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Equipment Discussions >> Mounts

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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: SWR]
      #5918299 - 06/13/13 07:43 AM

Quote:

An even better way is to have a non repeating code all the way round on the index track. This way you can tell the absolute position when you have moved a few counts in either direction.




"non-repeating code all the way around" = absolute encoder.

I mentioned this above already.

The ultimate solution is really a high-tick absolute encoder, which is the approach AP, 10Micron, etc. have all done.

The Heidenhain 2048-ppr EnDat absolute encoders are surprisingly inexpensive ($450). They have "32 million ticks" but since they are inherently only 2048 ppr, the actual angular accuracy is only +/- 20".

I wonder what is the actual internal accuracy of the "10 million tick" encoders on the 10Micron mounts... As we can see, for absolute encoders the tick count is meaningless.


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SWR
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Reged: 07/23/10

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5918403 - 06/13/13 09:20 AM

Yes, but the strange thing is that Heidenhain could change the cheaper 5000 PPR sin/cos encoder to be absolute just by changing the pattern on the index track. That wouldn't add any cost at all, and give us a low cost high resolution absolute encoder. Why don't they do that?

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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: SWR]
      #5918429 - 06/13/13 09:42 AM

I don't think it's as simple as changing the pattern on the index track. You would need an index track with a large number of unique gray code patterns.

I believe "all" the high-tic absolute encoders are really pseudo-absolute - they have a high-tic incremental grating, and an absolute index track with a much lower tic count.

Here's how Avago did it

http://www.avagotech.com/docs/AV02-0865EN

Incidentally the above article also discusses eccentricity error. Interestingly they don't use the Heydemann approach to fix non-circularity of the Lissajous pattern.

The trouble is you cannot print 32 million tics on a 3" diameter disk. It's trouble enough to print 5000 tics.

So you have the regular 5000 tic (or 2048 tic) grating, and you do interpolation on this grating to get your mega-tics.

Then you have a parallel index track with a gray code. But maybe you only have 1024 different gray codes, so the encoder has to rotate a bit before it can get the fine resolution.


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SWR
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Reged: 07/23/10

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5918880 - 06/13/13 02:11 PM

Quote:

I don't think it's as simple as changing the pattern on the index track. You would need an index track with a large number of unique gray code patterns.


Yes, you need a large number of unique codes, but not necessarily grey codes, and why isn't that simple? It's still only one track. You just replace the single index bit with a more intelligent pattern. No extra hardware is required.
Quote:

I believe "all" the high-tic absolute encoders are really pseudo-absolute - they have a high-tic incremental grating, and an absolute index track with a much lower tic count.


Why can't the index track have the same resolution as the quadrature track? You should be able to read both tracks whith the same resolution.
Quote:

The trouble is you cannot print 32 million tics on a 3" diameter disk. It's trouble enough to print 5000 tics.


A 3" track will have a length of 239,4mm. With a bit width of 130nm we have a pulse spacing of 260nm and a quadrature resolution of 920.728 pulses on the track. This is a quadrature resolution of 3.682.913 ppr corresponding to an angular resolution of 0,35". Not too shabby for a low cost solution.

The big question is: can the bearing tolerances and the difference in temperature expansion tolerances between the disk and the PCB be kept low enough to keep the Blu-ray track aligned without active adjustment?


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: SWR]
      #5918944 - 06/13/13 02:52 PM

Quote:

Yes, you need a large number of unique codes, but not necessarily grey codes, and why isn't that simple? It's still only one track. You just replace the single index bit with a more intelligent pattern. No extra hardware is required.




What will that pattern be?

Remember that the track is binary. So to have say 256 different gray codes, you need 8 tracks ( = 8 bits ). This is why high-bit absolute encoders are very expensive.


Quote:


Why can't the index track have the same resolution as the quadrature track? You should be able to read both tracks whith the same resolution.




Because the incremental track doesn't have slots. It has a diffraction grating. That's how the interpolation works.

Because the absolute tracks need to have exact bit values, it must have those multiple tracks with binary codes (gray codes) on them. So the resolution can't be as high as the incremental track.


Quote:

A 3" track will have a length of 239,4mm. With a bit width of 130nm we have a pulse spacing of 260nm and a quadrature resolution of 920.728 pulses on the track. This is a quadrature resolution of 3.682.913 ppr corresponding to an angular resolution of 0,35". Not too shabby for a low cost solution.




I don't know where that bit width came from. Normally encoders only have 5-20um physical resolution. Everything else is interpolated.

I think you are conflating the bit width of CDROM, DVD, or bluray with encoders. They are not the same.

These optical drives don't care about angular position. They just need the bits to come off the track at the correct rate. Variations in disk rotational speed are perfectly acceptable, because there's a big memory buffer where the bits are held before they are played out.

With an encoder, you don't have that option.


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SWR
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Reged: 07/23/10

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5919164 - 06/13/13 05:04 PM

Quote:

What will that pattern be?


Any unique serial pattern that doesn't repeat anywhere on the track.
Quote:

Remember that the track is binary. So to have say 256 different gray codes, you need 8 tracks ( = 8 bits ). This is why high-bit absolute encoders are very expensive.


