WO2010053392A1

WO2010053392A1 - Transducer for measuring vertical displacements

Info

Publication number: WO2010053392A1
Application number: PCT/PT2009/000058
Authority: WO
Inventors: Joaquim De Azevedo Figueiras; Carlos Manuel DA SILVA FÉLIX; Carlos Filipe Guedes Rodrigues
Original assignee: Universidade Do Porto; Instituto Politécnico Do Porto
Priority date: 2008-11-05
Filing date: 2009-11-05
Publication date: 2010-05-14
Also published as: WO2010053392A4; PT104250B; PT104250A

Abstract

The present invention relates to a transducer for measuring vertical displacement in civil engineering structures, such as bridges and overpasses. For that purpose, the transducer carries out hydrostatic levelling over the structure being observed, from a fixed reference position, thus enabling the relative vertical displacement over time to be determined at the various points equipped with said instrument. This transducer is based on the principle of Archimedes and is formed by an outer container (1) containing a liquid in a hydrostatic quasi-equilibrium (2) with the additional hydraulic circuit (3), said container representing the altimetric position of said body in relation to the remaining circuit. A partially immersed mass (6) is coupled by means of a load cell (4) provided with extensiometric sensors (5), allowing the liquid level to be transformed into a force (apparent weight of the body (6)) that can be measured by the load cell (4) in the signal from the extensiometric sensors.

Description

DESCRIPTION

"TRANSDUCER FOR MEASURING VERTICAL DISPLACEMENTS"

BACKGROUND OF THE INVENTION

Automatic observation of the behavior of viaducts and bridges is a growing theme, highlighting numerous potentialities and remarkable interest in any process of assessment, maintenance and structural reinforcement. Information from the observation of the behavior of these works of art is, in different contexts, a major element in the control of the quality and safety of the building and an important indicator in the management and maintenance of the infrastructure throughout its period. Shelf life.

Within the proposed scope, the measurement of relative vertical displacements in civil engineering structures - arrows, namely in the bridge deck, is one of the most representative quantities of the behavior of this structure and a reference quantity in the evaluation of its safety and integrity. However, the measurement of these arrows is generally hampered, in the specific case of bridges, by the impossibility of establishing physical references, particularly in the ground, to allow their autonomous and permanent measurement. Different techniques of arrow measurement have been tried and adopted in real structures allowing, depending on the structure typology and the terrain morphology, different levels of autonomy and precision. Among the techniques used for measuring arrows, the ones that contemplate, in the simplest cases, a vertical rigid or flexible plumbing to the ground, and corresponding measurement of the relative displacement next to the latter, in the most complex cases, by application techniques. using geodetic leveling or three-dimensional positioning by means of optical level or theodolite measurement systems respectively, until, in the most recent cases, they use the Global Positioning System - GPS. However, for the same purpose, one of the methods that has shown the most advantages and the greatest success in bridge monitoring, whenever physical reference to the ground is impossible, is the measurement of these arrows based on liquid level systems.

It is precisely through the use of a liquid level system that acts the transducer object of this patent. It is thus a liquid level measuring instrument which, in hydrostatic equilibrium along the structure and as a result of the methodology explained below, allows its relationship with the relative vertical displacements of the structure.

It should be noted that the exposed technique and use Measurement of liquid levels in the measurement of structure arrows is not new in itself and, over the last few years, different measuring instruments such as piezoresistive sensors or pressure / liquid level transducers have been tried for the same purpose. mechanical or electrical in nature, often not specifically developed for this application. In the foregoing, and seeking to refer to the use of liquid level systems for measuring vertical displacements in works of art, the following are some of the closest records referenced in the available databases:

- METHOD FOR MEASURING LEVEL OF LARGE-SCALED STRUCTURE Patent Number: JP63120213 of November 7, 1986

- DISPLACEMENT MEASURING DEVICE

Patent Number: JP56162009 May 19, 1980

- METHOD A D DEVICE FOR MEASURING DISPLACEMENT FOR EVALUATING LOAD RESISTANCE OF BRIDGE

Patent Number: R20010084318 of February 24, 2000

- DEVICE FOR MEASURING THE LEVEL OF A FLUID IN A CONTAINER Patent Number: EP0604670 of 19 December 1992

- MEASURING DEVICE OF SHAPE OF BRIDGE

Patent Number: JP10227633 of February 13, 1997 Throughout this document, we present the main innovative aspects of the object of this patent, called displacement transducer for measuring arrows in works of art, as well as its main advantages when compared to traditional equipment.

