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Monitoring structural displacement with millimetre accuracy

26 October 2010

Laser distance sensors, which feature a useful integral display of measured values, have proved their worth in a challenging civil engineering project high above San Francisco Bay

The Oakland Bay five-lane, double-deck toll bridge connecting Oakland and San Francisco, carries about 300,000 vehicles a day. Following a severe earthquake in 1989, the decision was taken to replace part of this vital link with a new bridge, a project that required the temporary realignment of a 100m long, 3,200ton section of the existing bridge in order to make space for the new construction.

The new 3,600ton bridge section was placed in position over three days in the Spring of 2009 by Dutch civil engineering contractor, Mammoet. Of necessity, this heavy-duty operation required continuous and accurate monitoring, so Mammoet appointed Syrinx Industrial Electronics (www.syrinx.nl) to develop a central monitoring system for the project that would allow all movements to be displayed graphically in real time on a large, 42in screen.

The measuring devices chosen by Syrinx for this challenging application were Leuze electronic type ODSL laser distance sensors, which feature robust construction and a useful integral display that allows measured values to be read at the sensor itself, greatly simplifying set-up. Once fixed to their mounting plates, the alignment of these sensors was easily adjusted using magnetic fixtures. A total of 31 laser distance sensors and some 16 pressure sensors were used in this monitoring application.

During the move, the bridge section was lifted by approximately 30cm and moved laterally by means of hydraulic rams, the positions of which were measured by Leuze’s type ODSL 96B laser distance sensors transmitting 4-20mA analogue signals. The horizontal movement, over a maximum displacement of 30m, was measured by Leuze type ODSL 30 laser distance sensors, chosen for their RS485 output. Syrinx designed a system to convert this high-resolution output into a high-accuracy 4-20mA analogue signal in order to achieve the degree of precision required over the entire 30m displacement. Indeed, this system proved capable of providing accurate measurements over a displacement of up to 50m.

In addition to these special converters, Syrinx also provided eight enclosures for the marshalling of all sensor outputs. These were ultimately networked to a laptop connected to a 42in monitor from which the movements of the rams could be tracked in real time.


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