Servo system is an important link in electromechanical products, which can provide the highest level of dynamic response and torque density. Therefore, the development trend of drag systems is to replace traditional hydraulic, DC, stepper, and AC variable frequency speed drives with AC servo drives, in order to achieve a new level of system performance, including shorter cycles, higher productivity, better reliability, and longer lifespan. In order to achieve better performance of servo motors, it is necessary to have an understanding of some of the usage characteristics of servo motors.

Common problems with servo motors in use.

Problem 1: Noise, instability

When customers use servo motors on some machinery, they often experience excessive noise and unstable operation of the motor driving the load. When this problem occurs, many users' first reaction is that the quality of the servo motor is poor, because sometimes it is replaced with a stepper motor or a variable frequency motor to drag the load, but the noise and instability are actually much smaller. On the surface, it seems that it is indeed the cause of the servo motor, but after carefully analyzing the working principle of the servo motor, we will find that this conclusion is completely wrong.

The AC servo system includes a servo drive, a servo motor, and a feedback sensor (usually the servo motor comes with an optical polarizer). All these components operate in a closed-loop control system: the driver receives parameter information from the outside, then sends a certain amount of current to the motor, converts it into torque to drive the load, and the load acts or accelerates or decelerates according to its own characteristics. The sensor measures the position of the load, allowing the driving device to compare the set information value with the actual position value, Then, by changing the motor current, the actual position value is consistent with the set information value. When a sudden change in load causes a speed change, the encoder will immediately respond to the servo driver after learning about this speed change. The driver then changes the current value provided to the servo motor to meet the load change and return to the set speed. The AC servo system is a fully closed loop system with a very high response, and the time lag response between load fluctuations and positive speed is very fast. At this point, the real limitation of the system's response effect is the transmission time of the mechanical connection device.

Problem 2: Inertia matching

In the selection and debugging of servo systems, inertia issues are often encountered!

Specifically manifested as:

When selecting a servo system, in addition to considering factors such as the torque and rated speed of the motor, we also need to first calculate the inertia of the mechanical system converted to the motor shaft, and then select a motor with appropriate inertia based on the actual action requirements of the machine and the quality requirements of the processed parts;

During debugging (in manual mode), setting the inertia ratio parameter correctly is a prerequisite for fully utilizing the best performance of mechanical and servo systems, which is particularly prominent on systems that require high-speed and high-precision (Delta servo inertia ratio parameter is 1-37, JL/JM).

Impact: The transmission inertia has an impact on the accuracy, stability, and dynamic response of the servo system. With large inertia, the mechanical constant of the system is large, and the response is slow, which can reduce the natural frequency of the system and easily cause resonance, thereby limiting the servo bandwidth and affecting the servo accuracy and response speed. An appropriate increase in inertia is only beneficial for improving low-speed crawling. Therefore, in mechanical design, without affecting the system stiffness, Inertia should be minimized as much as possible.

How to determine: when measuring the dynamic characteristics of a mechanical system, the smaller the inertia, the better the dynamic response of the system; The greater the inertia, the greater the load on the motor and the more difficult it is to control, but the inertia of the mechanical system needs to match the inertia of the motor. Different institutions have different choices and effects on the principle of inertia matching.

For example, when a CNC central machine performs high-speed cutting through a servo motor, when the load inertia increases, it will occur:

When the control command changes, the motor needs to spend a lot of time to meet the speed requirements of the new command; 2. When the machine performs rapid cutting along the two axis arc curve, significant errors will occur. 1. Generally, when the servo motor is under normal conditions, when JL ≤ JM, the above problem will not occur;

2. When JL=3 × JM, the controllability of the motor will slightly decrease, but it will not affect ordinary metal cutting. (High speed curve cutting generally recommends JL ≤ JM);

3. When JL ≥ 3 × JM, the controllability of the motor will significantly decrease, showing outstanding performance in high-speed curve cutting.

Different mechanism actions and processing quality requirements have different requirements for the size relationship between JL and JM. The determination of inertia matching needs to be determined based on the mechanical process characteristics and processing quality requirements.

Problem 3: Selection of servo motor

After selecting the mechanical transmission scheme, it is necessary to select and confirm the model and size of the servo motor.

(1) Selection conditions

In general, selecting a servo motor requires the following conditions

1. The maximum motor speed is greater than the maximum movement speed required by the system.

2. The rotor inertia of the motor matches the load inertia.

3. Continuous load working torque ≤ motor rated torque

4. The maximum output torque of the motor is greater than the maximum torque required by the system (torque during acceleration)

(2) Selection calculation

1. Inertia matching calculation (JL/JM)

2. Calculation of rotational speed (load end speed, motor end speed)

3. Load torque calculation (continuous load working torque, torque during acceleration)