That is the traditional way of doing it, and it will give you an absolute position without moving the disk.
If you can accept that you won't know the absolute position until you have moved the disk a certain number of counts, you can get by with just one index track, and the resolution will be the same as the pulse counting track.
Quote:

Because the incremental track doesn't have slots. It has a diffraction grating. That's how the interpolation works.


I agree, but if you have full digital resolution you don't need interpolation. Just feed the two quadrature signals into a quadrature counter input on the microcontroller.
Quote:

I don't know where that bit width came from. Normally encoders only have 5-20um physical resolution. Everything else is interpolated.


Here's an image from Wikipedia:
[image]http://en.wikipedia.org/wiki/File:Comparison_CD_DVD_HDDVD_BD.svg[/image]
The bit length is 150nm instead of 130nm, so the counts will have to be adjusted accordingly. I mixed the length and the width.
Quote:

These optical drives don't care about angular position. They just need the bits to come off the track at the correct rate. Variations in disk rotational speed are perfectly acceptable, because there's a big memory buffer where the bits are held before they are played out.

With an encoder, you don't have that option.


The speed is determined by the telescope mount. The microcontroller just have to count the pulses. Preferably with a hardware counter with quadrature inputs for high speed counting. The microcontroller will just have to read the counter at fixed intervals and perform the PEC adjustment before issuing guide pulses.

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SWR
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Reged: 07/23/10

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5920005 - 06/14/13 04:40 AM

Quote:

I think you are conflating the bit width of CDROM, DVD, or bluray with encoders. They are not the same.

These optical drives don't care about angular position. They just need the bits to come off the track at the correct rate. Variations in disk rotational speed are perfectly acceptable, because there's a big memory buffer where the bits are held before they are played out.


I know that this is using a Blu-ray disk for something it was not intended for or even designed for, and maybe this is just a crazy idea that won't work in a practical implementation?
By reading your comments I got the impression that you are looking for a low cost solution as an alternative to the expensive solutions that already exists on the market.

At least in my mind the idea ought to work in theory if the bearing tolerances can be made small enough.
I was just thinking about which low cost technology has the required resolution, and a Blu-ray disk was the only thing that came to mind, as I don't consider a 5000 pulse sine wave disk to be low cost, although they are comming down in cost.

The electronic decoding would also be much simpler if you don't have to convert two analog signals and interpolate with circle scaling and atan functions. You can just feed the two digital signals to a quadrature counter input on a microcontroller. You don't even need a controller with ADC. This counter can be read at precise time intervals to determine the speed, and it's quite cheap to get accurate timing with a low cost low PPM 32768Hz watch crystal on a microcontroller with built in PLL oscillator.

The most dificult part of dealing with high resolution mechanics is the very tight tolerances in mechanical alignment. The idea of installing the disk and laser PCB without doing any alignment at all, and then using the same lasers, that will later be used for reading the disk, to program the quadrature and index tracks after beeing fixed to the mount, was to try and remove as many tolerances as possible from the tolerance chain.

It will require three lasers to get two quadrature and an index signal, but they are cheap and the reduced complexity by not having movable parts seems like a good tradeoff.

Maybe there are too many problems with getting the bearing tolerances small enough on the RA axis? I like simple solutions and this one seems quite simple and low cost to me if the RA axis tolerances are adequate for this solution.


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: SWR]
      #5920055 - 06/14/13 06:31 AM

Actually to be honest I can't even imagine how the whole "burn the bluray disk while its on the RA axis" idea can he productized.. How to mount everything securely, protect it from dust, etc. Its just a bit too Rube Goldberg even for me (and I'm a big fan of Rube Goldberg!)

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SWR
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Reged: 07/23/10

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5920150 - 06/14/13 08:31 AM

The Blu-ray disk could be cut to shape and mounted under the orange ring like you did with the thin alu-plate in your current design. It must have the same outer diameter as the ring and the inner diameter should allow it to slide over the black ring in the centre. The dimensions does not have to be very accurate. It looks like there are a few mm distance between the inner diameter of the orange ring and the outer diameter of the black ring, where the disk would be visible. Three blu-ray tracks doesn't take up much space, so only a thin slice needs to be visible. The disk is held in place by tightening the screws holding the orange ring.

The PCB with the lasers and a small microcontroller must be fixed to the moving RA axis with an adapter that screws into the polar scope thread. It doesn't have to be aligned accurately, but it has to be fixed rigidly. The idea is that you don't have to place any of the components accurately, but you need them to not move once they have been mounted.

One way of burning the Blu-ray tracks would be to mount a large weight to the telescope and burn the pattern while slewing a complete revolution at maximum speed. The large weight could be a carefully balanced telescope with counterweights or a balanced weight added to the saddle. The weight should smooth out quick variations comming from the drive train.

There will still be variations in the speed, so the pattern won't be perfect, but it doesn't need to be. All these errors must be removed by PEC. The PEC curve can be recorded by guiding. Instead of issuing guide commands, you record the position of the guide star with 0,1 pixel resolution. A guide star is a very accurate reference to calibrate the PEC against. It can't remove flex, so that will have to be delt with mechanically.

