WO2003016891A2

WO2003016891A2 - A sensor and a method for measuring static and dynamic micro-deformations

Info

Publication number: WO2003016891A2
Application number: PCT/IB2002/003175
Authority: WO
Inventors: Luciano Lanotte; Roberto Germano; Amleto D'agostino; Vincenzo Iannotti; Giovanni Ausanio
Original assignee: PROMETE Srl; Istituto Nazionale per la Fisica della Materia INFM CNR
Current assignee: PROMETE Srl; Istituto Nazionale per la Fisica della Materia INFM CNR
Priority date: 2001-08-09
Filing date: 2002-08-08
Publication date: 2003-02-27
Anticipated expiration: 2004-02-09
Also published as: ITPD20010204A1; AU2002356000A1; WO2003016891A3

Abstract

A magnetoelastic sensor (1) for detecting static and dynamic micro-deformations is described and comprises a magnetoelastic ferroomagnetic element (11), an excitation coil (6) for sending an excitation signal to the ferromagnetic element (11), and a detection coil (7) for detecting a first signal emitted by the ferromagnetic element (11) as a consequence of the excitation signal. A sensor of this type is improved by the provision of a permanent magnet element (21) to be connected firmly to a body, the static and dynamic deformations of which are to be measured. A sensor of this type is also improved by the provision of a sheathing (12) of resilient material for the ferromagnetic element (11), fitted in a manner such that the ferromagnetic element (11) is at least partially embedded in the sheathing (12). A method of measuring static and dynamic micro-deformations is also described.

Description

A sensor and a method for measuring static and dynamic micro-deformations

The present invention relates to a sensor for detecting static and dynamic micro-deformations, including mechanical vibrations and, in particular, to a magnetoelastic sensor according to the preamble to Claim 1 or 2. The invention also relates to a method of measuring the above-mentioned deformations .

In the technical field of building and civil engineering, the need constantly to monitor in a non-destructive manner both static and dynamic deformations due to the stresses to which structures such as buildings, bridges, dams, etc. are subject is known. A similar need is also known in the field of the construction of transport means, in which it is necessary to know the magnitude of the vibrations to which predetermined parts of vehicles such as aeroplanes, trains, or movement means are subject.

In order to perform measurements of deformations, so-called magnetoelastic sensors, which use a ferromagnetic material, are used, amongst others; in these sensors, a variation in the internal stresses due to a mechanical deformation leads to a variation in the magnetization of the material, thus generating a magnetic flux owing to the so-called inverse magnetostrictive effect. By detecting the changes in magnetic flux, it is therefore possible to determine the deformations undergone by the material, by means of known physical laws .

It is known, from the article "Direct magnetostriction and magnetoelastic wave amplitude to measure a linear displacement" by R. Germano et al . , which appeared in "Sensors and Actuators" 81, 2000, pages 134-136, to form resonant magnetoelastic-wave sensors based on a core constituted by an amorphous ferromagnetic strip in which a magnetic and elastic wave is excited at the acoustic resonance frequency. The amplitude of the magnetic component of the wave (measured as the e.m.f. induced in a detection coil) can be correlated, by means of well-established models, with the stresses applied to the ferromagnetic strip.

This type of sensor has advantages in terms of sensitivity and speed in comparison with strain gauges, laser velocimeters, and Hall probes for the measurement of static and dynamic deformations transduced by direct contact of the sensor with the body subject to deformation.

However, the magnetoelastic sensors provided by the prior art are quite inaccurate for the measurement of dynamic deformations and, moreover, their response to the stresses may vary over time, compromising the soundness and reliability of the measurements taken.

A principal object of the present invention is to provide a sensor for detecting static and dynamic micro-deformations which is extremely versatile and can be optimized according to the type of measurement to be taken, and which is very sensitive, and at the same time inexpensive.

A further object is to provide a sensor in which the response to stresses is constant over time and is therefore reliable for the monitoring of buildings over long periods .

Yet a further object is that of providing an accelerometric sensor for measuring the amplitude and the frequency of vibrations in structures in the civil engineering and aeronautical fields. These and other objects which will become clear from the following description are achieved by the invention by means of a magnetoelastic sensor and a method of measuring static and dynamic micro-deformations according to the appended claims.

