We’ll take a guess that most readers have a set of digital calipers somewhere close to hand right now. The cheapest ones tend to be a little unsatisfying in the hand, a bit crusty and crunchy to use. But as [Matthias Wandel] shows us, these budget tools are quite hackable and a lot more precise than they appear to be.
[Matthias] is perhaps best known around these parts for making machine tools using mainly wood. It’s an unconventional material for things like the CNC router he loves to hate, but he makes it work through a combination of clever engineering and a willingness to work within the limits of the machine. To assess those limits, he connected some cheap digital calipers to a Raspberry Pi by hacking the serial interface that seems to be built into all of these tools. His particular calipers output a pair of 24-bit words over a synchronous serial connection a couple of times per second, but at a level too low to be read by the Pi. He solved this with a clever resistor ladder to shift the signals to straddle the 1.8 volt transition on the Pi, and after solving some noise problems with a few strategically placed capacitors and some software debouncing, he was gathering data on his Pi.
Although his setup was fine for the measurements he needed to make, [Matthias] couldn’t help falling down the rabbit hole of trying to milk better resolution from the calipers. On paper, the 24-bit output should provide micron-ish resolution, but sadly, the readings seem to fluctuate rapidly between two levels, making it difficult to obtain an average quickly enough to be useful. Still, it’s a good exercise, and overall, these hacks should prove handy for anyone who wants to dip a toe into automated metrology on a budget.
Thanks to [Dragan] for the tip.
It is using the ‘pigpio’ library, which is rasbperry specific:
https://github.com/Matthias-Wandel/caliper-interface/blob/master/decode_caliper.c#L16
gcc -o decode_caliper decode_caliper.c -lpigpio
Maybe using libgpiod would it make portable to other linux SBCs.
Not part of most readers. Only mechanical calipers here.
But this article might change that, because automation is awesome.
Same here! No flat or leaking batteries!
Getting micrometer ish readings is not giving anyone micrometer level accuracy.
Measurement uncertainty with calipers is usually within tenth of a millimeter in average measurements by average measurer. YMMV. To both directions.
Sorry I did not intend to reply here. For some reason cancel reply worked in ui level only and not in functionality.
Yeah, I don’t think I recognize “sorta-micron” in any mesaurement system I have seen.
It’s known as a midge’s bawhair and I’m fairly sure it’s an SI unit (Scottish Instrumentation)
I’m a bit disappointed about the very flaky resistor level shifting, and then attempting to compensate in software. It’s just the wrong way of doing things. It’s quite easy to use a real level shifter IC, or some simple BJT’s or FET’s and some pullups.
Also, the biggest problem with these cheap calipers is that they turn themselves “off” at the most inconvenient of times. When mounting them on a machine, you really don’t want them turning off, and loosing their position.
Some time ago I bought some calipers from the “Terma'” brand. These claim to have an absolute measurement, just like the Mitutoyo’s, but in reality they are somewhere in between the cheap garbage and the Mitutoyo’s. Overall I quite like the way they work. They turn off the display after a few minutes of non-use, and when you move the slider the display comes on again, so you don’t have to press any buttons. When they are not in use, they enter a deeper “power off / sleep” mode after a few days. You do have to press the on button to get them out of that mode, and then you also have to re-calibrate the zero position. It’s a quite nice compromise.
Some history:
After my trusty analog Mitutoyo’s I bought a bunch of different brands of (cheap) digital calipers. They were all garbage. From not sliding smoothly, to very short battery life and just quitting with no apparent reason. A bunch of years ago I threw the last of them away and bought a real Mitutoyo. Battery lasted 5 to 7 years. The low bat indicator was on for the last two years or so and I ignored that. A few months ago the contrast of the display started getting worse and that was the reason to replace the battery. Apart from the fading display, it still worked properly. I thought very hard of buying another Mitutoyo. Partly because the first does not have a data output, and partly because I use calipers in two different locations. After looking around I was quite irritated that Mitutoyo wants about EUR20 extra for the version with data output. (Don’t even look at the prices of their data cable). That prompted me to do a bit of research and that lead me to the Terma calipers. I paid around EUR50 for a 150mm caliper and as of yet I’m quite happy with them. I guess that iGaging is also of better quality then the “cheap chinese stuff”.
Better than the ones I got! Decided to spend a bit of money on proper steel ones after breaking too many of the flimsy $5 ones.
Feckin things turn themselves on, never off. Always have to leave a new battery in the case so I can trust I can use them when needed.
I have that problem with my Mitutoyo digital micrometer, the old arse thing I got for 99 cents at a good will outlet can repeatedly measure a “red one”, but the problem is, they LOVE to turn themselves back on.
It got to the point of I gave up on batteries (cause it sucks them faster than your…) and just use the scale 99% of the time
Strangely the 20 EUR no-brand digital calipers I have work pretty much perfectly. They automatically wake as soon as I move them, keep their absolute position even after days and go over a year on a LR44 battery.
Always a gamble buying this stuff, I have been hit by a few that drain batteries in months.
No, they never turn off. The microcontroller is running all the time. It switch only the LCD off.
But I agree with your objection about level shifting. This has to be done correctly. And it is also a good idea to use an external powersupply to replace the battery. But some, at least my one, use -1.5V at level, means the positive voltage is at the housing.
However, these callipers are now very old. I remember there problems are all discussed and solved 10-20years back in the past. So I wonder what is special about it toda. :)
yeah i am the standard by which i measure fumbling blindly at electronics, and even i can use a BJT for level shifting without any thought.
i guess i was lucky, the cheap digital calipers i had never lost their absolute position. i hated them for a ton of reasons but they didn’t have that flaw. i ultimately lobotimized my digital calipers and now it’s just a slide ruler and it’s 100% as useful as it ever was imo. i just cut up a piece of like milk jug plastic to make a window with a notch in it to indicate the measuring point. i am pretty confidently within +/-0.1mm if i squint, which is as good as my printer anyways. if i really care, i use a different tool
Next Step. ESP8266-powered WiFi-caliper: https://github.com/timonoko/Parkside-Caliper-Interface