Abstract:
A magnetic-field sensor, including: a die, a current generator in the die. The current generator generating a driving current. A Lorentz force transducer also in the die and being configured to obtain measurements of magnetic field based upon the Lorentz force is coupled to the current generator. The transducer having a resonance frequency. The current generator is such that the driving current has a non-zero frequency different from the resonance frequency.

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a magnetic sensor including a Lorentz force transducer, which is driven at a frequency different from its natural resonance frequency. Furthermore, the present disclosure relates to a method for driving a Lorentz force transducer. 
     Description of the Related Art 
     As is known, today available are the so-called magnetic sensors based upon the Lorentz force, which are also known as Lorentz force magnetometers and exploit, precisely, the Lorentz force to obtain measurements of magnetic field, as described, for example, in U.S. Pat. No. 7,642,692. 
     Lorentz force magnetometers represent a valid alternative, for example, to Hall sensors and to the so-called anisotropic magnetoresistive (AMR) sensors. In particular, Lorentz force magnetometers are suited to form single-chip triaxial sensors; moreover, these magnetometers can be integrated with gyroscopes, so as to form sensors with nine axes. However, Lorentz force magnetometers feature non-negligible levels of consumption, as well as not particularly wide bandwidths. 
     In general, the principle of operation of a magnetometer based upon the Lorentz force is exemplified in  FIG. 1 , where a magnetometer of this type is in fact shown, designated as a whole by  1 , and referred to in what follows for brevity as “magnetometer  1 ”. 
     The magnetometer  1  comprises a transducer  2 , which is of the MEMS (microelectromechanical systems) type and in turn comprises a stator  3  and a rotor  4 . The stator  3  comprises a first fixed electrode  6  and a second fixed electrode  8 , made of semiconductor material. 
     The first fixed electrode  6  comprises a first fixed-electrode subregion  7   a  and a second fixed-electrode subregion  7   b , which are electrically connected to one another and are fixed with respect to a substrate (not shown), which is made, for example, of semiconductor material. 
     The second fixed electrode  8  comprises a third fixed-electrode subregion  9   a  and a fourth fixed-electrode subregion  9   b , which are electrically connected to one another, are electrically separated from the first and second fixed-electrode subregions  7   a ,  7   b  and are fixed with respect to the substrate. 
     The substrate mechanically carries the first, second, third, and fourth fixed-electrode subregions  7   a - 7   b ,  9   a - 9   b.    
     The rotor  4  comprises a first suspended element  12  and a second suspended element  14 , which are physically suspended, at a distance, over the substrate. The first and second suspended elements  12 ,  14  have shapes, for example, of parallelepipeds with a length equal to L, measured along the axis y of an orthogonal reference system xyz. Furthermore, the first and second suspended elements  12 ,  14  are arranged so as to be parallel to one another and aligned along the axis x. 
     The first and second suspended elements  12 ,  14  may be made, for example, of semiconductor material. In addition, each of the first and second suspended elements  12 ,  14  has a first end and a second end opposite to one another, which are fixed, respectively, to a first anchorage element  16  and a second anchorage element  18 , which are in turn fixed with respect to the substrate. The first and second anchorage elements  16 ,  18  are made of semiconductor material. 
     The rotor  4  further comprises a third suspended element  20 , which is made, for example, of semiconductor material and comprises a first suspended-element subregion  22  and a second suspended-element subregion  24 , which are fixed with respect to one another. 
     The first suspended-element subregion  22  has an elongated shape, extends along the axis x and is provided with a first end and a second end, which are opposite to one another and are constrained, respectively, to the first and second suspended elements  12 ,  14 . In particular, the first end of the first suspended-element subregion  22  is fixed to a central portion of the first suspended element  12 , whereas the second end of the first suspended-element subregion  22  is fixed to a central portion of the second suspended element  14 . Furthermore, the first suspended-element subregion  22  extends between the first fixed-electrode subregion  7   a  and the third fixed-electrode subregion  9   a , on one side, and the second and fourth fixed-electrode subregions  7   b ,  9   b , on the other. 
     The second suspended-element subregion  24  includes a first cantilever element  30  and a second cantilever element  32 , each of which has an elongated shape, for example parallelepipedal. Furthermore, each of the first and second cantilever elements  30 ,  32  has a respective first end and a respective second end, opposite to one another; the first end is fixed to a central portion of the first suspended-element subregion  22 , whereas the second end is free. 
     In detail, the first and second cantilever elements  30 ,  32  extend parallel to the axis y, hence parallel to the first and second suspended elements  12 ,  14 , and are moreover arranged specularly with respect to the first suspended-element subregion  22 . Furthermore, the first cantilever element  30  is arranged, at a distance, between the first and third fixed-electrode subregions  7   a ,  9   a , whereas the second cantilever element  32  is arranged, at a distance, between the second and fourth fixed-electrode subregions  7   b ,  9   b.    
     In greater detail, in the magnetometer  1 , the first and second cantilever elements  30 ,  32  form a single piece with the first suspended-element subregion  22 . Furthermore, the first, second, and third suspended elements  12 ,  14 ,  20  form a single piece. 
     For practical purposes, the first and second fixed-electrode subregions  7   a ,  7   b  form a first plate of a first capacitor C 1 , the second plate of which is formed by the first and second cantilever elements  30 ,  32 . Furthermore, the third and fourth fixed-electrode subregions  9   a ,  9   b  form a first plate of a second capacitor C 2 , the second plate of which is formed once again by the first and second cantilever elements  30 ,  32 . 
     The magnetometer  1  further comprises a transduction circuit  35  and a current generator  40 . 
     The transduction circuit  35  is electrically connected to the first plates of the first and second capacitors C 1 , C 2 , as well as to the second (shared) plate of the second cantilever element  30 . 
     The current generator  40  is electrically coupled to the transducer  2 . In particular, the current generator  40  is electrically coupled to the first and second anchorage elements  16 ,  18  and is such as to generate, in use, a current i(t). 
     The current i(t) flows in part in the first suspended element  12  and in part in the second suspended element  14 , without traversing, to a first approximation, the first suspended-element subregion  22 , since the ends of this latter are at one and the same potential. More in particular, in each of the first and second suspended elements  12 ,  14  there flows substantially half of the current i(t). Consequently, in the presence of a magnetic field directed, for example, parallel to the axis z, each of the first and second suspended elements  12 ,  14  is subject to a Lorentz force F L (t), the modulus of which is
 
