Abstract:
A magnetic field sensor that can be manufactured using the technology of surface micromechanics, having a conductor loop that has at least one deformable segment; a deformation device for deforming the deformable segment of the conductor loop with a predeterminable time dependence; a voltage sensing device for sensing the voltage induced at the ends of the conductor loop upon deformation in the presence of a magnetic field; and a magnetic field determining device for determining the present static and/or dynamic magnetic field in consideration of at least the time dependence of the deformation.

Description:
FIELD OF THE INVENTION 
     The present invention concerns a magnetic field sensor, and in particular a magnetic field sensor that can be manufactured using the technology of surface micromechanics. 
     BACKGROUND INFORMATION 
     Although applicable to any magnetic field sensor, the present invention and the problem on which it is based are explained with reference to a magnetic field sensor that can be manufactured using the technology surface micromechanics, for use in automotive engineering. 
     Magnetic field sensors can be used in many ways in automotive engineering, for example in antilock braking systems (ABS) or in automatic slip control (ASR) systems as wheel sensors, as position sensors for needle valves or ignition pulse generators, as steering wheel angle transducers, as crankshaft position transducers, etc. The demands placed on these magnetic field sensors in terms of mechanical strength, e.g. shock resistance and temperature resistance, are usually very high. 
     There exist in the related art a large number of known magnetic field sensors which utilize the applied force of a magnetic field on a conductor through which a current is passing. The best known example thereof is the Hall sensor. 
     Disadvantages that have emerged with the aforesaid known approaches are that they cannot withstand the severe mechanical and/or thermal stresses, and/or have too little sensitivity, and/or are very costly. 
     SUMMARY OF THE INVENTION 
     The magnetic field sensor according to the present invention has the advantage, as compared to known approaches to the problem, that it can be designed in such a way that it withstands severe mechanical stresses and exhibits high sensitivity simultaneously with low temperature dependence. 
     The underlying idea behind the present invention is that a conductor loop whose area is modulated, i.e. that has a definable time dependence, remains at a voltage as long as a magnetic field is not passing through it. As soon as this conductor loop is exposed to a magnetic field, however, the result of the area modulation is that the magnetic flux Φ continually changes, causing a voltage induction that is proportional to the magnetic flux density and to the change in area over time. This voltage is picked off at the ends of the conductor loop and is analyzed in accordance with the mechanical excitation. The magnetic field sensor according to the present invention can be used to sense both constant magnetic fields and alternating magnetic fields, up to a system-related limit frequency. 
     According to a preferred development, in the undeformed state the conductor loop encloses an area having two longitudinal sides substantially parallel to one another, and the deformation device deforms the two longitudinal sides to change the enclosed area. It is thus advantageously possible to achieve a large change in area. 
     According to a further preferred development, the deformation device is configured such that it excites the two longitudinal sides to opposite-direction resonant flexural oscillations. It is thus advantageously possible to achieve a particularly large change in area utilizing resonance exaggeration. 
     According to a further preferred development, the conductor loop is configured such that the opposite-direction resonant flexural oscillations have a different resonant frequency from the corresponding same-direction resonant flexural oscillations. It is thereby advantageously possible to avoid interferences with the undesired same-direction resonant flexural oscillations, in which the net change in area is substantially zero. 
     According to a further preferred development, the deformation device is configured such that it deforms the deformable segments by way of a capacitive coupling. As a result, neither friction losses nor wear occur during excitation. In addition, capacitive excitation devices of this kind are known from other sectors, for example comb structures of acceleration sensors. 
     According to a further preferred development, the magnetic field sensor is manufactured using the technology of surface micromechanics, and has a substrate that is preferably made of silicon (or of another electrically conductive material). The conductor loop has substantially a rectangular shape, whose longitudinal sides are arranged floatingly above the substrate and are deformable by the deformation device, and whose widthwise sides are mounted floatingly on the substrate. This particular embodiment by high sensitivity simultaneously with low temperature dependence. 
     According to a further preferred development, the conductor loop has a continuous, nondeformable first widthwise side having a greater thickness than the longitudinal sides, which is connected, via at least one deformable floating strut having substantially the thickness of the longitudinal sides, to at least one connector pad anchored in the substrate. This rigid first widthwise side stabilizes the width of the surface. 
     According to a further preferred development, the conductor loop has a split nondeformable second widthwise side having a greater thickness than the longitudinal sides, whose parts is connected via a respective deformnable floating strut, having substantially the thickness of the longitudinal sides, to a respective connector pad anchored in the substrate. This split rigid second widthwise side also stabilizes the width of the surface, and allows the induced voltage to be picked off advantageously. 
