can easily take the battery out
| when storing and it always just works without having to buy
| overpriced button cells. It does look a bit janky.
| jstanley wrote:
| I assume the same, but possibly implemented more precisely.
| shimonabi wrote:
| If you leave the battery inside a cheap caliper, the battery
| will be dead in a couple of hours, even if the caliper is
| switched "OFF".
| MrBuddyCasino wrote:
| Not all of them. I can recommend this one, it has long
| battery life, good precision and smooth mechanics:
| https://www.amazon.de/gp/product/B002JU64F4/
| shimonabi wrote:
| A model that looks exactly the same costs 5EUR on
| Aliexpress. I don't like to be ripped off.
| brk wrote:
| That looks very similar to one I use that typically gets
| about 2 years of battery life.
| luma wrote:
| You're right but way off on timing. These common digital
| calipers have a design error which does in fact drain power
| while off, but it's nowhere near a couple hours. They
| reasonably should last several years but in reality it'll be
| on the order of 6 months or so.
| weaksauce wrote:
| i don't know if i'd call it a design error but it is a
| cheaper design to manufacture than the more expensive kind
| that requires calibration. could also be a patent
| workaround. for something like 10 bucks many years ago i
| got some nice stainless 12" ones that work fine other than
| the lower battery life thing. they do lose track if the
| battery is low though and you move the jaws quickly. just
| have to have a bunch of batteries on hand and/or take the
| battery out when not in use.
| Symbiote wrote:
| There may be different levels of "cheap".
|
| I bought a EUR20 digital caliper (this one[1]) and the
| battery is still fine, 21/2 years later.
|
| [1] https://www.bauhaus.dk/alpha-tools-digital-skydelaerer
| dsfyu404ed wrote:
| This is obvious BS. Many of them get packaged and shipped
| with the battery in them.
|
| That said, they absolutely do eat batteries faster than the
| expensive calipers.
| shimonabi wrote:
| It's not. I take the battery out after each use. Several
| users have reported this, for example:
|
| "Turns out they pull almost as much current off (15uA) as
| they do on (17uA)."
|
| https://www.reddit.com/r/harborfreight/comments/z7j8hp/fixe
| d...
| rsaxvc wrote:
| An LR44 supplying 15 microamps will last more than 9
| months before depleted. This base level of current draw
| obviously matters to different users differently.
| aidenn0 wrote:
| Possibly not if you buy the LR44 from Amazon. I have
| gotten batteries labeled "LR44" that were practically
| useless. I've also gotten "SR44"s that were actually
| LR44s.
| abruzzi wrote:
| my $10 Harbor Freight calipers are still on their original
| battery after 8 years. So I'm not sure about these claims
| about being constantly on and draining the battery. I think
| instead that you have to have them fully closed when you
| power it on so it can zero out. It also has a button to
| zero.
| skykooler wrote:
| It definitely depends on the model. In my experience, my
| cheap calipers from Harbor Freight run for far longer
| before needing a new battery than my cheap calipers from
| Home Depot.
| rsaxvc wrote:
| I think the worst offenders are the ones that use the
| measurement circuit to detect if you've started a
| measurement while the display is off.
| varjag wrote:
| They keep zero.
| DanTheManPR wrote:
| I think people might take your comment as flippant, but
| keeping zero is a very important capability that higher-end
| measuring devices can have. You can count on them not
| drifting when you open them up and take a measurement, and so
| you're not going to be off by a tenth of a millimeter every
| other measurement.
| throw0101a wrote:
| So what is some good equipment to buy (at various price points)?
| Has Project Farm (or someone else) done a ranking?
| theatrus2 wrote:
| The entry price of a pair of Mitutoyo calipers isn't that high,
| and is the standard instrument pretty much everywhere.
| pie314isi wrote:
| If they don't have a vernier scale they're just calipers
| somat wrote:
| Was going to make the same comment, but was uncertain, so I
| read the article, it does look like it uses a vernier sort of
| mechanism to get much higher precision than a plain measurement
| would.
