Low cost high resolution absolute rotary encoder - Page 1
Forum section: electronics / projects

author: raptor1956
author_rank: Frequent Contributor
timestamp: May 06, 2018, 09:26:29 pm
content: Asking for high resolution and low cost is a challenge but what's available at a reasonable cost that's capable of 5-arcsec or better? I'm looking at a project for an instrument something like a theodolite but not quite. I'd need two axis for azimuth and elevationWhat kind of outputs do they have for read by micro-controller?A second issue is bearings or bushings that are smooth enough with the axis of rotation that doesn't precess very much due to imperfections of the bearing -- you want the axis of rotation to always point in the same direction as the unit rotates. Perfection isn't possible but the better the better. I understand that some of the large telescopes use a fluid bearing with a fluid between two precision ground plates, but that would not be practical for my project I don't think.Brian

author: langwadt
author_rank: Super Contributor
timestamp: May 06, 2018, 09:47:36 pm
content: Quote from: raptor1956 on May 06, 2018, 09:26:29 pmmagnetic tape and decoder like a cheap caliper ?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 07, 2018, 01:15:23 am
content: Diameter is everything. If it's big enough, something cheap might do. If it's small, you'll have to pay. I've used Gurly for that sort of thing, and they have some easy to use USB interfaces. Probably too expensive if your'e looking for cheap. AMS makes some sensors that might be adapted-http://ams.com/eng/Products/Magnetic-Position-Sensors

author: jmelson
author_rank: Super Contributor
timestamp: May 07, 2018, 01:30:57 am
content: Quote from: raptor1956 on May 06, 2018, 09:26:29 pm5 arcsec is 259,200 counts, by my figuring. This is going to be a VERY expensive encoder to directly read a shaft to that resolution. Expect to pay, probably, over $10K each, if they can even be made. What you might be able to do is get an encoder that reads from a tape that you put on a very large drum. Look at Sony Magscales, Renishaw and Heidenhain.Jon

author: timpattinson
author_rank: Contributor
timestamp: May 07, 2018, 01:35:04 am
content: AFAIK, ams make encoders up to 14bit (0.022deg) compared to 5 arc seconds (0.00139 deg according to Google) which would be about 18 bits of resolutionIs absolute or just relative position important? Does it have to move in one or two directions?Are you moving the mechanism or does it have to move to a precise location?Sent from my SM-G930F using TapatalkEdit:Looking at the first page of Google results for "18 bit magnetic encoder" they do seem to exist, however in my experience linearity requires very precise alignment of the magnet and chip and may not be possible on a budget.

author: David Hess
author_rank: Super Contributor
timestamp: May 07, 2018, 02:14:33 am
content: Those AMS encoders are one of the first things I thought of but as timpattinson points out, they lack 1 part in 259200 (5 arcsecond) resolution.From what I remember of telescope mounts, gearing to multiply the angular resolution is used but this requires linearity correction for the various gear surfaces. I bet calibration is fun.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 07, 2018, 03:36:25 am
content: Look at the 9710 here-http://www.gpi-encoders.com/That will get you 5-10 arcsec. They have others that will do 3.5 but no doubt at a higher price. I think the last time we bought a 9710 with the USB interface it was in the $500 region. I used Newway air bushings for the bearing because I needed zero friction. IMO, you're not going to get close to what you want for a lot less unless you bound the problem tightly (say a short rotation) and use something like a capacitance gage or scale.

author: raptor1956
author_rank: Frequent Contributor
timestamp: May 07, 2018, 03:38:01 am
content: Well 14-bit is only going to give you about 5-arcmin resolution so that's out of the question. Many instruments like the theodolite have resolutions down to 5-arc-sec and can be had for less than $1000USD with two such encoders for alt and az. So, if an instrument like a theodolite with 5 sec resolution can be had for less than a grand and, presumably, the encoders represent a fraction of the total cost. I suppose they could use a cheaper 14-bit encoder and then attach that to the motor with a reducer in there, but it's not easy reducing backlash to the 5-arcsec range even with a harmonic reducer which is pretty pricey itself.The best total stations have resolutions down to 0.5 arcsec but they tend to cost north of $25K though, as before, the encoders would only form a portion of the total cost.Brian

author: timpattinson
author_rank: Contributor
timestamp: May 07, 2018, 04:56:22 am
content: Do you have a spec for accuracy? not just resolution. Some methods will have accuracy ~= resolution and some nowhere near.Gearing is definitely a possibility, especially if you approach the target from one direction only to reduce backlash etc.

