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Analysis of laser focus position detection and control technology for laser cutting

Analysis of laser focus position detection and control technology for laser cutting

The focal position of the laser focus cannot be measured directly, but it can be detected by indirect methods. For a laser cutting processing system, the focal position is determined by the optical focus of the focusing lens, so under certain conditions of the focusing lens, its position is unchanged (without considering the thermal effect of the focusing lens), so it can be detected by the focusing lens and the lens. The relative position between the processed objects is used to indirectly detect the positional relationship between the focus and the processed object.

The relative position between the laser focus and the processed object can be detected by an inductance displacement sensor and a capacitance sensor, and each has its advantages and disadvantages in use. Inductive sensors have a low response frequency, which is not suitable for high-speed processing and occasions that require non-contact detection such as! Affected by the interference of plasma cloud and slag spray generated during the laser cutting process.
This article will systematically discuss the generation path of laser focus position error in laser cutting and the composition of the control system that automatically eliminates the error. On this basis, the composition of the two sensor detection systems as well as the shortcomings in actual use and the methods to overcome are discussed.

1. The occurrence of focus position error during laser cutting

In the laser cutting process, there are many factors that cause the relative position between the focus and the surface of the processed object to change. The surface of the processed workpiece is uneven, the clamping method of the workpiece, the geometric error of the machine tool, the deformation of the machine tool under the load, and the workpiece Thermal deformation during processing will cause the laser focus position to deviate from the ideal given position (programmed position). Some errors (such as the geometric errors of machine tools) are regular and can be compensated by quantitative compensation methods, but some errors are random errors and can only be eliminated through online detection and control. These errors are:

1.1 Geometry error of the workpiece.

The object of laser cutting is plate or cover-type parts. Due to the influence of various reasons, the surface of the processed object has undulations, and the influence of the thermal effect during the cutting process will also produce surface deformation of the thin plate parts. In the case of three-dimensional laser processing, the cover will also have surface unevenness during the pressing process, all of which will cause random changes in the position of the laser focus and the surface of the processed object and the ideal position.

1.2 Errors caused by the workpiece clamping device

The workpiece processed by laser cutting is placed on a needle-like worktable. Due to processing errors, long-term wear and tear from the workpiece, and laser burns, the needle bed will appear uneven, and this unevenness will also produce a difference between the thin steel plate and the laser focus. Random error in the position between.

1.3 Errors caused by programming

In the process of 1D laser cutting, the processing trajectory on the complex surface is fitted by straight lines, arcs, etc. There are certain errors between these fitted curves and the actual curves, and these errors make the actual focus and the processed object The relative position of the surface and the ideal programming position produce a certain error. And some teaching programming systems will also introduce some deviations.

2. The composition of the focus position online detection and control system in the laser cutting process

Analysis of laser focus position detection and control technology for laser cutting

According to the relationship between the focus position detection control system and the system, the focus position detection control system is divided into two types: independent and integrated. The independent focus position detection and control system uses a separate coordinate axis to compensate and control the focus position error. The mechanical structure is complicated and the cost is high, but it can be used in conjunction with various numerical control systems and laser cutting machine tools. The integrated type uses a feed axis (for plane processing) or a composite of multiple feed axes (for one-dimensional cutting processing) of the laser cutting machine tool to compensate for the focus position error. This method has the advantages of simple structure, low cost, easy adjustment, etc., but it requires the same design as the CNC system, and requires higher openness of the CNC system.

2.1 Capacitance sensor detection circuit

As shown in Figure 2, the capacitance sensor detection circuit consists of a tuning oscillator, a signal amplifier, a crystal frequency stabilization oscillator, a synchronization circuit, a mixer circuit, a signal processing circuit, etc., which convert the capacitance signal into a pulse signal of the corresponding frequency. The signal is frequency sampled and processed to obtain the corresponding capacitance. The capacitance here is the capacitance formed by the two plates between the cutting nozzle and the cutting object. Obviously, its capacitance is not only related to the area of ​​the two pole plates, but also related to the medium between the pole plates and the spacing between the pole plates. And this interval is related to the distance between the laser focusing lens and the workpiece, that is, the distance between the laser focus and the workpiece, so the capacitance is approximately related to the distance between the focus position and the cutting object. This is how the capacitive sensor detects the focal position.

Analysis of laser focus position detection and control technology for laser cutting

It can be seen from the figure that the relationship between frequency and focus position error is a non-linear relationship and must be linearized by a computer. At the same time, since the capacitance is also related to the medium between the plates, the test results are easily affected by the plasma cloud and slag spray generated during the processing and must be overcome.

2.2 Inductive sensor detection circuit

As shown in Figure 3, due to the use of the latest large-scale integrated circuit, the detection circuit of the inductive sensor is relatively simple, and the integrated circuit adopts a new modulation and demodulation method and algorithm, which reduces the previous detection of heterodyne FM detection circuit methods. Due to the influence of the phase angle, frequency and amplitude drift of the sensor’s excitation signal on the detection result, the detection accuracy and stability are greatly improved.

