Magnetic sensor including a Lorentz force transducer driven at a frequency different from the resonance frequency, and method for driving a Lorentz force transducer

Abstract

A magnetic field sensor includes a die and a current generator in the die. The current generator generates a driving current. A Lorentz force transducer is also formed in the die and coupled to the current generator to obtain measurements of a magnetic field based upon the Lorentz force. The magnetic field has a resonance frequency and the current generator drives the Lorentz force sensor with the driving current having a non-zero frequency different from the resonance frequency.

Claims

1. A magnetic-field sensor, comprising: a die; a first Lorentz force transducer formed in the die, the first Lorentz force transducer having a first resonance frequency; and a current generator formed in the die and coupled to the first Lorentz force transducer, the current generator configured to generate a driving current having a non-zero frequency to drive the first Lorentz force transducer to sense a Lorentz force indicative of a magnetic field received by the magnetic-field sensor, the non-zero frequency being different than the first resonance frequency.

2. The magnetic sensor of claim 1, wherein the current generator comprises a micro-electromechanical resonator.

3. The magnetic sensor of claim 1, wherein the driving current comprises a square wave signal or a sinusoidal signal.

4. The magnetic sensor of claim 1 further comprising a second Lorentz force transducer formed in the die and coupled to the current generator, the second Lorentz force transducer having a second resonance frequency.

5. The magnetic sensor of claim 4, wherein the first and second Lorentz force transducers are coupled in series, and wherein the current generator is coupled across the first and second Lorentz force transducers.

6. The magnetic sensor of claim 5, wherein the first and second Lorentz force transducers are configured to be sensitive to components of the magnetic field oriented in a first direction and a second direction, respectively, the second direction being different than the first direction.

7. The magnetic sensor of claim 1, wherein the current generator is configured to generate the driving current independent of the first resonance frequency of the first Lorentz force transducer.

8. The magnetic sensor of claim 1, wherein the first Lorentz force transducer comprises a mobile element that is coupled to the current generator, and wherein the mobile element is configured so the driving current flows through the mobile element, the mobile element being subjected to the Lorentz force due to the magnetic field received by the magnetic sensor.

9. The magnetic sensor of claim 8, wherein the mobile element further comprises at least one cantilever element and first and second fixed-electrode elements arranged proximate each of the at least one cantilever element, the first Lorentz transducer configured to sense first and second capacitances between each at least one cantilever element and the corresponding first and second fixed-electrode elements, the first and second capacitances having values that are a function of the sensed Lorentz force.

10. A magnetic-field sensor, comprising: a die; a first Lorentz force transducer formed in the die, the first Lorentz force transducer having a first resonance frequency; a second Lorentz force transducer formed in the die, the second Lorentz force transducer having a second resonance frequency; and a current generator formed in the die and coupled to the first and second Lorentz force transducers, the current generator configured to generate a driving current having a non-zero frequency to drive the first and second Lorentz force transducers to sense a Lorentz force generated in response to a magnetic field applied to the magnetic-field sensor, the non-zero frequency of the driving current being different than the first and second resonance frequencies and the current generator not driving the first Lorentz force transducer with the driving current having the non-zero frequency equal to the first resonance frequency and not driving the second Lorentz force transducer with the driving current having the non-zero frequency equal to the second resonance frequency.

11. The magnetic sensor of claim 10, wherein each of the first and second Lorentz force transducers is configured to vary a corresponding mechanical quantity responsive to the Lorentz force generated in response to the magnetic field.

12. The magnetic sensor of claim 11, wherein magnitude of the Lorentz force in each of the first and second Lorentz force sensors is proportional to a magnitude of the driving current.

13. The magnetic sensor of claim 10, wherein the current generator comprises a micro-electromechanical resonator.

14. The magnetic sensor of claim 10, wherein the driving current comprises a square wave signal or a sinusoidal signal.

15. The magnetic sensor of claim 10, wherein the first and second Lorentz force transducers are coupled in series, and wherein the current generator is coupled across the first and second Lorentz force transducers.

16. The magnetic sensor of claim 15, wherein the first and second Lorentz force transducers are configured to be sensitive to components of the magnetic field oriented in a first direction and a second direction, respectively, the second direction being different than the first direction.

17. An electronic system, comprising: a magnetic sensor including, a plurality of Lorentz force transducers, each Lorentz force transducer having a respective resonance frequency; and a current generator coupled to the first Lorentz force transducer, the current generator configured to generate a driving current having a non-zero frequency to drive the first Lorentz force transducer to sense a Lorentz force indicative of a magnetic field applied to the magnetic sensor, the non-zero frequency being different than the respective resonance frequencies; and a processing unit electrically coupled to the magnetic sensor.

