Tag Archives: actuator

Shapeways dimensional tolerances

The first version of the planetary gearbox as 3d printed from Shapeways required a fair amount of post-machining to get all the pieces to fit together.  I wanted to get to a point where I could just order some parts and have a reasonable expectation of them mostly working out of the box.  To make that happen, I’d need to get a better understanding of where the tolerances were coming from.

Understanding the problem

Shapeways provides a fair amount of documentation on the processes and accuracy you can expect generally.  Most of this is detailed in “Design rules and detail resolution for SLS 3D printing“, however the results there have some limitations.  Primarily, they are only applicable to the specific geometries tested.  Shrinkage is qualified as +- 0.15% of the largest dimension, and is likely influenced by the exact printed geometry.  Secondarily, in the documented tests, the designers had full control over the part alignment in the print.  The standard shapeways platform does not allow you to orient parts, you are at the whim of their technicians where the Z axis will end up.

For the gearbox, I had numerous fit points that needed to have controlled tolerances.  The input and output bearing both needed a press fit for both sides.  The internal gear for the planetary gearing needed a press fit, and the front and back shells also have a lip which would be more rigid if the fit was snug.

Brute force

My solution?  Print slight variants of the relevant pieces of each fit point with each radial dimension printed in increments of 0.1mm.

Shapeways Dimensional Tolerance Test
Shapeways Dimensional Tolerance Test

For each part, I broke out the calipers to measure the as printed size, and also attempted manual press fits of each part.  I didn’t manage to put any identifying features on each of the prints, which probably annoyed the Shapeways technicians and made my life a bit harder.  I just assumed that the sizes came back in increasing order despite the part number markings, which I’m pretty sure were incorrect.  This resulted in the following table:

Measured dimensional accuracy of gearbox parts
Measured dimensional accuracy of gearbox parts

Conclusion

The second version of the gearbox had many other changes in addition to these, but this let me get a lot closer to the correct fit on the full assembly.

Improved actuators for SMMB

One of the major challenges SMMB had in Robogames 2016 was in overall walking speed.  It is using HerkuleX DRS-201 servos, which are roughly comparable to the Dynamixel servos that other entrants were using, but the physical geometry of the robot is such that is hard to get it to move quickly with that class of servos.  The center of gravity is too high, especially with the gimbal mounted turret. The R-Team bots all use very low slung machines that scoot along.  I could go that route, but why do things the easy way?

Instead, I’ve been working to take some ideas from fellow Boston-ite Ben Katz and build some actuators that would permit truly dynamic motion.  He got a leg jumping using Hobbyking brushless motors with some simple FOC control  The biggest differentiators vs the Dynamixel / HerkuleX class of actuators would be low mechanical inertia, high transient power, low backlash, and high speed.  I’m just getting started here, but have managed to build up a 5x planetary gearbox driven by a Turnigy Elite 3508 (so a fair amount smaller than what Ben did, but more appropriately sized for Mech Warfare), and a VESC 6 as an interim motor controller.  It is designed for electric skateboards, but has minimal position control support.  Although, as my bruised hand can attest, it isn’t super stable and flips out occasionally.

The first prototype is assembled and has been spun up, although a fair amount of dremel time and shims were required to get everything to fit together.

dsc_1056
Some SDP-SI gears for the prototype
dsc_1054
The 3508 with its stock shaft extracted
DSC_1057.JPG
New shaft installed, with spur gear alongside
dsc_1070
AS5047 wired up to the VESC
DSC_1072.JPG
Shapeways arrived!
dsc_1073
Housing mounted onto motor
dsc_1076
Planet carrier assembled
dsc_1077
Planet carrier inside housing
dsc_1081
Final assembled gearbox

And finally, the pretty videos: