Cable Bots, Arise! Domination of the Universe is at Hand

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

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.

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Raspberry Pi driven Polargraph exhibits high precision drawing ability

This polar graph draws some amazing shapes on a dry erase board. Part of that is due to the mounting brackets used for the two stepper motors and the stylus. But credit is also due for the code which takes velocity into account in order to plan for the next set of movements.

The Go language is used to translate data into step commands for the two motors. This stream of commands is fed over a serial connection between the RPi board and an Arduino. The Arduino simply pushes the steps to the motor controllers. The inclusion of the RPi provides the horsepower needed to make such smooth designs. This is explained in the second half of [Brandon Green’s] post. The technique uses constant acceleration, speed, and deceleration for most cases which prevents any kind of oscillation in the hanging stylus. But there are also contingencies used when there is not enough room to accelerate or decelerate smoothly.

You can catch a very short clip of the hardware drawing a tight spiral in the video embedded after the break.

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Drawing things very slowly, very easily

A while back, we saw [Euphy]’s polar pen plotter project. The mechanics of the build are very simple – just a pair of motors attached to a pen by a beaded cord. Even though the build is very simple, it’s possible to create awesome works of art albeit very slowly.

Since we featured [Euphy]’s polar pen plotter, a lot of improvements have been made. Now the Polargraph has an SD card slot for computer-less printing, a touch screen for manual control of the plotter, and a few new drawing styles that improve on the previous version a lot.

Right now the improved version of the Polargraph is set up in the front of a graffiti art supply shop in Edinburgh where it spends its time slowly drawing a window dressing. [Euphy] put up a few videos of what the Polargraph is capable of doing, very impressive and we hope he gets a few more PCBs in soon.

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