I fell in love with cable driven mechanisms a few years ago and put together some of my first mechanical tentacles to celebrate. But only after playing with them did I start to understand the principles that made them work. Today I want to share one of the most important equations to keep in mind when designing any device that involves cables, the capstan equation. Let some caffeine kick in and stick with me over the next few minutes to get a sense of how it works, how it affects the overall friction in your system, and how you can put it to work for you in special cases.
A Quick Refresher: Push-Pull Cable Driven Mechanisms
But first: just what exactly are cable driven mechanisms? It turns out that this term refers to a huge class of mechanisms, so we’ll limit our scope just to push-pull cable actuation systems.
For moving about in the real world, robots can crawl along the ground or take to the sky. Both options have disadvantages, with obstacles being a problem on the ground and flying being very energy intensive. What we don’t often see are robots that move along aerial cables, which can offer the best of both worlds for certain use cases. Taking inspiration from a sloth’s slow and efficient movement through the trees, researchers from Georgia Tech created a robot to crawl slowly along a cable network and monitor the world around it, and of course named it Slothbot.
Slothbot trades speed for efficiency, letting it operate for very long periods on solar power alone. It does require the set up and maintenance of a cable network, but that brings the advantage of no obstacles, and the ability to stop and recharge. To us the most interesting feature is the cable switching mechanism, that allows it to navigate its way along a web of interconnected cables.
Ring gears with a section removed hold the upper part of the pulley mechanism, but can rotate it’s opening to the left or right to allow an interconnecting cable to pass through, The body is in two pieces, with an actuated hinge in the middle to allow it to turn onto a different cable section. Each section of the body also has a powered wheel which pushes up against the cable and moves the robot along slowly. Not surprisingly, researchers say that making the cable switching mechanism reliable is the biggest challenge. It does look like the current design would not work well with thicker cable joints. Watch the video after the break for a better look at the mechanism Continue reading “Slothbot Lives Up To Its Name”→
Most CNC robots people see involve belts and rails, gantries, lead screws, linear bearings, and so forth. Those components need a rigid chassis to support them and to keep them from wobbling during fabrication and adding imperfections to the design. As a result, the scale is necessarily small — hobbyist bots max out at cabinet-sized, for the most part. Their rigid axes are often laid out at Cartesian right angles.
One of the exceptions to this common configuration is the delta robot. Deltas might be the flashiest of CNC robots, moving the end effector on three arms that move to position it anywhere in the build envelope. A lot of these robots are super fast and precise when charged with carrying a light load, and they get put to work as pick-and-place machines and that sort of thing. It doesn’t hurt that delta bots are also parallel manipulators, which means that the motors work together to move the end effector, with one motor pulling while the matching motor pulls.
But while Cartesian CNC bots are sturdy workhorses, and deltas are fly-weight racehorces, neither can really cut it when you want to go gigantic. In terms of simplicity and scale, nothing beats cable bots.
Cable bots use wires or strings pulled by reel-mounted motors, with dimensions limited only by the room to mount the motors and the tensile strength of the cables used. When the strings are tensioned you can get a surprising degree of accuracy. Why not? Are they not computer-controlled motors? As long as your kinematic chain accounts for the end effector’s movement in one direction by unwinding another cable (for instance) you can very accurately control the end effector over a very wide scale.
The following are some fun cable bots that have caught my eye.
When last we ran into [Daren Schwenke] he was showing off his 6-color delta printer that changes colors seamless mid-print. Right now he’s working on a printer that uses tensioned cables to precisely move a toolhead while maintaining enough solidity that [Daren] can tap on the toolhead without it budging at all.
It’s much more simple a rig than a gantry-style 3D printer, with a chassis shaped like a geodesic polyhedron consisting of fiberglass trusses (those driveway markers) secured by 3D-printed lugs, all controlled by a Beaglebone Green and four steppers. A key element of the build is the central steel rod, a 4′ repurposed garden stake which serves to stabilize the whole toolhead. In terms of build diameter it can scale from around 200 mm to 600 mm. [Daren] aims to using Machinekit’s tripod kinematics for control and he also learned a bunch from RepRap’s Flying SkyDelta project.
For more 3D-printing goodness, be sure to check out [Daren]’s aforementioned 6-color delta.
Straight from the Max Planck Institute for Biological Cybernetics, and displayed at this year’s Driving Simulation Conference & Exhibition is the coolest looking simulation platform we’ve ever seen. It’s a spherical (or icosahedral) roll cage, attached to the corners of a building by cables. With the right kinematics and some very heavy-duty hardware, this simulation platform has three degrees of translation, three degrees of rotation, and thousands of people that want to drive a virtual car or pilot a virtual plane with this gigantic robot.
The Cable Robot Simulator uses electric winches attached to the corners of a giant room to propel a platform with 1.5g of acceleration. The platform can move back and forth, up and down, and to and fro, simulating what a race car driver would feel going around the track, or what a fighter pilot would feel barreling through the canyons of the Mojave. All you need for a true virtual reality system is an Oculus Rift, which the team has already tested with driving and flight simulation programs
An earlier project by the same research group accomplished a similar feat in 2013, but this full-motion robotic simulator was not made of cable-based robotics. The CyberMotion Simulator used a robotic arm with a cockpit of sorts attached to the end of the arm. Inside the cockpit, stereo projectors displayed a wide-angle view, much like what a VR display does. In terms of capability and ability to simulate different environments, the CyberMotion Simulator may be a little more advanced; the Cable Robot Simulator cannot rotate more than about sixty degrees, while the CyberMotion Simulator can turn you upside down.
The Cable Robot Simulator takes up a very large room, and requires some serious engineering – the cables are huge and the winches are very powerful. These facts don’t preclude this technology being used in the future, though, and hopefully this sort of tech will make its way into a few larger arcades.