Friday, August 7, 2009

Design goals for 2009 walker:

1. No structural joints made with glue or epoxy.

The first walking robot I made wasn't much more than servos, a battery pack and a radio receiver held together with silicone glue. That worked well enough as those robots were too feeble and lightweight to need much structural strength. Later, as I went to stronger servos, heavier batteries, and more rigid structure I experimented with various epoxies. To keep the weight down, I wanted to use the servo cases and batteries themselves as structural elements, which meant ignoring the intended mounting tabs and epoxying directly to the servos.

The glue and epoxy kept failing. In the end I had the best luck with cyanoacrylate superglues, but even those didn't hold reliably. And when I started showing the robot at conventions, gluing everything together had a second downside in making it much harder to easily replace broken parts. The final straw was when I switched from the hefty NiCad cells I had been using to LiPoly packs with minimal structural strength. When I needed to add a steel frame to take the place of the batteries that had previously made up the structural core, there wasn't much point in using glue rather than screws to attach to the servos.

When I set out to completely redesign the structure on the latest rebuild, I set a solid rule. No glued or epoxied structural joints. Everything structural would either be fastened with machine screws (if meant to be disassembled), or welded or soldered (if meant to be permanent).

I did end up having to use superglue to attach weld nuts (used to provide a second hinge point on each servo joint) to the inside of the servo cases. The glue is only needed to hold the nuts in place during assembly, as the screw which holds the hinge pin in place also holds the servo nut from coming loose. I also had to use gaffer's tape and zip-ties in a few spots to hold wires in place. Not an elegant solution, but acceptable for non-structural parts.

2. No purely decorative components

I've made the mistake in the past of loading decorative bodywork and cosmetic detailing on convention robots. On the 2006 version of the robot I built up complex anodized titanium bodywork around the servos and joints, with rubber tubing covering the servo and battery wires and thick aluminum tubes for the legs and central chassis. The result - excess weight, stiffness and interference in the joints, for bodywork and cable sheaths that the robot would work better without. That version walked about ten seconds before stripping a gearbox.

Faster battery drain and more frequent gear replacement made for less time walking the robot around and having fun. Having a robot that works reliably for a long time had to be my first priority. My second strict rule on this redesign was to have nothing on the robot which didn't in some way help the performance or reliability of the robot. This didn't mean no cosmetic details. It meant making those functional parts of the robot cosmetically interesting so I wouldn't have to add completely useless cosmetic parts. I was also willing to stretch my definition of what counted as a justifiable functional purpose. For example, the copper body shell technically serves the purpose of holding wiring in place out of the way of the legs, so that counts.

3. Easy to maintain

I've tried, and failed, to design the robot such that nothing breaks over the course of a busy weekend. Instead I've designed the current version for easy maintenance. Those parts which I carry spares for can be easily removed and replaced without having to break or cut anything. The servo gears which are the most frequently failing point can be replaced without even having to remove the servos from the robot.

Those parts which I don't have spares for can still be either removed for troubleshooting, or at least easily accessed by removing any bodywork and components around them. I don't at the moment have spares for the radio, the battery, the servo-control board, or many of the other internal components, but I can get to and troubleshoot all those parts in place easily. Having a thin waterjet-cut steel frame supporting an easily removable outer shell versus the closed-box structure on some previous designs makes maintenance a lot easier.

4. Safe for human contact

This robot is meant to be shown off at conventions, walking up to people, shaking hands, and even being picked up and handled. It needs to be able to do all this without injuring anyone. No sharp points or edges. I designed it with spherical and cylindrical body sections, and rounded feet rather than points. The antenna had to stick out somewhat, but I replaced the stiff wire antenna that came with the radio gear with a length of super-flexible wet-noodle wire.

It also means I had to try to avoid pinch points. The servos I'm using now have amazing torque, enough that a finger in the wrong place could be crushed if the robot moved while being held. Although the XBee radios are pretty good about rejecting bad data, I didn't want to rely on the control system to prevent any of the servos from moving at the wrong time. This robot is designed with large openings around the joints, with rounded edges on all the metal surfaces facing openings. This also helps with reliability - wide open joints with lots of clearance are a lot less likely to bind or jam, and make it easier to reach in and work on mechanical bits without taking the entire joint apart.

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