Laser engraving machine has high precision carving, engraving speed, low thermal effect, no pollution, no noise and other advantages, has been widely used in automobile, shipbuilding, aerospace and electronics industries. The relative position between the Laser engraving quality and laser focus and the workpiece has a close relation, guarantee the relative position between the laser focus and engraving object fair is one of the hub laser engraving quality.
Focus position of laser focusing cannot be directly measured, but by indirect method of detection. For a laser engraving machine system, the focus position is determined by the optical focus focusing mirror, so there is some focusing mirror the position is unchanged (without considering the thermal effect of focusing mirror), so the relative position between the focusing mirror and by detecting the object to be machined to indirectly detect position between the focus and the processed object.
The relative position between the laser focus and the processed objects can be detected by inductance displacement sensor and capacitance sensor in use, each has advantages and disadvantages. The frequency response of inductance sensor is low, is not suitable for high speed machining and the like! This needs dimension machining non contact detection occasions; capacitance sensor, has fast response speed, higher accuracy of interference detection strengths, but there are clouds and plasma spray residue to produce nonlinear and vulnerable to laser engraving process in the process of using.
This paper will discuss the system composition way of laser focus position error of laser engraving processing and automatic control system to eliminate the error. On this basis, respectively, discussed the shortcomings of two kinds of sensors and detection system in practical use and the ways to overcome.
1. the focus position of laser engraving process error
In laser engraving process, causes changes in the relative position between the focus and the object to be machined surface a workpiece uneven surface, workpiece clamping, machine tool and machine tool geometric error, deformation in the load force under the workpiece in the process of thermal deformation will cause the laser focus position and ideal a given position (programming position deviation). Some errors (such as machine tool geometric error) has regularity can be compensated by quantitative compensation method, but some error is random error, can only be eliminated by online detection and control, the error is:
1.1 geometric error, workpiece
The object of laser engraving plate or cover piece type parts, because of various? Because of the influence of the processing object, the surface is rough, the surface deformation and thermal effect in the carving process will produce metal parts, for 1 dimensional laser processing, cover pieces during the pressing process will produce the surface uneven, all these will produce the laser focus and the object to be machined surface the position and ideal position change.
1.2 workpiece mounting clamp device
The laser carving is placed on the needle table, because the long time and the workpiece machining error, wear and laser burns, the needle will be thrown uneven, random error will produce this inequality between the sheet and the laser focus position.
1.3 the programming error
In 1 dimensional laser engraving machining process, machining path on complex surface is obtained by linear and circular fitting, there are some errors of these fitting curve and actual curve, the error of the actual processing focus and object surface relative position and ideal position programming errors. Some teaching programming system will also introduce some deviation.
2. the focus position of laser engraving in the process of online detection and control system
As shown in Figure 1, online laser engraving focus position detecting and control system by the controller, detection system, execution device and other departments.
According to the relationship between the focus position detection and control system and system, focus position detection and control system is divided into independent and integrated two.
Compensation control independent focus position detecting and control system using the coordinate axis of a separate focus position error, the mechanical structure is complicated, the cost is higher, but can be used with different numerical control system and laser engraving machine.
And integrated with a feed axis laser engraving machine itself (the plane processing) or a plurality of synthetic feed axes (for 1 dimensional engraving) movement to the focus position error compensation. This method has simple structure, low cost, easy adjustment and other strengths, but to the same design and CNC system, higher requirements for open CNC system.
2.1 detection circuit of capacitance sensor
Capacitive sensor detection circuit, signal amplifier, a tunable oscillator crystal resonator oscillator, synchronous circuit, mixer circuit, signal processing circuit and a capacitance signal into a pulse signal corresponding to the frequency, through the pulse signal frequency sampling and processing capacity, get the corresponding. Here the capacitance of the capacitor two electrode is formed between the nozzle and the object of carving carving. Obviously, the capacitance in addition to the two plate area outside, between the media, between the plate and plate spacing of about it. This interval is associated with between laser focusing lens and the workpiece is also related with the distance between the laser focus and workpiece interval, so between the capacitance and the focus position and the approximate interval of the carving of the object. This is the capacitance sensor to detect the focus position of the principle.
The relationship between frequency and focus position error is nonlinear, the necessary linear processing by computer. At the same time, because the capacitance between the plate and is related to the medium, and plasma spray residue impact test results so easily be in the process, must be overcome.
2.2 the detection circuit of inductance sensor
Because of the large scale integrated circuit new, detection circuit of inductance sensor is simple, and the integrated circuit with modulation demodulation method and new algorithm, reduce the detection heterodyne frequency modulation detection circuit of previous methods because of the effect of phase angle, the exciting signal frequency and amplitude drift on the test results, greatly improved the detection accuracy and stability.
