Using Servos

So, you think you'd like to use a RC servo in your robotics? Here, I'm going to discuss how servos work, how they "talk", and how you can use them in robotics. This work is based off of my own experience in servos, as well as countless conversations with others and discussions on the Usenet newsgroups and web-based forums.

(Note: From here on, when I refer to "servos", I am referring to RC hobby servo motors. While there are other types of servo motors out there, I don't wish to mislead the reader to believe that what is below is correct about all servo motors out there. Your mileage may vary.)

Servos: What are they?

Servo A RC servo is a small motor with a integrated gearbox and control electronics. They can range in size from under 1 sq. in. to 3 in. by 3 in., and some are even larger. They were originally designed for RC airplane and RC car use, and as such, are quite easily obtainable. You can buy a servo starting at around $15, although smaller and/or more powerful ones are more expensive.

Servos: What are they good for?

Servos are designed to move a control arm (called a control horn in some areas) to a specific angle, and keep it there against other forces. These are used to control the control surfaces and throttle of RC aircraft, and the throttle and steering of RC automobiles.

Servos can be considered a "closed-loop" system. That is, you tell the servo to go to a specific angle and it will move to that location and stop. If a force such as wind on the aerilons of a RC airplane pushes against it, the servo will respond by increasing the force to keep the control arm at the desired angle.

Servos: How do I control them?

A standard servo has a three-wire interface. Two of these wires supply power to the electronics and motor (positive and negative power leads), while a third wire is the control line. The following table tells you which color wire is which type:

Manufacturer Positive Negative Signal
Hitec Red Black Yellow
JR Red Brown Orange

Also note that the order of the wires does change from one manufacturer to another. Check and make sure you have the wiring correct, or else you may end up damaging the servo and the rest of your electronics.

The servo can be instructed to go to a specific angular position by using a type of electronic communication called Pulse Width Modulation, or PWM.

Servos: How do I use them in robotics?