For the uninitiated, harmonic drives, also known as strain-wave gears, are a compact, high-torque gearbox that has become popular with “robotic dog” makers and other roboticists. The idea is to have a rigid, internally-toothed outer ring nested around an externally-toothed, flexible cup. A wave generator rotates within the inside cup, stretching it so that it meshes with the outer ring. The two gears differ by only a couple of teeth, meaning that very high gear ratios can be achieved, which makes them great for the joints of robot legs.
[Levi]’s problem with the harmonic drive is that due to the depth of the flexible spline cup, compactness is not among its virtues. His idea is to couple the flex spline to the output of the drive through a flat spring, one that allows flexion as the wave generator rotates but transmits torque efficiently. The entire prototype is 3D-printed, except for the wave generator bearings and stepper motor, and put to the test.
As the video below shows after the excellent introduction to harmonic drives, the concept works, but it’s not without its limitations. Even lightly loaded, the drive made some unpleasant crunching sounds as the PLA springs gave out. We could easily see that being replaced with, say, a steel spring, either machined or cut on a water-jet machine. That might solve the most obvious problem and make [Levi]’s dream of a compact harmonic drive a reality. Of course, we have seen pretty compact strain-wave gears before.
In the video after the break, [Concrad] explains the concept behind the coupler and how he designed it. Put simply, the tabs inside of the coupler are designed to grab onto each other once the coupler is spun. When he demonstrates the action, you can see that both sides of the coupler are pulled together tightly with a satisfying little snap, but then can be easily removed just by rotating them back in the opposite direction.
The nature of desktop 3D printing means that the female side of the connection requires support when printing, and depending on your printer, that might mean a relatively rough mating surface. [Conrad] notes that you’ll need to experiment a bit to find how small your particular machine can print out the design before things get too gummed up.
Even the oldest of mechanisms remain useful in modern technology. [Skyentific] has been messing with robotic joints for quite a while, and demonstrated an interesting way to use a pulley system in a robotic joint with quite a bit of mechanical advantage and zero backlash.
Inspired by the LIMS2-AMBIDEX robotic arm, the mechanism is effectively two counteracting sets of pulley, running of the same cable reel, with rollers allowing them to act around the bend of the joint. Increasing the mechanical advantage of the joint is simply a matter of adding pulleys and rollers. If this is difficult to envision, don’t work as [Skyentific] does an excellent job of explaining how the mechanism works using CAD models in the video below.
The mechanism is back drivable, which would allow it to be used for dynamic control using a motor with an encoder for position feedback. This could be a useful feature in walking robots that need to respond to dynamically changing terrain to stay upright, or in arms that need to push or pull without damaging anything. With properly tensioned cables, there is no backlash in the mechanism. Unfortunately cables can stretch over time, so it is something that needs to be considered when using this in a project.
Pulley systems have been with us for a very long time, and remain a very handy tool to have in your mechanical toolbox. A similar arrangement is used in the Da Vinci surgical robots to control their tiny manipulators. It would also be interesting to see this used in the already impressive robots of [James Bruton]. Continue reading “Cable Driven Robotic Joint”→
The wonders of 3D printing don’t stop coming. Whether it’s printing tools on the International Space Station, printing houses out of concrete, or just making spare parts for a child’s toy, there’s virtually nothing you can’t get done with the right 3D printer, including spicing up your Halloween decorations.
Not only is this pumpkin a great-looking decoration for the season on its own, but it can also transform into a rather unsettling spider as well for a little bit of traditional Halloween surprise. The print is seven parts, which all snap into place and fold together with a set of ball-and-socket joints. While it doesn’t have any automatic opening and closing from a set of servos, perhaps we will see someone come up with a motion-activated pumpkin spider transformer that will shock all the trick-or-treaters at the end of this month.
It’s not too late to get one for yourself, either. The files are available on Thingiverse or through the project site. And we’ve seen plenty of other Halloween hacks and projects throughout the years too if you’re looking for other ideas, like the recent candy machine game, a rather surprising flying human head, or this terrifying robot.
Back in the 70s when I started getting interested in electronics, tons of magazines catered to the hobbyist market. Popular Electronics was my favorite, and I think I remember the advertisements more than anything, probably because they outnumbered articles by a large margin. Looking back, it seemed like a lot of ad space was sold to companies hawking the tools and materials needed for wire wrapping, which was very popular for prototyping in the days before solderless breadboards were readily available. I remember beauty shots of neat rows of small, gold posts, with stripped wires wrapped evenly around them.
To the budding hobbyist, wire wrapping looked like the skill to have. With a huge selection of posts, terminals, and sockets for ICs and discrete components, as well as a wide range of manual and powered wrapping tools, it seemed like you could build anything with wire wrapping. But fast forward just a decade or so, and wire wrapping seemed to drop out of favor. And today — well, does anyone even wire wrap anymore?
I was splitting wood one day a few years back, getting next winter’s firewood ready on my hydraulic splitter. It normally handled my ash and oak with ease, but I had a particularly gnarly piece of birch queued up, and the splitter was struggling. The 20-ton cylinder slowed as the wedge jammed in the twisted grain, the engine started to bog down, then BANG! I jumped back as something gave way and the engine revved out of control; I figured a hydraulic hose gave out. Whatever it was, I was done for the day.
I later discovered that a coupler between the engine shaft and the hydraulic pump failed dramatically. It was an easy fix once I ordered the right part, and I’ve since learned to keep extras in stock. Couplings are useful things, and they’re the next up in our series on mechanisms.
[Tim] needed very small, motorized joints for a robot. Unable to find anything to fit the bill, he designed his own tiny, robotic joints. Not only are these articulated and motorized, they are designed to be independent – each containing their own driver and microcontroller.
None of the photos or video really give a good sense of just how small [Tim]’s design is. The motor (purple in the 3D render above, and pictured to the left) is a sub-micro planetary geared motor with a D shaped shaft. It is 6mm in diameter and 19mm long. One of these motors is almost entirely encapsulated within the screw it drives (green), forming a type of worm gear. As the motor turns the screw, a threaded ring moves up or down – which in turn moves the articulated shaft attached to the joint. A video is embedded below that shows the joint in action.
[Tim] originally tried 3D printing the pieces on his Lulzbot but it wasn’t up to the task. He’s currently using a Form 2 with white resin, which is able to make the tiny pieces just the way he needs them.