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How do servo motors work? #Servo #Electronics #Robotics @TheSciJoy

Via SciJoy on Youtube who provides a wonderful description on how DC hobby servos work:

I did a teardown of a servo motor to see what it is made of and how it works. Inside the casing is the DC motor, gears, a potentiometer, and an integrated chip.

I couldn’t find great data sheets for the chips in my particular hobby servos. Instead, I looked up some popular RC servo chips. They all seem to have the same basic components – a potentiometer hooked to a voltage regulator and one shot generator, which converts position of the armature to a PWM pulse. This PWM pulse is compared to the original one sent by the microcontroller. This logic board finds the difference between the pulses, which is called the error.

The magnitude of the error is sent to a pulse stretcher and the direction of the error is send to a flip flop to be stored as a high or low. The magnitude of the error is stretched out by the pulse stretcher. Then both parts of the error are sent to the output driver, which is probably an h-bridge. Most servos are 50HZ, which means this control loop is happening 50 times every second until the error is “zero”. The dead band for the pulse stretcher sets a minimum pulse length that it will stretch. Anything below this limit is considered zero error.

There is also a resistor goes between the IC and the motor. The back EMF from the motor is used to dampen the control loop. I didn’t feel like I could explain this well enough so I left that out of the video.

Check the notes in the video for a bunch of great references to servo resources also.

How do you use servo motors? Let us know in the comments below.


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