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.
This is how a resolver[1] type encoder works. It's a rotary transformer with two stator phases at 90 degrees forming a sine and cosine output. The rotor has an AC current passed through, usually a few kHz, and the phase angle of the outputs vs the input is directly related to the shaft angle. There also exists a linear version called an Inductosyn which is similar to the calipers capacitive scale.
They are absolute for a single rotation meaning if power is lost and reapplied, you still know the angle of the shaft (good for robot arms). This absolute positioning is not possible with a quadrature encoder.
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.)
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 :)
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.
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.
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.
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
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.
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."
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".
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.
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.
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!"
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.
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.
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.
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.
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.
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. :)
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.
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.
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.
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.
Might be a 2xAA case. They’re the most popular, anyway (because 1.5v electronics are relatively new so it was either 3.3v running off the two batteries or boosted to 5v and you can’t really boost 1.5v to 5v cheaply, effectively, or for long).
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.
> 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?
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.
> They might also have slightly better electronics (some of these cheap verniers need a new battery every 3 months or so) or other differences.
This is because the cheap ones never power off, only turn off the lcd. They can’t power down without losing the position/calibration because they need to be aware when they move to a different section.
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?
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.
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.
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.
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.
What's really annoying is that they could have sidestepped the whole problem by simply putting a simple physical on/off switch on them, instead of the soft on/off button they use. No one uses these cheapo calipers for more than a few minutes at a time anyway, so the battery would probably last a lifetime with a normal switch and no one would notice that it's a power hog.
> simply putting a simple physical on/off switch on them, instead of the soft on/off button they use.
that's true but a physical switch needs to be placed and soldered while the soft on/off just needs to be placed atop it on the traces on the pcb. cost just probably was the biggest issue.
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.
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.
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.
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.
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.
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