Edited by SWR (06/14/13 08:50 AM)


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SWR
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Reged: 07/23/10

Re: Encoder-based PE Correction on the cheap new [Re: SWR]
      #5920223 - 06/14/13 09:25 AM

I forgot the dust protection: can't we just use the cover that comes with the mount as dust protection? I think the whole unit will fit inside the cover.

If there is not enough room for the disk to be vissible between the orange and the black ring, the disk could be glued to the top of the orange ring. The important thing is that is doesn't move when mounted.


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: SWR]
      #5923210 - 06/16/13 04:11 AM

At the risk of jumping the gun again.. I made up a much more robust encoder mount out of wood.



Did several iterations measuring the PE at different (and same) angles. Looks like the massive drift is gone. There still is a small drift (about 1 arc-second every 240 seconds) which I believe is due to eccentricity.



So the huge errors I was seeing before were due to the Baumer "anti-torque tab"... torquing.

I'll have a better encoder mount machined once I've validated that this fix is reliable.


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Raginar
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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5924671 - 06/17/13 12:01 AM

Woohoo!

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Mert
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Re: Encoder-based PE Correction on the cheap new [Re: Raginar]
      #5927986 - 06/18/13 05:41 PM

Now you are cooking Orlando, good news!!!!

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Mert
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Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5938071 - 06/24/13 01:54 PM

Have you been able to do some test-runs Orlando??
I'd be happy to see some of the results with your
new axis and mounting!


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5938170 - 06/24/13 02:47 PM

We've had absolutely terrible weather in Singapore this past week. Due to fires in Sumatra there has been very bad haze in Singapore and Malaysia. So, no... right now my main worry is being able to breathe.

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Starhawk
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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5938317 - 06/24/13 04:24 PM

Here's a basic question- Why is your mount not black?

That's good news on the installation fix.

-Rich


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Mert
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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5939910 - 06/25/13 02:40 PM

I didn't know that about the fires
so yes, you are right, keep breathing well!


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: Mert]
      #5943245 - 06/27/13 02:47 PM

Update. The haze has lifted temporarily, and I have been able to acquire some (actually a lot of) guiding data.

Good news: the random large errors are gone.

Bad news: the constant drift is still there. This must be the encoder concentricity/swash problem. This can be solved by adding a low-tic absolute encoder (which I already have) to index the PE of the main encoder. Or.. more elegant (and more expensive).. use an absolute encoder and calibrate it once by rotating the mount 360 degrees.

The problem with correcting the PE of the main encoder is, if there is some change in the concentricity of the shaft, the PE will change requiring another calibration.

The only way to (a) lessen the long slow PE; and (b) suppress variations which would force recalibration - is a very robust mechanical encoder mount. It is now clear to me that any shaft-mounted encoder is going to be hostage to mounting tolerances, and no amount of code or signal conditioning can fix that.

I am sick and tired of this approach. Having new encoder housings machined (to replace my wooden thing) would cost me another chunk of change.

I have decided to eat my pride and take the project in a different direction.

Currently you can buy Renishaw LM10 magnetic linear encoders on ebay for $80 a pop. These do the interpolation internally and have 1 micrometer resolution.

The tape is dirt-cheap, $20 for a 300mm length (which is enough for one mount). They will give about 200K to 300K tics for a half-rotation (180 degrees).

Based on the diameter of my RA housing (around which I will wrap the tape) the accuracy of the system is only 3" per tick. But - it is resilient to mounting inaccuracy. The SiTech guys are already using this design successfully.

The cost of tape and the read head is less than it would cost me to have a new encoder housing machined. One potential deal-breaker, the SDE (interpolation error) is as much as 40" peak-to-peak. But repeatability is under 1 tic as long as moving in one direction (which an RA axis does).

Anyway, now for another 2-week wait for the parts.. I will have to throw away all the code I wrote for robustly reading the ADC, doing sine-cosine interpolation, etc. etc. because the Renishaw read head has a digital output.

The good side is... these LM10 magnetic read heads and their corresponding tape are a new low in lows in pricing. I actually have a Heidenhain LIDA and Renishaw RGH optical read head lying around, but the (optical) tape is darn expensive and I can never find any...

So any of you folks who are curious about encoder interpolation, run, don't walk, to ebay and buy the LM10 read head ($80) and tape ($20). I don't know how long they'll last at these prices.

Worst-case, I can't get it to work and you wasted $100. But it's only $100 (as compared to the $400-plus for a rotary encoder).


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Starhawk
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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5943393 - 06/27/13 04:34 PM

What are you going to mount the tape on?

-Rich


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: Starhawk]
      #5944001 - 06/27/13 11:27 PM

the RA axis on the AP600 has a nice circular area right next to the worm. I can attach the self-adhesive tape to this area. The challenge is how to mount the read head.

If this works out I'll see about where to put tape on the CGEM (which arguably would benefit more from such a device).


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