Meazza Stadium Uses NI CompactRIO to Usher in a New Frontier in Structural Monitoring

Author(s):
Giovanni Moschioni - The Politecnico di Milano
A. Caprioli - The Politecnico di Milano
A. Cigada - The Politecnico di Milano
M. Vanali - The Politecnico di Milano

Industry:
Education, Construction

Products:
LabVIEW, CompactRIO

The Challenge:
Creating a new, continuous, real-time vibration monitoring system for the evaluation of the structural integrity of the Meazza Stadium in Milan.

The Solution:
Developing a sensor network in strategic points of the structure with distributed acquisition and data storage using the NI CompactRIO platform.

Meazza Stadium in Milan (also known as San Siro) suffers from the typical problems that plague large structures, including building stress induced by people using the facility, (think of synchronized choruses or pop concerts). Milan municipality asked the Politecnico di Milano (the largest technical university in Italy) to conduct a detailed study of the Meazza stadium and to design an innovative monitoring system to measure vibration within a tenth of a point, evolution of corrosion on metallic parts, and other physical parameters. It was important that the system be durable enough to withstand the stadium-environment and its high mechanical, thermal, and electromagnetic stresses.

The Meazza stadium in Milan was built in 1925 and was expanded in the 1950s with a second ring. Its current three-ring structure was completed in the late 1980s for the 1990 Soccer World Championships for a capacity of 80,000 people. The stadium was originally built for soccer, but starting in the late 1980s, its use was extended to nonsporting events such as music concerts.

Occasionally, during concerts and soccer matches, vibratory events have been noted. Such phenomena are associated with the jumping and movement of thousands of people in sync with the rhythm of the music. The Politecnico addressed this problem with a study of the state of the stadium with structural evaluations, modal analyses, dynamic and static measurements, and assessments of corrosive phenomena. In the first phase such issues have been systematically evaluated through detailed experimental campaigns to characterize the stadium and the physical phenomena involved. This first round of tests yielded a large amount of clear data.  These results proved the necessity of a monitoring system for vibrations, stress, temperature, and other physical quantities so as to assess these trends over long periods, to dynamically characterize the stadium, and to point out reliable indicators of hazardous conditions.

During concerts at the stadium, people sway, wave, or jump in rhythm with the songs, causing structural vibration. The vibration level is proportional to the alternated force applied by the audience. Moreover, if the rhythm of the song (and thus of the people) matches a natural frequency of the structure, the vibration amplitude gets significantly larger. During a concert, the vibration level depends on the size of the crowd and the rhythm of the music. For some songs, the rhythm is synchronized with the structure’s natural frequency and the vibration levels are higher than average. Thus, it is necessary to measure this phenomena, and keep it under control before to the vibrations reach hazardous levels.

This monitoring system has to continuously measure the parameters in the frequency range of 0 to 50 Hz, requiring reliable data acquisition, storage, and transmission. The Mechanical and Thermal Measurement Group of the Politecnico di Milano has a remarkable reputation for its creation of long-term monitoring systems for both modern and aged structures. For the Meazza Stadium challenge, we developed a state-of-art system based on the CompactRIO platform and powered by a multilayer software program developed using the NI LabVIEW graphical programming environment.

The role of the sensors is quite obvious; it is only to be outlined that in a measurement-quality perspective, the cable between a transducer and its conditioning unit must be as short as possible. From a network (and economic) perspective, it should be as long as possible since the number of conditioning units is thus minimized. Therefore, a compromise has to be reached. Since the measurements are currently carried out with accelerometers, the adoption of ICP standard is of great help under this point of view.

The PC, installed in a protected place, serves as the “master” of the system. Under normal operating conditions it primarily collects and analyzes data stored in peripheral nodes, thus becoming the final destination of all data. In setup phases, it acts as the user-interface for nodes configuration and sensor calibration. The PC is also the gateway for remote access to the system through the Internet.

The central idea of the proposed measurement system is to distribute the acquisition nodes so as to have the best solution logistically and economically. An essential goal is to reduce signal cable length to improve measurement quality in an environment with interference from tens of thousands of mobile phones, hundreds of television antennas, and numerous power lines.

With a network of CompacRIO nodes, the system enjoys the benefits of distributed data-acquisition nodes and the distribution of data storage independent from the network status. Normally, the master PC coordinates the nodes and controls data retrieval and storage. If the central unit or network fails, each CompactRIO node can retrieve and store data for several days independently from other nodes.
CompactRIO also provides superior computation and storage capabilities, small dimensions, and resistance to humidity and other environmental agents.

Additionally it was important that any choice regarding the measurement system does not prevent the addition of new monitoring solutions in the future. A scalable system in terms of nodes and type of signal conditioning was critical. A critical point for dynamic measurement systems, namely the filtering, was solved by choosing analog and digital filters with high frequency samplings and online decimation.

LabVIEW was chosen to meet the ease of use requirement. The group of mechanical researchers must be able to directly interact with the system. With LabVIEW, it is possible to easily change anything at any time without specific skills in software programming.
The software for the system management offers superior flexibility and usability at different levels and gives access to the system for routine controls and low-level configurations. Finally, over long periods of time, the autodiagnosis and calibration process implemented in the proposed solution gives the measurement system the desired reliability.

The authors thank the Milan Municipality and football teams of F.C. Internazionale and A.C. Milan for their support.

Author Information:
Giovanni Moschioni
The Politecnico di Milano
Tel: 39-0-2-2399-8584

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