Can I read a synchro with 3 analog inputs?

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I see lots of synchro to digital isa and vme and pci cards and solid state scott-t transformers that convert synchro to resolver sin-cos, but if the signal is 11vac 400hz, why cant I just snap s1 s2 and s3 with a 4 channel simultaneous sampling a/d chip and solve the equation for angle? I can see that this is sw intensive and would be a problem a decade ago, but not anymore. Do I have to sample the 3 signals at the peak of the reference signal? Gee, that doesnt get sent down the wire does it? Rebuttals?

Imagecraft compiler user

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Ho Bob -

Been many years since I even thought about synchros. My recollection is that they are wired in a delta configuration. You can measure the voltage difference between the nodes of the delta but your measuring circuit still needs a common connection point. My additional recollection is that these things are almost always in a floating system.

It MAY be adequate to use a simple resistor Y network. The common does not have to be at the exact electrical center of the delta, just some point (that may actually move, voltage-wise) that can be called common.

As for simultaneous sampling, not sure if that is needed or not. It would probably make it easier IF you had it. For the moment, lets suppose that you use an AVR Mega that samples at the maximum 13us rate. One period of 400Hz is 2.5ms. So, if you take 3 samples, round robin, around the nodes, you would have a new sample from each one every 3*13us= 39us. The voltage would have changed by 360 deg * 39us/2.5ms = 5.6 degrees. This would probably give you some bias to the relative phase angle, depending on which way you go around the nodes. Not sure if 6 degrees would be a big deal or not.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Try to figure out how to replace a bunch o hydraulic gauges that from from a synchro transmitter with a 3ATI AMLCD and a computer that reads the synchro ins and shows the PSI. I guess aircraft ground is the same all over. I guess the old gauge was 3000 lbs in 240 deg, 12.5 lb per deg. Need to get down to a lb, so need that minute and second too. Is S1 the same phase as the ref signal?

Imagecraft compiler user

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It sounds like a fun exploration. It is still somewhat amazing how many niche electro-mechanical apps like that that can still be found where you look at it and say "Hmmm--I could do this good enough with a micro (AVR8 of course) and a few thingamajigs (in this case, simultaneous sampling ADC [or, if the AVR's ADC is good enough then a simple sample-and-hold with enough channels])."

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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I think the ADC stuff is mentioned in this http://www.ddc-web.com/documents/synhdbk.pdf.

I wonder if it could be implemented using zero-crossing detectors and measure the phases directly?

I`m working on interfacing with an old antenna now, but will not make the synchro demodulator. Will instead take the easy route of using modules. The modern S/D modules are still replaceable with the original ones from 1976, same pinout and specs.

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Quote:

I think the ADC stuff is mentioned in this http://www.ddc-web.com/documents....

Wow--a nice document!

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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This patent seems interesting for Bob...

Quote:
Demodulator for angularly related signals
US 3639850 A
Abstract
A demodulator for signals angularly related to the position of a rotor of a synchro or resolver uses, in one embodiment, only one transformer in a circuit for subtracting, adding, and phase shifting the signals to produce a single signal phase shifted in proportion to the rotor angle. In another embodiment, a transformerless circuit uses an operational amplifier to derive the phase shifted signal.
http://www.google.com/patents/US...

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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We have the DDC book at work on the shelf. I just dled the pdf to look at at home.

Imagecraft compiler user

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As Jim suggests a Y divider may be used. The common node of the divider can be offset by a 2.5V. This offsets the AC for use with unipolar ADC. The coincident capture ADC, simplifies calculations. The L-L voltages calculated as differences. There are math tricks to simplify calculations and null the offset. DFT calculations are easy and accurate way to measure phases.

It all starts with a mental vision.

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Hi Bob,
if you send me your e-mail in a PM I can send you an ELEKTOR article about my avionics project.

I think an AVR can convert a lot of synchros simultaneously, it can also generate the apropriate
voltages to drive motors..

https://www.avrfreaks.net/index.p...

http://www.youtube.com/watch?v=2...

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The one under my picture goes right to me. Thanks!

Imagecraft compiler user