author: Marco
author_rank: Super Contributor
timestamp: May 07, 2018, 01:52:59 pm
content: I'm not convinced a USB microscope with a DIY patterned disc couldn't reach these resolutions ... with a 50cm diameter disc you're talking about 6 micron of displacement, that's not that small. Of course then you get into the problem of how much to charge for your own time, if you don't discount it down to zero it's hard to beat 500-1000$ for something off the shelf.

author: duak
author_rank: Super Contributor
timestamp: May 07, 2018, 03:57:41 pm
content: Canon developed some laser interferometer based rotary encoders in the last 80's for things like robotics. I see that Canon still offers them:https://www.usa.canon.com/internet/portal/us/home/products/list/optoelectronic-products/optical-digital-laser-rotary-encoder/optical-digital-laser-rotary-encoderI see that at least one has analog sine/cosine outputs so if you don't get the resolution you need, it should be possible to interpolate.BTW, another group where I worked developed an encoder with similar resolution that used a glass ring about 30-40 mm in diameter with some 30,000 lines imaged on it. I believe it was used instead of a Canon unit because it could be attached directly to the rotating drum for a large format imaging device.Cheers,

author: branadic
author_rank: Super Contributor
timestamp: May 07, 2018, 04:58:27 pm
content: Quote from: raptor1956 on May 06, 2018, 09:26:29 pm<5 arcsec is <0,00139° or 18bit+ resolution, no problem up to now. State of the art is 24bit and more.What is low cost for you? What quantities are we talking about? Only 2?What size is allowed? What speed? What accuray?Do you really need an encoder or are you looking for a 2-axis inclination sensor?With our technology we can provide e.g. encoders with a diameter of 30mm for the encoder disk and21bit resolution @ 24.000rpm22bit resolution @ 12.000rpm23bit resolution @ 6.000rpmwith ABZ or SPI digital signal output. If larger encoder disk are possible we can further increase the resolution.-branadic-

author: CM800
author_rank: Frequent Contributor
timestamp: May 07, 2018, 05:35:20 pm
content: Quote from: duak on May 07, 2018, 03:57:41 pmWoah. that's pretty cool. Havn't heard of such a thing before.... I wonder how much

author: tecman
author_rank: Frequent Contributor
timestamp: May 07, 2018, 06:12:42 pm
content: you can easily find absolute encoders with the resolution you are looking for. Most common is an SSI/SSD interface. 20-22 bits is not uncommon. Problem is you will spend $ 500~$1000 for it.for example:https://www.sick.com/us/en/encoders-and-inclination-sensors/absolute-encoders/afsafm60-inox/c/g409852paul

author: branadic
author_rank: Super Contributor
timestamp: May 07, 2018, 06:28:08 pm
content: Quote from: tecman on May 07, 2018, 06:12:42 pmFor a single unit this is presumably right. You can also look at:Broadcom/AvagoNikonRenishawHeidenhainbut also Yaskawa.-branadic-

author: woodchips
author_rank: Frequent Contributor
timestamp: May 07, 2018, 06:32:53 pm
content: I wonder if some of these position sensors mentioned are resolution or accuracy? They aren't the same.A glass disc theodolite, the H&W Microptic 2, goes to 1 arc second using glass discs, but you have to realise that it reads the disc 180 degrees apart to get over the eccentric mounting of the disc. Be interesting to see if this is needed on non-glass sensors. These discs were about 100mm in diameter. Possibly a similar arrangement could be created from a CD ROM?When I was looking at accurate position sensors an intriguing one used a rotating disc with lines and measured the time taken to move from sensor to sensor. Time, or frequency, is the most easily measured physical phenomena of course. CD ROM?

author: branadic
author_rank: Super Contributor
timestamp: May 07, 2018, 07:13:03 pm
content: QuoteThis is almost how our encoder technology works. We use CD/DVD process to create a glass master containing diffractive gratings. We make a nickel copy of the master, that is precisely aligned inside a molding tool, thus cheap encoder disks can be fabricated while grating and shaft hole are centered to each other and the disk needs no alignment when building the encoder.Different to Canon we use a laser with gaussian profile illuminating the grating of the rotating disk and if spot diameter of the laser and grating match, the convolution of the diffracted beams create modulated signals on photo diodes giving ideal sine and cosine signals.This way you can realize incremental and nonius encoders but with a little more effort also absolute encoders.-branadic-