The characteristics and the advantages of the invention will become clearer from the following detailed description, given by way of non-limiting example with reference to the appended drawings, in which:

Figure 1 is a schematic, perspective view of a sensor formed in accordance with the present invention,

Figure 2 is a schematic, sectioned, side elevational view of the sensor of Figure 1, and

Figures 3a and 3b are schematic, sectioned, side elevational views of variants of the sensor of Figure 1.

With reference to the drawings mentioned, a magnetoelastic sensor for detecting static and dynamic micro-deformations, formed in accordance with the present invention, is generally indicated 1.

The sensor 1 comprises a box-like casing 2 including a hollow tubular body 3 and two opposed walls 5a, 5b fixed to two opposite ends 3a, 3b of the tubular body 3. The boxlike casing 2 is made of plastics material, such as, for example, Nylon or PVC.

A first coil and a second coil 6, 7 are arranged a predetermined distance apart, coaxially with the tubular body 3, inside the box-like casing 2. The first, excitation coil 6 is connected to a generator 8 of electrical signals of selectable amplitude and frequency, and the second, detection coil 7 is connected to amplification and processing means 9, suitable for amplifying and processing a signal detected by the coil 7, and to display means 10, for displaying the signal thus processed.

A magnet 20 is incorporated within the cylindrical wall 3c defined by the tubular body 3 for generating a possible biassing static magnetic field.

A ferromagnetic element 11 extending predominantly longitudinally, in particular in the form of a wire or strip, is disposed inside the two coils 6, 7, substantially along the axis of axial symmetry of the tubular body 3 , indicated X in the drawings. The ferromagnetic element 11 is made of amorphous ferromagnetic metal, for example, of Fe₆₂.₅Co₆Ni₇.₅Zr₆Cu_ιNb₂Bi₅, treated thermally so as to provide the maximum amplitude of the resonant longitudinal magnetoelastic waves, as explained in detail below.

According to a preferred embodiment of the invention, the ferromagnetic element 11 is incorporated completely in a sheathing 12 of resilient rubber which is not homogeneous with the ferromagnetic element 11 and which is longer than the element 11. Two opposite ends 13a 13b, which project at the ends of the ferromagnetic element 11, are thus defined on the sheathing 12 and each is fixed to a respective wall 5a or 5b of the box-like casing 2. In particular, each end 13a, 13b is fitted in an opening 14 formed in the respective wall 5a, 5b, and is welded therein. The ferromagnetic element 11 is thus spaced both from the tubular body 3 and from the walls 5a, 5b but is connected to the latter by means of the sheathing 12.

Both the box-like casing 2 and the sheathing 12 may also be made of the same elastomeric material so that the sheathing 12 is completely homogeneous with the walls 5a and 5b.

The box-like casing 2 is also covered by a covering 15 of silicone material suitable for protecting the sensor 1 from impacts or external corrosive agents. The covering 15 also prevents oxidation of the ferromagnetic element 11 and of the electrical contacts existing between the various elements constituting the sensor 1.

Calibration means 17 are also connected to the detection coil 7 for the remote calibration of the sensor 1 in predetermined conditions, as explained by way of example below. The electrical-signal generator 8, the amplification and processing means 9, and comparator means 18 are subject to the calibration means.

In a variant of the present invention, not shown, the above- mentioned amplification and processing means 9 and the electrical-signal generator 8, as well as additional means for cutting any noise signals (not shown) are included in a single electronic control board.

In order to perform a measurement of the static and/or dynamic micro-deformations of a preselected object, the sensor 1 is placed in contact with the object or is incorporated therein. The sensor 1 can also perform measurements of deformations to which vibrating objects not directly in contact with the sensor are subject. The material of which the box-like casing 2 is made renders the sensor 1 easily deformable and affords it good resilience properties, at least within the estimated load range. Moreover, the covering 15 of silicone material renders the sensor 1 biologically compatible.

The excitation coil 6 is driven by an alternating-current signal having a sinusoidal curve, output by the electrical- signal generator 8. Because of this signal, the coil 6 generates a variable magnetic field having a predetermined frequency and amplitude. In response to this magnetic field, which is variable over time, the magnetic energy is converted by the ferromagnetic element 11 into elastic energy which is responsible for mechanical deformations of the element 11. Since the ferromagnetic element 11 is also magnetostrictive, as it deforms mechanically, at the same time, it generates a magnetic flux which can be detected by the detection coil 7.