     According to a further preferred development, the longitudinal sides are connected at their first end, via a deformable floating first resilient strut having substantially the thickness of the longitudinal sides, to a connector pad anchored in the substrate. With this embodiment, the split rigid second widthwise side is advantageously omitted, so that the structure becomes simpler. 
     According to a further preferred development, the longitudinal sides are connected at their second end, via a deformable floating second resilient strut having substantially the thickness of the longitudinal sides, to the continuous nondeformable first widthwise side. 
     According to a further preferred development, an additional mass is provided floatingly above the substrate in the middle of the deformable longitudinal sides. This additional mass is advantageously used to adjust the resonant frequency. 
     According to a further preferred development, a comb drive device connected to the longitudinal sides is provided as the deformation device. This connection can be either direct, by the fact that comb teeth are provided on the longitudinal sides; or indirect, by the fact that comb teeth are provided on an additional mass that is possibly present. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic depiction to explain the measurement principle of the magnetic field sensor according to the present invention. 
     FIG. 2 shows a schematic depiction of a first embodiment of the magnetic field sensor according to the present invention. 
     FIG. 3 shows a circuit diagram of the first embodiment of the magnetic field sensor according to the present invention. 
     FIG. 4 shows a schematic depiction of a second embodiment of the magnetic field sensor according to the present invention. 
     FIG. 5 shows a schematic depiction of a third embodiment of the magnetic field sensor according to the present invention. 
     FIG. 6 shows a schematic depiction of a fourth embodiment of the magnetic field sensor according to the present invention. 
     FIG. 7 shows a schematic depiction of a fifth embodiment of the magnetic field sensor according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     Description of Exemplary Embodiments 
     In the Figures, identical reference characters refer to identical or functionally identical components. 
     FIG. 1 is a schematic depiction to explain the measurement principle of the magnetic field sensor according to the present invention. 
     In FIG. 1,  1  designates in general a conductor loop of substantially rectangular shape,  2  deformable longitudinal sides of conductor loop  1 ,  3  and  3 ′ nondeformable widthwise sides of conductor loop  1 ,  4  and  4 ′ deformable struts, and  5  and  6 ,  6 ′ connector pads. B designates an external magnetic field, U an induced voltage, and v the movement direction or deformation direction of longitudinal sides  2 . 
     The measurement principle of the magnetic field sensor according to the present invention is simply that a suitable mechanical excitation creates an oscillating change in area dA/dt in area A of conductor loop  1 , which in the presence of a magnetic field B generates an induced voltage U at connector pads  6 ,  6 ′. The modulation function of area A, for example A=A 0  cos ωt, is utilized for demodulation of the correspondingly modulated induced voltage U, from which results a voltage which is proportional to the magnetic flux density B and whose sign indicates the direction of the magnetic field lines. Absolute values can be determined by suitable calibration or by the exact geometry of the arrangement. 
     The following special feature applies to an external magnetic field. If the condition Φ=B·A is set for the magnetic flux—where A=A 0  cos ωt and B=B 0  cos Ωt, ω being the angular velocity of the change in area and Ω the angular velocity of the change in magnetic field—the following is obtained for the induced voltage U: 
     
       
           U=−dΦ/dt=−d/dt ( A   0  cos ωt× B   0  cos Ωt) 
       
     
     The measured signal is then therefore an amplitude-modulated signal with a suppressed carrier. Only two sidebands appear in the frequency spectrum, at distances of +/−Ω from the carrier ω. Using demodulation methods that are very familiar from general communications technology—for example, multiplication by the carrier frequency ω followed by lowpass filtering—it is easy to draw conclusions as to the amplitude and direction, and the angular velocity Ω, of the change in magnetic field. 
     FIG. 2 is a schematic depiction of a first embodiment of the magnetic field sensor according to the present invention. 
     In FIG. 2, in addition to the reference characters already introduced,  20  designates a comb structure provided on longitudinal sides  2 , and  70  a comb structure that is provided in an anchoring element  7  and engages into comb structure  20 . 
     This first embodiment, like all the other embodiments described here, is manufactured using the technology of surface micromechanics, specifically with a silicon substrate. 
     Conductor loop  1 , which is substantially rectangular in shape, has its longitudinal sides  2  as thin bars, and its widthwise sides  3 ,  3 ′ as thicker bars, arranged floatingly over the substrate. The continuous, thick (i.e. nondeformable) first widthwise side  3  is connected via the thin (i.e. deformable) floatinq strut  4  to connector pad  5  anchored in the substrate. The split, thick (i.e. nondeformable) second widthwise side  3 ′ is connected via a respective thin (i.e. deformable) floating strut  4 ′ to the respective corresponding connector pad  6 ,  6 ′ anchored in the substrate. 
