Video: What Is a Servo Motor? Types of Servo Motors
Figure 1: Small servo motors.
In industrial applications, the most common motor is the servo motor. It is commonly used to drive motion axes to achieve high-precision positioning. For example, servo motors are widely used in movement platforms of material picking and placing, conveyor belts, robot manipulators, lens focusing, etc.
So what is a servo motor? What types of commonly used servo motors are there?
1. What Is a Servo Motor?
In fact, the servo motor is a motor in a closed-loop control system. As long as it is in a closed-loop system, it can be called a servo motor. So "servo" actually has nothing to do with the structure and type of the motor.
The motor in a servo system can be any of the motors we talked about before. For example, linear motors, synchronous motors, asynchronous motors, DC motors, voice coil motors, piezoelectric motors, hysteresis motors, and even stepper motors.
Figure 2: Stepper motor with feedback devices, image from Applied Motion Products.
The focus here is on “closed-loop”. The next article will introduce to you what a closed-loop system (servo system) is and its components. Next, let us continue to understand the types of servo motors.
2. Types of Servo Motors
Figure 3: Types of servo motors.
There are different types of servo motors according to different classification methods of motors.
● DC servo and AC servo motor
● Brush servo motor and brushless servo motor
● Synchronous servo motor and asynchronous servo motor
In fact, the exploration of servo motors originated in 1950, when the United States was committed to upgrading factory automation.
Early automated machinery and industrial robots used hydraulic or pneumatic mechanisms to control actuators, but they had problems in terms of accuracy, reliability, oil and gas leakage in pipelines, etc.
As technology advanced, DC servo motors grew in the 1950s and 1960s and began to be installed in industrial robots to replace troublesome hydraulic and pneumatic mechanisms.
At the same time, AC servo motors emerged in the 1980s. They make robots smaller and lighter, with less torque ripple, and as a result, the majority of servo motors used in modern industrial machinery are AC servo motors.
2.1 DC Servo Motor vs. AC Servo Motor
Depending on whether AC or DC power supply is used, servo motors can be divided into AC servo motors and DC servo motors.
From a performance perspective, the main difference between AC and DC motors is speed control. In a DC motor, the speed is proportional to the supply voltage at a constant load, whereas in an AC motor, the speed is determined by the frequency of the applied voltage and the number of poles.
Figure 4: Schematic diagram of AC servo motor, including rotary encoder.
AC servo motors are divided into synchronous servo motors and asynchronous servo motors (induction servo motors). Synchronous servo motors contain permanent magnets. In order to increase the motor output torque or force, the use amount of permanent magnets increases, so the cost is higher. They are widely used in low-power applications ( generally no more than 10kW).
However, with the advent of high-performance permanent magnets in recent years, synchronous AC servo motors have now become the default option, and indeed some of the most common high-performance industrial servo motors are three-phase synchronous brushless AC motors.
Asynchronous servo motors do not use permanent magnets and are often used in higher output applications, such as 10kW or more.
Although both AC and DC motors are used in servo systems, AC motors can handle higher currents.
At the same time, in a brushless DC motor (BLDC) motor, the stator windings are wound in a trapezoidal shape and produces a trapezoidal back EMF waveform, which usually produces audible noise, and commutation is achieved in six steps, which produces torque pulsation.
Synchronous AC motors, on the other hand, have a sinusoidally distributed winding and use continuous sinusoidal commutation, thus eliminating the torque ripple undergone by BLDC motors.
These make synchronous AC motors the first choice for high-performance industrial servo applications, such as robotics, online manufacturing and other industrial applications that require high repeatability and precision.
Figure 5: DC servo motor working block diagram.
2.2 Brushed Servo Motor vs. Brushless Servo Motor
According to the type of commutator used in the motor, servo motors can be divided into brushed servo motors and brushless servo motors.
Brushed motors are generally cheaper and simpler to operate, while brushless designs are more reliable, more efficient, and quieter.
Although most motors used in servo systems are brushless designs, brushed permanent magnet DC motors are sometimes used in servo motors because of their simplicity and low cost. The most common type of brushed DC motor used in servo applications is the permanent magnet DC (PMDC) motor.
Brushless DC (BLDC) motors are also used in servo systems.
Brushless DC motors replace physical brushes and commutators electronically, usually using Hall effect sensors or encoders.
Figure 6: Structure of brushless DC motor.
AC motors are usually brushless. The term brushless AC (BLAC) motors can be a bit confusing, as they are also known as permanent magnet AC (PMAC) motors or permanent magnet synchronous motors (PMSM).
2.3 Synchronous Servo Motor VS Asynchronous Servo Motor
Finally, depending on whether the stator's rotating magnetic field speed and rotor speed are synchronized, it is divided into synchronous motors and asynchronous motors.
In a synchronous motor, the rotor rotates at the same speed as the stator's rotating magnetic field.
In an asynchronous motor (often called an induction motor), the rotor rotates at a slower speed than the stator's rotating magnetic field.
When induction motors are paired with variable frequency drives, they can achieve speed control and performance, similar to servo motors, but because they typically do not include feedback, they are not true servo devices.