18. The electronic system of claim 17, wherein the plurality of Lorentz force transducers comprises first, second and third Lorentz force transducers coupled in series, and wherein the first, second and third Lorentz force transducers are configured to sense three different directions of the magnetic field applied to the magnetic sensor.

19. The electronic system of claim 18, wherein the first, second, and third Lorentz force transducers are formed on a single die.

20. The electronic system of claim 19 further comprising a display electrically coupled to the processing unit.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) For a better understanding of the disclosure, embodiments thereof are now described, purely by way of non-limiting example and with reference to the attached drawings, wherein:

(2) FIG. 1 shows schematically a cross section of a magnetometer based upon the Lorentz force of a known type;

(3) FIGS. 2 and 4 show plots of an example of a transfer function of a Lorentz force transducer;

(4) FIG. 3 is a schematic illustration of a cross section of an embodiment of the present magnetic sensor;

(5) FIG. 5 shows a block diagram of an embodiment of the present magnetic sensor; and

(6) FIG. 6 shows a block diagram of an electronic system including the present magnetic sensor.

DETAILED DESCRIPTION

(7) FIG. 3 illustrates a magnetic sensor 50, which comprises a Lorentz force transducer 55, referred to hereinafter as transducer 55, and a current generator 60. Purely by way of non-limiting example, it is assumed that the transducer 55 is the same as the transducer 2 illustrated in FIG. 1; moreover, components of the transducer 55 already present in the transducer 2 illustrated in FIG. 1 are designated by the same reference numbers, except where otherwise specified.

(8) In detail, the current generator 60 generates a periodic current i(t) with a frequency f.sub.i. The waveform of the current i(t) may be, for example, a square or sinusoidal wave.

(9) In greater detail, the transducer 55 has a resonance frequency f.sub.0. Furthermore, as illustrated in FIG. 4, the frequency f.sub.i of the current i(t) is fixed in time and differs from the resonance frequency f.sub.0 by a deviation f, the modulus of which may be comprised, for example, in the interval [500 Hz-1000 Hz], and in any case is not less than G.Math.f.sub.0/(2.Math.Q), with G equal to 10. More in particular, the current generator 60 generates the current i(t) in such a way that the frequency f.sub.i is independent of the resonance frequency f.sub.0. Consequently, the deviation f may undergo variations over time.

(10) Provided purely by way of example are possible embodiments in which f.sub.0=20 kHz and f=1 kHz, so that f.sub.i=19 kHz.

(11) In the above driving conditions, it is found that the bandwidth of the transducer 55, and hence of the magnetic sensor 50, is approximately |f|/3; consequently, it can be particularly wide. Furthermore, the bandwidth of the transducer 55 is independent of the damping coefficient of the peak of the transfer function H.sub.m(f) of the transducer 55 itself. Consequently, the damping coefficient can be reduced in order to reduce the impact of the Brownian noise, without this entailing any reduction of the bandwidth of the transducer 55. Furthermore, the operating point of the transducer 55 is affected marginally by the manufacturing tolerances of the transducer itself, since the latter operates at a point of the transfer function H.sub.m(f) where, in addition to assuming a value higher than the value at zero frequency, it has a limited slope.

(12) The fact that the transducer 55 will be driven with a current having a frequency different from the resonance frequency f.sub.0 entails a reduction in sensitivity as compared to the case of driving at the resonance frequency. This reduction in sensitivity can be compensated, for example, by modifying the conductive path along which the current i(t) flows within the transducer. For instance, embodiments of the transducer 55 are possible, which comprise a greater number of suspended elements, and/or a greater number of fixed-electrode subregions and of corresponding cantilever elements than what is illustrated in FIG. 3. In this case, it is possible to form one or more coils of conductive material, within which the current i(t) is made to circulate so as to increase the sensitivity, given the same current used.

(13) In general, as mentioned previously, moreover possible are embodiments in which the mechanism of transduction of the Lorentz force into a variation of a corresponding mechanical quantity, which corresponds, in turn, to a variation of a corresponding electrical quantity, is different from what is illustrated in FIGS. 1 and 3. Embodiments are hence, for example, possible that are sensitive to the components of the magnetic field directed parallel to the axes x and/or y, instead of the axis z.

(14) Provided purely by way of example are possible embodiments in which there is a rotation, instead of a translation, of a suspended element; this rotation is obtained once again by causing the current i(t) to flow within the suspended element. Furthermore, embodiments are possible in which the aforementioned corresponding electrical quantity is different from a capacitance; for example, this electrical quantity may be the electrical resistance of a piezoresistive element.

(15) As illustrated in FIG. 5, moreover possible are embodiments in which the magnetic sensor 50 is integrated within a die 70, made of semiconductor material, and comprises, in addition to the current generator (here designated by 90) and to the transducer 55, referred to hereinafter as first transducer 55, a second transducer 75 and a third transducer 80.