 F   L ( t )=½ ·i ( t )· L·B  
 
where B is the modulus of the magnetic induction.
 
     Under the action of the Lorentz force F L (t), each of the first and second suspended elements  12 ,  14  undergoes elastic deformation in such a way that its own central portion translates parallel to the axis x. 
     The first and second suspended elements  12 ,  14  hence function as springs, the deformation of which entails a translation of the third suspended element  20  parallel to the axis x, the extent and direction of said translation being, respectively, proportional to the modulus and direction of the magnetic induction B. For instance, with reference once again to  FIG. 1 , in the presence of a magnetic field, the translation of the third suspended element  20  is such that the first and second cantilever elements  30 ,  32  move away, respectively, from the first and second fixed-electrode subregions  7   a ,  7   b  and approach, respectively, the third and fourth fixed-electrode subregions  9   a ,  9   b.    
     In detail, assuming that, in the absence of magnetic field, i.e., in resting conditions, the third suspended element  20  is arranged in such a way that the first and second capacitors C 1 , C 2  have one and the same capacitance C 0 , in the presence of the magnetic induction B illustrated in  FIG. 1 , it is found that to a first approximation the first capacitor C 1  assumes a value of capacitance equal to C 0 −ΔC, whereas the second capacitor C 2  assumes a value of capacitance equal to C 0 +ΔC. 
     The transduction circuit  35  is designed to generate an electrical signal proportional to ΔC, which is hence proportional to the modulus of the magnetic induction B and moreover indicates the direction of the latter. This electrical signal is also known as “measurement signal”. 
     In greater detail, if we designate by x 0  the distance that separates, in resting conditions, one between the first and third fixed-electrode subregions  7   a ,  9   a  from the first cantilever element  30  (this distance being equal to the distance between one between the second and fourth fixed-electrode subregions  7   b ,  9   b  and the second cantilever element  32 ), we have that the sensitivity of the magnetometer  1  is, at a low frequency, 
                   Δ   ⁢           ⁢   C       Δ   ⁢           ⁢   B       ⁢     (   t   )       =           C   0       x   0       ·         F   L     ⁡     (   t   )         B   ·   k         =         C   0       x   0       ·     L   k     ·     i   ⁡     (   t   )                 
where k is the elastic stiffness of the deformable body formed by the first, second, and third suspended elements  12 ,  14 ,  20 , i.e., the constant that links a force to which the deformable body is subject to the corresponding translation of its centroid with respect to the resting conditions. The elastic stiffness k is a function of the elastic stiffnesses of the first and second springs  12 ,  14 .
 
