FR2656686A1 - Method and device for measuring the displacement of a point with respect to a reference. - Google Patents

Abstract

The present invention relates to methods and devices making it possible to measure the amplitude of the relative displacement of a point 2 and of a reference 3. The method and the device use two magnetic coils 4, 5 linked to one of the pair "point 2 and reference 3", the reactance of the coils varying during the displacement of a core which is linked to the other of the pair "point and reference". The method consists in carrying out sequences of measurements, of a first parametric value of a first element 4 by means of a first signal, a second parametric value of the second element 5 by means of a second signal, of a third parametric value of the two combined elements, by means of the first signal, of a fourth parametric value of the two combined elements, by means of the second signal, and in deducing the variation in position of the point with respect to the reference as a function of the variation, from one sequence to another, of the four parametric values. Application, in particular, to monitoring barriers (dams) tunnels, etc.

Description

Method and device for measuring displacement
of a point in relation to a repository
The present invention relates to methods and devices making it possible to measure the amplitude of the displacement of a point relative to a reference frame, more generally of the relative displacement of this point and of the reference frame.

 In many fields, it is often necessary to monitor the position of at least one point relative to a reference element, in order to possibly be able to intervene preventively.

This is for example the case of monitoring the relative movements between a point on the wall of a dam and points located on the banks on which this wall is hung, or else monitoring the wall of a tunnel. I1 is indeed known that, despite all the care taken in the realization, in particular, of important works of public works, their walls undergo deformations under the action of stresses of all kinds. These deformations are however tolerated, but on condition that they do not exceed certain limits. They must therefore be monitored, in particular by measuring their amplitude.

 This is how various devices have been developed which make it possible to detect such displacements and measure their amplitude, in order to be able to estimate for example the risks of a possible accident, the dramatic consequences of which are well known, and thus make the necessary security arrangements in time.

 Such devices include sensors essentially comprising two parts, a part linked to the point whose displacement is to be monitored, and the other linked to the reference frame with respect to which the possible displacement of the monitored point must be detected.

Among these sensors, there is one which is of the “variable reactance” type. The device then comprises at least one coil of the magnetic type mounted in series in a resonant oscillating circuit secured to one of the two elements, the point to be monitored or the frame of reference, and a core plunging into the coil and integral with the other element. When the distance separating the point to be monitored and the frame of reference varies, the core moves in the core of the coil, which has the effect of changing the value of its inductive reactance and, therefore, the resonance frequency of the oscillating circuit.

 However, a change in this type of frequency is very easily measurable using an electronic analyzer, for example a frequency meter whose calibration makes it possible to determine the amplitude of the relative displacement of the two elements.

 This device theoretically gives good results. But, in fact, the variation of the inductance of the coil does not involve a linear variation of the resonance frequency of the circuit. It is therefore very difficult to convert the variation of this inductance into the amplitude of the relative displacement of the two monitored elements. In addition, such a system, like others, is subject to a whole set of parameters which, in reality, distort the results and make the device less reliable than in theory.

 To overcome these drawbacks, an improved device was then produced based on the previous one, but in which two coils are used coupled on the same mandrel, with a single core plunging both into the two coils. These coils are mounted in two resonant oscillating circuits whose measurement of the variations of the resonant frequency is carried out according to the well-known method known as differential ". This makes it possible to eliminate the uncertainties due to the non-linearity of the measurements.

 On the other hand, this improvement does not make it possible, in particular, to overcome the intrinsic drifts of the coils themselves or of the other elements entering into the constitution of the electronic circuits used for the production of the device, that is to say variations characteristics of these elements. However, it is obvious that more and more reliability and fidelity are required of this type of device. Their disadvantages succinctly mentioned above therefore greatly limit their field of use.

 The object of the present invention is therefore to implement a method for measuring the amplitude of the displacements of a point relative to a frame of reference, which makes it possible to eliminate the drawbacks mentioned above concerning, in particular, the drifts of the constituent elements of the electronic means used for its implementation, as well as the parasitic phenomena, while preserving the linearity of the measurements.

 It also aims to produce a device implementing said method.

