ES2962882A1 - Device for measuring surface deformation (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Device for measuring surface deformation, for structures or structural elements, comprising at least a first closed loop with a first continuous optical fiber (1) located on a first surface (4.1) of the structure that has an initial end (1.1).) and a final end (1.2) located externally to the first surface (4.1) and means of connection integral with it, and a unit of measurement (6) arranged externally to the structure and formed by a base plate (7) with at least a first optical transceiver (8) capable of allowing the injection of a light pulse through the initial end (1.1) and its reception at the final end (1.2), a programmable digital integrated circuit (FPGA) (9) connected to the first transceiver (8) with a digital logic code (12), a distributor (10) for pulse multiplexing and a pulse counter (11). (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

Device for measuring surface deformation

Technical field of the invention

The present invention corresponds to the technical field of physics and structural engineering, specifically to a device for measuring deformation on surfaces of structures or any other structural element.

Background of the Invention

When we talk about deformation in structures, it refers to the magnitude that is used to describe the elongation (traction) or contraction (compression) that the material of the structure experiences due to possible factors such as the action of an external force, bending. of the structure, the temperature or even possible internal forces that cause residual deformation.

When a deformation exists, it has a non-dimensional value that represents the change in the length of a material with respect to its initial length.

Currently there are several types of gauges and sensors capable of measuring deformation, such as electrical strain gauges that have been used for more than eighty years and are capable of measuring fatigue and carrying out materials testing, for the purposes of productivity and safety.

More than a decade ago, optical fibers began to be used as sensor elements. These have many more advantages as sensors than traditional electrical devices because they present robustness, precision and versatility. One of the great advantages is that they are immune to electromagnetic fields and do not emit them, they resist high temperatures, they are fast and their use does not present danger in explosive or flammable environments.

Multiple sensor systems based on fiber optics are being used and developed, particularly in the measurement of structural deformations.

Depending on the size of the area that includes the sensor, it can be short, long or distributed.

The short gauge ones are similar to electrical strain gauges, since the measurement is carried out in a small area of the structure that can be considered almost punctual, while the long gauge ones simultaneously monitor the entire structure, as the sensor is embedded in it. . Finally, distributed ones allow specific measurements to be made, but within the same fiber distributed over very long distances, considering a combination of the previous two.

However, this type of optical sensors present a great drawback because the cost of the associated electronic equipment, known as an interrogator, is very high.

If what is considered is the method used to carry out the measurement, the existing sensors can be classified, on the one hand, into those that measure the curvature by variation in the intensity of light, which use the intensity dependence of the light transmitted in an optical fiber depending on the radius of curvature of the fiber. This is useful for small bend radii where significant attenuation losses occur in the fibers. In these devices, the interrogation equipment is cheap, but the precision obtained is low.

Another type of sensor is the one that performs measurements based on interferometry. With this method it is possible to obtain very high resolutions based on the interference patterns created by reflection in some part of the structure or optical fiber. The phase difference that is produced by the different transmission in a different path in the fiber produces an interference pattern that is modified when the fiber attached to the structure suffers elongations due to its deformation. In this case the interrogators used are Michelson or Fabry Perot type interferometers.

Likewise, there are multiple methods to carry out measurements that are based on light scattering techniques such as Rayleigh or Brillouin.

Finally, there are methods in which the measurements are based on the reflection that occurs in a diffraction grating that is created inside the fiber structure. This grating known as the Bragg diffraction grating is carved inside the fiber and depending on the separation between the grating marks, it reflects a different wavelength to a greater or lesser extent. Therefore, an interrogator must be available that is capable of distinguishing the intensity of different wavelengths, this being sophisticated and expensive equipment.

In almost all of these methods and devices mentioned, what is intended is a high resolution in the measurement. However, in most applications related to structural health, the element to be monitored is large, so a high resolution is not necessary, so relative resolutions of 10-4 may be sufficient for structures of several meters. .

It is, therefore, necessary to find a device that is capable of measuring this deformation with an adequate precision for the structure to be measured, that is reliable and that does not require complex and expensive measuring equipment. A significant reduction in the cost of the interrogator could allow the implementation of a system in which the interrogator is left connected to the structure, carrying out permanent, rather than punctual, monitoring of it.

Description of the invention

The device for measuring the deformation of surfaces, for structures or structural elements, presented here, comprises at least a first closed loop composed of a first continuous optical fiber located on a first surface of the structure forming a determined path that has a initial end and a final end between which a going branch and a return branch run.

This first fiber has means of attachment integral to the structure on its first surface, in addition its initial and final ends are located outside said first surface such that they are accessible.

The device also has a measurement unit formed by a motherboard comprising at least a first optical transceiver and a programmable digital integrated circuit (FPGA) connected thereto.

The first optical transceiver is connected to the first fiber loop at both the initial and final ends, and is capable of allowing the injection of a light pulse through the initial end and its reception at the final end.

For its part, the FPGA is connected to the first transceiver and implements a digital logic circuit that allows the generation of the electrical signal that is transformed into light pulses that circulate through the fiber loop and the reception of the electrical signal produced by the transceiver. after receiving said light pulses.

The logic circuit further comprises a multiplexing circuit and a received pulse counter, both connected to the FPGA and to the end of the loop.

With the device for measuring surface deformation proposed here, a significant improvement on the state of the art is obtained.

This is because a device is achieved in which the element that acts as a sensor is distributed throughout the entire structure, making it sensitive to variations at any point of it, thus achieving global monitoring at the level. structural with the assurance that no possible weak point has been ignored when it comes to guaranteeing the integrity of the structure, since the entire structure is continuously monitored.

This device uses the same measuring instrument or interrogator formed by a measurement unit that allows continuous and simultaneous measurement of the deformation over a large surface, without focusing on a specific or restricted area, thus providing global information of the same, without the need to use local voltage sensors. It is a very simple electronic device, based on a programmable logic device and a high-speed optical transceiver with an extremely low cost.

The measurement unit of this device uses programmable digital electronics, specifically an FPGA that, in addition to being low cost, can work with frequency transceivers in the MHz range, with the number of pins required being reduced. In addition, the FPGA can be multichannel, so that several closed fiber optic loops can be created, with the same or different path, to perform multiple measurements.

With this device, a methodology implemented for measuring the length of optical fiber is used by measuring the propagation time of a light pulse along it, adapting it for use in measuring the deformation of surfaces. , with the variation that a light pulse recirculation method is applied here. By using optical fibers, the device is insensitive to magnetic, electric or electromagnetic fields, so the presence of any of them will not affect the measurements made. Likewise, it can be used in structures that operate in extreme temperatures, the operating range of the structure being limited only by the temperature that an optical fiber withstands.

Another advantage of this device is that its operation is much more independent of the transmitted signal power than other existing devices, since what is important here is the arrival time so that, for the device to work correctly, it is enough for the signal to be transmitted. pulse and be received at the opposite end. This allows its use both in small structures and in very large structures, since fiber optic communication can be carried out over hundreds of meters without the need for repeaters, it can even be longer depending on the wavelength used and this device. It does not add any limitations in this regard.

Thus, a simple and practical device is achieved that allows the measurement of deformation on any type of surface, without dimensional limitation and achieving an effective measurement of deformation globally and continuously.

Brief description of the drawings

In order to help a better understanding of the characteristics of the invention, in accordance with a preferred example of its practical implementation, a series of drawings are provided as an integral part of said description where, with an illustrative and non-limiting nature, it has been represented the following:

Figure 1.- Shows a perspective view of a device for measuring surface deformation, for a first preferred embodiment of the invention.

Figure 2.- Shows a schematic view of the device for measuring surface deformation, for a first preferred embodiment of the invention.

Figure 3.- Shows a diagram of the digital logic blocks that are implemented within the FPGA of the device for measuring surface deformation, for a first preferred embodiment of the invention.

Figure 4.- Shows a perspective view of a device for measuring surface deformation, for a second preferred embodiment of the invention.

Figure 5.- Shows a perspective view of a device for measuring surface deformation, for a third preferred embodiment of the invention.

Detailed description of a preferred embodiment of the invention

In view of the figures provided, it can be seen how in a first preferred embodiment of the invention, the device for measuring the deformation of surfaces, for structures or structural elements, proposed here, comprises at least a first closed loop composed of a first continuous optical fiber (1) located on a first surface (4.1) of the structure forming a determined route that has an initial end (1.1) and a final end (1.2) between which a forward branch and a return branch.

In this first preferred embodiment of the invention, a structure is considered, which in this case is the blade (5) of a windmill. Thus, the first closed loop presents the first fiber (1) forming a path on a first surface (4.1) of the blade (5), as shown in Figure 1.

Said first fiber (1) has means of attachment integral to the structure on the first surface (4.1) thereof and both initial and final ends (1.1, 1.2) are located outside said first surface (4.1) such that they are accessible.

