Alt-BEAM Archive
Message #08878
To: beam@sgiblab.sgi.com
From: Bumper314@aol.com
Date: Fri, 7 Jan 2000 00:03:43 EST
Subject: [alt-beam] Re: SIMPLIFIED D1 CIRCUIT
In a message dated 1/6/00 9:59:44 PM Mountain Standard Time,
Wilf.Rigter@powertech.bc.ca writes:
> I hope this circuit will shine a little light on your D1 problems. Consider
> using this simplified D1 (SIMD1) type solar charger /darkness trigger. It
> uses 1/6 of a 74HCT14 with the remainder used for pummer, microcore, etc.
> If the rest of the hex inverter 74HCT14 chip is not used, then a single
> 74HCT14 inverter chip may be used instead, which is about the same size as
> the original 2N7000. The cap C1 can be a gold cap or if the current
> requirements are not high (ie pummer), one of the higher voltage/higher
> impedance coin type supercaps would be just fine. Alternately C1 can be
> replaced with a suitable NiCad battery. At dawn and throughout the day the
> SIMD1 will charge the cap/battery and then will turn on the circuit it
> controls at dusk and stays on until the cap/battery is discharged. The D1
> will trigger at the lower (+V/3) threshold of the Schmitt trigger and with
a
> 74HCT14 that point may be as low as 10% Vmax open circuit of the solar
cell.
> The SIMD1 "snaps" on when the solar cell is dark and will only reset at a
> much higher light level or when the cap/battery is deeply discharged
because
> of the Schmitt trigger hysteresis. The diode (D1) can of course be
replaced
> with the more efficient ZVD circuit.
>
is this more efficient then the original D1
Steve
8879 Thu, 6 Jan 2000 21:47:38 -0800 [alt-beam] Re: Pressure Sensors "'beam@sgiblab.sgi.com'" Wilf Rigter Yes, this is a good idea! A displacement transducer (ie linear or rotary
potentiometer) combined with a spring = pressure transducer.
Imagine a PC type joystick mounted on the center of a plate which is the
bottom of the foot. Now use centering springs from the 4 corners of the
plate to keep the joystick lever vertical with respect to the plate. The
springs should be chosen so that there can be a small displacement of the
lever with respect to the plate when unequal pressure is applied. This
displacement causes a change in the X and Y joystick potentiometers output
voltage which corresponds to the distributed pressure difference. While an
actual joystick is not practical the same principle can be applied to a foot
plate made from sandwich of springy and rigid layers. The displacement
transducers can be LED/LDR pairs at the 4 corners, mounted in such a way as
to change resistance for displacement of the top and bottom layers of the
foot sandwich. The sum of the signals provides a measurement of the total
pressure on the plate and the difference provides X and Y signals
differential pressure fron and back and side to side.
enjoy
wilf
> -----Original Message-----
> From: George Rix [SMTP:rix.g@bmts.com]
> Sent: Thursday, January 06, 2000 6:14 PM
> To: beam@sgiblab.sgi.com
> Subject: Re: Pressure Sensors
>
> > Hi, I was just looking around for pressure sensors to use on my biped
> > walker. The first place I looked was Jameco and I found one that looked
> > good. I thought to myself "I'll get on the net and order 8 of them".
> That's
> > when I looked at the price and noticed that they were $10 each. No way I
> > could spend that much money. I then checked DigiKey. Prices there range
> from
> > $13 to $150. Ouch! I then checked that web site that Evaristo posted but
> I
> > couldn't find any prices on their site. Anyone know of a cheap source
> for
> > pressure sensors? Thanks
> Well, if you didn't mind some extra effort, you could attach a small gear
> to
> the end of a pot, and then use a rack (like in rack and pinion steering)
> to
> change the resistance. Just give it a spring to go back to normal, and
> voila! A pressure sensor (I hope).
>
> Peace out!
>
> Rob Rix
>
> P.S. Was that clear? If not, I'll try better next time...
>
> God knew what he was doing when he deprived man of wings.
8880 Thu, 06 Jan 2000 21:54:37 -0800 [alt-beam] Re: Pressure Sensors (not about conductive foam) beam@sgiblab.sgi.com Bruce Robinson Hi, Ian. I guess everyone in the world read that article on conductive
foam pressure sensors. Here's a different idea: piezoelectric film.
