Alt-BEAM Archive
Message #12196
To: beam@sgiblab.sgi.com
From: Bruce Robinson [mailto:Bruce_Robinson@telus.net]
Date: Mon, 20 Mar 2000 11:31:21 -0600
Subject: Re: Funkie Bicore
Max Inggal wrote:
>
> Ok today I was determined to build a bicore walker
> using a Master/slave bicore circuit. But when I
> hooked the motors the the circuit for some reason the
> legs turn more in one direction and less the other
Hi, Max.
The simple answer is, random variation. The caps and resistors in your
circuit don't have precise values ... typically they could vary by as
much as +/- 10% each. If you get really lucky, they might average out.
Chances are, however, that one Nv stays active a little longer, so the
motor turns a little bit further. That's the electronic part.
Mechanically, it isn't unusual to find a gear motor turns a little
faster in one direction than the other ... once again, allowing it to
turn just a little bit further. I have one here that turns 12 degrees
further in one direction than the other direction, for exactly the same
running time.
> anyone know why and how I can fix it.
You need some kind of force that will try to return the gearmotor shaft
to a central position. Ideally you can use a method will not exert too
much force when the legs are nearly centered, so you won't be using up
all your walker's energy just fighting resistance. The geometry you
choose will make a big difference.
The most common method is to use a spring to center a set of legs.
Here's an ASCII layout of a very practical system.
pivot
_________________
0_______ o _______0
leg | | leg
| |
|.| <- arm
|
>
< <- spring
>
|
o <- anchor point
The arm is attached firmly to the legs. When the leg starts to swing
away from center, the spring is barely stretched, so it doesn't have
much effect. The further the legs turn, the more quickly the spring is
stretched, so it begins to exert a lot of resistance when the legs get
too far away from center.
Want a bit of mechanical feedback? make your legs like this, but hook
the two arms together with one spring instead of having two springs
anchored to the body. The legs will influence each other slightly as
they turn.
A second way to get a centering action is to use gravity (this only
works when the motor is tilted, or horizontal. Whenever a leg is off
center, it is lifting part of the robot, so the robot's weight tries to
return it to a centered position. This works best with efficient
(smooth) gearboxes. It also has a slight tendency to counteract a
gearmotor that runs faster in one direction than the other.
With a gravity design, the position of the feet relative to the motor is
important. Here's another ASCII layout. Imagine the motor is between you
and the screen, and the bottom end of the motor (where the shaft
extends) is higher up the screen than the top end (closest to you).
0\ /0
\\ //
\\ pivot //
\\_____________//
\_____ o _____/
leg leg
During the first part of its motion away from center, the motor has a
bit of a mechanical advantage as it lifts the weight of the robot. The
more it turns, however, the greater the resistance (unless it goes TOO
far).
In a typical bicore configuration with one motor vertical and one motor
angled, you can't use this arrangement on both motors.
> PS=I know about attaching springs to the motor shaft
> but looks ugly.
Hide 'em :)
> Beside do you see any springs on Mr. T's walkers?
Yup, sure do. Look closely. Some like Walkman, Lobster, and Strider have
a mechanical layout that can use gravity on all 4 legs. If you see one
with a vertical motor, look for a spring. It's probably there, and it
probably doesn't look ugly :)
Bruce
12197 Mon, 20 Mar 2000 19:03:53 +0100 [alt-beam] Festival 23-28 mai 2000...Vierzon France robotik@robotik.org robotik@robotik.org Cher amis,
Le festival Interantional des Sciences et Technologies approche, du 23
au 28 Mai 2000!
Merci de bien vouloir remplir vos bulletins d'inscriptions pour ceux qui
ne l'on pas encore fait, pour votre inscription d=E9finitive.
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Hello!
The Interantional festival of Sciences and Technologies approaches, from
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inscriptions for those which one not in made, for your final
inscription.
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