Whereas common electric motors run continuously and serve as a source of motive power in machinery, servo motors incorporate a control mechanism and are able to rotate by a specified angle and stop at a precise position. These capabilities have led to their use in equipment control in fields as diverse as industry and hobbies, with applications ranging from the positioning of joints in industrial robots to the control of rudder angle on radio-controlled aircraft.
Definition of servo motor
The term "servo" refers to the control mechanism. In terms of technology, the mechanism that performs control is called the "master" and the mechanism being controlled is called the "slave." Both terms, "servo" and "slave" derive from "servus", the Latin word for slave.
In other words, servo motors get their name from the fact that they can be relied upon to operate "exactly as commanded". Any electric motor capable of controlling parameters like position and speed is called a servo motor, regardless of how this control is achieved.
Accordingly, the use of this term sometimes encompasses stepper motors and coreless motors. But, for our purposes here, a servo motor will be defined as a motor that has an encoder (rotational motion detector) together with a driver that uses information from this encoder to control speed and position (rotational angle).
Differences between servo motors and stepper motors
Like servo motors, stepper motors have the ability to control their angle of rotation based on an external input, and therefore are also able to be used for positioning machinery and similar applications. However, these two types of motors differ in the following ways.
Servo motors have an encoder (rotational motion detector) that can determine their rotational position and use this information to perform feedback control of motor position. This provides precise stopping accuracy and the ability to move back to its original position should motor position deviation occur when stopped. For stepper motors, in contrast, the rotational angle of the motor is proportional to the number of input pulses. Accordingly, the driver controls position based on the number of input pulses it receives from the controller. Although this avoids the need for a position sensor, it also means that there is no way to detect position deviations. Unexpected changes in load, for example, can result in the motor getting out of synchronization (meaning the rotational angle of the motor differs from the angle specified by its input).
Torque and speed
Servo motors can operate at high speeds and also deliver reliable torque over a wide speed range, from low to high. While stepper motors can deliver high torque at low speeds in particular, their torque diminishes with speed and therefore are not well suited to high-speed operation.
Because servo motors require a rotary encoder and servo controller (driver), they cost more than stepper motors.