Abstract:
A sensor for measuring an input signal is provided. The sensor includes a transducer having a soft magnetic material. The transducer may be disposed on a spring element. The soft magnetic material produces a change in impedance when the transducer is stimulated by the input signal. The impedance change is representative of a magnitude of the input signal. The sensor further includes a circuit coupled to the transducer, which is operable to measure the impedance change to determine the magnitude of the input signal. A method of operating the sensor is also provided.

Description:
BACKGROUND  
       [0001]     The invention relates generally to sensors, and more particularly, to high sensitivity pressure sensors fabricated using soft magnetic materials.  
         [0002]     Pressure sensors are used in a wide range of industrial and consumer applications. Bourdon-tube type, diaphragm based, and strain gauge based pressure sensors can measure pressures across many orders of magnitude. A variation of the diaphragm-based pressure sensor is a cantilever-based pressure sensor that may be constructed by micro-machining techniques.  
         [0003]     Several sensing techniques and devices have been developed for specific pressure sensing applications. Although attempts have been made to improve desirable sensor properties, such as high sensitivity, high stability, linearity, low hysteresis, high reliability, fast response and long lifetime, sensors typically suffer from limitations regarding one or more of the aforementioned properties.  
         [0004]     Furthermore, micro-machined pressure sensors may include cavities filled with oil or other substances for transferring the pressure to the sensing element. Such pressure sensors are costly to manufacture and have limited ranges of operation.  
         [0005]     It would therefore be desirable to develop a pressure sensor that exhibits high sensitivity to changes in pressure, high stability, linearity, low hysteresis, high reliability, relatively fast response and long life while reducing the need for packaging that is expensive or difficult to manufacture.  
       SUMMARY  
       [0006]     According to one aspect of the present technique, a sensor for measuring an input signal is provided. The sensor includes a transducer having a soft magnetic material. The transducer may be disposed on a spring element. The soft magnetic material undergoes a change in its impedance when the transducer is stimulated by the input signal. The impedance change is representative of a magnitude of the input signal. The sensor further includes a circuit coupled to the transducer that is operable to measure the impedance change to determine the magnitude of the input signal. A method of operating the sensor is also provided.  
         [0007]     In accordance with another aspect of the present technique, a sensor for measuring an input signal is provided. The sensor comprises a transducer having a soft magnetic material that exhibits stress-impedance properties. The soft magnetic material is disposed on a spring element. The spring element is operable to resonate at a resonant frequency in absence of the input signal and to resonate at a responsive frequency upon being stimulated by the input signal. The sensor also includes a circuit coupled to the transducer that is operable to measure magnitude of shift in the resonant frequency to the responsive frequency. The magnitude of shift in the resonant frequency to the responsive frequency represents a magnitude of the input signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings.  
         [0009]      FIG. 1  is a cross-sectional view of a pressure sensor with a cantilever-based capacitive pressure sensing mechanism constructed in accordance with an exemplary embodiment of the invention.  
         [0010]      FIG. 2  is a cross-sectional view of a vertical diaphragm pressure sensor array illustrating measurement of pressure using soft magnetic material transducers, constructed in accordance with an exemplary embodiment of the invention.  
         [0011]      FIG. 3  is a cross-sectional view of the vertical diaphragm pressure sensor array of  FIG. 2  taken along line III-III of  FIG. 2 .  
         [0012]      FIG. 4  is a cross-sectional view of a diaphragm-based force-compensated pressure sensor illustrating measurement of pressure, constructed in accordance with another exemplary embodiment of the invention.  
         [0013]      FIG. 5  is a cross-sectional view of a cantilever-based force-compensated pressure sensor illustrating measurement of pressure, constructed in accordance with another exemplary embodiment of the invention.  
         [0014]      FIG. 6  is a top view of a diaphragm-based pressure sensor illustrating measurement of pressure by measuring change in electric impedance of the soft magnetic material, constructed in accordance with another exemplary embodiment of the invention.  
         [0015]      FIG. 7  is a side-view of the diaphragm-based pressure sensor of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0016]     In accordance with certain aspects of the present technique, pressure sensors that utilize transducers constructed using soft magnetic materials for gauging pressure will be explained below. One example of such a pressure sensor may employ a transducer made from a soft magnetic material (such as a giant stress impedance material). The transducer may be disposed on a spring element, such as but not limited to, a cantilever, a diaphragm, a metallic foil, a beam, a tube, a cylinder, or any structure that can induce stress in the transducer due to its elastic properties. Such a transducer may be used as a strain gauge. The soft magnetic material used to construct the transducer may be partially or entirely a crystalline microstructure, an amorphous microstructure, a nanocrystalline microstructure, or any combination thereof.  
