· Walker 1

June 2005
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My first attempt on building a biped.

After seeing so many different walking robots on the internet, I finally decided to try and build my own. This walker falls under the category of the "COG-Shifters". Unless you have built one of them yourself, you probably won't know what this means.
COG stands for "center of gravity". This type of bot walks by shifting this COG, thus forcing one leg to remain still (being pressed against the ground by something really heavy) while the other one moves. Mine is one of the more primitive models, not really raising its legs but rather dragging them over the surface it's standing on.

As you can see, the RCX is a little inclined on top of the construction. This is not an accident, the only reason for it to be like this is that the RCX itself is being used as the COG. It moves to one side of the robot (over one of the legs) so the other one can skid and move forward a bit. The RCX then swivels to the other side and the leg that was trapped under its wheight can now move. This process is repeated to make the robot move.

You can see four motors on this picture, plus one that is visible on the previous one. The motors on the legs do not move (can you see any cable?). They are additional wheight so the overall COG is not too high. Imagine a very tall and light robot with a very heavy object on top. It will fall to one side very easily. If you place the main wheight (the COG) further down, it will not be as sensitive to this. Another solution to this problem would be to make the base of the robot wider. I used both of these techniques to make the robot pretty stable.
If you want a more scientific explaination about the COG, ask Wikipedia.

The motor centered above the legs moves them by gearing down the motor and therefore achieving more torque (power). This is needed because the robot is very heavy and has to be moved by only this motor. The topmost motor inclines the RCX to either side. Why is this really necessary?
The "leg-moving motor" always moves both legs. One moves forward a bit and one moves backwards at the same time, while the motor stays essentially still. By moving the COG above one leg, it is forced against the ground and won't move due to friction. Because of this, the other leg will move further.

To make all this physics a little less obvious, I decorated the bot a little to make it look like a little hiker. Of course, the principle stays the same, and so does the motion of the robot. By the way, a little video can be found here. (823 KB; without decoration)

The following three pictures show the robot partially disassembled so you see the different systems involved in the walking process:

The RCX was removed to show the gearing necessary to move a mass as heavy as the RCX from one side of the robot to the other (It's a microprocessor and 6 AA batteries, after all!). The clutch gear (white) is of little use here, as the worm gear (hidden inside the gearbox) reduces the torque that the motor needs to apply to a minimum, even when the RCX is on the limit of movement and starts tearing the whole robot apart. (So be careful with worm gears as they are very powerful!)
See the touch senors? They allow the robot to know whether the COG (i.e. the RCX) is centered or is leaned to one side, thus pressing the touch sensor on that side. This activates a self-centering routine before doing the actual walking.

All the COG-shifting gears were removed, only the walking device remains. The clutch gear lets the motor spin even if the legs can't move any further. This eliminates the need of a rotation or touch sensor to determine the exact position of the legs and lets the robot rely entirely on timing to adjust the leg position. This means the motor simply turns on and moves the legs until they can't go further, and it doesn't damage the motor if it stays on longer than needed.

These are only the legs. The stick in the middle is the one attached to the crown gear visible on the picture above. The large gears cannot turn, so the whole leg has to move when the small gear in the middle starts turning.
I recommend you build your own robot so you understand all this COG-theory and leg movement better. Watching the video can also be useful.
I didn't create this robot from scratch. A lot of inspiration comes from Joe Nagata and other walking robot websites. However, I built the robot myself; I did not have concrete building instructions, just some basic ideas.

One more picture of my creation, because I'm so damn proud of it ;-)
It took me about 4 days to concieve it, 3.5 of them were probably dedicated entirely to the wheight-shifting mechanism. I tried like 5 different designs ranging from little cars that drove from side to side to some complete disasters that made the robot fall over because they were too high mounted.
You can get the source code for this bot (walking + self-centering prior to start) here.

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