SYNCHROS "SELSYNS" as Output Devices

@robotbuilder

Only tested by an ohm meter. So with DC current.

Hi, robotBuilder.

I only want to give you a little update.

In the meantime, I refreshed my Arduino basic programming knowledge and did some tests wit the RMI.

I applied PWM driven signals powered by 2N2222 NPN transistor and 3.5 to 7.5 external voltage to 2 stator coils of the RMI. Duty cycle 50% gave the best results. I kept the sum of the voltages from S1 to S2 + S3 to S2 = constant. Best results were obtained by 5V input voltage to the collector side of the NPN transistor. The current is about 0.06 amps.

The results are promising: The needle position could be regulated in a range of 60° (half of 120°) without of changing the polarity of the inputs.

An interesting fact is, I don´t need to power up the rotor coil! The needle reacts, as there would be a permanent magnet inside. So further tests I will do without power on the rotor.

Powering the rotor with 3.5V (=1.75 V at 50% duty cycle) the RMI keeps silent. Rising the input voltage to 5.0 V or 7.5 V, appears the 490 Hz sound heard in the videos that you send (at less intensity). So I guess the guys that made the videos applied a far higher voltage to the coils. Applying voltage only on the 2 stator coils, there is no noise.

At some intermediate positions, the needle trembles a bit (maximum +/- 1.5 degrees on the scale). I will test to use a ceramic capacitor to stabilize the needle. I tested some uF and nF capacitors but they seem to be too big. So I ordered some pF capacitors. Next week I will get them and I will continue the tests.

Until soon.

Peter

@peter-elsler

Of course the system is a transformer thus the stator ac currents would be inducing a current in the rotor coil.

It would be interesting if you could get hold of an oscilloscope an look at the wave forms. That would also show the exact voltages and frequencies.  In theory you could generate a sine wave from the Arduino with a sine wave table of pwm values and smooth them out.

I imagine the choice of capacitor would need to match with the frequency to smooth out any jitters. Again an oscilloscope would make that visible.

You are figuring out a working solution by trial and error instead of calculations 🙂

Of course the system is a transformer thus the stator ac currents would be inducing a current in the rotor coil.

That sounds good and simplifies the problem.

It would be interesting if you could get hold of an oscilloscope an look at the wave forms. That would also show the exact voltages and frequencies. 

Where do you need the measurements? I suppose on the coils S1 to S2 and S3 to S2. Is that correct?

 In theory you could generate a sine wave from the Arduino with a sine wave table of pwm values and smooth them out.

That would be great. But it comes to my limits of actual programming abilities. Please could you give an example of the program lines to use that sine wave table as parameters.

Many thanks.

Peter

Another interesting fact:

If I shortcut the Rotor coil, the jittering reduces drastically. Maximum less than 0.5 degrees on a small part of the 60° sector.

In this case, I need 7.5 V resulting in 0.17 amps to get the full range of 60° on the scale.

At any case, the sine wave table would be helpfull.

I have nearly no experience with the oscilloscope. Therefore I will need a little bit of time to work through the manual.

@peter-elsler

Where do you need the measurements? I suppose on the coils S1 to S2 and S3 to S2. Is that correct?

Yes. 

That would be great. But it comes to my limits of actual programming abilities. Please could you give an example of the program lines to use that sine wave table as parameters.

It is not something I have ever done with the Arduino. The last time I generated sine waves was a long time ago on an old computer where I attached a Digital to Analog Circuit (DAC) to the parallel port of the computer to play music. You can't really afford to assign 8 Arduino pins to such a project  and also you would need to set the bits all at once not one at a time with the digitalWrite() function if you used a DAC. 

This talks about it on the Arduino if you can follow it. There are many other links just google something like "generate sine wave using an Arduino"

However I wouldn't complicate your project more than you need to.

Posted by: @peter-elsler

I have nearly no experience with the oscilloscope. Therefore I will need a little bit of time to work through the manual.

So you have an oscilloscope!  Yes read the manual and learn how to use it.

@peter-elsler

If I shortcut the Rotor coil, the jittering reduces drastically. Maximum less than 0.5 degrees on a small part of the 60° sector.

Remember I am not an electrical engineer just a self taught hobbyist so I am just making suggestions which you use at your own risk!  It would be nice to have a professional provide you with the answers.  You might damage your hardware when you experiment with it.  You don't want your coils to go up in smoke!

Was your rotor open circuit before?

The math can be complicated when it comes to inductance and capacitance.  Your stator and rotor have the same relationship as coils in a transformer.

Many thanks for the video link.

Very interesting, but at this moment still a liitle bit complicated for me.

One thing that I noted is, that the Arduino stops executing all the rest, when generating the sine waves. I have a lot of "needles" to move. This means it could become  complicated / expensive. OK, I have to study a little bit more.

First the oscilloscope.

Many thanks.

Peter

Yes, the rotor circuit was open before. 5V input voltage to the collector side of the NPN transistor. The current was about 0.06 amps.

Hello @peter-elsler, I am working on the same kind of project.

Because my knowledge on electrics/electronics is rather limted I seeked for help on the WWW and did find it by Mike Powell (MikesFlightDeck.com).
He had created a DigitalToSynchro converter several years ago and published that one in his book. After asking him form some advicde (Oct'21) he started an updated of his project. The last picture on his blog is showing the actual design.
The empty PCBs (made by JLCPCB) arrived here already, actually Mike is writing the documentation. When he is finished I can order the needed parts and start the soldering. Implementing it into the system will be the next big step.
The solution of Mike is working with a 400 Hz power source (the "easy solution" as per @robotbuilder).

In one reply @robotbuilder used some pictures to show a "D.C. Selsyn System" (also known as Desynn System). Such devices are not using synchros and are working on DC only. I have such a device (Flaps Position Indicator) and I managed to get it working WITHOUT changing anything in the device. Powered with 5V DC it can be controlled with a simple potentiometer. I used a digitial poti, controlled via SPI. That way there is no moving part (wear&tear) outside the indicator.

I'd like to share my findings!

Hello JueBag,

many thanks for your comment!

Unfortunately, I saw your post only today. Very interesting, your approach.

Please share your findings.

Many thanks,

Peter

@peter-elsler 

The actual state of the project concerning synchro driven instruments is "on hold". That is caused by the "Chip crisis" (the LTC1590CN and TL084ACN are said to arrive at the end of this year). Mike Powell is actually writing the documentation for his DTS board and will publish it as an EBook on Amazon.

In order to have some progress for whole system I did set up a spare synchro driven by a stepper motor. This transmitting synchro and the Airspeed indicator are supplied with 400hz. The stepper motor is controlled by the software Mobiflight. That will do as an intermediate speed indication until those Chips arrive. 

Hello JueBag,

thanks for the update.

I am still working with the ilumination. Then synchros are still in stand-by.

Stay safe,

Peter

Hi, @juebag very much interested in you Selsyns output device. Would you share your circuit and code please. Regards Devan