Sensor fusion: Low-cost, high-end

December 21, 2015  - By
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Integration for infrastructure monitoring, navigation

By Desislava Staykova and Nico Zill

Rapid development in the technology of combined sensors within complex systems has taken place over the last decade. Such systems provide different accuracy levels, offering the possibility of use in application areas such as surveying, railway and automotive engineering, land administration, and for navigation purposes.

Multi-sensor integration and fusion is a comprehensive process of reading and combining sensor signals to ensure a higher level of data reliability and accuracy. Input data from every sensor and further combination with specially developed algorithms ensures the complete identification of observed features, which would be impossible with data from each individual sensor operating separately.

Because of its flexibility and the possibility for fast and continuous data measurement, multi-sensor integration and fusion has evolved rapidly in different areas. The object of this article is to overview the use of high-end and low-cost system complexes and software solutions for the purposes of the engineering geodesy, transportation and navigation.

Deformation Monitoring

Geodetic measurements for monitoring and displacements analysis of various engineering objects have always played an important role in maintaining structures like bridges, dam walls, building columns, wind power generators, and other construction.

This requires properly designed network schemes enabling continuous and highly accurate measurements. For such angular and length measurements of millimeter-level accuracy that must be performed in intervals of minutes, hours or a day, standard total stations are being replaced by automated ones (ATS) comprising precise servomotors, automatic target recognition sensors, electronic inclinometers, self-calibration control systems and other sensors.

The synchronized process of high-accuracy measurements (angular accuracy better than one second and distance accuracy better than one millimeter) and simultaneously adjustment software enables real-time or post-processing deformation monitoring and analysis. This type of hardware and software combination is often used during the life cycle of a project for construction and reconstruction of objects and for regular monitoring of the object’s stability.

Terrestrial Laser Scanning. The need for precise modeling and geometrical characterization of large structures and open areas as dams, mines, landslides and others cannot be covered by traditional surveying methods which require the use a huge number of points for describing the object’s surface. The development of laser scanning technology in the last decade offers a new way for deformations measurements and becomes part of the infrastructure monitoring.

The high scanning speed, dense measurement of huge numbers of points and high accuracy gives terrestrial laser scanning (TLS) an advantage other technologies used for large structural monitoring. Compared with the technologies using single point monitoring approaches where the displacements detection is limiteded to specific benchmarks, TLS provides high data redundancy. Combined with proper software products, this technique offers the possibility for high-accuracy surface modeling and displacement detection at the millimeter level. The scanned object consists of a large number of points, which allows implementation of mathematical algorithms for modeling and analyzing the object’s behavior.

Another advantage of TLS as a remote sensing measurement tool is the minimized impact of the operator over the observed points and network.

A new method for structural monitoring has emerged recently, comprising the advantages of the TLS, GNSS, geotechnical and meteo-sensors, enabling wide-area coverage and surface monitoring. One such tool is shown in Figure 1.

Figure 1. Terrestrial laser scanning combined with GNSS and other sensors enables wide-area coverage and survace monitoring. (Images courtesy of Leica Geosystems)

Figure 1. Terrestrial laser scanning combined with GNSS and other sensors enables wide-area coverage and survace monitoring. (Images courtesy of Leica Geosystems)

Mobile Laser Scanning

For different navigation purposes, for monitoring and investigation of wide areas, static measuring methods are being replaced by complex mobile measuring combinations of both high-end and low-cost sensors, to ensure fast, continuous and accurate data acquisition.

Recently mobile laser scanning (MLS) has experienced rapid development and proved its usage particularly in the railway and automotive sectors, for deformation analysis, for monitoring and documentation of as-built street and railway networks and the corresponding infrastructure objects.

MLS for Rail and Road. The advantages of MLS for fast, high-accuracy and complete scanning of the surroundings make it an important part of current railway and road conditions monitoring.

Continuous data acquisition and processing minimizes operator errors, and significantly reduces the time for performance of the surveying work and a-posteriori data analysis.

Localization and recognition of infrastructure objects forming part of railway and road environment has long been of primary importance in the transportation sector.

For determination and documentation of as-built railway and street networks from acquired data, Technet-Rail (Berlin, Germany) developed two software solutions, SiRailScan and SiRoadScan, for point-cloud analysis. The integrated mathematical algorithms ensure high-accuracy extraction and adjustment of the as-built left rail, right rail and center line, as well as of the roads’ border lines.

The adjusted geometry forms the basis for driving speed control tests, determination of the as-built environment for clearance detection and documentation, investigation of catenary wire deviations, ballast and road settlements, traffic signal positions, and any changes in the existing situation (see Figures 2 and 3).

Figure 2. Adjusted as-built rail geometry with SiRailScan used as basis for performance of clearance analysis and documentation in chainage based railway system.

Figure 2. Adjusted as-built rail geometry with SiRailScan used as basis for performance of clearance analysis and documentation in chainage based railway system.

Figure 3. Adjusted with SiRoadScan road border lines. Detection and recognition of the roads signals.

Figure 3. Adjusted with SiRoadScan road border lines. Detection and recognition of the roads signals.

 

In response to the growing interest in application of the MLS technique and a-posteriori data adjustment for monitoring purposes, Technet-Rail developed additional tools for deformation analysis of structures such as tunnel bodies, railway bridges and road surfaces. The integrated software solutions enable comparison between the designed and as-built situation, epoch-wise analysis, modeling of the structure, development into 2D followed by color-coded deformation map (see Figures 4 and 5).

Figure 4. Tunnel deformation analysis performed with SiRailScan based on the as-built rail geometry. Automated calculation of differences between designed and as-built tunnel structure.

Figure 4. Tunnel deformation analysis performed with SiRailScan based on the as-built rail geometry. Automated calculation of differences between designed and as-built tunnel structure.

Figure 5. Tunnel deformation analysis with SiRailScan based on a pre-defined form and direction.

Figure 5. Tunnel deformation analysis with SiRailScan based on a pre-defined form and direction.

MLS for Navigation. Multi-sensor integration is the basis for operation of the moving measuring systems integrating hardware devices such as laser scanning devices, GNSS, inertial measurement units (IMU), distance measuring instruments (DMI) and specific software algorithms for data synchronization. A milestone in the development of such systems is the measurement and navigation in indoor places or in areas with low or no GNSS coverage.

The need for safe and reliable navigation in transportation systems such as train control systems, intelligent vehicle systems, system tracking, in urban environments, underground areas, and other areas with no available GNSS signal stimulated much research in the area of multi-sensor integration and fusion. the main scope of some studies is the integration of different sensors delivering information for the attitude, velocity, acceleration such as the IMUs, inclinometers, wheel sensors, and correspondent filtering algorithms to achieve the best possible position accuracy without usage of GNSS signals.

Conclusion

For decades, infrastructure objects such as dam walls, bridges, tunnels, roads and railway tracks form a substantial part of civil engineering and engineering geodesy. The integrity of their structure requires deep knowledge of the behavior of these objects and the various methods for their optimal and high accurate monitoring. The rapid development evolution of multi sensor integration in combination with laser scanning technology makes it an essential method for accurate, continuous and dense measurement for the purposes of the engineering surveys.


Desislava Staykova and Nico Zill are engineers with Technet-rail 2010 GmbH, Berlin, Germany.

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