Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 1 hour 9 minutes 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 hours 6 minutes 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. yesterday

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. yesterday

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. 3 days 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. 4 days 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. 5 days 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. 5 days 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. 7 days 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. 1 week 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. 1 week 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. 1 week 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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Wolfgang Birkfellner replied to the topic 'Rpi DIY mount stepper control upgrade?' in the forum. 2 weeks ago

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 ( www.youtube.com/watch?v=effn5Dxg43g ) 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 ... www.youtube.com/watch?v=fHCLndB8eG0 ... 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

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... made a little video - youtu.be/effn5Dxg43g ... mainly about slewing my big phat scope :)
yours
wolfi

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