The winning combination for high-speed laser marking
Tan Li, Alex Luk, Sammy Woo, Mark Lucus, David Freihofer, John Tinson |
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The advent of robust, high-speed and accurate laser marking system for production lines that demand automation control and monitoring, high throughput and minimal laser maintenance, is made possible by digital state space servo control and "low order mode" high repetition rate pulsed fiber laser technologies. The significance of this type of laser technology is its ability to match up with a high-speed scanner to deliver high-quality markings.
 | | Figure 1a |
 | Figure 1b
Figure 1a and 1b show the laser marking
patterns by the digital and analog system
respectively. |
A digital servo controller consists of high-speed digital signal processor (DSP) to perform all calculations necessary for digital control of servo motors in torque, velocity or position mode. The controllerÕs interface with control and feedback signals, such as motor currents and voltages, position encoder measurements is provided by high-resolution analog-to-digital conversion (ADC) integrated circuits. Tuning parameters that can be extracted from an auto-tuning process are stored digitally inside the hardware to eliminate manual potentiometer tuning and problems associated with analog circuit drift and aging.
Furthermore, high performance DSP technology also allows for the implementation of advanced motor control algorithms such as model based predictive control for high bandwidth performance. The predictive model is derived from the state space equation of motion of laser scanner motor with motor position as observable and other dynamic variables such as current and voltage simulated. The servo predicts ahead of time the movement of laser scanner and constructs motor voltage signals to ensure that the incoming signal can be followed within the constraints of the power-supply system. The drive that incorporates state space model can deliver substantial bandwidth enhancements over an analog servo.
Pulsed fiber lasers have a lot of advantages, such as excellent beam quality with the laser parameter M2 < 2[Ref1], extremely high conversion efficiency and very long life time in comparison with existing lasers such as Nd:YAG , Nd:YVO4 and CO2 lasers. It can also be operated minimal maintenance such as water-cooling and optical alignment. Materials processing has been improved very much by the usage of high repetition rate pulsed fiber lasers. The extreme short pulse width makes it easy to achieve very high peak laser intensity with low pulse energies For example, the peak power of a 20W pulsed fiber laser can reach 6KW at a repetition of 100KHz. Together with low M2 value, the laser pulse can be focused to a spot size < 100um in diameter. Because of the extremely high intensity and short laser-matter interaction times, heat diffusion is limited to the very small region and the concentration of the deposited laser energy causes rapid vaporization of the material.
Hence, pulsed fiber laser can ablate high-quality and precise patterns on selected material surface in laser marking application. Since, the distance between two laser marked dots along a scan path is proportional to scanner speed and inversely proportional to pulse repetition rate, so high repetition rate pulsed fiber laser is a key element in designing a high-quality and high-speed laser marking system when the laser scanner is controlled by digital state space servo.
 | | Figure 2a |
 | Figure 2b
Figure 2a and 2b show the horizontal line
marking patterns by the digital and analog
system respectively with both servos
executing same marking speed (10Kmm/sec). |
Experimental Results: Laser Marking Speed and Servo Bandwidth
The digital laser marking system under test consists of a DC2000 digital state space servo with 6230 galvanometers with 10mm mirror set from CTI and a 20W fiber laser from SPI. The laser is running at 125KHz repetition rate. Stainless steel plates are used for the laser marking processing study. The marking performance of this system is contrasted against the output from a corresponding analog marking system driven by an optimally tuned CTI 671 analog servo. For both laser marking systems, the best performance on a particular pattern is determined by proper selection of a set of correlated parameters, such as laser power, marking speed, laser mark delay and jump delay.
Figure 1a and 1b show the laser marking patterns by the digital and analog system respectively. The marking pattern is sufficiently complicated to include features such as hatching, cusps, spirals, straight and curved lines of different lengths. Hence, it is intuitively obvious that the overall performance of a laser marking system on this particular pattern is a reliable indicator of the fundamental system capability. According to the experimental results, the digital system is able to complete the marking process within 25.6 sec whereas the analog system takes 52.0 sec to finish the same operation. These are the best results from both systems because any extra effort to reduce the processing time of both systems will deteriorate the making quality. Therefore we claim that the digital system demonstrates a speed increase of 200 percent on a moderately complex pattern when compared the analog system.
 | Figure 3, the dark line and the red line
represents the time dependent angular path
using the digital servo and analog servo
controller respectively. |
High- performance servo is also characterized by its ability to exert torque to control motor through periods of rapid acceleration and deceleration. Figure 2a and 2b show the horizontal line marking patterns by the digital and analog system respectively with both servos executing same marking speed (10Kmm/sec). Two regions of different dot spacing can be seen in both figures. Region I, the region with varying dot spacing, corresponds to the acceleration / deceleration phase of the scanner. Region II, the region with constant dot spacing, corresponds to the steady scanning phase of the scanner. Since the length of Region I (~ 310 um) of Figure 2a is shorter than that of Figure 2b (~2600 um), we can infer from this observation that digital servo can handle short bursts of torque impulses better than the analog servo. This inference is also an argument to the previous marking results to reinforce our claim that the digital state space system out -performs analog systems in speed.
The difference in galvanometer angular response with respect to an input command between the digital and analog system illustrate the advancement in digital state space servo technology. Both servos are driving a CTI 6230 galvanometer with a 10mm mirror as before. In Figure 3, the dark line and the red line represents the time dependent angular path using the digital servo and analog servo controller respectively. The angle of mirror rotation is plotted along the Y-axis and is represented in arbitrary internal units. The path of the digital servo matches closely to the input command that the both traces are indistinguishable. The distortions in the red line happen because the analog PID servo is not able to follow the input signal precisely due to bandwidth limitations.
The performance of high repetition rate pulsed fiber laser marking systems driven by a model based digital state space servo and an analog PID servo and find that the digital system is able to deliver a substantial gain of 200 percent in speed performance. Hence, the digital laser marking system can offer valuable economic utility of low maintenance cost and high throughout for manufacturers looking for new or replacement laser marking systems.
Acknowledgement:
The authors would like to thank Modetech (Hong Kong) application lab for providing the laser marking results quoted in this paper.
References:
[1] SPI G3.0 Pulsed Fiber Laser Installation Guide and User Manual.
[2] CTI DC2000 Operation Manual.
[3] CTI MicroMax Series 671 Instruction Manual.
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