Integrated power measurements in automated laser systems
Increasingly higher laser powers are being used in material processing. Laser systems in the kW range have become the rule in laser cutting, welding and grinding. In many of the automated laser systems developed for this purpose, data is obtained to ensure the quality and efficiency of the processes. Sensors are constantly checking the material properties or visually scanning the machined part. However, the parameters of the — usually invisible — laser beams are not so easy to measure, especially in the higher power ranges. Many sensors are simply too delicate to withstand the laser powers or the harsh production conditions. It also often proves too difficult to connect to production networks, although this is a prerequisite for uninterrupted collection and analysis of the data. However, MKS Instruments has successfully developed new measurement technologies that rise to this challenge; read on to learn more details.
High quality, maximum throughput
Repeatable machining processes and consistently high part quality have always been key business requirements. Issues such as the ever-rising raw material costs, just-in-time production and the increasing importance attached to sustainability keep the focus on process quality. At the same time, these efforts must not compromise the availability of the systems in automated production processes. The solution? Record as many measured values as possible directly in the running process and analyze them immediately. Not only does this continuously verify the process quality, it makes it possible to carry out proactive maintenance and avoid machine downtime – not to mention adherence to occupational safety requirements, which ensure protection of the operators while measuring. So, we ask: which laser parameter provides the best initial assessment with regard to process stability?
Laser power and power density
The decisive factor in material processing is the power density of the laser beam when it strikes the workpiece; this power density is expressed in watts per square centimeter (W/cm2 ). In order to weld metal, a laser beam in the kilowatt range must be focused on a point in the millimeter range, which puts the power density at several MW/cm2. If the laser power decreases due to a faulty component, or if the beam diameter increases due to thermal effects of the laser system, the power density decreases very quickly – which has detrimental effects on the welding process. The particular challenge lies in the fact that this faulty welding very often goes undetected. On the surface, neither the depth of the weld nor the breadth of the heat-affected zone can be seen.
Caption picture 1: Dust particles from the manufacturing process have gotten onto this protective glass. Such contamination can cause a thermal effect in the laser system that leads to a shift in focus. This would cause the power density to drop; in turn, the processing quality of the laser system would deteriorate.
Especially in sensitive manufacturing processes, such as autobody welding, faulty welds can have disastrous consequences. Even regular maintenance of the systems at short intervals is not enough on its own to assure the quality. Since this can only be achieved through pass/fail testing at each shift change or before each machining operation, the power sensor must be integrated into the system. The challenges are manifold:
- The sensor itself must be protected from dust and dirt
- The sensor must not overheat
- Industrial interfaces and protocols must allow direct control of the sensor via the system
- Installation should be very flexible, because every system is designed individually
Along with laser applications, measurement technology has also developed significantly in recent years. New technologies for measuring the laser beam, such as the non-contact method based on Rayleigh scattering or the pulsed power method, enable reliable measurement of today's high-power lasers. In the past few years, MKS Instruments has already developed several Ophir devices that employ these approaches while taking the needs of industrial users into account.
The compact power meter Helios Plus determines laser powers up to 12kW by measuring in short bursts. Because it takes measurements within seconds, the unit can be operated without needing any water cooling. On the other hand, the Ophir BeamWatch Integrated System is based on non-contact measurement of the laser beam. It combines a beam analyzer with a power meter and has various industrial interfaces. And, like the Helios Plus, is has successfully been used directly in numerous production lines.
Recently, the company introduced the Ophir IPM-10KW sensor, another solution for measuring laser power in automated laser systems. Like the two developments before it, this sensor also offers industrial interfaces and measures a wide range of laser powers between 100W and 10kW. A new feature of this system is its modular design. The measurement concept is based on three components that can be combined as required by the integrators or the users:
- 1. Water-cooled sensor head
- Automatic protective cover (IPM shutter)
- Interfaces for Profinet or EtherNet/IP
Integrating the power measurement into the laser cell is thus greatly simplified. Whereas previously, it was necessary to run a cable up to 30 meters long from the cell to the display unit – with an A/D converter forwarding the result from there to the robot's control unit – now, the measurement setup is much more straightforward. The measurement data is reported via the communication interfaces to the control unit, which communicates directly with the laser robot.
Caption picture 2: The Ophir IPM-10k power meter consists of three components that can be combined as required.
Due to their durability and efficiency, high-power lasers offer myriad possibilities, especially in material processing. Still, experience shows that regular measurement of the laser parameters has a protective effect on the entire process. Deficits can usually be quickly corrected once they're discovered. The best assurance of processing quality is provided by robust measuring devices that can be integrated directly into the laser processing cell.