Re: Re: [ATM] Telescope direct drive

["matt" <electro_optic@bellsouth.net>]

I'm not sure what your point is . Are you arguing that it is doable by ATM's
or that it's not doable by ATM's .

I have not used the Nanomotion but have used Newfocus Picomotors .
Definitely not years of continuous operation . I have used homebrew PZT
actuators and one thing they have in common is they're very fragile compared
to electric motors. Drop it and it cracks, control it with the wrong pulse
shape and it cracks , etc. High voltage drivers are also not that fun or
inexpensive to build and repair .

Assuming the Nanomotion are much more robust and lifespan is not an issue ,
force and price are. 30N means 6 lbs , not sure if that's enough to drive a
mount , definitely not a large mount .

I have posted in the past regarding Renishaw, Heidenhain, etc encoders and
met very little enthusiasm due to price. I'm not sure what you consider the
avergae ATM but I've just seen a post from someone stating that at $3/inch
or so the USDigital optical encoder strip was expensive .

If we add $1k for the piezo actuators, another $1k for encoders, plus some
more for the motor drive electronics , that amounts to $3k just for some of
the drive parts. Add to that a mount and you're in the stratosphere for
mount alone. Not average ATM. Doable by very few.

If Don comes with a novel idea that uses parts he could build himself or
farm out to local machine shops like regular mechanical parts and not
expensive optoelectronic assemblies, then he will certainly advance state of
the art mount technology .

best regards,
matt tudor

-----Original Message-----
From: Matthew L. Brown <mlbrown@everstrive.com>
To: atm@atmlist.net <atm@atmlist.net>
Date: Saturday, April 09, 2005 6:57 PM
Subject: Fwd: Re: [ATM] Telescope direct drive


>
>>From: "matt" <electro_optic@bellsouth.net>
>Matt,
>  I've used the nanomotion actuators at work.  These are small fingers that
>'walk' along a surface in tiny steps, 10 or so nm each at
>http://www.nanomotion.com/hr_series.asp
>
>1. I've run them for years in a factory with no sign of wear.
>
>2. I could easily get 0.25 m/s slewing rate.
>
>3. I used Renishaw linear encoders, 50 nm/step.
>web page for rotary encoders capable of 0.01 arc sec:
>http://www.renishaw.com/client/product/UKEnglish/PGP-117.shtml
>These are moderately priced, just out of reach of the average ATM (but
>inexpensive by industrial standards)
>I think the last linear encoder I bought was around $300.  The rotary ones
>are more expensive.
>
>However, this particular motor doesn't have a lot of force, around 30N or
so.
>On the other hand, it is safe to manually drag the motor around manually.
>
>I did have problems getting them to move *very* slowly, say less than 2
>microns/sec -- they would stick and slip a bit.
>
>And they aren't cheap, somewhere around $1K.
>
>=Matt
>
>
>>Don,
>>there are several problems with the piezo motors as direct drive systems:
>>
>>#1-life span - in order to work as direct drive, a piezo actuator cycle
must
>>produce considerably less than 1 arcsec telescope motion .
>>It takes then a lot more than 1.3 million actuations just to complete a
>>single 360 deg telescope slew  . Piezos are not rated for that kind of
>>lifespan. They would last about for a total of about 10 such slews after
>>which the direct drive piezo would need replaced. Expen$ive .
>>
>>#2-step frequency range not wide enough to allow both tracking and fast
>>slewing. True, a differential or a dual clutch arrangement , in
conjunction
>>with a second electromechanical actuator would fix that but would add
cost.
>>Expen$ive.
>>
>>#3-no intrinsic angle measurement . DC motors with cheap encoder wheels on
>>their shaft are a simple position feedback solution. Steppers even open
loop
>>are also a simple position control solution . Piezo actuators don't have
>>this intrinsic quality , their step size is not exactly constant, which
>>means a separate encoder is required. Since this is direct drive, there
are
>>no intermediate shafts and the encoder must measure telescope motion to
>>subarcsec accuracy . Encoders that can measure subarcsec or better than
1ppm
>>, more like 0.1ppm are Expen$ive.
>>
>>So bottom line,
>>if you create a direct drive that has at least 1000 times longer lifespan
>>than piezos ,
>>and come up with a drive that accepts a 2000:1 step frequency ratio
between
>>tracking and fastest slewing,
>>and come up with an encoder with about 2 million counts per scope
revolution
>>,
>>and make all these about 100 times cheaper than current industrial
products
>>,
>>then you're in business .
>>
>>best regards,
>>matt tudor
>>
>>
>>
>>
>>-----Original Message-----
>>From: Don Clement <clement.focuser@verizon.net>
>>To: atm@atmlist.net <atm@atmlist.net>
>>Date: Saturday, April 09, 2005 11:30 AM
>>Subject: Re: [ATM] Telescope direct drive
>>
>>
>> >Perhaps the piezomotor (PZT motor) may not exactly be a direct drive but
>> >would act as a replacement for the roller or friction drive acting to
drive
>> >on the circumference of a large disc or curved sector to move the
telescope
>> >axis. There are two types of PZT drive ideas: The inch worm motor drive
>> >(Nanomotion) and the compliant friction shoe (EDO). I have been playing
>>with
>> >a direct drive idea similar to the compliant friction shoe but not
>> >necessarily using PZT as the driver.
>> >
>> >
>> >
>> >I am surprised at the responses from those on this list to the
piezomotor
>> >drive concept. I believe the same arguments could be about large amateur
>> >optics in general. The cost of commercial PZT motors will come down
soon.
>> >PZT motors should soon be making the jump out of the expensive
>>semiconductor
>> >manufacturing  and lab positioner arena to commercial products like DVD
>> >players, etc.
>> >
>> >
>> >
>> >Andy: I am sure that if there were some real benefit for a cheap radar
>> >stealth coating on cars, that an enterprising amateur with a knowledge
of
>>RF
>> >network analyzers (S parameters) and RF antennae measurements could
hobble
>> >some surplus RF test equipment together to test common materials. Black
>> >rouge comes to mind as a starting material. However in our present
climate,
>> >one might expect a visit from men in black.
>> >
>> > :-)
>> >
>> >
>> >
>> >Don Clement
>> >
>> >Running Springs, California
>> >
>> >_______________________________________________
>> >ATM mailing list http://www.atmlist.net/
>> >
>>
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>
>
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