(16) In detail, the first, second, and third transducers 55, 75, 80 are such that the first transducer 55 is sensitive, as mentioned previously, to the magnetic fields directed parallel to the axis z, whereas the second and third transducers 75, 80 are sensitive to magnetic fields directed, respectively, parallel to the axis x and to the axis y. In this way, the magnetic sensor 50 is of a triaxial type.

(17) For instance, one between the second and third transducers 75, 80 may be the same as the first transducer 55, but oriented in a way different from the latter. In general, in any case, in each from among the first, second, and third transducers 55, 75, 80 the modulus of the Lorentz force is proportional to the modulus of the current i(t).

(18) In greater detail, the first, second, and third transducers 55, 75, 80 are connected in series to one another.

(19) Furthermore, the current generator 90 is connected to the terminals of the series formed by the first, second and third transducers 55, 75, 80. Consequently, the current i(t) traverses in succession the first, second, and third transducers 55, 75, 80. Furthermore, if we designate, respectively, by f.sub.0z, f.sub.0x and f.sub.0y the resonance frequencies of the first, second and third transducers 55, 75, 80, the frequency f.sub.i of the current i(t) differs from these resonance frequencies, respectively, by a first deviation f.sub.z, a second deviation f.sub.x, and a third deviation f.sub.y, each of which has a modulus comprised, for example, in the interval [500 Hz-1000 Hz]; in particular, if we designate by f.sub.i any one of f.sub.z, f.sub.x and f.sub.y, the relation f.sub.i>G.Math.f.sub.0/(2.Math.Q) still applies.

(20) Furthermore, the frequency f.sub.i of the current i(t) is such that each from among the first, second, and third transducers 55, 75, 80 operates at a point of its own transfer function, in which the transfer function itself assumes a value higher than the value assumed at zero frequency.

(21) In practice, the first, second, and third transducers 55, 75, 80 are not driven at the respective resonance frequency, thus, it is possible to use the same current for driving all the transducers. Furthermore, the current i(t) is generated using an oscillator circuit 90 (FIG. 5) of a known type, which forms the current generator, is integrated in the die 70, and has a nominal operating frequency that differs from the nominal resonance frequencies of the first, second, and third transducers 55, 75, 80, respectively, by the aforementioned first, second, and third deviations f.sub.z, f.sub.x and f.sub.y.

(22) In greater detail, the oscillator circuit 90 is of a MEMS type; namely, it includes a resonator 91 of a MEMS type, which functions as frequency-selective element and includes a resonant electromechanical structure. In this way, the oscillator circuit 90 has process tolerances similar to the tolerances that afflict the first, second, and third transducers 55, 75, 80, since they are all integrated in the die 70, if possible close to one another. Consequently, the relations present between the nominal values of the resonance frequencies of the first, second, and third transducers 55, 75, 80 and the nominal frequency of the current i(t) are substantially equal to the relations present between the corresponding real values.

(23) FIG. 6 shows an electronic system 100, which comprises any embodiment of the magnetic sensor 50, a display 110, and a processing unit 120, for example of the microcontroller type.

(24) The processing unit 120 can receive appropriate external control signals through an interface (not shown) provided for this purpose. Furthermore, the processing unit 120 is electrically connected to the magnetic sensor 50 so as to receive the measurement signal. Moreover, the processing unit 120 is connected to the display 110 so as to supply to the latter a processed signal, generated by the processing unit 120 itself on the basis of the measurement signal. The processed signal is then displayed on the display 110.

(25) The advantages that the present magnetic sensor affords emerge clearly from the foregoing description. In particular, the present magnetic sensor features low levels of consumption and a good resolution (low noise), as well as an appreciable bandwidth. Furthermore, the present magnetic sensor is characterized by the possibility of including a number of transducers supplied in series and integrated in one and the same die.

(26) Finally, it is evident that modifications and variations may be made to the magnetic sensor described herein, without thereby departing from the scope of the present disclosure.

(27) For instance, the first transducer 55 and the oscillator circuit 90 may be integrated in one and the same die even in the absence of further transducers; also in this case, the oscillator circuit 90 may be of a MEMS type. In general, moreover, it is possible, irrespective of the number of transducers present, for part of the oscillator circuit, and hence of the current generator, to be integrated in the die. In particular, it is possible for the resonator 91 to be integrated; further components of the oscillator circuit may then be formed outside the die. On the other hand, it is also possible for the resonator not to be of a MEMS type, but, for example, to be an electronic resonator of a known type.

(28) The magnetic sensor may moreover comprise one or more MEMS gyroscopes, which may be integrated in the same die as that in which the first transducer 55 and, if present, the second and third transducers 75, 80 are formed.

(29) Finally, each from among the current generator and the first, second, and third transducers may be of a tunable type; for example, in the case of the transducers, these may be electrostatically tunable. In this way, it is possible to obtain a precise control of the deviation f.

(30) The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.