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Old 18-08-2018, 12:23 PM
rthorntn (Richard)
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Adding steppers to EQ3

Hi All,

I want to motorise my EQ3 by adding two NEMA 17 steppers.

I'm a bit confused with all the ratios and the advantages & disadvantages of direct, belt or geared connection, please be gentle!

I think we want R.A. and Dec. to be quick and accurate to slew but R.A. to be slow and accurate when tracking, so it would be great to use microsteps?

What are all the gearing solutions trying to do, slow down or speed up R.A. and Dec.?

Ideally I would directly connect the steppers to the R.A. and Dec. control shafts.

So I need brackets, does anyone sell what I need, just the brackets?

I found this:

https://www.astroeq.co.uk/tutorials.....php?id=ratios

Supposedly the worm gear ratio for R.A. is 130:1 and Dec. is 65:1.

I want to use "uStepper S":

https://www.kickstarter.com/projects...stepper-s-line

It uses the TMC2208 which has a 1/256th microstepping interpolation algorithm.

Thanks for looking!

Cheers
Richard
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Old 18-08-2018, 12:54 PM
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Have a look at onstep by Howard Dutton. Its excellent. A few here, including me, have done it.
https://groups.io/g/onstep
http://www.iceinspace.com.au/forum/s...d.php?t=133677
Onstep is arduino/teensy based and works really nicely by itself via Bluetooth with an android app from your phone. Or you can align with the app then use standard apps like stellarium (on a computer) or skysafari (on phone). Take some time to go through the ONSTEP group and there's a lot of helpful people there.

I used NEMA14 steppers on my EQ5 which is supposed to be pushing it a bit, but it works fine. So I imagine they would be fine with an EQ3. Get 400 step/rev ones if you can get them - I got some on au.ebay and even started with 200 step/rev ones which would be fine for visual (probably). I used GT2 pulleys/belts with 2.5:1 and have moved to 3.75:1 (40:16 teeth and 60:16 teeth). Very easy to build up. I made metal holders and used the standard stepper mounts. Now I've just got a 3D printer so will rebuild the holders with this. As far as stepper drivers the TMC2130s are the best around right now - super quiet and expect 1/32 interpolation. With my gears I'm supposedly getting 0.2arcsec per step.
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Old 18-08-2018, 01:21 PM
rthorntn (Richard)
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Thanks Chris!

Great IIS link.

I had looked at onstep and will probably use that.

Why 400 step/rev?

Why belts, did you explore planetary gearboxes?

What are GT2 pulleys/belts?

3.75:1 that means for one turn of the stepper shaft the control shaft turns 3.75 times, so you are speeding up the steppers, why did you move from 2.5:1 because the slewing was too slow?

You made metal holders, do you have a drawing?

You used standard stepper mounts, which ones?

With the 3D printer versions do you have a design for the mounts?

Did you take a look at uStepper, they have encoders?
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Old 18-08-2018, 01:51 PM
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Hi Richard,



Belts are "quieter" than gearboxes, and cheaper.
400s/rev is to have better resolution (smoother movement).


As to EQ3... have a look at this one.
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Old 18-08-2018, 02:12 PM
rthorntn (Richard)
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Thanks bojan.

With the link you posted, did you post it to illustrate that the EQ3 is not a great candidate for AP?

I also have an EQ6, the EQ3 is for visual, as you mentioned it's very portable, which for me translates into "it gets me out more" .
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Old 18-08-2018, 03:16 PM
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EQ3 is great for wide fields... 200mm FL max.
Eq6 is better.. especially if mounted on pier.


BTW, I had a look at uStepper S....
It is a bit of overkill for our purpose in some respect (it has uP, USB), and not adequate for precise tracking (clock frequency stability - I can't see quartz crystal anywhere, maybe it is not shown).
It is also not h/w or s/w compatible with OnStep, or AstroEQ or similar.


For my EQ6 (belt modified 1:5), I have stepper drivers (Pololu, A4988) and 400s/rev motors inside the mount.

Interface is STEP/DIR (via D15 connector, provides power from mount for controller as well) for both motors so I can use anything to drive it: onSTEP, Bartels, SoundStepper (all controlled with CdC or Stellarium), my own driver (handpad with processor which generates 15ms pulses for tracking and faster pulse streams for slewing).
I intend to do the same thing to my Bresser/LXD75 mount one day.

The whole thing is very low cost, but reasonably high performance in operation.

