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Art Bianconi
Post Laureate
Reged: 03/06/06
Posts: 4659
Loc: Delaware River Valley, New Jer...
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Until recently, my experience with stepper motors was limited to large format plotters used in engineering and architectural offices.
In the early days of AutoCAD, a stepper driven plotter would give you lovely straight lines in the x or Y axis but if you tried to plot a line diagonal to either, what you got was a staircase; a horrible collection of zig-zags.
I realize that this is 25 years later and that steppers have come a long way. Still, even at their finest, they do stop and start. I have trouble giving steppers a fair shake knowing that.
I've never ever read a comparison between the various drive motors and wondered if those of you who are more knowledgable in this domain might share some thoughts on the relative differences, their features, the benefits and how to choose.
Thanks
Art
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“Everything is on its way to somewhere. . . . . everything!"
____________________ George Malley (John Travolta)
________________________ "Phenomenon"
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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It depends on what your trying to do. If it is just make your 'scope track, then all three will work well. Steppers may have slight advantage that they can be powered by batteries and accurately controlled with a simple control circuit and also allow for a wider range of speeds. You can get 16x or greater slewing rates out of them. They are can be half stepped very easily and even microstepped for very fine movement. They also comes with gear boxes. I use one made by Hurst that is geared to have 14400 steps per rev and works very well driving my Polaris mount. They have been used for years on many commerical telescopes. They also can be used for GOTO but it is a trade off in slewing speed vs tracking accuracy. Plan DC motors without feed back have bit more error to them then a stepper. Add servo feed back and you have a very nice system for both tracking and GOTO but this take more electronics.
AC synchronous motors also work well for tracking. With inexpensive DC to AC converter on available they can easily used for a transportable system. They lack the dynamic range that a DC system has. Your not going to be able the slew at 16x standard rate with them. They are good for around plus or minus 20% of there standard output which is fine for guiding.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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ccs_hello
Post Laureate
Reged: 07/03/04
Posts: 3298
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Dave stated the subject very well.
I'd just like to add the recent hybrid stepper option:
http://www.cloudynights.com/ubbthreads/showflat.php?Cat=&Board=lxd55&Number=2074255#Post2074255
Clear Skies!
ccs_hello
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bbriggs
sage
Reged: 01/09/06
Posts: 217
Loc: Graham, Wa.
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I have considered upgrading my stepper motor drive to DC for the following reason: When I have the steppers tracking there is a very complex resonance that occurs because the motors are running at different speeds. It causes an intermittent fuzzy vibration at the eyepiece. I have it on an ALT/AZ mount and this may not happen on an equatorial mount where only one motor is needed.
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14.5 f4.5 homemade newt w/Zambuto
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ccs_hello
Post Laureate
Reged: 07/03/04
Posts: 3298
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Stepper vibration/jitter can be fixed by replacing the standard (full-step/half-step) driver with a microstepping driver. E.g., 1/64 microstepping is to divide the step into 64 fine mini-steps simulating the sinusoidal wave. For example, legacy EQ6 (non-GOTO, non-SynTrek) folks are replacing the controller IC with a new controller which has a different firmware.
Clear Skies!
ccs_hello
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Timing motors do not have any vibration problems, are very cheap and can run off house current or an inverter drive like Art is making and draw little power. If you don't need a lot of speed change or reverse they are ideal such as an equitorial platform.
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Ed Jones
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Art Bianconi
Post Laureate
Reged: 03/06/06
Posts: 4659
Loc: Delaware River Valley, New Jer...
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The Planet Killer is an exceptionally heavy scope. The four steel pillow blocks on which the two axles are mounted, each weigh 18 pounds!
All the weight of the 12.5" standard thickness mirror, the cast aluminum cell, the scope tube et al, has been spun by this little AC synchronous motor.
The addition of Ed's variable cycle inverter will not only elliminate the need for power chords in the back yard but will give me a fine speed adjustment that the current system lacks.
Art
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“Everything is on its way to somewhere. . . . . everything!"
____________________ George Malley (John Travolta)
________________________ "Phenomenon"
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