I. Overview of AC Servo Systems
An AC servo system includes a servo driver, a servo motor, and a feedback sensor (typically, the servo motor has a built-in photoelectric encoder). All these components operate within a closed-loop control system. The driver receives parameter information from the outside and then supplies a certain current to the motor. The motor converts this current into torque to drive the load. The load moves or accelerates/decelerates according to its own characteristics. The sensor measures the load's position, allowing the drive to compare the set information value with the actual position value. Then, it adjusts the motor current to maintain consistency between the actual position value and the set information value. When a sudden change in load causes a speed change, the encoder detects this speed change and immediately responds to the servo driver. The driver then adjusts the current supplied to the servo motor to accommodate the load change and returns to the set speed.
An AC servo system is a highly responsive, fully closed-loop system with extremely fast time lag response between load fluctuations and speed correction.
II. Vibration Fault Analysis of AC Servo Motors
The following analysis of vibration faults in AC servo motors mainly focuses on mechanical and electrical aspects.
1. Mechanical aspects
(1) Excessive clearance due to wear of the bearings at both ends of the motor and on the lead screw bearing housing, or severe wear of the bearing rolling elements and cage due to lack of grease, resulting in excessive load. Excessive clearance after bearing wear will cause coaxiality error between the center of the motor rotor and the center of the lead screw, causing vibration in the mechanical system. Severe wear of the bearing rolling elements and cage will increase friction and cause "stalling". "Stalling" will increase the response time of the servo system and cause vibration due to excessive load, even if it does not cause "overload alarm".
(2) The motor rotor is unbalanced. If the dynamic balance of the motor rotor is defective during manufacturing or deteriorates after use, it will produce a vibration source like a "vibrating motor".
(3) The shaft is bent. The situation of the shaft is similar to the rotor imbalance. In addition to generating a vibration source, it will also generate a coaxiality error between the center of the motor rotor and the center of the lead screw, causing the mechanical transmission system to vibrate.
(4) Manufacturing defects or wear after use can cause coaxiality errors between the two parts of the coupling. In particular, cast rigid couplings are more prone to coaxiality errors due to their poor manufacturing precision, which can lead to vibration.
(5) Poor parallelism of the guide rail during manufacturing will cause the servo system to be unable to reach the specified position or stay at the specified position. At this time, the servo motor will keep trying to find the position and fluctuate between system feedback, causing the motor to vibrate continuously.
(6) Parallelism error between the lead screw and the guide rail plane. If there is a parallelism error between the lead screw and the plane where the guide rail is located during the installation process, the motor will vibrate due to uneven load.
(7) When the lead screw is bent, in addition to the axial thrust, the lead screw will also be subjected to a changing radial force. When the bending is large, the radial force is large, and when the bending is small, the radial force is small. Similarly, this radial force that should not exist will also cause the mechanical transmission system to vibrate.
2. Electrical aspects
The main electrical issues with AC servo motors are related to the parameter adjustments of the servo driver.
(1) Load inertia. The setting of load inertia is generally related to the size of the load. Excessive load inertia parameter will cause the system to vibrate. General AC servo motors can automatically measure the load inertia of the system.
(2) Speed proportional gain: The larger the setting value, the higher the gain and the greater the system stiffness. The parameter value is determined according to the specific servo drive model and load conditions. Generally, the larger the load inertia, the larger the setting value. Under the condition that the system does not generate vibration, the setting value should be as large as possible. However, the larger the gain, the smaller the deviation, and the easier it is to generate vibration.
(3) Speed integral constant. Generally, the larger the load inertia, the larger the set value. If the system does not generate vibration, the set value should be as small as possible. However, reducing the integral gain will make the machine tool response slow and the rigidity worse.
(4) Position proportional gain: the larger the value, the higher the gain and the greater the stiffness. Under the same frequency command pulse conditions, the position lag is smaller. If the value is too large, it may cause motor vibration.
(5) Acceleration feedback gain: When the motor is not rotating, a very small offset will be amplified by the proportional gain of the speed loop, and the speed feedback will generate corresponding torque, causing the motor to vibrate back and forth.
III. Troubleshooting AC servo motor vibration faults based on on-site assessment.
Knowing what factors can cause vibration faults in AC servo motors, the challenge in actual repair is to further narrow down the scope of the fault and pinpoint the cause, which requires comprehensive judgment based on specific on-site information.
(1) The fault occurs after the new equipment is started and debugged. The faults that occur during this period are the most complex. They may be due to mechanical manufacturing reasons or incorrect parameter adjustment. It is necessary to eliminate them step by step. The principle of elimination is to eliminate the simple reasons first and then the complex ones. If the CNC system is equipped with two or more identical drivers and AC servo motors, and one of the motors vibrates, the simplest "swap method" can be used to swap the servo drivers of the two AC servo motors. This method can be used to quickly determine whether the problem is in the servo driver parameter settings.
(2) The fault occurs after the equipment has been in operation for a long time. In this case, the problem of servo drive parameter setting can be basically ruled out, because if the parameter setting is incorrect, the problem should have been reflected long ago.
(3) The fault occurs immediately after powering on. If the AC servo motor vibrates immediately after powering on, it can be determined that mechanical jamming occurred when the CNC system automatically seeks the origin of the machine tool, causing the motor to be unable to reach the specified position or to repeatedly move after reaching the specified position. In this case, it is generally a mechanical fault.
(4) The fault occurs when the machine tool is processing the workpiece. In this case, the first consideration is that the vibration is caused by the increased load during processing. Check the cause around the increase in load.
(5) The fault occurs continuously or intermittently. When the fault occurs continuously, it means that the cause of the motor vibration has always existed. When it occurs intermittently, it means that the cause of the motor vibration sometimes changes. In this case, if the load does not change much, the cause of the servo drive parameter setting can be basically ruled out.
IV. Conclusion
Vibration faults in AC servo motors are caused by a variety of complex factors. Practical experience has shown that mechanical faults or motor failures caused by mechanical faults account for a large proportion of the causes. When troubleshooting these faults, it is necessary to understand the working principle of the AC servo system, know which factors are likely to cause motor vibration faults, and make a comprehensive judgment based on the on-site situation in order to completely solve the vibration faults of AC servo motors.
