Laser processing utilizes the energy of light, which is focused by a lens to achieve a very high energy density at the focal point, and processes materials through a photothermal effect. Laser processing requires no tools, has a high processing speed, minimal surface deformation, and can process a wide variety of materials. It uses a laser beam to perform various processing operations on materials, such as drilling, cutting, scribing, welding, and heat treatment.
As customers' requirements for laser equipment processing increase, the demands on servo drives are also rising. However, some problems have emerged in practical use, particularly the deformation of the processed contours, which is a major concern.
In fact, whether it's a sheet metal cutting machine, a pipe cutting machine, or a complex 3D cutting device, the key to ensuring the equipment processes according to a predetermined shape lies in the dynamic responsiveness of each axis involved in the process and their coordination. If, during processing, the overall response of each axis is too slow, or if there are discrepancies where one axis has a smaller deviation than another, then the processed contour will be deformed. There are many reasons for this inconsistency, including mechanical factors, external forces, servo responsiveness, control system issues, and the combined effect of multiple factors . Therefore, the key to solving this problem is ensuring that each axis has good dynamic responsiveness and good coordination, enabling it to perform processing actions more strictly according to the predetermined target.
1. Mechanical factors
Mechanical problems are relatively common and mainly manifest in aspects such as design, transmission method, installation, materials, and mechanical wear.
2. Mechanical resonance
The biggest impact of mechanical resonance on servo motors is the inability to further improve their responsiveness, resulting in the overall equipment operating at a relatively low response level. This problem is common in synchronous belt drives, and sometimes also occurs with long-distance ball screws. The main reasons are the low rigidity and low resonant frequency of synchronous belts, the large inertia of long-distance ball screws, and the increased likelihood of deformation, especially when the motor capacity is too small. The quality of assembly processes and materials used during installation also affects mechanical resonance. If this problem occurs, in addition to mechanical optimization, it can be compensated for through servo adjustments. The main idea is to utilize the servo's resonance suppression function.
3. Mechanical vibration
Mechanical vibration is essentially a problem of the machine's inherent frequency, and it commonly occurs in cantilever structures that are fixed at one end, especially during acceleration and deceleration. Low-frequency vibrations in the machined parts will appear as large, wavy patterns, while higher-frequency vibrations will appear as sawtooth patterns. The basic approach to this situation is to add multiple filters to stagger the torque command frequency.
4. Factors such as mechanical internal stress and external forces
Due to differences in mechanical materials and installation, the internal mechanical stress and static friction of each drive shaft on the equipment may vary. If the internal stress or static friction of one of the two axes involved in trajectory interpolation control is greater, it will consume servo torque to some extent, causing that axis to accelerate more slowly, thus leading to deformation of the machining contour. We can usually observe the internal stress problem of the drive shaft through the waveform curve generated by the servo driver feedback.
The situation is similar when external forces act on the shafts. In general sheet metal cutting machines, the axes are not in contact with the workpiece, so the external forces they experience are limited. However, in some pipe cutting machines, the pipe feeding axis participates in the interpolation during cutting, while the other axis is generally not in contact. In this case, the pipe, due to the influence of the clamp, will exert a reverse force on the pipe feeding axis. Thus, the two axes involved in interpolation control experience inconsistent force, which will definitely affect the cutting effect.
5. Servo Factors
Even under normal mechanical conditions, significant deviations in servo parameters can lead to unsatisfactory cutting contours. Therefore, the principles to be followed during servo debugging are:
Servo responsiveness should be kept at a high level within permissible limits.
Each axis involved in interpolation control should have a relatively similar dynamic response, and the actual pulse deviation level can be observed through waveform curves.
The inertia ratio should be set according to a more realistic value. If the inertia automatically calculated by the driver accurately reflects the actual load, this value should not be changed arbitrarily.
When adjusting responsiveness, automatic gain should be used as much as possible. Even if manual adjustment is required, it should be done on the basis of automatic adjustment, which will simplify the adjustment process.
6. CNC system factors
In some cases, the servo adjustment may not be effective, requiring intervention to adjust the control system. Laser cutting machines typically maintain a relatively constant linear speed, the same speed on both straight lines and curves. This is not a major issue for linear motion, but it can lead to contour deformation due to excessive acceleration when cutting curves, especially small arcs. Therefore, speed and acceleration must be limited when cutting small circles or similar shapes. Furthermore, adjusting parameters such as acceleration and interpolation sampling period is more effective in suppressing jitter than simply adjusting the servo when encountering jitter issues.
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