Speaker
Description
Summary
The CMS Barrel Muon Alignment System is composed of a series of opto-mechanical
and opto-electronical elements. These elements can be arranged into two main
groups, the chambers and MABs (Module for Alignment of the Barrel). The starting
element of the chamber group is the light source (called "fork") containing 10 LED
light sources. The forks (1000 pieces altogether) are mounted on the barrel muon
chambers (4 forks on each of the 250 chambers). The chambers are mounted on the CMS
detector. The starting element of the MAB group is a 2D video-sensor that is -
together with an imaging lens - enclosed in a camera-box. The camera-boxes (600
pieces in total) are mounted on the MABs (large-scale rigid carbon-fibre mechanical
support structures, 36 pieces in total). The MABs are mounted on the CMS detector
in a way that the camera-boxes can observe the LEDs on the forks. The images
captured by the video sensors serve as an input to reconstruct the locations of the
chambers inside CMS with submillimeter accuracy.
To achieve this goal the position of the elements within the system
must be known with a precision beyond the manufacturing accuracy, ie:
- the location of the centre-of-mass of the emitted light spot of the LEDs on
the forks with <10 micrometer;
- the location of the forks on the chambers with <70 micrometer
- the homogeneity, sensitivity and linearity of the video-sensors with <1%
- the lens-sensor distance in the camera-box with <10 micrometer and the
perpendicularity of the sensor to the optical axis of the box with <3 millirad
- the location of the camera-boxes on the MABs with <50 micrometer and 50
microrad.
Therefore each piece of all element types have to be identified, calibrated and the
position of each embedded element on the enveloping object must be known and
followed.
Given the large number of elements and the complexity of the system, the data
handling procedures of the calibration and assembly were automated as much as
possible.
Five calibration facilities have been built to calibrate the forks, the
chambers, the sensors, the camera-boxes and the MABs. They are very different in
size and complexity but similar in nature: in all of them LED-type light sources
have to be switched on and off and driven with given current and their images have
to be captured by camera-boxes. These functions are complemented -depending on the
task- by other control and DAQ functions like 2D-motion table control, video-
multiplexing or environmental (temperature and humidity) measurements.
Therefore all the facilities are also similar from control, data acquisition and
data handling point of view.
The core hardware elements of the control and data acquisition system are the
microcontroller-based modular units that - together with the control software - can
easily be adapted to any particular calibration step. As the primary result of all
the measurements are either full video-images or light spots, the image processing
and transfer solutions can be reused at each calibration step.
The intermediate and final calibration values are stored in a relational
database. Each step of the calibration procedure heavily relies on the results
obtained in the previous steps, so the online query and update features offered by
the database system are vital to our design. A dynamic WEB-based reporting tool
makes it possible to follow the status of calibration and assembly and to search
for any data in an easy way without direct database-operations. In the paper these
points are discussed in detail.