     This having been said, today two different techniques are known for driving Lorentz force magnetometers, these techniques being described in what follows once again with reference to the magnetometer  1  illustrated in  FIG. 1 . 
     According to a first driving technique, the current generator  40  operates in d.c., in such a way that the relation i(t)=I C  applies. This driving technique is simple to implement; however, it entails that the magnetometer  1 , when driven in this way, has a somewhat reduced sensitivity; moreover, the magnetometer  1  operates in a region where the electronic noise is rather high. To overcome at least in part this problem, it is possible to increase the value of the current I C , with consequent increase in consumption, and/or to increase the surfaces of the plates of the first and second capacitors C 1 , C 2 , with consequent increase in the area of semiconductor material occupied by the magnetometer  1 . 
     Furthermore, the first driving technique entails that the magnetometer  1  is sensitive also to external vibrations and accelerations. 
     According to a second driving technique, instead, the current generator  40  operates in a.c., and in particular operates in such a way that the current i(t) has a periodic waveform, for example of a sinusoidal or square-wave type, at a frequency f i  equal to the resonance frequency f 0  of the transducer  2 . 
     In greater detail, the transducer  2  is characterized by a respective transfer function H m (f), also known as frequency response, which sets in relation, as the frequency varies, the values of amplitude, in sinusoidal regime, of the Lorentz force F L (f), to which each of the first and second suspended elements  12 ,  14  is subject, with the corresponding values of amplitude of the translation X(f) of each of the first and second suspended elements  12 ,  14  with respect to the corresponding resting position, i.e., the position assumed in the absence of magnetic field. In particular, the transfer function is equal to the ratio X(f)/F L (f). 
       FIG. 2  shows an example of the transfer function H m (f), which has a peak at a value of frequency equal, precisely, to the aforementioned resonance frequency f 0 . 
     This having been said, the resonance frequency f 0  is not exactly known beforehand, in the sense that, even though it is possible to estimate, based on the characteristics of design of the transducer  2 , a nominal resonance frequency f N , i.e., an estimate of the resonance frequency f 0 , this nominal resonance frequency f N  does not coincide perfectly with the resonance frequency f 0 . Furthermore, over time, the resonance frequency f 0  may vary, for example on account of temperature variations. Consequently, in order to guarantee that the frequency f i  of the current i(t) is effectively equal to the resonance frequency f 0 , the current generator  40  is controlled in closed-loop fashion in such a way that the frequency f i  of the current i(t) follows the resonance frequency f 0 . 
     The second driving technique enables to obtain a sensitivity that is higher than that obtained by the first driving technique. However, the implementation of a closed-loop control, which is based upon generation of an oscillating signal with an amplitude that varies together with the amplitude of the magnetic field, entails an increase in the circuit complexity. In addition, high sensitivity and resolution may be obtained at the expense of bandwidth. In this connection, in fact, it should be noted how the impact of Brownian noise on the performance of the transducer  2 , when the latter is driven with the second driving technique, is directly proportional to the damping coefficient of the peak of the transfer function H m (f). Consequently, by reducing the damping coefficient, the effects of the noise are reduced, and hence the resolution increases; this means that the peak narrows and hence the sensitivity increases, given that the latter may be expressed as: 
                 Δ   ⁢           ⁢   C       Δ   ⁢           ⁢   B       =           C   0       x   0       ·       F   L       B   ·   k         =         C   0       x   0       ·     L   k     ·          i   ⁡     (   t   )            ·   Q             
where Q is the quality factor of the peak of the transfer function H m (f). However, the fact that the peak narrows moreover means that the bandwidth is reduced, since the latter is approximately equal, in this driving conditions, to f 0 /(2Q). For these reasons, generally the bandwidths of magnetometers driven with the second driving technique are in the region of a few hertz.
 