     Comb devices  20 ,  70  are also provided floatingly above the substrate, anchoring element  7  being anchored in the substrate. 
     The resilient attachment of conductor loop  1  to the substrate by way of the thin struts  4 ,  4 ′ makes it possible to achieve a mode separation between a same-direction movement of the bendable longitudinal sides  2  in which both are deflected in the same direction, and an opposite-direction movement in which the two are deflected in opposite directions. This mode separation is based on the fact that in the same-direction movement, struts  4 ,  4 ′ and widthwise sides  3 ,  3 ′ co-oscillate to some extent, whereas in the opposite-direction movement these components remain stationary. The mechanical resonant frequency of the system will thus be lower for the same-direction movement than for the opposite-direction movement. 
     Excitation of conductor loop  1  is achieved via comb structures  20 ,  70  which are known from acceleration sensor drives and comb drives. Electrostatic attractive forces are generated by way of a preferably sinusoidal voltage, causing comb structure  20  to be pulled into or forced out of comb structure  70 . The mechanical deflection of longitudinal sides  2  can be measured by way of the same comb structure or an additional one of similar configuration; based on these measurement results, the mechanical excitation can be kept constant in terms of its frequency and amplitude. For accurate measurement, it is important that these mechanical parameters be kept constant. Suitable control circuits that serve this purpose are widely known in the existing art. Advantageously a resonant excitation is performed, since then on the one hand the aforesaid mode selection is possible, and on the other hand the mechanical quality exaggeration can be utilized, resulting in a reduction in the electrical excitation amplitude. In order to reduce damping as much as possible, the magnetic field sensor can also be operated in vacuum. 
     FIG. 3 is a circuit diagram of the first embodiment of the magnetic field sensor according to the present invention. 
     In FIG. 3, in addition to the reference characters already introduced, U 1  and U 2  designate the excitation voltages of comb device  70 , C 1  and C 2  the capacitive coupling between comb devices  20  and  70 , R 1  through R 4  equivalent resistances of the corresponding conductor loop segments, and MASSE the zero or ground potential. 
     When longitudinal sides are at rest, there is applied to conductor stop  1  a voltage U that is proportional to the change dB/dt in magnetic field B. This corresponds to the trivial formulation according to which known induction measuring instruments operate. If the conductor loop is driven electrostatically with voltages U 1  and U 2  via comb structures  20 ,  70 , then the equivalent circuit diagram shown in FIG. 3 applies. 
     An important point to be noted in this context is the following: If a sinusoidal alternating current is applied to comb structures  20 ,  70  in order to excite conductor loop  1 , then a sinusoidal alternating current also flows through resistances R 3  and R 4 , generating a corresponding voltage drop which falsifies the measurement signal of the induced voltage U. 
     The actions set forth below can be taken to counteract this falsification. 
     First, a minimization can be accomplished by generating a common-mode signal. Since a differential signal is measured at the output of conductor loop  1  (connector pads  6 ,  6 ′), the influence can be minimized by also generating a similar voltage drop. For that purpose, U 1  is set equal to U 2 . Assuming C 1 =C 2  and R 3 =R 4 , this results in identical voltage drops which cancel one another out when the differential voltage is measured at the output of conductor loop  1 . 
     Second, a phase discrimination in the context of resonant excitation can be performed. The current i through capacitive structure C 1 , C 2  has a cosine profile if the excitation voltage is sinusoidal, since i=C dU/dt for capacitors C 1 , C 2 . If comb structure  20 ,  70  is excited with a sinusoidal voltage, then the electrostatic force relevant for drive purposes also has a component with the time function F 0  sin ωt. With resonant excitation, the movement amplitude lags this force with a 90° offset, i.e. with a time dependence cos Ωt, like the interfering current. What is relevant for the voltage U induced by the magnetic field, however, is the velocity of the movement, which again occurs at a 90° offset, i.e. with a time dependence—sin ωt. The result is a phase difference of 90° between the induced voltage U and the parasitic voltages dropping through resistances R 3  and R 4 . The interfering current can thus be filtered out by phase-sensitive demodulation. 
     Third, discrimination in the frequency range can be performed. As stated, the voltage U induced by a magnetic field B is proportional to the velocity of the movement, i.e. has, for example, a time dependence—sin ωt. The electrostatically generated force F 0  sin ωt is proportional to the square of the electrical excitation voltage U 1  or U 2 . In other words, in accordance with known trigonometric identities, the excitation frequency that generates a force profile F 0  sin ωt equals 1/2 ωt. The voltage drop across resistances R 3  and R 4  then also has a time dependent cos 1/2 ωt. The undesired voltage drops on thus be removed by electrical high-pass filtration. 
