How to judge the performance of servo motor
Servo motors are generally divided into stepping motors, DC brushless servo motors, DC brushless servo motors and AC servo motors. With the increasing progress of science and technology, many special servo motors came into being, such as piezoelectric ceramic motors, linear motors and voice coil motors. Here we mainly talk about the selection of servo motors in the general sense. What kind of servo motor to choose depends largely on the physical characteristics of the load, the working characteristics of the load, system requirements and working environment.
Once the system requirements are determined, no matter what form of servo motor is selected, the first thing to consider is how suitable the motor is, mainly considering the physical characteristics of the load, including load torque, inertia, etc. In the servo motor, the size of the motor is usually measured by torque or force, so when selecting the motor, first calculate the load torque or force converted to the motor shaft end. After calculating the torque, it is required to set aside some margin. Generally, the continuous torque of the motor is > = 1.3 times the load torque, which can ensure the reliable operation of the motor. In addition, it is also required to calculate the load inertia converted to the shaft end. Generally, the load inertia: motor rotor inertia < 5:1 is selected to ensure the fast response of the servo system. If the inertia and torque between the motor and the load do not match, you can only sacrifice the speed and increase the reducer between the motor and the load. At this time, you need to weigh. After selecting the motor with what torque, what needs to be done is to understand the working characteristics and working environment of the load. The working characteristics of the load include, for example, whether the load operates at high speed or low speed, how much acceleration needs to be achieved, whether it needs to start and stop frequently, how much frequency needs to be achieved, system operation accuracy, etc. At this time, there is no specific law to follow when selecting servo motor. The key is that the motor you choose must meet the working requirements of your load movement. For example, stepping motor is a good choice when the system accuracy is not high, the movement speed is less than hundreds of revolutions (no more than 500 revolutions) but not too low (more than 1 revolution), and there is no need to start and stop frequently. This is due to the open-loop control of stepper motor, low control accuracy and high speed. The motor torque will drop rapidly, which will bring no load. If the speed is too low, there will be discontinuous crawling phenomenon of rotation, and the response of stepper motor is not fast, so it is not suitable for frequent start-up applications. When the motion speed is less than 3000 revolutions, the control accuracy is relatively high. DC or AC servo motor can be selected. Generally, the low-speed characteristics of AC servo motor are not as good as DC servo motor. If the load works at a low speed, it is recommended to choose DC servo motor. Due to brush commutation, there will be commutation ring fire, which can not be used in vacuum anti riot underwater and other occasions. Moreover, because the ring fire expands the motor shaft and transmits it to the connecting parts, it can not be used in occasions with high system accuracy requirements. At present, the AC servo motor widely used in industrial applications is AC permanent magnet synchronous motor. Because of its constant torque characteristic below the rated speed, it is mostly used in occasions where the load torque is constant or does not change much, such as machine tool feed system. The choice is relative. For the same application, you can use AC or DC. Sometimes it depends on the environment. For example, for some robot projects, AC power supply is relatively difficult to obtain, so you can only use DC servo motor.
In many special applications, it is difficult for the conventional servo motor to complete the task, such as ultra-low speed and smooth operation, and some are even as low as a few revolutions a year. The general servo motor can not meet this requirement, so it can only choose the torque motor to complete the task. For example, the demand for frequent start and stop, rapid response and high acceleration is also difficult to meet the requirements of ordinary servo. Generally, the frequent start and stop frequency of AC servo motor with load will not be higher than 5Hz, while that of linear motor is almost the same. It can achieve high acceleration, some up to 30g, and the start and stop frequency can reach 20Hz. The only rule for selecting the motor is to understand the load characteristics, working environment and motor characteristics. Only in this way can we select the appropriate servo motor.