Thread: SMM moco

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    #21
    Senior Member Jim Arthurs's Avatar
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    Quote Originally Posted by morgan_moore View Post
    [I]
    I was trying to shoot 200mm macro and vibration/backlash are problematic and have wasted my whole week.. trying to shoot a box content for a national brand.. a bit scary as I delivered late.

    I was in a box of hell.. too slow and the motors vibrated, too fast and the physical system vibrated causeing jello.
    So, what little info and advice I can give is at least first hand advice, so here we go...

    First, take any of the cute shiny Syrp, Edlkrome, and similar "moco" rigs and slap a 200mm lens on them and watch the same happen. For real fun, get one of those "one man crew" Redrock units and try and shoot at telephoto... they all fail. There's a reason you never see anything other than wide angle shots in those slick promo videos they all put out.

    So, what to do? First on the gearing. The BEST gearing boxes for this work are wave motors from the original creator of the concept, Harmonic Drives. Zero backlash. They are expensive, but sometimes can be found on ebay. Here's one of three I bought for pan/tilt/roll...

    http://files.datausa.com/imageshoppe...RMONIC_003.jpg

    They also sell "very low" backlash planetary motors that I can personally vouch for. I've got two GIANT Harmonic Drive branded units I bought off ebay for the crane and boom axes of a 4' span crane. Here's a test I did with the planetary that covers these two issues, the backlash and focal length... 120 frames to cover both 1/2 and 1 degree of travel with smooth in and out, with a return to "home"...

    http://files.datausa.com/imageshoppe...fdegreePan.mp4

    Next up, slower framerates for shooting the telephoto stuff. Take it down to 3 or 6fps instead of real time. Even the mighty Dykstraflex I linked to earlier shakes like a drunk at happy hour when moving fast. If you're running into frequencies with your stepper motors when moving slow that seem to cause vibration it's time to upgrade your motor drivers. There are basically three categories of stepper drivers; small circuit board based drivers like you'll find on CNC cutters and 3D printers, the mid level US10-15 dollar Chinese drivers that seem better because they're laid out with heatsinks and break-outs (they are not better), and finally, micro-processing controlled drivers like Leadshine or the Geko brands. They are your ticket to motors running smoothly at different rotational speeds as some can profile and learn your motor to provide the optimal voltage and amperage for any task. Here are representational links for all three types...

    Circuit board drivers...

    https://www.sparkfun.com/products/12859 (useful for testing and small projects)

    Chinese mid-level drivers... (cosmetically they look like digital drivers, but they are not! Stay away from these)

    https://www.ebay.com/itm/Single-TB66...IAAOSwYxBZ6jW7

    Digital drivers... (this is what you want for Nema 17 when using low voltage micro-controllers for signal generation...

    https://www.omc-stepperonline.com/di...-Stepper-Motor

    Finally, a word on effective micro-stepping as this can be an issue when moving slow and working telephoto can show off the issue. Most stepper motors are rated to 5% positional accuracy per step, meaning than when you rotate the stepper, it will only fall on the position you want + or - 5% of that steps angular value. It's not cumulative, it doesn't add up, it just means that when you land, each and every position has inherent slop of that amount. With a 200 step motor, than means that you can only be accurate to .09 degrees of motion. Microstepping is a method of slewing the rotor between magnet positions in a fractional fashion with the benefit of giving you more motor positions in 360 degrees, and delivering a smoother motion experience at the expense of some torque. All well and good, except that some motor drivers advertise crazy amounts of microstepping that enter into that +/- 5% inaccuracy range. Let's look at 1/16th microstepping, where each of the motor's 200 real steps are broken into 16 addressable positions. The math on that is .1125 degrees per 1/16th microstep... getting VERY close to the .09 degree of the +/- 5% positional uncertainty. Any greater microstepping than 1/16th is simply too fine a division for positional accuracy. Just something to keep in mind when fiddling with the jumpers.

    Regards,
    Jim Arthurs


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    #22
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    Jim
    That is some useful hints. Thank you.

    Interestingly my best motor/geardown made by my mate 'mr accurate' looks very similar to the the harmonic drive in configuration

    I just need another one.. but Boing poached him

    I use the TB6600 drivers and grbl.


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    #23
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    Quote Originally Posted by ReelFilmGeek View Post
    Right now I'm trying to found out the best way to attach the stepper motor shaft to some 8020 Aluminum Extrusions so I can rotate them. The big issue is finding a solid mounting solution but I have a feeling I will need to get something custom made for it. Though been on the road for a few shoots lately and haven't had as much time as I would like to play with the setup.
    All my 'extrusions' are solid aluminium with press fit steel shaft (another work of Mr Accurate)

    But I dont have extrusions I just have an aluminium cube with bolt holes then I can strap on..

    camera
    long extrusion
    circular shooting table.


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    #24
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    Reel

    I dont know if you can get this but my rotating part is held by bearings and the motor/cog are on a seperate shaft.

    arm1.jpg

    arm2.jpg

    The pink and blue plates move along slots in the main component allowing tensioninng of the belt.
    The bottom of the shaft has a bearing not shown
    the belt goes to a large pulley wheel (cog) , not show on the shaft
    You will note that the gearing can be adjusted with a gears change to taste.

    I doubt that 'direct drive' will work
    Last edited by morgan_moore; 07-14-2019 at 11:40 AM.


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    #25
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    my tilt arm (not all parts)

    https://a360.co/2XJqEUp


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    Im prepping for an up and over you can see a long section bolted onto the 'block' on the main usit body - which is mounted sideways on the truss.

    https://www.instagram.com/p/Bz6IBRdA..._web_copy_link


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    #27
    Senior Member Jim Arthurs's Avatar
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    Quote Originally Posted by morgan_moore View Post
    I use the TB6600 drivers and grbl.
    Morgan, I believe most if not all your issues in regards to vibrations and drive harmonics will go away with the digital motor drivers. At least get one for your more troublesome axis. It's actually rather amazing to see a stepper motor whose performance you're familiar with under an inexpensive driver suddenly "go quiet" and behave nicely when using the better drivers.

    Best of luck, it all looks like fun!
    Jim Arthurs


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    #28
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    $23 - it has to be well worth a try

    BTW are you aware or GRBL - could that be the culprit.

    At even, fast speeds it is good, but for creeping in come the buzzes and wobbles.


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    #29
    Senior Member Jim Arthurs's Avatar
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    At the end of the day it's all just step and direction signals flowing out to the drivers, it's what the drivers do with power management that is the key. You'll see improvements in motor response even just running a simple speed ramp from an Arduino using a simple sketch with the AccelStepper library, etc. Yes, they are more expensive than the motors, but worth every penny. Stepper motors are amazing, but they are simple and "dumb" devices, digital drivers are the "smarts" needed to get great performance from them.

    Regards,
    Jim Arthurs


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    Quote Originally Posted by Jim Arthurs View Post
    Digital drivers... (this is what you want for Nema 17 when using low voltage micro-controllers for signal generation...

    https://www.omc-stepperonline.com/di...-Stepper-Motor

    Regards,
    The link doesnt work.. could it be this?

    https://www.omc-stepperonline.com/di...-Stepper-Motor


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