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"Hammerhead" Biped Walker
DAT File
About
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There are at least two distinctive types of center-of-gravity (COG) shifting Lego biped robots. Those whose legs bend at the "ankle", and those that move the RCX at "hip" level, using the smart brick as counterweight. For a time, Kazuhiro Umeda's walking biped robot looked to me as the coolest thing ever done with Lego. And although Kazuhiro-san graciously gives away a DAT file of his model, it remained elusive to me. I didn't want to copy it, yet I felt it was far too complex to serve as inspiration. For several reasons, I believed that you could only accomplish a two-leg walking machine by fairly sophisticated computer control, not necessarily outside the scope of the Lego RCX brick , but maybe a bit too sophisticated for a first-time player like me. And then one day, feeling pretty confident after coming up with the DDK biped , and while searching the Lego Mindstorms website for other DDK-based bipeds, I saw Yoccha's extraordinary Demigod of Pure White . This was a true epiphany, as I quickly sorted out the single-engine walking mechanism from the photos. This proved to be too juicy no to try to build right away. And I did. This being the second biped I make, there are a few things that I already got nailed down. I will outline them loosely following the sequence of events that went into its design. Note: I created Hammerhead about 3 weeks ago. Since then, I've been working on a steerable biped, and more to the point, have compiled a list of links to Lego bipeds on the internet. By doing this I've discovered life beyond Kazuhiro-san's still excellent bot. Among my favorites in the same complexity level as Biped1 are SPE672's Two-Legged Walker and Explorer1's SpOC-Bot 
I first "sketched" the walking mechanism several times, first mentally and then using Lego elements to make sure everything was working. The picture to the right shows how it is finally implemented. The key to constructing an ankle-level COG-shifting robot using only one engine is to use the hip joint both as an axle to the back and forward walking movement and as a means to transfer circular movement to the weight-shifting mechanism located at the ankle. So rule one would be figure out the general but complete walking cycle, and only then start fitting it into Lego standards. Rule two is to incorporate worm gears as a means to have a better control of the mechanism. Backtracking is huge once the model starts to grow. The first "construction" step comprises the design of the feet. I wanted a roughly square footprint, and needed to accommodate a reinforced axle and several bevel gears that are part of the walking mechanism. This worked out to fit in a platform 10 studs long by 8 studs wide.
One significant departure from Yocha's model is the use of a longer beam as the lever for the ankle-shifting mechanism. This reduces the span of the side-to-side weight-balancing movement. Rule three: design solid feet and check for their stability. Rule four: be prepared to change them later as needed. Rule five: assume that breaking a piece or two is part of the price. The lower part of the leg is a necessary part of the foot / ankle contraption, but instead of finishing up the leg, I chose to first create a box 4 studs wide (distance between the legs at hip level) with two single-motor-driven axles. I spent very little time here, just enough to come up with the necessary gear reduction that would be powerful enough to lift the bot on one leg. The battery box on top was a latter addittion. Once the robot was assembled, the battery box attachment made it auto-powered as well as raising the COG and thus greatly helping the walking cycle. This black, red and yellow battery box comes with set 8082 . While testing the robot, the battery box received close to ten bangs against a hard ceramic tile surface. This produced some very nasty noises of plastic hitting hard rock, but the box shows no scratches. I dread what would happen to the RCX in similar circumstances. Rule six: come up with a simple body with two rotating axles sticking out of it. Rule seven: concentrating the weight on the head helps, but also produced hair-raising situations. Rule eight: Be prepared to modify even the simplest motor box you can come up to fine tune the bot later on.
When constructing the robot, it is important to remember to place the bevel gears at ankle level on the opposites side in each leg. See picture for details. Finally, it would have been much cooler to design a robot with a knee joint, so that the foot is raised from the ground vertically as well as laterally. But as I said elsewhere, I am a first-time player. Rule nine: Prepare to spend some time in the leg-to-motor block attachement. Transfering motion through an axle is tricky. Rule ten: completing the construction doesn't mean the robot is ready to walk . |
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This single footprint had to be stable enough to support the whole robot, so
throughout the process of integrating the weight-shifting mechanism into a
sturdy structure, I mercilessly tested it by attaching a couple of Technic 16 X
1 beams and hanged battery boxes and motors off them via an 12-stud axle. This
axle ended up bended, probably damaged beyond repair.
The final step was attaching the legs to the motor block. This called for a
somewhat tricky design, as it needed both to be sturdy but also easily
accessible in order to make the necessary gait adjustments. The small beveled
gears were fortunately up the task and by placing them on the outside of the hip, I force
the weight of the robot on them, thus helping to engage the teeth of the two gears.