[skround]

Hi everybody! Newbie here in the forum faithfully in need of some advice.

Myself and some fellow astronomy club members are building a 0.8m telescope for an observatory.
For tracking accuracy encoders are key to detect the position of the telescope mount - you can think of that as a 2-axis machine.

Now the accuracy to take nice pics of the sky -and to do some science with that- translates into a linear accuracy of an incremental encoder of 0.3micron or better.

There are 2 main round surfaces that rotate with the axis and they are/will be ground to -hopefully- less than 0.01mm run-out.
The idea is to attach linear scales to these rotating surfaces and utilize an encoder.
The challenge is to do that on a budget i.e. buying used components.

For example I just bought a used RGH22 – it was too cheap not to buy it and just having a closer look at that.
To start with silly questions: can a Renishaw encoder work with a different linear scale as specified?

Again on linear scales – can it be salvaged from machinery? Is it as ‘expensive’ as the encoder or it’s the cheap part of the equation? I would probably need in excess of 1m for each axis. On ebay it looks like you can find encoders for less than £100 but there aren’t many scales…

Btw the motor control sys is already there and has convenient phase A & B inputs.

Thanks in advance!
Michele

[peteeng]

Hi SKR - why use linear in a rotary application? Better to get some rotary instruments? Peter So if you need 0.003mm and you are aiming at 0.01mm ground your not going to be happy? Grinding surfaces is usually spec'ed in the 0.001mm range you can machine something to 0.01mm grinding is usually better... Peter

[Jim Dawson]

Normally the scale tape is the inexpensive part. I have been buying the magnetic scale tape at about US$35/meter. Not sure what the optical scale costs.

I think the scale tape is a one use because of the sticky backing. I don't think you would be able to reuse it after removing.

I do have to say that accurate rotary positioning is an order of magnitude more difficult than liner positioning in an application like yours.

Renishaw does have rotary read heads.

[hanermo]

I don´t see the problem.
Use small cheap timing belts like HTD3 or similar to link your axis rotation to a common cheap rotary encoder.

A CUV102 encoder is about 40€ for 10.000 counts, not so accurate or precise, but cheap and cheerful.

After that, 100x better accuracy and precision is available for not too much money.
Using same timing belt drives, perhaps with a high precision belt coupler.
And very high rez encoders.

A slow-speed encoder readout is 50€, digital, to 25 kHz.

You can get resolution, (not accuray) to about 0.1 degrees for 200€,
0.01 for 800€,
0.001 for 4000€.
Costs, if selling I would expect about 3x + 5k for a pack.

[skround]

Hi SKR - why use linear in a rotary application? Better to get some rotary instruments? Peter So if you need 0.003mm and you are aiming at 0.01mm ground your not going to be happy? Grinding surfaces is usually spec'ed in the 0.001mm range you can machine something to 0.01mm grinding is usually better... Peter

Hi Peter, you're referring to the accuracy of the surface that I'm planning to attach the linear scale to 0.01mm run-out should be enough to not mess up with the readhead - need to check Renishaw datasheet to confirm that

[skround]

Normally the scale tape is the inexpensive part. I have been buying the magnetic scale tape at about US$35/meter. Not sure what the optical scale costs.

I think the scale tape is a one use because of the sticky backing. I don't think you would be able to reuse it after removing.

I do have to say that accurate rotary positioning is an order of magnitude more difficult than liner positioning in an application like yours.

Renishaw does have rotary read heads.

Hi Jim -
The challenge is the level of resolution – target is to get 10 ticks – or pulses- each arcsecond which means 36,000 ticks each degree.
As a consequence a full revolution of an axis would call for a 36,000 x 360 = ca.12 million ticks rotary encoder.

There are rotary encoders that can go that high but they are top of the bunch and I suppose pretty rare and expensive (I guess 1,000/2,000£ easily, maybe much more).

On the other end I can fit a linear scale to a 1200mm diameter ground surface that is solid to the axis and read with a read-head.

Read-heads seems fairly attainable on ebay ( like less than 100$/£ for a RGH22/RGH24Series).

I hope you are right that linear scales are the inexpensive part of the system but I seem not to find any of them – the optical ones with 20/40microns gratings, at least.

I suppose that If they are usually taped into their Alu enclosing it should be viable to dissolve the glue with some solvent – maybe – but if that’s a cheap way to get them I’d give it a good try.


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[skround]

I don´t see the problem.
Use small cheap timing belts like HTD3 or similar to link your axis rotation to a common cheap rotary encoder.

A CUV102 encoder is about 40€ for 10.000 counts, not so accurate or precise, but cheap and cheerful.

After that, 100x better accuracy and precision is available for not too much money.
Using same timing belt drives, perhaps with a high precision belt coupler.
And very high rez encoders.

A slow-speed encoder readout is 50€, digital, to 25 kHz.

You can get resolution, (not accuray) to about 0.1 degrees for 200€,
0.01 for 800€,
0.001 for 4000€.
Costs, if selling I would expect about 3x + 5k for a pack.

Hi Hanermo, there is basically already such a system and it's an encoder on the motor shaft. But that's not enough and the telescope needs a true direct angular position with no backlash or slop.

As per the previous post I need a resolution of 0.000002 degree.

If I can pull together a system with just a used 100/150$ encoder + scale for 100$ it would do for me.

Now I just can't see to find a used optical linear scale anywhere - it's weird because ebay is full of encoders