| causality0 wrote:
| _Thank you._
|
| That was driving me nuts.
| fortran77 wrote:
| No, that's what a socket set is used for.
| geocrasher wrote:
| Reminds me of the time I went into a pub right on the docks
| of a fishing port. An old seafarer came in and sat next to
| me. When I asked him about the ships steering wheel shoved
| down the front of his pants he just looked at me and said
| "Yar, it's drivin' me nuts!"
| jameshart wrote:
| I mean, from reading this the way they work is by detecting the
| relative offset of two slightly differently pitched scales by
| measuring a series of capacitances.
|
| Honestly it's a much more 'vernier'-like mechanism than I'd
| given it credit for.
| DanTheManPR wrote:
| This must be a regionalism; I've never heard of them called
| "verniers" before.
| cmiller1 wrote:
| I've run into several older engineers that call all calipers
| "verniers." It used to bug me but I guess it's okay for
| language to be a bit flexible like that. I still call the shelf
| in front of me in my car the "dashboard" even though it's not
| blocking the rocks kicked up by a dashing horse anymore.
| zrobotics wrote:
| The reason this is annoying is that there are other measuring
| devices that can have a vernier scale though. For example, in
| my shop I have a cheap micrometer that only measures to
| thousandths of an inch, and a more expensive one with a
| vernier scale that measures to ten-thousandths. Offhand,
| height gauges also commonly have a vernier scale.
|
| The reason this is annoying in a shop environment is that
| there are likely other tools around with a vernier scale.
| That being said, I've also been around older machinists &
| engineers who exclusively refer to gauge blocks as "jo
| blocks", even when there wasn't a single set manufactured by
| the Johansson company in the building. This is common enough
| that searching for "jo blocks" returns a dedicated page on
| mc-master: https://www.mcmaster.com/products/jo-blocks
| SoftTalker wrote:
| Do you blow your nose with tissue or Kleenex?
| martyvis wrote:
| It's funny how this comes up often. Even though Kleenex
| is by and far the most popular brand here in Australia, I
| don't ever recall anyone ever saying something like "pass
| me a Kleenex". Maybe it's our penchant for either
| shortening things (afternoon becomes arvo) or adding
| synthesising suffixes for name (David is always Davo,
| McDonald's are Maccas) that Kleenex doesn't work for us.
| cwillu wrote:
| There are regions that refer to any carbonated beverage
| as "coke". As in "Hi, can I get a bottle of sprite coke"
| dreamcompiler wrote:
| Many years ago I did a stint at a big American car company.
| The engineers there were adamant that the shelf you speak of
| is properly called the "instrument panel." The "dashboard" is
| a sheet of steel that separates the passenger compartment
| from the engine compartment, and it's only accessible under
| the hood.
|
| I guess this is a car engineer's version of "that thing you
| call the Internet is actually the web."
| sokoloff wrote:
| I'd have called that the "firewall" rather than the
| "dashboard".
| pbhjpbhj wrote:
| I think the problem is the dashboard was originally an
| optional panel seperating a buggy driver from the horse
| pulling it; preventing dirt from getting dash-ed on to
| them (from hooves; or horse-farts, I imagine). The part
| on a tram, between the driver and road, was also called a
| dashboard -- so it might have come to cars [automobiles]
| via trams or omnibuses?
|
| Dashboards were kept, and as gauges fell into use the
| gauges were mounted on or about the dashboard. So the
| dashboard is the ancestor to the gauge display holder,
| and to the firewall.
|
| Patent US652940 (1900) shows a water-level gauge mounted
| to a dashboard in a "Whitney motor wagon".
| InitialLastName wrote:
| A few years ago, I found myself needing ready access to
| disposable latex gloves in my car and was so happy to finally
| be able to use the "glove compartment" for its nominal
| purpose.
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