SENSOR FOR MEASURING THE ABSOLUTE POSITION OF A MOVING PART

Abstract

The invention provides a measurement sensor for determining the position of a moving body, the sensor comprising a series of at least four detector probes for detecting a physical magnitude coming from a target comprising at least one track for creating a physical magnitude that is measurable by the detector probes and that varies along the path of the target with a function that is continuous and that includes a first harmonic and a second harmonic, the probes being connected to a processor unit for processing signals delivered by the probes, the processor unit including a reconstruction system for performing a linear combination of the signals and for obtaining firstly two quadrature signals including solely the first harmonic, and secondly two quadrature signals including solely the second harmonic, the unit also including a calculation system for processing the quadrature signals in order to determine the position of the moving body.

Claims

1. A measurement sensor for determining the position of a moving body (11) moving along a determined path F, the sensor comprising a series of at least four detector probes (12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N) for detecting a physical magnitude coming from a target (13) securely mounted on the moving body, the target comprising at least one track (14) for creating a physical magnitude that is measurable by the detector probes and that varies along the path of the target with a function that is continuous and that includes a first harmonic (N.sub.c) and a second harmonic (N.sub.d), the probes being connected to a processor unit (16) for processing signals delivered by the probes, the processor unit including a reconstruction system (19) for performing a linear combination of the signals delivered by the detector probes and for obtaining from the liner combination of those signals at least firstly two quadrature signals (a.sub.1, a.sub.2) including solely the first harmonic, and secondly two quadrature signals (a.sub.3, a.sub.4) including solely the second harmonic, the unit also including a calculation system (20) for processing the quadrature signals in order to determine the position of the moving body.

2. A measurement sensor according to claim 1, characterized in that the calculation system (20) for determining the position of the moving body calculates the A tan 2 of the two quadrature signals including the first harmonic and the A tan 2 of the two quadrature signals including the second harmonic, the calculation system giving two relative positions for the moving body.

3. A measurement sensor according to claim 1, characterized in that the calculation system (20) calculates the difference between the two relative positions for the moving body modulo 2 in order to obtain the absolute position ({circumflex over (x)}) of the moving body.

4. A measurement sensor according to claim 1, characterized in that the first harmonic and the second harmonic present respectively a first spatial frequency (N.sub.c) and a second spatial frequency (N.sub.d) such that the ratio of the spatial frequencies is given by the following relationship;
N.sub.d=N.sub.c1 where is an integer greater than 1.

5. A measurement sensor according to claim 1, characterized in that the target (13) creates a magnetic field that varies continuously and that includes the first and second harmonics, the amplitude or the direction of the magnetic field being detected by the detector probes.

6. A measurement sensor according to claim 1, characterized in that the reconstruction system (19) performs a linear combination of the signals delivered by the probes by applying weighting weights, these weighting weights being programmable in such a manner as to enable the quadrature signals to be reconstructed for a spacing of given value between the probes.

7. A measurement sensor according to claim 1, characterized in that the weighting weights of the reconstruction system (19) are selected so as to obtain a zero contribution from the uniform component of the magnetic field to the reconstructed signals (a.sub.1, a.sub.2, a.sub.3, a.sub.4).

8. A measurement sensor according to claim 1, characterized in that the maximum distance between two of its detector probes (12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N) is strictly less than one half-period of the first spatial frequency (N.sub.c).

9. A measurement sensor according to claim 1, characterized in that all of the detector probes (12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N) are grouped together in a single microelectronic integrated circuit.

10. A measurement sensor according to claim 1, characterized in that it includes at least two probes (12.sub.1, 12.sub.2) located on the path of the target that enable the component of the magnetic field that is tangential to the path to be measured, and at least two probes (12.sub.3, 12.sub.4) located along the path of the target that enable a component of the magnetic field that is perpendicular to the path to be measured.

11. A measurement sensor according to claim 1, characterized in that the target (13) has two tracks (14), each track being magnetized with one of the two harmonics, and in that the detector probes (12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N) are positioned substantially centered relative to the two tracks.

Description

[0023] Various other characteristics appear from the following description given with reference to the accompanying drawings which show embodiments of the invention as non-limiting examples.

[0024] FIG. 1 shows a sensor of the prior art.

[0025] FIG. 2 is a functional representation of an embodiment of an angular position sensor of the invention.

[0026] FIG. 3 is a diagram showing a linear position sensor of the invention.

[0027] FIG. 4 shows an embodiment of a magnetized target for an angular sensor of the invention.

[0028] FIG. 5 shows an angular position sensor of the invention with probes measuring two orthogonal components of the magnetic field generated by the magnetized target.

[0029] As can be seen more precisely in FIG. 2, the invention relates to a measurement sensor 10 for determining the position of a moving body 11 moving along a determined path that is represented by arrow F. In the example shown in FIG. 2, the moving body 11 moves along a circular path such that the sensor is an angular position sensor. In the example shown in FIG. 3, the moving body 11 moves along a rectilinear path so that the sensor is a linear position sensor. Naturally, the measurement sensor of the invention is suitable for determining the position of a moving body that follows some other path, such as a curvilinear path.

[0030] The sensor 10 of the invention has a series of at least four detector probes 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N for detecting a physical magnitude coming from a target 13 mounted securely to the moving body 11. The target 13 has one or more tracks 14 for creating a physical magnitude that is measurable by the detector probes. This measurable physical magnitude varies along the path of the moving body with a function that is continuous and it has a first harmonic N.sub.c and a second harmonic N.sub.d. In a preferred embodiment, the measurable physical magnitude is a magnetic field such that the sensor has a magnetized target and Hall effect probes. Naturally, the measurable physical magnitude may be of some other kind. Thus, the target 13 may have conductive tracks of varying width and the probes may be coils powered at high frequency so as to be capable of measuring variation of inductances as a function of the width of the track facing the coil as a result of eddy currents. In order to simplify the description, the detailed description below describes the embodiment having a magnetized target and Hall effect probes.

[0031] In the example under consideration, the target 13 has a magnetized track 14. The amplitude or the direction of the magnetization of the track varies in the travel direction of the target with a function that is continuous and that includes a first harmonic N.sub.c and a second harmonic N.sub.d.

[0032] The detector probes 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N are connected to a processor unit for processing signals s.sub.1, s.sub.2, s.sub.3, s.sub.4, . . . s.sub.N delivered by the probes.

[0033] As described in greater detail in the description below, the processor unit 16 has a system 17 for acquiring and processing signals delivered by the detector probes, and connected to a reconstruction system 19 for performing a linear combination of the signals delivered by the probes. The reconstruction system 19 serves to perform a linear combination of the signals s.sub.1, s.sub.2, s.sub.3, s.sub.4, . . . , s.sub.N to obtain at least firstly two quadrature signals a.sub.1, a.sub.2 including solely the first harmonic N.sub.c, and secondly two quadrature signals a.sub.3, a.sub.4 containing solely the second harmonic N.sub.d. The processor unit 16 also has a calculation system 20 for processing the quadrature signals in order to determine the position of the moving body. Typically, the calculation system 20 gives two relative positions X.sub.c and X.sub.d for the moving body 11 by calculating the two-argument inverse tangent (A tan 2) of the two quadrature signals having the first harmonic N.sub.c (i.e. the signals a.sub.1 and a.sub.2 in the example shown) and the A tan 2 of the two quadrature signals having the second harmonic N.sub.d (i.e. the signals a.sub.3 and a.sub.4 in the example shown). The calculation system 20 then takes the difference modulo 2n between the two relative positions X.sub.c and X.sub.d for the moving body in order to obtain the absolute position x of the moving body.

[0034] The description below describes in greater detail the unit 16 for processing the signals s.sub.1, s.sub.2, s.sub.3, s.sub.4, . . . , s.sub.N delivered by the probes. It should be considered that the detector probes 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N are mutually offset in the travel direction of the target. The magnetic field b measured at a point x in a given direction may be described as follows:

b(x)=V.sub.c cos(w.sub.cx)+V.sub.d cos(w.sub.dx)

where w.sub.c=2N.sub.c/L and w.sub.d=2N.sub.d/L, where N.sub.c and N.sub.d are integer numbers corresponding to the number of periods of each harmonic over the stroke L of the target 11, and where x is the position taken along the path. The first harmonic thus presents a first spatial frequency N.sub.c (e.g. considered to be low frequency) and the second harmonic presents a second spatial frequency N.sub.d (e.g. considered to be a high frequency). The parameters V.sub.c and V.sub.d correspond to the amplitudes of these two harmonics.

[0035] Typically, the sensor 1 of the invention uses the acquisition and processor system 17 to take N simultaneous measurements s.sub.k of the magnetic field generated by the magnetized track 14. In the example shown in FIGS. 2 and 3, these measurements are taken using the probes 12.sub.1 to 12.sub.4 (N=4 in this example), which probes are mutually offset in the travel direction F. These various measurements s.sub.k are defined by:

s.sub.k(x)=b(x+.sub.k)

where .sub.k corresponds to the position of each measurement, i.e. to the positions selected for the detector probes 12.sub.1 to 12.sub.4 (FIG. 3). By means of conventional trigonometric formulae, s.sub.k may be rewritten as follows:

s.sub.k(x)=V.sub.c cos(w.sub.c.sub.k).Math.cos(w.sub.cx)V.sub.c sin(w.sub.c.sub.k).Math.sin(w.sub.cx)+V.sub.d cos(w.sub.d.sub.k).Math.cos(w.sub.dx)V.sub.d sin(w.sub.d.sub.k).Math.sin(w.sub.dx)

[0036] It is possible to rewrite this formula in the following matrix format:

[00001]$\left(\begin{array}{c}{s}_1\left(x\right)\\ \mathrm{.Math.}\\ s_N\left(x\right)\end{array}\right)=\left(\begin{array}{cccc}{V}_c.Math.\cos \left(w_c.Math.{}_1\right)& -V_c.Math.\sin \left(w_c.Math.{}_1\right)& V_d.Math.\cos \left(w_d.Math.{}_1\right)& -V_d.Math.\sin \left(w_d.Math.{}_1\right)\\ \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}\\ V_c.Math.\cos \left(w_c.Math.{}_N\right)& -V_c.Math.\sin \left(w_c.Math.{}_N\right)& V_d.Math.\cos \left(w_d.Math.{}_N\right)& -V_d.Math.\sin \left(w_d.Math.{}_N\right)\end{array}\right).Math.\left(\begin{array}{c}{a}_1\left(x\right)\\ a_2\left(x\right)\\ a_3\left(x\right)\\ a_4\left(x\right)\end{array}\right)$

where the new variables a.sub.P are given by:

[00002]$\{\begin{array}{c}{a}_1\left(x\right)=\cos \left(w_c.Math.x\right)\\ a_2\left(x\right)=\sin \left(w_c.Math.x\right)\\ a_3\left(x\right)=\cos \left(w_d.Math.x\right)\\ a_4\left(x\right)=\sin \left(w_d.Math.x\right)\end{array}$

[0037] This formula can be written more simply using matrix notation:

S(x)=.Math.A(x)

where S corresponds to the N1 column vector containing all of the measurements s.sub.k depending on the position x, and where M is an N4 matrix depending on the position .sub.k and on the periodicities w.sub.c and w.sub.d or spatial frequencies N.sub.c and N.sub.d. Finally, A is a 41 vector containing the variables a.sub.p depending on the position x in simpler manner than the variables s.sub.k. Since the relationships between the position x and the variables a.sub.p are simpler, the purpose is thus to determine these values a.sub.p from the various measurements s.sub.k. With N=4 different measurements, and if the matrix M is a full rank matrix, the new variables a.sub.p can be determined as follows as a function of the measurements s.sub.k:

A(x)=M.sup.1.Math.S(x)

where M.sup.1 is the matrix that is the inverse of the above-described matrix M, and is also referred to as the weighting matrix.

[0038] It should be observed that the weighting matrix M.sup.1, like the matrix M, depends solely on constant and known parameters of the measurement system, i.e. on the position .sub.k of the detector probes and on the periodicities w.sub.c and w.sub.d. For a given measurement system, it is thus possible to determine a matrix M.sup.1 that transforms the vector S(x) of N measurements, each comprising both harmonics, into a vector A(x) of four signals a.sub.1, a.sub.2, a.sub.3, a.sub.4 in the example shown.

[0039] This matrix M.sup.1 defines the weighting weights that are applied to the signals s.sub.1, s.sub.2, s.sub.3, s.sub.4 by the reconstruction system 19 in order to obtain the signals a.sub.1, a.sub.2, a.sub.3, a.sub.4. The four signals of the vector A(x) comprise firstly two quadrature signals a.sub.1, a.sub.2 including solely the first harmonic N.sub.c, and secondly two quadrature signals a.sub.3, a.sub.4 including solely the second harmonic N.sub.d.

[0040] In other words, the reconstruction system 19 performs a linear combination of the signals delivered by the probes by applying weighting weights selected as a function firstly of the spatial frequencies of the two harmonics and secondly of the distances between the detector probes.

[0041] These weighting weights are preferably programmable so as to enable the quadrature signals to be reconstructed for a spacing of given value between the probes and for different spatial frequencies. Thus, a standard subassembly comprising probes 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N with constant spacing between the probes can be used for several variant sensors with targets having a variety of diameters and spatial frequencies.

[0042] In the general situation where the number of measurements N is greater than 4, the vector A(x) can always be determined by the method of least squares:

A(x)=(M.sup.TM).sup.1M.sup.T.Math.S(x)

where M.sup.T is the transpose of the matrix M.

[0043] Once the vector A has been determined, the calculation system 20 can determine the position x very easily. Initially, two relative positions X.sub.c and X.sub.d are calculated:

[00003]$\{\begin{array}{c}{X}_c\left(x\right)=a.Math..Math.\tan .Math..Math.2.Math.\left(a_2\left(x\right),a_1\left(x\right)\right)=\mathrm{modulo}\left(x,\frac{L}{N_c}\right)\\ X_d\left(x\right)=a.Math..Math.\tan .Math..Math.2.Math.\left(a_4\left(x\right),a_3\left(x\right)\right)=\mathrm{modulo}\left(x,\frac{L}{N_d}\right)\end{array}$

where the function a tan 2 is the function linking the sine and cosine functions to the angle over a period of 2.

[0044] Finally, the absolute position x of the target 13 may be estimated as follows:

{circumflex over (x)}=modulo(X.sub.cX.sub.d,2)

[0045] It is possible to improve the sensor so that it cancels out an external magnetic field that is uniform. For this purpose, assume that the total field b(x) is now written as follows:

b(x)=V.sub.c cos(w.sub.cx)+V.sub.a cos(w.sub.dx)+V.sub.e

where V.sub.e is the amplitude of the uniform external magnetic field. The measurements s.sub.k are now written as follows:

[00004]$\left(\begin{array}{c}{s}_1\left(x\right)\\ \mathrm{.Math.}\\ s_N\left(x\right)\end{array}\right)=\left(\begin{array}{ccccc}{V}_c.Math.\cos \left(w_c.Math.{}_1\right)& -V_c.Math.\sin \left(w_c.Math.{}_1\right)& V_d.Math.\cos \left(w_d.Math.{}_1\right)& -V_d.Math.\sin \left(w_d.Math.{}_1\right)& 1\\ \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}& \mathrm{.Math.}\\ V_c.Math.\cos \left(w_c.Math.{}_N\right)& -V_c.Math.\sin \left(w_c.Math.{}_N\right)& V_d.Math.\cos \left(w_d.Math.{}_N\right)& -V.Math..Math.\sin \left(w_d.Math.{}_N\right)& 1\end{array}\right).Math.\left(\begin{array}{c}{a}_1\left(x\right)\\ a_2\left(x\right)\\ a_3\left(x\right)\\ a_4\left(x\right)\\ V_e\end{array}\right)$

[0046] The matrix M now possesses an additional column. Consequently, it suffices to perform N=5 distinct measurements so that the matrix is once more square and invertible. Thus, it is possible to determine separately the contributions of the useful variables a.sub.P and of the external magnetic noise V.sub.e. In this variant embodiment, the reconstruction system 19 uses the linear combination of the weighted signals to obtain firstly two quadrature signals a.sub.1, a.sub.2 including solely the first harmonic, and secondly two quadrature signals a.sub.3, a.sub.4 containing solely the second harmonic, together with the signal V.sub.e including solely the uniform component of the magnetic field. The weighting weights of this reconstruction system 19 make it possible to obtain a contribution of zero from the uniform component of the magnetic field to the reconstructed signals a.sub.1, a.sub.2, a.sub.3, a.sub.4.

[0047] So long as the entire processing system remains under linear conditions, the signals a.sub.1, a.sub.2, a.sub.3, a.sub.4 are not influenced by the uniform magnetic field, and consequently the position measurement is not disturbed by a uniform external magnetic field. In certain applications, the external magnetic field may reach extreme values that cause the magnetic probes to saturate or that cause a portion of the processing system to saturate, which can lead to an erroneous measurement. In applications that require a high level of operating safety, it can therefore be useful to validate the determined position only when the signal V.sub.e representing the external magnetic field remains within acceptable limits, and to issue an alert signal otherwise.

[0048] The weighting matrix M.sup.1 is determined so as to completely eliminate the unwanted harmonics in the signals a.sub.1(x) to a.sub.N(x). It is possible that this weighting matrix simultaneously gives rise to large attenuation of the useful harmonic, which would have the consequence of degrading the signal-to-noise ratio and thus the accuracy of the measurement. This unwanted attenuation depends on the spacing between the measurement points and on the spatial frequencies used. In general manner, for a given spacing of measurement points, a weighting matrix that enables one given spatial frequency to be cancelled completely is likely to greatly attenuate spatial frequencies that are nearby. The conventional vernier method makes use of two frequencies associated by the equation:

N.sub.d=N.sub.c1

[0049] In order to obtain good measurement resolution, it is desirable to select numbers N.sub.c and N.sub.d that are high, and under such circumstances it can be considered that the frequencies are close together and that the attenuation of the useful signal runs the risk of being considerable. In order to remedy this problem, it is proposed in a preferred version of the invention to use a relationship between the two spatial frequencies N.sub.c, N.sub.d in compliance with the following equation:

N.sub.d=N.sub.c1

where is an integer greater than 1.

[0050] This makes it possible to obtain a greater difference between the spatial frequencies used, while maintaining the possibility of finding the absolute position. The absolute position x is then obtained from two relative positions X.sub.c and X.sub.d by using the following equation:

{circumflex over (x)}=modulo(X.sub.cX.sub.d,2)

[0051] According to an advantageous embodiment characteristic, the maximum distance between two detector probes 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N is strictly less than one half-period of the first spatial frequency N.sub.c, i.e. the lower frequency.

[0052] According to another advantageous embodiment characteristic, all of the detector probes 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N are grouped together in a single microelectronic integrated circuit 22. Such an advantage is made possible by the fact that all of the detector probes can be mounted close to one another. Typically, all of the detector probes can be accommodated on an area of the order of a few square millimeters (mm.sup.2).

[0053] In the examples shown in FIGS. 2 and 3, the target 13 has a single track 14 for creating a physical magnitude that is measurable by the detector probes and that varies along the path of the target with a continuous function including a first harmonic N.sub.c and a second harmonic N.sub.d. FIG. 4 shows an embodiment of a magnetized target 14 having a single magnetized track. The arrows represent the direction of the magnetic flux and the gray shades represent its intensity. Making the target with a single track contributes to making a measurement sensor that is compact.

[0054] It should be observed that the measurement sensor of the invention may be used with a target 13 having two tracks 14 mounted side by side, each being magnetized with a respective one of the two harmonics. In a preferred embodiment variant, the two tracks 14 are arranged to be side by side while the detector probes 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N are positioned to be substantially centered relative to the two side-by-side tracks. In other words, the detector probes 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4, . . . , 12.sub.N are placed over the junction between the two tracks.

[0055] FIG. 5 shows another sensor in a variant of the invention. In this variant, instead of using four probes distributed over four positions that are shifted in the measurement direction, two pairs of probes 12.sub.1, 12.sub.2 and 12.sub.3, 12.sub.4 are used that are located at only two different positions in the travel direction, each pair of probes measuring two components of the magnetic field in two perpendicular directions, comprising a component b.sub.T that is tangential to the path and a component b.sub.P that is perpendicular to the path. Firstly, the magnetic field around the target may be approximated in the same manner as above in two directions in three-dimensional space:

b.sub.P(x)=V.sub.Pc cos(w.sub.cx)+V.sub.Pd cos(w.sub.dx)

b.sub.T(x)=V.sub.Tc sin(w.sub.cx)+V.sub.Td sin(w.sub.dx)

where b.sub.P and b.sub.T are two mutually perpendicular components of the magnetic field, and where the coefficients V.sub.xx are constants which can be obtained by simulation or by measurement. In this variant of the invention, the four measurements s.sub.k are defined as follows:

s.sub.1(x)=b.sub.P(x+.sub.1)

s.sub.2(x)=b.sub.P(x+.sub.2)

s.sub.3(x)=b.sub.T(x+.sub.1)

s.sub.4(x)=b.sub.T(x+.sub.2)

where .sub.1 and .sub.2 are the two measurement positions in this variant of the invention. By using conventional trigonometric formulae, it is possible as above to write a matrix system:

[00005]$\left(\begin{array}{c}{s}_1\left(x\right)\\ s_2\left(x\right)\\ s_3\left(x\right)\\ s_4\left(x\right)\end{array}\right)=\left(\begin{array}{c}{V}_{\mathrm{Pc}}.Math.\cos \left(w_c.Math.{}_1\right)-V_{\mathrm{Pc}}.Math.\sin \left(w_c.Math.{}_1\right).Math.V_{\mathrm{Pd}}.Math.\cos \left(w_d.Math.{}_1\right)-V_{\mathrm{pd}}.Math.\sin \left(w_d.Math.{}_1\right)\\ V_{\mathrm{Pc}}.Math.\cos \left(w_c.Math.{}_2\right)-V_{\mathrm{Pc}}.Math.\sin \left(w_c.Math.{}_2\right).Math.V_{\mathrm{Pd}}.Math.\cos \left(w_d.Math.{}_2\right)-V_{\mathrm{pd}}.Math.\sin \left(w_d.Math.{}_2\right)\\ V_{\mathrm{Tc}}.Math.\sin \left(w_c.Math.{}_1\right)-V_{\mathrm{Tc}}.Math.\cos \left(w_c.Math.{}_1\right).Math.V_{\mathrm{Td}}.Math.\sin \left(w_d.Math.{}_1\right).Math.V_{\mathrm{Td}}.Math.\cos \left(w_d.Math.{}_1\right)\\ V_{\mathrm{Tc}}.Math.\sin \left(w_c.Math.{}_2\right)-V_{\mathrm{Tc}}.Math.\cos \left(w_c.Math.{}_2\right).Math.V_{\mathrm{Td}}.Math.\sin \left(w_d.Math.{}_2\right).Math.V_{\mathrm{Td}}.Math.\cos \left(w_d.Math.{}_2\right)\end{array}\right).Math.\left(\begin{array}{c}{a}_1\left(x\right)\\ a_2\left(x\right)\\ a_3\left(x\right)\\ a_4\left(x\right)\end{array}\right)$

[0056] The procedure is then identical to that described above. Thus, the external field can be cancelled in similar manner by adding a third measurement on the component b.sub.P and a third measurement on the component b.sub.T.

[0057] It can be seen from the above description that the detector probes are characterized either by their locations in three-dimensional space by being mutually offset relative to the travel path of a moving body, or else by the component of the measured physical magnitude, such as for example the radial and axial components of the magnetic field.