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Performance

front view Click in the photo to see a larger version (33K)

Yes, I know, some animation would be appropriate. I'm working on that. For the time being, you'll have to make do with a description and some numbers. First the size. The body measures 12 x 11 x 3.5 cm. Width including legs is 22 cm. In resting position, its belly is 2.3 cm off the ground, rising to 3.3 cm at the high point of a step. As you see it, the Spider weighs 92 grams, and consumes around 17 mA at 3.6 V (61 mW) when walking on a flat surface. It has been tested at an all-up weight of 158 grams, climbing a `stairway' made of cassette boxes; power consumption went up to about 30 mA at 3.6 V (108 mW). One full step (eight legs making their moves) takes 4 seconds to complete. Forward speed is 70 cm/minute.

In solving problems, the Spider is aided by the spring quality of its 1 mm steel wire legs. Hold one of its feet in place relative to the body and the mechanism keeps turning, the obstructed motor consuming less than 40 mA while it bends the leg. Let go and the leg springs back into shape. As I write this, the Spider is scrambling up and over my keyboard. Some of its feet get temporarily stuck between keys, springing loose again as others push down. It has no trouble whatsoever with this obstacle, nor with any of the others on my cluttered desk - even though it is still utterly brainless.

Click in the photo to see a larger version (30K) side view

Limits

As the feet rise to a maximum of 2 cm off the floor, a cassette box is about the tallest vertical obstacle that the Spider is able to step onto. Another limitation is slope. When asked to sustain a climb angle of more than about 20 degrees, the Spider rolls over backwards. And even this fairly modest angle (extremely steep for a car, by the way) requires careful gait control, making sure that both rear legs do not lift at the same time.

Improvements are certainly possible. Increasing step size would require a longer body (more distance between the legs) and thus a different gear train. A better choice might be more legs, like 10 or 12 on a longer body, but with the same size gear wheels. That would give better traction and climbing ability. And if a third motor is allowed, one might construct a horizontal hinge in the `backbone'. Make a gear shaft the center of a nice, tight hinge joint. Then the drive train will function as before. Using the third motor and a suitable mechanism, the robot could raise its front part to step onto a tall obstacle, somewhat like a caterpillar. But turning on the spot becomes difficult...

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