Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 4 days ago

Yeah. That's the first thing Mark said, and it satisfies the law of conservation of energy and Occam's razor, being the simplest solution. "The energy used to create motion is subtracted from the heat produced", meaning that when it is turning the shaft some of the energy is converted to motion instead of heat, but it doesn't really solve all the variables involved or jibe with the fixed current (P=I2R), which should lower power output at standstill due to the loss of inductive reactance (XL=2piFL) ...and I Never did like that Occam guy. :P
He also mentioned that every stepper he has seen has drawn dramatically more current at standstill than while running if not current regulated. This led to us doing what we do all too often in repair, trying to guess from a distance what is wrong with something, which is sometimes helpful. In this case we both agree that your phidgets might not be limiting current correctly at zero or possibly set too high and running in voltage mode. You might try setting the limit lower while at zero (or possibly in tracking mode?), since they draw less current while moving they should still be okay at speed. Of course, if the heat isn't damaging, there's no need to change anything.

Interestingly, I've done repairs on industrial stepper drives for decades without actually knowing that much about the quirky little motor. This thread has been very educational.

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 4 days ago

Let's see if I can get this right... In my own words and hopefully avoiding jargon, a stepper holding a position has one or a set of coils energized and holds it so the power is constant and duty cycle 100% on that coil, where as a stepper in motion is changing coils and direction of current flow, making a 50% duty cycle and spreading out the heat load. Also, you would think the lower impedance of the coil at rest should make it roll back the output on its own in order to maintain the current setpoint. So why does it still heat up? Flux driven Hysteresis buildup in the core from the DC applied to the coil without switching. When running, the current flow changes direction every step which cancels out any residual magnetic flux/eddy current/ hysteresis, while at rest the core effectively gets a solid shot of direct current and magnetizes which creates eddy currents in the core and lots of heat...I think. :D
A tech that works with me is actually better at physics and theory, after 20+ years working together I still routinely bounce theory off him. I'll ask if I got it right when he comes over this evening. 8-D

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 6 days ago

Wow, got a link to that one Rob?
I've seen spi enabled units (dspin) but they're usually more expensive. Case in point www.sparkfun.com/products/13752

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 6 days ago

Score one for the Phidget!, The ability to control current remotely means you can determine needed holding current, often less than running current - especially vs. slewing, and turn the thing down when parked or possibly even use PEC (periodic error correction) or full position feedback from encoders to autotune for best minimum results for each mode. You would want to run it as a tuning process and then lock it though, I've seen tuning loops do some crazy stuff.

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 1 week ago

I fear clogged roads and crowds at the dome, I want to go there but I also don't want to have to watch from the roadside in a survival mode transportation disaster area(take the lightweight camera gear and a bike?). Oddly enough I can actually make knoxville in about the same time, but then it's another hour to get in shadow from there.
Quick response to the stepper question. The nema23/400/265 is quite a bit stronger than normal and 400steps gets you to 2:1 over a direct single stepping motor so you can half the microstep number in the spreadsheet. If you can figure out the count on that wheel and plug it into the worm slot in the spreadsheet, then you can see the results of different gear arrangements and levels of microstepping. It's big enough that the 400s might just be enough to go direct...did I already write that? I looked for the ratio of the lx3 last night but that photo was all I could come up with fighting sleep. Might be a good tihng to add to the list if we do find out, lx200 as well. knowing that is a big part of calculating needed gearings.

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 1 week ago

The Smokies! Are you in the shadow of the eclipse? I'm planning a trip in that general direction next month. Either to Clingman's done or a "secret" rest stop near Anderson sc. either is about 4 hours one way for me. It's going to be a very long day.
There has been much back and forth about which is better but I think all of them have merit. I've also heard from some folks with larger systems that they have had good results from large servo motors with encoder feedback, but that's a different thread entirely.
That nema 23 400 step motor looks nice. Too big for my small rig but on a larger one like that lx3 it may work out nicely. When you start getting into torque figures things branch out quickly due to all the choices out there. with a well balanced rig, the stepper basically needs to be able to overcome the stiction(static friction) of the bearings and the of the worm gear/hob, or the bearings and the gearhead, or the mount bushings and the belts...etc. In larger units inertia comes more into play as well.
The higher powered 1.7amp nema17 200 step motor is rated at 51 oz-in holding power, the 400step nema17 that I found (sadly) had the lighter 300ma coils and only around 25 oz-in ..[figures and rambling erased]
so in short, yeah...that monster should have plenty of pull for the big guy. Looking at that spreadsheet I posted might give you an idea of what speeds you can get depending on your final ratio. You might want to count the teeth on the existing gear in the lx3 to get some idea what ratio you need to get it to the right speed, and if this photo is any indication, you only need a couple to get things going. That's a line frequency(60hz) synchro timer with a gearhead in that thing. :P You might be able to read the rotational speed of that motor if you have it out. RPM is usually on the side of the gear section.
Remounting the yoke on an equatorial angle(wedge) will take the rotation out of your photos. I think big yokes are Superior to gem mounts. No meridian flip is a big factor. This is a photo of one on a wedge mount. Your machinist could probably make you something for it, just a thought.
www.cloudynights.com/topic/497182-the-re...ic-–-meade-2080-lx3/ Getting somthing on the DEC axis might be a bit harder.

