Development program improves laser processing
In 1989, the Italian National Research Council (CNR) funded a five-year project on electro-optical technologies (PET) designed to develop laser sources, systems, and optical components that could provide immediate spin-offs to industry. As a consequence, a large part of the resources were dedicated to activities concerning laser material processing. The results of the project were presented during a demonstration day held in Turin, Italy, last March.
Integrating lasers into robots results in a compact unit with increased mobility and simplifies system operation at different workstations or over large areas. In particular, two anthropomorphic robot-laser prototypes, equipped with CO2 lasers, were designed and produced during the PET by Comau (Beinasco, Torino), Stola (Rivoli, Torino), and EL.EN (Sesto Fiorentino, Fir enze). Comau and Stola produced the robotic structures, and EL.EN was responsible for laser development.
The first system features a 3-kW carbon dioxide (CO2) fast-axial-flow laser mounted in the robot base (see Laser Focus World, Feb. 1994, p. 28). Because the laser-beam quality at the work site is essential for effective manufacturing, significant attention has been devoted to the optical chain. The robot is capable of moving along a 5-m slide and can accept different types of focusing heads suitable for welding, surface treatment, and cutting. The laser is radio-frequency (RF) pumped and can operate as a conventional stable resonator or an unstable resonator by using a variable reflectivity output coupler designed and tested by CISE (Segrate, Milan) and CEQSE/CNR at Milan Polytechnic. The second system, developed for cutting applications, features a robot arm modified to house a compact, 1-kW CO2 laser (see Fig. 1). The laser is diffusion-cooled and RF pumped with a planar electrode configuration.
Various electro-optical systems to control laser processes and to extend the robot-laser application range were developed. RTM (Vico Canavese, Torino) produced a system for weld-seam tracking and two focusing heads capable of maintaining the maximum trajectory error below 0.1 mm. These heads extend the range of laser-robot applications to those requiring the high-precision trajectory control attainable using dedicated motion units with the robot as a static positioning unit.
A focusing head capable of measuring surface temperature during laser heat treatments was developed by Comau. Centro Laser (Bari) produced a real-time process control sensor for CO2 laser cutting based on the fact that the erosion front temperature variations have the same frequency as the cut-edge roughness. The company also developed a new off-line controller for laser welding based on the correlation between welding plasma electronic temperature to weld-bead quality via statistical analysis.
Systems to control laser welding also have been developed by CISE, including an on-line process-monitoring system capable of noncontact detection of cracks by means of a sensitive fiberoptic interferometer (see Fig. 2). Based on a heterodyne Michelson-type configuration and a diode-pumped Nd:YAG source, the system measures the workpiece surface displacements due to the propagation of the defect-generated ultrasonic wave.
One of the initial PET-based industrial developments was a special laser line for welding steel-plate-tailored blanks, according to L. Caprioglio of Stola and A. M. Verga Scheggi, the project director. It will produce laser-welded tailored blank components for the rear doors of a new FIAT (Torino) automobile to be launched in the fall of 1995.
The Program for Electro-optical Technologies has been very successful. According to I. Pera of Fiat Research Center (Orbassano, Torino), the technical advancements described above "will undoubtedly assure Italy a very important position in laser robotics."
FIGURE 1. CO2 laser mounted in robot arm eliminates constraints due to the optical paths of externally mounted sources.
FIGURE 2. Fiberoptic interferometric probe detects cracks during the welding process.