Last edited by bojan; 18-08-2018 at 03:53 PM.
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Old 18-08-2018, 04:10 PM
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+Belts pulleys much less expensive than steppers with gear boxes. And less backlash.

It's step down for the pulleys to get more accuracy. Work it out. 130 teeth per 360deg. Then 2 or 3:1 for pulleys. Then stepper 400 steps/rev. Then x32 interpolation. I think that's 0.4deg per step. There's a nifty spreadsheet on the onstep site.

I don't have drawings for my current mount. It was suck and see. Depends on the size of pulleys and what belt size you get. It was a fiddle for me as I'm not very mechanical.

I can pm about the steppers. But go for 1A or under as you are limited by the driver tmc2130/2100 or the drv8825 Bojan mentioned. I like the TMCs.

If you go onstep I have a spare MiniPCB (look it up) you can have. That's uses the teensy though and you more prefer arduino ?
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Old 18-08-2018, 04:17 PM
rthorntn (Richard)
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Thanks again bojan!

Great info, uStepper is overkill for sure but I like how it just attaches to the back of a stepper and away you go, being able to attach shields to them is also nice. Can you please elaborate on the potential clock frequency stability and why exactly it matters, I thought I read the uStepper takes in STEP, DIR and ENA signals from a controller, I was planning on using a teensy for onstep?

The clock frequency stability probably won't be an issue for an EQ3.

Thanks again ChrisV, can I please take the MiniPCB, if I do something with uStepper then that won't be until the end of the year, I want to build something now. I already have a teensy.

What about the TMC2208?
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Old 18-08-2018, 04:46 PM
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Quote:
Originally Posted by rthorntn View Post
.... I thought I read the uStepper takes in STEP, DIR and ENA signals from a controller, I was planning on using a teensy for onstep?
....
Yes, uStepper_S takes STEP/DIR but from it's own uP.. which to my understanding uses internal RC clock, not stable enough for our applications. you will have to modify the board in order to provide external signals from teensy or Arduino MEGA), which would then be similar h/w configuration that I am using.


BTW, onSTEP f/w can be configured for Arduino MEGA + RAMPS1.4 (all that available for couple of $$ on ebay, widely used for 3D printers and CNC machinery)

Last edited by bojan; 18-08-2018 at 05:03 PM.
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Old 18-08-2018, 05:50 PM
rthorntn (Richard)
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Thanks bojan.

The image below shows uStepper accepting STEP and DIR from a stepper controller.

https://drive.google.com/file/d/1HdA...w?usp=drivesdk

So planetary gearboxes have backlash. I'm still interested in whether it's feasible to directly connect the stepper shaft to the control shaft (1:1) would it be painfully slow, could increasing the supply voltage (increasing the stepper drivers ability to overcome the back EMF voltage generated by the motor, and hereby increasing the available torque at increasing velocities) speed up the stepper?

Does the TMC5130 look like a good driver?

Last edited by rthorntn; 18-08-2018 at 07:22 PM.
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Old 18-08-2018, 07:44 PM
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Tmc2100 is cheapest of the tmc drivers and would be fine. The 2130 is better but all the eBay ones need a bit of a solder mod - and is more expensive. I've used both - great voltage control to get the most out of your motor.

Teensy is good as has stable oscillator. But some arduinos do also - I think. A few people use ramps etc. Read the onstep wiki. And definitely go through the spreadsheet - helps with determining gears/motors for what accuracy and max goto speed.

I pushed the supply voltage on mine from 12v to 24v. Helped get extra torque.

I bread boarded mine to get it all going. It's a simple circuit though
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Old 18-08-2018, 08:21 PM
rthorntn (Richard)
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Thanks ChrisV.

I think I'm missing something fundamental, I have never done AP so I don't really understand arc-sec that well. OK so there are 3600 arc-secs in a degree.

I found the OnStep Configuration Calculator excel spreadsheet but I don't really understand it. Adding a gearbox ratio seems to improve the tracking resolution but increasing microsteps doesn't, if every step is 0.9degree and I microstep a 256th of that surely that should improve my tracking resolution?

3.75:1 is speeding things up right, how can tracking resolution improve when your 0.9degree turns in to 3.375degrees a step?

I don't think I even understand gearing very well, if the worm gear ratio for R.A. is 130:1 and Dec. is 65:1, R.A. is the important one, what does that actually mean 130:1, 130 rotations of the axis knob moved the axis what amount?

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Old 19-08-2018, 05:31 AM
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Gearbox (belt transmission) improves resolution because it is pretty linear and also increases moment (torque), while microstepping is not linear and positional accuracy depends on load (it is best with no load).

