Depth profiling method for detecting focal position in laser cutting

The focal position of laser focusing cannot be measured directly, but it can be detected indirectly. For a laser cutting system, its focal position is determined by the optical focus of the focusing lens, so its position is constant under certain conditions (not considering the thermal effect of the focusing lens). Therefore, the positional relationship between the focus and the object being processed can be indirectly detected by detecting the relative position between the focusing lens and the object being processed.

The relative position between the laser focus and the object being processed can be detected by inductive displacement sensors and capacitive sensors, each of which has its own advantages and disadvantages. The response frequency of inductive sensors is low, and they are not suitable for high-speed processing and occasions that require non-contact detection such as 3D processing; capacitive sensors have the advantages of fast response speed and high detection accuracy, but they are nonlinear in use and are easily affected by the interference of plasma clouds and slag generated during laser cutting.

This paper will systematically discuss the generation of laser focus position error in laser cutting and the composition of the control system for automatically eliminating the error. On this basis, the composition of two sensor detection systems and the shortcomings in actual use and the methods to overcome them are discussed respectively.

1. Generation of focus position error during laser cutting

During the laser cutting process, there are many factors that cause the relative position between the focus and the surface of the object being processed to change. The uneven surface of the workpiece being processed, the clamping method of the workpiece, the geometric error of the machine tool, the deformation of the machine tool under load, and the thermal deformation of the workpiece during processing will all cause the laser focus position to deviate from the ideal given position (programmed position). Some errors (such as the geometric error of the machine tool) 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 Workpiece geometric error The object of laser cutting is a plate or a cover-type part. Due to various reasons, the surface of the processed object is uneven, and the thermal effect during the cutting process will also cause surface deformation of thin plate parts. For one-dimensional laser processing, the cover will also have an uneven surface during the pressing process. All of these will cause the position of the laser focus and the surface of the processed object to change randomly from the ideal position.

1.2 Errors caused by workpiece clamping devices

The workpiece for laser cutting is placed on a needle-shaped workbench. Due to processing errors, long-term wear between the workpiece and laser burns, the needle bed will appear uneven. This unevenness will also cause random errors in the position between the thin steel plate and the laser focus.

1.3 Errors caused by programming In the process of 1D laser cutting, the machining trajectory on the complex surface is fitted by straight lines, arcs, etc. There are certain errors between these fitting curves and the actual curves. These errors cause certain errors between the relative position of the actual focus and the surface of the processed object and the ideal programming position. Some teaching programming systems will also introduce some deviations.

2. Composition of online detection and control system of focus position during laser cutting

As shown in FIG1 , the laser cutting focus position online detection and control system is composed of a controller, a detection system, an actuator and other parts.

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 for the focus position error. The mechanical structure is complex and the cost is high, but it can be used with various CNC systems and laser cutting machines. The integrated type uses a feed axis (for plane processing) or a combination of multiple feed axes (for 1D cutting processing) of the laser cutting machine itself to compensate for the focus position error. This method has the advantages of simple structure, low cost, and easy adjustment, but it requires the same design as the CNC system and has high requirements for the openness of the CNC system.

2.1 Capacitive sensor detection circuit

As shown in Figure 2, the capacitance sensor detection circuit is composed of a tuning oscillator, a signal amplifier, a crystal frequency-stabilized oscillator, a synchronization circuit, a mixing circuit, a signal processing circuit, etc., which converts the capacitance signal into a pulse signal of the corresponding frequency, and obtains the corresponding capacitance by frequency sampling and processing the pulse signal. The capacitance here is the capacitance formed by the two plates between the cutting nozzle and the cutting object. Obviously, its capacitance is related to the medium between the plates and the interval between the plates in addition to the area of ​​the two plates. This interval is related to the interval between the laser focusing mirror and the workpiece, that is, the interval between the laser focus and the workpiece, so the capacitance is approximately related to the interval between the focal position and the cutting object. This is the principle of the capacitance sensor detecting the focal position.

As can be seen from the figure, the relationship between frequency and focus position error is nonlinear and must be linearized by computer. At the same time, since capacitance is also related to the medium between the plates, the test results are easily affected by the plasma cloud and slag spraying generated during the processing, which must be overcome.