Hope all my late night rambling helps somehow. :D

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 1 week ago

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

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 weeks ago

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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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 weeks ago

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.

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 weeks ago

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

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Ray Wells replied to the topic 'Gphoto command structure, vs. the new Pentax pslr updates.' in the forum. 2 weeks ago

I recently helped get the Pentax k50 working in gphoto2 by suggesting that they reintegrate pktriggercord then testing a zillion commits for MMiesner. Gphoto's pentax driver and pktriggercord were both based on pkremote and those earlier efforts. you might want to try compiling from git source to see if your camera is now listed. you can also start an issue there if your camera is not yet listed and it would be very helpful to offer to help test. Marcus is very enthusiastic and great to work with. The docs page was still out of date last I looked and many of the newer cameras are now at least partly functional at the command line.
Hope this helps!
github.com/gphoto/libgphoto2
As for my problem i'm pretty sure it's something in the indi-driver, or i'm not holding my head right. I already started comparing code to try to find the whacky bit.

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 weeks ago

I tested my share link and realized it need explaining. To use the worksheet you need to either download the .ods file and use a spreadsheet or open it in google sheets and then you can change the gear ratios and see calculated output right away. I found it especially useful for balancing planetary gearheads and or belt/pulley ratios against my fixed wormgear(130) at various microstep ratios. Also be aware that torque drops as you increase steps/per step which could lead to stalling when you hang that SBIG camera on the back. lol

Everyone please feel free to check my math on this. Any errors found would be appreciated. Expect some argument though, especially late on a Saturday night so be ready to show your work. :P
I based the targets on getting a sub 1 arc second per step ratio, which theoretically should not show up on sensors due to atmospheric limits. That said, aiming low is better for AP, and getting a fair median of around .5 seems like a good plan for observational use where goto may need to be faster to avoid sleep. :D

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Ray Wells created a new topic ' Gphoto command structure, vs. the new Pentax pslr updates.' in the forum. 2 weeks ago

Hi guys,
I've been working on my motor systems for a while and just turned back to the camera. Now that Pentax k-50 is working in Gphoto2 at the command line I waa wondering if anyone has tried with the indilib driver yet. I'm currently getting some errors that appear to be related to the enumerator - the bit that translates commands for each model. The K-50 and other later models have different numbers there. Before I start hacking up a k-50 version of I thought it might be good if I asked around first to see if anyone else is working on it. I haven't sent them to Marcus at Gphoto yet since the command line and most of the driver working, it just fails to trigger the shutter

Here's the errors:
Driver indi_gphoto_ccd: pentax/pslr.c:1560:scsi_write(fd, cmd, sizeof (cmd), 0, 0) failed: 2
2017-07-11T15:08:58: Driver indi_gphoto_ccd: pentax/pslr.c:1216:command(p->fd, 0, 8, 0) failed: 2
2017-07-11T15:08:58: Driver indi_gphoto_ccd: pentax/pslr.c:484:ipslr_status_full(p, &p->status) failed: 2

If there's a fix that doesn't require forking the driver that would be great too.
I'm familiar with pslr.c but not the way the indi driver interacts with it. am I right that it is calling it as a class library? Also - Still not a programmer so try to understand my ignorance when appropriate. :)

ps. fixed my mount sync problem. Next up goto and meridian flip(fear)

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 weeks ago

On the gear ratio front, I cooked up this gear ratio spreadsheet using a similar one I found online. It also has some mount's shaft ratios listed. Could be handy for anyone in the design stages.
drive.google.com/file/d/0B7RyjEB940IOMm9...VjA/view?usp=sharing

As for my system, I've pushed back the upgrades till after the eclipse and have been chasing down other bugs getting ready for the big trip.
Besides Wolfi's extensive work(cool vids) I also found out that the Onstep guys, who started about when I did, have put together some very nice looking code for arduino. It's much easier to read than the "classes" warfare I inherited.

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Ray Wells replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 3 months ago

Voltage differences in interfacing aren't a big deal as long as they are current limited and age old standard practice is to avoid fanout issues by operating pull down (logic low) on things anyway. As for the motor regenerative issue, the h-bridge used in most controllers isolates from that pretty well, but the optos are a good idea anyway as they allow you to drive with isolated supplies, which keeps the cpu supply more stable...see L293 or L298N for a good example. It's starting to sound like the Arduino is already ahead of the game at the control end if the RPI gpio can't reliably output pwm to make the sign/cosign needed for microstepping. Hope none of that comes off as snarky, as it's not my intention. I'm on the fence and any discussion is wonderfully helpful. :D


I could leave most of my hardware as is if I reuse the arduino and if there's no control stability to be gained by making the change to the pi then i'm inclined to take the familiar path and just rewrite for that, using the arduino as an lx200 emulator and leaving the pi in place as the indi/ccd comm hub. The dspin is on my mind at this point in either case, if I can find a dip one or a driver board for cheap it'll be even better, pretty much making the hardware swap to steppers a snap to recode.

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