The sensor signal is processed to obtain a voltage signal proportional to the displacement of the sensor probe, which is converted into a corresponding frequency signal by a conversion circuit, and the position error signal of the focus is obtained by computer processing.

Analysis of laser focus position detection and control technology for laser cutting

Due to the inherent characteristics of the inductive sensor, the frequency of the measured signal is limited (a few hundred), which is not suitable for high-speed processing occasions. At the same time, because it is a contact detection method, it can only be used for plane processing occasions.

3. The influence of plasma cloud on the focus position detection system during the cutting process

At the moment when the workpiece has not been cut through, the laser interacts with the metal to generate cloud-like plasma between the nozzle and the processing object, which changes the medium between the capacitor plates, thereby causing interference to the capacitive sensor. In the normal cutting process, the auxiliary gas blows the plasma from the slit, which has little effect on the capacitive sensor. However, if the processing speed is too fast and at the beginning of cutting, because the workpiece is not completely cut through, a plasma cloud will be generated around the laser irradiation point, which will interfere with the capacitive sensor, and even make the sensor unable to work normally in severe cases, which will seriously affect the processing quality. Figure 4 is a schematic diagram of plasma interference.

According to the principle of electromagnetics, the capacitance between two adjacent plates is C=εS/h, where ε—the dielectric constant between the plates) is generally (1), S—the plates are relative Effective area, h—the distance between the two plates. If there is no interference from the plasma, then the capacitance measured according to formula (1) is inversely proportional to the distance between the plates (nozzle and processing object), which is determined by the capacitance It is convenient to find the distance between the two plates, and then find the relative position between the focus and the processed object. However, when there is plasma or slag spray between the nozzle and the processed object, the dielectric between the capacitor plates is not air, and its dielectric constant changes. According to the capacitance principle formula, the capacitance between the two plates at this time is:

C“”=ε S1 /[(h-h1)+h1ε/ε1 ]+εS2/h (2), where ε1—the dielectric constant of the plasma, h1—the thickness of the plasma cloud , S1 + S2 = S is the area of ​​the area with plasma cloud or slag spray and the area without plasma cloud or slag spray. If the plasma cloud is evenly distributed within a certain height range between the nozzle and the processed object, the capacitance The distance between the two plates measured by the sensor is:

h“”=(h-h1)+ h1ε/ε1 (3)

Theoretical value of detected error:

Δh = h“”-h

= h1 (ε/ε1 -1) (4)

It can be seen from equation (4) that the size of the error is determined by the thickness of the plasma cloud between the plates and the dielectric constant of the plasma. The plasma dielectric constant has a very large value, which can reach the order of 105. Therefore, it can be seen from equation (4) that the plasma cloud or slag spray has a very large impact on the detection result. The literature [2~4] concludes that if the thickness of the plasma cloud is 1~2mm, the capacitance sensor detects The theoretical error of the distance between the two plates also reaches 1~2mm, which obviously does not reach the accuracy index of the laser focus position detection (±0.2mm).

4.Sensor optimization design technology reduces the influence of plasma cloud on detection results

The interference of plasma on the capacitive sensor is due to the plasma changing the medium between the two plates of the capacitor. Therefore, in order to eliminate the interference of plasma on the capacitive sensor, it is necessary to make the medium between the two electrode plates of the capacitor not be affected by the plasma. The central hole of the circular plate can be enlarged and the capacitive sensor can be moved outside the plasma cloud. Method to achieve.

  • (1) To eliminate the influence of plasma on the capacitance, the plasma must be placed outside the electrode plate of the capacitance sensor. Considering that the plasma cloud is distributed around the cutting point, it can be shown in Figure 5: The diameter of the central small hole of the circular plate is enlarged to 2~3mm and embedded in the high temperature resistant ceramic material at the edge, due to the capacitive sensor The electrode plate is hollow. Without considering the edge effect, the plasma cloud around the irradiation point has no effect on the capacitance and detection value of the sensor, so this method can effectively reduce the interference effect of the plasma cloud.
  • (2) For plane laser cutting, it can also be measured indirectly through mechanical transmission. That is, a mechanical device is used to follow the movement of the processed object, and the upper end of the mechanical device and the detection sensor form a polar plate, and the position between the laser focus and the processed object is indirectly detected by detecting the distance between the sensor and the mechanical device. This method can avoid the impact of ion cloud and slag spray on the detection accuracy to the greatest extent, and also takes advantage of the rapid response of the capacitive sensor.

5.Conclusion

Laser focus position detection and control is one of the key technologies of laser cutting processing. For fast cutting processing, the focus position detection accuracy and rapidity will directly affect the control accuracy and processing quality of the focus position. Capacitance sensing has high detection sensitivity, The advantage of fast response, the linearization of the computer system can be used to overcome its non-linearity; the special sensor structure can eliminate the influence of plasma cloud and slag spray generated during the processing on the detection results, and improve its performance in the laser cutting processing system. Effect.

Original Link:https://lasercutting-service.com/analysis-of-laser-focus-position-detection-and-control-technology-for-laser-cutting/

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