These specialty motors have electronics, gears, and a motor to turn what is connected to them. Unlike other motors, servos have a limited range of rotation, usually half way around a circle (180 degrees). A servo can be told at what angle to turn to so movements can be controlled by the user.
A servo with an electronic servo controller is great for moving things in a very controlled fashion. Most robot joints along with robot car steering systems use servos. They can also be used to move something in and out a short distance dependent on the type of horn and connection to the horn.
Servos come in different sizes. They are often a "standard size" although when you buy one you should check the dimensions to make sure it is the size you want. Typical names for sizes are standard and micro although other terms like nano, ultra-nano, and giant might be used.
Inside a servo is a continuous DC motor, control electronics, a variable resistor (potentiometer) and gears for the range of motion.
On the left is a nice photo we took of the internal gear-box of a servo!
Some servos come with metal gears, which are more rugged, but they may emit a bit more noise.
The gearbox means a servo is strong, and somewhat slow. It can only move back and forth about 180 degrees. The movement might be more limited on certain servos, perhaps 170 degrees on some, only 90 degrees on another.
There are three wires on a standard servo, two for DC power and one for control. A servo requires some other electronics to tell it what angle to turn to.... like a Crickit!
Servos come with various screw on pieces, called horns, to help you attach the servo to something else, making connections easier than with Continuous DC motors.
One of the benefits of the servo form factor in general are the number of accessories available to mount on the shaft. Nearly all servo shafts have grooves so attachments fit in without spinning. The shaft also has a hole at the end to secure attachments.
The attachment items for servos are almost always called horns. Horns can come in various shapes to allow users to make various types of connections depending on their project. Most servos you buy will come with two or three horns that fit that specific servo but often you can mix or match other horns and shops have alternatives to the ones in the package.
Horns most often have holes in them to help make attachments, often screws or wires.
There are specialty accessories for continuous rotation servos like wheels. For standard/hobby servos, a wheel would only turn 180 degrees, limiting the usefulness of a wheel.
As the cable of a servo has a connector on the end, there are extension cables available which extend the connection if your servo is not close to your controller board.
When looking for standard servo for your project, you will want to keep in mind these questions:
- What range of motion are you looking for? Most standard servos rotate in about a 180 degree arc but some have a smaller range (older or specialty models).
- What type of load are you trying to move? Cardboard and paper have different requirements than metal or other heavy loads. Look at the servo torque (rotational force) rating to see how much mechanical force the servo can handle safely. If your project needs more force, consider a larger servo or one with metal gears for more durability.
- How will you control the servo? Most often a servo is hooked to a control circuit to tell the arm what position it should be at or travel to. This requires more circuitry compared to a continuous DC motor. Will you use a microcontroller or a special purpose robotics board like Crickit to use the servo?
- What are the voltage and current ratings for the servo? Are they compatible with the controller?
Here are some hobby servos in the Adafruit Shop which have been hand-picked for various applications you may be looking for:
For high torque applications:
Two servos in a pan & tilt configuration: