Ultrahigh Vacuum Pump Performance
Edwards, a leading manufacturer of vacuum and abatement equipment, services some of the world’s most advanced industries, including semiconductor, pharmaceutical, and solar, and facilitates cutting-edge research in fields such as renewable energy, genetics, stem cell research, and particle physics.
Among the company’s most innovative new products is the nEXT Turbo Molecular Vacuum Pump, which provides ultrahigh vacuum performance through the utilisation of a turbine rotor spinning at 60,000 rpm, with a blade-tip velocity approaching 90% of the speed of sound.
A key application for this type of pump is in the laboratory, where the pump is incorporated into mass spectrometry and electron microscopy equipment to provide the ultrahigh vacuum environment needed to allow ionised particles to reach detectors. However, these types of instruments are very sensitive to vibration and electromagnetic interference. With this in mind, the nEXT pump was designed to offer significantly lower levels of vibration and stray magnetic fields than any alternative pump.
Creating an In-House Solution
Manufacturing a pump capable of achieving class-leading vibration performance, while also rotating four times faster than a Formula 1 racing car engine, is not an easy task. It requires the pump’s rotor to be balanced to within extremely tight tolerances, with residual vibration levels so low that measuring this vibration, to achieve the balancing tolerances required, was beyond the capability of any of our existing balancing rigs.
Naturally, the vendors of these rigs were approached, with a view to purchasing new systems or upgrades, but it soon became apparent that none of these standard systems would be able to satisfy our specialist requirements.
With only weeks until our product launch, the challenge became to develop an in-house solution. Although we have used several test hardware vendors and programming languages over the years, National Instruments products, including LabVIEW system design software, have emerged as accepted standards within our company—not just for production test but also within our service centres around the world and in our labs to facilitate ongoing research and development.
The widespread use of LabVIEW within our company, together with its ease of use and unparalleled compatibility with NI and third-party hardware, made it an obvious choice for this application. To meet our demanding signal-capturing requirements, we chose the NI USB-9234 four-channel DSA. Whilst its 51.2 kS/s sample rate, 24-bit resolution, and excellent 102 dB dynamic input range more than satisfied our requirements, this module also offered the advantages of seamless integration with LabVIEW and IEPE signal conditioning. We can directly connect to an accelerometer, thus simplifying the overall system build.
Work began by using the DSA module, along with the NI Sound and Vibration Measurement Suite, to put together a prototype system that could be used to prove out the concept. An accelerometer was attached to the pump body and a photo sensor was used to detect the pump rotor position and provide a tachometer speed reference. The photo sensor is a digital device that outputs a single short pulse on each rotation of the rotor. Connecting a digital sensor such as this to an analogue input is not normally viable. However, the measurement suite software made this possible by offering several functions for reconstructing the original tacho signal from the analogue domain. Using both the NI Sound and Vibration Assistant included in the measurement software suite and NI hardware, we put together a working prototype system in a matter of hours, proving the feasibility of an in-house solution.
The next step was to move the prototype to LabVIEW, so that work on the full application could begin. The Sound and Vibration Assistant made this easy by generating the LabVIEW code for us and providing a foundation for developing the software.
From here, it was relatively simple to add a user interface, calibration functions, dialogues for adjusting product-specific parameters, and the code for calculating the pump imbalance from the measured vibration data. The methodology for these calculations is well-known, with extensive literature available on this topic. However, when calculating imbalance and the masses that must be added to correct it, it is generally taken for granted that any measurements will be made as closely as possible to the source. In our case, this was not possible because the rotor is contained within a pump housing that prevents measurements from being made directly, which makes the process of estimating the actual imbalance less straightforward. Off-the-shelf solutions are not designed to cope with these deviations from standard practice, which forces operators to compensate by performing the calculations manually. Our in-house solution allowed us to incorporate company know-how and practices to make the system more intuitive and improve production workflow and manufacturing efficiency. Furthermore, by adding an interactive software tool to help operators make up the required correction mass from standard weights, we enhanced the customer experience and gained further workflow improvements.
Finally, using the inherent parallelism in LabVIEW and the two remaining DSA inputs, we scaled the software to support balancing on two pumps at the same time. Therefore, we developed two balancing rigs for a fraction of the price of one. Full development, from proof of concept through to validation and final delivery of the working solution, took just under five weeks.
Building on the Benefits
Using LabVIEW, NI data acquisition hardware, and the NI Sound and Vibration Measurement Suite, we developed a robust and user-friendly balancing system with a dynamic input range far exceeding that of any commercially available system, at a fraction of the cost and within an extremely tight project timeline.
In addition to significant cost savings, producing a custom solution provided several benefits including improved measurement accuracy, enhancements in production workflow and efficiency, and greatly simplified support (should it ever break, we can fix it ourselves).
Since introducing the solution last year, we have moved beyond the original brief and adapted the application to support balancing across our entire turbo pump product range to make it suitable as a universal replacement for off-the-shelf balancing rigs. As our company expands, the built-in distribution tools of LabVIEW make it easy to roll this solution out globally to new service and manufacturing facilities, thereby offering the substantial ongoing cost savings and production process improvements needed to help ensure Edwards’ place as a global leader.
Gerard Johns, Senior Development Engineer
15 Marshall Road, Hampden Park
East Sussex BN22 9BA
Tel: 01323 525327