Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

Hi! No personal experience, just hearsay from an experienced friend:
- unhardened, stainless steel
- bearings should be massive from my gut feeling :)
Yours wolfi

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

hi!
well, I learned a lot in the past 9 months, especially as this stepper controller stuff popped up in my academic work when working with a lung dosimetry phantom (i am a medical physicist), and TSC became the spin-off of that stuff. so yes, there is of course more than the first order approximation (heat vs. motion) as we do not have 100% efficacy. However, in my application it is solved by disabling the drive (this is a software command for the phidgets or a logical i/o for the drv 8815) when it is not needed. the current regulation thing sounds attractive at first sight, but my experience is that the correct current is the one where the stepper operates smooth under load without loosing steps. i never saw a reason (heating or insufficient precision at higher speeds) to tinker with current byond that. too muc current simply causes rough operation and overheating. so far for the constant output transformer - it has it's sweet spot for the load and voltage given ...

friction drives - i know a few ones here in Austria - funny enough we have quite a number of rather big fork mounted ATM instruments here, the reason might be a detailed plan that is circulating for the past 30 - 40 years called the "Austrian Precision Mount" which was designed by the head technician of Vienna University Observatory, Mr. Pressberger. Having built a fork mounted 60" RC himself, quite a number of people have made instruments in the 35-70 cm aperture range following his concepts. my houghton is one of the smaller instruments, but it is very heavy because of its front mounted corrector. my friend richard gierlinger has driven a 70 cm telescope with a friction drive - he says they are easy to control, but the roller and disk have to be made of stainless steel, and they must not be hardened. that's it, more, or less ....
yours
wolfi

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

Hi! I simply think that - with constant power output, you can either put some power into motion ... force*travel/time ... or all into heat ...
Yours wolfi

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

hi!
one more note: being a simple physicist and not an electronics engineer, I usually take for granted what my colleagues from EE explain to me, start to think about some general physics rhubarb why it makes sense and beyond that, I simply believe them. i did the same thing with the statement "a stepper motor at rest is integrating" ... especially as it somehow was my observation that the enabled stepper at rest gets hot whereas the moving stepper does this not to the same degree.

here is the explanation - www.geckodrive.com/support/step-motor-basics.html ... "constant output power transducer" says it all :D
yours
wolfi

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

hi!
- heating of the chip never was an issue with the 1067, as opposed to other drivers. I assume they dissipate the load in a clever fashion :D.
- 1/16 microsteps are fixed with that board.

I have to say that i am quite happy with the 1067, my only concern is the size (little bit smaller than a Eurp PCB) and the price (100USD per board).
yours
wolfi

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

here is a link - www.sparkfun.com/products/13752 ...
yours
wolfi

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

hi rob!
1.) well, heating is an issue, mostly when the drive is NOT running. BUT - it only occurs if the drive is enabled/powered up. this can be controlled both for the DRV 8825/RAPS 128 and the phidget 1067 - and the drives should be disabled when they are not running as long as you don't need their holding force. the bigger issue is the heating of the driver chips. the phidget board solves this in an elegant fashion, the DRV and RAPS need massive heaters. i doubt whether this problem on the chip has a positive effect on the life expectation of those things ... i never had problems with hot motors when driving the telescope, but i had hot drives on the bench when not running them :)
2.) form my experience, the only power that you can run the system on is the current that allows for smooth operation under the given load. this is easier with the phidget controller as you can control the current via software. for the DRV / RAPS, this requires turning a little pot ...
3.) you NEED motor controllers with interfaces because you need a microcontroller to drive motor as a realtime application. i remember the interfaces with RS232 when i was a young guy, we drove them with a microVAX! fortunately those days are over ... BUT the phidget 1067 is controlled via usb, this is how TSC works. there is an interesting driver that already has a microcontroller on chip and can be directly addressed via spi - STMicro L6470 dSPIN, and there is a breakout board from spark fun. i made a pcb that features an arduino mini pro and 2 drivers and also has connectors for controlling the arduino via spi ...
4.) well, you may have seen my video on that thread - this is 350 kg moving at a rate of ~ 0.7°/per sec in two axes. the motors are set to 1.5 and 1.7 A, and the are powered by 12 V - let us assume a power consumption of ~20W per drive. No heating problems on the drive. Big heating problems with boards other than the phidgets as the thermo protection switch on the driver chip drops with in seconds if you don't take additional measures (cooling bodies, ventilator...)
yours
wolfi

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

hi rob!
indeed, the pi zero has only one core. as soon as one starts more than one thread. one is in trouble. On TSC, I run each routine that monitors a stepper in a separate thread. Therefore the program consumes 3 cores when in goto mode. I assume that in Python, you have something similar to the Concurrent Thread mechanism of Qt.
But my question was directed to the operating system. On Bare Bones Raspian, you will always lose steps as long as you do not take any special measures. You lose them at any point in time when the driver expects a digital input but the OS scheduler assigns a another process at that point in time ...
yours
wolfi

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

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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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

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

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

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

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

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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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

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

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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 months ago

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

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

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

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