Why Do We Call It a Servomotor?


You may call it a servo motor, servo-motor, servomotor, or just a servo. Indramat called their 20th century original an “AC Servo.” Why?

Servo or cerveau?

While it has been suggested that the French cerveau (brain) might be the root for this term, it’s more likely to be the Latin servus, which gives us (and the French) words like “service” and “serve.” It appears that the term “servomotor” in English did come from the French servomoteur in the late 1800s, and it’s therefore conceivable that the French use of the word included a pun on cerveau, but it’s clear that the “servo” part is related to “serve” (or servir).

In fact, when J. J. L. Farcot used the term servomoteur in 1868 in reference to a ship’s engine, English speakers translated it as “slave motor,” referring to the fact that the servomoteur replicated the movements of a controller. Farcot’s book Le Servo-Moteur ou Moteur Asservi, published in 1873, used asservi, meaning “subservient” or “enslaved,” as a synonym.

Subserviant motors

The modern servomotor, therefore, is a motor that serves the desires of the motion control system instead of just doing whatever it was first built to do. The name “servomotor” doesn’t just celebrate the usefulness of servomotors, though. It means that the motor or drive in question has feedback in a closed loop.

That is, the motor monitors the feedback from the rest of the system and responds. If the settings require a higher RPM than the sensors detect, the motor will provide more torque to speed things up.

The motor itself doesn’t know what’s going on, of course. It responds to the feedback from an encoder, which is reported to a drive, which gives the motor its instructions. The servomotor might be said to serve the drive, which is its master — except that the motor, drive, and control function together as a servo drive system.

The personality of the servo

Indramat servos have a programmable module known as a “personality module.” At this point, you’ve got to be thinking of the the Sirius Cybernetics Corporation, which made machines with “Genuine People Personalities.” All the doors on the spaceship in The Hitchhikers Guide to the Galaxy, being from the new GPP range,  had sunny dispositions which made it a pleasure for them to serve you.

That’s not what Indramat meant. What they did mean — that’s another story.

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