Laser material processing drives new servo control technology

Figure 4. During the auto-tuning process, actual system feedback is compared with the model’s expected results to adjust model parameters; shown is the system response before (a) and after (b) the auto-tuning process, with the top curve showing the position and the bottom The curve shows the current.

Simplify production and field service

The galvo system must be closely aligned with the model’s preset state for the automatic adjustment process to be successful. Therefore, automatic adjustment can help identify edge parts during assembly. For example, large changes in resonant frequency may indicate that the mirror is installed incorrectly. High noise levels during trimming may indicate a marginal cable connection. In short, auto-tuning is a comprehensive verification step that ensures high-quality assembly.

Compared to digital servos, the analog servo tuning process is largely isolated from component-level changes. The tuning process essentially monitors the position feedback of the black box system while adjusting the servo gain term to achieve the desired response to a predetermined command input. As long as moderators can be adjusted to achieve the desired response, the system is considered validated. Even an experienced tuner may not see slight changes in the resonant frequency of an edge-attached mirror.

The analog servo tuning process varies greatly depending on the type of control architecture and the specific criteria of each application. Flexible tuning strategies for analog servos require extensive user experience before proficient tuning is achieved and are typically performed by trained technicians using expensive test equipment. Therefore, analog servo tuning is almost always performed by the servo board vendor, which tends to limit field serviceability options.

Automatic tuning of digital servo drives greatly simplifies the tuning process and makes it easier for new users. It also enables flexible inventory control: since the digital servo system can be adjusted at any time, it can be effectively delivered separately from the galvanometer and reflector. For example, if the configuration differs only by mirror coating type, the laser tool manufacturer can redirect the galvanometer and servo to the appropriately coated mirror as needed. In contrast, analog servo systems require a complete galvanometer-servo-mirror configuration for both mirror coating types in stock.

Autotuning simplifies field service as the process requires minimal technical investment for successful calibration. Once field service personnel identify a faulty part, they simply replace the defective part and initiate a re-alignment procedure. This limits component removal to only what is needed and leaves the rest of the configuration in a ready-to-run state, minimizing customer downtime and making replacement components easier to stock. In the same situation, the analog servo system would require the entire configuration to be replaced and returned to the factory for repair and re-tuning.

Enhance monitoring and system protection

Using a model of the digital galvo and its thermal resistance, the power dissipated in the galvo can be continuously monitored to verify that the temperature remains within safe operating limits. Servo systems can also use thermal models to incorporate changes in the galvanometer model as it heats up during operation. Because the galvanometer’s resistance, inductance, and torque are all affected by higher power consumption, the servo system can compensate for changes in these parameters to maintain a consistent stable response from initial power-up to high duty cycle operation.

Digital servo technology continues to enhance existing and emerging laser materials processing applications. The next generation of digital servo systems is expected to improve and support new application requirements. This could include better modeling, faster update rates, higher-resolution command and position feedback, predictive maintenance algorithms, overall system safety checks, and enhanced system controller and laser synchronization.

Eric Ulmer([email protected]) is a product manager at Cambridge Technology, a Novanta company located in Bedford, Massachusetts; Cambridge Technologiesgy.com.

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