How do servo motors work? | Teardown and explanation beyond the pulse.

Описание: 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.

Let us know what you are building and have a wonderful day!


Servo chip part numbers:
KC5188 -- The chip on one of my servos
NE544
Mitsubishi M51660L
AA51880 -- http://www.agamem.com.tw/productPDF/A...


References:
-Build Your Own Quadcopter: Power Up Your Designs with the Parallax Elev-8by Donald Norris (Chapter 7)
-Analog Circuit Design Volume 2 by Jim Williams; Bob Dobkin (Chapter for pulse stretcher)
-DC Servos: Application and Design with MATLAB by Stephen M. Tobin (Appendix)
-Modern Control Engineering by Katsuhiko Ogata (read but too advanced for this video. A servo is actually a control loop where you are minimizing error over time)
-Practical Electronics for Inventors by Simon Monk and Paul Scherz (amazing book!)
-http://www.seattlerobotics.org/encode...
-http://sound.whsites.net/articles/ser...

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