The integration of advanced digital servo control technology with high-repetition rate fiber lasers delivers exceptional laser marking quality. This innovative system combines precise digital state space control with a "low-order mode" pulsed fiber laser, enabling fast and accurate marking in automated production environments that demand high throughput and minimal maintenance. The key advantage of this technology lies in its compatibility with high-speed scanners, ensuring superior marking results even at high speeds.
At the heart of the digital servo controller is a high-speed digital signal processor (DSP) that manages all necessary calculations for torque, speed, or position control of the servo motor. The controller uses a high-resolution analog-to-digital conversion (ADC) integrated circuit to handle control and feedback signals, such as motor current, voltage, and encoder position data. Auto-tuning processes allow parameters to be stored digitally in hardware, eliminating the need for manual adjustments and reducing issues caused by analog drift or aging. Additionally, advanced control algorithms like model-based predictive control can be implemented using the DSP, enhancing performance through real-time predictions based on the motor’s dynamic behavior. These models use state space equations derived from observed motor movement, current, and voltage, allowing the system to anticipate scanner motion and optimize motor voltage signals accordingly. Compared to traditional analog systems, these digital drivers significantly increase bandwidth and responsiveness.
Pulsed fiber lasers offer several advantages over conventional laser technologies such as Nd:YAG, Nd:YVO4, and CO2 lasers, particularly in terms of beam quality and stability, as measured by the M² parameter. This makes them ideal for precision marking applications.
In our testing, we used a digital laser marking system composed of a Cambridge Technology DC2000 digital state space servo, a 6230 scanning galvanometer, a 10 mm mirror, and a 20 W fiber laser from SPI. The system operated at a repetition rate of 125 KHz. We tested it on stainless steel plates and compared its performance to an optimized analog system driven by a CTI 671 analog servo. Both systems were fine-tuned to achieve optimal results for specific patterns, including laser power, marking speed, delay times, and more.
Figures 1a and 1b illustrate the marking patterns produced by the digital and analog systems, respectively. The pattern included complex features like hatching, spirals, lines, and curves, making it an effective test for overall system performance. The digital system completed the marking in 25.6 seconds, while the analog system took 52 seconds. This represents a 200% improvement in speed without compromising quality, as further reductions in time would have led to lower output quality. This result confirms the digital system’s superior efficiency for moderately complex patterns.
Another experiment focused on the acceleration and deceleration phases of the scanning process. Figures 2a and 2b show horizontal line markings at a constant speed of 10 Kmm/sec. The graphs reveal two distinct regions: one with variable point spacing during acceleration/deceleration and another with consistent spacing during stable scanning. The digital system showed a shorter acceleration phase (310 µm) compared to the analog system (2600 µm), demonstrating better short-term torque control and response. This supports the earlier findings that the digital system outperforms analog systems in speed and accuracy.
In a third test, we examined the angular response of the scanning galvanometer for both systems. The digital and analog servos drove the same CTI 6230 galvanometer with a 10 mm mirror. As shown in Figure 3, the digital system closely followed the input command, while the analog system exhibited noticeable distortion due to limited bandwidth. This highlights the improved tracking capability of the digital state space servo, reinforcing its overall superiority.
In conclusion, the comparison between the model-based digital state space servo-driven fiber laser system and the analog PID system clearly demonstrates that digital solutions offer up to 200% faster performance. For manufacturers looking for efficient, reliable, and cost-effective laser marking systems, this digital technology provides significant economic benefits, reduced maintenance needs, and higher productivity. It is a strong choice for modern industrial applications where speed and precision are critical.
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