The sensor signal through the processed voltage signal and the sensor is proportional to the displacement, into the corresponding frequency signal through the conversion circuit, has been the focus of the position error signal is processed by computer.
Because of the inherent characteristics of inductive sensors, there are certain restrictions on the frequency of the measured signal (hundreds), a little used for high-speed processing occasions, at the same time, because of its contact detection method, can only be used for plane processing occasions.
3. plasma cloud in the carving process of focus position detection system influence
The workpiece has not been cut through the moment, and the laser metal interaction, produce clouds of plasma between the nozzle and the processing object, changing the capacitor medium, resulting in interference of capacitance sensor. In the carving process, auxiliary gas plasma from the slit blowing, have little effect on the capacitance sensor. But if the processing speed is too fast and the beginning of carving, because not be completely cut through the workpiece, laser produced plasma cloud around, the interference to the capacitance sensor, severe or even make sensor does not work, serious influence the quality of products.
The electromagnetic principle shows that the capacitance between adjacent plates is two:
C= e S/h, e - type between plate dielectric constant) is (1), S--- plate relatively effective area of h--- between the two electrode plates interval.
If no plasma interference, then, according to equation (1) measured by the capacitance and the plate (nozzle and processing object) between intervals is inversely proportional to the capacitance can be calculated easily between the two electrode plates interval, and then calculated the relative position between the focus and the processed object.
However, when the existence of the plasma or spray residue between the nozzle and the object to be machined, the dielectric capacitor is not air, the dielectric constant changes. According to the principle of formula of capacitance, capacitance between the plates at two for:
C‘=ε S1 /[(h-h1)+h1ε/ε1 ]+εS2/h (2)
The dielectric constant of 1--- plasma in h1---, plasma cloud thickness, S1 + S2 =S respectively with plasma cloud or spray residue area and no plasma cloud or spray residue area.
If the plasma cloud is distributed evenly in the nozzle and the object to be machined to a certain height range, while the capacitance sensor is measured between the two electrode plates interval:
h’=(h-h1)+ h1ε/ε1 (3)
The theory of error detection value:
Δh = h‘-h= h1 (ε/ε1 -1) (4)
From the type (4) shows that the error is decided by the size of the dielectric constant and the thickness of plasma cloud between the plates. While the plasma dielectric constant is very large, can reach number 105. So by the formula (4) can be seen in the plasma cloud or spray residue effects on the testing result is very large, if the plasma cloud thickness is 1 ~ 2mm, while the theoretical error by capacitance sensors between the two electrode plates interval has reached 1 ~ 2mm, is clearly not up to the accuracy of laser focus position detection (+ 0.2mm).
4. sensor optimization design techniques to reduce the impact on the detection results of plasma cloud
Interference plasma on the capacitive sensor is because the plasma changes between the two electrodes of a capacitor dielectric. Therefore, in order to eliminate the interference of plasma capacitance sensor, influence to make between the two electrodes of a capacitor medium by plasma, can the central hole with round ring plate and the capacitance sensor to outside the plasma cloud two methods to achieve.
4.1 to eliminate the effects of plasma on the capacitance, will be placed outside the plate plasma capacitance sensor. Taking into account the plasma cloud is distributed along the carved near the point, so it can be shown in Figure 5: the central circular hole plate diameter expanded to 2 ~ 3mm and embedded in the insulation resistance of high temperature ceramic materials, because the electrode plate is hollow, without considering the edge effect, the cloud point around the plasma irradiation the value does not affect the sensor capacitance and interference detection, so using this method can effectively reduce the plasma cloud.
4.2 for the planar laser engraving process, can also be used for indirect measurement by mechanical transmission method. By means of a mechanical device followed by processing object motion, mechanical device and upper detection sensor is formed between the plates, and the mechanical device by detecting sensor intervals to indirectly detect the laser focus and the processing of object location. This method can avoid the maximum effect of the ion cloud and spray residue on the detection accuracy, also performed rapid response strengths of capacitive sensor.
Laser focus position detection and control technology is one of the hub of laser engraving, carving for rapid processing, the focus position detection accuracy and speed will directly affect the control precision and the processing quality of the focal position detection, with high agility, fast response speed advantages with capacitance sensor, can overcome the nonlinear linearization by computer system.
Through the special structure of the sensor to eliminate the influence of plasma spray and slag produced by the process of the test results, the effect of its use in the progress of laser engraving machine system.
Jinan Style Machinery Co.,Ltd
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Factory: Huashan Industrial Park, Licheng District, Jinan, Shandong, China