#precision kinematic couplings and related work. “Just using the most ordinary quality kinematic hardware will produce repeatability between the two kinematic platforms in the micrometer (40 micro inch) range. By using super stiff cermet component parts with the geometry and surface texture held to the highest possible quality, repeatability in the nanometer range can be achieved.” The micron range is ordinary bearing-ball tolerances, so I suspect that as long as you’re not swapping out your bearing balls, you can get much better repeatability than a micron. Lots of ideas about #exact-constraint design that I wasn’t familiar with! Not very well written, though; lots of repetition. Practical tips: “A complete ball can be economically installed in a kinematic platform by first machining a pocket that is nearly hemispherical. A female cone can be used, but it is much less effective. This pocket is wetted with a high strength epoxy glue, and the ball is clamped in place with enough force to squeeze out the excess glue. Wipe the excess epoxy glue away with some absorbent material wetted with Isopropyl Alcohol.” Unfortunately this advice is repeated almost verbatim in almost every single entry! Also not very reliable; it claims EDM uses an electrolyte, for example. #manufacturing #mechanisms
on 02024-09-09#Harrison-Low #video about a #mechanical linear actuator design for a juggling robot, capable of a full 263mm stroke in 100ms. He’s mostly using carbon-fiber reinforced plastic tubes, with six 6001RS cartridge bearings for rollers (one too many for #exact-constraint), held in place by a piece made by #3D-printing, with interchangeable spacers to adjust the fit in 100μm units to the imprecisely-sized tubes. He’s doing the actual actuation with a kevlar-kite-string cable drive, with the cables actually run through teflon Bowden tubes so the motors don’t have to add mass to the actuator joints; the motor pulley has two V grooves for the two ends of the cable. This seems like a good way to do robotic #mechanisms in general. This replaces his first hydraulic design made out of syringes and a second iteration using a ballscrew which was too slow and (I assume) also super expensive. He has an AS5048 magnetic encoder for positional measurement, coupled to the actual moving parts with a constant-force spring, whose rotation is what it measures, but so far he’s just using encoders on the motors themselves (which I guess are brushless since he’s using #ODrive; in fact, the motors are ODrive-branded, model D6374-150KV). He says ODrive's current sensing “completely removes” the need for limit switches. His attempts to clamp the three frame tubes with a flexure from the inside failed, but I don’t know why he doesn’t clamp them with a flexure on the outside, instead relying on hammering them in. He uses a thin wire with a hook bent on the end as fish tape to pull the kevlar string through the teflon Bowden tubes, but doesn’t always need it because of how stiff the kevlar string is. On one occasion the kevlar string was able to friction on the 3-D printed plastic and just cut right through it, presumably from heat (a big advantage for kevlar over UHMWPE, except when you want to cut it; presumably kevlar will creep less for the same reason). #robotics
on 02024-04-28