[TowerTalk] Telrex rotator

Fri Nov 16 09:00:32 EST 2018

On 11/16/18 4:16 AM, Steve Lott wrote:
> On the unit I had years ago,
> 
> The selsyn on the tower
> was driven by a small chain from a gear sprocket
> on the rotor assembly
> 
> That selsyn drove another selsyn that is in the shack on the control box
> 
> basically the selsyn are dc motors
> the one in the shack has a pointer on it
> that is on a dial face showing headings (bearings of the compass)
> 
> The unit on the tower drives the one in the shack
> (the unit on the tower becomes a small generator)
> The down side was chain slippage

Selsyn, or synchro, are actually AC motors, either 2 phase or three 
phase, with a excitation winding. You put AC power on the excitation 
winding and the voltages and phases on the other windings correspond to 
the angle of the shaft. at the receiving end, those voltages cause the 
rotor of the receiving synchro to turn to match.

https://en.wikipedia.org/wiki/Synchro

They're pretty cool to play with.

They're also potentially very high precision and very long life 
(compared to a potentiometer) and have no issues with wrap around. 
There are commercial synchro to digital converters with 16 bit accuracy 
(0.005 deg, about 1/3 arc minute) so a synchro and converter is WAY more 
accurate than a pot and an adc. The system is also insensitive to things 
like dc biases and resistive losses, so it works well with long wires.

See below...

> 
> I would recommend a more modern approach
> either a wireless GPS Compass module around an Ardunio or Rasperry Pi box
> with wifi transmitter/receiver
> Which is available off the shelf from a place called Ham Station if my
> memory on it is any good
> 
> I once built a ring of very small relays
> and placed a very small but powerful button magnet on the mast
> when the magnet was perpendicular to the relay it closed the normally open
> contacts
> and the relay contacts were wired to the shack panel to illuminate a small
> LED
> on a rose Compass diagram showing my present beam heading
> It was visually appealing in the shack
> 
> Cheers!
> Steve
> KG5VK
> http://sdxa.blogspot.com/
> KG5VK Blog
> 
> http://www.KG5VK.com
> My Ham Radio Friends
> 
> 
> On Thu, Nov 15, 2018 at 11:04 PM Martin Sole <hs0zed at gmail.com> wrote:
> 
>> Hi,
>>
>> I'm restoring a Telrex A3695RISX rotator. It's quite a monster but if I
>> can figure out a way to use it I think it's unlikely I would ever need
>> anything bigger, 1/3hp reversible AC motor, dual worm reduction gearbox,
>> chain drive to 3" output shaft and it weighs about 125 lbs, around 57kg.
>>
>> I have no control box which poses a small problem. Wiring to the motor
>> is straightforward enough though I think the limit switch arrangement
>> could be handled better.
>>
>> It uses a selsyn for position indicating. In this case actually a
>> General Electric Selsyn Control Transformer 2J161. This is marked
>> 57.5-57.5 V 400CY. The device is working in that I can get the three
>> outputs from the stator winding when I excite the rotor winding, all
>> three stator outputs rise and fall with rotation so I'm pretty sure its
>> good.
>>
>> The problem is what to do at the other end. Without a matching selsyn,
>> not something I expect to ever turn up around here, I am wondering how I
>> might be able to take the selsyn outputs and interpret them by some
>> other means. My understanding is the 3 stator outputs are AC voltages
>> varying in amplitude and with a fixed phase relationship in order to
>> produce an unambiguous value related to absolute position. I'm not 100%
>> on this yet though. They are in a delta configuration though so I'm
>> thinking anything I do first needs to make them Y configured

You can probably use almost any other synchro as a readout,  Although 
yours is a 400 Hz part, a lot of folks use them with 50 or 60 Hz 
excitation (at *lower* voltage. if it's rated at 100V 400Hz, you want to 
use 12.5 V at 50Hz).  I've run a lot of synchros off a 6.3 V filament 
transformer.  You don't get as much torque out, but in this kind of 
application it works.

Or there's a variety of synchro to digital converters.  The DDC parts 
are kind of expensive.

Ultimately, what you need is a 3 (or more) channel ADC. Then it's a 
matter of calculating position from the 3 numbers.  I'd hunt around and 
someone has probably done a synchro readout using an Arduino or similar 
microcontroller.

Here's someone doing the opposite (generating synchro signals)
https://www.eevblog.com/forum/projects/trying-to-emulate-three-synchros-using-arduino/

Here's an article on Reddit about reading a synchro

https://www.reddit.com/r/AskElectronics/comments/2lq0pn/measuring_angle_of_synchro_on_an_arduino/

Here's someone who built a 3 channel resolver to number converter using 
a Arduino

http://wiki.linuxcnc.org/cgi-bin/wiki.pl?ResolverToQuadratureConverter

>>
>> What are my options?
>>
>> I had thought about an I2C compass module but that will entail placing
>> some other conversion hardware at the rotator to convert to something
>> like RS422. Any other thoughts. Just looking for ideas to think of what
>> might work well enough. Ideally keeping the selsyn as it's all purpose
>> built around that.

The synchro is rugged, reliable, and accurate.

>>
>> Martin, HS0ZED
>>
>>
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