An AC servo system includes a servo driver, a servo motor, and a feedback sensor. The following analysis of AC servo motor vibration faults focuses on both mechanical and electrical aspects.
Mechanical aspects
1. Bearings
Excessive wear and clearance in the lead screw bearing housing, or insufficient lubrication leading to severe wear of the bearing rolling elements and cage, can cause overloading. Excessive clearance due to bearing wear can cause coaxiality errors between the motor rotor center and the lead screw center, resulting in vibration in the mechanical system. Severe wear of the bearing rolling elements and cage can increase friction, leading to "stalling." Even if "stalling" doesn't trigger an overload alarm, the excessive load can increase the servo system's response time and cause vibration.
2. Motor rotor imbalance
If the dynamic balance of the motor rotor is defective during manufacturing or deteriorates after use, it will produce a vibration source similar to a "vibrating motor".
3. Bent shaft
A bent shaft is similar to a rotor imbalance. In addition to generating a vibration source, it also causes a coaxiality error between the center of the motor rotor and the center of the lead screw, resulting in vibration in the mechanical transmission system.
4. Coupling
Manufacturing defects or wear after use can cause coaxiality errors between the two parts of the coupling. This is especially true for cast rigid couplings, which are more prone to coaxiality errors and vibration due to their poor manufacturing precision.
5. Parallelism of the guide rails
Poor parallelism of the guide rail during manufacturing can cause the servo system to fail to reach or stay at the designated position. In this case, the servo motor will continuously search for the position, causing the motor to vibrate continuously.
6. Parallelism error between the lead screw and the guide rail plane
Parallelism error between the lead screw and the guide rail plane: If the lead screw has a parallelism error with the plane where the guide rail is located during the installation process, it will also cause the motor to vibrate due to uneven load.
7. Lead screw bending
When a lead screw bends, it is subjected to a changing radial force in addition to the axial thrust. The radial force is large when the bend is large and small when the bend is small. Similarly, this radial force, which should not exist, will also cause the mechanical transmission system to vibrate.
Electrical aspects
The main electrical issues with AC servo motors are related to the parameter adjustments of the servo driver.
1. Load inertia
The load inertia setting is generally related to the size of the load. An excessively large load inertia parameter will cause the system to vibrate. A typical AC servo motor can automatically measure the load inertia of the system.
2. Speed proportional gain
The higher 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. If the system does not produce vibration, the setting value should be as large as possible. However, the higher the gain, the smaller the deviation, and the easier it is to produce vibration.
3. Velocity integral constant
Generally, the set value should be as small as possible when the system does not vibrate. The smaller the set value, the faster the integral speed and the stronger the system's resistance to deviation, i.e., the greater the stiffness. However, if the set value is too small, it is easy to cause overshoot, which will cause the motor to vibrate.
4. Position Proportional Gain
The higher the setting value, the higher the gain and the greater the stiffness. Under the same frequency command pulse conditions, the position lag is smaller. However, if the value is too large, it may cause motor vibration.
5. Acceleration feedback gain
When the motor is not rotating, even a 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.
Based on the 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 occurred after the new equipment was started and debugged.
The faults occurring during this period are the most complex. They may be due to mechanical manufacturing issues or incorrect parameter adjustments. It is necessary to troubleshoot step by step. The principle of troubleshooting is to eliminate the simpler causes first, and then eliminate the more complex ones. If the CNC system is equipped with two or more identical drivers and AC servo motors, and one of the motors is vibrating, the simplest "swap method" can be used to swap the servo drivers of the two AC servo motors. This method can quickly determine whether the problem lies in the servo driver parameter settings.
2. The malfunction occurred after the equipment had been in operation for a long time.
This situation basically rules out servo drive parameter settings as the problem, because if the parameters were set incorrectly, the issue would have been reflected long ago.
3. The fault occurred immediately after powering on.
If the AC servo motor vibrates immediately upon powering on, it indicates a mechanical jamming during the CNC system's automatic search for the machine tool's origin, preventing the motor from reaching the designated position or causing it to repeatedly move after reaching the designated position. This is generally a mechanical fault.
4. The malfunction occurred while the machine tool was processing a workpiece.
In such cases, the first consideration should be vibration caused by increased load during processing; the cause should be investigated by focusing on the increased load.
5. The fault occurs continuously and regularly or intermittently and irregularly.
When a fault occurs continuously, it indicates that the cause of the motor vibration has always existed. When it occurs intermittently and irregularly, it indicates that the cause of the motor vibration may change at times. In this case, if the load does not change significantly, the cause of the motor vibration can be ruled out as a problem with the servo drive parameter settings.
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.
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