More specifically, the subject of the present invention is a method for measuring the amplitude of the relative displacement of a point and a frame of reference, by means of two active measurement elements linked to one of the two "point and frame of reference ", each active element having a parameter value which can vary during the movement of a body of a given nature, this body being linked to the other of the two" point and frame of reference ", the value of said parameters being able to be determined by means of a suitable signal, a method characterized by the fact that it consists of
measuring a first parametric value of a first active element by means of a first signal,
measure a second parametric value of the second active element by means of a second signal,
measuring a third parametric value of the two associated active elements, by means of said first signal,
measuring a fourth parametric value of the two associated active elements, by means of said second signal, and
deduce the relative position of said point and said reference system as a function of the variation of said four measured parametric values.

 The present invention also relates to a device allowing the implementation of the method defined above.

Other characteristics and advantages of the present invention will appear during the following description given with reference to the drawings annexed by way of illustration, but in no way limitative, in which
FIG. 1 represents the block diagram of an embodiment of the device according to the invention making it possible to measure the amplitude of the relative displacement of a point and of a reference frame, by implementing the method according to the invention, and
FIG. 2 represents a cutaway view of a detail of a part of the device in the embodiment illustrated by FIG. 1.

 The method according to the invention makes it possible to determine the amplitude of the relative displacement of a point and of a reference frame. This determination is carried out by means of two active measurement elements linked to one of the two "point and reference frame", each element having a parameter value which can vary during the movement of a body of a given nature linked to the other of the two "point and reference frame", the values of the two parameters can be determined by means of a suitable signal.

 The method can be implemented using various means of the electronic type, in particular electrical capacitors or capacitors and magnetic coils.

 FIG. 1 represents an embodiment of the device making it possible to implement the method according to the invention, in which the two active elements used are elements with variable reactance, more particularly two magnetic coils. But it is obvious that this type of active element should only be considered as an example chosen for the drafting of this description.

 The method consists in carrying out sequences of four measurements and in determining, from the variation, from one sequence to another, of the results of these measurements, the amplitude of the relative displacement between the point and the reference frame.

 The first measurement carried out, in each sequence, is that of the value of the parameter of one of the two active elements, this value being determined by means of a first suitable signal. In the example chosen for the present description, the measurement of the reactance L4 (1) of a first coil is therefore carried out, for example by means of a first frequency signal.

 The second measurement is that of the value of the parameter of the other active element, by means of a second signal, in this case the reactance L5 (2) of the second coil by means of a second frequency signal.

 The third measurement is that of the parameter value of the two active elements associated by means of the first signal. In the example illustrated, the reactance of the two coils connected in series is measured, by means of the same first frequency signal as that which was used for the measurement of L4 (1) alone. The reactance value is obtained: (L4 + L5) (1).

 The fourth measure in each sequence is similar to the third, but using the second signal. The reactance value is then obtained: (L4 + L5) (2).

 Any variations in the values of parameters L4 and L5, from one sequence to another, due to the intrinsic drifts of the magnetic coils are corrected using the third and fourth measurements.

 I1 is then possible, knowing the transfer function of the sensor, which is generally not linear but simply one-to-one, to calculate the amplitude of the relative displacement of the point and the reference frame, in the case obviously where the corrected values of L4 and L5 have varied from sequence to sequence.

 To do this, a microprocessor, for example, calculates the corrections, then the displacement, either by interpolation from a table memorized by sensor or by a polynomial or transcendent function.

 Figures 1 and 2 show an embodiment of a device for implementing the method defined above.

 The device 1 illustrated makes it possible to measure the amplitude of the displacement, for example, of a point 2 relative to a reference frame 3, these two elements having been represented diagrammatically for the simplification of the drawings.

 The device 1 comprises two magnetic coils 4, 5 wound on a single mandrel 6, FIG. 2, consisting, for example, of a tube made of a material which undergoes very little deformation under the effect of temperature. This material can for example be fiberglass. The first outputs 7 and 8, respectively of these two coils, are electrically connected at the same point 9. The two coils and the mandrel are securely fixed at point 2 by any mechanical means 10.

 On the other hand, the reference frame 3 is rigidly linked, for example, a rod 11 supporting a core 12, this core and the rod 11 being arranged so that the core 12 is located inside 13 of the mandrel 6 and that, when potential relative displacements of point 2 and reference frame 3, this core 12 moves along the axis 14 of the mandrel 6. In addition, the length of the core 12 is determined so that it is less than the distance separating the two ends further away 15, 16 from the windings of the two coils 4 and 5. The core is in known manner made of a material making it possible to vary the magnetic coupling between the two coils when, during its movement along the axis 14 of the mandrel, its lengths respectively located inside the coils vary.To obtain this effect, the core is therefore made in a known way of a material like soft iron, an alloy of nickel, chromium and iron of the type of that like specialized under the name of "Inconel", copper, aluminum, etc.