2.2 Inductive sensor detection circuit

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

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

Due to the inherent characteristics of inductive sensors, there are certain limitations on the frequency of the measured signal (several hundred), which makes them not suitable for high-speed processing. At the same time, due to their contact detection method, they can only be used in plane processing.

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

At the moment when the workpiece has not been cut through, the laser and the metal interact to generate cloud-like plasma between the nozzle and the object being processed, changing the medium between the capacitor plates, thereby interfering with the capacitive sensor. In the normal cutting process, the auxiliary gas blows the plasma away from the cut, which has little effect on the capacitive sensor. However, if the processing speed is too fast and at the beginning of the cutting, since the workpiece has not been completely cut through, a plasma cloud will be generated around the laser irradiation point, interfering with the capacitive sensor. In severe cases, the sensor may not work properly, seriously affecting the processing quality. Figure 4 is a schematic diagram of plasma interference.

According to the electromagnetic principle, the capacitance between two adjacent plates is C=εS/h, where ε---the dielectric constant between the plates is generally (1), S---the relative effective area of ​​the plates, and 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 (the nozzle and the object being processed). The capacitance can be used to easily calculate the distance between the two plates, and then calculate the relative position between the focus and the object being processed. However, when there is plasma or slag spraying between the nozzle and the object being processed, 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 is:

C''''=ε S1 /[(h-h1)+h1ε/ε1 ]+εS2/h (2), where ε1 is the dielectric constant of plasma, h1 is the thickness of plasma cloud, S1 + S2 =S are the areas of the area with plasma cloud or slag spraying and the area without plasma cloud or slag spraying, respectively. If the plasma cloud is evenly distributed within a certain height range between the nozzle and the object being processed, the distance between the two plates measured by the capacitive sensor is:

h''''=(h-h1)+ h1ε/ε1 (3)

Theoretical value of the detected error:

Δh = h''''-h

= h1 (ε/ε1 -1) (4)

From equation (4), we can see 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 dielectric constant of the plasma has a very large value, which can reach the order of 105. Therefore, from equation (4), we can see that the influence of the plasma cloud or slag spray on the detection result is very large. References [2-4] show that if the thickness of the plasma cloud is 1-2 mm, the theoretical error of the interval between the two plates detected by the capacitive sensor will also reach 1-2 mm, which obviously cannot meet the accuracy index of the laser focus position detection (±0.2 mm).

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

The interference of plasma on the capacitive sensor is due to the fact that plasma changes the medium between the two plates of the capacitor. Therefore, in order to eliminate the interference of plasma on the capacitive sensor, the medium between the two plates of the capacitor must not be affected by the plasma. This can be achieved by enlarging the central hole of the annular plate and moving the capacitive sensor outside the plasma cloud.

(1) To eliminate the influence of plasma on capacitance, the plasma should be placed outside the electrode of the capacitive sensor. Considering that the plasma cloud is distributed around the cutting point, it is possible to expand the diameter of the central hole of the annular electrode to 2-3 mm and embed the high-temperature resistant ceramic material at the edge as shown in Figure 5. Since the electrode of the capacitive sensor is hollow, without considering the edge effect, the plasma cloud around the irradiation point has no effect on the sensor capacitance and detection value, so this method can effectively reduce the interference of the plasma cloud.

(2) For plane laser cutting, indirect measurement can also be performed through mechanical transmission. That is, a mechanical device follows the movement of the object being processed, and the upper end of the mechanical device and the detection sensor form a plate. The distance between the detection sensor and the mechanical device is used to indirectly detect the position between the laser focus and the object being processed. This method can minimize the impact of ion cloud and slag spray on the detection accuracy, and also give full play to the advantage of the rapid response of the capacitive sensor.

5 Conclusion

Laser focus position detection and control is one of the key technologies in laser cutting processing. For fast cutting processing, the focus position detection accuracy and speed will directly affect the focus position control accuracy and processing quality. Capacitive sensing has the advantages of high detection sensitivity and fast response. Its nonlinearity can be overcome by the linearization of the computer system; the special sensor structure can be used to eliminate the influence of plasma cloud and slag spraying generated during the processing on the detection results, thereby improving its use effect in the laser cutting processing system.