Servo Slave and Master?

J. J. L. Farcot coined the term “servomoteur” in 1868 in his book Le Servo-Moteur ou Moteur Asservi.  “Asservi” is the French word for “enslaved,” so it was no surprise that English speakers used the phrase “slave motor” along with “servo motor.”

The servo motor replicated the movements of a controller. It still does. It may be that the flow of paper into a printing press is the master to which the motion control for printing and cutting must be synchronized. For more than a century, the motors that replicated the master’s movements were called “slaves.”

“Master-slave configuration” was common parlance, and the use of this language wasn’t controversial. Now Wired magazine reports that tech companies are beginning to feel uncomfortable chatting about slaves.

Master-slave configuration

When Farcot wrote about a steamship motor that was enslaved to its controller, slaves were a reality of life. In July of the same year, the 14th amendment gave full citizenship to former slaves in the United States, and slavery had been outlawed in France twenty years earlier, but most people were familiar with the idea. “Slave” was a meaningful word, and saying that the motor was enslaved by the control was a description.

A century later, most engineers setting up master-slave configurations for motion control didn’t think about human slavery at all.

Today, as the Black Lives Matter movement increases awareness of African Americans’ experience in the United States, Americans in general are becoming more aware of the casual racism that pervades our workplaces, schools, and cities.

People are realizing that the word “slave” probably shouldn’t be tossed around as though it has no particular meaning in our history and no particular emotional content for the descendants of slaves.


Some tech companies already did away with master-slave terminology. “Primary” and “replica” or “leader”and “follower” are in use by some companies.

The Washington Post pointed out, however,  that machine learning and AI systems have to be able to communicate with each other. Those industries have to change together or not at all, because the code won’t work if some versions use “slave” and the others use “replica.”

A user at Github said, “Trying to bring feelings into the clean technical world is one of the most stupid things i can think of.” It’s possible that the feelings are already there but not being spoken.

With servo motors, it might be time to make the change. In the meantime, if you need support for your Rexroth motion control, we can help you fast. Call right away for immediate assistance.

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