Moreover, when the frequency of the exciting magnetic field is equal to the mechanical resonance frequency of the ferromagnetic element 11, the conversion of the magnetic energy into mechanical energy is maximal and a stationary magnetoelastic resonant wave is excited in the ferromagnetic element 11.

The magnetic component of the magnetoelastic wave, which is not necessarily at resonance frequency, and which is designated the first signal emitted by the ferromagnetic element 11 in the appended claims, induces an electromotive force in the detection coil 7 and various parameters of this magnetic component can therefore be measured by the processing means 9. For example, the amplitude of the magnetoelastic wave can be correlated, by means of well-established theoretical models, with the stresses applied to the ferromagnetic element, and the deformations of the object monitored can be derived from these stresses.

In fact, the resilient rubber sheathing 12 is connected to the opposed walls 5a, 5b in a manner such that the ferromagnetic element 11 is pretensioned so that, for example, compressions of the box-like casing 2 caused by external deformations acting on the object to which the sensor 1 is applied cause a reduction in the tensioning and hence an increase in the amplitude of the magnetoelastic wave in the element 11.

For dynamic measurements, a small permanent magnet (for example 2x2x2 mm³) is fitted in the object which is subject to vibrations and the main body of the sensor 1 (irrespective of whether it is formed in accordance with the prior art or as described in the above-described preferred embodiment) is placed in the immediate vicinity and is excited by the oscillations of the above-mentioned permanent magnet so that the permanent magnet modulates the magnetoelastic wave induced by the excitation coil. As in the previous embodiment, parameters relating to the magnetoelastic wave excited in the ferromagnetic element 11, which are correlated with the movements of the magnet applied to the vibrating object, and hence with the deformations undergone thereby at the point at which the magnet is fitted, are measured. In particular, the electrical signal emitted by the detection coil 7 is demodulated to derive the modulating component due to the mechanical vibration of the object. With reference to vibration measurements in dynamic conditions (accelerometric sensor) , Figures 3a and 3b show variants of the sensor for measurements in which it operates in direct contact with the object to be monitored. In this case, the permanent magnet is an integral part of the sensor system and is arranged therein in accordance with two possible solutions. In the solution of Figure 3a, the permanent magnet, indicated 21, is applied directly to a base wall 22 of the box-like casing 2, inside a closed extension portion 23 of the casing, and the sensor is restrained firmly on the body 24 to be monitored through the wall 22, in an operative condition.

In the solution of Figure 3b, the casing of the sensor has an open extension portion 25 of its own lateral surface and the free edge of the extension portion is restrained directly on the body 24 to be monitored, in an operative condition, so as to surround the permanent magnet 21 which is applied firmly to the body.

With further reference to the general diagram of Figure 1, the measurement of deformations in dynamic conditions is more accurate in particular by virtue of the stability achieved by the elastomeric sheathing 12. The response of the sensor 1 is thus rendered substantially stable with respect to movements of the ferromagnetic element 11 due to external mechanical actions such as rigid oscillations of the sensor 1.

During the taking of a measurement by means of the sensor 1, an on-line calibration stage, by means of the calibration means 17, is also provided for according to the invention. This stage provides for the transmission to the excitation coil 6 of a series of alternating-current signals which have frequencies of the order of the mechanical frequencies, that is, between 1 and 500 Hz, and which are superimposed on the exciting signal . Upon transmission of these alternating- current signals, which are referred to below as calibration signals, the change in the signal emitted by the ferromagnetic element 11 (designated the second signal emitted in the claims) and detected by the detection coil 7 is checked. Since the response to these calibration signals is known, if comparison by the comparator means 18 between the exciting signals (that is, the exciting signal on which the calibration signals are superimposed) and the signal detected matches predetermined standards, that is, by comparison with known values in an optimal operating situation, the working conditions are left unchanged; otherwise, the amplitude of the exciting signal generated by the generator 8 and the gain of the detection coil 7 are automatically changed so as to re-establish the working conditions for which the sensor 1 was calibrated. This enables the sensor 1 to be used as calibrated, even if some conditions boundary change, in particular, if the local magnetic field is changed for some reason, owing to the presence of magnetic material which was absent at the time when the sensor was installed.

