Why spend so much effort to achieve an "exact" gear ratio? Having more zeros does not equal to being more "precise".
Also, I wonder how resistant this mechanism is to wear and fatigue.
Also, I wonder how resistant this mechanism is to wear and fatigue.
And you can't really declare your design is "high precision" and present yourself as someone others should take transmission design advice from if you aimed for a gear ratio of 8 and achieved "somewhere around 7.9 to 8.2"
It's also interesting because competing actuators with strain-wave, cycloidal, or planetary gearboxes will state exactly what the ratio is. The actual gear teeth may not be spaced out perfectly around the circumference, but the number of teeth is an integer with an infinite number of zeros.
He could also go with narrower rope, and spread the load over more windings, which would give him more throw.
Notice that the fastener bearing the weight of the cord is wrapped around two bends. If I recall my physics properly, each of those direction changes behaves like a pulley, halving the strain on the bight (dyneema is difficult to fasten though. Slippery bugger.)
No wait, he is already using 2mm: https://www.aaedmusa.com/projects/cara but it’s 2 ropes per leg, so maybe he could use 1/16” rope (1.6mm).
He actually discussed this in an earlier video for his initial tests on the capstan drive. He ended up testing the rope he used for around 358 hours (two weeks) on continuous use in the drive itself with very low backlash
That's why pro crews don't use gears and ropes. At high impulses deformations and elasticity throw the kinematics off what's actually happening. Modeling the deformations and the elasticity is a computational no no. Instead what you see is the motors right on the joints.
At least that was the case last time I had a look at robotics.
the motors were so sloppy the company wasted a ton of money [0] having me write heuristics to tackle the errors they accumulated over several hours.
one of his whole points is that by using dyneema (rope), there's almost no elasticity at all in the capstans.
[0] relative to the cost of better motors
The answer here, as with so many things in robotics, is: It Depends.
UR10e robot arm that can lift a 4kg object with a reach of 1m and has sub-1mm repeatability? Strain wave gears in the base and shoulder joints, 100:1 ratio.
MIT Mini Cheetah robot dog that can do backflips? 6:1 planetary gearbox.
Shadow Hand with 20 degrees of freedom? Tendon driven, with the 20 motors in the forearm to keep the fingers slim.
Little dinky Huggingface SO-101? Servo motors, integrating 1:345 gearing with a series of 6 tiny brass gears.
Mid-price CNC milling machine, if you call that a robot? Really long ballscrews, driven by stepper motors.
>Mid-price CNC milling machine
A ball-screw is mostly decorative on small machines... =3
Back-drivability is the enemy of precision, so many robotic applications can do without it.
The old "fast, cheap, or good... choose any two joke is mostly still true. =3
You’re right that it has a freedrive mode, and force control modes. But it’s a rigid, low-backlash robot with the compliance achieved in software afterwards.
Expensive, naturally, but none of the problems that come with things like series elastic actuators.
I think the X-Carve 3-axis wood carver uses stepper motors with belts of all things. The Shapeoko Pro is leadscrews and stepper motors. Wazers, I believe, are belt-and-servo driven. And a lot of 'CNC conversion kits' you can order online use stepper motors. Plus of course laser cutters have really low torque requirements, so they've got a lot of design freedom.
Arguably those are "cheap" rather than "mid price" it just felt weird to declare a $4000 machine to be "cheap"
Note that belts were continued through the Shapeoko 3 (I got a machine as a "thank you"), though the Z-axis got a leadscrew in the Z-Plus upgrade, then the Pro (since, I got a job with the company and got an XXL as part of my employment), then the 4 (the original Pro is referred to as a 4 Pro sometimes), and it is only with the 5 pro that ball screws were switched to for all axes (and I now have a 5 Pro).
For an example of what a belt-drive CNC can do see:
https://community.carbide3d.com/t/hardcore-aluminum-milling-...
Sure. Yet evolution has achieved astonishing kinematics with all manner of deformation and elasticity inherent to the materials, and also constantly changing physical properties, using low resolution data. We cannot build permanently lash free mechanical devices at reasonable cost and reasonable size/weight. Eventually, the answer must be pervasive real-time compensation throughout the kinematic model.
> Modeling the deformations and the elasticity is a computational no no.
Why? Nervous systems do this. That's why you can change your shoes and still walk upright.
That sounds like “It’s not wrong, we just don’t do it”. There are some amazing examples of imprecise drive systems compensated for by excellent control systems all over the world, for millions of years.
Isn't having more decimal places the exact definition of precision (vs accuracy)?
2. Do you actually say fourteen-zero-zero or just fourteen? If the latter, that's your answer right there.