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Author Topic: Light weight rotor that uses a stepper motor  (Read 4782 times)
K2YO
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Posts: 436




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« on: March 12, 2012, 11:09:01 PM »

Has anyone seen a light weight rotor that uses a stepper motor? I'd like to be able to have precise direction control of a 4 or 5 element VHF yagi or quad. The antenna will be fairly light, so the rotor doesn't have to be real big. I want to be able to control it remotely some how, ie RS232, USB, ethernet, etc.

Thanks,
Bernie
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K5BOB
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« Reply #1 on: March 12, 2012, 11:29:35 PM »

Im sorry but i do not know of one,
BUT stepper motors are not the best pic, for what your are asking
A stepper motor does not have a reference point. They uses step's to calc the position
A better pick for what your are asking is Servo's. A servo's never "drop  Steps".

bob
k5bob
www.k5bob.com
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K2YO
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« Reply #2 on: March 12, 2012, 11:33:29 PM »

Bob,
Do they make servo motors big enough to swing a VHF yagi? I'm only finding the smaller ones.

Bernie
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K5BOB
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« Reply #3 on: March 13, 2012, 12:07:30 AM »

Again i have never seen a stepper or servo driving rotor.
But because of the original question. if he was asking for steppers, why not do servo's

Servos are very accurate it would be possible to home brew something i think.

Hmmm.......  may be a good project for me, one day, I have servo's and Drivers (Controls) laying around. I think i have the stuff to build a very low gear ratio to turn it.. maybe one day. All i would need are thrust bearing and i could cut those on my CNC Mill. haha


bob

ps yes servo's can be VERY powerful from grams of force to several tons of force.
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G3RZP
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« Reply #4 on: March 13, 2012, 02:54:24 AM »

A servo system in conjunction with such things as a Metadyne can easily control a turret with three 15 inch naval guns.
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AC4RD
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« Reply #5 on: March 13, 2012, 04:06:19 AM »

A servo system in conjunction with such things as a Metadyne can easily control a turret with three 15 inch naval guns.

Yes ... but it's sure gonna be hard to get a QSL out of the DX after that.    Wink
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K3GM
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« Reply #6 on: March 13, 2012, 06:48:47 AM »

....I'd like to be able to have precise direction control of a 4 or 5 element VHF yagi or quad......

Let's work on this from the other end, so to speak.  The beamwidth of the example antennas is wide enough that you don't need to aim them with such precision. "That way" is generally enough accuracy for a bearing.
« Last Edit: March 13, 2012, 01:00:14 PM by K3GM » Logged
WB4SPT
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« Reply #7 on: March 13, 2012, 07:15:43 AM »

THere is nothing inherent with either servos or steppers that gives position.  Many motors CAN be supplied with back of shaft mounted encoders for position, giving rotor position.  There are a few that give absolute position.  In most cases, a remote encoder is used since what you need is the position of the geared down output (low velocity) shaft, not the motor anyway.  In the big naval gun mounts, it was done with syncro 3 phase systems, you will also find resolver type systems that monitored the rotary position of shafts. 
There is no technical reason why either a stepper or servo could not make a good antenna rotor drive.  Both need a good controller, one that has preset acceleration, speeds, etc.  A stepper has no brushes, which is some advantage outside.  Lower end servos are DC brushed motors, but many now can be had with polyphase brushless drives. 
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N7NBB
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« Reply #8 on: March 13, 2012, 07:18:05 AM »

Just use a light-weight TV antenna rotator.  It uses a stepper motor (sort-of) the only down side is the CLICK CLICK CLICK (very noisy) from the control box. You can find them used at garage sales, and NEW from many sources.  Maybe even Radio Shack still sells them.  I've got on on my 5 element 6 meter beam that's been up for about 5 years, with NO problems other than a burnt out indicator bulb in the control head.  (and I might add it's survived 60MPH wind gusts, and still CLICKS away just fine.  I got mine for $15.00 at a garage sale, so even if it FAILS, five bucks a year was a good investment. Good Luck finding what you need.
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K8AXW
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« Reply #9 on: March 13, 2012, 07:56:07 AM »

One more thing.  Every time you "step" the antenna you have starting and then stopping inertia.  If you watch a beam as it's being turned you will see it stop and bounce back and forth a bit.  I feel with a stepper motor there is going to be a lot of "whiplash" with your beam and you risk a short mechanical life.

It might be minor..... but the bottom line is the beam width on the receive end is broad enough that precise accuracy isn't necessary.
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W4DRR
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« Reply #10 on: March 13, 2012, 09:58:27 AM »

...Maybe even Radio Shack still sells them.

Only if they can be used in conjunction with a cellphone.  Wink

73,
Bob
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73,
Bob
K2YO
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Posts: 436




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« Reply #11 on: March 14, 2012, 06:43:33 AM »

Thanks for the input everyone.

Quote
"That way" is generally enough accuracy for a bearing.

In a general application true, but not in this application.

Quote
One more thing.  Every time you "step" the antenna you have starting and then stopping inertia ... you risk a short mechanical life.

Interesting point. I had considered the whiplash from a data collection standpoint but not from a mechnaical reliablity standpoint. The application has the antenna walking through all of it's know points and recording the rf signal received at that point. I had figured on maybe needing some software smoothing of the data to correct for this problem. I'll have to consider the mechanical issues.

The original plan included spinning the antenna with a traditional motor and using a shaft encoder to determine location. The down side is the shaft encoding and rotary coax connector hardware can get expensive. Also the rotary connector introduces loss into the setup.

Thanks again for the ideas.

Bernie
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K8AXW
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« Reply #12 on: March 14, 2012, 09:18:40 AM »

Quote
The original plan included spinning the antenna with a traditional motor and using a shaft encoder to determine location

Now that's an interesting concept that I've never heard before! 

Quite often I'll hear a DX station while my beam is pointed in he "wrong" direction.....think that if I point the beam to the "correct" direction the signal will be stronger.  Quite often this isn't the case because the signal is coming in "long path".  A lot of time is lost rotating the beam 180 degrees and then back 180 degrees to learn this.

Then there is the hassle of rotating the beam searching for what part of the world is on.  If a guy could rotate the beam, say slowly but constantly with signal activity levels being recorded then this could be very interesting.

Good luck!

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KB1GTX
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Posts: 460




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« Reply #13 on: March 14, 2012, 09:22:10 AM »

A Pulley reduction to a motor that makes on turn which has one magnetic sensor and a solenoid break on the shaft of the small motor.

 Larson mno mount might be modified for the turning rf connector. with some type of rf friendly lube.
« Last Edit: March 14, 2012, 09:25:06 AM by KB1GTX » Logged
K0JEG
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Posts: 638




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« Reply #14 on: March 17, 2012, 09:50:34 AM »

I've done some experimenting with telescope Az/El mounts for satellite tracking. They typically have a DC motor attached to a gearbox, that has an encoder/turns counter of some sort. This setup is very accurate (more than necessary for small beams on 2M/70CM), repeatable and predictable.

With good gear reduction you should be able to have a fairly good position repeatability, but you'll need to keep the coils energized to maintain position, and even then if you're running at the high end of recommended voltage. Once the coils are de-energized the motor will turn very freely. Of course, keeping the coils energized will likely limit their life.

I would also recommend using servos instead. More expensive, but easier to interface and they stay put. If you're not set on microcontrollers, there are a lot of Arduino libraries and shields to interface all types of motors, so it shouldn't be too difficult to try different motor types without having to adapt code.
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