GB2307986A

GB2307986A - Force measurement using an interferometer

Info

Publication number: GB2307986A
Application number: GB9625347A
Authority: GB
Inventors: Simon Larcher; St Phane Teral
Original assignee: Alcatel Contracting SA France
Current assignee: Alcatel Contracting SA France
Priority date: 1995-12-07
Filing date: 1996-12-05
Publication date: 1997-06-11
Anticipated expiration: 2016-12-05
Also published as: DE19649877A1; DE19649877C2; GB2307986B; FR2742227B1; GB9625347D0; FR2742227A1

Description

1 2307986 A METHOD OF PROCESSING AN INTERFEROMETER SIGNAL FOR MEASURING A

FORCE The invention relates to a method of processing an interferometer signal for measuring a force by means of the 5 birefringence imparted by stress in a light waveguide.

The invention generally applies to monitoring road traffic, and in particular to weighing moving vehicles.

The principle of measuring a force by means of the birefringence imparted by stress in a monomode optical fiber or light waveguide is well known. Figure 1 is a very diagrammatic view showing the various elements of a piezooptical sensor serving to measure a force using that principle. A light beam having rectilinear polarization Ei and generated by a laser diode 1 is injected into the optical fiber 2 via a polarization splitter 3 which splits the light beam into two waves having crossed polarizations, one Ev in the (vertical) direction of the stress applied to the optical fiber by the force to be measured, the other Eh in an orthogonal direction. Since monomode propagation conserves polarization, each wave propagates independently as a function of the refractive index corresponding to the direction of the polarization. The difference between the optical path lengths of the two waves thus depends on the difference between the directional stresses along the paths. A variation in the birefringence of the optical fiber, due to a dynamic load such as the tire 4 of a vehicle running over the optical fiber thus leads to a relative phase shift between the two waves (this phase shift increasing with increasing load, then remaining constant at a certain level when the load is at its maximum in the vertical direction, and finally decreasing with decreasing load, i.e. as the tire leaves the optical fiber). In the circuit shown in Figure 1, the incident light beam is reflected by a mirror 5 disposed at the end of the optical fiber so as to be recovered by a detection photodiode 6 via the polarization splitter 3. The interferometer electrical signal S resulting from additive recombination of the two waves at 2 the detection photodiode 6 has an instantaneous amplitude level that varies as a function of the instantaneous phase difference between the two waves Ev and Eh.

Figure 2 shows the overall waveform of an interferometer signal S. This signal has an amplitude level that, over an interference period T corresponding to the influence of the load, has a succession of maxima and of minima defining polarimeter fringes. Starting from the origin of time t, the figure shows a first succession of polarimeter fringes that correspond to the increase in phase-shift angle, then a central polarimeter fringe that corresponds to a maximum angular phase shift, and then a second succession of polarimeter fringes that correspond to the phase-shift angle decreasing.

In Document EP-O 153 997, it has already been proposed to count the number of polarimeter fringes present in an interferometer signal for measuring a force using the aboveindicated principle. Unfortunately, such a method has a level of accuracy that is not suitable for applications to weighing moving vehicles. Merely counting the number of polarimeter fringes offers accuracy that is intrinsically limited to 0.5 fringes, it being possible for the accuracy uncertainty to represent a few percent of the force to be measured. 25 An object of the invention is to provide another method of processing an interferometer signal for measuring a force, in particular a dynamic load, more accurately. To this end, the invention provides a method of processing an interferometer signal for measuring a force by means of the birefringence imparted by stress in a light waveguide, said method being characterized in that it comprises steps consisting in:

digitizing the interferometer signal so as to extract a chronological succession of amplitude extrema in digital form; processing the succession of extrema so as to obtain a corresponding chronological succession of first data items 3 representative of variation in phase rotation over time; and using a physical model relating phase rotation to force for processing the succession of first data items so as to obtain a corresponding chronological succession of second data items representative of variation in force over time.

The invention makes use of the fact that the expression for the phase shift W between the waves Ev and Eh, resulting from a directional pressure stress P being applied over the length L of optical fiber of a piezo-optical sensor, is of the following form:

W = KPOIL where K and a are constants dependent on the characteristics of the piezo- optical sensor (geometrical shape of the optical fiber, elasticity of the silica, wavelength of the laser source, etc.), which constants can be obtained by calibrating the piezo-optical sensor with known weights.

