Hi Ralf,

After thinking about the idea of controlling the Dec directly from a mount encoder (no servo encoder) and Dan's comment about the problem of backlash, I think any backlash will be a big problem because during any backlash, the servo motor would be running free (no feedback) and probably trip the protection in SiTech that detects a loss of the encoder signal.  Also during any backlash motion, there would be no way to control the speed of the servo.  I think the only way Astrophysics is able to run with mount encoders is that they eliminate all backlash through precision design.

With your backlash, I just don't see mount encoders able to control Dec.

Don W

Ralf,

Can you take your DEC worm gear on a lathe, cut out the center, then press in a bushing and cut it back to correct specifications?

Russ

Hello Ralf,
  A few thoughts if you want to save some money:

If you are willing to not have full 360 degree rotation in DEC:
On eBay you will find RGH41 sin/cos analog read heads for $25.  Someone is selling a Parker MX80L-T04  linear motor for $250 that includes the Renishaw REE 2000x interpolator (you only need the interpolator and not the stage, perhaps you could negotiate with the seller).  Finally, Renishaw RGS40-PC 40um period gold grating scales in 400mm length are available for $150.  You will also need an index mark magnet.

If you need 360 degree rotation:
Someone is selling an Aerotech ADRT 150 stage on eBay.  Internally, these stages includes the RESM20USA075 ring with Signum analog sin/cos readhead. You could extract the ring and the readhead from the ADRT and mount it on your shafts.  You'll still need the REE2000 interpolator in this case. The index marks are part of the ring.

Good luck!
Jan

Hi Ralf,

Using a mount encoder on Dec with Cascade mode should help a lot.  The accuracy of that encoder determines the quality of the result.  Don't confuse accuracy with resolution.  Dan has said that Cascade Mode requires (or is best) with around 10 million tics/rev.  That resolves about 1/8 arc-sec, but the accuracy of SiTech ability to control the servo is at least 3 times that (assuming the error can be held to ±1 tic).  So more tic/rev should be "better".  However more tics is also a "diminishing return" compared to your original problem of Dec Backlash.  If your backlash is tens of arc-sec, then the difference between 1/8 and 1/80 arc-sec is moot.  Either would be a great improvement.

I personally do not have any experience with Renishaw interpolated encoders.  My experience is with the Gurley 320K encoders (made in Russia) that have been used on SiTech mounts since about 2008.  The interpolation scheme may not be the same as Renishaw, but the Gurley encoders had a definite and very repeatable error when adding 64 or 100 tics to the 5,000 tic optical encoder.  Dan developed a software scheme to measure that error and compensate for the error which is invoked using the IPE in SiTechexe.  This is exactly why I don't fully trust interpolation.  But I have been and continue to use my Gurley on RA, which is a definite improvement in tracking accuracy for me.

So what ever encoder you decide to use, please be sure to let us know how well it works for you.

Don W

I agree with Don..... If SiTech can position within a "tick", and I believe it can based of my SiTech focuser, than the positioning will be no better than the servo motor tick count.  Again I say, what good is a lot more or less ticks on the mount encoder if the servo motor can't get there, especially when stationary.  Perfect would be the same number of ticks on both the mount and motor encoder, but we work with what we can get.......or close to it.  That's the way I see it.  What is your servomotor tick count? Maybe your can up that too, I did.

Good luck my friend!
Russ

Hi to all!
Mounted the scope encoder on DE. Seems to work. But where on hell can I set the ticks per rev. The only thing I found in Servo Config:
EDIT PARAMETERS - Get Data from Controler - Auto Tracking - Numer of Altitude Scope Encoder Tick Per Scope Rev.
But I cant change the value.... Must be 32.000.000 Ticks, but after ENTER, switching back to the old value....

Thanks Ralf

Hi Ralf,

Go to ServoConfig.exe and edit parameters.  Then under Auto Tracking you can add the tics/rev for the Azimuth Scope encoder (RA) and the Altitude scope encoder (DEC).

Don W

Hi Ralf,

Be sure to save the new tics settings to your controller, both send to controller and flash memory.

Don W

Ralf,

Try this:
 

All commands must be in CAPS.

Open Servo Config and select the correct COM port.

Click Terminal Window

A window should pop up. Make sure you are communicating with the controller by pressing ENTER on the keyboard.  A string should appear in the format: X# Y# XZ# YZ# XC# YC# V# T# X[AM] T[AM]

For Alt/Dec mount encoder type: XXT32000000 and press ENTER

Type XXT and press ENTER, you should see XXT32000000

To save configuration, type XW and press ENTER.

Good Luck!
Russel R.

Hi Don,
thats exactly what I did, but I cant edit any value in this section: after pressing ENTER the value always switches back to the old value (= ticks/rev of DE Motor encoder). I cant remember what I did to set the value for the RA Scope encoder, this value is right there. I have to try terminal comands...
Ralf

Hi,
thanks to Russell: it's so easy if you know how to do it. These Renishaw encoders are simply great: precise and simple setup. When the axis is at rest, the encoder stands like a one, nothing jumps, at most 1 digit and that at 36,000,000 ticks / rev !!!

Yes, these encoders are pretty awesome.  Now that you have the hi-rez Renishaws installed and running, you can check errors in Controller Stuff.
Most likely you will use cascade mode. In cascade mode, you want to set the backlash to "0", otherwise the Dec axis could oscillate.
Congratulations on building the mount, now.............ENJOY the fruits of your labor!

Clear skies your way
Russell