Ok, this is about a project - and it's about ENCODERS!

[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 a silly question: 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

[Neale]

Michele - I'm wondering why you are looking at linear encoders and not rotating? I would have assumed that you want to record the position of rotating joints. Rotary encoders with, say, 10K pulses per rev are readily available but without knowing the geometry of your telescope, I'm not sure how linear and rotating resolution translate.

[skround]

Hi Neale, very good question - you’re right, what I need to detect is the angular position of an axis.

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.

There are 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).

I’d love to salvage an encoder like that but I think it’s not realistic.

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.

If you want to have an idea of how the arrangement looks like I'm posting pics and vids here:

Instagram: #800mm_telescope
Facebook https://www.facebook.com/800mm.telescope
YOUTUBE https://www.youtube.com/channel/UCC4...IGmvUwCV9ASGUQ

[AndyUK]

Well, yes, but in that application most encoders would be geared so you wouldn't need to buy one with enough ticks for a 1:1 ratio.

Also look at multiple potentiometer systems. We have a few devices that use course single turn pots in combination with fine multi turn pots. By correlating the signal between the two you can end up with a very sensitive position sensor that also has reference to an absolute position.

[hanermo2]

Adv:
Forget the linears.

Use timing belts like HTD3 or GT3 with a big head and a small end, like 9:1 ratio.
Someone can cnc drill you a large circular head for little money.
From a hydraulic pipe end or similar, in a production machine shop.

50.000 count encoders are not expensive, and 2M counts seem around a few hundred €.
The best cnc cut bit belt end will still have errors in linearity and accuracy.
So what.
Software map them out, because the repeatability is 100x or more better than the accuracy.
All based on what I have seen others do.


*You* cannot make the system *accurate* to a high level.
It will bend and flex if changing direction.
But You *can* most definitely get extremely accurate incremental positioning for this use.
0.01 - 0.001 degrees angular increments are certainly possible - depending on your mechanicals.

Your system could get 100x more accurate in angular resolution via double crossed geartrains, preloaded, error-mapped, using ac servos with feedback.
Some potential private systems could make it fast in tracking.