In this first preferred embodiment of the invention, the means for integrally joining the first fiber (1) to the structure are formed by integrating the fiber into the material for forming the structure, that is, the first fiber ( 1) is embedded in the resin that makes up the structure of the blade (5), so that when this resin solidifies the first fiber (1) is integrated into the body of the blade (5). In other embodiments, these joining means can be formed by an adhesive that fixes the fiber on the first surface or by screwing a structure for holding the fiber onto the structure, for example.

The device also has a measurement unit (6) formed by a base plate (7) that comprises at least one first optical transceiver (8) connected to the first fiber (1) of the first loop at both initial and final ends (1.1, 1.2) and, a programmable digital integrated circuit (FPGA) (9) connected to this first transceiver (8), as can be seen in Figure 2.

The first transceiver (8) is capable of allowing the injection of a light pulse through the initial end (1.1) of the first fiber (1) and its reception at the final end (1.2), while the FPGA (9 ) implements a digital logic circuit (12) that allows the generation of light pulses that circulate through the fiber loop and the reception of the electrical signal produced by the transceiver after receiving said light pulses.

As shown in Figure 3, to measure the optical length of the fiber, the measurement unit (6) also includes a multiplexing circuit (10) and a counter (11) of received pulses that allows knowing the number of pulses that have traveled through the fiber optic loop in a given time, and which provides the measurement that is intended to be obtained from the system. Both the multiplexing circuit and the counter are connected to the FPGA (9) and the final end (1.2) of the fiber of the first loop.

For its part, the multiplexing circuit allows the selection of the output signal to the transceiver between two possible sources. During the measurement or integration regime, the multiplexing circuit is configured to direct the signal received from the transceiver back to it, producing a self-sustained oscillation of pulses circulating through the fiber optic loop. During the start-up regime, the multiplexing circuit is configured to send an electrical signal suitable for the initiation of self-sustained oscillation to the transceiver. In its simplest implementation, this start signal is the sending of two logic values 1 and 0, each for a time equivalent to half the propagation time of the pulse in the entire loop.

In this way, the injection of a pulse of light circulates through the fiber to be measured and the signal received at the receiver is fed back to the transmitter, thus creating a self-sustained oscillation. Thus, the pulses received at the receiver are injected on the one hand back into the first loop and on the other hand they are taken to a counter (11) which is the one that increments with each pulse received.

Having precise knowledge of the frequency of the self-sustained oscillation from the counting of a number of pulses during the appropriate integration period, the length of the measured fiber can therefore be estimated from the period of the oscillation and a Once the other terms have been characterized, the fiber propagation delay can be calculated, which is one of the terms that contribute to said period of this oscillation. A fiber deformation is visualized as a change in this propagation time and, therefore, the existence of a change in length is determined.

As shown in Figure 1, the measurement unit (6) is arranged externally to the structure and attached to it by means of fastening means.

In this preferred embodiment of the invention, the device comprises a housing (13) to protect the measurement unit (6). In the event that it is expected that the temperatures outside are going to be extreme, and therefore it is decided to implement a system for active control of the operating temperature of the measurement unit, the casing (13) meets thermal insulation conditions. .

Likewise, in this first preferred embodiment of the invention, the measurement unit (6) comprises independent power means, formed in this case by a battery, and wireless data transmission means.

In a second preferred embodiment, which can be seen in Figure 4, the device comprises a second closed loop parallel to the first loop and composed of a second continuous optical fiber (2) located on a second surface (4.2) of the structure, opposite to the first surface (4.1). This second fiber (2) has means for connecting it to the structure, forming a specific path that has an initial end (2.1) and a final end (2.2) connected to a second optical transceiver of the measurement unit (6).

By having in this case two loops on both first and second opposite surfaces (4.1, 4.2), each of them measures the deformations on one of the surfaces, these deformations corresponding with expansions and contractions respectively, so that, for the same deformation of the blade (5), one of the loops measures the elongations due to dilations while the other measures the compressions caused by contractions. Furthermore, by having two loops it is possible to deduce the direction of curvature of the blade (5) from the relative measurement of both. The fact of having two channels on opposite surfaces of the blade allows obtaining a differential measurement that is truly related to the bending of the blade, and becomes more independent of possible disturbances that affect both surfaces equally, such as that due to thermal expansion of the blade assembly.

In this second preferred embodiment of the invention, the FPGA (9) has at least two channels for the separate measurement of elongation and compression.

