Multiple instruments inside an aircraft’s cockpit are dependent on AC transducers. Transducers include synchros, resolvers, and linear/rotary variable differential transformers (LVDTs/RVDTs) and are used in numerous applications including navigation reference units, automatic direction finders, distance measurement equipment, and landing gear position and control. Synchros have been used in both commercial and military purposes; they are the transducer of choice when reliability is important and environment conditions are unforgiving.
Synchros and resolvers are essentially transformers in that they have primary winding and secondary winding. Just like a transformer, their primary winding is driven by an AC signal. Synchros, however, have a primary winding and three secondary windings, with each secondary winding mechanically oriented 120 degrees apart. A resolver has two primary windings and two secondary windings, spaced 90 degrees from each other. While electrically similar to transformers, they are mechanically more like motors, where the primary winding in a synchro or resolver can be rotated with respect to the secondary windings. For this reason, primary windings are also called rotors, while secondary windings are referred to as stators, due to their fixed position. In an automatic direction finder (ADF), the resolver or synchro is used to drive an indicator. As the aircraft turns the amount of coupling in the transducer changes proportionally, thus indicating for the pilot just how far their aircraft has actually turned.
Synchros are used to track the rotary output angle of a closed-loop system, which uses feedback to achieve accuracy and repeatability. A synchro can be turned continuously, and since its secondary winding outputs are analog signals, provide infinite resolution output. As the shaft of a synchro turns, the angular position of its rotor winding changes in comparison to the secondary (stator) windings. The relative amplitude of the resulting AC output signals from the secondary windings indicates the rotary position of the synchro’s shaft.
The analog output signals that synchros generate must then be converted into digital form by a synchro-to-digital converter. Conventional analog-to-digital converters do not work well in this task, as synchros have inductive characteristics that affect such readings, synchro output signals can be distorted due to nonlinearities in the synchro and phase-shift the transducer, and synchro output signals typically contain lots of electric noise due to their working environment. Therefore, a synchro-to-digital converter must use transformer-isolated inputs and outputs.
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