You can get it through Images at:
http://www.imagesco.com/catalog/sensors/piezo.html
A 1/2" x 1" piece is about $2.50
They also have a nice article on how piezo film works, and how to use it
in various ways. The article starts at:
http://www.imagesco.com/articles/piezo/piezo01.html
and goes on for several pages. Around page 4/5 of the article they
explain how to use it to sense pressure (it produces a voltage when
distorted).
See what you think.
Bruce
8881 Thu, 06 Jan 2000 22:25:05 -0800 [alt-beam] Re: Bi ped beam@sgiblab.sgi.com Bruce Robinson Well, since everyone else is talking about it, I may as well get in my 2
cents worth. This is a subject I actually know something about (unlike
my robotics and electronic skills).
First, human leg joints, from top to bottom. Begin with the hip joints;
each has 3 DOF (ball joints) separated by a relatively wide pelvis. That
pelvis is very important. Next the knee joints, 1 DOF (hinge joint).
Skiping down to the ankle joint, 2 DOF (double hinge joint). Then the
feet, 5 flexible joints inside, not used for motion, but makes a
wonderful triple arch suspension system. Finally, the toes, not to be
underestimated for smooth walking. You don't need all 14 toe joints to
make a walker (humans don't use most of them anyway); you can
approximate the toes with a 1 DOF hinge joint, simulating the toes with
a stiff but flexible material.
OK, now you all want to object about my description of the ankle joint
:) You sit there, twisting your foot from side to side. The knee's not
twisting (I hope), so there's got to be a third degree of freedom down
there. The answer: as you twist your foot, look down at those two bumps
that stick out from either side of your ankle: those are the ends of the
two bones in your lower LEG ... they aren't part of the foot. So where's
that extra degree of freedom? Can't be in the ankle, or those bony
protrusions wouldn't twist like that. Remember, TWO bones in the lower
leg, side by side.
so, does it matter anyway, exactly where between the knee and the foot
that rotating motion comes from? Read on.
If you look at human musculature, you'll discover that the muscles that
control a walking limb are always as high up as they can be. Thigh
muscles are near the hips. Lower leg muscles are just below the knee.
Most of the muscles that produce large foot motion are also just below
the knee, connected by a network of tendons to the foot joint.
There's a reason for this. If you keep all the mass low down, when you
walk, you'll have to accelerate it to get your limb moving, and stop it
when you want to reverse the limb direction. That takes a lot of energy,
and will tend to upset your balance. To complicate things, when the feet
move, they accelerate to twice the speed the upper body is moving (think
about it).
With the weight up high on the limbs, there's a lot less force involved
to move the legs. Try swinging a baseball bat or golf club at your
favourite robot; stop the swing just before you hit the 'bot (oops,
sorry about that). Now grip the bat or club at the wrong end and swing
it. You can stop it a lot faster.
That's why it's important in a robot where you put the servos. The lower
they are, the more unbalancing momentum you'll have to deal with.
Furthermore ...
Humans are top heavy -- we ought to fall over, but we don't (because we
have really great muscle control). Now look at Evaristo's biped -- very
top heavy with that lead-acid battery up there. Look at Ian's
description of his walker motion -- top leans to one side, shifting the
center of gravity over one foot. The thing is, you can't pick up one
foot if most of the mass is down near the ground -- you need enough mass
up high to counterbalance the foot you're lifting. More to the point,
the higher up the mass is, the less you have to lean over to the side in
order to pick up a foot.
So the rules for effective biped design are: separate the hips, lots of
joints (but not to begin with ... learn how to c-ordinate the joints
first), keep the mass in each limb high, keep the non-moving mass
(batteries, controller, etc) up at the top, and cushion the foot
slightly.
-----------
Here's a couple more tidbits.
You don't need an motion in the upper body to walk. Just a solid lump
from the hips up. I spend a good deal of time teaching people to move
this way: torso erect, hands on hips, no leaning or swaying to the side,
no up and down motion. Fast & very efficient.
You don't need arms or a tail to get up off the floor. You can get up
with your arms crossed on your chest, or held out in front of you. I
teach people to do that, as well. It IS pretty hard to stand up without
using your arms if you don't have flexible toes on your foot.
-----------
The great thing about using the human body to study pibed motion is,
there's always one handy to examine.
Regards,
Bruce
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