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Equipment Discussions >> Mounts

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Mert
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Encoder Resolution new [Re: orlyandico]
      #5436232 - 09/23/12 02:16 PM

For really fast ADC, it would be best to use a Flash ADC
chip.
For example the MAX1114 can do 150Msps conversion rate.
Then just use an interrupt to notice the data ready and
read the data.
I think I'll go this route ( still waiting for prices on
encoders )


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Encoder Resolution new [Re: Mert]
      #5436865 - 09/23/12 07:56 PM

Mert, I chickened out and got a Heidenhain IBV 660.

Up to 400X interpolation. Input is 1Vpp sine/cosine, output is digital quadrature. Cost a hundred bucks. Although I believe they are $600+ new. And if it's broken, I'd be back where I started.


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Mert
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Encoder Resolution new [Re: orlyandico]
      #5438527 - 09/24/12 05:44 PM

Sounds good, I hope for you that it works like it
should!
Did you get it on Ebay??
Please let us know how you do with it!

Now that I read the product spec, what is
that input frequency they state for the interpolation
to work? 0.78KHz is the lowest frequency I see in
the table.
How many cycles does your encoder have?? Remember the RA
axis only rotates at a rate of 15 arc-second per second!


Edited by Mert (09/24/12 05:53 PM)


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orlyandico
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Re: Encoder Resolution new [Re: Mert]
      #5438709 - 09/24/12 08:12 PM

no 0.78 kHz is the maximum speed when interpolating at that rate... (400X) it's not the minimum.

anyway will see.. if the IBV 660 ends up being broken or a China fake (it came from an HK seller) I'll do the usual paypal complaint. And would then end up needing to use an Arduino to read the Baumer encoder...


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Mert
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Encoder Resolution new [Re: orlyandico]
      #5439979 - 09/25/12 02:19 PM

I see, that's interesting news!
Which reference of the Baumer encoder do you have??


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Encoder Resolution new [Re: Mert]
      #5440498 - 09/25/12 07:38 PM

I downloaded the data sheet from their web site. Fairly standard, 1Vpp which is the more recent Heidenhain "standard". Some older H. Interpolation boxes like the EXE series use the older 11uApp standard, we don't want that...

Basically it's not a matter of choosing the right encoder, but rather trawling ebay for the cheapest possible analog encoder and then working back from that..


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orlyandico
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Reged: 08/10/09

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Re: Encoder Resolution *DELETED* new [Re: Mert]
      #5440501 - 09/25/12 07:40 PM

Post deleted by orlyandico

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Mert
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Encoder Resolution new [Re: orlyandico]
      #5441762 - 09/26/12 02:19 PM

I just got a request out for a 5000 cycle sin/cos
encoder, but it must be expensive I fear!
The Heidenhahn was quit expensive compared to others,
so right now I'm checking cheaper alternatives.
The interpolation scheme will depend on the strategy
to follow, we'll see.

[edit]Typo, wasn't 10.000 but 5.000 cycles

Edited by Mert (09/26/12 02:26 PM)


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orlyandico
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Re: Encoder Resolution new [Re: Mert]
      #5442272 - 09/26/12 07:45 PM

Mert, yes they are expensive, $800 range. EBay is your friend. :-). I saw a Heidenhain ERN180 for $160. But I got the Baumer for $100. New the cheapest is Gurley (used in the SiTech).

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Mert
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Encoder Resolution new [Re: orlyandico]
      #5443497 - 09/27/12 01:08 PM

Just got a quote for a new Tamagawa 2048 sin/cos encoder
which is very apealing to me, eventhough they ask 220€
plus shipment ( another 60€ !! ).
I'm looking at some external hardware to hookup between
the arduino and the encoder to speed things up.
With a modest 864x interpolation I'll have 8.640.000
pulses ( quadratured ) which should be enough I believe.
Some interesting Flash-ADC chip does the conversion in
only 1.18usec! ( 8 bit only ), so using 2 of them
for sin and cos.
I'm still thinking which way apeals most to me to setup
the whole thing.


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Encoder Resolution new [Re: Mert]
      #5443664 - 09/27/12 02:51 PM

864X interpolation isn't modest!

the key issue is that any form of interpolation will be inaccurate. So the lower the interpolation, the better.

with a 2048-ppr and 8-bit interpolation (256X) you should get 524288 tics per rev, or 2.47" per tic, or 6 tics per second. I think that should be enough...

I am planning to run my 5000-ppr encoder at 200X only = 1 million tics per rev, 1.296" per tic.


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Mert
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Encoder Resolution new [Re: orlyandico]
      #5443746 - 09/27/12 03:54 PM

Are you using 1 channel or 2?? ( sin and cos output
available )


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Mert
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Reged: 08/31/05

Loc: Spain, Pamplona
Re: Encoder Resolution new [Re: Mert]
      #5445406 - 09/28/12 03:24 PM

Hi Orlyandico,

Thinking it over again, with a 5000 cycle encoder:
Sidereal day has aprox. 86164.091 seconds.
That means 1 turn of the encoder in a sidereal day gives
us 0.058028814 cycles per second.
Doing 259x interpolation on 1 channel would give us
aprox. 15.0294628 pulses or ticks, which equates quit
nicely to the sidereal rate of the mounts.
Accumulated error in 1min and 1 second would be
aprox. 0.7716" which can be compensated for.
360º = 1296000 seconds
5000*259 = 1295000 pulses So we will miss 1000 pulses
each 360º with this scheme.
Looks like it isn't too hard to get there.


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orlyandico
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Loc: Singapore
Re: Encoder Resolution new [Re: Mert]
      #5445884 - 09/28/12 09:41 PM

I think if we can do about 10 tics per second that is adequate.

Since there are 1296000" per sidereal day and 86164 seconds, that is 15.04" per second.

To get 10 tics per second (which means we can update the RA speed 5x / second - much better than any autoguider!) we would need a resolution of 1.5" per tic. Which translates to 861K tics. The 5000ppr analog encoder, with 4X digital quadrature and 50X analog interpolation, would be 1 million tics. That would be sufficient.

So even a 2048ppr analog encoder would do.


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brave_ulysses
super member


Reged: 04/19/09

Loc: far outside the wire
Re: Encoder Resolution new [Re: orlyandico]
      #5460804 - 10/08/12 03:10 PM

here are some prices from the us renishaw distributor:

A-9523-6051 RGS20-S scale 510mm long
Price $ 197.00/ea

9531-0342 – epoxy - stock
Price $ 5.00

A-9523-4015 – end clamps – stock
Price $ 20.00

A-9531-0239 – scale applicator – stock
Price $ 58.00


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Encoder Resolution new [Re: brave_ulysses]
      #5461169 - 10/08/12 07:47 PM

given that there's a Heidenhain ERN 180 on ebay right now for $157 USD, and you can buy the encoder for 220 EUR new... I'm not so hyped up about tape that costs $200.

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