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Motor control for vacuum variables and Rotary Inductors




 
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Author Topic: Motor control for vacuum variables and Rotary Inductors  (Read 724 times)
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pwfallon
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« on: October 22, 2022, 08:54:05 AM »

Hi Folks,

I am considering building a remote  antenna tuner (Balanced LC)  and am looking to motor control  the rotary inductors and vacuum caps. Familiar with stepper motor, etc. but I am interested in specific design and components recommendation as well as experiences.

73's

Joe (PW Fallon) Wa1iwq
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K8DI
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« Reply #1 on: October 22, 2022, 09:18:51 AM »

Hi Folks,

I am considering building a remote  antenna tuner (Balanced LC)  and am looking to motor control  the rotary inductors and vacuum caps. Familiar with stepper motor, etc. but I am interested in specific design and components recommendation as well as experiences.

73's

Joe (PW Fallon) Wa1iwq

My first attempt at this process on my RCA BTA-1R1 conversion.  Note I used three different couplings. One is wrong/cannot use, the spiral slit aluminum type on the roller grounds the roller as the shaft is hot. Also note that the caps are mounted such that their shafts are at ground; I used the couplers for flex/alignment reasons.  Iíve since replaced the roller coupler with a big porcelain one.  Note the roller motor has a planetary gear reduction.  The normal motors arenít strong enough to smoothly turn the big roller.  Iíve since changed the load cap (metal cased Comet gas cap) to a reduction style as well, but that was for precision, that cap is 180 degrees from fully meshed to unmeshed, while the vacuum cap has many turns and is easy to adjust.

My first batch of motors and control boards came from Pololu, I used the cheap basic TIC T500, and then rotary encoders off Amazon.  While the high RF environment did not affect the motors, I did put bypass caps on every pin of the controllers and ferrites on the motor cables where they entered the chassis theyíre in. Receive, on the other hand, had noise issues.  I ended up killing the controller/motor power after tuning up.

Ed


* F95B2A28-1E1F-4778-9E5D-1B237B55FF66.jpeg (3570.14 KB, 4032x3024 - viewed 86 times.)
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W3SLK
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« Reply #2 on: October 22, 2022, 11:41:24 AM »

Joe, I am in the process of building the same thing but with dual roller inductors. I'm getting a pair from Blue Ridge Amateur Radio @ $300. But they already have the linkage between the two. https://www.blueridgeamateurradio.com/home
I have a couple of Comet vacuum variables from an old Marconi antenna unit I intend to switch front/back of the inductors with vacuum relays. All to be driven from an Arduino control which will be the last part of the project.
W7FG at https://www.trueladderline.com/ has already built a box but you would have to supply the stepper motor drivers for the caps and the inductors.
Finally, Rich Measures, AG6K(SK) did a nice article about a balanced line tuner in QST about 30 years ago. A fellow by the name of E.F. Bigbie, W4MMQ(SK) copied the article without attribution to Rich or QST. Rich's website is now defunked but another ham has copied the data and posted it here: http://kb3jqq.com/bbat.html
I would be open to swapping ideas and notes as either of us come upon them. I have a full-size 160M dipole so realistically I don't need the extra L. 20uH should be enough for where I care to operate.
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Mike(y)/W3SLK
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« Reply #3 on: October 22, 2022, 02:53:38 PM »

Like Mike, I started a  remote tuner using 2 Arduinos in a wireless master-slave configuration. I got the Arduinos working. They can send and receive one-way but I would like 2-way.  I can send commands but receiving feedback (slave sending status) like inductor and cap position, I have not had much luck and haven't had much time to work it. I think it's a duplex vs simplex thing.  Got the hardware for the tuner.  Once the Arduino is settled the tuner will be tackled. the remote unit will be battery operated and charged by a small solar panel. tentatively, considering just DC motors with pulley or gear reduction drive for simplicity and since that is how the arduino is setup at the moment.   The one mystery is will the Arduino require RF hardening. Initially thought about putting everything in one box but I'm thinking 2 separate metal boxes might be in order.
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pwfallon
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« Reply #4 on: October 23, 2022, 10:46:45 AM »

Hi Folks,

Thanks for the replies.

 In terms of RF susceptibility of the uP stepper controllers that was something that i was concerned about ( especially when I am running the 4-1000A rig....). With this in mind I am leaning towards a dc motor approach with simple analog style controllers....

Joe
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« Reply #5 on: October 23, 2022, 11:58:00 AM »

Joe, I have an RCA SB1K 2X 3-400Z's 1K amp. The tuning in that is remote from the front panel using shade pole motors and feedback resistors to tune the RF section of the amp. There are several settings on it starting from the lowest freq. to the highest in that order. However, the PI-L section was tapped. The PLATE and TUNE caps were tuned with the aforementioned set-up. I don't see why you couldn't adopt it for turning your inductor. Let me get the schematics and I will try to post them here so you will have at least an idea.
EDIT: Here is the schematic as it is in the print. It is referred to as a servo system that kind of works under the same principle as a Wheatstone Bridge.


* SBA-1K servo Circuit 1.jpg (1261.25 KB, 2550x3506 - viewed 45 times.)

* SBA-1K servo Circuit 2.jpg (1248.27 KB, 2550x3506 - viewed 33 times.)
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Mike(y)/W3SLK
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« Reply #6 on: October 23, 2022, 06:07:08 PM »

Sorry for the delay. I had to do some 'family things' and reduce the scan resolution so I could post it. Here is the write up for how the circuit works. The only thing I can think of is you would need to incorporate an interlock to keep you from going too far on your roller inductor. I would place a physical stop with a limit switch that would de-energize the circuit and force you to manually go and reset the unit in the field. Of course that is me. I haven't seen them in a while but when I worked in the bio-med field we used pots that had a retractable line on them to physically use for positioning a patient in our device. They were extremely accurate and such a device could be connected to a flag on the roller contact. Also, if you look at the AT4K remote tuner here: https://www.trueladderline.com/atu4k-balanced-antenna-tuner-atu-outdoor/ you will see that he is using a stepper motor in his RF environment. I am doing my tuner in a ABS type of box. I thought there would be too much capacitance if I used a metal box like him.

