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Rpi DIY mount stepper control upgrade?

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I just got some drv8825 boards to play with but haven't tried them yet. Things are no bigger than a postage stamp!
The new ultra low on resistance power components are TINY. The first time I replaced one at work (industrial repair) I didn't believe the spec sheet. I read somewhere that most steppers will be at max smoothness around 1/10 and that more doesn't really help, but then the paper wasn't trying to do AP on a portable gem. :P
0,15! I imagine slew rates would need to be controlled on a large mount like that but I was aiming for .8 to .5 on mine.
mine is an omni cg-4 which I hear is similar to an eq3. The RA worm is 130t and I was looking at using a 5:1 ratio with 16-32s/s to get under the 1 arc second per step margin.

This stuff takes a while to get in your head but gets easier once you see it all working, in theory anyway. LOL
6 years 8 months ago #17843

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hi! I pesonally also doubt the benefits of 1/128, but taste may differ. i can do 0.5 deg/s on both axes...
the drv get's very hot :/
yours wolfi
6 years 8 months ago #17849

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I'll add my tuppence worth:

Assuming standard bi-polar motors: NEMA stepper motors have two important descriptions you will need to review for your application. 1. The physical size of the mounting face (NEMA 8, 11,17,23,34) all refer to the dimension of the front face. 2. The Force/current rating, which will in part determine the number of coil stacks used in the windings and so defines the height of the motor. (lower the impedance, higher the force, higher current requirement, more windings = bigger motor). Most of the standard motors are designed to 1.8 deg full step, but driven by a micro-stepping unit you can improve the resolution further.

I have chosen NEMA-8 for a telescope focuser, with 0.25Nm with roughly a 50:1 gearing ratio . This tiny (6cm) motor is perfect for the job, but still well over powered/engineered. A stepper motor from a CD player would have just about been enough.


I would expect that NEMA 17 would be about the size (maybe even smaller would do) for your application (assuming you are using worm drive gears), but you might need a higher force depending on your mount loads you are planning.

My 3d printer uses NEMA 17, but others work well on NEMA11.


I'm running NEMA 23, 8Nm for a CNC machine, it has over 300kg of force on the spindle to drive the axes.

You might also consider motors with built-in position decoders - they do exist but only for the large motors - a bit pricey too, and interfacing them to a raspberry pi might prove interesting. This would be the approach I would investigate if I were considering this project.

One final point. I would strongly recommend using a proper stepper motor micro-controller units which is able to drive higher voltages (and I would recommend using 24v - all motors will tolerate this). These controllers avoid burning the steppers out by regulating the current flow allowed into the motor, and so get far more force/power for the motor size (important if size is an issue for you). These controllers also lower the power consumption with idling and current profiling on load, which keeps them cool (a point that wbirk has made), and a good quality one will also opto-isolate the inputs/outputs to protect your equipment from motor back EMF.
I've not used the DRV8825 boards myself, they are going to be much better than and Easy Driver boards, and will profile currents, but I'd be concerned about the heat/size. My 3d printer uses DRV8825, and they have been known to fail under loads for long periods of time. I have sourced drivers off ebay for £5 that are the size of 4 flat stacked of iphone 6s, and seem to me to be more robust and suitable for passive cooling. I would only use DRV8825 with heatsinks stuck in the IC and a small fan running over them.
Last edit: 6 years 8 months ago by Rob Jones.
6 years 8 months ago #17869

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Good info Rob!, Your CNC can almost pick up Wolfi's telescope! :P i'm gonna fry these drv8825's and then worry about transients. LOL The fet h-bridges in the chip have flyback diodes to shunt power to the supply which should prevent most problems.
@wbirk: Do you have that RPI TSC/raspian ISO online somewhere I can get a copy? I just happen to have an Rpi3 laying fallow at the moment after replacing it with my notebook pc and VNC for the indi system and I'm thinking about building a mockup of the upgrade to try things out.
6 years 8 months ago #17874

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hi! i can try to give you a link, but i need an e-mail address. be aware that tsc works with the 1067 phidget board only right now...
yours
wolfi
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6 years 8 months ago #17878

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I messaged email for that TSC Raspian RPI3 image thank you! I am attending an out of town conference early next week but I will take a look at it as soon as possible.

