Microradian motor shaft angle control

On Fri, 05 Feb 2010 14:29:28 -0500, Phil Hobbs wrote:


There may be an aerospace quality resolver that would do that, but the
ones that I know of are about an order of magnitude too noisy and about
an order of magnitude bigger than your itty bitty motor.

Jewel bearings may not be the way to go.  A properly working jewel
bearing leaves the shaft not quite centered in the jewel.  This ambiguity
in shaft position and 'normal' angle will complicate any attempt to
measure the 'radial' angle.

--
www.wescottdesign.com

On 2/5/2010 7:49 PM, Tim Wescott wrote:

Thanks.  I dimly recall that the shaft rolls around the interior of the
sapphire doughnut, so I suppose you're quite right.

I might also try two encoder sensors mounted at (say) 90 degrees from
each other, which would allow me to take out tilt and decentration of
the encoder wheel.  Avago makes some nice 2500-7500 pulse/rev
incremental encoders that might be able to be hacked like that.

The other nice thing about multiple pickups, ISTM, is that it would be
possible to sort out motor jitter from encoder jitter--the motor jitter
could be made common mode and the encoder jitter is all 90 degrees out
of phase.

Then there's Mr. Newton, who constrains some of this stuff pretty
tightly.  (I have lots of ideas, just not much motion control experience
to help me weed out the stupid ones.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

On Fri, 05 Feb 2010 20:12:58 -0500, Phil Hobbs wrote:


One arc second is 1/1296000th of a circle; I don't think that a 2000
count encoder is going to have the accuracy you need.

For that matter, one arc second is 1/1296000th of a circle; if you put a
one meter lever arm on your shaft one arc second of rotation will move
the end of the arm by a hair more than one wavelength of red light.

Not that I'm trying to naysay or anything...

--
www.wescottdesign.com

On 2/5/2010 9:09 PM, Tim Wescott wrote:

I'm trying to do Radon inversion of phase shift plots from beams
rattling round inside a rotating prism, with a 40-50 mm path difference,
and am operating at 350 nm.  So given that I want my data equally
spaced, I get right up into that territory.

A 6000 line encoder with Moiré readout, with the tilt and decentration
fixed, needs about 100x-200x interpolation to do the job.  My hope is
that the actual irregularities of the motion are on a coarser scale than
that (e.g. the first 10 harmonics of the number of motor poles), so with
a bit of help from Newton's laws, I can do pretty well by interpolating.
  I don't mind measuring and remembering the error of the individual
encoder lines, and with independent readouts at different phases, I'm
hoping I'll have enough info for the job.

How good are bearings?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

Phil Hobbs wrote:

   ...


Bearings are better when moving continuously without reversing. I built
a laser interferometer to provide position feedback for the carriage of
a ruling engine driven by a leadscrew*. Position had previously been
measured by a rotary encoder on the leadscrew. The first thing we
learned from the interferometer was that each time the carriage was
restarted, it actually backed up a few microns before going ahead. The
behavior was traced to the dynamics of oil films.

Jerry
____________________________
* Two inches diameter, and the support for one side of the carriage.
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On 2/5/2010 11:46 PM, Jerry Avins wrote:

Interesting, thanks.  What did you wind up with by way of bearings?
Preloaded ruby balls?  Steel balls?  Precision sleeves?  The more
detailed the wisdom, the more useful in this case.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

Phil Hobbs wrote:

I supplied oil under pressure to the existing bearings to keep the film
from collapsing when the machine was still. Normally, the turning shaft
wedges oil into the gap at the bottom. Large machines like power-plant
generators and turbines settle into metal-to-metal contact when still
for a while and suffer damage with every restart. Typically, power
plants use "turning gear", electric motors that keep machinery like
peaking generators moving slowly when not in use. They must know about
pumping just the oil because that is done in the thrust bearings of
vertical turbines. (It takes a lot of power to put a generating station
on line.)

Correcting the anomalous motion wasn't high priority once the
interferometer was in the feedback loop. Only rarely were lines needed
very close together. After all, the carriage motion was erratic only in
detail. The accuracy of the leadscrew guaranteed the accuracy of the
carriage's gross motion.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On Fri, 05 Feb 2010 23:32:14 -0500, Phil Hobbs wrote:


I thought you were quoting 1000 lines, and given the size of of your
motor figured you couldn't go to a larger encoder.  6000 lines may do it
-- I'd check noise, though.  You can get some encoders with impressively
high resolution if you don't mind something that's over a foot in
diameter.  And note that I say "resolution" -- getting that sort of
accuracy is going to be a huge science project no matter what you do.


Whenever I ask a mechanical engineer that question he says "how much
money do you have?"

If you're designing the machine for production -- even low volume -- you
can find a custom bearing supplier and explain what you want and why, and
they will help you specify a bearing that they can sell to you for huge
piles of money.

If you really want to know more ask me off list and I will ping my
mechanical engineering friends for names.  Expect a few more zeros on the
price over equivalent-sized skateboard bearings.

--
www.wescottdesign.com

Phil Hobbs wrote:

Take a look at Sony BH-20 Encoders. A little larger than 9mm (16mm dia).
With a little interpolation they will get you in the range. At 115200
lines, a 16x interpolation gets your resolution below an arc-second and
is easily accomplished with a simple lookup table.
Also I' not sure if your low speed will work, but consider a self
generating aerodynamic bearing like laser scanners use.

--
Jeff Lowe,
Moore Nanotechnology Systems.

On 2/8/2010 7:52 PM, jeff wrote:

Thanks, those are really cute.


Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

jeff wrote:

Passive air bearings are very good and can be very stiff. Some large
pumped air bearings are called hovercraft. Pumped oil bearings work well
also.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On 2/9/2010 11:14 AM, Jerry Avins wrote:

Thanks.  I'm trying to keep this gizmo light on the hardware and heavy
on the software.  Sony doesn't seem to want to talk to mere mortals
about their encoders--you have to register before they'll even send you
a data sheet.  I'll probably have to wear a pigtail and bang my forehead
three times on the floor before they'll speak to me.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

Phil Hobbs wrote:


they are tough getting information from, and you may find that they
don't really understand what they have. Stay away from their
interpolation electronics as they are pricey.

--
Jeff Lowe