 The second output 20 of the coil 4 is connected to a first input 21 of an oscillator 22, the second input 23 of which is connected to the midpoint 24 of a three-position switch 25, 27, and 28. The first input 26 of the switch 25 is connected to the common point 9 between the two coils 4 and 5. In this way, a first oscillating circuit 101 is produced comprising in series the oscillator 22, the coil 4 and the switch 25 taken between its two terminals entry 24 and 26.

 The terminal 28 of the switch 25 is connected to the second output 31 of the coil 5, so as to be able to produce, as a function of the position of the switch 25, a second oscillating circuit 201 comprising in series the oscillator 22, the switch 25 and the two coils 4 and 5 connected in series.

 The device further comprises a second oscillator 29, a first input 30 of which is connected to the second output 31 of the coil 5, while its second input 32 is connected to the midpoint 33 of a second three-position switch 34, 36 and 37. Its terminal 35 being connected to the common point 9, a third oscillating circuit 102 is thus produced, comprising in series the oscillator 29, the coil 5 and the switch 34 taken between its two terminals 33 and 35.

 The terminal 37 of the switch 34 is connected to the second output 20 of the coil 4, so as to be able to produce, as a function of the position of the switch 34, a fourth oscillating circuit 202 comprising in series the oscillator 29, the switch 34 and the two coils 4 and 5 connected in series.

 It is first mentioned that by the term "oscillator" is understood an electrical circuit capable of entering into resonance when it is connected to an active element such as a capacitor or a magnetic coil. It follows that the four oscillating circuits have a resonant frequency which depends on the reactance of the coils 4 and 5 which can be connected to the two input terminals, respectively 21, 23 and 30, 32, of the two oscillators 22 and 29.

 The device further comprises means for measuring the resonance frequency of the oscillating circuits 101, 102, 201 and 202 defined above. These means are known in themselves and can for example be constituted by frequency meters.

 In the illustrated embodiment, the device 1 comprises a single frequency meter 40 the two outputs 41 and 42 of which are connected to a first set of input terminals 44 of a reversing switch 43 comprising two other sets of input terminals 45, 46 respectively connected to the two output terminals 21, 23 of the first oscillator 22 and 30, 32 of the second oscillator 29.

 In FIG. 1, the two switches 25 and 34, as well as the reversing switch 43, have been illustrated by electromagnetic relays in order to better highlight their functions. In addition, each of them has control inputs making it possible to position them, in one of the three states for the two switches 25 and 34, and in one of the two pairs of states for the reversing switch 43. These control inputs have been schematically illustrated by means such as those found in electromagnetic type relays. However, it is obvious that, in a preferred embodiment, these three switches will be produced by elements of the electronic type, which will not be described here, since they are well known in themselves.

 The device I also comprises means 50 making it possible to control, in a determined manner, as explained below, by control lines 51, the various switches.

 In an advantageous embodiment, the device comprises a processing member 60 of the computer-calculator type in which the frequency counter 40 defined above and the control means 50 of the three switches are integrated. This processing unit further comprises memories 70 making it possible to store the results of the various measurements carried out as described below. I1 also includes an output 61 connected, for example, to the input 62 of a display 63.

The device as described above operates as follows
It is assumed first of all that the core 12 is stable, in a determined position, that is to say that it is assumed that it does not move relative to the two coils 4 and 5. A first measurement is then carried out of the oscillation frequency of the first oscillating circuit 101 by positioning the switch 25 so that the output 23 of the first oscillator 22 is directly connected to the common point 9 of the two coils, the switch 34 being in the "open" position for the oscillating circuit 102. In addition, the reversing switch 43 is controlled so that the two outputs 41, 42 of the frequency meter 40 are connected directly, and only, to the two outputs 21, 23 of the oscillator 22. The oscillation frequency of this first oscillating circuit is therefore representative of the reactance of the coil 4 for a determined position of the core 12. The result of this first measurement is stored in memory at 70.

 The two switches 25 and 34, as well as the reversing switch 43, are then controlled to measure the oscillation frequency of the third oscillating circuit 102 comprising the second oscillator 29, the measured frequency being representative of the reactance of the coil. 5, for the same position of the core 12. The result of this second measurement is stored in memory at 70.

