All aboard!

Every year, CERN's surveyors take detailed measurements to check the alignment of the LHC components. This year, from 16 to 18 January, they took some of those measurements for the first time using a brand-new remotely controlled train in one of the long straight sections.


From left to right: Thierry Feniet, Patrick Bestmann and Cédric Charrondière in the arms of the measuring wagon.

This train doesn’t take people, it takes pictures. Its purpose? To save CERN’s surveyors from having to take the alignment measurements manually, particularly in areas where operators are subject to constraints due to radioactivity (in line with the ALARA principle of keeping radiation exposure to a level that is “as low as reasonably achievable”).

The surveyors’ train, over four years in development, is the joint brain-child of several groups from the EN and BE Departments. The result is a state-of-the-art device which, as Thierry Feniet, the EN-HE Group member who designed the vehicle, explains, “can be controlled from the surface using a network of optical fibres and WiFi antennas”.

Let’s step on board for a virtual tour of this train, fitted out with ultra-modern surveying equipment (see photo). Suspended from a monorail attached to the ceiling of the LHC tunnel, the engine pulls its four wagons (two containing the measuring equipment, one for the controls, and one for the batteries) in the direction of Point 7 and its 38 collimators, clocking up speeds of up to 6 km/h.

“The measuring wagon is equipped with four photogrammetric cameras and two moveable arms,” says Patrick Bestmann, the surveyor in the BE-ABP Group who designed the measurement system. “The arms have sensors that are used to track a reference wire which is strung along the section being measured. When the train pulls up next to a component whose alignment is to be measured (e.g. a collimator or a magnet), it halts and triggers simultaneous read-out of the different sensors including the four cameras. The data serve to reconstruct the precise coordinates in three dimensions for collimators and magnets.” By combining the 3-D data for a section, the surveyors can calculate the relative positions of all of the components.

“While measurements are being taken, the calculations are performed inside the control wagon and forwarded in real time to operators on the surface," says EN-ICE Group member Cédric Charrondière, who developed the measurement software. “This allows us to verify the measurements on a continuous basis, and to compare them with earlier measurements so as to detect any movement that may have taken place."

Given that the train's payload and operating régime can be customised easily, it is likely to be joined by others like it in the near future. Thierry Feniet is gratified: “The Radiation Protection Service already has plans to order three trains for the purpose of radiation monitoring. Civil Engineering has also shown a keen interest. They have demonstrated that it is possible to do a tunnel inspection very easily using a special camera fitted to the train.” As for the surveyors, they are already looking at the idea of using this kind of train in the future to measure the alignment of the magnets in the LHC arc sections.


by Anaïs Schaeffer