[600MRG] Suggestion for Stepper or Servo for Variometer?

[Roger Rehr W3SZ]

Hi All,

I am looking for some specific recommendations on what specific model of
stepper or servo motor to use for remotely tuning (via an Arduino) a
homemade variometer that I am building.  I have zero experience with
steppers and servos, and so I need some help. 

Here are the details of the variometer.  I am building a variometer for
my 475 kHz inverted L which is 40+ ft tall and 200+ ft long (estimated),
which has a capacitance of 663 pF measured using a BK Precision LCR
meter.  I need an inductance of approximately 169 microHenries.  So I am
using a "6 inch" PVC sewer pipe (o.d. 6.275 in) with 32 turns wound over
a 3.2 inch span, giving 167 uH.  My inner coil will use a "3 inch" Sch40
PVC pipe (o.d. 3.5 in) with 20 turns wound over 2 inch span for 34 uH. 
Thus the variometer will cover from 133-201 uH.  I don't have questions
about any of that.

The plastic shaft that the smaller coil is mounted on and that will be
rotated by the motor is 0.375 in diameter:
https://www.lowes.com/pd/Hillman-48-in-x-0-375-in-Staff-Sign/3025196
I can fashion a coupling to match the shaft to almost any motor shaft
diameter.

I need to be able to rotate thru a span of 180 degrees, and I want
rotation to be limited to about 200 degrees at most, because I don't
want the wires to become twisted on themselves by rotating through more
than 360 degrees.  Thus I specifically do NOT want to use, instead of a
stepper or servo, a low-RPM motor that has continuous rotation beyond
360 degrees.  And I do NOT want to use the "solution" of using such a
low-RPM continuous rotation motor with mechanical stops to limit its
rotation to 200 or 360 degrees or whatever.  A simple low-RPM motor that
by design only rotated through 360 degrees or less would be OK though; 
I haven't found any of those though.

That is the "background" material for my question.

I think that a (bipolar) stepper is what I want rather than a servo,
because I think that the steppers have greater resistance to turning
when not powered and I think for reasons of EMI I want to have the motor
and control electronics powered up only when adjusting the variometer. 
So, most of the time the motor will not be powered up.  Or maybe all of
these motors have enough turning resistance for this applicationwhen not
powered up so that this is not an issue.  And then there is the issue of
servo "twitch" when first powered up.

As examples of what motors I have been considering, here is a stepper
that costs $16.95:

https://www.sparkfun.com/products/10846

Here is a servo that costs only $12.95:

https://www.sparkfun.com/products/11965

There are hundreds (or thousands) of other choices on the web;  too many
choices!  Thus my request for some specific recommendations.

I am figuring that spending up to $50 for the motor is reasonable.  The
main criterion is to get a motor that will give sufficient torque when
turning and sufficient turning resistance when stopped. 

Using microstepping with something like the Sparkfun Big Easy Driver

https://www.sparkfun.com/products/12859

I can get steps of a tiny fraction of a degree for a bipolar stepper
motor.  I don't have any idea about the rotational precision of the
servo that I listed above.

At this point unless advised otherwise I will get the SparkFun 10846
stepper and 12859 driver listed above for this project.

Finally, for those who like me are newbies in the world of steppers this
"how to" for the Big Easy Driver might be interesting:

https://learn.sparkfun.com/tutorials/big-easy-driver-hookup-guide

Thanks in advance for any advice and

73,

Roger
W3SZ