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

Mon Apr 30 13:59:23 EDT 2018

Has anyone tried one of the servo's out of the bottom of a microwave
Oven?

-----Original Message-----
From: 600mrg-bounces at mailman.qth.net [mailto:600mrg-bounces at mailman.qth.net] On Behalf Of Brian Pease
Sent: Monday, April 30, 2018 9:28 AM
To: 600mrg at mailman.qth.net
Subject: Re: [600MRG] Suggestion for Stepper or Servo for Variometer?

The reality is that once the antenna has been tuned and matched in this 
narrow band, I have found that the direction to re-tune is always 
obvious with a quick nudge of the switch in either direction while 
glancing at reflected power on an analog meter.  The cheap 0.5 RPM 12VDC 
Ebay gearhead motors work just fine and hold position when off.  I would 
worry about RF getting into the electronics during tuning.  I actually 
tried using an RC servo first, but it would not work smoothly with the RF.

On 4/30/2018 12:05 PM, Roger Rehr W3SZ wrote:
> 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
>
>
>
>
>
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