Optical scanning laser triangulation sensors for 3D inline metrology

Abstract

High precision 3D in-line measurement systems play a pivotal role in shaping the future landscape of production. They present a key technology to meet the ever growing demands for increased productivity and precision in manufacturing systems. Optical measuring principles are particularly suited for these systems, due to their non-contact properties, their ability to achieve high through put rates and their outstanding precision. To overcome the performance limitations of common optical in-line measurement systems, which are based on the combination of standard automation components, comprehensive opto-mechatronic design concepts are essential, which enable fast and precise 3D measurements. The foundation of this approach is built upon the holistic system design and integration of advanced high performance mechatronics with optical components, which are undeniably fundamental technologies for cutting edge imaging and measurement systems.To obtain viable measurement geometries for scanning the optical path of a laser line triangulation sensor, scanning systems that either scan solely the illumination path or both optical paths using optical beam steering devices are developed and validated. The good agreement between the measurement results of these optical scanning systems and those of a mechanical scanning system, which moves the entire optical sensor, emphasizes the feasibility of an optical scan. Scanning only the illumination path introduces an additional measurement error due to the violation of the Scheimpflug condition. However, this approach allows for a smaller aperture size, leading to higher scan speeds and a more compact design. The measurement error is further analyzed and a correction method employing a correction polynomial to partially mitigate it is developed.The resolution of the system is restrained along the laser line by the limited lateral resolution of the sensor. To overcome this limitation, an optical scanning point sensor system is proposed, which can perform raster and Lissajous trajectories to scan the area of interest. Depending on the measurement application, these trajectories can be tailored to optimize measurement time or enhance lateral resolution. Tailored feedback controllers are designed for each scan trajectory, to achieve a high tracking performance. Especially Lissajous trajectories are highly favoured, due to their multi-resolution properties and applicability of dual tone controller, which features an exceptional tracking performance. The design for a mechanically tunable resonant fast steering mirror (FSM) is introduced and validated, demonstrating a potential reduction in power consumption by a factor of 13.83.In addition, a novel system design is proposed that maintains the Scheimpflug condition even though only the illumination path is scanned. This is achieved by positioning the mirror surface at the intersection line of the lens and object plane. For this arrangement, both a scanning point and a line sensor system are developed and validated, offering either superior lateral resolution or enhanced scan speed. The optical scanning point sensor system employs a compact, high-performance, hybrid reluctance-actuated FSM, capturing a measurement area of 15×23×25 mm in 10 seconds. Conversely, the optical scanning line sensor system utilizes a galvanometer scanner to manipulate the optical path, covering a measurement area of 31×36×50 mm in 0.67 seconds. Both systems achieve resolutions in the tens of micrometers, making them highly suitable for demanding high precision measurement applications. Additionally, an uncertainty framework is proposed which enables to determine the achievable resolution of the optical scanning system in advance. To enhance system flexibility, machine vision is applied to automatically detect and scan features on a sample with a high resolution.