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BEAM From the Ground Up is a BEAM Reference Library site.
Convergence
The fine art of
falling
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.....
Methods of
adjustment... 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. 1: Nv Time
value 2: Weight
Redistribution 3: Leg
Shape Baby Steps:
Be patient. What It should
be achieving: 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).
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.
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.
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).
All parents take
great joy in watching their children take their
first steps.Joy is not usually the emotion that
roboticists feel....
The easiest way
to get an idea about what the legs should be
doing is to step through the motions
manually.
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