Synchro Signal Frequency changes

[grasscut]

Hi there! i have a question regarding signal and hope someone can help me out.
Currently, i have a system that outputs synchro signal to other system. Apparently when the cables are connected to the system, the frequency of the synchro signal changes. From around 400KHz, it became around 1.2KHz for S1S2 and S2S3, and 4KHz for S1S3.
When my system is standalone without connecting to the system, the output voltage and frequency is correct. But i changes when is connected to the system. What could be the possible cause that will change the frequency of a synchro signal?

 

[sophiecentaur]

What sort of "system" are we talking about here? What is being synchronised with what?

 

[grasscut]

What sort of "system" are we talking about here? What is being synchronised with what?

Its a glide slope indicator, which takes in synchro signal based on S1S2S3 output

 

[sophiecentaur]

Sorry. This far too specialised for me to pick up on with so little information. I'd suggest that ILS systems are a bit too 'Safety of Life' to risk taking the sort of advice you can expect to get on a general EE forum. You could be lucky, of course. Someone may come in and prove me wrong. Goo luck.

 

[grasscut]

Sorry. This far too specialised for me to pick up on with so little information. I'd suggest that ILS systems are a bit too 'Safety of Life' to risk taking the sort of advice you can expect to get on a general EE forum. You could be lucky, of course. Someone may come in and prove me wrong. Goo luck.

Thank you for your reply anyway. Anyway, my system is just giving a synchro output with reference voltage of 115V 400Hz to the system to drive it.

 

[sophiecentaur]

115V and S1S2S3 in the same post? Where does an ILS system involve Cardiology?
It could be April 1st for all you have written so far.

 

[berkeman]

Its a glide slope indicator

Hmm, we should page some of our engineers who are pilots... @anorlunda @boneh3ad @Borek @rcgldr @scottdave others?

 

[anorlunda]

ILS? Not me.

@grasscut , can you post a link to the manual for this system, and the manual for what you are connecting it to? I'm having a hard time visualizing what you are connecting to what.

 

[jim hardy]

S1S2S3

all i can find for that acronym. is medical

You'll have to define your terms.

What is this - an old Narco or King GA autopilot ?
They used variable capacitor sensors and tuned circuits and special low capacitance cables to interconnect the pieces. You can't poke around without high z probes.

old jim

 

[Tom.G]

ILS = Instrument Landing System

 

[grasscut]

ILS? Not me.

@grasscut , can you post a link to the manual for this system, and the manual for what you are connecting it to? I'm having a hard time visualizing what you are connecting to what.

Apologies but i don't not have access to the manual for the system. Let me briefly explain and i hope you all can understand. I have a system that has a Synchro- Digtial Card. This card will output a certain synchro signal based on the degree of pitch and roll from a Gyro. This synchro signals will then drive the glide slope indicator.
An example of how the synchro outputs are calculated are shown below.

 

[anorlunda]

Thank you @grasscut , that is much better. But it is still missing a critical piece, about what you are driving with that output signal?

There must be reasonable limits on how much power the outputs deliver to their "load", and if those limits are exceeded, it won't behave normally. For example, if there was a near short circuit in the instrument being driven. If it is something like that, descriptions of what the resolver is normally supposed to do are not very useful.

Are you able to plug the same panel instrument into a different resolver to verify that it works normally? Are you able to try your resolver's outputs to drive a different but similar instrument to verify that it works normally? Those are among standard troubleshooting techniques.

 

[grasscut]

Thank you @grasscut , that is much better. But it is still missing a critical piece, about what you are driving with that output signal?

There must be reasonable limits on how much power the outputs deliver to their "load", and if those limits are exceeded, it won't behave normally. For example, if there was a near short circuit in the instrument being driven. If it is something like that, descriptions of what the resolver is normally supposed to do are not very useful.

Are you able to plug the same panel instrument into a different resolver to verify that it works normally? Are you able to try your resolver's outputs to drive a different but similar instrument to verify that it works normally? Those are among standard troubleshooting techniques.

thank you for your reply. the synchro signal will drive the glide slop indicator and keep it stabilized.
An example of the indicator are shown below.
Apparently, my synchro outputs to other systems are working fine. As shown in the previous excel picture, I am able to achieve the voltage value at the designated degree. So I am not quite sure what happened that causes the change in the signal voltage and frequency

 

[jim hardy]

What could be the possible cause that will change the frequency of a synchro signal?

