High resolution large displacement/crack sensor

A structural displacement sensor that includes an arched member having two ends, where each end is attached to a fastening member. In an embodiment of the invention, the fastening members are configured to be attached to a structure. Further, a strain gauge is attached to the arched member, and the strain gauge is operatively connected to a signal processing device. In an embodiment, the arched member and strain gauge are configured to measure a displacement of the structure based on the amount of strain detected on the strain gauge.

FIELD OF THE INVENTION

This invention generally relates to structural displacement sensors.

BACKGROUND OF THE INVENTION

In general, structural displacements and openings caused by cracks or deformations in civil structures are on the order of a few micrometers to a few centimeters. Conventionally, mechanical systems using springs and magnetic transduction methods or other similar techniques are employed to measure large displacements such as these. These systems are not easily deployable in some structures because of the lack of conformity with structural shapes. Further, these systems tend to have a low degree of resolution with respect to the displacement measurements. Strain-gauge-type systems, such as fiber optic Bragg gratings, are more easily deployed but are highly sensitive and enjoy high resolutions, such that their strain measurement range is small—on the order of 5,000 microstrains, which translates to a fraction of a millimeter once converted to displacement over their gauge length. The displacement measurement range for these strain monitoring systems is low when they are configured to be attached to straight mechanical elements. In this type of arrangement, the displacements due to crack openings are directly transferred to the strain gauge and provide a limited displacement range, i.e., 5,000 microstrains times the length of the mechanical element providing at best a fraction of millimeter in displacement range. Higher displacement readings may damage the strain sensor, or just simply are not transduced.

It would therefore be desirable to have a fiber optic system for measuring structural displacements and crack openings in civil structures that has a greater range of measurement than conventional displacement-measuring systems employing optical fibers.

The invention provides such a displacement-measuring system. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

In one aspect, embodiments of the invention provide a structural displacement sensor that includes an arched member having two ends, where each end is attached to a fastening member. In an embodiment of the invention, the fastening members are configured to be attached to a structure. Further, a strain gauge is attached to the arched member, and the strain gauge is operatively connected to a signal processing device. In an embodiment, the arched member and strain gauge are configured to measure a displacement in the structure based on the amount of strain detected on the strain gauge.

In another aspect, embodiments of the invention provide a structural displacement measurement system that includes a plurality of structural displacement sensors, where each structural displacement sensor is attached to a structure at a different location on the structure. In an embodiment, each of the plurality of structural displacement sensors has an arched member having two ends, where each end is attached to a fastening member. In a particular embodiment, a fiber Bragg grating strain gauge is attached to the arched member, and the fiber Bragg grating strain gauges are operatively coupled to an optical signal processing device. An amount of structural displacement distance in the structure is determined by using the optical signal processing device to measure the difference between the wavelength of an optical signal propagating through the fiber Bragg grating strain gauge after the structural displacement, and the wavelength of the optical signal before the structural displacement.

In yet another aspect, embodiments of the invention provide a method of measuring structural displacement that includes attaching a structural displacement sensor to a structure. In an embodiment of the invention, the structural displacement sensor includes an arched member having two ends, where each end is attached to a fastening member. The structural displacement sensor further includes a first strain gauge attached to the arched member, wherein the first strain gauge is operatively connected to a signal processing device. In an embodiment, the method of measuring structural displacement also includes coupling the first strain gauge to a signal processing device, measuring the strain placed on the first strain gauge, and calculating the structural displacement distance based on the strain measurement.

DETAILED DESCRIPTION OF THE INVENTION

In an embodiment of the present invention, the shortcomings of conventional mechanical structural displacement systems are overcome through the use of a structural displacement sensor configured to de-amplify the effect of structural displacement on a non-mechanical, high-resolution, strain-gauge. As will be described more fully below, such non-mechanical, strain-gauge-type systems are well-suited for measuring structural displacement in large civil structures for example, such as bridges, multi-story buildings, dams, and various concrete structures. In a particular embodiment of the invention, the non-mechanical strain gauge is attached to an arched member, and the arched member secured to a location on the structure. The arched member is configured to de-amplify the strain placed on the strain gauge, thus allowing the strain gauge to measure strain over a larger dynamic range than would be possible if the strain gauge were affixed directly to the structure, or a straight element. By placing the strain gauge at the crown of the arch where the strain is at a minimum, it is possible to use a highly sensitive strain gauge and, depending on the size and shape of the arched member, it is possible to increase the displacement measurement range up to 10 mm or greater.

