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Analysis of Vibration Faults in AC Servo Motors

Home > Analysis of Vibration Faults in AC Servo Motors

Analysis of Vibration Faults in AC Servo Motors

2023-12-27 11:00:19

An AC servo system consists of a servo driver, servo motor, and a feedback sensor. The analysis of vibration faults in AC servo motors mainly focuses on mechanical and electrical aspects.

Mechanical Aspects

  1. Bearings

Excessive clearance due to bearing wear on the bearing seat of the screw, or severe wear of the bearing rolling elements and cage due to lack of lubricating grease, can cause overload. Excessive clearance in the bearings can result in misalignment between the motor rotor center and the screw center, leading to mechanical system jitter. Severe wear of the rolling elements and cage can increase friction and cause "stalling". Under heavy loads, this can increase the response time of the servo system and generate vibrations.

  1. Rotor Imbalance

Defects during the manufacturing of the rotor's dynamic balance or degradation over time can create vibrations similar to a "vibrating motor".

  1. Shaft Bending

Shaft bending is similar to rotor imbalance. Besides generating vibration, it can also cause misalignment between the motor rotor center and the screw center, leading to mechanical transmission system jitter.

  1. Couplings

Manufacturing defects or wear and tear of the couplings can result in misalignment between the two parts of the coupling, especially with rigid couplings made of cast iron. Due to their poor manufacturing precision, they are more prone to misalignment, leading to vibrations.

  1. Parallelism of Guide Rails

Poor parallelism of the guide rails during manufacturing can prevent the servo system from reaching or staying at the specified position. In such cases, the servo motor will continuously search for the position, causing continuous vibrations.

  1. Parallelism Error between Screw and Guide Rail Plane

Parallelism error between the screw and the plane where the guide rail is located can cause vibrations in the motor due to uneven loads.

  1. Screw Bending

When the screw bends, it not only experiences axial thrust but also varying radial forces. Larger bending leads to greater radial forces, which, in turn, generate vibrations in the mechanical transmission system.

Electrical Aspects

The main electrical cause of vibration faults in AC servo motors lies in the adjustment of servo driver parameters.

  1. Load Inertia

The setting of load inertia is generally related to the size of the load. Setting a large load inertia parameter can cause system vibrations. Most AC servo motors can automatically measure the system's load inertia.

  1. Velocity Proportional Gain

The higher the set value, the higher the gain and system stiffness. The parameter value is determined based on the specific servo driver model and load conditions. In general, the larger the load inertia, the larger the set value. However, a larger gain leads to smaller deviations and easier generation of vibrations.

  1. Velocity Integral Constant

In situations where vibrations do not occur, it is advisable to set a smaller value. The smaller the set value, the faster the integral speed, resulting in a stronger system resistance to deviations, i.e., greater stiffness. However, setting the value too small can lead to overshoot and motor vibrations.

  1. Position Proportional Gain

The higher the set value, the higher the gain and stiffness. Under the same frequency command pulse conditions, a larger set value leads to smaller position lag. However, setting the value too large can cause motor vibrations.

  1. Acceleration Feedback Gain

When the motor is not rotating, even a small deviation is amplified by the proportional gain of the velocity loop, generating corresponding torque feedback and causing motor oscillation.

Based on On-Site Judgement

Knowing the factors that can cause vibration faults in AC servo motors, narrowing down the range of faults and identifying the root cause during actual maintenance is a challenging task. It requires comprehensive judgment based on specific on-site information.

  1. Faults Occurring After New Equipment Commissioning

Faults occurring during this period are the most complex, possibly due to mechanical manufacturing issues or incorrect parameter adjustments. The principle of troubleshooting is to first eliminate simple causes before complex ones. If the CNC system is equipped with two or more identical drives and AC servo motors, and one motor experiences vibrations, the simplest "swap method" can be used to interchange the servo drivers of the two motors. This method can quickly determine whether the problem lies in the servo driver parameter settings.

  1. Faults Occurring After Long-Term Equipment Operation

In this case, servo driver parameter settings can be largely ruled out because if there were incorrect settings, the problem would have manifested much earlier.

  1. Faults Occurring Immediately After Power-On

If vibrations occur immediately after power-on, it can be determined that a mechanical obstruction has occurred during the CNC system's automatic search for the machine tool origin. This is usually a mechanical fault.

  1. Faults Occurring During Workpiece Machining

In such cases, the first consideration is increased vibration due to increased load during machining. The causes should be investigated based on the increased load.

  1. Regular or Irregular Intermittent Faults

When faults occur continuously, it indicates that the cause of motor vibration is consistently present. On the other hand, when faults occur intermittently and without a pattern, it suggests that the cause of motor vibration may vary. In such cases, if there are no significant changes in the load, the servo driver parameter settings can be largely ruled out as the cause.

The causes of vibration faults in AC servo motors are complex and multifaceted. Based on practical experience, it has been found that mechanical faults or faults caused by mechanical issues account for a significant proportion of motor faults. When troubleshooting such faults, it is important to understand the working principles of AC servo systems and identify the factors that are prone to causing motor vibration faults. Comprehensive judgment based on the on-site situation is necessary to fully resolve vibration faults in AC servo motors.

Conclusion

In conclusion, analyzing and addressing vibration faults in AC servo motors requires a systematic approach. Both mechanical and electrical factors must be evaluated through methods like parameter testing, component inspection, and load simulation. While some faults like improper settings can be identified through simple tests, complex issues involving system integration may require meticulous diagnosis. The level of difficulty also varies depending on factors like the fault occurrence timing. Overall, having a strong understanding of servo operation principles and applying logical troubleshooting processes according to the specific condition are key to pinpointing the root cause efficiently. With precision motion control becoming increasingly important across many industries, continued advancements in vibration fault analysis will be crucial to maximize productivity and product quality in automated manufacturing.

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