I'd be suspect of test equipment error.

There must be reasonable limits on how much power the outputs deliver to their "load", and if those limits are exceeded, it won't behave normally.

Does the voltage amplitude change when you connect the indicator? First thing i'd check is "Does voltage drop below counting threshold of my frequency counter?"

Question:: ..did you really mean 400khz ? I assumed you'd have just 400 hz like on old airplanes, so my alleged brain suspects a typo. K and H are only a finger apart on the keyboard... Would you confirm for us?

second thing - if this is like an old Selsyn, until the indicator comes to final position it draws current so voltage may sag.while it's in transit..

This tutorial covers a lot of servos, i suspect yours is a classic described in beginning of section 2.1.
http://www.moog.com/literature/MCG/synchrohbook.pdf

old jim

 

[sophiecentaur]

second thing - if this is like an old Selsyn,

That's what I thought. The selsyn (and Magslip?) is a very clever (almost magical) way of doing what they do these days with complicated position encoders and stepper motors, very little electronics involved and it just relies on relative phases to give angle information. If this is what's involved, it could be a problem in the windings. Are there any signs of the effects of heat? Could be a job for eBay??

 

[grasscut]

I'd be suspect of test equipment error.Does the voltage amplitude change when you connect the indicator? First thing i'd check is "Does voltage drop below counting threshold of my frequency counter?"

Question:: ..did you really mean 400khz ? I assumed you'd have just 400 hz like on old airplanes, so my alleged brain suspects a typo. K and H are only a finger apart on the keyboard... Would you confirm for us?

second thing - if this is like an old Selsyn, until the indicator comes to final position it draws current so voltage may sag.while it's in transit..

This tutorial covers a lot of servos, i suspect yours is a classic described in beginning of section 2.1.
http://www.moog.com/literature/MCG/synchrohbook.pdf

old jim

My apologies, its my typo, its 400Hz, not KHz.
The voltage measured from my system seems to be okay, as the measured voltage values are pretty close to the excel table which i have attached in the previous post. I measured the frequency with a multimeter. Supposedly if is correct, the frequency across S1S2S3 will be roughly 400Hz.
The indicator is uanble to move to its final position as the synchro signals received are wrong, thus, its unable to stabilize. Another about the voltage drop and change in frequency, this happened even with the indicator is not switch ON. Which is what i can't comprehend.

 

[anorlunda]

What evidence do you have that the glide slope indicator is not faulty?

 

[grasscut]

What evidence do you have that the glide slope indicator is not faulty?

i do not have evidence is not faulty, i think it might be the indicator as well.
However, the glide slope indicator is able to function and moved to its final position when it received synchro signal from its simulator

 

[sophiecentaur]

@grasscut Would you have an oscilloscope available? It should be possible to look at the relative phases between two legs as you rotate the sender unit. With two channels, you could compare a with B and A with C phases. Imo, the variation should be smooth and all three signals should have similar / same amplitude.

 

[grasscut]

@grasscut Would you have an oscilloscope available? It should be possible to look at the relative phases between two legs as you rotate the sender unit. With two channels, you could compare a with B and A with C phases. Imo, the variation should be smooth and all three signals should have similar / same amplitude.

unfortunately, i do not have a oscilloscope available for me. Do you know what are some of the reason that can cause an AC frequency to change?

 

[sophiecentaur]

unfortunately, i do not have a oscilloscope available for me. Do you know what are some of the reason that can cause an AC frequency to change?

The only reason I can think of is a heavy load slowing down the alternator. Frequency shifting is pretty difficult to achieve, in most cases.
If you actually own (or share) an aeroplane, I should have thought the cost of a cheap second hand 'scope would pale into insignificance with the running costs of any aircraft.
Test equipment will always prove useful for things you never imagined that can turn up.

 

[jim hardy]

Do you know what are some of the reason that can cause an AC frequency to change?

as indicated by a DMM ? Distortion of the waveform. No anti aliasing so it's apt to count harmonics, or maybe plain old noise.