Rob I appreciated your comments about sizing and other considerations for steppers. I have a garage sale LX3 8 inch SCT that I have done a full tear down on. I was able to replace a connector and the power supply and restore it to full electronic functionality. l think it is probably worth keeping it intact as is, given that even old hand controller still works, but I thought while having it torn down might allow me to play with a stepper and see how RPI tracking might work. I suspect something in the NEMA-17 15:1 as recommended might handle the load just fine properly balanced but if I were investing already in a geared stepper maybe a 23 would be good for other experimentation.THe phidgets board I want to have anyway has isolation and safety features.

After attempting to image on ALT/AZ on a wedge for abit I am at that point many people arrive where they feel they want to drop the money for an ATLAS or CGEM or such more robust mount. I frankly would rather use that investment as an opportunity to fashion a custom mount at my fixed dark sky location. Portability is not an issue. I would love to build something hardware wise simiar to this astro.neutral.org/homemade-diy-gem-telescope-mount.shtml using rpi stepper control.

I have access to a motivated and talented machinst who owns a professional shop with astronomy interest but no experience in mount design. Have any of you seen a ATM mount design you love that would support payload similar to the ATLAS or CGEM mounts that I could use as a good starting point to specify my own design? I liked the demo of the TSC GUI and the fact it still operated with Kstars etc.
6 years 8 months ago #17888

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How about building a wedge base for the lx3 to convert it to EQ? You could incorporate it into the pier easy enough. If you do decide to go completely from scratch watch out for bearing play with ball bearings. Doubling up on them or using double block bearings like car axles at each end of each axis use should help, but I think using conicals would be a better fit for the purpose, especially if you nave a machinist handy. - (pining for a 3d printer and flat mill - and a concrete floor to put them on)
On the motor selection front, I walked into the other room at work yesterday and another tech was working on a nema 23 stepper from a large valve control that turned out to have bad driver chips. The 23 would be pretty husky for portable rigs but I wouldn't hesitate to use them on any fixed mount project. The flange is only about 15mm bigger than the nema17. The extra torque availability shuld equate to smoother micro-stepping and better balance and weight handling as Rob mentioned. The encounter moved them to the top of my list when I build "The big one". I'm going with nema17 on my portable though. I was tempted by 11 and even 14 but applied basically the same logic as above through the old engineering addage build at max+50%
I've been posting and gathering intel from a facebook group which has a ton of DIY projects going on you guys might want to check out(shameless plug). It also covers "covers"...lol. Home observatory builds. Search for Ray Wells IV to see all my cg4 related posting.
www.facebook.com/groups/1108595349155243/
Now I'm off to burn up some driver boards! ,,,I mean test them. :D
Cheers!
Ray
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6 years 8 months ago #17899

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hi!
@blueshawk @lynxsky: image is under way, hopefully you can download it, it is 7 GB ... contact me once you have it ...
for the homebuilt mount: an 8" SCT is usually not that heavy a telescope, NEMA 17 should work fine. here is a mount i once built that should be sufficient. features double conic bearings, brass axles and a design with a little extravaganza as it is an inverted fork mount. the worm wheels are pretty small as it is designed for short focal lengths ...
yours
wolfi
6 years 8 months ago #17917
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Lynxsky: I like the equipment in the link, and would love a setup like that. Further thoughts for your consideration:

If space/portability isn't an issue, i would tend to go with the bigger motor; not only does it take away the 'will-it/won't-it worry, but if you can wear the increase costs of power supplies etc, then the motors are better driven well within their working envelope, than on the ragged edges and run the risk of a missed step.
Also, further to a point made earlier in the thread, these motors will get warm if in use at full power to periods of time. An over sized motor would allow you to run them at lower power, but it still has a greater thermal mass to help reduce the effect of any temperature issues.

I would tend to stay-off motor gearing if possible, since using them will increase the backlash of the system. Motors direct onto the worm drives are probably best.

Also, although the majority of stepper motors are 1.8degs step, that are others. On ebay you can source 0.9degs for similar prices. This will double your resolution without degrading backlash.

