Topics

Encoders

Matthewabey@...
 

I tried to decipher the different posts on encoders and the wiki. I see the encoder signals are sent to the wifi module? I'm in the middle of upgrading my first onstep computer to the latest max. If I put an encoder on the RA only like a few main stream mounts do, will that make my RA closed loop with correction from the encoder? I'm not running a wifi module, I will be running the W5500 Ethernet instead. Full V2 MAX pcb with a teensy 3.6.

Very sorry if this has been covered.

Howard Dutton
 
Edited

On Sat, Jun 29, 2019 at 06:20 AM, <Matthewabey@...> wrote:
I tried to decipher the different posts on encoders and the wiki.

I see the encoder signals are sent to the wifi module?

Yes.

 

I'm in the middle of upgrading my first onstep computer to the latest max. If I put an encoder on the RA only like a few main stream mounts do, will that make my RA closed loop with correction from the encoder?
There is experimental code for rate control.  This is a pet side project of mine that hasn't reached its conclusion yet.  Additionally only CW/CCW encoders are supported... I wouldn't want to even attempt A/B encoder support for this without a high res. encoder to work with so it probably won't work for you.  Perhaps you could get around the issue with h/w that converts A/B to CW/CCW?  And then the RA axis encoder would need to have some serious specs.  The one I've been experimenting with is a Gruley 8235S with 10,000 lines and 4x A/B and another 5x interpolation (200,000 count.)

I'm not running a wifi module, I will be running the W5500 Ethernet instead. Full V2 MAX pcb with a teensy 3.6.
In theory encoders should work with the W5500/Teensy3.2...  I've tested this but only for pointing not rate control.

Take a look at this Topic to see what's been done...
https://groups.io/g/onstep/topic/26142320#5810

Matthewabey@...
 

Thanks Howard. This mount already has encoder mounts on it with a 4 to 1 reduction setup. It was for a ngc max from back in the day. So very easy for me to install an encoder on the RA, I can be the guinea pig with the Ethernet teensy setup.

Two questions will it work with only a RA encoder or do I need a Dec and RA Encoder. And where can I buy that 10,000 line encoder. I looked it up and didn't see any for sale. I would rather have the same encoder you are running.

Matthewabey@...
 

With the reduction and effective 200,000 count encoder. I would get 1 count every 2.16 arc seconds.

Matthewabey@...
 

Is your encoder on your RA a Sinusoidal, Pulse, or Square wave output. I'm trying to figure out a comparable Gurley encoder to your's that will work. I finally found a data sheet on the 8235s, it looks to be no longer in production.


Howard Dutton
 

Mine has CW/CCW output, an eBay find at ~ $100.  I think they are still made (I see them on their site just called "835" since there are 8135 and 8235. 1 or 2 read heads.)

Good, you seem to know not just any 8235 will do as there are a wide variety.  I see two used 8235's on eBay with 10800 line discs, not sure about the other outputs.  I toy with the idea of pulling the electronics out of mine to get to the sinusoidal signal(s?) so I could do more data processing on it to get higher resolution.

Sitech has a similar thing and their users often go with the Gurley R158 I believe.

Again this is very likely to not work in OnStep - experimental.

Matthewabey@...
 

Agreed chopping the Sinusoidal wave form up, I think would be very hard with an arduino but would add almost as much resolution as you could want theoretically. But the build variance and the peak to peak variance i'm sure varies from physical tick to tick would limit this.

So i'm assuming your encoders are out putting a square wave? 

Butchf
 

Howard is the expert but I wanted to pass on that I got US Digital 10,000 CPR quadrature encoders directly from US Digital for about $100.00 each about a month ago. So 10,000 Cycle Per Revolution Quadrature encoders gives you 40,000 Pulses Per Revolution.  if you have a 4:1 reduction setup that would be 160,000 PPR.  Not sure if this helps.

