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[DPRG] Velocity Profiling move of RC Servos

Subject: [DPRG] Velocity Profiling move of RC Servos
From: Randy M. Dumse rmd at newmicros.com
Date: Sun Dec 8 16:11:00 CST 2002

> your success is a wonderful accomplishment.

Thanks Chuck.

> Once an arm is completely profiled, the data is
> installed in a kinematics
> model of the manipulator and you use inverse
> kinematics to accomplish
> specific work cell goals.

You, or anyone, got a good site or reference on kinematics and
inverse kinematics?

> Fortunately, the servos are *NOT* open loop, in fact
> everything you need is
> right there but you have to go and get it.
> ...
> solder it to the middle pin of the
> potentiometer and bring it out through the hole.

Courtesy of Bill James and David Peterson, we got that signal on
one RC servo. Those Hitec 422's are the hardest servo I've tried
to open. Failing to get the circuit board out, Bill found the
pot signal by probing the board and we have that brought out on
the one we experimented with.

> install a 1/8 watt ... .1 ohm precision
> resistor in series with it.

Haven't tried to go after that yet. Good. Another thing to try.

> It would actually be an excellent use of
> one of those Motorola Nitron processors.

Yes it might be. I'm way behind on using the Nitron. I'm still
going full steam on the IsoPod(TM), and haven't gotten past Code
Warrior entry hurdle yet.

> Alternatively
> with appropriate trimming components and a dual
> op-amp you could bring out
> a 3 wire (position, load, ground) feed back connector
> and hook it into the
> control processor (probably an Iso-POD in your case :-)

Yes, that sounds better. We have the 8 ch of 12bit A/D there.
Might as well make use of it. Atleast I can direct read 4 of the
eight servos. Have to think about what that means for the 5th
servo. None are non-critical, in any sense. I'd have to run one
open loop, or come up with an alternative plan... hum...

Well, our '807 IsoPod(TM) in layout has 16 ch. of A/D.

We looked at the swing on the center tap of the pot, and it was
limited from .5 to 2.3 or so. That's a nice fit for the Pod's
A/D being limited to 3V inputs.

The supply signal presents a different problem. That opamp would
have to be special, CMOS as a minimum I'd think. It would have
to work up very close to its top rail, and output the signal
relative to its bottom rail.

> Another point, there was a guy (NetMedia?) selling a
> servo controller that
> sensed the current to the servos locally so with that
> hack you need only
> the position data to be completely wired in.

I've heard of this. Is Net Media's whole thing just reading
supply current? Or do you know if he has a more elaborate scheme
than that?

> At that point Jim
> and you would have to figure out if you ran this
> thing 24x7 how long before
> the gears in the servos turn to dust :-) I'm guessing
> you probably wouldn't
> get more than a month or two out of it but would be
> love to be proven wrong.

You're probably right.

Well, two things. I'd use higher torque servos, and metal gear
ones, if I was looking for any kind of reliability. I suggested
a 100 oz. in. BB servo to Jim rather than the 422 he chose. Now
that I've seen the arm, I see what he meant about some
components designed for a specific servo size. Assembly in
places is critical on servo size, so I'm not sure how flexible
substitutions could be. I'd also wonder about the plastic gears
in the gripper and the cable delivering power to it. The cable
gave us some problems, bending over on openning of the grippers.
I don't know if it was we over travelled it (yep, I admit we did
that, ...or should I say I did that) or if it is inherently
going to be that way.

We could set up a rig where it pushes buttons at opposite ends
of its range of motion, then set it going, and record when the
buttons stop getting pushed. Jim, you want to run one of these
to failure to find out? What do you think?


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