     FIG. 4 is a schematic depiction of a second embodiment of the magnetic field sensor according to the present invention. 
     In FIG. 4, in addition to the reference characters already introduced,  1 ″ designates the conductor loop according to the second embodiment,  2 ′ and  2 ″ a respective thin (i.e. deformable) first and second resilient strut, and  5 ′ two connector pads that are connected via a respective thin (i.e. deformable) strut  4 ″ to the wide (i.e. nondeformable) widthwise side  3 . 
     In contrast to the aforementioned first embodiment, in the second embodiment two connector pads  5 ,  5 ′ are connected to the rigid widthwise side  3  via struts  4 ″. 
     Longitudinal sides  2  are connected at their first (lower) ends, via the deformable floating first resilient struts  2  having substantially the thickness of longitudinal sides  2 , to connector pads  6 ,  6 ′ anchored in the substrate. In other words, in this embodiment only one upper rigid widthwise side  3  is provided. 
     In addition, longitudinal sides  2  are connected at their second (upper) ends, via the deformable floating second struts  2 ″ having substantially the thickness of longitudinal sides  2 , to the continuous nondeformable first widthwise side  3 . 
     The second embodiment is otherwise identical in function and configuration to the first embodiment. 
     FIG. 5 is a schematic depiction of a third embodiment of the magnetic field sensor according to the present invention. 
     In FIG. 5, in addition to the reference characters already introduced,  1 ″′ designates the conductor loop according to the third embodiment,  2 ″′ a respective thin (i.e. deformable) resilient strut, and  8  an additional mass integrated into the longitudinal sides. 
     In contrast to the aforementioned first and second embodiments, in the third embodiment there is provided, floatingly above the substrate in the middle of deformable longitudinal sides  2 , additional mass  8  by way of which the resonant frequency of longitudinal sides  2  can be tuned. Comb structure  80  is also provided on additional mass  8  instead of comb structure  20 . 
     In addition, the attachment of longitudinal sides  2  at their upper edge to rigid widthwise side  3  is accomplished via the odified resilient struts  2 ″′. Rigid widthwise side  3  is anchored only by way of one strut  4  to a connector pad  5 . 
     The third embodiment is otherwise identical in function and configuration to the second embodiment. 
     FIG. 6 is a schematic depiction of a fourth embodiment of the magnetic field sensor according to the present invention. 
     In FIG. 6, in addition to the reference characters already introduced,  1 ″′ designates the conductor loop according to the fourth embodiment. This fourth embodiment corresponds to the second embodiment with the addition of additional mass  8 . 
     FIG. 7 is a schematic, depiction of a fifth embodiment of the magnetic field sensor according to the present invention. 
     In FIG. 7, in addition to the reference characters already introduced,  1 ″″ designates the conductor loop according to the fifth embodiment, and  9  designates respective thin (i.e. deformable) connecting struts to corresponding additional anchoring elements  90  to the substrate. 
     This fifth embodiment corresponds to the fourth embodiment with the addition of the additional connecting struts and the additional anchoring element  90  to the substrate. These additional design features stabilize additional mass  8  with respect to interfering torques. 
     Although the present invention was described above with reference to preferred exemplary embodiments, it is not limited thereto but rather is modifiable in many ways. 
     In particular, the conductor loop can of course have a geometry that is other than rectangular. 
     The micromechanical base material also need not be silicon, but rather can be any desired base material. 
     Furthermore, excitation of the deformable segments of the conductor loops need not be accomplished capacitively, but can also be effected inductively, magnetically, or in any other manner. 
     Analysis of the measurement signals need not be accomplished in the time domain, but can also take place by way of corresponding Fourier transforms. 
     Lastly, the magnetic field sensor need not necessarily be a micromechanical component, but can also, depending on the magnetic fields to be sensed, be a macromechanical component. 
     LIST OF REFERENCE CHARACTERS 
       1 ,  1 ′,  1 ″,  1 ′″,  1 ″″ Conductor loop 
       2  Longitudinal sides 
       3 ,  3 ′ Widthwise sides 
       4 ,  4 ′,  4 ″,  9  Struts 
       5 ,  6 ,  6 ′ Connector pads 
     B Magnetic field 
     U Induced voltage 
     v Velocity 
       7 ,  90  Anchoring element 
       20 ,  70 ,  80  Comb structures 
       2 ,  2 ″,  2 ″′ Resilient struts 
       8  Additional mass 
     R 1 -R 4  Equivalent resistances 
     C 1 , C 2  Equivalent capacitances 
     U 1 , U 2  Excitation voltages 
     MASSE Ground potential