Howard Dutton
 
Edited

On Sat, Jun 29, 2019 at 11:37 AM, <Matthewabey@...> wrote:
Agreed chopping the Sinusoidal wave form up, I think would be very hard with an arduino but would add almost as much resolution as you could want theoretically. But the build variance and the peak to peak variance i'm sure varies from physical tick to tick would limit this.
I have hope since the top end 8235 configuration comes close to the R158 specs (perhaps better depending on how you look at it) and I know the R158 works w/Sitech.

So i'm assuming your encoders are out putting a square wave
Nope, pulse.

Here's the 835 info.
https://www.gurley.com/s/835S-bndr.pdf

Butchf
 

wow really impressive specs.   

Howard Dutton
 

On Sat, Jun 29, 2019 at 02:35 PM, Butchf wrote:
wow really impressive specs. 
Not bad for what I paid for it but I've yet to see another one (w/similar configuration) again online for near that price.  And I check every once in a while.

Matthewabey@...
 

Yeah $600 for the one on Ebay. How big are these things they look pretty big in the pictures.

Howard Dutton
 

Yes, pretty big.  I think the R158 is smaller.

https://groups.io/g/onstep/message/5629

Matthewabey@...
 

Wow that is alot bigger than I thought it would be. I'm starting to think of just putting A/B 10000 line encoders for now just for the pointing accuracy. Then see how it goes the mount tracks really well and I've never done a PEC chart. So I'll try doing pec and once it's all together with the new onstep max.

Thanks for the info Howard.

Thomas_Jacobson
 

An interesting alternative to the usual optical and capacitive shaft encoders are the recent Hall based 14 bit position sensors from AMS. They are low cost and small, easy to mount, and provide both incremental and absolute position via ABI and/or SPI. See:

https://ams.com/as5047u#tab/features

 

Digikey stocks them, including the eval kit that costs $17. See:

https://www.digikey.com/product-detail/en/ams/AS5047P-TS_EK_AB/AS5047P-TS_EK_AB-ND/5452344

 

I have been testing it using an uno board and the fairly simple/clean step/dir to closed loop servo package called “deservo” that has interactive console control of the servo P,I, and D parameters and position, and a simple tuning graph. See:

https://github.com/misan/dcservo

 

I just glued the magnet on the end of one of the motors I am using as part of my Hydrogen line radio telescope project, and mounted the eval board 1mm behind that. Seems to work so far.

 

It might be interesting to explore using them in ABI incremental mode with the OnStep WiFi approach, and then migrate to the direct AMS SPI interface and absolute.... With SPI you might be able to avoid the need for the WiFi assist.

 

Khalid Baheyeldin
 

The Hall Effect Encoders were mentioned on this list previously.

Maybe be useful to refer to that discussion, here.

Khalid Baheyeldin
 

On Thu, Jul 4, 2019 at 03:18 PM, Thomas Jacobson wrote:
part of my Hydrogen line radio telescope project
The 16bit resolution means a resolution of ~ 19 arc seconds (360 X 60 X 60 / 2^16).

This is more than adequate for radio astronomy, given the 21 cm wavelength of the Hydrogen line.

But for optical (400 to 700 nanometers), it is too coarse for tracking corrections. It is useful for pointing at most.

George Cushing
 

Where is OnStep as a simple simple interface for encoders? Say I want to just replace my Losmandy 492 board with a ST-4 enabled OnStep PCB, using my Losmandy handbox and encoders. Can OnStep read the encoders and produce RA and Dec output? 

Matthewabey@...
 

I'm pretty sure that drive is Hurst steppers with no encoders. I converted one of these digital drive mounts a few months ago. There are alot of files Howard has made for motor mounts and instructions for the g11 / g8 and they work great. I would grab 2 400 step nema 17 motors, Howard's drawing for motor mounts and pickup a mini or max pcb he has designed and start soldering. Encoders are not necessary with steppers. I was even able to put the onstep in the original digital drive case with a 3d printed cover.

Matthewabey@...
 

If you are looking for a max pcb v2.1 I have a few from a recent order.