FIG. 1illustrates a structural displacement sensor100according to an embodiment of the invention. The structural displacement sensor100includes an arched member102, which, in at least one embodiment, comprises a thin strip of pliable material, such as spring steel for example, formed into the shape of an arch. However, it is also envisioned that the arched member102can be made from suitable materials other than spring steel. At each end of the arched member102, there is attached a fastening member104for attaching the structural displacement sensor100to a structure (not shown) under evaluation. In at least one embodiment, the fastening member104is made of spring steel or some other suitable material having similar properties. Also, in particular embodiments of the invention, the fastening member104is flat and somewhat rigid, though it is contemplated that a relatively more flexible material could also be employed. In an alternate embodiment, the fastening member104is curvilinear to allow for attachment to rounded structures, such as columns, rounded beams, and curved walls for example. In a particular embodiment of the invention, the fastening member104has an opening105therein to facilitate the attachment of the sensor100to a concrete or metal structure (not shown), using nuts and bolts (not shown) for example. In another embodiment, the fastening member104can be attached to the structure using other means, such as adhesives.

FIG. 3is a schematic diagram that shows the typical widening and flattening of an arched member202resulting from the movement of an exemplary structure204in the direction indicated by arrows206. As can be seen, the structural displacement is caused by a crack201in the structure204. The solid-line arch shows the arched member202before the structural displacement, while the broken-line arch shows the arched member202after the structural displacement. As stated, the arrows206show the direction of the displacement. The strain at the crown208of the arched member202is less than at any other point on the arched member202, thus providing the de-amplification of strain that permits the increased range of measurement when using a high-resolution strain gauge.

Referring back toFIG. 1, a strain gauge106is affixed to the underside of the arched member102at a crown108of the arched member102. The strain gauge106may be affixed to the arched member102using any suitable means that does not damage the strain gauge, such as adhesives. In at least one embodiment of the invention, the strain gauge106is routed through holes110in the arched member102. Depending on the type of strain gauge106used, wires or optical fibers connect the strain gauge106to external devices (not shown) that provide power or signal processing capabilities. In some embodiments, the signal processing device, includes, but is not limited to, an optical interrogation unit, an electrical interrogation unit, a voltage meter and a current meter, which is coupled to the strain gauge106to measure the amount of strain placed on the strain gauge106.

In one embodiment of the invention, the strain gauge106is an optical fiber, such as a fiber Bragg grating, wherein the fiber Bragg grating (FBG) is coupled to an FBG interrogator (not shown) at one end and it is capable of sending and receiving optical signals to the FBG from one end. When a crack or deformation occurs in a structure having an attached structural displacement sensor100, depending on the nature of the deformation, the arched member102may widen and flatten or, in some instances, compress, thereby placing a mechanical strain on the strain gauge106attached to the crown108of the arched member102. In the case where the strain gauge106is a fiber Bragg grating, the mechanical strain on the optical fiber caused by the structural displacement produces a slight, but detectable, change in the wavelength or frequency of an optical signal propagating through the fiber Bragg grating. The optical signal processing device determines the extent of the wavelength shift, as compared to the wavelength of an optical signal before the structural displacement. Based on the extent of the measured shift in wavelength, the amount of strain on the optical fiber and, therefore, the extent of the structural displacement can be calculated.

When using fiber Bragg gratings as strain gauges, it should be noted that temperature can affect the strain measurements from the strain gauge. These thermally-induced strains can introduce a degree of error into the strain measurements. With respect to the present invention, changes in temperature due to the change of season for example, may cause the results from a structural displacement sensor in an outdoor environment to overstate or understate the actual structural displacement. A particular embodiment of the invention is fashioned to reduce the effects of thermally-induced strain. Referring again toFIG. 1, a second fiber Bragg grating strain gauge112(shown in phantom) may be added to the structural displacement sensor as shown. The second fiber Bragg grating strain gauge112is attached to the top side of the crown108of the arched member102. The first fiber Bragg grating strain gauge106is attached to the bottom side of the crown108of the arched member102. When a structural displacement causes the arched member102to extend or contract, the two fiber Bragg grating strain gauges106,112measure equal strains at any instant, but with opposite signs. But the thermally induced strains in the two sensors are equal. Therefore, by subtracting the strain measurement from of the second fiber Bragg grating strain gauge112from the strain measurement from of the first fiber Bragg grating strain gauge106and dividing the outcome in half, the effect of the thermal strains is removed from the measurement. The resulting figure is the strain due to the structural displacement.

