Phase measurement of Synchros

[Clint1000]

Anyone familiar w Synchros?  I'm trying to measure the phase difference (PXI-6229-Differential Analog Ins) between S1>S2, S2>S3 and S1>S3 using the "Amp and Freq.vi" that is shipped w LV2014.  I took the idea from an earlier post. (http://forums.ni.com/t5/LabVIEW/Phase-issue/m-p/3079552#M879880). When I run the vi the phase is all over the place w the synchros stationary.  One time its say 79 degrees, then  its -135 deg etc.  Can I use a PXI-6229 using differential inputs to measure the phase difference of a synchro?  Once I get this working I need to measure the different legs of the synchros (S1>S2, etc) relative to a ref signal.  The synchro (R1>R2) input signal is 5.25 VAC@400 hz. My sample rate is 4000 samples/sec...1000 samples.  To see if I was using the Amp and Freq.vi right I created a Sinewave vi and a cosine vi and measured the phase difference (90) and it worked like a charm.  Why would the phase jump all over the place.  If I look at the output of the synchros on a vi graph from the analog card they look pretty stable and "clean".

Thanks..

[Ben]

http://www.ddc-web.com/documents/synhdbk.pdf

I have not worked with synchros for almost 40 years but I still remember the general ideas.

Spoiler

They were used in the Naval Fire gun and missile fire control systems to measure angles of the guns, gyros, and similar

This link looks like it has good references to support the task.

The first issue could be that the NI 6229 does not do simultaneous sampling which is normally required for measuring phase relationships. Additionally sample at 4KHz of a 400 sine wave will only give you 10 samples per cycle which it barely good enough to sample and display let alone analyze.

Consult that link above for the details on the math but as I recall...

The three stator winding are physical oriented about the rotor with 120 degree between each stator winding. The signal in the stators are either in phase or 180 out if the rotor signal. The amplitude (taking into consideration the sign - negative when 180 degrees out) end up being the length of  a vector with the angle of the vector being 0, 120, or 240. The angle of the resulting vector when the three stator vectors are added is the angle of the rotor which is generally what is being measured.

So resorting to just RMS amplitude measurements adjusted for in-vs-out of phase may let you resolve (excuse the bad pun) the angle of the synchro.

I admit I am doing some hand waving when it comes time to decide if the stators signal is in or out of phase but I figured I would reply to get the discussion going.

But start out by seeing if you can crank up the sampling rate.

Ben

[Clint1000]

Since you now seem to be the Synchro Gru..  Question: Each S1, S2  and S3 are out of phase w each other by 120 deg..this..to me...is a fixed component.  So why do I need to look at the other 2 pair if I know the voltage in one pair..I found a formula on the net.  Sine ( angle of interest)= ratio of S1>S2 voltage divided by R1>R2.  So if I put 2.34vac on R1>R2 and measure 2.34 on say S1>S2 then my angle is InvSine of 1 or 90 deg..  and S2>S3 is 120 deg out from S1>S2 so Sine (angle of interest+120)= -1.17/2.34 which also = 90 deg..check!  I realize 6229 isn't simultaneously sampling but thought it would sample each channel fast enough for my measurements.  Tell me if I'm wrong but I was just treating the synchros as a transformer w 1:N windings.

I'm confused...You said " The amplitude (taking into consideration the sign - negative when 180 degrees out) end up being the length of  a vector with the angle of the vector being 0, 120, or 240. The angle of the resulting vector when the three stator vectors are added is the angle of the rotor which is generally what is being measured." So the angle of interest is just the vector ( RMS amplitude) of each leg of S1,S2 and S3 (relative to each other) added up ???  And the negative sign is 180 out ref to the supplied (R1, R2) voltage?? phewwww !

Thanks..So summing up do you think I can measure my synchros w a PXI6229 and determine the angle ( using Labview 2014).

[Clint1000]

By the way..I had the DDC and North Atlantic handbooks you linked to..they didn't answer my questions.

[Ben]

A synchro gru I am not!

