Exact spectrometry

B at Sill Optics GmbH & Co. KG, a specialist for high-precision optical components, every anti-reflective and every dielectric mirror coating system is tested using a spectrophotometer. However, the sensitive measuring instruments repeatedly failed, resulting in production downtimes and costly recalibrations. As a

Highest precision is necessary

analysis showed that there were frequent power fluctuations at this point, the optics expert installed two power conditioning systems at the end of 2011. The Security Plus systems from Powervar catch disturbances, voltage peaks and dips on the one hand and ensure the power supply in the event of a power failure on the other. Power problems can thus be ruled out as a source of error and the often lengthy quality tests run through without interruptions.

Around 1,400 precision lenses with diameters from 4 to 300 mm are manufactured daily at Sill Optics in Wendelstein. In addition, there are illumination lenses up to 650 mm as well as custom-made products. These are used, among other things, in lenses and complete systems for laser, measuring and medical technology as well as for image processing, lighting and photonics. Since the lenses are decisive for the function of the subsequent devices, quality is the top priority - especially in terms of coating. "The majority of our spherical and aspherical precision optics are produced with AR or dielectric mirror coatings," explains Andy Stufler, process engineer at Sill Optics. The surface treatment makes it possible to change the properties of the lens by selectively creating interference. For example, an anti-reflective (AR) coating can reduce light losses at air-glass interfaces from four percent down to 0.05 percent and prevent unwanted reflections.

Most accurate controls

However, the individual layers of the coating must be applied very precisely in order to generate the necessary interference. "Measuring with the spectrometer is essential for this. The vapor-deposited layer systems are precisely checked after each disposition process," says Stufler. This is the only way to detect production errors in time. Two UV/Vis/NIR spectrophotometers from Perkin Elmer are used for this purpose, which are specially designed for the analysis of difficult samples such as highly absorbent glass or optical coatings. At the same time, the processes of the coating systems are also subsequently analyzed and the material parameters for their adjustment are determined.

Power fluctuations trigger outages

However, the measurement modules failed sporadically for no apparent reason, resulting in time-consuming and costly error analyses. In addition, there were frequent power failures that interrupted the measurement processes, as the process engineer reports: "The spectrometers then had to be reinitialized, and after recording the baselines, 15 to 45 minutes of measurement time were then often lost. " The testing process also had to be started from the beginning. A conventional, upstream UPS system could not solve the problem because it reacted too slowly and the measuring systems still crashed in the event of a power failure.

Since no technical trigger for the problems of the spectrophotometers could be found and in order to exclude disturbances in the power supply as a source of error, the service technicians of Perkin Elmer worked together with the power quality expert Powervar. "In the process, a week-long current measurement revealed repeated voltage dips on the phase as well as interference voltages between the neutral and ground conductors," Werner Karau, Managing Director of Powervar Germany, explains the result. "Such phenomena are usually caused by the operation of adjacent large consumers in the production process. " The interference voltages were far beyond the maximum values recommended by the semiconductor industry, so that in the long run damage to the measuring devices would have been to be feared. "I was somewhat surprised that the voltage fluctuations were so high that they could even lead to defects," says Stufler. As a result, previous damage to the coating equipment was also attributed to voltage spikes when the power grid restarted after an outage.

Power without disturbances

To ensure a consistent power supply, Sill Optics therefore decided to install so-called Security Plus systems on the two spectrophotometers. These contain various barriers against interference from the power grid, including effective overvoltage protection, a low-impedance isolation transformer that stops common-mode interference voltages, a filter network against high-frequency noise and a voltage regulator. Even frequency fluctuations can be compensated. In this way, the sensitive measuring device is provided with exactly the current required for operation in constant quality, regardless of events or disturbances on the supply side.

In addition, the systems also include a UPS system that seamlessly bridges power failures and thus prevents crashes of the measurement modules. For this purpose, the Security Plus devices are connected to the power supply via a double-converter circuit.

Time-intensive error analyses

Power supply integrated, in which the current flows in any case - whether mains or battery operation - through the internal inverter, so that there are no interruptions when switching between the modes. In an emergency, the systems provide stored power for at least 15 minutes at full capacity; for longer bridging phases, they can be coupled with external batteries. In order to reduce the operating costs, attention has been paid to high efficiency: The output power factor is 0.9, which means that around 30 percent more usable power is delivered than with conventional UPS systems.

Less time required

Time-consuming downtime after a power failure is no longer an issue in quality assurance at Sill Optics. The time-consuming re-setup that used to take up to 45 minutes is avoided. "You could pull the plug even during a sensitive measurement process without affecting the process in any way," reports process engineer Stufler. Also possible damage due to voltage peaks

Uniform power supply

are now reliably prevented. Above all, however, in the event of problems with the measuring devices, the power quality factor no longer has to be considered as a potential cause in the fault analysis, which previously resulted in many technician deployments and a lot of lost production time. For the future, the company is even thinking of installing further protection systems from Powervar: "If it is feasible in terms of cost, I would also like to protect the sensitive parts of the quenching and tempering system, as a defect is almost guaranteed here in the event of any power failure."