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Rpi DIY mount stepper control upgrade?

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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 cooling bodies. 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.) from my experience, the only current 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.) when using a non - RTOS like Raspian, 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 DRV-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
Last edit: 6 years 8 months ago by Wolfgang Birkfellner.
6 years 8 months ago #18005

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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.
6 years 8 months ago #18007

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Wow, that dSPIN unit is a whole load of features for £8!

Take your point of energised non-moving motors being the issue, I would have assumed that the speeds of side-real tracking would present very similar heating problems - but I defer to your experience :)
6 years 8 months ago #18008

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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
6 years 8 months ago #18010

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No eBay APP ID and/or Cert ID defined in Kunena configurationThe site won't let me post the ebay link - its too long.
All i did was search on "L6470 Stepper Driver Breakout 3A 8-45V" on the Uk ebay site, and loads of them popped up.
Chinese imports for arround £7

Last edit: 6 years 8 months ago by Rob Jones. Reason: add link
6 years 8 months ago #18011

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6 years 8 months ago #18012

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Great stuff to listen in on guys! That 350 KG video and the implied near degree per second movement at that weight is awesome to ponder, given the stingy load limits and performance of off the shelf mounts and what they cost to get even decent AP performance.

I have used perhaps a dozen different phidget products in non-astronomical projects so the 1067 "feels" right in addition to liking the usb programmatic control and the safety features. WBIRK I bet there would be general interest for all 1067 users in a well designed heat sink for that board. I am assuming that if the stepper is driving a worm then back EMF and torque holding issues are not the issues they would otherwise be.

I can see some limited advantages in the .9 degree 400 step NEMA 23 motor in an attempt to avoid or limit microstepping, but I would think I would still need a planetary gearing and couldn't go direct at 400 steps even with 360 teeth. Although I can brute force torque, the smoothness I need is only going to come from stepping/planetary ratios (I think). Doesn't the Phidgets 1067 automatically implement stepping through microsteps anyway?

Assuming you wanted to create a mount to drive a 20-30 KG load and didn't mind a slow slew rate, is there any hope for avoiding a planetary ratio in your stepper for RA tracking? Direct drive mounts are such an illusive goal in general, but even direct stepper drive of a 360 tooth 12 inch worm would seem illusive given the desire for arc-seconds smoothness regardless of the motor size or choice. I am going to either do planetary/microstepping or both.

BTW, I have a Phidgets servo motor controller and a nice high torque CR servo with metal gears.. I have a good python program to control rotation and acceleration (use it on a lesser CR servo for focus control)..how would a high end hobby servo do for mount control I wonder.
6 years 8 months ago #18013

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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
6 years 8 months ago #18022

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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
6 years 8 months ago #18023

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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

The torque curves on that page are pretty neat!
Last edit: 6 years 8 months ago by Ray Wells.
6 years 8 months ago #18034

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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
6 years 8 months ago #18042

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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.
I should add that I don't see them all that often in manufacturing, mostly brushless DC(really ac servo) and ac servos these days.
Last edit: 6 years 8 months ago by Ray Wells. Reason: because time speeds up as you age.
6 years 8 months ago #18061

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