* Servo1.pdf (1153.16 KB - downloaded 29 times.)
* Servo2.pdf (1331.59 KB - downloaded 19 times.)
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Mike(y)/W3SLK
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« Reply #7 on: October 23, 2022, 11:54:10 PM »

Hi Folks,

Thanks for the replies.

 In terms of RF susceptibility of the uP stepper controllers that was something that i was concerned about ( especially when I am running the 4-1000A rig....). With this in mind I am leaning towards a dc motor approach with simple analog style controllers....

Joe

I agree with you on that point. Simple and easy to RF-proof. Limit switches and relays seldom affected by RF and provide a definite end point for travel. Pots can be geared to give some DC feedback. This said I have worked analog servos using 400Hz and DC, have but not built a tuner using a servo.
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« Reply #8 on: October 24, 2022, 10:18:07 AM »

My Harris RF-103 has the ability to remote tune.  It's fixed caps but a roller in a standard pi config.

The roller shaft is coupled via a couple gears to a Bourns 10 turn pot.

Feedback from that pot is fed to a comparator (for auto alignment on the presets) and drives a meter you can use for position setting.


Might be worth looking into.

--Shane
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« Reply #9 on: October 24, 2022, 11:42:31 AM »

KD6VXI said:
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The roller shaft is coupled via a couple gears to a Bourns 10 turn pot.
I forgot all about those Bourns pots. They are pretty accurate. I think that was what they used with a retractable line and a swivel for position. I guess if you really want to get anal you could use a LVDT and a comparator circuit but that is really getting out there, (and probably expensive too!)
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Mike(y)/W3SLK
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KD6VXI
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« Reply #10 on: October 25, 2022, 06:53:57 AM »

KD6VXI said:
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The roller shaft is coupled via a couple gears to a Bourns 10 turn pot.
I forgot all about those Bourns pots. They are pretty accurate. I think that was what they used with a retractable line and a swivel for position. I guess if you really want to get anal you could use a LVDT and a comparator circuit but that is really getting out there, (and probably expensive too!)

That's pretty much what the Harris does.  It has a preset potentiometer and a jack plug.  The jack plug sets the preset C combinations for Ctune and Ckoad.

There is an internal pot that is switched in that allows you to preset the roller position.  As soon as you flip the switch for that preset, the roller starts spinning to get the Bourns pot at the same resistance as the preset pot....  All driven through a comparator circuit from the late 70s or early 80s (judging from the typeset of the manual).

Pretty ingenious idea.  I used a multiposition switch as a bandswitch and a 10 turn pot in an external box and pack the refrigerator (dorm size) box into the laundry room of a rental I had.  ERP via the dryer socket!

I'm sure this could all be duplicated with 3 dollars in parts today.....



--Shane
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« Reply #11 on: November 02, 2022, 03:42:30 PM »

FWIW, I'm doing a similar thing with my Johnson KW Matchbox. At present I have the Matchbox at my operating position about 15 feet from the entry point of the OWL. That means I have 15 feet of window line tastefully draped across the ceiling. (Outdoors is homebrew OWL.) It works fine but leaves somewhat to be desired.  I want to put the Box just inside the entry wall and then bring coax to the desk. I have no need for automatic operation. I just need a "long shaft extension" with two tuning knobs at the desk. (I can handle walking 15 feet to flip the bandswitch.) My system has two stepper motors to drive the Matchbox caps. Ten-turn pots coupled to them produce a position voltage which is read via cable by a couple of cheapy little LED panel meters at the desk (2 for 5 bucks). Also at the desk are two small rotary encoders which drive a very simple CMOS gate circuit to produce step and direction pulses. Said pulses are sent through an RS422 transmitter chip via CAT cable to an RS422 receiver, then a line buffer. The buffer drives two (cheap) stepper drive board inputs. The boards drive the motors. I have produced a couple of little pc boards for the simple gate stuff at each end.

At present I am doing the mechanical work to couple the motors and pots to the tuner. The electronic stuff is largely done and it all works on the bench. I turn the encoder knobs back and forth and the motors mimic my action on the other end of 200 feet of CAT 5.  But, I have NO IDEA as yet as to RFI susceptibility once this system is tuning RF at power since that's still being assembled. I'm using shielded CAT cable. The stepper driver boards are optically isolated. Etc, etc.

This is not meant to be elegant. It's just to substitute for my short arms. There is no programming. There is nothing automatic. I'm the servo. I have schematics and board designs and a few spare boards left over from the overseas vendor if anybody is interested.

I'll let you know how it all works out.

Best,
Chuck
NA3CW
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pwfallon
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« Reply #12 on: November 07, 2022, 06:07:44 PM »

 Hi Folks,

A couple of follow ons;
 - Any specific recommendations for dc motors to drive the roller inductor and vacuum cap?
 - What is a good source for gears and dc motors? I see a lot of stuff online but it's a bit hard to judge quality, etc.

73's

Joe
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NA3CW
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« Reply #13 on: November 07, 2022, 06:14:42 PM »

I've never bought any motors from McMaster-Carr but I've always gotten good quality for anything I've bought from them. They're reputable and traceable which is more than most of the online items:

https://www.mcmaster.com/dc-gearmotors/

73,
Chuck
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