AC servo motor vibration fault analysis

Mechanical aspects

1. Bearings

The clearance of the bearing on the screw bearing seat is too large after the bearing is worn, or the bearing rolling element and cage are seriously worn after the bearing lacks grease, causing excessive load. The clearance is too large after the bearing is worn, which will cause coaxiality error between the motor rotor center and the screw center, causing the mechanical system to vibrate. Severe wear of the bearing rolling element and cage will increase friction and cause "stall". Under the condition that "stall" does not cause "overload alarm", due to excessive load, it will increase the response time of the servo system and cause vibration.

2. The motor rotor is unbalanced

If the dynamic balance of the motor rotor is defective during manufacturing or deteriorates after use, a vibration source similar to a "vibration motor" will be generated.

3. Shaft bending

The bending of the rotating shaft is similar to the imbalance of the rotor. In addition to generating a vibration source, it will also cause coaxiality errors between the center of the motor rotor and the center of the screw, causing vibrations in the mechanical transmission system.

4. Coupling

Manufacturing defects or wear of the coupling after use can cause coaxiality errors between the two parts of the coupling, especially when using cast rigid couplings. Due to their poor manufacturing accuracy, coaxiality errors are more likely to cause vibration.

5. Parallelism of guide rails

Poor parallelism of the guide rail during manufacturing will cause the servo system to be unable to reach the specified position or unable to stay at the specified position. At this time, the servo motor will keep searching for the position, causing the motor to vibrate continuously.

6. Parallelism error between the screw and the guide rail plane

The parallelism error between the lead screw and the guide rail plane. If the lead screw has a parallelism error with the guide rail plane during installation, the motor will also vibrate due to uneven load.

7. Screw bending

After the screw is bent, in addition to the axial thrust, it will also be subjected to changing radial force. The radial force is large when the bend is large, and the radial force is small when the bend is small. Similarly, this radial force that should not exist will also cause vibration in the mechanical transmission system.

Electrical

The main cause of electrical problems in AC servo motors is the parameter adjustment of the servo drive.

1. Load inertia

The setting of load inertia is generally related to the size of the load. Excessively large load inertia parameters 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. When the system does not vibrate, 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. Velocity integral constant

Generally, when the system does not generate vibration, the set value should be as small as possible. The smaller the set value, the faster the integration speed and the stronger the system resistance to deviation, that is, the greater the stiffness. However, if the set value is too small, overshoot may occur easily, causing the motor to vibrate.

4. Position proportional gain

The larger the setting value, the higher the gain, the greater the stiffness, and the smaller the position lag under the same frequency command pulse conditions. A too large value may cause motor vibration.

5. Acceleration feedback gain

When the motor is not rotating, a small offset will be amplified by the proportional gain of the speed loop, and the speed feedback will generate a corresponding torque, causing the motor to shake back and forth.

Based on on-site judgment

Knowing what aspects can cause vibration failures in AC servo motors, it is difficult to further narrow the scope of the failure and then identify the cause of the failure in actual maintenance. This requires a comprehensive judgment based on specific on-site information.

1. The fault occurred after the new equipment was started up and debugged

The faults occurring during this period are the most complicated. They may be due to mechanical manufacturing reasons or incorrect parameter adjustment. They need to be eliminated step by step. The principle of elimination is to eliminate simple reasons first and then 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 lies in the servo driver parameter settings.

2. The failure occurs after the equipment has been in operation for a long time.

This situation can basically rule out the problem of servo drive parameter settings, because if the parameters are set improperly, the problem should have been reflected long ago.

3. The fault occurs just after the machine is turned on

If the AC servo motor vibrates right after it is turned on, it can be determined that a mechanical blockage occurred when the CNC system was automatically searching for the machine origin, causing the motor to be unable to reach the specified position or to vibrate repeatedly after reaching the specified position. In this case, it is generally a mechanical failure.

4. The fault occurs when the machine tool is processing the workpiece

In such a case, we first consider whether the vibration is caused by the increase in load during processing, and check the cause of the increase in load.

5. The fault occurs continuously or irregularly

When the fault occurs continuously, it means that the cause of the motor vibration has always existed. When the fault occurs intermittently and irregularly, it means that the cause of the motor vibration sometimes changes. In this case, if the load does not change greatly, the servo drive parameter setting can basically be ruled out as the cause.

There are many complex reasons for the vibration failure of AC servo motors. From actual operations, it is found that mechanical failure or motor failure caused by mechanical failure accounts for a large proportion. When troubleshooting such failures, it is necessary to master the working principle of the AC servo system and understand which reasons are likely to cause motor vibration failures. At the same time, comprehensive judgment based on the on-site situation can completely solve the vibration failure of the AC servo motor.