Monday, June 20, 2005

Knewt Bulks Up

Knewt Bulks Up
Keith Rowell Design

After a long absence from the project, I am once again back in the saddle, and all the happier for it. We are adjusting the design to overcome some bugs revealed during testing and experimenting with a new driver ( more on the driver later).

Mostly this means larger motors. The balance motor (1 below) has had to replaced repeatedly, and the leg motors (2,3) show high stress under load but have not yet failed outright. These 3 were HS55's with a torque of 15in/oz's. They've all been replaced with 81's which have 49in/oz's of torque.

It appears that with the bulkier body, the head might could stand some scaling up. The image above shows Knewt 2.0 with modifications to the backplate and chest plate to accommodate these larger servo motors. There's also a new motor added for "active" balance at the ankles. I had hoped that "passive" linkage would do this sufficiently, but alas, the "play" in so many joints causes Knewt to "lean" toward the foot that's off the ground, ruining the "level body" appearance I wanted, not to mention making it that much harder to balance during walking. The "active" balance motor (4) will replace the passive balance linkage, and work in unison with the weight shifting motor (1).

Larger Motors

larger servo motors
Keith Rowell Design

The width of the 81 motors are the same as the 55's, but the length and depth are larger by 20% to 50%. The weight shifter (1) now protrudes further through the breast plate, and the leg motors (2,3) had to be shifted down and out under the leg retainer bar.

The width of the stance was left in it's original position, but the body mass is now thicker and longer.

By the way, I've pictured Knewt with an old camera body for a head, but can't find any trashed cameras. A defunkt micro cassette video camera or digital camera body would be perfect because of the brushed metal cases they tend to have. If anyone knows where to get trashed camera bodies, please let me know.

rear view
Keith Rowell Design

In this rear view, the new leg motor brackets can be seen. They move the motors lower (further from the center point of the pivot) to give more leverage. There was plenty of travel here, so the travel lost by moving will not be missed. I'm confident that the tripled power combined with the extra leverage will overcome the hesitancy the 55's exhibited in the legs. The weight shifter however is in such a high torque position, (close to the pivot point) and needs so much travel, I'm keeping my fingers crossed that it wont have to be upgraded yet again.

Also visible in yellow is the lithium battery back pack. The origianl space inside the chest cavity is still available for the hydride prism cells, but the current plan is to use that space as a wire way and take advantge of the higher power density privided by lithium.

The active balance motor (4) had to be mounted underneath. I originally placed it vertically in the space between the leg motors (2,3). I liked the symetrical, vertical positon, but the space between the leg motors (2,3) is used during weight shifting as those motors rotate about the bearing center point shown in "red". With the current configuration, a weight shift rotation of 20deg is possible. The old design was only 14deg.