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[DPRG] Velocity Profiling move of RC Servos

Subject: [DPRG] Velocity Profiling move of RC Servos
From: Earl Bollinger earlwbollinger at attbi.com
Date: Wed Dec 11 18:15:01 CST 2002

Today's servos seem to have much better pots over the servos from years ago.
When I worked at Proline Electronics (for anyone who doesn't know, they used
to make the best RC systems at that period in time), they used a special
wiper arm that had a carbon button to ensure the pots would be reliable in
model airplanes, and to improve the accuracy of the feedback measurements.
Most all the competition airplane and boat people used their radio systems.
During that period the pots were attrocious, really crummy stuff.
The current crop of radio systems seem to have much better quality pots
today. Of course the el-cheapo servos are excluded from this statement. I've
flown a number of RC models, some with considerable hours on them and the
"newer" servos never gave me any problems. Of course since the models were
very expensive, I used good, contest rated, ball bearing servos that cost
more than the cheapie ones. Back in the Proline days, I used to have to
disassemble the servos about every 20 hours and replace the pot element and
carbon button, to prevent problems from occurring.

The most common problem is the center, neutral position would get a bad spot
worn into the pot element, as that is the place where the servo is
positioned most of the time, thus the vibration and wear would eventually
get to it. Today, the better servos use a multi-fingered wiper that ensures
the pot wears more evenly over the single contact wiper units. I still see
the single wipers in the cheaper servos though, so that might be a problem
if you get a lot of hours on a servo.

So Randy, I don't think you need to worry about the pots, that'll be the
least of your problems.
Unless they gave you el-cheapo servos.

I don't think it's probably worth the effort to put in expensive ball
bearing servos, unless you need to reduce any slop of play in the arm
joints. Most of the high torque servos are taller over the regular ones, so
you may have problems getting them to fit.

It is a neat arm, nice job coding it up. Forth is still a mystery to me.

-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org]On Behalf
Of Randy M. Dumse
Sent: Wednesday, December 11, 2002 7:51 AM
To: Jim Frye; Dprglist at Dprg.Org
Subject: RE: [DPRG] Velocity Profiling move of RC Servos

I'll have to look closer at the instructions and what we've got
in the fine details. The gripper joints are almost frictionless
(very loose) but the rod doesn't seem to move easily, so perhaps
it is caught in the cable or crimps as you suggest.

I was wrong about the servos. Apparently I've got all HS-322's
instead of 422's, which is surprizing.

I had over looked the life of the pot. Certainly from my old
days fixing radios and TV's (most of them were tube based back
then, if you can imagine!!!) after filment burn out, the rasping
of the volume control was the next most likely problem.
Certainly the pot must be the weak link in a servo (if not over
stressed otherwise).

I still like the idea of higher torque servos. The two servos
you have at the base to lift like a shoulder will either be able
to or not able to lift something at the end of the lever arm
based purely on how much torque they can generate. Then the
elbow above them, being a single servo out on the arm, again the
same is true. A substitution of a 100 oz. in. servo set would
allow the arm pick up over twice the load it can currently. But
as you say, if the potentiometers are crappy, and there's no
life to the arm... maybe that doesn't matter greatly, because
without a larger gripper, I don't have an application for the
arm beyond the learning experience (which is very valuable in
its own right, however).

You know, a bit of market feedback. Down here in Dallas, lots of
us dream of advancing some day to the point where we can give
David P Anderson a challenge at can can. That takes an arm which
can grip a can. And of course the next step up, is to be able to
pick up a fill canned drink of some sort. I wonder if you
wouldn't sell more arms if you had a kit big enough to deal with


> It isn't inherent to the design. I have found it to
> be very reliable when
> set up properly. The cable is set up as a poshrod. It
> will work reliable
> only if if you minimize friction when opening. If you
> have trouble with
> this, simply loosen the hardware associated with the
> gripper assembly. If
> this doesn't help then perhaps the gripper cable
> itself has friction. If
> that is the case then the crimp could be at fault.
> It's easy to fix, just
> apply a slight crimp in the opposite direction to
> loosen the crimp a bit.
> All of this applies to the cable bending up instead
> of the gripper opening.
> If however the gripper opens all the way and then the
> cable bends, then
> this is caused by overtravel of the servo. This can
> be eliminated with
> adjustment of the hardware and or software on the
> gripper servo. Once this
> is fine tuned it will deliver long term trouble free
> operation.
> >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?
> I seam to remember a Hitec rep telling me the servos
> have a lifetime of a
> few hundred thousand cycles? But the most limiting
> factor is the
> potentiometer.
> About servos.
> In my testing the Hitec units perform and behave much
> better than their
> Futaba cousins. There has been an inrush of GWS
> servos on the market. My
> personal experience with them tells me they're not
> ready for prime time
> yet. Gears that have less than perfect quality
> control means they can lock
> up under normal operation, and the pots are the
> cheapest thing that will do
> the job. Cheap pots means low lifetime... They also
> have larger overall
> dimensions as compared to other comparable spec
> servos. This can be a real
> problem when trying to standardize mechanical
> components. As for using
> higher torque servos in the arm. Yes it can be done,
> but I'm not convinced
> that it will be money well spent. In other words I
> don't think that higher
> torque servos will automatically mean you can all of
> a sudden pick up large
> loads. I know the PID in the servos aren't designed
> around what people like
> us want to do. The arm with 422 servos can easily
> pick up as much as 8 oz.
> But the real test is when you try to put the object
> down in a graceful
> manor. This usually results in a slam even when the
> gripper is lowered
> relatively slowly. Obviously there is still much work
> and experimentation
> to do. Thanks, Jim
> -
> Lynxmotion, Inc.
> http://www.lynxmotion.com
> 866-512-1024 Sales
> 309-382-1816 Sales
> 309-382-2760 Support
> 309-382-1254 Fax

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