Older blog entries for NateW (starting at number 20)

Whoops... That ED-209 link again. My bad.

I've finally started construction on a biped - something I've been wanting to do for years but never quite got around to doing. It's a made of Lynxmotion servo brackets and laser-cut polycarbonate, and the overall shape was inspired by ED-209. However my "ED-1" uses no linear actuators in the legs, and will probably have longer arms, though I haven't yet made up my mind about that. I think I'll be departing from the ED-209 form factor in order to add an extra degree of freedom to the arms.

The legs, hips, and torso are built; the arms should be done by the end of this week, the feet will follow in another week. I have no pictures (as usual) but a couple people took photos at the Seattle Robotics Society meeting yesterday and if those images find their way onto the web I'll link to them from here.

The dead hard drive I mentioned in my last entry turned out to be not much of a tragedy. I booted to Linux, mounted the drive as a file, then copied the raw data to a good drive. Then Googled for "GPL" and "FAT32" and found some tools... After fixing some corrupt data on the boot sector, I was able to recover about 90% of the data from the dead drive - and as far as I can tell, the other 10% was completely unimportant.

Good news: I got my friend's scout walking yesterday.

Bad news: the boot drive on my desktop PC just failed.

I have backups for the really important stuff, but I have a sinking feeling that there's a "long tail" of stuff that wasn't quite important enough to back up religiously... but which, in aggregate, adds up to a lot of lost work. Only time will tell.

Have been learning about Kalman filtering in hopes of providing my robot with a good sense of balance... In case anyone else is interested in learning about Kalman filtering, here are the best links I've found so far. These are not the most in-depth, but they are (far as I can tell) the ones with the best "plain english" description of what all the formulas mean.

This one presents Kalman filtering of scalar values, rather than vectors, which cuts down on the greek notation but preserves the key concepts. The second link explains how the scalar version is extended to work with vectors:

http://www.swarthmore.edu/NatSci/echeeve1/Ref/Kalman/ScalarKalman.html
http://www.swarthmore.edu/NatSci/echeeve1/Ref/Kalman/MatrixKalman.html

I found the links above to be the most helpful, but these are also pretty good:

http://www.cs.unc.edu/~welch/kalman/Levy1997/Levy1997_KFWorkhorse.pdf
http://www.cs.unc.edu/~welch/media/pdf/maybeck_ch1.pdf
http://www.cs.unc.edu/~welch/media/pdf/kalman_intro.pdf
http://www.uno.edu/~SAGES/publications/PractitionersKalman.PDF

After a couple nights of reading these papers, the light bulb suddenly went on... I get it! At least, I think I do. I'll find out soon, as I just ordered a SparkFun 6DOF IMU, so I should soon have some signals to filter.

I have to plug Sparkfun here because this product appeared on their web site just a couple months after I proposed the idea in their online forum. I beg, they build, I buy. I wish more companies worked that way. :-)

http://www.sparkfun.com/shop/index.php?shop=1&cart=209119&cat=1&itemid=406&

Here come the movies and screen shot I promised a while ago... except that instead of my hexapod, this is a MegaRobotics dog, loaned to me by a friend of mine.

moive: side view
movie: front view
screen shot: source code

The software is coming along nicely, but defining a single sequence of operations (for example, the stand-up routeine at the start of each video) is more trouble than it should be. It might be nice to have the state machine system interpolate from one pose to another when transitioning from state to state... I still need to think on that for a while, but it seems promising.

The four-legged heapod took its first steps tonight. As you might guess, it walks with a limp.

Still no photos... Still working on a new web site to host them.

The hexapod is up to four legs now. One of my parrots chewed up the servo leads on three of the servos, so it's going to have to be a quadropod until I replace those.

The leg design I'm using now is built from hand-bent metal rods, and I'm not satisfied with it. Between the inconsistent bending and the interference between the rods and the chassis, I've decided to revise the legs to use straight rods, with another set of laser-cut plastic pieces to get the ends of the rods in the right places. Hard to explain, but I'll post a photo before long.

The control system software is coming reasonably well. Photos to come...

The laser-cut parts arrived from www.pololu.com and the hexapod is taking shape. There were a couple of minor errors in the plans... no show-stoppers, but it's surprisingly hard to use 2D CAD to design flat parts that get assembled in 3D. :-) Only two legs built so far, the rest will probably have to wait until after the holidays. I post some pictures when it starts to look like a robot. :-)

I just wanted to publicly thank www.picobotics.com for putting out a good product and backing it with good support. I bought a PicoPic 20-servo controller and had some trouble getting it to work at first, but Mark answered my emails promptly and got me going.

(The problem? I held the unit upside-down when I configured it like the picture in their manual, so I removed the wrong jumper. Oops.)

After a couple years of not doing much robot-related stuff, I'm getting back into it again...

Hexapod progress: Have received servos, arms, and linkages for the hexapod, and pololu.com will be cutting the chassis pieces this coming week.

Biped progress: Finally came up with a hip joint configuration that I know I can build and I think will work well enough. I really want to do a belt drive system similar to the robot on this page - http://www.ixs.co.jp/index-j.html - but am not convinced I can actually pull it off. The design I have in mind is based on Lynxmotion's brackets though so there's no real fabrication involved. The 3-DOF hip has been one of the more challenging parts of the design, if this really works I may move from design to construction soon.

Software progress: I'm working on a visual programming language to make it easier to develop software for both robots, it's coming along pretty well. It's similar to the control system diagrams in Juice, a bit more sophisticated, completely rewritten, and not (yet?) integrated with the simulator. I can simple draw diagrams and execute them, but am still fine tuning the "syntax" and optimizing the execution code.

With any luck, the software and hexapod will come together in the next two or three weeks and I'll have it walking before christmas. But it's probably optimistic of me to think so. :-)

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