Think of a continuous servo as a hybrid of a continuous DC motor and a standard servo.
A continuous servo can turn around a full 360 degrees like a motor. A continuous servo also has three wires like a standard servo, 2 for power and one for control. The speed of a continuous servo is controllable. Due to the gears, continuous servos generally are slower than a continuous DC motor or a geared motor.
You can get servo horns in the shape of wheels to build things that roll on the ground.
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.
Servos most often have mounting points so the body can be screwed down for stability. The rectangular shape also helps in mounting compared to round 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. The wheels have servo style mounts and can be secured with screws.
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 continuous rotation servos for your project, you will want to keep in mind these questions:
- What type of load are you trying to move? 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 servo 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 continuous rotation servos in the Adafruit Shop which may work for various applications you are planning: