Method and system for exploiting the signals of linear inductive sensors, which system is adaptable for different sensor structures

A method and system for exploiting the signals of linear inductive sensors, which system is adaptable for different sensor structures. The system is formed of a linear inductive displacement sensor provided with a primary coil and two identical symmetrical secondary coils which are in series and in antiphase. A mobile core and an alternative voltage source are mounted between the terminals of the primary coil wherein the amplitude of the voltage at the primary coil terminals is greater than the maximum amplitude of the voltage that can be taken at the secondary coil terminals. The method has the steps of forming the sum of the two alternative voltages present at the primary and secondary coils. These voltages are rectified by means of full wave rectifying and deriving the difference between the sum of recitified voltages. A continuous means valve corresponding to the difference is also derived.

BACKGROUND OF THE INVENTION 
The present invention concerns a method and system to exploit signals 
derived from linear inductive displacement sensors. 
Linear inductive sensors are very commonly currently used, especially as 
regards angular attitude sensors and rectilinear displacement sensors. 
Based on the variational principle of an inductive resistor, they present 
the advantage of possessing extremely good linearity and excellent 
resolution. 
Linear inductive sensors are generally preferred to other non-linear types 
of sensors; in fact, the linear relation, which links the output signal to 
displacement, allows for simpler exploitation of the signal. Moreover, 
their conception renders these sensors sturdy and suitable for being 
installed in harsh environments. 
The most elementary embodiment of this type of sensor includes a primary 
winding and a secondary winding. The displacement of a magnetic core 
allows for an inductance variation which induces a secondary voltage 
variation. 
In this type of embodiment, the signal delivered by the secondary is 
sensitive to variations of the primary voltage, the variation of the 
winding ratio according to the temperature, and to its constructional 
inaccuracy. 
So as to minimize influence quantities, new sensors have been developed; 
these have been conceived on the basis of the principle relating to 
differential methods. The embodiment of these sensors comprises a primary 
winding and two secondary windings in antiphase. At this stage, it is 
possible to distinguish two types of sensors, namely 4-wire sensors and 
6-wire sensors. 
In the 6-wire sensors, the two secondary windings are independent. On the 
other hand, the two secondary windings of the 4-wire sensors are connected 
in series so as to be electrically in opposition. 
Thus, it can be seen that, because of the structural differences of these 
sensors, the systems for exploiting the signals they generate are specific 
to a particular structure and may not be used for a sensor with a 
different structure. 
SUMMARY OF THE INVENTION 
The object of the present invention is to overcome this drawback and to 
provide a system for exploiting the signals of linear inductive sensors 
and appropriate to the structure of said sensors. According to the 
invention, this result can be attained by virtue of the fact that the 
Applicant has discovered an original method and system for processing 
signals generated by a sensor with two series secondaries, this system 
having a particular structure enabling it to be adapted to sensors with 
different structures. 
To this end, the invention offers a method for exploiting signals generated 
by a linear inductive displacement sensor is provided with a primary, coil 
two identical symmetrical secondary coils in antiphase and connected in 
series and a mobile core. The invention being distinctive in that, as the 
amplitude of the voltage at the terminals of said primary is greater than 
the maximum amplitude able to be taken by the voltage at the terminals of 
the series connection of said secondaries: 
the sum is formed of the two alternating voltages respectively present at 
the terminals of said primary and at the terminals of the series 
connection of said secondaries; 
said sum of alternating voltages is double-alternation recitified; 
the alternating voltage present at the terminals of the primary is 
double-alternation rectified; 
the difference is formed between said sum of rectified voltages and said 
rectified voltage at the terminals of the primary; and 
the mean continuous value corresponding to said difference is formed. 
As shall be clearly shown by the following, the amplitude and the sign of 
said mean continuous value are respectively characteristics of the 
amplitude and direction of displacement of the core. 
In order to implement this method, the invention provides a remarkable 
system in that it comprises: 
first input means for a first alternating voltage (the primary voltage); 
second input means for a second alternating voltage (the secondary 
voltage); 
first addition means so as to form the sum of said first and second 
alternating voltages; 
first means for double-alternation rectification of said sum; 
second means for double-alternation rectification of said first alternating 
voltage; 
substraction means so as to form the difference between said rectified sum 
and said first rectified voltage; and 
first integration means so as to give the mean continuous value of said 
difference. 
Preferably, said system comprises floating input means for said alternating 
voltages respectively present at the terminals of said primary and at the 
terminals of the series connection of said secondaries. 
This is particularly advantageous as one of said floating input means is 
also used to form the sum of the two alternating voltages respectively 
present at the terminals of said primary and at the terminals of the 
series connection of said secondaries. 
So as to be able to allow for exploitation of signals generated by linear 
inductive displacement sensors with a single secondary or two independent 
secondaries, it is advantageous that the system according to the invention 
also comprises: 
a first switch disposed between said second rectification means and said 
substraction means; 
a circuit arm connected to the output of said first rectification means and 
comprising in series a second switch, second addition means and second 
integration means intended to give a mean continuous value; 
a link or coupling between the output of said second rectification means 
and said second addition means; and 
a third switch controlling the feeding of said first alternating voltage to 
said first addition means. 
As shall be seen from the following description, it is possible to process 
the signals of linear inductive sensors with one secondary (4 wires), with 
two independent secondaries (6 wires) or with two series secondaries (4 
wires) by connecting said sensors to said input means and by suitably 
activating said first, second and third switches.

The system 1, according to the invention and represented on FIG. 1, is 
designed to exploit the signals issued from an inductive displacement 
sensor 2 of the linear type, namely supplying an electric signal whose 
amplitude is proportional to the displacement. 
This inductive sensor 2 comprises a primary coil 3 associated with two 
identical secondary coils 4 and 5. Between the primary 3 and the 
secondaries 4 and 5, a mobile core 6 is disposed able to be joined by a 
link 7 to an object (not represented) for which it is desired to know the 
displacement; said secondaries 4 and 5 are disposed in such a way that, in 
the absence of said core 6, the voltages induced there by the primary 3 
are of equal amplitude and in antiphase. 
An alternating voltage source 8 is mounted between the terminals 9 and 10 
of the primary 3 so that the latter is the seat or receiver of an 
alternating primary voltage VR. 
The two secondaries 4 and 5 are series connected so that the voltage VX, 
which appears at the terminals 11 and 12 under the action of the primary 
voltage VR, is the algebraic sum of the voltages induced in each of said 
secondaries. For a median position of said core 6, these two induced 
voltages are equal and opposed so that the voltage VX is nil. By 
construction, the maximum amplitude of VX is less than that of VR. 
The exploitation system 1 comprises two differential input stages 13 and 
14. The terminals 15 and 16 of the differential stage 13 are intended to 
be connected to the terminals 9 and 10 of the primary 3, whereas the 
terminals 17 and 18 of the differential stage 14 are intended to be 
connected to the terminals 11 and 12 of the series connected secondaries 4 
and 5. 
The outputs 19 and 20 of said input differential stages 13 and 14 are 
connected to an adding device 21 whose output 22 is joined to a full wave 
rectifier 23. In addition, the output 19 of the stage 13 is connected to 
another full wave rectifier 24. 
The output 25 of the rectifier 23 is connected to the positive input of a 
subtracting device 27 whose negative input is joined to the output of the 
rectifier 24. 
The output 28 of the subtracting device 27 is joined to a filter 29 capable 
of supplying at its output 20 the mean value of the continuous signal it 
receives at its input 28. 
There now follows an explanation of the functioning of the system of FIG. 1 
with reference to FIGS. 2a to 2g and 3a to 3g. 
The alternating source 8 generates the primary voltage VR in the primary 3 
(see FIG. 2a). Whilst the core 6 is in its median position, the voltages 
induced in the secondaries 4 and 5 are of equal amplitude, but in 
antiphase, so that the signal VX is nil (see zone I of FIG. 2b). 
Accordingly, the signal at the output 20 of the input stage 14 is also 
nil. As a result, the signal at the output 22 of the adding device is 
constituted by the single voltage VR. The subtracting device 27 thus 
carries out the subtraction of two identical rectified voltages, namely 
the rectified voltage VR noted .vertline.VR.vertline.. The signal 
appearing at the output 30 is therefore nil. 
If at the instant t=to (see FIG. 2b), the magnetic core 6 is now moved to 
draw close to the secondary 4 (assumed to be in phase with the primary 3) 
and be distanced from the secondary 5 (assumed to be in antiphase with the 
primary 3), magnetic coupling is gradually increased between the primary 3 
and the secondary 4, and magnetic coupling is gradually reduced between 
the primary 3 and the secondary 5. Accordingly, as the displacement d 
gradually increases in the direction of the arrow F1 from the median 
position of the core 6 between the terminals 11 and 12, a voltage VX 
appears in phase with the voltage VR, but whose amplitude gradually 
increases (see FIG. 2b). As a result, the sum VR+VX appearing on the 
output 22 of the adding device 21 is thus also in phase with VR and 
presents an increasing amplitude with d (see FIG. 2c). 
At the output 25 of the full wave rectifier 23, a rectified voltage VR+VX, 
noted .vertline.VR+VX.vertline., thus appears and illustrated on FIG. 2d. 
This voltage .vertline.VR+VX.vertline. is thus constituted by a series of 
positive alternations of increasing amplitude. 
In the subtracting device 27, the rectified voltage VR, shown by FIG. 2e 
and noted .vertline.VR.vertline., is subtracted from the voltage 
.vertline.VR+VX.vertline.. Thus, at the output 28 of said subtracting 
device the d.c. voltage .vertline.VR+VX.vertline.-.vertline.VR.vertline. 
is obtained, as shown by FIG. 2f, and constituted by a series of positive 
alternations of progressively increasing amplitude (see FIG. 2f). 
The filter 29 thus supplies at its output 30 the positive and increasing 
continuous signal 31, as shown by FIG. 2g, and formed by the mean value of 
the signal .vertline.VR+VX.vertline.-.vertline.VR.vertline. of FIG. 2f. 
Thus, it can be seen that the displacement of the magnetic core 6 in the 
direction of the arrow F1 is expressed by the appearance of a positive 
continuous and increasing signal 31 on the terminal 30. The amplitude of 
this signal 31 is representative of the amplitude of the displacement d in 
the direction of the arrow F1. 
Conversely, if at the instant t=to (see FIG. 3b), the magnetic core 6 is 
displaced (see the arrow F2 of FIG. 1) in order to move it away from the 
secondary 4 (in phase with the primary 3) and bring it closer to the 
secondary 5 (in antiphase with the primary 3), magnetic coupling is 
increased between the primary 3 and the secondary 5 and progressively 
reduces magnetic coupling between said primary 3 and the secondary 4. 
Accordingly, as the displacement d increases in the direction of the arrow 
F2 from the median portion of the core 6 between the terminals 11 and 12, 
a voltage VX appears in antiphase with the voltage VR, but whose amplitude 
progressively increases (see FIG. 3b). Since the amplitude of VR (FIG. 3a) 
is selected so as to be greater than the maximum amplitude able to be 
taken by VX, the sum VR+VX of the two voltages VR and VX in antiphase is 
in phase with VR, but presents a decreasing amplitude with d (see FIG. 
3c). Such a signal VR+VX appears at the output 22 of the adding device 21. 
The rectified voltages .vertline.VR+VX.vertline. and .vertline.VR.vertline. 
respectively appearing at the outputs 25 and 26 of the rectifiers 23 and 
25 are respectively shown by FIGS. 3d and 3e; the voltage 
.vertline.VR+VX.vertline. is constituted by a series of positive 
alternations but of decreasing amplitude, whereas clearing the voltage 
.vertline.VR.vertline. is constituted by a series of positive alternations 
of constant amplitude. Moreover, the amplitude of the alternations of 
.vertline.VR.vertline. is much greater than those of the alternations of 
.vertline.VR+VX.vertline.. 
Accordingly, at the output 28 of the subtracting device 27, the d.c. 
voltage .vertline.VR+VX.vertline.-.vertline.VR.vertline., shown on FIG. 
3f, is constituted by a series of negative alternations of increasing 
amplitude. The filter 29 thus supplies at its output 30 the continuous 
negative decreasing signal 32, as shown by FIG. 3g, and formed by the mean 
value of the signal .vertline.VR+VX.vertline.-.vertline.VR.vertline. of 
FIG. 3f. 
Thus, it can be seen that displacement of the magnetic core 6 in the 
direction of the arrow F2 is expressed by the appearance on the terminal 
30 of a negative continuous decreasing signal 32 whose amplitude is 
representative of the displacement d in the direction of the arrow F2. 
Thus, at the output 30 of the filter 29, a signal (31 or 32) appears whose 
amplitude is representative of the size of the displacement d and whose 
sign is representative of the direction (F1 or F2) of said displacement. 
FIG. 4 shows an embodiment variant 40 of the system according to the 
present invention. This embodiment variant shows the elements 13, 15-19 
and 23-30 described with reference to FIG. 1. As one can see, the 
differential input stage 14 and the adding device 21 have been replaced by 
a double differential input stage 41 receiving both the voltage VX from 
the terminals 17 and 18 and the voltage VR from the terminals 15 and 16. 
Accordingly, the link 33 between the output 19 of the differential stage 
13 and the adding device 21 have been suppressed and replaced by a link 42 
between the terminals 15 and 16 and the double input stage 41. At the 
output 43 of the latter, the sum VR+VX appears directly. This output 43 is 
connected to the double-alternation rectifier 23 instead of the output 22 
of the adding device 21. 
Of course, the functioning of the system 40 is similar to that of the 
system 1 described with reference to FIGS. 2a to 2g and 3a to 3g. 
The embodiment variant 50 represented on FIG. 5 shows the elements 13, 
15-19, 23-30 and 41-43 of the system of FIG. 4. In addition, this system 
50 comprises: 
a switch 51 inserted in the link 26 between the rectifier 24 and the 
subtracting device 27; 
a circuit arm 52-55 connecting the output 25 of the rectifier 23 to a 
supplementary terminal 55 and comprising in series a switch 52, an adding 
device 53 and an averaging filter 54; 
a link 56 between the output 26 of the rectifier and the adding device 53; 
and 
a switch system 57 able to open the link 42. 
It can easily be seen that when the switch 51 is closed at the same time 
when the switches 52 and 57 are closed, the system 50 of FIG. 5 is 
identical to that of FIG. 4 and thus functions in a similar way. In this 
configuration, the system 50 is thus suitable for exploiting the signals 
of an inductive sensor with two series secondaries, as illustrated on FIG. 
1. In this case, a continuous signal appears which is representative of 
the rectified voltage .vertline.VR.vertline. on the terminal 55 by virtue 
of the link 56 and the action of the filter 54 (similar to the filter 29). 
Thus, it is possible to place a divider (not represented) between the 
terminals 30 and 55 so as to form the ratio of the signals respectively 
representative of .vertline.VX.vertline. and .vertline.VR.vertline. so as 
to be freed from the effect of any possible variations of the voltage 
generated by the source 8. 
FIG. 6 shows a further known type of inductive sensor comprising a primary 
61 and a single secondary 62. The primary 61 is fed between its terminals 
63 and 64 by the alternating source 8 so that an alternating voltage VX, 
representative of the displacement of the core 6 disposed between said 
primary and secondary, appears at the terminals 65 and 66 of said 
secondary 62. The terminals 63 to 66 of the sensor are respectively 
connected to the terminals 15 to 18 of the system 50. In the latter, the 
switches 51 and 52 are kept open, whereas the switch 57 is open. 
Accordingly, a continuous signal representative of the displacement 
appears on the terminal 30, whereas a continuous signal representative of 
the voltage VR between the terminals 63 and 64 of the primary 61 appears 
on the terminal 55. The system 50 thus makes it possible to know said 
displacement and subsequently to carry out the ratio of the signals 
appearing at the terminals 30 and 55, as mentioned above. The terminals 63 
and 64 could subsequently be connected to the terminals 17 and 18 and the 
terminals 65 and 66 to the terminals 15 and 16. Moreover, it is not 
essential to have the link between the terminals 63 and 64 and the 
terminals 15, 16 or 17, 18, so that it is possible to simultaneously 
connect two sensors 60 to the system 50. In this case, a continuous signal 
representative of the displacement of a core 6 of a sensor 60 appears at 
each of the terminals 30 and 55. 
The inductive sensor 70, of a known type and shown on FIG. 7, comprises a 
primary 71 and two independent secondaries 72 and 73. The primary 71 is 
fed between its terminals by an alternating source 8 so that the 
alternating voltages V1 and V2, representative of displacement of the core 
6 disposed between said primary and secondary, respectively appear at the 
terminals 74 and 75 of the secondary 72 and at the terminals 76 and 77 of 
the secondary 73. The terminals 74 and 77 are respectively connected to 
the terminals 15 to 18 of the system 50. In the latter, the switches 51 
and 52 are kept closed and the switch 57 is kept open. 
Accordingly, a continuous signal representative of the difference 
.vertline.V1.vertline.-.vertline.V2.vertline. between the rectified 
voltages .vertline.V1.vertline. and .vertline.V2.vertline. appears on the 
terminal 30, whereas on the terminal 55, there appears the continuous 
signal representative of the sum 
.vertline.V1.vertline.+.vertline.V2.vertline. of said rectified voltages. 
It is a simple matter to verify that the ratio of these two continuous 
signals, namely the one concerning the sum and the other the difference, 
is representative of the displacement d of the core 6. Accordingly, a 
divider (not shown) disposed between the terminals 30 and 55 also makes it 
possible to know said displacement d.