Telescope Drive Master - The Truth of the Matter
Posted 21 September 2012 - 01:09 PM
Also quadrature is already a form of (digital) interpolation. I do not think 4096X interpolation would give very good results.
Best to find an analog (voltage mode, not current mode) encoder like the Heidenhain, with 5000 tics, and built an analog interpolation module like the TDM folks did. You can find 5000 tic analog encoders on ebay for $200 range.
Posted 21 September 2012 - 01:53 PM
use that in the Arduino ADC to get 1024 samples from 0-5V.
That would cover a half cycle or 2048 samples per cycle.
Each sample could generate a pulse maintaining quadrature
and then with a simple quadrature circuit x4.
At least this is what I was thinking for the moment!
Posted 22 September 2012 - 02:35 AM
I get what you are thinking - use two A/D converter channels to get the 2 sinusoids. Now you have A and B, take the arc-tangent which then gives you the angle between the two encoder ticks.
You probably don't even need to up-sample to 0-5V, 0-1V is probably OK but the resolution is less.
I have calculated with a 5000-ppr encoder, if you do 256X interpolation (8-bit) that will give you 1280000 counts/rev, which is pretty close to 1" per rev.
Since the sidereal rate is 15.04" / second, with 1280000 counts/rev you will get basically one tick per second. Should be enough, since then you can do 1 correction per second.
Posted 27 March 2013 - 04:53 PM
A lookup table is a good way to do trig quickly, if you have the memory. You need tangents for 0 to 45 degrees, then do some jiggery pokery to get this to 360 degrees.
You have given me an idea, thank you! I'm not sure the Arduino has enough processor power for that... but can the Arduino compute arc-tangents quickly?
Posted 27 March 2013 - 07:23 PM
The AVR math library can calculate an arc-tangent in 5700 clock cycles (!!) which is about 0.4 ms. That's what am doing..
Posted 27 March 2013 - 08:38 PM
Posted 28 March 2013 - 12:40 AM
Posted 28 March 2013 - 03:51 AM
Here's my time budget so far...
1) reading the A, B, and N channels 16 times - takes 13-15 milliseconds
2) calculating the arc-tangent - takes <1 millisecond
I only aim to do corrections every 100ms (that's 10X per second) so as can be seen I still have lots of CPU time left over. Futzing around with the flash, SPI ROM, etc. is simply not worth it.
Posted 29 March 2013 - 06:28 AM
Units in deci-arcseconds. So that's between 0.8" and 1.2" p-p and 0.23" RMS PE. On a CGEM. With the following parts count:
1x Arduino Uno
1x MCP3304 AD converter
1x ACPL 847 optocoupler
miscellaneous wires and a breadboard
1x Baumer-Sinus Thalheim ITD 42 encoder
The encoder is by far the most expensive component at over $700, but the equivalent Heidenhain ERN480-5000 is $325. All the other parts are probably less than $50.
Posted 27 December 2015 - 02:56 PM
Your point is a good one. However, $1000 is a $1000. I can only cite Paul Burke's success with his Gurley encoder, 20 minutes unguided at 2400mm focal length, as an example. That's pretty good performance for an $850 investment plus his time to make an adapter. Also, the adapter for the TDM is extra, so it's $1800+.
David - you say "I don't know what it would require to adapt a Gurley to a Gemini powered system, but it would be a lot less money than the TDM."
in the other thread, the 500K tick Gurley encoder R158S05000Q5L25AN18SP04EN is $850. I'm not sure that's comparable to the TDM encoder which is >1M ticks (note also the AP3600 encoder is 1.5M ticks).
And at $850 for the encoder alone (vs $1800 for the TDM) I wouldn't say it's "much less" if the encoders are not comparable.
At any rate, it comes down to what we really need to accomplish. Like I said, my MI250 is a good enough mount that guiding takes care of the minor errors. I'd have to think long and hard whether or not to spend the money for incremental improvement.
For those of us that don't have sitech, you'd need that. gemini2 won't support it any time soon. Downside of buying a product updated by one guy whose getting old.
The part I can't figure out is how you mount the encoder to the mi-250. Does the cap at the end remove some how? I see that the TDM often screws on or uses some side of pressure related screw to mount the encoder shaft.
I asked atilla awhile ago if he'd make an adapter and never got back to me.
Speaking of, any of your Mi 250 owners wanna do a group buy on RA extensions? I'd like one and imagine it'd be cheaper that way. Feel free to come over to the yahoo group or PM me. Maybe we could figure out the encoder thing too.