From: evm@opo.vanc.wa.us
Newsgroups: sci.astro,sci.space,sci.electronics
Subject: Re: Telescope Drive Systems.
Date: 10 Apr 92 19:10:20 GMT
Article-I.D.: opo.1992Apr10.111020.17
Organization: OuterPlane Observatory, Vancouver, Wa

In article <cjjhbh0b@cck.coventry.ac.uk>, esf007@cck.coventry.ac.uk (Gorth) writes:
> In article <1992Apr6.011453.1@pomona.claremont.edu> jdishaw@pomona.claremont.edu writes:
>>In article <10192@inews.intel.com>, dpeterso@mipos2.intel.com (David Peterson) writes:
> 
>>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.)
> 
> 
> Stepper motors would be easier to control for your computer system,  the 
> interfacing is standard tech.   One of the closed loop problem you should watch 
> is deadband and backlash non-linearities when using a gear system.  These
> effects can cause very serious accuracy problems in this particular application
> and also instability in tracking.  You could have your telescope 'hunting'
> and 'limit cycling'  (look it up).  If you want highly accurate control
> I believe the DC motor is the best option,  if you can afford it and have the 
> right design parameters,   (damping factor,  omega-n,  etc).
> 
> However,  the stepper motor option is cheaper and easier to program, but
> if your using a gear system (even one), then use CLOSED LOOP and feedback
> the position and velocity from a point in the system AFTER the gearbox
> Then your software will be the key to how good the system is.
> 
> 
> Chris Grice(Gorth),,,,,,MSc  Computer Control Systems.
-- 

	I've been following this thread and would like to throw my bit into
the fray.

	I'm designing a closed loop DC drive servo system for a very fine
mount that I picked up. Now, as has been noted the software and such to control
a DC servo loop can be a real bear but there is another solution.

	National semi makes the LM628 / LM629 precision motion controller
chips. The 628 is a 12 bit analog output device and the 629 an 8 bit PWM
(er, pulse width modulation) output device. These chips do the math in hardware
for you with a fully programmable 32 bit PID filter (Proportional Integral
Derivative).  See the Special Purpose Linear Devices databook for details.

	Also if you choose PWM the LMD18201 driver hooks right up to the 629.

	After setting the filter parameters ( and I like to be a bit
underdamped ) the actual control micro just needs to issue position or velocity
commands and read the current position out for reporting every now and then.
 
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