Furthermore, the instantaneous amplitude level I(t) of the interferometer signal is of the following form:

(1) I(t) = A + Bcos(p(t) (2) where W represents the phase difference between the two waves Ev and Eh, and where A and B are constants (A and B are calculated on the basis of the relationships A=k(Imax+Imin) and B=(Imax-Imin), where Imax and Imin are respectively the mean maximum value and the mean minimum value of the interferometer signal over a period of interference activity). The phase shift in fact corresponds to phase rotation that is counter-clockwise as the load increases, and that returns clockwise as the load decreases.

By differentiating the expression (2) with respect to time, it can be seen that the relative extrema of the signal I(t) correspond either to an angular determination threshold (W=kw) being crossed, or to an extremum of the phase rotation (dcp/dt=O).

Therefore, on the basis of the relationships (1) and (2), it can be seen that a.variation in a dynamic load over time (defined by different values of P as a function of 4 time) can be derived from the variation in phase rotation over time corresponding to the phase shift tp, the variation in phase rotation over time being simply defined for the extrema of the interferometer signal.

According to a feature of the method of the invention, the processing of the extrema consists in taking into account alternately an extremum from the beginning of the succession of extrema, and an extremum from the end of the succession of extrema, and continuing towards the middle of the succession of extrema after each cycle, so as to obtain said succession of first data items representative of variation in phase rotation over time.

The processing takes advantage of the fact that the variation in phase rotation over time starts from a rest level and returns thereto after going to a maximum value typically corresponding to the extremum in the middle of the succession in the simple case of the variation in phase rotation over time being of the bell-curve type.

When variation in phase rotation over time is more complex, e.g. of the saddle-curve type, the phase rotation maximum no longer necessarily corresponds to the central extremum in the succession of amplitude extrema of the interferometer signal, and it is necessary to identify each extremum as corresponding to an angular determination threshold being crossed, or as a phase rotation maximum. This ambiguity can however be removed simply by noting that the angular determination threshold being crossed corresponds to an extremum close to the limits (A+ B) or (AB). Conversely, a phase rotation maximum generally corresponds to an extremum observed well outside said limits. The invention is described in detail below with reference to the accompanying drawings, in which: Figure 1 is a diagram showing a piezo- optical sensor; 35 Figure 2 shows an example of an interferometer signal; Figure 3 is a flow chart showing the main steps of the method of the invention; Figure 4 shows another example of an interferometer signal;.

Figure 5 shows the variation in the phase rotation over time in the form of a curve, which variation over time is 5 reconstructed from the signal shown in Figure 4; and Figure 6 is a detailed flow-chart of the step in which variation in phase rotation over time is reconstructed.

In the method of the invention, in order to measure a force, in particular a dynamic load, variation in the force over time is reconstructed on the basis of variation in phase rotation over time, which variation is itself reconstructed on the basis of the amplitude extrema in the interferometer signal. Compared with merely counting the polarimeter fringes, this offers measurement accuracy that is much finer, i.e. about 0.01 fringes. Furthermore, when the method of invention is applied to weighing moving vehicles, the variation in the dynamic load over time makes it possible to work back more accurately to the value of the static load, because such variation shows up the dynamic effects of suspensions, accelerations, etc, which can then be taken into account when determining the static load. As shown in Figure 3, the method of the invention of processing an interferometer signal 5 starts with a digitizing step 30 in which the signal is digitized. 25 Step 30 is preferably followed by a digital filtering step 31 making it possible to reduce interference by smoothing the interferometer digital signal. Step 31 is followed by a detection step 32 for detecting the beginning and the end of the period of interference activity in the interferometer signal in digital form. For example, detecting the beginning and the end of interference activity may be based on a logic circuit for monitoring the instantaneous level of the digital signal and for comparing the instantaneous level with a mean level of the interferometer digital signal as measured in the absence of stress and updated after each period of interference activity.

6 Step 32 is followed by a sample-processing step 33 for processing samples recovered for the period of interference activity of the digital signal, so as to extract those samples which correspond to amplitude extrema.

Figure 4 shows the variation in the amplitude level of an interferometer signal over time. The beginning and the end of the interference period are indicated by D and F.

The amplitude extrema are indicated by Ext[l],..., Ext[n], where Ext designates a vector in which the chronological succession of amplitude extrema is stored. In this figure, the extremun Ext(j) corresponds to a phase rotation maximum.

The chronological succession of extrema in the vector Ext is processed in step 34 so as determine a corresponding chronological succession of data items representative of variation in phase rotation over time. It should be noted that step 34 also requires each extremum to be associated with its time position in the interference period to reconstruct the variation in the phase rotation over time and the variation in load over time. In order to make the description clearer, no further reference is made to the time positions of the extrema.

