Magnet Control with Revolution Pi: A Reliable Solution for the Mu3e Experiment at PSI
Challenge
Unstable magnet control at PSI
The Mu3e experiment relies on a superconducting solenoid magnet (nicknamed "HULK") from Cryogenic, with a field of around one tesla. Four coils are excited with currents of up to 200 amperes. To stay superconducting, the magnet has to be cooled to about 4.2 kelvin, the temperature of liquid helium. Four load cells measure mechanical forces of up to 4000 newtons on the coils.
All of these values need to be read out, displayed and controlled around the clock. The magnet power supplies are also addressed through the control system. The existing setup ran on a Windows 10 PC with a LabVIEW application. In day-to-day operation, this caused problems again and again. LabVIEW crashed every now and then, Windows kept asking for updates, and the screen could not easily be accessed over the network. For an experiment that needs to run 24/7, this was not a sustainable solution.
The Paul Scherrer Institute (PSI) is the largest research institute for natural and engineering sciences in Switzerland. At its site in Villigen, it operates the world's most intense continuous muon beam. This beam is used by the international Mu3e experiment, which is looking for an extremely rare decay of a positive muon into two positrons and an electron. Observing it would be a clear sign of physics beyond the Standard Model.
Solution
DIN rail solution with Revolution Pi
The team at PSI replaced the previous setup with a compact industrial solution on a DIN rail. A RevPi Core and a RevPi AIO expansion module handle the acquisition of the analog signals.
The device runs the open-source data acquisition software MIDAS. MIDAS has been used in particle physics for over 30 years. It comes with a modern web interface and integrates seamlessly into the existing Mu3e infrastructure. As a result, the magnet can be monitored and controlled from any computer on the PSI network.
Implementierung
So funktioniert die Magnetsteuerung
Auf dem RevPi Core läuft das KUNBUS Standard-Image, MIDAS wurde nativ kompiliert. Drei parallel laufende Frontend-Prozesse decken den Funktionsumfang ab:
- Lakeshore für die Temperatursensoren
- Loadcell für die mechanischen Lastzellen
- Cryogenic SMS für die Magnet-Power-Supplies
Die Lastzellen liefern 0 bis 10 Millivolt. Verstärker vom Typ TXDIN1600 von Omega wandeln das Signal in 0 bis 10 Volt um, das RevPi AIO Erweiterungsmodul digitalisiert es. Power-Supplies und Temperatursensoren laufen über USB-to-RS232-Konverter, gebündelt durch einen USB-Hub auf der Hutschiene.
Ergänzt um Schutzkomponenten und ein 24-Volt-Netzteil entsteht eine kompakte Installation ohne externen PC. Im MIDAS-Webinterface zeigt eine eigene HULK-Seite den Magnetzustand in Echtzeit und erlaubt kontrolliertes Hoch- und Herunterfahren. Über das Mu3e-Backend ist die Steuerung zudem Teil der globalen Datenerfassung.
Verwendete Technologien:
- RevPi Core als zentrale Steuerung auf der Hutschiene
- RevPi AIO Erweiterungsmodul für die analogen Eingänge
- KUNBUS Standard-Image als Betriebssystem
- MIDAS als Open-Source-Datenerfassungs- und Steuerungssoftware
- TXDIN1600 Lastzellen-Verstärker (Omega)
- USB-Hub auf Hutschiene mit USB-to-RS232-Konvertern für Power-Supplies und Lakeshore-Temperaturcontroller
- 24-Volt-Industrienetzteil
"At the Paul Scherrer Institute, we switched the magnet control of the Mu3e experiment from a PC and LabVIEW solution to a Revolution Pi from KUNBUS GmbH. The new solution runs much more reliably, is more compact and robust thanks to its DIN rail mounting, and offers convenient web-based control via the MIDAS system, all at lower costs."
– Dr. Stefan Ritt, Head Muon Physics, Laboratory for Particle Physics, Paul Scherrer Institute
Results & Outlook
Reliable control, lower costs
Switching to the RevPi Core solved several problems for the PSI team at once. The control system runs stably in continuous operation. Unplanned restarts caused by Windows updates or crashed LabVIEW processes are no longer an issue. The MIDAS web interface lets the team monitor and control the magnet directly over the network. Instead of a full industrial PC with a LabVIEW license, a compact DIN rail setup is now enough.
The new solution is also attractive from a cost perspective. Compared to proprietary hardware in the range of an NI CompactRIO setup with LabVIEW, PSI gets by with a RevPi Core, a RevPi AIO expansion module and a 24-volt power supply, all at a fraction of the previous cost. On top of that, the open-source nature of MIDAS gives the research team full control over the software.
The group is already planning further applications on a RevPi basis. Control systems for beam line separators and X-Y scanners at PSI are being discussed. Since MIDAS is widely used in the global particle physics community, the combination of RevPi and MIDAS has potential well beyond PSI.