SUMMARY OF THE INVENTION AND ADVANTAGES

The object of this patent, hereinafter referred to as the "vertical displacement measuring transducer", is for the measurement of liquid levels associated with measurement of arrows in civil engineering works on transient or permanent structural monitoring systems.

For this purpose, the transducer uses a hydrostatic leveling that runs through the structure under observation and a fixed reference position, enabling the relative vertical position of the various instrumented points to be determined over time by measuring the height. of the column of liquid at each of these same points.

0 liquid level sensor shown here is designed and framed in the foregoing context, it takes advantage of a principle more than 2000 years, widely ^'known Archimedes principle, and translates, as a result of its internal architecture, directly, perfectly linear and highly sensitive, the variation of a hydrostatic level (namely of water or possibly of a comparable fluid) in the signal of extensometric sensors (of any nature).

It is a transducer that makes it possible to compensate, through its architecture, all the effects of temperature variation on its internal components. This leads to a sensor with high temporal and meteorological stability, particularly suitable for permanent structural monitoring applications.

It has a measurement field that can be adjusted to the specificity of the structure to be monitored and a sensitivity of about 0.05% of that same field of measurement when associated with a hydraulic circuit in the measurement of arrows in civil engineering works. These two characteristics are essential in view of the range of quantities to be measured and are achieved with expected changes in the dimensions of some critical transducer elements.

It is a versatile transducer in the nature and technology of its signal which, depending on the nature of the installed strain gauges, may be associated with both electrical based and optical base monitoring systems. When equipped with optical base strain gauges, it has the unquestionable advantages of monitoring arrows in civil engineering works. The most emerging optical technology, namely the fact that the optical signal emitted by them can be autonomously acquired, with high precision and stability, are insensitive to electromagnetic fields, may be associated with multiplexing techniques and signal loss over the long term. beam path can be compensated by increasing the power of the source.

BRIEF DESCRIPTION OF DRAWINGS

The following description is based on the accompanying drawings which without any limitation represent:

Figure 1 - Transducer for measuring vertical displacements: measuring mechanism and general internal constitution - Section A-A;

Figure 2 - Transducer for measuring vertical displacements: measuring mechanism and general internal constitution - Section B-B;

Figure 3 - Detail of the base of the transducer body for attachment to the structure - Cut C-C;

Figure 4 - Detail of the removable cover of the transducer body;

Figure 5 - Suspension force receiving suspended body: front view;

Figure 6 - Suspension force receiving suspended body: lateral view;

Figure 7 - Suspension force receiving suspended body: top view; Figure 8 - Load cell: front view;

Figure 9 - Load cell: lateral view;

Figure 10 - Load cell: D-D section;

Figure 11 - Load cell: section E-E;

Figure 12 - Lower element for immersible body suspension: front view;

Figure 13 - Lower element for immersible body suspension: side view;

Figure 14 - Upper element for immersible body suspension: front view;

Figure 15 - Upper element for immersible body suspension: side view;

Figure 16 - Detail of the support conditions of the immersible body in the load cell;

Figure 17 - Schematic principle of the method of measuring arrows using liquid levels;

Figure 18 - Diagram of forces on the body partially immersed in a liquid;

Figure 19 - Load cell: single-line, deformed diagram, bending moment diagram and axial stress diagram.

DETAILED DESCRIPTION OF THE INVENTION

Operation principle

Based on Figure 17, the measurement of relative vertical displacements (arrows) in civil engineering works is performed using the principle of the communicating vessels (Stevin's Law) to a circuit that, through the structure under analysis, carefully contemplates all the sections to be observed. The free surface of this circuit, filled with a liquid in a state of hydrostatic equilibrium (static), will be at the same absolute altitude throughout its length, thus establishing a possible reference for the assessment of relative vertical displacements. In turn, the vertical deformation of the structure will lead to changes in the altimetric position of the structure and consequently to changes in the height profile of the liquid column along the solidary circuit therewith. Point-to-point monitoring of the deformed structure and the circuit itself is accomplished by measuring variations in the relative height of the liquid column at a set of points of interest. The liquid height at a reference point, usually at a negligible vertical displacement point, for example on a frame support, is used as a reference for determining other relative displacements.

Thus, safeguarding that the system is in hydrostatic equilibrium as a result of a slow deformation of the structure, the relationship between the structure arrow and the apparent liquid height is given at a generic point A and at a time point ti by the variation of the height of liquid at that point A deducted from the change in height of liquid at that time in the reference section, as follows: general formulation (see Figure 17):

Where is the arrow at time t (vertical displacement variation); _the liquid height at the point

A at time t1 is the liquid height at point A at the initial time (t); ¾ 'the height of liquid in the (fixed) reference section at time t ±; and "0 o and the liquid height in the (fixed) reference section at the initial time (t ₀ ).

