Phase Difference

[Henrik_Volkers]

In you current setup you can't seperate 0° (3o'clock) position and 180° (9°o'clock) and all amplitude/phase combinations have two possible positions.

If you excite the rotor (with a sine) you need to measure all three legs of the stator to get an absolute position, or the exciter voltage and two legs.  (not 100% shure)

No matter wich stator leg you measure in relation to the rotor (exciter) the phase will be 0° or 180° ! Just for the two positions where the signal it about zero you can see different phase values due to imperfection of the synchro. The direction from 0° to 180° could run via 90° or -90° depending on the unit you use! Replace the syncro and with 50% chance your cw ccw detection goes crasy 😄

Or you excite the stator with a 3 phase signal and measure one phase and the rotor output (always a high amplitude signal and a nice absolute  360° phase shift 🙂 ). This is the only configuration where you really need to measure the phase. In all other cases you just need to detect wheter the phase is 0° or 180°. 

@Ben:  (Amplitude of Rotor) / (Amplitude of Stator winding) = Cos (Rotor Angle) ...  

Shure? Stator amplitude can go to zero Oh oh 😉

So if you measure S1 and S2 in your setup  both amplitudes should be the same at your 0° , (easy to measure with tone detection) , the ratio S1/S2 ( or which amplitude is greater ) will tell you your CW or CCW off positioning.

[Ben]

---

@Henrik_Volkers wrote:

...

@Ben:  (Amplitude of Rotor) / (Amplitude of Stator winding) = Cos (Rotor Angle) ...  

Shure? Stator amplitude can go to zero Oh oh 😉

...

---

Let me step back and try to recover by talking this through a bit looking at how the "used to be used".

Synchros were often used in pairs with the same excitation applied the rotor of both synchros and the stator winding wired to the conterpart of the other (S1 of control wired to S1 of the slave...)

In that configuration there was a magnetic field generated in the rotor windings of both control and slave syncros.

Looking at the control synchro, the magnetic field of the rotor would then induce a current in each of the stator windings. The induce current (sine wave) magnitude and phase (in phas or 180 out) in the stator windings depended on the angle between the rotor and each stator.

When the rotor was aligned exactly with one stator winding there would be maximum coupling into that stator windind. The other two winding would have a diminished based on the angle between the windings with one of them being 180 out of phase. That is what Clint observed when he reported that the difference between two winds was 7 V which is larger that the applied excitation.

Now casting our attention to look at what happens in the slave synchro...

The rotor is driven with the same excitation as in the master. Each of the stator windings driven with the signal induced in the master.

The three stator windings generate an magnetic field that is the result of the the windings adding together.

In fewer words it is vector addition of the three fields.

In our case we are attempting to determine the direction of that vector.

So to solve the question of try to detect the angle of the rotor we need to...

1) Measure amplitude of the induced signal in each stator winding. We need only know the amplitude of each signal. RMS, peak to peak, peak we don't care.

Scratch that! We need to know in phase or out of phase to be able to determine the sign of the magnitude!

2) We know the angle displacement between each stator wind which will be -120, 0, 120 degrees.

3) We can construct a vector for each stator signal to create a complex number for each stator which would be =;

Length of vector for a winding = Amplitude of the signal in that winding

Angle = -120 (or 0 or 120 in radians)

Add those three complex numbers together (like what was happening inside the slave synchro from the old days)

The sum of those three vectors will yield the angle of the resulting vector of the rotor of the master (the sensor in this case).

So in LV we need only measure the amplitude of each stator winding signal, define the angle of the vector to get a complex representation, add them together (Compound arithmetic configured for add) and then pull out the angle of the complex sum.

Maybe five nodes on a diagram?

But we do have to measure all three stator windings.

For what it is worth,

Ben

Discalimers:

Spoiler

I do not have a license for LV. I have to wait for the free version to come out.

It has been at least 15 years since I had to used a synchro and then I used an appropriate DAQ device to make it simple.

It has been about 43 years since was using synchros in the radars used for Missile Fire Control.

[Henrik_Volkers]

Just for clarification: When I said compare S1 to S2, compare  the voltages S1/rtn and S2/rtn ...

[Clint1000]

Not to keep dragging this out and probably should have explained earlier but I don't have a control and slave synchro.  I'm inducing 5 vrms on the rotor and using my daq to measure the amplitude and phase..ONlY the phase relative to the rotor.. on the stator S1 to rtn.  It may not be the correct way of doing this but we are upgrading a test set and thats what I was given.  

My biggest issue is ..I think..is being able to measure the phase change relative to my ref voltage on the rotor.  

