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.


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.

What is the current state of your 2 motor stepper control library-- seems an issue with the latest commit?

I am interested in using the phidgets 1067 boards to control two stepping motors for RA and DEC control. I have used phidgets quite a bit for varios projects including the 1061 servo motor controller and prefer to work in python just because I am more familiar. Unfortunately, I only have 180 tooth worm drive in an old Meade LX3 to work with for my experiment. However, I have disassembled and the condition of the drive and worm is excellent and was well machined at the time.

I was thinking of swapping the 1/8 rpm motor with a Stepper and giving it a try to see what kind of results I might get For deep sky tracking, this drive would need to advance 1 tooth every 480 seconds with 2.5 steps per second without microstepping (based on 1.8 degree step angle?) Obviously, I am a complete newb to this but very attracted to the idea of stepper control. The 1067 micro stepping of 16 (while coarse by professional standards) would give me 40 steps per second theoretical? which would seem to limit vibration. Can you recommend a specific NEMA motor and a barebones strategy for a newbie approaching this task (assuming to start...DEC LOCK, one 1067 phidges controller and a raspberry pi 3 or windows machine available?? Thank you for any information.

hi!
are you referring to my project - TSC - tscatm.wordpress.com/? well, the current status is that the controller part is quite complete, it works with skysafari, cdc, kstars and stellarium. it does not yet fully work with ASCOM as this is not relaible. in other words -the advanced ascom LX200 driver works sometimes, which is kind of frustrating. there was an issue with a windows virus on the repository, but this was clarified. what is not yet working is the autoguider. there is activity towards using cheaper stepper controllers for smaller scopes, but this is not my main goal right now. hope to bring this to life in the next few weeks/months. but actually, 2 of my sons are leaving our house and we have ongoing movin/piantin/flat reconstruction work going on all the time.

the 1/16 microsteps of the phidgets are a problem for very small gear ratios. however, with my 288 teeth wormwheel and a 1:9 planetary gear, i do not see any issues with that. the phdigets can drive steppers down to 70 microsteps/ sec (=7.9° / sec) without any visible effect...

in case you missed it - here is a little video that demonstrates TSC () alongside with a little GoTo action using skysafari.
and - here is a video that shows 15 minutes of following sirius in the prime focus of my 33cm/f/4 ... ... one sees PEC but that is it ...

if you wnat to give it a try, i will provide you with a sd-card running the system.

yours
wolfi

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.

Ray

Sorry for the delay in my response. Thank you both for your helpful replies. Wolfi, I would love an SD card image to experiment with-- are you speaking of an SD image suitable for Raspberry Pi 3 Hardware? I need to order the appropriate stepper and the phidgets controller board.. A recommendation for a good stepper motor choice would be helpful as well as any sourcing recommendation for a planetary gear. I would just love to be pointed in the right direction sourcing wise. I am stuck with a 180 tooth gear RA only drive for now but i am looking for additional Worm drives to play with. Blueshawk thanks for the Gear ratio info and I'll peek at your solution as well.

hi!
yes, it is about 8 GB, and you can directly dd it on a 16 GB SDcard, or i will send you a card for the cost of the card+postage. what do you prefer? this runs raspian directly on a raspi3 model B.

and if you have 180 teeth, you should go for a 1:15 planetary and a bipolar stepper. how heavy is your mount? the drivers are two phidget 1067 boards ...
yours
wolfi

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.
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.

hi!
the spreadsheet looks reasonable. with my setup (288 teeth worm wheel, 1:9 planetary, 1.8° per step, 1/16 microsteps) i have a theroetical resolution of 0.15 " ...
however, there is a version int making that uses lower currents and standard drivers that support 1/32 or 1/128 microsteps. still it is to be said that the phdiget boards are more sophisticated than the DRV 8825 or the RAPS 128, which are tested right now. i have no experience, how these work on the telescope, on my desk, they do ...
yours
wolfi

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.
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

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

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.

Good info Rob!, Your CNC can almost pick up Wolfi's telescope! 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.