In the case in which it is necessary to monitor the variable temperature independently of the variable deformation, as occurs in this second preferred embodiment of the invention, the device comprises a third loop parallel to the first loop and composed of a third fiber ( 3) continuous optics that have means of connection not secured to the structure on one of the surfaces, that is, it is simply superimposed on the surface or embedded in it in a non-joint manner, for measuring the effect of temperature on the fiber optical fibers independently of other deformations of mechanical origin to which the other optical fibers are subjected. In this second preferred embodiment, this third fiber (3) is located on the first surface (4.1), as shown in Figure 4 and has initial and final ends (3.1, 3.2) connected to a third optical transceiver of the unit of measurement (6).

This third loop, shown in Figure 4, allows for easier calibration of temperature-dependent fiber variations. In this case, furthermore, the device comprises temperature sensors (not represented in the Figures) connected to the FPGA and the structure to improve calibration aspects.

As can be seen in Figure 5, in a third preferred embodiment of the invention, the device comprises an additional loop formed by an additional fiber (15) integrally attached to the first surface (4.1) of the structure, and where the FPGA (9) comprises an additional transceiver for the connection of said additional fiber (15). In this case, by measuring the differential propagation times (and therefore lengths) of both fiber loops, a measurement of the bending of the first surface (4.1) is obtained directly.

In other embodiments, it may have more than one additional loop or these may be located on both surfaces and the same number of additional transceivers for the corresponding connection of each additional loop to one of them.

Likewise, in other embodiments, at least one of the additional loops can be formed by an additional fiber (15) that has a planar path equal to that of the first fiber (1) and is located at a depth different from that of said first fiber (1), in the order of magnitude of hundreds of microns to a few millimeters.

Claims (1)

1- Device for measuring the deformation of surfaces, for structures or structural elements, characterized in that it includes - at least a first closed loop composed of a first continuous optical fiber (1) located on a first surface (4.1) of the structure forming a determined path that has an initial end (1.1) and a final end (1.2) between which a going branch and a return branch run, where the first fiber (1) has means of connection integral with the structure on its first surface (4.1) and both initial and final ends (1.1, 1.2) are located exterior shape to said first surface (4.1) such that they are accessible, and; - a measurement unit (6) formed by a base plate (7) comprising at least one first optical transceiver (8) connected to the first loop at both initial and final ends (1.1, 1.2), capable of allowing the injection of a light pulse through the initial end (1.1) and its reception at the final end (1.2), a programmable digital integrated circuit (FPGA) (9) connected to the first transceiver (8) and implementing a digital logic circuit (12) which allows the generation of an electrical signal that is transformed into light pulses and the reception of the electrical signal produced by the transceiver, a multiplexing circuit (10) and a counter (11) of received pulses, where the distributor (10) and the counter (11) are connected to the FPGA (9) and to the end end (1.2) of the first fiber (1), to measure the optical length of the first fiber (1). 2- Device according to claim 1, comprising a second closed loop parallel to the first loop and composed of a second continuous optical fiber (2) located on a second surface (4.2) of the structure, opposite to the first surface (4.1), which has means of connection integral with the structure forming a determined path that has an initial end (2.1) and a final end (2.2) connected to a second optical transceiver of the measurement unit (6). 3- Device according to any of claims 1 or 2, which comprises a third loop parallel to the first loop and composed of a third continuous optical fiber (3) that has means of attachment not secured to the structure on one of its surfaces. for measuring deformations due to temperature and initial and final ends (3.1, 3.2) connected to a third optical transceiver of the measurement unit (6). 4- Device according to claim 3, comprising temperature sensors connected to the FPGA (9) and/or to the structure. 5- Device according to any of the preceding claims, which comprises one or more additional loops similar to the first loop, each of them formed by an additional fiber (15) integrally attached to the first and/or the second surface of the structure, where The FPGA (9) comprises the same number of additional transceivers for the corresponding connection of each of them with one of the additional fibers (15). 6- Device according to claim 5, where at least one additional loop is formed by an additional fiber (15) that has a planar path equal to that of the first fiber (1) and is located at a depth different from that of said first fiber. (1). 7- Device according to any of the previous claims, wherein the means of attachment to the structure are formed by integrating the fiber into the structure-forming material. 8- Device according to any of claims 1 to 6, where the means of attachment to the structure are formed by means of fixing thereto. 9- Device according to any of the previous claims, where the measurement unit (6) comprises independent power means. 10- Device according to any of the previous claims, wherein the measurement unit (6) comprises wireless data transmission means. 11- Device according to any of the previous claims, where the measurement unit (6) is arranged externally to the structure and attached to it by means of fastening means. 12- Device according to claim 11, which comprises a housing (13) to protect the measurement unit (6).