In an alternate embodiment of the invention, the strain gauge106is a thin-film resistor that is connected to a voltage source (not shown) and a measurement device, such as a current meter or voltage meter (not shown). In this embodiment, when a crack is formed in the structure having the displacement sensor100, the displacement causes the arched member102to widen and flatten, which places a mechanical strain on the thin-film resistor at the crown108of the arched member102. The mechanical strain on the thin-film resistor produces a slight, but detectable, change in the resistance of the resistor. The current meter or voltage meter determines the amount of strain on the thin-film resistor and, therefore, the structural displacement, by evaluating the change in resistance.

FIG. 4is a schematic illustration of a strain measurement system300employing a plurality of structural displacement sensors302. In the embodiment shown, the strain measurement system300includes twenty structural displacement sensors302evenly divided between two columns304that are part of, and supporting, a structure306. The structural displacement sensors302are placed on different locations on the columns304. In an embodiment of the invention, the strain gauges (not shown) for each of the structural displacement sensors302is a fiber Bragg grating strain gauge. In a particular embodiment, the strain gauges of the twenty structural displacement sensors302are connected in series and serially multiplexed at the optical signal processing device310, which, in an embodiment of the invention, is also the generator, or source, of the optical signal. The optical signal processing device310could be located in or near the structure306, however, it is also contemplated that the optical signal processing device310could be at a remote location, thus allowing the monitoring of multiple remote structures from a central location. In at least one embodiment, the strain measurement system300includes a fiber Bragg grating accelerometer308to measure vibrations in the structure306. The fiber Bragg grating accelerometer308allows for monitoring of structural displacement related to seismic activity, for example, or from vibrations associated with use, such as might be caused by vehicle traffic on a bridge or elevated roadway.

Referring once again toFIG. 1, the structural displacement sensor100will typically be calibrated before use. For example, when using an optical fiber strain gauge106, the wavelength shift for a given displacement may vary with the size and shape of the arched member102, with the material used to fabricate the arched member102, and with the specific type of fiber Bragg grating used. Similarly, with a thin-film resistor-type strain gauge106, the shape, width, and length of the resistive element, or the material used to make the resistive element may affect the change in resistance for a given structural displacement. Hence, the need to calibrate the structural displacement sensor100.

When calibrating a structural displacement sensor100with a fiber Bragg grating strain gauge106, the calibration process typically calls for subjecting structural displacement sensor100to a displacement of a known distance and measuring the resulting shift in wavelength of an optical signal propagating through the fiber Bragg grating strain gauge106. This process will result in the calculation of a gauge factor, which is defined as the displacement-induced change in the wavelength of the optical signal propagating in the fiber Bragg grating, divided by the displacement distance. In an embodiment in which the strain gauge is a thin-film resistor, the gauge factor may be defined as the displacement-induced change in the resistance of the thin-film resistor, divided by the known displacement distance.

FIG. 5is a graphical illustration of the wavelength shift of an optical signal as measured in an exemplary fiber Bragg grating strain gauge as a function of displacement distance, and shown by line402. Such a graph400could be generated during the calibration process. As can be seen inFIG. 5, the wavelength of the optical signal in the fiber Bragg grating strain gauge is 1527.7 nanometers (nm) when there is no displacement of the structural displacement sensor. At one millimeter (mm) of displacement, the wavelength increases approximately 0.6 nm to 1528.3 nm. At 2 mm of displacement, the wavelength of the optical signal is approximately 1528.9 nm. and at 3 mm of displacement, the wavelength is approximately 1529.6. In this example, a 3 mm displacement causes about a 1.9 nm upward shift in the wavelength of the optical signal.