Real-Time Laser Focus Tracking for Compressive Testing: Modular Rust Architecture with OPC UA and Web GUI for Laser Ultrasonics

Compression testing combined with laser ultrasonics enables non-destructive, in situ material characterisation during thermo-mechanical loading, but the shortening and radial expansion of the specimen displace the laser focal points during deformation and therefore require active repositioning of the laser-ultrasonic optics. This work addresses this engineering problem by providing a modular positioning-control system that couples a Gleeble 3800 thermo-mechanical simulator and a laser-ultrasonic measurement unit. The Gleeble provides two analogue shaft-position signals, which are digitised and used as real-time reference signals for stage-position control. The excitation and detection optics are mounted on motorised Zaber linear stages; two stages translate the optics along the compression direction, while a third perpendicular stage adjusts the detection focus to compensate radial specimen expansion and the limited depth of field of the detection optics. The control software was implemented in Rust and computes the stage target positions from the digitised Gleeble voltages output using user-configurable mathematical expressions. It further provides process-data access through an embedded OPC UA server and a browser-based graphical user interface through an integrated HTTP server. A custom ADC-based acquisition chain was introduced because the built-in ADC inputs of the Zaber controllers showed a significant noise level when used for the Gleeble shaft-position signals. Profiling showed that the control-loop runtime was dominated by ADC read/write operations; by reorganising acquisition into a parallel setup, the mean control-loop time was reduced from approximately 130 ms to approximately 27 ms. The resulting system provides an active, configurable, and retrofit-capable link between Gleeble compression testing and laser-ultrasonic measurement. It improves the temporal response available for laser-focus tracking on deforming specimens and provides a foundation for future quantitative validation of tracking error, positioning uncertainty, and LUS signal quality during real compression experiments.