Wireless sensors for infrastructure health monitoring

Concrete

Bestech Australia has released wireless sensors and data acquisition systems from BeanAir Germany for structural health monitoring applications. It is a mature technology with solar energy harvesting and capability to withstand freezing temperatures in winter conditions. In one of the applications it has been used in, BeanAir deployed the wireless sensor networks for large-scale condition monitoring of highway bridges in Montreal, Canada.
The Turcot Interchange is a three-level stack freeway interchange within the city of Montreal in Quebec, Canada. Located southwest of downtown, the interchange provides access to Champlain Bridge, one of the largest bridge infrastructures in the Northern America.

It is the third busiest interchange in Montreal (after Decarie and Anjou interchange, respectively), as of 2010. The numbers averaging a north-southbound flow of about 300,000 daily drivers and more than 350,000 west-eastbound in total.  It sees about 50 million crossings annually, of which 200,000 are buses.

In June 2007, the Quebec government announced the demolition and reconstruction of the structure. This project was estimated to be completed by 2016. The announcement came four years after a study on the interchange revealed that the Turcot structure was crumbling. After careful studies, it was discovered that the concrete structure has been degraded by the excessive use of de-icing salt. Huge concrete slabs up to one square metre were reported falling from the overpasses.

A new interchange was also proposed to be built lower to the ground with a large segment to be rebuilt towards the north. Reconstruction of the interchange is expected to cost the government between $1.2 billion and $1.5 billion. It was proposed to use wireless sensors for remote condition monitoring of this interchange to reduce costs and complexity.

Wireless Sensor Network for interchange monitoring
The wireless sensors from BeanAir are mature technology for long-term structural health monitoring of large infrastructure such as bridges, dams, stadiums, tunnels, churches and other buildings. The wireless sensors network (WSN) technology was deployed for assessing the impact of construction-related activities and evaluating the effect of structural retrofitting. The inherent monitoring of environmental loads and influences, operational loads and traffic patterns were used to collect a database of field measurements. They are useful to provide feedback for interchange structure extension.
A total of 300 wireless sensors were installed on the interchange to monitor the vibration, inclination and deformation of the interchange structure. These fit customer requirements to monitor several physical parameters which included vibration, tilt, cracks, shocks and deformation, at the same time.

The vibration response and shock level of the bridge was measured with wireless accelerometers mounted on the interchange pillars. Sampling rate was configured to 100Hz on concrete structure monitoring and to 400Hz on steel/metallic structure monitoring. A wireless data acquisition system connected to a displacement sensor was also used to measure the bridge displacement for vibration analysis. Wireless inclinometers were used to monitor bridge foundations and to detect structure sinking during the construction works. The inclinometer has smooth accuracy of ±0.1 per cent and measurement range of ±15o.

For crack measurements, several wireless crack metres utilising LVDT technology were mounted on the interchange pillars. They were used to measure the existing cracks, and evolution were compared to the resonance frequencies of the interchange pillar. This provides indication on when the structure is due to fail.

To coordinate these wireless sensors, more than 25 wireless network coordinators with the same NTP clock source (provided by a 3G/4G modem) were deployed on the monitoring site. The wireless sensors nodes were synchronised using a two-way ranging technique.
For power supply options, the combination of solar panel and lead acid battery were used. They are mature technology and are suitable for long-term monitoring solutions.

Technical Challenges
When implementing the wireless solutions, experts discovered the issues with network bottleneck and aggregation capacity of the wireless network. Networks have generally relied on either local data logging and post-sampling transmission of sensor data or on low sampling rate and/or limited number of sensors in order to address the issue with bandwidth limitations.

By managing several WSN together with a unique supervision system, BeanAir wireless sensors were able to address these limitations and deliver optimised sampling duration, data acquisition rates and spatial resolutions for accurate monitoring.

During operations with WSN, clock drift may be observed which requires time re-synchronisation of the networks. This can be managed by bringing a time-triggered on-demand synchronisation which allows the network to obtain sensor data from multiple sensor nodes at a specific time. Using this method, there is no event set to trigger the sensor nodes, allowing the nodes to sample data at a right time.

Integrating WSN gives numerous customer benefits
Wireless sensors prove to be serious competition to wired sensors for structural health monitoring applications. Especially in monitoring large structure/infrastructure, wireless instrumentations greatly reduce the wiring complexity and overall cost required for installation.

The deployment of WSN in highway bridge monitoring in Canada oversees a reduction of 60 per cent in total costs and shorter duration needed for installation as compared to the use of wired sensors. It also provides new added value services to the users in terms of accessing and monitoring the real-time data remotely through a cellular network. Perhaps, the greatest benefit of WSN in this case is its ease of integration and flexibility to adapt with changes in the bridge’s behaviour. As a full wireless system, the sensor position can be easily interchanged depending on the bridge behaviour. In terms of scaling-up, new wireless sensors can be easily added to the same network or by adding a new network without interfering with the existing wireless network.

The system can also be easily integrated to the customer IT system through BeanScape monitoring software. For this project, BeanAir integrated an OPC DA server in the supervision software as it is particularly well suited for real time measurement and data sharing.