Servo motors are the ultimate controllable motor on the market. They are a closed-loop device, meaning that there is a feedback mechanism that allows the controller to have a constant update on speed and position of the motor. This is in contrast with stepper motors which do not have a feedback system and rely on a calibration strategy when first powered up to maintain an accurate location of the rotor. Servos also differ from other motors in that they have a very complex controller and sensing system, which allows them to perform these functions. They are usually far more expensive than traditional motors but are critical when there is an industrial need for their capabilities.The motor takes an input signal from an outside source to decide where to place the drive shaft (if a stationary location is desired) or what speed to run the motor. They usually do have the capability of variable speed operation, although there are some that have only a single operational velocity. As the motor is powered up, it requires a command signal that tells it what state the motor should be in, there is no calibration required. As soon as it's fully activated the desired state will be achieved. Read more Read less