In the above context, it is notorious that, as a transducer element of the vertical displacement at each of the monitored points, a sensor that assumes, independently and permanently, with high accuracy and stability, the respective height of the liquid column is of paramount importance. .

Inherent in this method, the circuit dimensions as well as the characteristics of the liquid used in the circuit will condition the stability and the response of the system. Most of the time, water is used to fill the hydraulic circuit. It has an appropriate density and viscosity for this application and is extremely easy to obtain. However, problems related to water volatility, the possibility that it contains diluted air bubbles, the development of microorganisms, as well as the variation their density with temperature should not be ignored and may even justify combining additives or even replacing them with a more suitable liquid.

Despite the intrinsic presence of some difficult to quantify errors mainly due to environmental changes (temperature and atmospheric pressure) acting differently along the hydraulic circuit or even due to the presence of air bubbles inside the hydraulic circuit, it is possible to take measures to minimize and control it. Following a careful design and installation procedure of the hydraulic system under normal use, the use of liquid levels has been shown to be a stable and accurate (accuracy greater than 0.2mm) measurement of structural arrows at steady state with easy, low cost and efficient instrumentation.

The target vertical displacement transducer of this invention, developed for the purpose of measuring arrows in civil engineering works, thus bases its operation on measuring the height of the liquid column at a given point in a hydraulic circuit. . The structure of the present relative vertical displacement transducer is shown in Figure 1 and Figure 2.

The basic principle for obtaining the height of liquid is to measure the apparent weight of a body of previously studied geometry and properties (6), partially immersed in a liquid and whose immersed volume is associated with its relative liquid height (position (2)). Thus, the Archimedes Principle is used to deduce that, being a solid body, partially immersed in a fluid, by the action of an upward pushing force of hydrostatic origin (see Figure 18), the value of its weight, within that liquid is apparently less than its weight measured in air (resulting from the forces? and 1}. The difference between the value of the actual weight (P) measured in air and the impulse exerted by the liquid ('), corresponds to apparent body weight (¾?) ·

Considering also the body under analysis, immobile, slightly denser than the liquid

Patrpe and created from the linear translation of its base, it can be demonstrated that there is a direct proportionality relationship between the liquid height {*) and the respective immersed volume (¾m), therefore, by the above, a variation of the height of net corresponds to a linear variation of the thrust force and consequently of its apparent weight. Uniquely and linearly, the variation in liquid height, *, with the apparent weight of this partially immersed body can be related to this transducer. expression: m <3 ~ Akgp

(iii)

Where Vi is the immersed volume of the cylindrical or prismatic body; A is the base area of the body; * is the immersed height of the body; ^m is the mass of the body; ã is the acceleration of gravity and P is the density of the liquid in which the body is immersed.

Taking advantage of the above property, the present transducer consists of a watertight container (1) connected to the respective hydraulic circuit (3) by means of a tubular nozzle (9) near the base (8). This container (1) comprises inside it the free surface of the liquid column (2) as well as suspended through its upper cover (18) the partially immersed rigid cylindrical body (6) and a load cell (4) which It will be possible to measure, over time, through a pair of extensometric sensors (5) the respective apparent weight of the different body (6).

Given the desired stability and sensitivity, coupled with the small variations in force that it is intended to measure, a force transducer, load cell (4) has been designed with innovative features. A U-geometry transducer element was then developed, receiving at its ends the apparent weight of the rigid coupled body. The acting apparent weight (external force in the cell naturally deforms this element generating tensions and mechanical deformations. In this particular case, in the case of an isostatic part, that is, with an internal stress distribution dependent exclusively on static equilibrium conditions, the theoretical mechanical behavior of said part can be characterized. The schematic structure of the present load cell as well as the internal forces installed therein are shown in Figure 19 and are given in the central region by:

«= It = P _ep .a (iv)

Where is it ^? and ¾ correspond to the forces installed on the part, namely axial stress and bending moment, respectively; ¾ is the apparent weight of the body suspended in the load cell (acting external force) and 'the respective arm of the load cell.

Given the internal distribution of efforts, one. A pair of strain gauges, arranged in opposite fibers of the thinner central vertical zone (see Figure 9) subjected to composite circular flexion, form the basis of this sensor and lead to an element which, in addition to its high sensitivity, has high temporal and meteorological stability. , making it particularly suitable for permanent structural monitoring applications. Generally, the mechanical strain, «, is numerically given by the sum of the axial strain portion, * m, and the bending strain portion, * m, given

Where R is the modulus of elasticity of the material, ^A is the cross-sectional area; 'is the moment of inertia of the cross section the local coordinate of the part perpendicular to its axis.