So, steppers driving worm gear directly will work (EQ8) but steppers must have high torque and be of better linearity - therefore more expensive.
However, linearity is not such an issue if guiding is used... which is almost a norm anyway.

For worm gear reduction, 130:1 means 130 turns of worm for one rotation of worm gear.
It also (usually( means worm gear has 130 teeth (if this ratio is not known, you can always count teeth or rotations.. it will always be a whole number)
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Old 22-08-2018, 06:49 AM
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Hi,

I am one of the uStepper developers, and I just wanted to jump into this thread, as I can see there have been some misunderstandings, which I would like to correct

The uStepper (both S and S lite) takes in step/dir/ena signals from any outside controller (as indicated in the picture attached by rthorntn), just like if it was a regular step stick. However, it can also provide these signals from the onboard microprocessor (arduino compatible), or via the SPI/UART bus to the driver chips.

As for the oscillator, the uStepper does NOT use the internal RC oscillator. both versions use a stable crystal oscillator for the clock

whether the uStepper is overkill or not for this particular application is not something i am qualified to have an opinion about, as I don't know anything about the actual application you are discussing. I just wanted to be sure that wrong information about the uStepper is not provided based on false assumptions

Best Regards,
Thomas Olsen
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Old 22-08-2018, 07:30 AM
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Hi Thomas,
Thank you for the clarification - my assumptions was based on what and how is presented on the website, all images seem to be computer graphics (generated by CAD package), and I simply did not see those details (clock, inputs) on them.
Personally, I like this project, it will certainly have many applications.

As to overkill or not for our particular application (full telescope control) - it is, because we need to move two (or 3 in case of ALT-AZ mount) stepper motors at the same time from the same controller running Arduino or whatever firmware (via STEP-DIR inputs), which makes on-board processors on uStepper board somewhat redundant (only stepper driver module on carrier board is required (like this one, similar modules were available on ebay, but not any more), which is probably cheaper and more flexible solution compared uStepper (AstroEQ needs access to /ustep control pins on Pololu modules).


On the other hand, if only simple tracking is required (controlled movement of the stepper motor, only one axis), then uStepper module is adequate.

Last edited by bojan; 22-08-2018 at 09:59 AM.
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Old 22-08-2018, 08:14 AM
rthorntn (Richard)
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Thanks all!

I'm thinking (out loud) about how the encoder feedback loop would work in this application, on a 400 step/rev if we ask UStepper to move 1/64th of a step (0.014) degrees and because of load it doesn't move that amount, what will happen, we wouldn't want it to shudder?

Can you get 400 step/rev Nema 17's with more torque than normal?

I think I want to just directly connect the steppers to the R.A and Dec control shafts, it's simpler and this is just for visual.

I wonder would the master/slave bus for uStepper be useful for this application, the R.A. uStepper could be the master issuing commands to the slave Dec. uStepper when it was time to slew?
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Old 22-08-2018, 09:38 AM
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Richard,

Do not overdesign if you want only visual and no GoTo, 200s/rev and 16 /usteps should be OK as it will give you ~3arcsec resolution (see attached excel spreadsheet.... please note, ustepping is treated the same as "gearbox"). Even crystal accuracy is not the issue.

Of course, 3arcsec is not enough for AP, it will result in star trails, but for visual it should be enough - you will see your object move in small steps ~5 times every second.

If this annoys you, you can increase ustepping, ~ 32 or 64 should be OK


So, if you want to only track, but not necessaarily GoTo etc, uStepper module is OK.
And of course. you can also consider possibly cheaper options - Maybe Digispark board and Pololu module?



My spreadsheet will give you the required pulse rate, and all you have to do is programming of the uStepper_S board.



However, if you want GoTo and use only uStepper_S, as far as programming is concerned, you will be on your own.

If you still want uStepper_S, and if you are happy with not using onboard proceesors (and Arduino MEGA or Teensy running OnSTEP) then you are also OK.
Attached Files
File Type: zip Gear_ratios.zip (17.5 KB, 17 views)

Last edited by bojan; 22-08-2018 at 10:14 AM.
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Old 22-08-2018, 10:45 AM
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As to encoder loops... they are OK because they could prevent skipping steps (which theoretically could happen, if motor is prevented from moving or supply voltage (or current) is *just* a bit too low for motor to operate reliably.
Some mounts use this concept (EQ5? some variations of LDX75 or Vixen).
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