I'm suspect this is a selsyn which is just two synchronous machines with common AC excitation to their fields..
I'd be suspect of rusty slip rings or shorted rotor turns on one of them.

i notice one frequency you reported is 1.2khz which is third harmonic of the 400 hz i think is on your rotors.old jim

 

[jim hardy]

The simpler your test equipment the less apt it is to fool you.
That's why i dislike DMM's for general purpose troubleshooting. I much prefer analog.
In the power plant i used a Simpson 260 much to the amusement of my colleagues.

nowadays they're a bit pricey for home hobbyists but used ones abound on Ebay.

For 95% of my work i use this $10 meter from Walmart

for less than the cost of a DMM you could connect three of these to your Selsyn and read all three phases simultaneously while you rotate the shaft with your fingers. That'd give you an intuitive "Feel" of the device. You'll see the needles move according to that Excel table of voltages.

I solder in longer leads with alligator clips which makes it real handy for automobile work - you can connect it to something in the engine compartment and read it sitting in the driver's seat .

As Mr @sophiecentaur says , an oscilloscope is the Excalibur for troubleshooting analog electronics.

old jim

 

[grasscut]

@sophiecentaur @jim hardy thank you all for your reply.

I having a hard time trying to prove that my system, which output the synchro signal via a synchro-digital card is working fine. because somehow the indicator simulator is able to work with the indicator. And based on my observation, the voltage drop and changes in frequency is likely the cause that my system is unable to integrate with the indicator. I have limited resource, and my findings are mostly measure with DMM. So I am trying to seek any advice from someone who has experience or encountered sound situation

 

[jim hardy]

i'd try an avionics forum. Perhaps pprune.org has one or EAA.org.

I having a hard time trying to prove that my system, which output the synchro signal via a synchro-digital card is working fine. because somehow the indicator simulator is able to work with the indicator. And based on my observation, the voltage drop and changes in frequency is likely the cause that my system is unable to integrate with the indicator.

well you've given scant little information about it.
You haven't even told us if it's an electromechanical selsyn-like gizmo or a computer pretending to be one.
If the former it should be incapable of changing frequency.
If the latter anything is possible- that's the downside to high tech.

I've guessed as best i can.
When you describe what is this mysterious machine you're inquiring about somebody might be able to help you.

To your original very general question, 'what can make voltage decrease', the answer is "asking the source of that voltage to deliver too much current."

Why are you reluctant to give details or a photograph ? Surely it has a nameplate with make, model and serial number. If not, is it "hot" ?

 

[grasscut]

i'd try an avionics forum. Perhaps pprune.org has one or EAA.org.

well you've given scant little information about it.
You haven't even told us if it's an electromechanical selsyn-like gizmo or a computer pretending to be one.
If the former it should be incapable of changing frequency.
If the latter anything is possible- that's the downside to high tech.

I've guessed as best i can.
When you describe what is this mysterious machine you're inquiring about somebody might be able to help you.

To your original very general question, 'what can make voltage decrease', the answer is "asking the source of that voltage to deliver too much current."

Why are you reluctant to give details or a photograph ? Surely it has a nameplate with make, model and serial number. If not, is it "hot" ?

my deepest apologies, its not that I am reluctant to give the details, but i myself do not have much information. Moreover, its not meant for commercially use. Thats why i can only share what i have observed thus far.

 

[jim hardy]

Well, then understand your question from my side:

" I have this secret thing that makes voltage. When i plug it in the voltage goes away. Why? "
to which the only possible answer is
"Must be broke."

 

[sophiecentaur]

my deepest apologies, its not that I am reluctant to give the details, but i myself do not have much information. Moreover, its not meant for commercially use. Thats why i can only share what i have observed thus far.

You can at least share the name of the manufacturer and a model number.
How can we help without that? You have to assume we know nothing until you tell us.

 

[grasscut]

You can at least share the name of the manufacturer and a model number.
How can we help without that? You have to assume we know nothing until you tell us.

I honestly do not have the information. and i really wish i could share more.
i try to make do with what i have and the information you guys had shared with me thus far.

 

[sophiecentaur]

So the equipment has no markings on it?