CERN's eagle-eyed movement hunters in action

Vibrations, movements, strains - nothing escapes the eagle eyes of CERN's Mechanical Measurements Laboratory, which helps groups needing mechanical testing and delicate transport operations.

They are on the look-out for anything that moves, shakes or changes shape. The slightest movement, however minute, will attract their attention. The Mechanical Measurements team, which is part of the Installation Coordination Group (TS-IC), specialises in all kinds of vibration studies, for design projects as well as for the transport of fragile objects.

The Mechanical Measurements Laboratory was created in 1973 and, after a lull at the end of the century, was given a new lease of life by the LHC project. The team uses a battery of sensors and software tools to scrutinise every kind of movement, stress, strain, buckling and vibration, in real-life situations. The Laboratory's aim is to assist groups in understanding the behaviour of structures (vacuum chambers, physics detectors, etc.), validating computer calculations through experimentation, and helping with the transportation of particularly fragile objects.

With the LHC installation in full swing, the team is working flat-out. All the LHC machine components, in particular the magnets, are fitted with stand-alone acceleration recorders for their various journeys above ground and in the tunnel. 'Our main concern is to prevent shear failure of the cryogenic support posts that bear the load of the cold masses inside the cryostats,' explains laboratory head Michael Guinchard. No fewer than 6,300 magnets were fitted with this kind of device in 2006. All the data recorded are analysed and integrated into a database for monitoring purposes. This work occupies two people full-time.

The team is also involved in all major transport operations for the experiments, installing sensors throughout the fragile detectors to provide real-time measurements of accelerations, tilt, temperature and humidity. The team has been involved in spectacular operations such as the transport of the LHC Cherenkov detector, the ALICE TPC, and the ATLAS and CMS trackers, ensuring that these precious detectors do not incur any damage during transport or installation.

A major field of study associated with the team's work is experimental modal analysis, which provides a dynamic characterisation of the structure under consideration. Each structure has its own natural frequencies (where the amplitude of the system's response is much greater than the amplitude of the excitation) and natural modes of vibration (or 'mode shapes,' the manner in which it deforms). This type of analysis is essential to ensuring that machine and detector sub-assemblies operate according to their design. The necessary micrometric precision of a detector could be compromised by the vibration of a neighboring piece of equipment (pump, ventilation system, etc.).

Experimental modal analysis is also a key element for successful handling operations. 'These studies give us an essential insight into how structures behave and help us avoid potentially harmful vibrations,' Michael Guinchard emphasises. For instance, excitations produced by the environment must not coincide with one of the structures' natural modes of vibration - this would be akin to the phenomenon of a bridge dangerously oscillating when crossed by a troop of marching soldiers. As a result, for the transport of the ATLAS inner detector end-caps (SCT and TRT), the team had to determine the structures' natural mode shapes with the utmost precision (see illustration). This process serves to identify the critical points of the structure, where accelerometers are then positioned. During transport, a wireless monitoring system will be used to check that the tilt is at no time greater than 1° and that, at the detectors' natural frequencies, their acceleration does not exceed 0.1 g ('g' being the unit of acceleration of the Earth's gravitational field, equal to 9.81 ms^-2).

The Mechanical Measurements Laboratory has other strings to its bow. For example, it contributed to the design of the LHC magnets, using strain gauges and capacitive gauges to study the stresses generated in the cold masses during operation (see below). Here, the Service uses measurements obtained through experimentation to adjust the calculations performed by design offices.