We have come with increasingly creative uses of servodrivers. For example, there are now little vacuum cleaners that can travel around your house unsupervised. These robot vacuums can travel at a constant speed, so they know the floor is clean.
They are used in hard disc drives and your DVD player to maintain constant disc speed. A servodriver mechanism is a motor with a speed sensor attached.
In large ships (aircraft carriers etc.) if the sea makes the ship roll a mechanical or laser gyro sends a signal to the underwater vanes that maintain the horizontal attitude of the ship so that aircraft can land more safely on deck.
It's exactly the same as your brain making you swim in a straight line.
This type of mechanism is met at NC machines.
This kind of description is vast, but i'll make a summary of this type of mechanism:
servo motor: encoder + rotor => coupling + bearing+ball screw+nut => table and workpiece
The servo motor is rotating according to the command from NC. This rotation of the servo motor is transmitted to a ball screw with the help of a coupling.
A nut is transforming the rotation of the ball screw into linear movement. The result is that the table with the workpiece moves linearly.
Briefly, the servo driving mechanism controls the velocity and torque of the table through the servo driving device of each axis based on the velocity commands received from NC.