     Moreover, the adoption of the second driving technique entails that the current generator  40  and the corresponding closed-loop control cannot be used for supplying further transducers additional to the transducer  2 , for example integrated in a single chip together with the transducer  2  to form a multiaxial magnetic sensor. In fact, each of these further transducers has a respective resonance frequency, which is inevitably different from the resonance frequency f 0  of the transducer  2 . Consequently, each of the further transducers is coupled to a respective current generator, controlled in closed-loop fashion in such a way that the frequency of the current generated thereby will follow the resonance frequency of the further transducer. Consequently, the current used by a triaxial magnetic sensor is three times the current used by a uniaxial magnetic sensor. 
     BRIEF SUMMARY 
     The present disclosure is directed to providing a magnetic sensor including a Lorentz force transducer that will enable at least a partial solution of the drawbacks of the prior art. 
     One embodiment of the present disclosure includes magnetic-field sensor that includes a die, a current generator in the die and configured to generate a driving current, and a first Lorentz force transducer in the die, coupled to the current generator and having a first resonance frequency, the driving current has a non-zero frequency different from the first resonance frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       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: 
         FIG. 1  shows schematically a cross section of a magnetometer based upon the Lorentz force of a known type; 
         FIGS. 2 and 4  show plots of an example of a transfer function of a Lorentz force transducer; 
         FIG. 3  is a schematic illustration of a cross section of an embodiment of the present magnetic sensor; 
         FIG. 5  shows a block diagram of an embodiment of the present magnetic sensor; and 
         FIG. 6  shows a block diagram of an electronic system including the present magnetic sensor. 
     
    
    
     DETAILED DESCRIPTION 
       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. 
     In detail, the current generator  60  generates a periodic current i(t) with a frequency f i . The waveform of the current i(t) may be, for example, a square or sinusoidal wave. 
     In greater detail, the transducer  55  has a resonance frequency f 0 . Furthermore, as illustrated in  FIG. 4 , the frequency f i  of the current i(t) is fixed in time and differs from the resonance frequency f 0  by a deviation Δf, 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·f 0 /(2·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 i  is independent of the resonance frequency f 0 . Consequently, the deviation Δf may undergo variations over time. 
     Provided purely by way of example are possible embodiments in which f 0 =20 kHz and Δf=−1 kHz, so that f i =19 kHz. 
     In the above driving conditions, it is found that the bandwidth of the transducer  55 , and hence of the magnetic sensor  50 , is approximately |Δf|/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 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 m (f) where, in addition to assuming a value higher than the value at zero frequency, it has a limited slope. 
     The fact that the transducer  55  will be driven with a current having a frequency different from the resonance frequency f 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. 
     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. 
     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. 
     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 . 
     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. 
     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). 
     In greater detail, the first, second, and third transducers  55 ,  75 ,  80  are connected in series to one another. 
     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 0z , f 0x  and f 0y  the resonance frequencies of the first, second and third transducers  55 ,  75 ,  80 , the frequency f i  of the current i(t) differs from these resonance frequencies, respectively, by a first deviation Δf z , a second deviation Δf x , and a third deviation Δf y , each of which has a modulus comprised, for example, in the interval [500 Hz-1000 Hz]; in particular, if we designate by Δf i  any one of Δf z , Δf x  and Δf y , the relation Δf i &gt;G·f 0 /(2·Q) still applies. 
     Furthermore, the frequency f 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. 
     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 Δf z , Δf x  and Δf y . 
     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. 
       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. 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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 Δf. 
     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.