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Senin, 07 Februari 2011

Andrew Miller's 2-Motor Walker Tutorial ( Part 5 )

Convergence, or the subtle art of falling over

Well this is where we try to make your pile of wire and batteries walk -- the counterintuitive part of the whole thing.

Most people are of the misunderstanding that in order for a robot to remain on its feet it has to be balanced at all times. This is the train of thought that leads to so many 6 leg walkers (three legs is a stable platform from which to move your other three). Although this is in some cases successful, it makes for a robot that doesn't adapt well. Walking should be thought of as controlled falling. Static balance is not the key, rather it's dynamic imbalance.....

There are several ways to make your walker stumble around right, all of which are in some way related to the center of gravity of your bug. None is more important then the others, nor is it possible to make it walk without adjusting all of them.

Nv Time value

You'll note that in its raw state the Nv is just an RC time value (but remember it is not a constant, it adjusts itself according to load). So at a base level you can change the duration of rotation of each of the motors. One Meg Ohm is the default value, it is just a good starting point. You can adjust the values as big as 20 Meg Ohm or as low as a few KOhms. By changing the duration of the leg's movement you change where it stops. If you think of the Walker as a first class lever and the feet as fulcrums, you'll see that the position of the leg when it stops is crucial to which direction it tips.

Weight Redistribution

By moving the components from front to back you change the center of gravity. The batteries are a good candidate for this since they are invariably the heaviest thing on the bug.

Leg Shape

This is the thing most likely to change dramatically; by bending the legs back and forth you change the fulcrum point and thus the balance. Remember that contact point is more critical than leg shape (refer to the leg mechanics Section).

Baby Steps

All parents take great joy in watching their children take their first steps. Joy is not usually the emotion that roboticists feel....

Be patient.

What It should be achieving

The easiest way to get an idea about what the legs should be doing is to step through the motions manually.

This is where that compliance thing comes in. If you can't move the legs manually then the motors are not going to provide an appropriate feedback to the microcore.

By twisting the front leg CW about 45 degrees off center and the back leg about 30 degrees CW you have what we'll call start position. The walker should now be balanced with it's front left foot in the air and be just on the verge of tipping forward onto it.

This is where the dynamic imbalance thing comes in, the robot literally falls over onto its front foot.

You should be able to tell if it's at the tip point by giving it's butt a little tap, it should tip to the front foot and stay tipped. If it doesn't, try moving the battery front or back in order to find the balance point (leg configuration will come later). The next step (literally) may or may not be obvious. By moving the back legs CCW you will move the fulcrum back thus making the front tip down and the rear right foot raise off the ground. Keep rotating it until you have moved 60 degrees or so, the front two feet and the back left one should be flat on the ground and the rear right will be just off the ground towards the front of the walker.

The walker has just completed a half a cycle (two Nv processes).

Now by moving the front leg CCW 90 degrees you will provide lift and drive with the front left foot and raise the front right in the air just to the tipping point. Now its time for the rear to produce the drive forward that tips the walker forward and raise the rear left foot while stepping. This is done by rotating the rear CW 60 degrees.

TADAA! One walking cycle and a full loop around the microcore.

Now go through that a few times manually with power off and familiarize yourself with what it should be achieving. You may have to move the battery around and change the legs a little. But remember that symmetry is very important. In order for both sides to be doing the same thing you have to have the feet contacting in the same place with respect to the body (how the leg gets there isn't as important). See leg mechanics.

The body should also sit flat when all feet are down and the legs are straight out. If the body leans, then one leg is shorter than the others and your bug will limp (but then, so would you if one leg was shorter than the other).

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