From: jdishaw@pomona.claremont.edu
Subject: Re: Telescope Drive Systems.
Organization: Pomona College
Date: 6 Apr 92 01:14:53 PST

In article <10192@inews.intel.com>, dpeterso@mipos2.intel.com (David Peterson) writes:
> 1) Are closed loop (position or speed feedback) systems common for telescope
>    drives?
Well that depends.  In some setup's people use a CCD camera to provide tracking
feedback so that the controller can determine if it is doing its job 
effectively.  However, the cost of the CCD and the requiste electronics can
easily become expensive ($500+).  You can also use a photometer to help
determine if you drifted off your star, but that doesn't help the controller
get back on!  Speed feedback is commonly used.  Many computer control
setups use stepper motors which have a non-zero probability to miss pulses.
By using optical encoders, you can provide a secondary count reference to
verify speed and the number of pulses received by the motors.
 
>    a) Are these systems cost effective? Is it worth the matterial and work 
>       to build compared to open loop systems considering the stability of
>       a closed loop system?
It depends what you mean by open loop and closed loop systems.  Unless you
want a fully automatic telescope (which Trueblood and Genet describe in
their book), your system will be open loop to some extent.  Tracking
and acquiring stars are not that difficult to have a computer.  It would
be pretty wasteful to have a human control tracking since a human's reaction
time is about a sixth of second.

>    b) Would a DC motor with a gear reduction system be effective, or are
>       stepper motors about the only thing that run well enough at low
>       enough speeds to be usefull? (It seems to me that a DC motor 
>       would have to run at a fairly high speed relative to the speed
>       of the RA axis.)
I believe stepper motors are the way to go.  At the 40" that I designed
the control software for we have stepper motors that have a resolution of
25,000 steps/rev (Compumotor).  In the hour angle axis we have 180 000 steps
per degree and in the dec we have 90 000 steps per degree.  Obviously we have
a substantial amount of gearing!  You could use other types of motor, but
stepper motors are much easier to work with.  If you do go with stepper
motors, make sure that the hardware can support a streaming velocity mode.
If, on the other hand, you are not using a computer to control the telescope, 
there exists literature describing how to build an analog circuit that will
turn the hour angle motor at the right rate.

> 
> 2) I can think of four types of driving devices available: DC feild
>    controlled motor, DC armature controlled motor, AC motor, and
>    stepper motors. 
>    a) I understand that a stepper motor moves in small steps of a specific 
>       angle. What is the advantage of stepper motors? Wouldn't they result 
>       in a small amount of "jerkiness" at high magnifications? How, exactly, 
>       do they work? (Any good references?)
If you can command the stepper motors to move a constant velocities and the
stepper motors are high enough resolution, you won't see jerkiness.  We use
them on the 40" and they work fine.

>    b) What would the advantages and disadvantages of the other motors be?
>       What would be a typical horsepower range and shaft speed for a
>       telescope drive motor?
The maximum shaft speed for our motors is 20 rps, which moves the 40" at
2.8 degrees per second in hour angle and 5.6 degrees per second in dec.
At this rate we could literally move accross the sky in a minute.  However,
we generally don't run them past 6 rps.  We modified the card that controls
the hour angle motor by replacing the crystal with one that makes it more
in "tune" with the sidereal second.  I believe we have set it up so that
750 pulses/second will move the hour angle at the sidereal rate.

> 
>        I realize there may not be simple answers to these questions.
> If anybody could at least point me to some good references, I would
> be gratefull. Please excuse me if this is not the best group for this
> discussion, and for any ignorant errors. I am cross-posting to 
> sci.electronics.
> 
> -------------------------------------------------------------------------
> David Peterson 
> Intel Corp., M/S SC4-69, 2625 Walsh Ave.
> Santa Clara, Ca. 95052-9940
> dpeterso@mipos3.intel.com
> Dave Peterson
> SC4-69
> 765-5109
> dpeterso@mipos3
One of the best books is by Trueblood and Genet and is available from
Wilmann-Bell.  I'm writing my thesis on computer control of telescopes;
however, I'm not sure how applicable it is to some of the questions
you are asking.  You are right that there are no simple answers to these
questions.  As the designers, you and your friend need to consider *many*
design factors and make your decisions on which tradeoffs to consider.

-James R. Dishaw
JDISHAW@POMONA.CLAREMONT.EDU