Re: Buying an SCT - Servo Motors
From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 01/23/05
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Date: Sun, 23 Jan 2005 21:44:45 GMT
"Larry G" <no.one@nowhere.com> wrote in message
news:opsk1uqhyee5g1ia@newshost.vvm.com...
> On Sat, 22 Jan 2005 10:25:26 GMT, Roger Hamlett
> <rogerspamignored@ttelmah.demon.co.uk> wrote:
>
>>> It is my understanding that the newer Meades have stepper motors which
>>> make an awful racket under the most peaceful skies. And Celestron's
>>> single armed fork isn't stable enough for serious long exposure
>>> photography. However, the larger Celestron scopes (esp. 9.25") are
>>> rumored to have excellent optics these days.
>
>> Both the Meade units, and the Celestrons, use servos. The difference is
>> that on the Celestron NS family, these are professionally made
>> industrial servo systems. On the Meades, the older units used what were
>> basically 'slot car' motors, and mouse encoders for the position, the
>> latter units have retained the mouse encoders, but switched to slightly
>> improved motors. The Celestron motors are more accurate, and
>> smoother/quieter, _but_ they still have the downside, of no working
>> PEC, in the NS units...
>
> Thanks to all who responded to the earlier part of this thread.
> I haven't kept up with consumer telescope technology, at least
> not concerning the apparent distinction between stepper motors
> and "servo systems".
>
> A quick Google search for "DC servo motor theory principle operation"
> provided a link to a very enlightening page on different actuator types:
>
> < http://www.ee.polyu.edu.hk/staff/eencheun/WebSubject2/chapter2.htm >
>
> At the very bottom, the author(s) discuss the 'Brushless DC Servomotor'.
> This IS nothing more than a stepper motor with built-in control
> electronics. As with most stepping motors, run in full-step or
> half-step mode, they will generate characteristic audible noise.
> Using a micro-stepping controller, and perhaps a greater drain
> on battery capacity, is one way to quiet these things. Other
> possibilities are heftier, more rugged construction, or better
> mounting.
>
> I very much doubt that Meade or Celestron has given much effort into
> doing so. Has anyone noticed a change in drive noise over
> the past decade or so?
Basically, cost.
The servo system, using a simple optical encoder on an early shaft in the
gearbox, is currently the cheapest way of giving the sort of accuracies
required. The cost differences involved, are not small either.
Microstepping, if it is to work well, requires a motor designed to support
it. The problem with the astronomy market, is that the production runs
involved, are tiny compared to most other markets, and as such do not
justify custom development. Hence the only way to make a product at a
reasonable price, is to use existing parts made for another market.
Brushless motors, are great for applications requiring constant revs (they
are used in just about all hard drives), but high torque units, suitable
for use as the drive motor in a servo system, are expensive. I have a
couple sitting here, from a positioning system, used to control a laser.
The control system alone, with motors and controllers, runs to over
$19000...
Brush motors, offer one of the most compact packages for the torque
available (in small units), and have the nice characteristic, of
increasing the delivered torque, if loaded. If you are prepared to spend
enough, they can be superb. The units used on the mass production scopes,
reflect just how small the budget is for this part of the system.
Best Wishes
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