Reliable sensor technology
The basic instruments used for the measurement are strain-gauges, which are attached directly to the structure of the bridge or as sensing element in the force sensors. Strain-gauge based sensors have the advantage of high long-term stability. This is important in this project because they are operational during the whole lifetime of the new bridge and it is not possible to stop the monitoring for recalibration.
In addition to this, the HBK strain gauges of the LY41-series, which are used in this application, offer a good temperature compensation, which is a must, as the structure is faced with several weather and temperature conditions.
For data acquisition, HBK's PMX system is being used.
After the installation, the measuring point is covered to prevent damage and electromagnetic interference (EMI).
Critical bridge sections
This project comprises of a distributed multichannel configuration for permanent observation of the critical bridge sections:
- Experimental stress analysis (durability testing): 4 mid span zones, 8 support zones and 2 zones during launching;
- Force measurement in hangers: 12 hangers;
- Vertical displacements: 12 mid span zones and 8 support zones;
- Longitudinal displacements: 4 support zones;
- Inclinations: 12 mid span zones and 9 support zones;
- Acceleration, frequency and damping parameters: 12 mid span zones and 9 support zones and
- Temperature measurement in the bridge structure: 12 mid span zones and 9 support zones.
472 sensors installed
The total number of installed sensors and transducers is 472:
- 328 HBK strain gauges (LY41-series) in 14 sections (the experimental stress analysis),
- 80 force transducers (force monitoring in hangers)
- 12 displacement transducers in 4 movable support sections (for horizontal displacements),
- 32 bi-axial inclinometers (for bi-axial inclination and vertical displacement from acceleration measurements) with embedded tri-axial accelerometers
- and temperature sensors in 20 structural sections and one section of central pier zone.
Adjustments can be made directly at the measurement point
The measurement and data acquisition and control system has to fulfill several tasks: Fast and reliable measurements, easy set-up, real-time calculations, diagnosis information, no additional software installation and efficient pricing. All this can be achieved with the PMX measurement and control system.
As an additional protection against dust and EMI, the PMX data acquisition unit was mounted in a cabinet, which is a cost-effective solution and also includes the power supply.
In the first step, all signals are sampled with 19.2 kHz, which guarantees a high measurement-bandwidth and evaluation of the measurement signals. All adjustments to the PMX system are handled via the standard Ethernet-interface and the internal PMX webserver.
This solution offers a great benefit: Adjustments can be done directly at the installation point or via the Ethernet network at the control room or even remotely if additional WiFi is available. So each engineer is able to have a real-time view of the application and the status of the test. An integrated operator-control prevents unsecure operations.