Jim Arthurs
Well-known member
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/outgoing/motion/HARMONIC/HARMONIC_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/outgoing/motion/HARMONIC/HARMONIC_drive_halfdegreePan.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-TB6...356233&hash=item1a7d3b7225:g:9IIAAOSwYxBZ6jW7
Digital drivers... (this is what you want for Nema 17 when using low voltage micro-controllers for signal generation...
https://www.omc-stepperonline.com/d...rent-1.4-5.6A-for-Nema-17-23-24-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,