The invention also provides for the parameters of the magnetoelastic wave detected by the coil 7 and processed by the processing means 9 during the taking of a measurement to be the amplitude of the wave, its phase, and the phase displacement relative to the exciting wave. According to the type of application of the sensor 1, the use of one parameter rather than another in fact increases the sensitivity of the sensor.

As a further innovative characteristic with respect to known magnetoelastic sensors, the sensor of the present invention does not operate exclusively by generating waves at the basic resonance frequency in the ferromagnetic element 11, but a wave having a frequency slightly offset from the resonance frequency is preselected as the excitation frequency (and hence the vibration frequency of the ferromagnetic element 11) since, in these conditions, variations in the amplitude of the signal detected by the coil 7, in dependence on the local magnetizing field generated, for example, by a permanent magnet which marks the object to the monitored, as in the case of Figures 3a and 3b, or on the deformations of the ferromagnetic element 11, have the maximum derivative.

In particular, the excitation signal emitted by the excitation coil 6 has a frequency within a range around the basic resonance frequency having a width calculated as follows. With reference to a graph of the amplitude of the magnetic component of the magnetoelastic wave as a function of the frequency, where v^* indicates the resonance frequency and A* the corresponding amplitude (maximum) , the value of the frequency (< v^*) corresponding to A*/2, which is referred to as Vp_P, is determined at a single resonance peak. The range in question therefore has a width equal to [v^*-v_PP,

V^*+Vpp] .

This arrangement permits operation with greater sensitivity. The optimal working frequency is re-established automatically from time to time by means of the same self- calibration system just described, by automatically varying the frequency in the above-mentioned range.

The invention thus achieves the objects proposed, affording the above-mentioned advantages over known solutions.

Moreover, a further advantage is represented by the fact that the covering of silicone material protects the ferromagnetic element 11 from possible corrosion due to external agents .

Furthermore, the resilient rubber sheathing makes the coupling between the ferromagnetic element and the box-like casing more stable .

One of the main advantages is that the sensor thus designed is very sensitive and its response is stable over time.

Applications for performing continuous or periodic monitoring of deformations and of the consequent tension state in civil-engineering structures or mechanical members, as well as of vibrations at strategic points of volcanic and archaeological areas are envisaged.

Sensors according to the invention may also be used as travel-limit sensors or level sensors with variable travel for control systems for industrial processes.

Claims

1. A magnetoelastic sensor for detecting static and dynamic micro-deformations, comprising:

a ferromagnetic element (11) with magnetoelastic properties,

- an excitation coil (6) for sending an excitation signal to the ferromagnetic element (11) , and

- a detection coil (7) for detecting a first signal emitted by the ferromagnetic element (11) as a consequence of the excitation signal, characterized in that it includes:

- a permanent magnet element (21) intended to be connected firmly to a body, the static and dynamic deformations of which are to be measured.

2. A magnetoelastic sensor for detecting static and dynamic micro-deformations, comprising: a ferromagnetic element (11) with magnetoelastic properties,

- a detection coil (7) for detecting a first signal emitted by the ferromagnetic element (11) as a consequence of the excitation signal, characterized in that it includes: a sheathing (12) of resilient material for the ferromagnetic element (11) such that the element (11) is at least partially embedded in the sheathing (12) .

3. A sensor according to Claim 1 or 2 in which the excitation coil (6) can send to the ferromagnetic element (11) an excitation signal having a frequency equal to the mechanical resonance frequency of the element (11) .

4. A sensor according to Claim 1 or 2 in which the excitation coil (6) can send to the ferromagnetic element

(11) an excitation signal having a frequency in a region around the resonance frequency.

5. A sensor according to Claim 4 in which the excitation signal has a frequency such that the variations in the amplitude of the first signal, emitted by the ferromagnetic element (11) and detected by the detection coil (7) in dependence on a local magnetizing field or on deformations of the element (11), have the maximum derivative.

6. A sensor according to any one of the preceding claims in which the ferromagnetic element (11) is an element made of amorphous ferromagnetic material in wire or strip form.

7. A sensor according to Claim 1, comprising a sheathing

(12) of resilient material for the ferromagnetic element (11) such that the element (11) is at least partially embedded in the sheathing (12) .

8. A sensor according to Claim 2, comprising a permanent magnet element (21) to be connected firmly to a body, the static and dynamic deformations of which are to be measured.