Figure 6 shows the processing performed in step 34. In this figure, PHI designates the vector containing the chronological succession of data items representative of variation in phase rotation over time. The vector PHI contains the various values of the phase rotation respectively corresponding to the extrema of the vector Ext. The processing 34 merely consists in a loop with indices ig (pointing to the beginning of the vector Ext) and id (pointing to the end of the vector Ext) initialized at 61 respectively to 1 and to n (where n designates the number of extrema in the vector Ext), in which loop, alternately, an extremum of index ig of the vector Ext is taken into account at 62 to determine a discrete value PHI[ig] corresponding to phase rotation, and an extremum of index id of the vector Ext is taken into account at 63 to determine another discrete value PHI[id] of phase rotation. The indices id 7 and ig move together at 64 towards the middle of the vector Ext. When the index ig becomes greater than or equal to the index id, comparison at 65 causes the processing to exit from the loop and continue at 66. The test id k ig is used, in preference to the test id = ig, in order to ensure exiting from the loop, even if the number n of detected extrema should accidently be even in spite of this number normally being odd.

In step 62, the rank ig of the extremum taken into account serves directly to determine the corresponding value of phase rotation. The same applies in step 63 for the index id. At this stage of processing, only the maximum value of the phase rotation remains to be determined. It is determined in step 66 on the basis of the phase rotation value in the absence of stress and using the following relationship:

PHI[id] = PHI[id-1] + Arccos((Ext[id]-A)/B) - Wo (3) where (po is the phase rotation value in the absence of stress. The value Wo depends on the settings of the piezo- optical sensor and may, g nrinyi, be arbitrary. It is estimated by analyzing the mean level Io of the interferometer signal in the absence of stress, using the following relationship:

xpo = Arccos(Io-A)/B (4) At 65, id and ig correspon d to the index j in Figure 4, i.e. they point to the extremum in the middle of the succession of extrema.

Figure 5 shows the shape of the variation over time as reconstructed for the interferometer signal shown in Figure 4. In this example, the shape is that of a single bell curve, where PHI[1] = v, PHI[2] = 2r, PHI[3] 3r, PHI[n] = v.

The processing shown in the flow chart in Figure 6 can be refined for more complex variation in phase rotation over time. In which case, in steps 62 and 63, the value of the extremum. Ext[ig] or Ext[id] must be compared with the values A+B or A-B (see Figure 4). When this extremum is close to 8 A-B or A+B, it is an extremum corresponding to the phase rotation crossing a determination threshold, and the rank ig or id of the extremum is also used to determine the value of the phase rotation. Otherwise, a particular value, e.g. 0 is assigned to the data item PHI[ig] or PHI[id], which value is easy to find again by analyzing the vector PHI. The vector PHI is then processed so as to replace the particular values with values calculated using the relationship (3).

After step 34 in Figure 3, the vector PHI is processed at 35 so as to obtain a final chronological succession of data items representative of the variation in load over time on the basis of the following relationship:

P[i] = (PHI[i]/KL)l/c (5) where P designates the vector containing said final chronological succession of data items.

It should be noted that the value of the force or of the dynamic load corresponds to the data item of maximum value in the vector P.

9

Claims

CLAIMS 1/ A method of processing an interferometer signal (S) for

measuring a force by means of the birefringence imparted by stress in a light waveguide, said method being characterized in that it comprises steps consisting in: digitizing (30) the interferometer signal so as to extract a chronological succession of amplitude extrema in digital form; processing (34) the succession of extrema so as to obtain a corresponding chronological succession of first data items representative of variation in phase rotation over time; and using a physical model relating phase rotation to force for processing (35) the succession of first data items so as to obtain a corresponding chronological succession of second data items representative of variation in force over time.

2/ The method according to claim 1, in which the processing of the extrema consists in taking into account alternately (62) an extremum from the beginning of the succession of extrema, and (63) an extremum from the end of the succession of extrema, and continuing towards the middle of the succession of extrema after each cycle, so as to obtain said succession of first data items (PHI) representative of variation in phase rotation over time.

3/ The method according to claim 2, in which, if the extremum taken into account in the processing is identified as corresponding to the phase rotation crossing an angular 30. determination threshold, the rank of the extremum in the succession of extrena serves to determine a corresponding value for the phase rotation.

4/ A method of processing an interferometer signal, substantially as herein described with reference to the accompanying drawings.