Servo Motor Operating Principles
Introduction

A "servo" is a generic term used for an automatic control system. It comes from the Latin word "servus" - slave. In practical terms, that means a mechanism that you can set and forget, and which
adjusts itself during continued operation through feedback. Servo control is a closed loop control system for electric motors. The motor used in servo control are usually DC motors
(although AC servo is also possible).
The servo system uses a sensor to sense motor position/speed. Servo control has a feedback circuit which changes the drive power going to motor according the control input signals
and the signal from sensors.

Disk drives, for example, contain a servo system insuring that they spin at a desired constant speed by measuring their current rotation, and speeding up or slowing down as necessary to keep that
speed. Many robotics applications contain servo circuits that use motors to position some mechanical parts to desired location. In a servo positioning system the encoder gives the motors position
to the servo amplifier and it compares this with the desired position to get the error. The amplifier then sends current to the servo motor to make the motor move into the proper position, reducing
the error.The opearating power fed to the motor is usually controlled usign PWM method. Servo control is usable over varietyof compled motion profiles. Those may involve the following: control
of either velocity and/or position; high resolution and accuracy; velocity may be either very slow, or very high;
and the application may demand high torques in a small package size. Because of the additional
components such as feedback device (usually encoder or tachometer), complexity is considered by some to be the weakness of the closed loop approach. Those additional components add
to the initial cost and complexity of the control system. A typical servo unit consists of a small motor, a gearset, a feedback potentiometer, and some control electronics. Servo motors are typically
controlled with analogue control interface, most often using +-10V control signal, but there are also ther options available (current loop, digital numerical control etc. on some devices).
There are many applications where there is possible to use servo or stepper motor. While the operating concept is similar, in that they're both able to position an object ot a given orientation,
the mechanism of the two is entirely different, and has distinct limitations on the accuracy available when using each type. To understand which one is better, here are some details of differences
between those two:

The stepper's resolution is based on the steps (typically 1.8 deg or 3.6 deg per step). In the stepper system, the driver advances one step, and the stepper motor follows. For example
a 1.8 deg. stepper will turn a full circle in 200 steps. No matter how you gear it, a stepper motor still moves in discrete steps. Each step covers a specific range of "swing". In a nutshell, a stepper
(with or without gear-train) is a set of "preset" positions you can move to. Any positon that's not part of the "presets" is unattainable by that motor or motor-and-gear-train combination, and can
only be reached as an approximation
. Stepping motors can be used in simple open-loop control systems; these are generally adequate for systems that operate at low accelerations with static loads, but closed loop control may be
essential for high accelerations, particularly if they involve variable loads. If a stepper in an open-loop control system is overtorqued, all knowledge of rotor position is lost and the system must
be reinitialized; servomotors are not subject to this problem.

In a servo system the encoder gives the motors position to the servo amplifier and it compares this with the desired position to get the error. The amplifier then sends current to the servo motor
to make the motor move into the proper position, reducing the error. The servo's resolution is based on the encoder attached to it, and the servo amplifier's error. A servo is a motor that can
be stopped anywhere
you want it, with no "detents" either needed or present. You can turn it to any position you like (within its range, of course), and assuming it's been properly "dialed in", it's reasonable
to expect that when you say "turn to 4.6 degrees" and punch the "go" button, it's going to turn whatever it controls to point at a reasonable approximation of 4.6 degrees.

The term servo motor is used by electric motor manyfacturers to define a motor that is specifically designed to operate in a closed-loop control environment where a feedback device, usually
monitoring speed, current, position, etc. is used to control the performance of the motor. Servo motors are usually designed to be particularly sensitive to the systems control signal
voltages, especially at or near zero speed.