autosyn

An autosyn is a type of synchro, an electromagnetic coupling device that transmits angular position between two shafts without mechanical connection. Where a standard synchro converts shaft rotation into electrical signals for remote indication or control, an autosyn goes further: it uses a servo motor to automatically rotate its output shaft to match the input shaft of a distant autosyn, creating a follower system. The two units are wired together through three signal wires, and they stay synchronized as long as power is applied.

The autosyn consists of a rotor (a permanent magnet or electromagnet wound to produce a rotating magnetic field) and a stator with three coils arranged 120 degrees apart. When the input rotor turns, it induces voltages in the stator coils whose magnitude and phase depend on the rotor angle. These signals travel down a three-wire cable to the receiver unit. There, the received voltages drive a small servo motor that rotates the output rotor and its mechanical load until the voltage pattern it generates matches the input pattern exactly, at which point the motor torque drops to zero.

Autosyn systems appeared widely in aviation and military fire-control equipment from the 1930s onward. A typical setup might link an aircraft compass heading indicator to the flight engineer's panel, or couple a gun turret training dial to its actual physical position. Common operating frequencies were 50 Hz and 400 Hz, with 400 Hz favored in aerospace because smaller transformers and faster response times were possible. Standard resolver-type autosyns operated at low power, producing only enough torque to move lightweight pointers and indicator dials; heavier loads required servo amplification.

Failure modes and drift

Mechanical play in the servo gearbox and magnetic hysteresis in the rotor core produce lag: the output shaft may lag the input shaft by a few degrees under dynamic conditions, or hunt (oscillate around the correct position) if servo gain is set too high. Friction in the output mechanism also causes static error, particularly when the system is asked to hold position against spring force or gravity. These limitations made autosyns unsuitable for precision applications requiring better than 0.5-degree accuracy over long runs.

Autosyn systems fell out of favor after the 1970s as digital angle encoders and stepper motors became reliable and affordable. A few niche applications in aerospace and maritime navigation still use them because they require no electronics, tolerate vibration well, and continue to function if power fluctuates. Modern equivalents use resolver-to-digital converters paired with stepper or brushless DC motors, which offer better accuracy and no hunting.