 These two measurements being carried out, the two switches 25 and 34 are again controlled to, for example, produce the second oscillating circuit 201 defined above, the fourth oscillating circuit 202 being open. This resonant frequency of the second oscillating circuit 201 comprising the first oscillator 22 is representative of the reactance of the sum of the two coils, always for the same position of the core 12. It is stored in memory at 70.

 Similarly, switches 25, 34 and 43 are controlled to determine the oscillation frequency of the fourth oscillating circuit 202 comprising the second oscillator 29. This frequency, stored in memory at 70, is also representative of the reactance of the sum of the two coils for the same position of the core, but for an oscillating circuit comprising the oscillator 29.

 It is thus possible to determine the values of the reactances of the two coils by eliminating errors due to possible variations in the characteristics of the various electronic elements of the device.

 We have just described above a first sequence of measurements controlled from the processing member 60.

 A second sequence of measurements is carried out, in the same way as defined above, to determine the value of the reactances of the two coils, a value which is compared with that determined in the previous sequence. If the processing unit detects a variation in the value of the reactances, it follows that there has been, during the space of time separating the two sequences, a relative displacement, along the axis 14 of the mandrel 6, of the core 12 and of the coils 4, 5, and therefore of point 2 and of the reference system 3.

 The processing unit 60 makes it possible, with the different values stored in memory, to determine the amplitude of the linear displacement of the point 2 relative to the reference frame, amplitude which can be indicated on the display 63. If it exceeds a predetermined threshold, it can trigger an alarm, in particular for applications such as those mentioned in the preamble to this description.

 The present description does not include a more detailed example of a processing member 60, but a technician specialized in the electronic field and knowing the functions to be implemented as defined above will be able to very easily realize this.

Claims (9)

 1. Method for measuring the amplitude of the relative displacement of a point and a frame of reference, by means of two active measurement elements linked to one of the two "point and frame of reference", each active element having a value of parameters which can vary during the movement of a body of a given nature, this body being linked to the other of the two "point and reference frame", the value of said parameters being able to be determined by means of a suitable signal, method characterized by the fact that it consists of  measuring a first parametric value of a first active element by means of a first signal,  measure a second parametric value of the second active element by means of a second signal,  measuring a third parametric value of the two associated active elements, by means of said first signal,  measuring a fourth parametric value of the two associated active elements, by means of said second signal, and  deduce the relative position of said point and said reference system as a function of the variation of said four measured parametric values.  2. Method according to claim 1, characterized in that said parameter is the reactance of a magnetic coil.  3. Method according to claim 1, characterized in that said parameter is the capacity of a capacitor.  4. Method according to one of claims 2 and 3, characterized in that said adapted signal is an electrical frequency signal in a resonant oscillating circuit.  5. Device for implementing the method in accordance with the preceding claims, characterized in that it comprises  at least two active measurement elements (4, 5) linked to one of the two so-called "point (2) and reference frame (3)", each element having a parameter value which can vary during the movement of a body (12 ) of a given nature, this body being linked to the other of the two so-called "point and referential", the value of said parameters being able to be determined by means of a suitable signal,  first means (22) for delivering a first supply signal,  first means (101,43,45,60) for measuring a first parametric value of a first active element (4) by means of said first supply signal, said first measurement means being capable of delivering a first measurement signal ,  second means (29) for delivering a second supply signal,  second means (102,43,46,60) for measuring a second parametric value of the second element (5) by means of said second supply signal, said second measurement means being capable of delivering a second measurement signal,  third means (201,43,45,60) for measuring a third parametric value of the two associated elements (4,5), by means of said first supply signal, said third measurement means being capable of delivering a third signal measured,  fourth means (202,43,46,60) for measuring a fourth parametric value of the two associated elements (4,5), by means of said second supply signal, said fourth measurement means being capable of delivering a fourth signal measure, and  means for processing (60, 40, 70, 63) of said first, second, third and fourth measurement signals to deduce the amplitude of the relative displacement of said point (2) and said reference frame (3).  6. Device according to claim 5, characterized in that said active elements (4,5) are constituted by magnetic coils and that said parameter is the reactance of these coils.  7. Device according to claim 5, characterized in that said active elements consist of capacitors and that said parameter is the capacity of these capacitors.  8. Device according to one of claims 6 and 7, characterized in that said means for measuring the parametric values of said active elements consist of a resonant oscillating circuit (101,102,201,202) in which is mounted in series at least one of said active elements.  9. Device according to one of claims 5 to 8, characterized in that said processing element is constituted by a microprocessor computer.