author: raptor1956
author_rank: Frequent Contributor
timestamp: May 07, 2018, 07:48:54 pm
content: Thanks for all the input guys, I do appreciate that. It's also interesting to note how many of us with lots of electronics experience are not really up on where encoders are these days with guesses to over $10K.The question of accuracy versus resolution is an important one and part of the issue is how you use the encoder. My understanding of how survey crews operate there 1sec total stations to get the most accuracy out of them is to do field calibration every time they do a new setup. First they level to within 15 seconds or so using the tribrach knobs but then, to get better than that they shoot an object then rotate the scope 180 degrees in elevation and azimuth and re-sight the same object. Do that again at a target about 90 degrees different in az and the smarts in the total station now knows where level is within less than an arcsec.So, although pretty much all things are less accurate then the resolution would indicate but with appropriate operation you can claw back a good bit of the difference.Brian

author: Dubbie
author_rank: Unknown
timestamp: May 07, 2018, 07:56:04 pm
content: Are you absolutely sure you need an absolute encoder? For the price difference, you could afford to keep a zeroed incremental encoder powered on permanently.

author: raptor1956
author_rank: Frequent Contributor
timestamp: May 07, 2018, 07:57:19 pm
content: Quote from: branadic on May 07, 2018, 04:58:27 pmThanks for the input branadic...Let me first clarify by saying that this is an idea that may go no where but I'm looking at something that might be made, if its made at all, in perhaps units of 50 so 100 encoders. I'd really want 5sec as a minimum but 1 sec would be even better.Without going into detail let me just say that what I'm looking at is something of a cross between a theodolite and a tracking camera so the rpm requirements while tracking would be very slow and during slewing limited to 20rpm.Brian

author: raptor1956
author_rank: Frequent Contributor
timestamp: May 07, 2018, 07:59:45 pm
content: Quote from: Dubbie on May 07, 2018, 07:56:04 pmThat's a possibility, however, zeroing with an accuracy of 5arcsec isn't easy. OTH, with the appropriate field calibration the zero reference may not be so critical.Brian

author: branadic
author_rank: Super Contributor
timestamp: May 07, 2018, 08:26:21 pm
content: Quote from: raptor1956 on May 07, 2018, 07:57:19 pmWhat we do is custom encoders, so if you are interested in, please feel free to contact me.Quote from: raptor1956 on May 07, 2018, 07:59:45 pmBut possible. 2-axis inclination sensors e.g. with a range of ±2°, accuracy in the order of x µm/m is another field we are in. Competing products are Leica Nivel 230 (2 axis, optical, range ±1.1mrad, accuracy 2arcsec, resolution 0.2arcsec) or Wyler Zerotronic (1 axis, range ±1°, accuracy 1.8arcsec).Sometimes it's worth combining different sensors.-branadic-

author: raptor1956
author_rank: Frequent Contributor
timestamp: May 07, 2018, 09:38:48 pm
content: Quote from: branadic on May 07, 2018, 08:26:21 pmI've used Wyler 0.01 degree levels when installing AMHS track in many FAB's as we had very tight tolerances for track installation, but that level of accuracy is nothing like what's needed to get to low arcsec's.Could you post or PM me some links?Brian

author: wholder
author_rank: Regular Contributor
timestamp: May 08, 2018, 09:42:54 pm
content: That's a very hard spec for any encoder to meet, much less a "low cost" one. 3600 arc seconds/degree times 360 divided by 5 means the encoder would have to be rated at 259,200 counts/revolution... But, that speaks only resolution and nor accuracy or repeatability.Wayne

author: Dubbie
author_rank: Unknown
timestamp: May 08, 2018, 09:56:13 pm
content: I just built a DRO for my mill rotary table using a 80,000 count encoder and I thought that was high

author: Smokey
author_rank: Super Contributor
timestamp: June 06, 2018, 09:55:28 pm
content: You need one of two things. Both of which can be had off ebay for around $2000USD or less.1) An absolute rotary encoder with digital output such as Renishaw Resolute series. Easily goes up to 32bit. This is the better solution, but costs way more. Works great even in electrically noisy environments.2) A incremental rotary encoder with sine/cosine analog output. I got one recently off ebay for about $80USD. Worked great.Both of these solutions will need some interface. The sine/cosine will need analog interpolation to get the resolution up, which can increase the total cost well above the physical encoder cost. You can use the following TI reference design for the whole interface, which makes it easy, and it will even hook up to a cheap micro. Use the index pulse for homing. Hope you aren't putting it in a noisy environment.http://www.ti.com/tool/TIDA-00176As others have mentioned, your mechanical system is likely to be the limiting factor here.