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C00002 00002 \\M1BDR25\M2SIGN57\M3NGR25\M4NGR20\F2\CVICARM
\F3\C154 EAST DANA STREET
\CMOUNTAIN VIEW, CA. 94041
Professor John Birk
Department of Electrical Engineering
University of Rhode Island
Kingston, R.I. 02881
My apologies for the delay in this reply. I have been very busy
recently, and have been lax at replying to your letter. I hope the following
information will suffice for a while.
You asked several questions, I will try to answer some of them now.
Joint travel- 600 degs. vs. 330 degs. Most everyone is using 330
degs. now although the original arms at Stanford had 600 degs. of
travel. To resolve 600 degs., I add another wiper to the pots. This
then requires an extra A/D channel. When one of the wipers is in the
dead band at the ends of the pot element, you switch to the other
wiper. This requires some computer bookkeeping. In addition, you
have a startup problem, as you dont know which stop you are near.
Because of the extra travel,wire failure rate is potentially greater
too. I've thought of designing a proper 600 deg. pot, but have not
gotten to that yet. More likely, I will eventually get to intalling
encoders before fancy pots. Encoders will resolve continuously over
Yes you can damage the arm by bumping into things. My
suggestion is to put some styrofoam around while testing it out.
Most likely to fail in this mode are the fingers or hand slides.
Both are easily replaced. The arm uses permanent magnet D.C. motors
with Alnico 5 magnets. These magnets will become permanently
demagnetized should you ever exceed the maximum allowable current
rating. Remagnetizing requires dissassembly and is a hassle. Thus
your power supply and amplifiers must have a current limit
with protection from turnon and turnoff transients, etc.
Brakes are used to hold the arm in position when not moving.
This allows the motors to be turned off thus reducing motor heating.
At Stanford we use a software servo. Thus the computer turns the
brakes off only when moving the arm. At all other times the motors
are off and the brakes on. With a hardware position servo you can
probably turn the brakes on when the corresponding joint is within
the error tolerance region.
Tachometers are used to provide damping inside of the
backlash on the joints. Yes, you can differentiate the pots signal,
but its good to have extra motor damping. You can do this two ways-
with tachometers, and by using back EMF to indicate motor speed.
Both systems work. In fact, I can deliver arms either with or
without tachs. The no tach version costs $75 less per joint. For
your wrist thats $225 less as the hand doesnt have provision for a
Power amplifiers are not supplied in the price I quoted you.
You must provide them, or else I can provide them. At the present time I
am trying to come out with a power amplifier with hardware servo
electronics all in one unit. Price is $1200 for the four wrist
joints. You then interface this to your computer thru an A/D and D/A
interface ( plus some brake bits). I can also send you the
schematics of what we have at Stanford, so you can build it yourself.
The Stanford system is a software servo based system, and the power
amplifers acccept current commands from the DACS rather than position
Speeds are based on power supply voltage, and are independent
of duty cycle. but they are for unloaded joints. Basically, a
motor's speed is a function of the voltage supplied, the back EMF and
the load. The power supply is nominally about +-30vdc. With more,
you can go faster, but power dissipation goes up too.
My power supply keeps track
of motor heating with a thermistor attached to a resistor model of the
motors. This cuts out the supply in the event of motor overheating.
At Stanford we just use software to limit duty cycle.
The wrist unit is provided with 8 spare internal wires for your
instrumentation. Extra wires can be added, but if they are bulky
coax, they probably will have to be run outside the arm structure.
I have set a delivery of 24 weeks ARO because I am always in
the process of improving the arm. At the present time I am hesatant
to start on a new arm order until I get a little feedback from the
three systems I am currently finishing up. Very possibly, I can get
something to you sooner. But from past experience, I can only say
that I am striving for the best system I know of, and this takes
Pricing. The complete arm with power supply runs around $20,000.
It costs just as much to make a wrist joint as a shoulder joint.
I am trying to keep the costs as low as possible consistent with quality and
performance. In addition the volume of these arms is not yet to the point where
I can justify production runs.
I will be comming to the East coast in about two weeks. I'll plan on
calling you and stopping by for some furthur conversation.
I hope this information is adequete.\.