9. A sensor according to Claim 2 or 7, comprising a box-like casing (2) in which the ferromagnetic element (11) is housed.

10. A sensor according to Claim 9 in which the box-like casing (2) comprises a covering (15) of silicone material.

11. A sensor according to Claim 9 or 10 in which the boxlike casing (2) is made of the same resilient material of which the sheathing (12) is made.

12. A sensor according to any one of Claims 9, 10 or 11, in which the ferromagnetic element (11) is connected to the box-like casing (2) solely by means of the sheathing (12) .

13. A sensor according to any one of Claims 9 to 12 in which the ferromagnetic element (11) is completely embedded in the sheathing (12) of resilient material.

14. A sensor according to Claim 13 in which the sheathing (12) has a longitudinal extent greater than that of the ferromagnetic element (11) , the ferromagnetic element (11) being connected to the box-like casing (2) solely by means of two opposite ends (13a, 13b) of the sheathing (12) which project from the element.

15. A sensor according to Claim 1 or 8, comprising a boxlike casing (2) in which the ferromagnetic element (11) is housed and a portion (23; 25) for housing the permanent magnet element (21) .

16. A sensor according to Claim 15 in which the box-like casing (2) comprises a closed extension portion (23) carrying the permanent magnet element (21) , applied to a base wall (22) , the wall (22) being suitable for being applied to the body (24) to be monitored.

17. A sensor according to Claim 15 in which the box-like casing (2) comprises an open extension portion (25) suitable for being applied firmly to the body (24) to be monitored, and for housing the permanent magnet element (21) , applied directly to the body (24) .

18. A sensor according to any one of the preceding claims, comprising: - a current-signal generator (8) connected to the excitation coil (6) and arranged to send to the excitation coil (6) current signals suitable for generating the excitation signal,

- calibration means (17) to which the generator (8) is subject, the calibration means (17) being adapted to bring about the generation of a plurality of calibration signals by the excitation coil (6) , and the modification of the excitation signal, and comparison means (18) adapted to compare with predetermined values a second signal emitted by the ferromagnetic element (11) and detected by the detection coil (7) as a consequence of the calibration signals.

19. A sensor according to any one of the preceding claims, comprising processing means (9) for processing signals detected by the detection coil (7) , the processing means being adapted to measure the amplitude and/or the frequency of the first and/or the second signal emitted by the ferromagnetic element (11) and detected by the detection coil (7) .

20. A sensor according to any one of the preceding claims, comprising processing means (9) for processing signals detected by the detection coil (7) , the processing means being adapted to measure the phase shift of the first signal emitted by the ferromagnetic element (11) relative to the excitation signal .

21. A method of measuring static and dynamic micro- deformations by means of a sensor as defined in Claims 1 to 20, comprising the steps of: - exciting a magnetoelastic wave in a ferromagnetic element (11) by means of an excitation signal sent by an excitation coil (6) , and

- detecting a first signal emitted by the ferromagnetic element (11) as a consequence of the excitation signal, characterized in that it comprises the steps of: sending, by means of the excitation coil (6) , at predetermined time intervals, a plurality of calibration signals superimposed on the excitation signal,

- detecting a second signal emitted by the ferromagnetic element (11) as a consequence of the calibration signals superimposed on the excitation signal,

- comparing the second signal detected with predetermined values, and modifying the excitation signal if the second signal detected differs from the predetermined values by a quantity greater than a preset value .

22. A method according to Claim 21 in which the signal sent by the excitation coil (6) has a frequency equal to the mechanical resonance frequency of the ferromagnetic element

(11) •

23. A method according to Claim 21 in which the signal emitted by the excitation coil (6) has a frequency in a region around the resonance frequency such that the variations in the amplitude of the first signal, emitted by the ferromagnetic element (11) and detected by the detection coil (7) in dependence on a local magnetizing field or on deformations of the ferromagnetic element (11) , have the maximum derivative .

24. A method according to any one of Claims 21 to 23, comprising the step of measuring the amplitude of the first and/or of the second signal detected by the detection coil (7) .

25. A method according to any one of Claims 21 to 24, comprising the step of measuring the frequency of the first and/or of the second signal detected by the detection coil (7) .

26. A method according to any one of Claims 21 to 25, comprising the step of measuring the phase shift of the first signal detected relative to the excitation signal .