         [0017]     Furthermore, the soft magnetic material may include iron, cobalt, or nickel alloys. The alloys formed thereof may comprise combinations of silicon (Si), boron (B), zirconium (Zr), niobium (Nb), copper (Cu), aluminum (Al), molybdenum (Mo), tungsten (W), chromium (Cr), manganese (Mn), phosphorus (P), and carbon (C) in varying proportions. Transducers constructed out of a soft magnetic material, when excited by an electrical signal, may exhibit a large change in impedance even with small changes in stress. This characteristic makes a pressure sensor constructed with the transducer highly sensitive. The electrical signal that may be utilized to excite the soft magnetic material transducer for producing a response may be in the range of about 10 kHz to about 1 GHz. Transducers constructed out of a soft magnetic material may also be disposed in an environment having a magnetic field which may be generated by a magnetic source such as a hard magnetic material or an integrated coil, as will be understood from the following description.  
         [0018]      FIG. 1  is a cross-sectional view of an exemplary pressure sensor  10  illustrating a cantilever-based capacitive pressure sensing mechanism. The pressure sensor  10  comprises a substrate  12  on which a cantilever  14  is constructed. A fixed end  16  of cantilever  14  may be disposed on a block  18 . The substrate  12  and the block  18  may be micro-machined on an integrated chip or may be constructed directly on a semiconductor substrate. In one embodiment, the pressure sensor  10  may be disposed in a gaseous atmosphere and is subjected to an external magnetic field.  
         [0019]     A pair of actuation electrodes  20  may be disposed on the base substrate  12  and the cantilever  14 , such that one of the actuation electrodes  20  is positioned on the base substrate  12  while the other is positioned on the cantilever  14 ; the pair forming the plates of a capacitor, as will be appreciated by one skilled in the art. The actuation electrodes  20  may be coupled electrically with an external circuit that may be utilized to excite or actuate the actuation electrodes. At a given external gaseous atmosphere, and an external magnetic field, the actuation electrodes  20  have a reference resonant frequency. The external circuit may control the electrical resonance occurring in the actuation electrodes  20 . At resonant frequencies, the amplitude of the mechanical vibration or motion of the cantilever  14  may be enhanced. A transducer  22  made of a soft magnetic material, may be fabricated on a surface of the cantilever  14 , as illustrated. Strain caused in the soft magnetic material transducer  22 , because of the mechanical vibration or motion of the overhanging end of the cantilever  14 , causes a corresponding change in impedance of the transducer  22 . The impedance change of the transducer  22  is an indirect measurement of amplitude of oscillation of the cantilever  14 , as the cantilever  14  is driven by the electrostatic actuator or actuation electrode  20  disposed on the base substrate  12 .  
         [0020]     When the pressure sensor  10  is subjected to an external pressure within the range of about 1 psi to about 30,000 psi, the viscosity of the gas around the cantilever  14  changes. A change in viscosity of the gas affects the resonant frequency of the cantilever  14 , so that the resonant frequency of the cantilever  14  shifts from the initial reference resonant frequency to a different resonant frequency. The shift in the resonant frequency may depend on the external pressure to which the cantilever  14  is subjected, because, in a gaseous atmosphere, at a given external magnetic field, the viscosity of the gas may change when the external gas pressure is changed. At resonant frequencies other than the initial reference resonant frequency of the cantilever  14 , the magnitude of electrical response produced by the transducer  22  may attain maximum values at frequencies different from the initial reference resonant frequency. This phenomenon enables sensing of the attainment of the different resonant frequencies.  
         [0021]     In one embodiment, the soft magnetic material transducer  22  can be extended to cover the entire length of the cantilever  14 . Thus, the transducer  22  and the actuation electrode  20 , fabricated on the base substrate  12 , together form a capacitive pair.  
         [0022]     Referring to  FIG. 2  and  FIG. 3 , an exemplary vertical diaphragm pressure sensor array  24  using soft magnetic material transducers for measuring pressure is illustrated. A spring element  26  comprises one or more pressure blind cells  28 , which are sealed cavities comprising a gas at a known pressure or a reference pressure. Alternatively, the pressure blind cells  28  may be sealed under vacuum also. The pressure sensor array  24  may be affixed, such as by bonding, to the bottom of the container or vessel that contains gas whose pressure is to be determined. As illustrated in  FIG. 2 , the pressure sensor array  24  may include a plate  30  to block the pressure blind cells  28  from exposure to the gas under pressure. The plate  30  may be made using a gas impermeable material such as, but not limited to, silicon, silicon carbide, germanium, stainless steel, alumina, aluminum nitride, or the like. The spring element  26  may further include one or more pressure sensitive cells  32  in which the gas whose pressure is to be determined is allowed to enter. As illustrated in  FIG. 2 , the arrows  34  and  36  indicate the entry of the gas whose pressure is to be determined, into the pressure sensitive cells  32 .  
