How can I understand a servo well enough to build one?

Question

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.

Answer 1

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.

Answer 2

Here is the datasheet: http://www.kcsemitech.com/upLoad/down/month_1403/201403091839011423.pdf You can use google translate to get some info out of it, but some data is in english.

Answer 3

"Is there perhaps a book with the above information?"

I think, there are many books in Control Theory/Control Systems describing servo loops.

Examples: Classical Feedback Control (B.J. Lurie); Modern Control Systems (R. Dorf, R. Bishop).

Don`t overlook the importance to select a suitable controller/compensator for ensuring closed-loop stability with a good step response (without ringing) at the same time.

Comment (edit): The transfer function of a dc motor (Output rotation angle-to-input voltage) can be approximated by an I-T2 function (integrate with two first-order delay elements). Hence, compensation with a PD-T1 controller works good.

Answer 4

OpenServo (Internet Archive link) was an open hardware project that aims to upgrade a servo by replacing the controller board. The chip they use is some variant of the ATtiny chips.

The board designs are still available in Github.

Sparkfun used to sell a custom implementation of these boards.

Answer 5

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.

That's not hacking a servo. That is a servo. Sure, rc servos are a commodity now and there are all-in-one chips to control them, but there is no secret sauce. They're doing exactly what you described.

Add to your description a motor driver of some kind, and a PID loop between the input and output and you have a servo that's exactly the same as a commercial one.

Once you've got that down, your questions are no longer servo specific, they're things like: "how do I tune a PID loop" or "how do I make a compact circuit board".