Electronic – How to understand a servo well enough to build one


I am trying to understand exactly how a servo works, so that I can build one.

I bought and built a small robot arm kit, and it has five geared DC motors to actuate it. I would like to control the robot arm from a computer, but in order to do that, I need to make the actuators smarter, essentially turning them into servos.

I opened up a couple of inexpensive analog RC servos, and the design looks simple. It has a dc motor, a potentiometer on the output shaft, and circuitry to turn the PWM signal to a position for the output shaft.

My problem is that after opening a couple of servos, I got a range of controller chip models ranging from KC2462 to KC8801. These all seem to be made by the same Chinese company. Unfortunately, I can't find the data sheets, and even if I did, I do not understand any Chinese dialect.

My question can be summed up as this:

  • Is there another servo control chip datasheet that I can read?
  • Is there a website with enough information to learn how exactly a typical analog servo does what it does?
  • Is there perhaps a book with the above information?

Please understand that I already understand how I could hack a servo motor together – I can reprogram an ATtiny45 to take some kind of positional input from one pin, an analog input from a potentiometer on the output shaft, and then output direction and speed to the DC motor. Speed accomplished with PWM in this case. Maybe with a huge cap on the supply to ensure the DC motor doesn't cause too big of a voltage drop when it starts moving.

However, I don't know if the above would damage the chip with inductive feedback from the motor, or if there's a different way to do the speed control.

I would like to understand how the problem is solved in a real-life professionally designed and built servo motor.

As for my background, I did some electronics in school, and I've been playing with Arduino and ATtiny45/85 microcontrollers for a few weeks.

Best Answer

Another RC servo IC was made by Signetics - the NE544 (and some similar models, perhaps NE5044) for which quite a bit of information can be found.

This is the old style pulse width control which is easy to generate and still seems to be quite common; I don't know anything about newer "digital" servo interfaces.