Ideas for rotating head

Where does the error come from? Is the shaft wobbly, thus making the
rotating platform unstable? Is the drive mount hardware, specificaly what
looks like thin metal holding the motor in place, too flexable?

While there are a number of issues with the design, if you can code around
the inaccuracies, you should focus on correcting the mechanical slack.
Basically correct the mechanics so that it doesn't get too tight or too
loose when it turns.


I don't know what you are using to support the rotating platform, so it is
hard to really say.

: I don't know what you are using to support the rotating platform, so it is
: hard to really say.

A hardware store lazy susan bearing -- this:
http://www.herbach.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=HAR&Product_Code=TM92MEC2204

Rather than try to center the shaft on the whole platform, I decided to
motorize the lazy susan as a separate problem. I reasoned that the shaft
need only be centered on the bearing to work. The platform itself could be
slightly off-center and give only a visual problem, so that was less
critical.

I cut that square plate you see to the same size as the lazy susan, found
the center of that (standard, low-tech, draw an X corner to corner), and
mounted the shaft to the plate.  The plate screws through the platform to
the bearing in the corners.

: Where does the error come from? Is the shaft wobbly, thus making the
: rotating platform unstable? Is the drive mount hardware, specificaly what
: looks like thin metal holding the motor in place, too flexable?

I think it's a combination of any error in my centering of the shaft, any
slight angle the shaft as to the plate, and the cog-gear has on the shaft
itself.  The metal holding the motor is actually steel (it was an OLD CD-ROM
drive) and doesn't give at all.

: While there are a number of issues with the design, if you can code around
: the inaccuracies, you should focus on correcting the mechanical slack.

If I put the tension to the point where it doesn't go slack at any point, at
the tightest point it won't turn with the motor hooked directly to power, so
I didn't see a way to code out of it.

The other solution I didn't mention was a bigger motor, but brute forcing
it didn't seem like a great idea.


--
==========================================================
Chris Candreva  -- chris@westnet.com -- (914) 967-7816
WestNet Internet Services of Westchester
http://www.westnet.com/

On Fri, 20 May 2005 14:15:48 GMT, "Christopher X. Candreva"



In most situations like this a spring loaded idler wheel is run
against the belt between the motor and the pulley to keep ~ the
same tension on the belt while allowing it to account for off
center pullies and such. The motor can also be put on a hinged
mount with a spring keeping it tensioned on the belt allowing the
motor to move some to account for the pulley being off center. If
you are using a cog belt, I'd look into using one of the big
quarter scale servos to turn the lazy susan (but wouldn't have
the power of the motor setup).  

There are a couple of changes I'd suggest.

The most critical is the radial load being applied to the potentiometer.
Even timing belts require a certain tension, and the tension must be
increased as torque increases. As torque increases, so does the overhund
loads on all the shafts. I wouldn't worry about the motor, but the pot
may die an early death if the load is too high. It'll start getting
flakey, and then eventually go out altogether.

So I'd recommend doing two belts: a drive belt between the motor and
turntable, and a separate one from the turntable to the pot. The
separate belt can be a smaller cogged belt. Because you probably have
more flexibility regarding the length of the turntable shaft, you might
mount the sprocket for the pot belt there, instead of on the motor. You
can add a spring-loaded tensioner, if you need to, on the
motor-to-turntable belt, and it won't affect accuracy or load on the
pot.

You might also want to think about scrapping the metal mounting thingie
and create a mounting plate out of 1/8" plastic, like ABS or
polycarbonate (PVC would probably be too soft unless it'sx 1/4", and
acrylic cracks too easily). Mount some aluminum standoffs of the desired
length. That ought to be a lot more rigid than the metal frame you have
now. The flex in the sheetmetal might be causing some of your problems.
You can also afford to remake the mounting plate a couple of times to
improve accuracy. Cut out a couple squares, drill the mounting holes in
all of them, and then try out a few until you get it right.

If the shaft to the turntable is slightly off center, you'll get a
misalignment that may be what you're seeing. I think you might be able
to mitigate some/all of this misalignment if you enlarge the holes on
the turntable so that you can adjust the centering of it over the lazy
susan flange. Loosen the screws (they should have large heads, or use
fender washers), adjust the centering, retighten, and try again.

One way to check for alignment is to remove the center shaft, and shine
a flashlight down the hole, and onto the floor. Spin the turntable. If
the spot on the floor wobbles, the turntable is not centered on the lazy
susan.

-- Gordon

    --Couldn't find a photo of Carl's commutators, but here's a
drawing from his site that you might find useful.
http://www.carlpisaturo.com/diagrams/OctopusStudyB_Main.JPG
    

--
        "Steamboat Ed" Haas         :  The other night I  
        Hacking the Trailing Edge!  :  dreamed about wasabi...
                   http://www.nmpproducts.com/intro.htm
                   ---Decks a-wash in a sea of words---

That would be a way to fix your problem, but as Gordon pointed out, it will
eventually kill your pot. Modify your idea slightly and I think you'll have
a winner...

Use your idea of a spring-loaded lever as a tension arm, but run the belt to
an idler wheel, not the pot. Move the pot farther up the tension arm and run
a second (obviously much smaller) belt (like a thick rubber band or a large
O-ring?) between the pot and the idler wheel. This will allow the arm to
keep a constant tension on the drive belt, but not subject your pot to the
strain of that tension, which would ruin it. The pot and idler wheel will
always be a fixed distance apart so you can make the tension between them
just enough to work without putting undue stress on the pot.

Alternately, you can try an entirely different method - put a large encoder
disk (maybe made from a large painted can lid?) on your shaft and read the
position that way - eliminate the pot entirely. I'd still use the tension
arm, but it would be a simpler design. It means redesigning some of your
control circuits, but you may gain more functionality, so there are pro's
and con's to the idea...


~WEC

Thanks to everyone for the ideas. I hadn't thought of the load on the pot
being a problem, or at least didn't think of the load being that much of a
problem.

In fact, I've already someone damaged this pot putting the gear on. It's
such a tight fit, I pulled the shaft loose trying to pull the gear off
again, so the shaft now slides up and down in the pot.  I was surprised to
find it still worked after that, but it does, and I've been using it since
I figure I'm going to destroy it completely if I ever try to take it out now.

I'm going to try to set aside a night of tinkering, with a printout of all
your suggestions, and see what I can come up with.




--
==========================================================
Chris Candreva  -- chris@westnet.com -- (914) 967-7816
WestNet Internet Services of Westchester
http://www.westnet.com/