View Full Version : Reading Control Transformers and Differential Resolver Synchros

10-12-2009, 01:10 PM
Well I aquired a few L1011 autopilot controls. The Altitude select panel is the one I am currently working on. Will post pictures later when I get home.
There are two sets of Control Transformers, and 2 sets of differential Resolvers(1 set for each autopilot I presume). These are control synchros, and not Torque synchros which everyone seams to be only concerned about.
I need some suggestions as to how to wire these together. There is also a much smaller item that looks like a syncro with only 2 wires comming out of the back, each wire is labled 1, and 2 respectively. It is not a motor, and it turns 1:1 with the external control knob for selecting altitude. I assume this one is for hundreds of feet. The CT, and DR are geared to turn much more slowly...

I think I will modify a computer PSU to output 24v, or simply -12 and + 12. These synchros are rated at for 28v but as long as I get a signal from them that my Atmel AVR can read, it should be fine. After all I don't need torque to drive anything, rather just read a signal. Pictures to follow.

10-13-2009, 06:42 PM
Control transformers and resolvers are AC devices. A modified PSU probably is not the best choice.

It is possible to supply AC power to the R1 and R2 leads, read the resultant AC voltages on the S1, S2, and S3 leads, and determine the rotational position of the rotor. You must make phase sensitive voltage measurements, however. That is to say, you must be able to tell the difference between an AC signal that is in phase with the rotor voltage or inverted with respect to it. Once you have these voltage measurements, you'll have to do some calculating to convert from voltages to degrees of rotation.

Another approach is to supply a synchro signal to the S1, S2 and S3 leads and sweep until you pick up the zero output from the R1 and R2 leads. This also must be made in a phase sensitive way as there are two zero points per rotation.

Depending upon how the synchros/resolvers are used, yo umay want to consider replacing them with some other rotation sensing control like a rotary encoder or potentiometer.

10-13-2009, 10:19 PM
Thanks Mike, I must say I am pleased with your latest book, and thanks for the help. I have a few potentiometers with no stops in them which would be perfect for this.. I will use one pot for 100's of feet, and another for 1000's of feet using the built in gearing already in place!

The little blue thing, which all I can really call it is an encoder... Only two wires coming from it... If I spin it, I get varying readings from an ohm meter, but it certainly is not a potentiometer... must be a coil, and probably in the synchro family...