But since I am waiting for a new version of LV to install I will try to act as a sounding board based on old memories from my youth. Borrowing an image from Wikipedia...

Starting with just the first stator and simplifying by saying the winding are 1 to 1 and there is perfect coupling...

When the rotor is aligned with S1 (stator winding 1) we would expect the signal induced in S1 to be the same as what is in the rotor.

If we rotate the rotor 180 we would expect the same amplitude but now the signal is inverted.

When the rotor is 90 degrees out of alignment with S1 we would expect the signal to be zero.

So the amplitude in S1 is related as a cosine function.

Returning the rotor to aligned with S1...

In S2 we expect the signal to be out of phase with an amplitude determined by cosine of 60 degrees.

Similarly with S3.

So all three stator tell us something about the orientation of the rotor. The three signals taken together give us the full picture.

Still with the rotor aligned with S1

S1 signal would be maxed out and point would be a vector pointing "Up".

S2 and S3 would be much smaller pointing away but with a negative length so those vectors would also be pointing up and when those two are added together point straight up.

So when the three signals are mapped to vectors... got run!

Ben  

[Ben]

OK that meeting was short.

You may be able to get way with that board using RMS of each stator winding. Simultaneous sampling is going to be need I believe because we need to know what the amplitude of all of the singal as the same time. Multiplexed sampling is going to not let us compare if the rotor is Plus when the stator is Plus.

Cranking up the sample rate will force the samples to be closer in time. At the very lest crank up the sample rate to max. You could subtract the rotor signal from the stator signal (point by point) and if the amplitude is greater than the rotor signal that would indicate out of phase...

But eventually you will want to know the angle as the rotor moves so... what I pondered above may be useless.

I think I would first be asking NI if they have a board that will handle this situation. It amounts to same thing as a LVDT with three simultaneous LVDTs being monitored.

Done doing the sounding board thing!

Please feel free to update when you find more. I enjoy having the old gray matter awaken.

Ben 

[Ben]

Thinking a bit more about what I tried to talk through above...

The vector addition I tried to talk through is what happens when you have a two synchro system where one is the master and the slave follows the master as the magnetic field produced by the three stators add together vector-wise and the rotor follows same.

Ben

[Ben]

Still installing ... part 23 of 47... and still thinking...

Your terminology eg. S1-S2 hints that your synchro is wired as a delta scheme (?).

If you look back where I was thinking through the voltages measured across each winding the theory still holds true if you can measure all of the voltages at the same time. But having to multiplex them, ... that will not go well.

Increasing the sample rate will force the samples to be closer together. Additionally, back in the day of Traditional DAQ it was possible to set the inner-channel delay. By default DAQmx will try to spread-out the sampling evenly in order to avoid inter-channel cross talk when measuring from high impedance sources. if that setting is still available, it may help.

Another thing to think about is CMR Common mode rejection and making sure there is a ground path somewhere to the DAQ device (a 1 meg resistor for example) that will ensure the signals are within the CMR of your DAQ device.

 Now on part 26 of 47...

Ben

[Ben]

PXIe-4340

See page 2-6 here.

Ben

[Clint1000]

Thanks Ben...you're a life ( career )  saver.  I can't find anyoneor anywhere  that can put this in laymen's terms..while I TRY and digest the rest of your info and you said you didn't mind...here is what I'm trying to measure.  See attached JPEG.

I have 4 different fins that go from 0 deg to +/-30 deg w 0 deg being the middle.  I stimulate R1>R2 on all 3 w 5.25 VAC@400 Hz.  North Atlantic has a chart that converts voltage to angle..see attached excel sheet. In my current design I have (for simplicity I'll just describe 1 ch setup, the others are identical ) a PXI6229 w S1>S2 connected differentially to say AI0. S1>S3 to AI1 and S2>S3 to AI2 etc.  I have R1>R2 connected as a ref to gnd signal on another analog in channel.  No 1 meg to gnd anywhere to eliminate CMRR.  I measure each channel, one after another. So you're saying I can't do it this way...at least correctly??  I have to use the PXIe-4340 and I'd have to get 4 of them??