Re: [APML] Mount for C-14 at a University

[Gene Horr <genehorr@texas.net>]

Jerry Lodriguss wrote:

> I thought I heard something about stepper motors not working on computer
> controlled goto mounts because they don't have enough torque to move the
> scope when used at a high rate of speed when slewing.
> 
> Is that right?  I don't know much about motors.

Well, you're kind of right for the wrong reasons <g>.  Steppers have a
lower RPM limit, and so to maintain high slew speeds they generally
have a lower reduction gearing, which means each step is "coarse",
which in poorly designed mounts can lead to vibration.

If anything steppers have too high a torque.  Each full step can
shake the entire mount at normal operative voltage.  This is why 
variable voltage and microstepping controllers came into more 
widespread use.  Both have disadvantages, though.  Variable voltage
increases cost and microstepping introduces errors (there is a 
minor chance (.1%?  I don't remember) of a microstep not moving for
each microstep).

Servo motors are cheaper and have a far higher RPM limit (tens of 
thousands versus hundreds).  But you have to add some sort of feedback
loop to measure the RPMs which eliminates some of the cost advantage
and adds something else that can fail.  They also don't have anywhere
near the vibration problems.

Gene Horr
genehorr@texas.net

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