# Principles and Methods of Servomotor Control: Comparative Analysis and Applications
## 2 Servomotors: Structure, Operating Method, Types, Main Characteristics
### Servomotor Types  
Currently, servomotors are divided into several types based on six different criteria: the type of motor used, the type of current, the type of construction, the function performed, the signal processing method, and the type of gearbox.  
Based on the type of motor used and the type of current, servomotors are classified as synchronous or asynchronous, and using alternating or direct current, respectively. Considering that asynchronous motors are more powerful, this type of servomotor is produced only for alternating current. Synchronous motors have lower power but provide greater accuracy; in conjunction with direct current, they allow for achieving smaller motor dimensions and using this type of servomotor for autonomous mechanisms [31; 32; 33; 34; 35].  
In terms of construction, servomotors are divided into brush motors, coreless motors, and brushless motors. Unlike brush motors, brushless models have a wider range of rotation speeds, allowing them to be used in processes requiring high-speed movement. However, controlling a brushless motor requires the presence of a PLC, regardless of the tasks it performs [33; 36; 37; 38].  
According to the function performed, servomotors are divided into two groups: maintaining a specified rotation angle or rotational speed. Based on the names, the first group of servomotors is used to bring mechanisms to the required position, such as locks, dampers, cranes, etc. The second group of servomotors is used to move objects in the working area and is employed in manipulators, various CNC machines, etc. Depending on the function performed, the main control parameter in the servomotor will differ: the motor's rotation range or moment of inertia for the first or second group of motors, respectively [15; 17; 39; 40; 41; 42].  
In terms of signal processing, servomotors are divided into analog and digital motors. The main difference between these groups is the control principle. Analog motors use microchips, while digital ones use microprocessors. Due to technological advancements, digital servomotors have replaced analog ones due to their increased response speed to the control signal. Consequently, these servomotors have increased positioning accuracy and the ability to maintain a constant torque [43; 44; 45; 46].  
For example, in Figure 2, a servomotor used in a Hirata Cartesian robot (Hirata Corporation, Kumamoto, Japan) is depicted. It is a synchronous alternating current motor with the function of maintaining a constant rotational speed, as it is used to move the robot axis along the working area [47; 48].  
Motors with the function of maintaining a constant rotational speed are also used in laboratory setups of a digital twin of a wind generator [49], as well as in diagnosing damage to bearings [50; 51] depicted in Figure 3a,b. Motor control is carried out using a frequency converter, allowing the selection of an appropriate motor control mode, adjustment of controller settings for different operating conditions, calibration of control to eliminate malfunctions, and more.  
For example, in bearing fault diagnosis setup, discrete motor control is used to maintain a constant rotational speed. The use of a potentiometer to set an analog speed signal is not appropriate because there is a high probability of additional disturbances that would affect the rotational speed of the output shaft. For the digital twin setup of a wind generator, modeling methods are used, since, to maintain a constant rotational speed, the input speed signal is converted from a database stored in the cloud.