         [0023]     A dielectric material  38  may be disposed on a surface of the spring element  26 . Transducers  40  are disposed on the dielectric material  38  or directly on the spring element  26 . The transducers  40  may include a variety of geometries. For example, the transducers may be radial (as shown in  FIG. 3 ), spiral, serpentine, or straight in shape. The transducers  40  are electrically coupled to connectors  42  that enable powering of the transducers  40 . Whenever a gas whose pressure is to be determined is allowed to enter the pressure sensitive cells  32 , the pressure developed by the gas in the pressure sensitive cells  32  causes deformation of walls  44  and  46  that enclose, respectively, cells  28  and  32  in the directions indicated by reference numeral  48 . The deformation of walls  44  and  46  causes a corresponding horizontal force F p    50  to be reflected on the transducers  40 . The horizontal force F p    50  causes the transducers  40  to deform or distort from their original shape, thereby causing a corresponding strain to be developed in the transducers  40 . Consequently, the change in impedance of the transducers  40  with respect to the known or reference pressure in the pressure blind cells  28  is indicative of the pressure of the gas that enters the pressure sensitive cells  32 .  
         [0024]     Another class of pressure sensors in accordance with aspects of the present technique includes force-compensated pressure sensors that employ transducers made from soft magnetic materials, such as stress-impedance materials. Two exemplary types of force-compensated pressure sensors that may be implemented using soft magnetic materials are diaphragm-based force-compensated pressure sensors and cantilever-based force-compensated pressure sensors.  
         [0025]      FIG. 4  is a cross-sectional view of an exemplary diaphragm-based force-compensated pressure sensor  52 . The diaphragm-based force-compensated pressure sensor  52  has a diaphragm  54  that is formed on blocks  18 , which are in turn formed on a substrate  12 . On one surface of the membrane that forms the diaphragm  54 , a thin layer of soft magnetic material  56 , such as a stress-impedance material is disposed. The thin layer of soft magnetic material  56  may be a part of the diaphragm  54 . Defined by substrate  12 , blocks  18  and diaphragm  54  is a cavity  58  that is filled with a fluid such as air or an inert gas. An integrated coil  60  may be disposed on a surface of the substrate  12 . The integrated coil  60  is utilized to provide an opposing force to the force developed when the diaphragm  54  is subjected to external pressure. The integrated coil  60  may be fabricated using an electrically conductive material such as copper, aluminum, or other electrically conductive metals.  
         [0026]     When the pressure sensor assembly  52  is subjected to an external pressure, the force developed by the pressure  62  deflects the magnetic structure or soft magnetic material  56  in a direction perpendicular to the plane of diaphragm  54 , such that the diaphragm  54  will deflect up or down. An electrical signal is fed into the integrated coil  60  so that a magnetic force F magn    64  is developed in soft magnetic material  56 . The electrical signal that is fed into integrated coil  60  is modulated so as to compensate for the force developed by the pressure  62 . For example, if the force due to pressure  62  causes diaphragm  52  deflect downwards, the electrical signal fed into integrated coil  60  may be modulated so that magnetic force F magn    64  developed in soft magnetic material  56  will cause diaphragm  54  to move up to compensate for the force developed by pressure  62 . Similarly, if the force attributable to pressure  62  causes diaphragm  54  to deflect upwards, the electrical signal fed into integrated coil  60  may be modulated so that magnetic force F magn    64  developed in soft magnetic material  56  will cause diaphragm  54  to move down.  
         [0027]     A measure of the electrical signal fed into integrated coil  60  for compensation of the force due to pressure  62  will therefore be indicative of the amount of pressure applied to the pressure sensor assembly. Thus, the amount of electrical signal may be modulated to provide a compensative magnetic force F magn    64  and the same may be calibrated to read the pressure applied.  
         [0028]      FIG. 5  is a cross-sectional view of an exemplary cantilever-based force-compensated pressure sensor  66 . The cantilever-based force-compensated pressure sensor assembly  66  may be constructed on a substrate  12 . A cantilever  68  is disposed such that a fixed end  70  of the cantilever  68  is positioned on a block  18 . Substrate  12 , block  18 , and cantilever  68  may be constructed via micro-machining techniques known in the art.  
