Monday, December 20, 2010

Design is Iterative 1 of N

Design is an iterative process, that rarely produces optimal or even usable results on the first try. This series of posts will show some of that design process including the flaws of the initial prototypes and how they were resolved.

So this arm design looked like a good idea for a robot, if it had some more mobility.

So, how do you design a cable drive continuum robot appendage? Build a prototype.
How do you build a prototype? Build a cable drive prototyping system.

The initial design of the winch drum uses 1/16" steel cable with an stop attached to the end. So far things look good.

The first design flaw. The exit hole is normal to the surface of the drum which keeps the cable from lying flat. The next version places the hole much closer to tangent to the surface of the drum. If you are familiar with the three body problem then you will understand that ideally the exit hole should be shaped like a transfer orbit. A curved hole of the required shape can be made with a 3D printer, but it is difficult to manufacture with injection molding. Design for manufacturing.

Hope you're wearing your safety glasses, because that cable is about to unspool violently. Here you can also see another problem where the hole was placed in the center along the axis of the drum, which leaves little room for the cable to spool. The next design moves the hole closer to one side.

The smaller pulley is much smaller than the minimum bend radius of the cable.

String is more flexible than the steel cable, so testing can continue while new parts are being made.

Here is the drum and a idler pulley mounted with the cardboard cable drive prototyping system. The drum is designed to be attached to a servo in the actual robot.

The hole in the cardboard is made with a hole punch, and the idler pulley is attached to the cardboard with metal brad and a Protobrad™.

Here is a video of the system in action. Future versions will support two cables on the drum, with one being wound as the other is being unwound. This configuration will remove the need for the spring.

On to the next iteration.

Update: U.S. Navy Wire Rope Handbook (6.2MB PDF)

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