Final thoughts on motor drivers: Using micro stepping will improve your resolution, but it is associated with a few issues:
1. The stepper motors are designed to step forward one complete step (say 1.8 degs). Using microstepping progressively energises the coils in graduated sequence to 'fight one another' to hold the angle of movement between the designed steps that motor has. As soon as you de-energise the motor, the motor will snap back to a 1.8deg integral angle.
2. you loose torque using micro stepping - so re-enforces the case for over-sizing.
3. Obvious really, but gives a much smoother angular rotation with less stepper vibrations, which (unless using a geared system) might be material issue in the system you design.
4. Higher the micro-steps means your microprocessor needs to work harder. I played around with Raspberry Pi systems, with driver code written in Python (being interpreted) will struggle to operate high slewing rates - i've tried 128 microsteps, and rotational limits were imposed by software (900MHz processors!). Ardinuo systems (8MHz) quite evidently would struggle. So: use multi-core-RPi-3 (not Zeros) and consider compiling your code!
Last edit: 6 years 8 months ago by Rob Jones.
6 years 8 months ago #17920

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hi!
rob, which os on the pi did you use? with raspian, all the joy of having such fast clock cycle is limited to the time slice allocated to the process running the stepper :)

as far as microcontrollers are concerned, you are righ, the 16 -25 MHz of most arduinos are an issue. Teensies are faster, i think.
yours
wolfi
6 years 8 months ago #17924

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Thanks for that Wolfi, ii'll check it out. Brass with conical bearings! Nice!

Rob, I looked at a .9 degree 400 step motor but the torque was quite a bit lower than similar nema17 200 step motors and it was still equivalent to a 2:1 which wasn't enough to get my rig into the sub arc second step range. I also thought about using a 400s with a 3:1 timing belt set I found to get to 6:1 which might be enough to make up for the lower current/torque specs.
I'll add to the microstepping vs snap back note that the RA track doesn't snap to index because it never stops, or shuts off in normal operation, but plan on holding current on the DEC axis as well even though it is usually stopped during tracking. Sleeping a peripheral stepper like a focuser could be problematic as well, though the gearhead on the one i'm working on should mask any snapping that goes on...probably in the massive lash it has.. Whaddya want for 5$ LOL they're originally little valve control units making them ideal for an open loop low torque slow operation like focus.
With full DIY systems, you have the "luxury" of choosing a large worm gear(check those prices !0.0!), but portable units or crossovers like mine that use slow motion axis worms on an existing mount don't have that option so we're stuck with some type of geartrain to make up the difference. The timing belt systems are a good alternative to the planetary, which is reported to have 1 degree backlash(the loose bit during direction changes).
I suggest dropping out of micro-step mode when slewing
in order to keep from overtaxing the processor or heating the drivers with tons of pwm and inductive skew,. I've been studying these DRV8825 modules and they have 3 input lines (8 modes) which can be tied to Arduino or gpio outputs to change microstepping modes.
Many of the stepper drivers have them in some arrangement making it pretty easy to drop into single step mode when you start slewing. Be sure to change the position return data accordingly if you try this. i.e. a slew of 2000 at 32:1 would need to become 2000/32 if you drop to single..or something like that. :)

This may not be quite on topic, but i'll include it for reporting aobut the prospective use of the new driver boards. I put together a prototype focuser on the on the bench using a 24BYJ-48 stepper and an Arduino nano for a test of the new boards but it will be a bit before I get to any real testing because I'm rewriting the motor section of an existing Indilib compatible focusing program that currently uses the out of date AF_motor library...while tiptoeing around a nested accelstepper library mostly out of curiosity...I'm apparently a glutton for programming punishment. :-:woohoo: D
6 years 8 months ago #17938

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hi!
- well, the planetary does add backlash, but that can be compensated for; also a drive belt/pulley combination may have some backlash, but this is less predictable, I would assume.
- in guiding mode, the RA drive may stop, but it does not invert direction usually, which minimizes backlash problems. denote also that, unless, the drive is disabled (on the DRV8825, this is the first pin on the left hand side) the motor does not snap back. it heats up. however, a worm wheel is usually self locking (i hope that is the right word, it is a literal translation from german).
- changing microstepping mode for GoTo might be a good idea. the phidget 1067 cannot do that and it is not necessary as its microcontroller is fast enough to provide sufficient rotation speeds. other solution (for instance rDuinoScope) that rely on DRV 8825 realize their ramps however by increasing step size.
- I have a pcb here for driving two steppers with an arduino mini pro and DRV 8825/RAPS 128 breakout boards, if anyone is interested. primary control is via SPI ...
yours
wolfi
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6 years 8 months ago #17942

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