One additional thing I thought of.  As my ailerons move through the zero position the voltage goes to near zero.  When they are at the extremes the voltage is ~7 Vp.  So I set my daq card range to +/- 10 .  As it approaches zero it is still at +/-10.   Is there a way to change that to +/-5 ..+/- 1..+/-0.2 as the voltage decreases when I don't know when this will be happening in the test??  Sort of on the fly?

thxs.

[Henrik_Volkers]

Sorry for repeating myself: Your approach is the wrong way of using the synchro. No signal means no phase! and the residual voltage you measure could even be some wire coupling.

I found a (nearly correct) figure of the output signals of a synchro

https://de.wikipedia.org/wiki/Drehmelder#/media/Datei:Drehmelderspannungen.png

Your zero migth be at 270°, the S3 amplitude zero crossing. As you can see the amplitude of S1 and S2 have the same magnitude at this point but different slopes. So again my proposal: Instead of trying to measure uncertain phase in the noise, use the S1 and S2 magnitudes .  I worked a lot with linear differential transformers ... and the resolution you can achieve are incredible ... except at the zero position!

If you hoock up a scope on S1 and S2 , trigger with the exiter signal , you should easely see that.

Kudos for the first who find the error(s) in that figure.

[Henrik_Volkers]

Ben explained a nice way to resolve the absolute angle if you measure the three legs (and the ref to determin the sign of the magnitude)

the nice thing: the absolute magnitudes are not needed, just the 0° or 180° phase information and the relative magnitudes...

(and if you know and compensate the phase lag between ref and the Sx signals, a simple linear fit of the two signals would give you the ratio incl. the sign , feed that into the calculation above and done 🙂 )

What angle resolution do you need?

You said you run into problems below 100mV ... at 7V peak that is arcsin(0.1/7) ~0.82deg

S1 and S3 have an amplitude of coarsly 6V and there difference is  nearly (~0.87)  as sensitive to an angle change as the imperfect zero crossing. 

[Ben]

Catching up...

If the rotation is limited to only 180 degrees of motion, the extra winding can be ignored (I believe).

---

@Henrik_Volkers wrote:

Sorry for repeating myself: Your approach is the wrong way of using the synchro. No signal means no phase! and the residual voltage you measure could even be some wire coupling.

I found a (nearly correct) figure of the output signals of a synchro

https://de.wikipedia.org/wiki/Drehmelder#/media/Datei:Drehmelderspannungen.png

Your zero migth be at 270°, the S3 amplitude zero crossing. As you can see the amplitude of S1 and S2 have the same magnitude at this point but different slopes. So again my proposal: Instead of trying to measure uncertain phase in the noise, use the S1 and S2 magnitudes .  I worked a lot with linear differential transformers ... and the resolution you can achieve are incredible ... except at the zero position!

If you hoock up a scope on S1 and S2 , trigger with the exiter signal , you should easely see that.

Kudos for the first who find the error(s) in that figure.

---

It hard to see in that image but the phase should switch at each zero crossing.

Now as far as measuring when the signal is small.

I am confused since I would hope the small signal would only happen at the extremes of the motion not in the middle.

While there are ways to measure smaller signals, there are some cludges involved.

You could configure multiple task with different ranges defined at the same time, but just can not have two tasks running at the same time. You would have stop a "Large Range Task" and then start the pre-configured "Small Range Task" is one approach.

Not sure it it work but you may get away with wiring the signals to multiple inputs where the multiple inputs are configured for different ranges. Provided that acquisition runs without throwing out of range errors... Check the large range channel and if the value is below the upper range of the low range input, use the low range else use high range. I did say cludgy didn't I?

For what it is worth...

Ben

[Henrik_Volkers]

Ben, you are correct about the phase change at each zero crossing. I contacted the author and already got a confirmation.

[Henrik_Volkers]

Clint1000: can you capture S1-3 and ref from some 'critical' positions .. 20 periods would be fine.. and post it?

or some seconds with an angle sweep?

tdms or lvm or csv or a vi containing a  graph and the data (make current values default, save) 

[Ben]

---

@Henrik_Volkers wrote:

Clint1000: can you capture S1-3 and ref from some 'critical' positions .. 20 periods would be fine.. and post it?

or some seconds with an angle sweep?

tdms or lvm or csv or a vi containing a  graph and the data (make current values default, save) 

---

I like where you are going there Henrik.

Switching from S1-S2 to S1-S3 could push the zero point outside the range of motion. We can always adjust the angle after the measurement is done.

Ben