However, in addition to mechanical deformations, even taking into account apparent deformations of thermal origin. These apparent deformations do not depend on the stresses installed, but are the result of variations in environmental conditions, namely temperature variations, which also lead to changes in the dimensions of the workpiece. With the load cell free to deform due to temperature variations, AT _f these variations of apparent deformation are given by:

(i)

Where is the corresponding coefficient of thermal expansion of the material and is the temperature variation.

These two strain components act on the present transducer simultaneously and potentially vary over time. The total strain, ½, measured by a strain gauge is given, as shown above, by the sum of the said portions and corresponds to:

Given the existence of a pair of strain gauges (5) installed on opposite faces of the element subjected to composite circular flexion, the possibility of compensating for the effects of thermal nature by applying the decomposition of the respective measured extensions to their identical axial portion is shown here. in the two opposite fibers, and in the flexural portion, symmetrical in those same fibers. We then consider the strain measured at each strain gauge given by:

From the algebraic combination of the two previous relations, it is proved that:

C ^ -A ^ _t) ^and t ^^ p ^ -A ^ £ _t _i) ^and _M¾pS? . (A _{) t} -Aeâ,) 'à i _x )

By static equilibrium, it is deduced, therefore, that the amount of deformation resulting from the differential flexion of the workpiece varies exclusively and proportionally with the installed weight, thus, in the foregoing, linearly with the respective liquid height. , *. In this way and isolating this effect, it becomes possible to compensate the above disturbing effects and there is a direct proportionality between the water height of interest and the variation of the difference in measured lengths. A convenient calibration of each sensor makes it possible to experimentally find the proportionality constant that best fits the relationship between the vertical displacement and the respective measured extensions, or rather the vertical displacement and the signal from the weighting of the respective extensometers.

It is also noteworthy that this load cell, besides having a geometry and dimensions adjusted to its function and the desired sensitivity and field of measurement (7), presents perfectly defined boundary conditions (12/13 and 14/15) that allow a clear and stable characterization of the internal stresses installed. With the proposed conditions and based on the detailed details and the materials used less than the elastic material proportionality limit extension) it is also possible to guarantee the absence of nonlinearity phenomena related to the state of deformation of the part (geometric and material nonlinearity). ). The fact that all moving parts of the transducer are suspended from the cover,. No contact with the transducer body ensures no disturbing frictional forces.

Transducer Constitution

Given the above operating principle The measurement of relative vertical displacements in structures based on liquid levels is made by means of a mechanical device (transducer), all of which is made of stainless steel with adequate mechanical properties and durability. contained in Figures 1 to 16.

A cylindrical or prismatic outer container (1), connected to the reference hydraulic circuit (3) by a mouth (9) of similar diameter to the circuit itself, internally contains its fluid in imminent hydrostatic equilibrium. Access to the interior of this outer container (1) is by means of a removable lid (18) rigidly secured to the main body using a sequence of securing screws (20). Under service conditions, it is within the container (1) that the free surface of the liquid column (2) of the respective hydraulic system is within a pre-defined measuring range (7). Stainless steel also ensures the attachment of all internal mechanical components by means of a central fastener (17) located on the top cover (18).

A partially immersed body (6), i.e. the suspended mass subjected to an eminently variable thrust and whose apparent weight is to be measured, is coupled to a load cell (4) and suspended from the lid. top (18) of the outer body (1) of the transducer. This element (6) consists of a cylindrical surface, closed at the tops and as a whole slightly denser than the liquid.

The driving force acting on this element is directly proportional to the height of liquid installed inside the container. Its dimensions are defined so that, on the one hand, it covers the desired field of measurement (7) - height, and on the other hand, it ensures that the pushing force, proportional to the planar area of this element, is consistent with the sensitivity. intended. It is a target element that can be adapted to either the field of measurement or the sensitivity of the sensor. A conical shape in the base further ensures the elimination of unwanted air bubbles in contact with the surface of this body.

The load cell (4), specifically developed for this purpose, takes advantage of a studied geometry in order to allow a high sensitivity and stability in measuring the apparent weight of the suspended body. This element has a "U" geometry, with the points of application of the forces (12/13 and 14/15) at their ends and subjected in the central region to compound circular bending forces. In addition to Moreover, as a result of a combination of two thicknesses, it is a high rigidity element, thus contributing to a negligible deviation resulting from the consideration of the suspended body as integrally with the structure over the entire measuring range of the sensor. A pair of extensometric sensors (5), located on opposite sides of the central region of the load cell (4) (zone subjected to compound circular flexion), gives it greater accuracy in the measurement and allows the compensation of temperature effects. be performed either numerically or metrologically.

