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Experimental Estimating Deflection of a Simple Beam Bridge Model Using Grating Eddy Current Sensors

Abstract: A novel three-point method using a grating eddy current absolute position sensor (GECS) for bridge deflection estimation is proposed in this paper. Real spatial positions of the measuring points along the span axis are directly used as relative reference points of each other rather than using any other auxiliary static reference points for measuring devices in a conventional method. Every three adjacent measuring points are defined as a measuring unit and a straight connecting bar with a GECS fixed on the center section of it links the two endpoints.

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In each measuring unit, the displacement of the mid-measuring point relative to the connecting bar measured by the GECS is defined as the relative deflection. Absolute deflections of each measuring point can be calculated from the relative deflections of all the measuring units directly without any correcting approaches. Principles of the three-point method and displacement measurement of the GECS are introduced in detail.

Both static and dynamic experiments have been carried out on a simple beam bridge model, which demonstrate that the three-point deflection estimation method using the GECS is effective and offers a reliable way for bridge deflection estimation, especially for long-term monitoring. Keywords: three-point method; deflection estimation; relative deflection; absolute deflection; grating eddy current sensor (GECS) OPEN ACCESS Sensors 2012, 12 9988 1. Introduction. After a bridge is put into use, gradual deterioration is inevitable because of loading, temperature changes or other environmental factors. In order to guarantee the safety and durability of those bridges which are expensive and closely related with people’s livelihood, long-term and continuous structural health monitoring is an essential part of the maintenance management. Among the various structural performance evaluations, vertical deflection is an important parameter that can directly and effectively indicate a bridge’s behavior.

In terms of instrumentation for deflection estimation, there are contact and non-contact deflection estimation methods. Traditional displacement sensors such as mechanical dial gauges or linear variable differential transducers (LVDTs) are used in contact measurement, through which static or real-time displacement values can be obtained directly or fed into a computer for processing and displaying via a data cable. This method, however, requires access under the bridge and installation of a temporary supporting system to mount sensors, which is time consuming and not very efficienct.

In addition, it might even be unavailable when bridges are over rivers, highways or have high clearance. Another contact sensor is the fiber optic Bragg-grating (FBG) sensor through which the deflection is calculated from the measured strain data and displacement-strain relationship [1,2]. In this way, however, the calculated displacement from strain data is sensitive to noise, and the sensors are expensive and must be embedded into the structure, which to a certain degree is difficult for bridges in service.

To cope with those inconveniences in contact measuring methods, various non-contact approaches have been proposed. Based on the detection of the Doppler shift of the laser light, a laser Doppler vibrometer (LDV) equipped with displacement and velocity signal decoders can measure both bridge deflection and vibration simultaneously [3]. In this way, a static reference point (usually underneath the bridge) is needed for device mounting, and the device should be attended, which limits it’s usability for long-term monitoring.

Among image methods, dynamic deflection with high resolution of the bridge can be obtained through using digital image processing techniques [4], while deflection distribution from the images of the bridge girder surface recorded by a digital camera before and after deformation can be evaluated by digital image correlation techniques [5], and digital close-range terrestrial photogrammetry (DCRTP) can measure the spatial coordination in three-dimensions [6,7].

Like the LDV, devices such as video cameras used in image methods cannot be left unattended and they are easily affected by weather conditions. Use of a Global Positioning System (GPS) can provide spatial locations of the measuring points on the bridge in real-time by comparing with a continuing operational reference station (CROS). It offers a long-term monitoring approach without being affected by climatic factors [8,9], but due to its relatively low accuracy, it is only applied to those bridges with significant deformations.

All the non-contact methods mentioned above, although they differ in instrumentation, have one thing in common, a static reference point or CROS that is kept a certain distance away from the bridge is selected for installation of the measuring device, otherwise measurements cannot be carried out. Another method is using inclinometers which can be installed on the bridge directly along a line paralleling the bridge span axis [10,11], and both static and dynamic deflection time history curves can be calculated through curve-fitting technology based on the accurate angle records of the inclinometers.

An outstanding feature of the inclinometer is that static reference Sensors 2012, 12 9989 points mentioned above are no longer needed. This approach reduces the dependence on environmental conditions and it is suitable for long-term monitoring. To avoid those deficiencies in conventional estimating deflection methods mentioned above, a novel three-point deflection estimation method is presented in this paper. Measuring points along lines paralleling the bridge span axis are chosen equidistantly.

Among these measuring points, every three adjacent measuring points are defined as a measuring unit in which a straight connecting bar linking the two endpoints is taken as a relative reference line. Relative deflection of the mid-measuring point relative to the intermediate point of the connecting bar on which a displacement sensor is fixed can be measured, and thus the absolute deflection of each measuring point can be calculated from the relative deflections of all the measuring units.

Compared with the contact and non-contact methods mentioned above, only real spatial positions of the measuring points are taken as relative references without any other static reference points. Moreover, the selected displacement sensor is the grating eddy current absolute position sensor (GECS) which is different from traditional eddy current sensors based on vertical characteristics [12,13]. Since the structure of grating reflective conductors is adopted, the measurement range is extended but without compromising the accuracy.

In addition, as an inductive sensor, the GECS is waterproof and dustproof in principle, thus it can work under bad weather conditions, which makes it ideal for long-term monitoring. In this paper, both the principles of the three-point method and displacement measurement of the GECS are presented. Then, this three-point method for deflection estimation is verified in a simply supported girder bridge model in the laboratory. Comprehensive static and dynamic experiment results on the laboratory tests demonstrate this method is effective and offers an alternative way for bridge deflection estimation.