From every angle

For nearly 60 years, CERN’s surveyors have measured the Laboratory’s tunnels and caverns both in preparation for the accurate positioning of magnets and detectors, and to plot their shapes, dimensions and other lumps and bumps. Now that they have the very latest 3D laser scanner, nothing will get past them.

A measurement campaign carried out using laser scanners in the ISOLDE Hall.

About 15 years ago, the arrival of 3D laser scanners, digital devices which can reconstruct various objects in the form of 3D images, opened the door to a whole host of possibilities in the topography world. These new tools have been used at CERN since 2004 to produce increasingly detailed digital images of the LHC tunnels and experiments.

The CERN surveyors’ high-performance laser scanner has become an irreplaceable tool in many instances. It is capable of measuring every 2 millimetres from a distance of 10 metres (and therefore 4 millimetres from a distance of 20 metres or 1 millimetre from 5 metres) with a precision of 3 millimetres... and at a speed of 1 million measurements per second! “It works by measuring the difference in phase between the electromagnetic wave emitted by the laser and the wave received in return,” explains Tobias Dobers, an engineer in the SU Section of the ABP Group. “This method means the device is able to make very precise measurements." The laser scanner is controlled remotely and can rotate 360° horizontally and up to 310° vertically (it can’t see its own feet!). This huge range of movement means that complex and large objects – such as a detector – can be measured in a matter of minutes.

At CERN, where requirements are often very specific, 3D laser scanners have three main missions: creating 3D models of the accelerator tunnels for the teams who need to install new equipment, particularly as part of the R2E (Radiation to Electronics) project; scanning the detectors of the LHC experiments in order to get a precise plan of the locations of cables, the way in which components fit together or even what space is available; and finally to check the conformity of a building after civil engineering work is completed.

Cloud of 3D points on the CMS experiment obtained by the laser scanner. The colours are based on the texture (colour, brilliance, etc.) of the objects measured. Black or reflective surfaces for example are more difficult for the laser detector to handle and appear in red in the image, whereas neutral surfaces appear blue-green.

Although this tool saves precious time in the field, there is still plenty of work for the surveyors to do. “With millions of measurement points being recorded every second, you can imagine the size of the files we are left with after a measurement campaign,” notes Aurélie Maurisset, a surveyor for the CMS experiment. “In the post-processing phase, we therefore have to clean the files to make them digestible for the computer.” And this involves several stages: first, manually cleaning any points which are obviously incorrect. Then the data passes through several filters until a clean and relevant set of data is obtained. “Then there is the meshing stage,” continues Tobias. “This operation consists of taking a cloud of individual points and turning them into a consistent surface. This is also the stage where we assemble the various pieces of our puzzle.” To reconstruct a 3D object, readings of course have to be taken from several different positions. The surveyors must move the laser scanner around their target so that they can scan every part, and then, like a panoramic photo, join the different sets of readings together.

The last stage of the process is to integrate the newly digitised object into CERN’s coordinates system. This geo-referencing allows all of CERN’s teams to work from the same reference source. “It allows us to compare our results thoroughly," adds Aurélie. “It also sometimes reveals errors in the models… which the experts, of course, are quick to correct.”

by Anaïs Schaeffer