The use of a pair of strain gauges (5) installed on opposite sides of the thinner central load cell (4) subjected to compound circular flexion, in turn, isolates the flexing effect, leading to the total elimination of deformation due to axial stress installed on the part and the effects associated with temperature variation. Convenient treatment of the signal from strain gauges, compensating for all thermal effects, leads to a direct proportionality relationship between the installed liquid height and the respective weighting of the measured signals. Also noteworthy is the fact that it is a versatile transducer in the nature and technology of its signal, which, depending on the nature of the installed strain gauges (5), may be associated with both electric-based and optical-based monitoring systems. . The connections between the load cell (4) and the suspended body (6) and between the load cell (4) and the transducer cover (18) are punctually connected via a conical system. (12/13 and 14/15) on both the load cell (4) and its supports (17 and 10). This detail contributes to a repeatable and stable positioning of the load application points in the load cell. A pair of transverse dowels (11 and 16) further guarantee the integrity of the system against sudden movements, for example resulting from system installation.

The connection to the hydraulic circuit (3) has adequate architecture and characteristics which, in hydrostatic equilibrium, allows the leveling of all points of the lower mouth (9) by means of commercial stainless steel hydraulic pipe fittings.

A fastening system to the rigid base (8) of the transducer by means of a three-point screw arrangement (22) shall ensure rigid attachment of the transducer to the frame and allow for vertical adjustment of the sensor body. The verticality of the transducer will ensure no physical contact between the moving elements of the transducer, ensuring no friction and resulting deviations. An air inlet valve (21) at the top of the transducer allows, when open, the air pressure inside the transducer to be identical to the local atmospheric pressure.

Two mechanical connectors (19) (optical or electrical depending on the nature of the adopted strain gauges) allow the connection of this transducer to a sensor network and its acquisition equipment.

Lisbon, 5 November

Claims

1. Transducer for measuring relative vertical displacements in civil engineering works using hydrostatic leveling which connects the points of the structure under observation to enable the measurement of relative vertical displacement at the various points concerned by means of a direct relationship. and linear between the relative variation in the height of the liquid and the relative vertical displacement at those points, characterized in that it has a cylindrical or prismatic outer container (1) in connection with the remaining hydraulic circuit (3) in imminent hydrostatic equilibrium, which container holds internally the liquid used for leveling and its free surface (2), a rigid or prismatic body (6) partially immersed in that liquid, subjected to a pushing force whose magnitude is proportional to the height of the liquid inside the container (1) , and means for measuring the apparent weight of that body (6) by means of a load cell (4) provided with a pair of strain gauges (5) located on opposite faces of the central region of the load cell (4).

Vertical displacement transducer according to Claim 1, characterized in that the load cell (4) indirectly measures the height of the liquid by measuring the apparent body weight (6) with a sensitivity of better than 0,05% FS. (7) with compensation of thermal effects on the sensor element.

Transducer for measuring vertical displacements according to the preceding claims, characterized in that the load cell (4) suspends the partially immersed rigid body (6) and transduces the variation of the liquid level (2) by means of pair of strain gauges (5),

Transducer for measuring vertical displacements according to the preceding claims, characterized in that the nature of its signal is a function of the nature of the strain gauges (5) used and may be associated with either electrical monitoring systems or optical base systems.

Transducer for measuring vertical displacements according to the preceding claims, characterized in that said load cell (4) has high stability and for that purpose has a "U" geometry with a combination of thicknesses which gives it a high degree of stability. rigidity and sensitivity, with the points of application of the forces (12/13 and 14/15) at their ends and subjected in the central region to composite circular flexion efforts.

Transducer for measuring vertical displacements according to the preceding claims, characterized in that the connections between the load cell (4) and the suspended body (6), either between the load cell (4) and the transducer cover (18), are punctually realized by means of a conical locking system (12/13 and 14/15 ) in both the load cell (4) and the respective supports (17 and 10), which contributes to a repeatable and stable positioning of the load application points in the load cell.

Transducer for measuring vertical displacements according to the preceding claims, characterized in that their geometry, boundary conditions, sensing elements and negligible friction between their components result in better sensitivity than 0,05% FS and temporal stability. allowing the monitoring of the measured quantities over time with self-compensating thermal effects.

Transducer for measuring vertical displacements according to the preceding claims, characterized in that it is associated with temporary or permanent structural monitoring systems and manual or automatic acquisition systems.

Lisbon, 5th November 2009