Tuesday, May 11, 2010

Twitch, the part where I try to build something.

Twitch (or, Twitch, Jr., since the original Twitch was much bigger) is finally underway. (I decided it would be a good idea to finish writing my thesis first so I could graduate.) It should be a relatively quick project, especially considering that I got most of the important parts sent out for water jet cutting.

Mail-order robot.

Actually, this is the first time I've tried to make a drive like this, with so many moving parts. Here's a quick reminder of what it should look like:

Both sets of diagonal wheels are coupled by parallelogram linkages. The two wheels on each side can also be coupled by a cross link as well, though with two linkage drive servos this is not entirely necessary. The most critical pieces are the bearing blocks that support each wheel and motor module. These are made from Delrin for low friction and ease of machining. There are a total of eight, for four wheel modules:

Four identical wheel modules, mounted in the corners.

The bearing blocks sit on aluminum posts, hopefully aligned correctly between the top and bottom plates. With the two diagonal linkages in place, the wheels can be moved in pairs. Normally, both pairs will be moved together to switch between the "long" and "wide" orientations.

 "Long" orientation (top) and "wide" orientation (bottom.)

Besides these two orientations, there are other possibilities. One is to put all four wheels at 45º. Another is to put one set "long" and one "wide." Since they are omni-wheels, either of these would be capable of full 3DOF movement. If the cross links are used to couple the two diagonal sets of wheels, the range of intermediate wheel positions is more limited. Needless to say, this project will become an exercise in input mapping and control at some point.

For now, though, there are still a few mechanical details to work out. After the first assembly, it's clear that there is a bit too much friction in the wheel module bearing blocks for the servos to drive the linkage. They were made to be fairly tight, for stiffness, but after seeing it in real life I think I would trade a little slop for less friction. Unfortunately, this linkage gives the servo the least amount of mechanical advantage at the extremes, so if the bearing friction is constant, the servo sees the most torque at these points. I'm crossing my fingers that it's actually the wheels, driving against each other during the transition, that can create the torque for overcoming this friction. (Again, why does everything converge to a controls problem?!)

Next step is creating hubs for the Vex 4" omni-wheels that allow them to attach to the gearmotor shafts. Then, wiring and programming. Okay, so it wasn't quite a two-week job, but it's progressing.


  1. hi.. just dropping by here... have a nice day! http://kantahanan.blogspot.com/

  2. Who does not love HUGE servo? I really like you clean design. I see nice application for this type of vehicule, army stuff and big guns :)

    keep up the great work!