TDM vs Autoguider?

[AndrewJ]

Gday Mark

Andrew the EQ6 is a Chinese copy of the EM200 so I imagine fitting encoders to that mount would not be too difficult. The encoders on the EM200 are outside of the case directly attached to the worms (see pic below).

I assume the EM200 has extended shafts on the worm itself????
My EQ6 has no free section of worm axle to easily hook anything onto, and as the new unit would have to provide feedback via guide commands, i would still need to make a software black box to drive it.

With the Meades, its simpler to hack into the standard processing loops of the firmware, ie semi seamlessly replace the current encoder with the new one, but it still leaves a problem of how to deal with backlash in an "elegant manner". Ive been looking at how that works in the Meades for several years, and it is not a trivial exercise.
I guess if it was simple, it would have been done by now.

I still like the precision of a direct drive motor with an absolute encoder on the output axle as being pretty close to optimum.
Just hope the prices come down with time and scale of manufacture,
and all this conjecture will become moot.

Andrew

[marki]

Originally Posted by AndrewJ

Gday Mark


I assume the EM200 has extended shafts on the worm itself????
My EQ6 has no free section of worm axle to easily hook anything onto, and as the new unit would have to provide feedback via guide commands, i would still need to make a software black box to drive it.

I still like the precision of a direct drive motor with an absolute encoder on the output axle as being pretty close to optimum.
Just hope the prices come down with time and scale of manufacture,
and all this conjecture will become moot.

Andrew

Yes the worms are slightly longer but the encoders slip inside the worm and are fixed with a grub screw. Would it be possible to drill a hole in the centre of the EQ6 worms and do the same? Most likely I would think, wouldn't prove too difficult. Now the black box, well that's a different story unless you worked out a way to wire them back into the circuit board internally and made corrections in the software to compensate for the slower rotation.

Yes direct drive is the future no doubt.

Mark

[AndrewJ]

Gday Mark

Would it be possible to drill a hole in the centre of the EQ6 worms and do the same?

Possibly, but to get good precision of the bored hole would require
an accurate setup, plus modifying the preload endplugs.
Ie probably not worth the effort on the EQ6.

Andrew

[Poita (Peter)]

I'm having a brainfart, how would the encoder on the worm work?

[AndrewJ]

Gday Peter

I'm having a brainfart, how would the encoder on the worm work?

Same as per usual, but the position of the encoder determines how much "guessing" the system has to do.
There are also 2 scenarios here
a) Use an external Hi accuracy encoder to give tracking corrections via the guider port.
b) Use a mid accuracy encoder on the worm to directly replace the feed from the existing encoder on the motor.

I am familiar with the Meades ( which already use encoders ) so would choose option b) and explain it the following way.
With an LX200, the encoder is mounted to the motor.
There is then a 50:1 gearbox then a 180:1 worm drive.
Assuming we had an absolute encoder on the final axis,
to get say 1 arcsec resolution, it needs 1,296,000 counts per rev
If we put the encoder on the worm, it needs 7200 counts per rev
If its on the motor, we only need 144 counts per rev.
If you put the encoder on the output axle, then if it moves, the axis is moving, ie there are no backlash or PE errors between the encoder and the OTA.
If its on the worm, ( and the worm is springloaded so always in contact ), there is virtually no backlash, but there is still the worms PE.
If its on the motor, you have all the lash and PE in the gearbox plus the PE in the worm.
When tracking, the system needs to know the PE, and when guiding ( esp in DEC ), the lash needs to be constant ( and its not )
The Meade LX200 gearbox requires three revs of the output axle before all the gears resynchronise ( hence the three turn PEC model they use )
The LX90s etc require 128 turns or the worm for the gearbox to resynch.
As such "modelling" PEC when it includes a gearbox like this is a non trivial problem, esp if the gearbox PE is large relative to the worms PE.
An encoder on the output axle is optimum, but horribly expensive ( at present ).
A mid range encoder on the worm is a much cheaper proposition, but still needs a PEC model to be used to get good tracking. However, as the encoder is on the worm now, no PE effects from the gearbox contaminate the model, hence an accurate model can be generated.

So its just looking at it on a cost benefit analysis of what method gives the best bang for the buck.

Andrew

[marki]

Originally Posted by AndrewJ

Gday Mark



Possibly, but to get good precision of the bored hole would require
an accurate setup, plus modifying the preload endplugs.
Ie probably not worth the effort on the EQ6.

Andrew

Andrew I think it would actually be pretty straight forward mechanically. Simply drill a 3.5 mm hole in the desired end and thread m4 then make up a sleeve to suit the encoder shaft with a m4 male thread on one end. You would not need to alter the worm block at all. Now the software that's another matter entirely.

Mark

[AndrewJ]

Gday Mark

Simply drill a 3.5 mm hole in the desired end and thread m4 then make up a sleeve to suit

Sounds simple in practice, but to drill "a perfectly concentric" hole into the end of an existing worm, without any runout is not a simple exercise. And i make this comment based on the tolerances we are talking about.
At the precision we are talking about, getting the encoder mounted without any eccentricity is a critical requirement to getting repeatable results.
My lathe, even with a collet fixing probably isnt up to this

Andrew

[marki]

Originally Posted by AndrewJ

Gday Mark



Sounds simple in practice, but to drill "a perfectly concentric" hole into the end of an existing worm, without any runout is not a simple exercise. And i make this comment based on the tolerances we are talking about.
At the precision we are talking about, getting the encoder mounted without any eccentricity is a critical requirement to getting repeatable results.
My lathe, even with a collet fixing probably isnt up to this

Andrew

Andrew, the tolerance you have stated is fine for the worm and worm wheel but are not necessary when mounting encoders. For example when I run the EM200 at full slew you can plainly see the encoders oscillating around the central axis as a result of them being fixed to the worm by grub screws. My EM 200 delivers +/- 2arc sec PE without PEC (you don't have a choice on the PEC, there isn't any nor does it need it) and when properly polar aligned drops the target smack bang in the middle. I build miniature engines with my lathe and mill and my latest project, a V8 required me to drill the camshaft hole 100mm long with a max tolerance of 1 thou run out over the length. With careful setup and patient drilling with a sharp drill and lots of coolant I got the job done. Mounting an encoder would not require anywhere near this accuracy and the hole would at most only need to be 5mm deep. I base this statement on using similar encoders to those found on the EM200 which are basically glorified volume pots. You may have a different system in mind.


Mark