Definition of servo motor
The "servo" in servo motor refers to the control mechanism. In the field of technology, the controlling side is called the master, and the controlled side is called the slave. It is said that both "servo" and "slave" originate from the Latin word "servus" (slave).
In other words, the name "servo motor" means a motor that "works faithfully according to instructions." As long as the rotational position and speed can be controlled, any motor, regardless of its mechanism, is called a "servo motor."
Therefore, stepper motors and coreless motors are sometimes included in servo motors. Here, we will explain the type of servo motor that is equipped with an encoder (rotation detector) and uses a driver to control the speed and position (angle) based on this information.
The difference between servo motors and stepper motors
Like servo motors, stepper motors are equipped with a mechanism that controls the rotation angle by receiving external signals. They can both perform operations such as positioning of equipment, but they have the following differences.
1. Control method
Servo motors detect their rotational position using an encoder (rotation detector) and feed this information back to the controller to control the position. This allows for high-precision stopping; even if stopped during rotation, it can return to its original position if there is a deviation. On the other hand, stepper motors' rotation angle is proportional to the number of pulses, and the driver controls its position by receiving this pulse signal from the controller. Therefore, it doesn't actually need a position detection mechanism and cannot identify positional deviations. Consequently, it may experience step loss (a state where the indicated rotation angle is out of sync with the motor's rotation) due to unexpected load fluctuations or other reasons.
①Servo motor
Program → Controller (pulse signal) → Driver (current conversion) → Motor → Encoder (encoder signal) → Driver (motion information) → Controller
② Stepper motor program → Controller (pulse signal) → Driver (current conversion) → Motor
2. Torque/Speed
Servo motors can generate stable torque in both low-speed and high-speed ranges, thus enabling high-speed operation. Stepper motors, on the other hand, can only generate high torque in the low-speed range, with torque decreasing in the high-speed range, making them unsuitable for high-speed rotation applications.
3. Cost
Servo motors are more expensive than stepper motors because they require expensive rotary encoders and servo control devices (servo drives).
4. Acceleration performance
Stepper motors require 200–400 milliseconds to accelerate from a standstill to their operating speed (typically several hundred revolutions per minute). AC servo systems, on the other hand, have better acceleration performance, reaching their rated speed of 3000 RPM from a standstill in just a few milliseconds, making them suitable for control applications requiring rapid start and stop.
The history of servo motors
Servo motor technology has seen significant development along with the advancement of industrial robots.
The United States began to emphasize factory automation in the 1950s, and started an automation process represented by belt conveyors, automated devices, and industrial robots. Early automated devices and industrial robots used hydraulic and pneumatic positioning actuators, which had problems such as accuracy, stability of use, pipelines, and oil and air leaks.
Subsequently, with the continuous development of DC servo motor technology in the 1950s and 1960s and its increasing practicality, it began to be used in industrial robots to replace the problematic hydraulic and pneumatic mechanisms.
In the 1980s, AC servo motors emerged, and there was a trend towards smaller and lighter robots. Due to their high practicality, AC servo motors are now used in almost all industrial equipment products.
Types of servo motors
Servo motors can be divided into two categories: DC servo motors and AC servo motors.
DC servo motor
Servo motors driven by brushed DC (direct current) motors. DC motors were easier to control than AC motors, and were smaller and cheaper, so they were widely used in the past. However, with the development of AC motor control technology, they are now used less and less.
AC servo motor
A servo motor driven by an AC (alternating current) motor. Its control is more complex than that of a DC motor, but with the development of control technology, it is now the most widely used servo motor.
AC servo motors can be divided into two types according to the type of drive motor: synchronous (SM) and inductive (IM). The difference between the two lies in whether or not a permanent magnet is used.
Synchronous (SM) type motors use permanent magnets. The higher the motor's output power, the more expensive permanent magnets are used, resulting in higher costs. Therefore, they are mostly used in applications with lower output power (less than 10kW). Nowadays, with the advent of high-performance permanent magnets, synchronous type motors have become the mainstream product.
Induction (IM) type does not use permanent magnets and is used in applications with higher output power (above 10kW).
