Investigating Electromagnetic Magic In Obsolete Machines

February 5, 2025 by Bryan Cockfield 15 Comments

Before the digital age, when transistors were expensive, unreliable, and/or nonexistent, engineers had to use other tricks to do things that we take for granted nowadays. Motor positioning, for example, wasn’t as straightforward as using a rotary encoder and a microcontroller. There are a few other ways of doing this, though, and [Void Electronics] walks us through an older piece of technology called a synchro (or selsyn) which uses a motor with a special set of windings to keep track of its position and even output that position on a second motor without any digital processing or microcontrollers.

Synchros are electromagnetic devices similar to transformers, where a set of windings induces a voltage on another set, but they also have a movable rotor like an electric motor. When the rotor is energized, the output windings generate voltages corresponding to the rotor’s angle, which are then transmitted to another synchro. This second device, if mechanically free to move, will align its rotor to match the first. Both devices must be powered by the same AC source to maintain phase alignment, ensuring their magnetic fields remain synchronized and their rotors stay in step.

While largely obsolete now, there are a few places where these machines are still in use. One is in places where high reliability or ruggedness is needed, such as instrumentation for airplanes or control systems or for the electric grid and its associated control infrastructure. For more information on how they work, [Al Williams] wrote a detailed article about them a few years ago.

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Not A Pot, Not An Encoder: Exploring Synchros For Rotational Sensing

February 1, 2023 by Dan Maloney 25 Comments

We’re all familiar with getting feedback from a rotating shaft, for which we usually employ a potentiometer or encoder. But there’s another device that, while less well-known, has some advantages that just might make it worth figuring out how to include it in hobbyist projects: the synchro.

If you’ve never heard of a synchro, don’t feel bad; as [Glen Akins] explains, it’s an expensive bit of kit most commonly found in avionics gear. It’s in effect a set of coaxial transformers with a three-phase stator coil and a single-phase rotor. When excited by an AC reference voltage, the voltage induced on the rotor coil is proportional to the cosine of the angle between the rotor and stator. It seems simple enough, but the reality is that synchros present some interfacing challenges.

[Glen] chose a surplus altitude alert indicator for his experiments, a formidable-looking piece of avionics. Also formidable was the bench full of electronics needed to drive and decode the synchro inside it — a 26-volt 400-Hz AC reference voltage generator, an industrial data acquisition module to digitize the synchro output, and an ESP32 dev board with a little OLED display to show the results. And those are impressive; as seen in the video below, the whole setup is capable of detecting tenth-of-a-degree differences in rotation.

The blog post has a wealth of detail on using synchros, as does this Retrotechtacular piece from our own [Al Williams]. Are they practical for general hobbyist use? Probably not, but it’s still cool to see them put to use.

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Retrotechtacular: Synchros Go To War (and Peace)

April 25, 2018 by Al Williams 41 Comments

Rotation. Motors rotate. Potentiometers and variable capacitors often rotate. It is a common task to have to rotate something remotely or measure the rotation of something. If I asked you today to rotate a volume control remotely, for example, you might offer up an open loop stepper motor or an RC-style servo. If you wanted to measure a rotation, you’d likely use some sort of optical or mechanical encoder. However, there’s a much older way to do those same tasks and one that still sees use in some equipment: a synchro.

The synchro dates back to the early 1900s when the Panama Canal used them to read and control valves and gates. These devices were very common in World War II equipment, too. In particular, they were often part of the mechanisms that set and read gun azimuth and elevation or — like the picture to the left — a position indication of a radar antenna. Even movie cameras used these devices for many years. Today, with more options, you don’t see them as much except in applications where their simplicity and ruggedness is necessary.