Publication: ‘97 Proceedings of the Symposium on Lasers and Their Applications (SLA'97), Campinas, SP, Brazil; 12/1997 Geometrical Superpulse for CO2 Laser Cutting of Steel Edgardo Gerck - LASERTECH1S/A - DEF/FEM/UNICAMP2 Jorge L. Lima - Lasercam1- DEF/FEM/UNICAMP Abstract This work describes a new superpulse regime, called geometrical superpulse, that allows the laser pulse power to be controlled as a function of distance traveled, not of time. This regime is conveniently implemented as a standard extension of the DIN norm command set for computer numerical control (CNC). This allows the geometrical superpulse to be applied in a standard production environment. 1 . Introduction The favorable features of CO2 lasers for their use in manufacturing processes includes the ability to have their temporal power output modulated in superpulse mode [1]. The laser superpulse mode has allowed a remarkable increase of laser material processing capacity, and it consists of a sequential application of trains of very short duration and high peak power pulses, with constant frequency as a function of time. This work describes a new concept in laser superpulse power control, that has been successfully applied [2], which is based on a constant frequency of pulse train in relation to space, which we call geometrical superpulse. 2 . Discussions In the temporal superpulse regime, the mean power depends on the duty cycle, which is expressed by the relation [Rt = t0 / t b], where t 0 is the laser turn-on time and t b is the pulse train base (total time). For a short t 0, that depends on the structural type of the CO 2 laser and its design features, the peak power can reach up six times the laser power output in CW mode [1], as in the case of the MT-1000® laser [3]. As was reported in a previous paper [2], the laser cutting process for steel sheets can benefit from the use of the geometrical superpulse regime, which improves the final quality of the laser cut parts. This is possible because the geometrical superpulse regime provides for controlled laser power per unit distance as a function of the feed rate, even for varying acceleration. In contrast, temporal superpulse offers controlled power as a function of time, resulting in uncontrolled laser power delivery per unit distance, as the CNC accelerates and disaccelerates around the corners.
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[email protected] © Copyright by the authors, 1997 1 2
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3 . Results Figure 1 shows the laser power output variation as a function of time and space (with constant cutting speed) for both ideal and real geometrical superpulse power output (a), and the difference between the temporal and the geometrical superpulse modes in a cutting path region wherein the laser head movement suffers a disacelleration (b).
Figure 1 - (a) Schematic diagram showing both the ideal and real pulse train; (b) behavior of the pulse train relative to time and space in function of the laser cutting speed variation, with the laser power output operating in geometrical superpulse regime (upper part) compared to temporal superpulse (lower part). How can this behavior be implemented in the praxis? The answer was to extend the usual CNC commands, as the DIN norm allows, with a set of procedures that relate power with traveled distance. The table 1 bellow exemplifies the new M65 command, as used in two sets of geometrical superpulse parameters in the experimental laser cutting of zinc- and aluminum-coated steel sheets [2].
TABLE 1: Experimental parameters used for geometrical superpulse compared to temporal of steel CO2 laser cutting. [2] Set of parameters Geometrical superpulse parameter code with experimental attributed values. 1 M65 Q0.1 S0.05 T6 D2 A30 F3000 L1550 2
M65
Q0.2
S0.05
T6
D2
A30
F3000
L1550
2
Actually, two new commands called M65 and M66 are needed, with the following parameters: M65 ® turn on the laser geometrical superpulse feature (with parameters). To actually turn on the laser, the usual command M67 is used. M66 ® turn off the laser geometrical superpulse feature (with no parameters). To actually turn off the laser, the usual command M68 is used. ·
Parameters for M65 (all values are real numbers with decimal point):
S[x] ® distance x (in mm) traveled with laser off before and after each corner of the cutting path Q[x] ® distance x (in mm) traveled with laser on before and after each corner of the cutting path T[t] ® laser turn-on time t during the [ms] D[t] ® laser turn-off time t during the pulse trains [ms] A[n] ® number n of repeated application of the superpulse parameter sets before and after each cutting path corner F[v] ® linear speed v of laser beam on the cutting path [mm/min] L[t] ® initial time t of the laser beam, for entry [ms] 4 . Conclusions We have discussed a new superpulse regime, called geometrical superpulse, that allows the laser pulse power and laser average power to be controlled as a function of distance traveled. For example, this is adequate for CO2 laser cutting of complex steel parts because it allows the laser power to adapt to the part geometry. As a result, the laser is able to deliver constant power to the part, even during acceleration or disacceleration around corners, automatically. 5 . References [ 1 ] - Sasnett, M. W., CO2 laser design considerations for pulsed material processing, in: Proceedings of the 3rd International Conference on Lasers in Manufacturing, pp. 279-292, June 1986. [ 2 ] - Gerck, E., Lima, J.; An experimental study of laser cutting of mild steel sheets with metallic surface coatings; October 1997, in preparation. [ 3 ] - CO2 Laser/CNC Machine MT-1000Ô - Technical Characteristics and Operation Manual, LASERTECHÒ S/A, 1993.
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