         [0029]     A thin layer of soft magnetic material  56  may be disposed on cantilever  68 , while an integrated coil  72  may be disposed on the substrate  12 . Once an external pressure is applied to the cantilever  68 , the force  74  that is developed due to the pressure will cause the cantilever  68  to vibrate in a direction perpendicular to the plane in which cantilever  68  resides. An electrical signal may be fed into integrated coil  72  so that a magnetic force F magn    76  is developed in soft magnetic material  56  overlying cantilever  68 . The electrical signal that is fed into integrated coil  72  may be modulated to compensate for the force  74 . For example, if the force  74  causes cantilever  68  to deflect downwards, the electrical signal fed into integrated coil  72  may be modulated so that magnetic force F magn    76  developed in soft magnetic material  56  will cause cantilever  68  to move up so as to compensate for the force developed by pressure  74 . The magnitude of electrical signal that is fed into integrated coil  72  for compensation of the force due to pressure  74  may therefore be utilized as a measure for the external pressure applied to pressure sensor assembly  66 .  
         [0030]     Referring to  FIG. 6  and  FIG. 7 , an exemplary diaphragm-based pressure sensor  78  is illustrated. The diaphragm-based pressure sensor  78  comprises a diaphragm  80  that may be fabricated or micro-machined on a substrate (not shown). On a top surface of the diaphragm  80 , a thin layer of soft magnetic material  82 , such as a stress-impedance material, may be disposed. If the diaphragm  80  is constructed out of an electrically conducting material, then a layer of an insulating material  84  may be used to isolate the soft magnetic material from the diaphragm  80 . The insulating material  84  may also serve as a bonding material between the diaphragm  80  and the layer of soft magnetic material  82 . The layer of soft magnetic material  82  may be connected to an electrical signal/circuit via electrical connectors  86 .  
         [0031]     In one embodiment, the insulating or bonding material  84  may be disposed on the diaphragm  80  below the ends of the soft magnetic material  82  where electrical connections  86  are made. In another embodiment, the insulating or bonding material  84  may be disposed in a ring pattern such that the soft magnetic material  82  rests above the bonding material  84  and may be connected by electrical connectors  86 . In a different embodiment, the diaphragm  80  may be modeled such that the soft magnetic material  82  may not be completely in contact with the surface of the diaphragm  80 .  
         [0032]     When the pressure sensor  78  is subjected to an external pressure, the force developed by the pressure deflects the diaphragm  80  and soft magnetic material  82  in a direction perpendicular to the plane in which the diaphragm  80  resides. Therefore, the diaphragm  80  will deflect up or down. An AC current is delivered to the soft magnetic material  82 . As the soft magnetic material  82  deflects, the stress developed in the soft magnetic material  82  produces a change in the impedance of the soft magnetic material  82 . A measure of the change in amplitude of impedance or phase angle of the change in impedance of soft magnetic material  82  may therefore be indicative of the amount of pressure applied to the pressure sensor  78 .  
         [0033]     Because soft magnetic materials, such as stress-impedance materials, exhibit a large change in impedance when the material is subjected to a small amount of stress, the sensitivity of the materials in detecting stress is very high. The application of soft magnetic materials in gauging input signals or stimulating forces such as pressure, force, motion, mechanical vibration or the like by utilizing this property of the material is advantageous. The teachings of the present techniques may be applicable for gauging force, motion, mechanical vibration, weight, position, acceleration, or the like in addition to pressure, by modifications to the described embodiments that would be apparent to one of ordinary skill in the art.  
         [0034]     Those of ordinary skill in the art will appreciate that strain gauges or transducers constructed using soft magnetic materials in accordance with aspects of the present technique may be arranged in a wide array of geometric patterns depending upon the specific application. For example, the strain gauges may be arranged in a rectangular pattern as illustrated in  FIG. 1  and  FIG. 6 , or radial pattern as illustrated in  FIG. 3 . Other geometric patterns may also be used, such as but not limited to, a spiral pattern, a serpentine pattern, a rectangular pattern, a ring, a disc, an arc and other patterns formed by connecting strips of soft magnetic material strain gauges together that would enable the measurement of strains in specific directions. Furthermore, the soft magnetic material may be constructed to provide the functionalities of a spring element.  
         [0035]     In all the embodiments noted above, the substrate  12  and the block  18  may be micro-machined on an integrated chip using semiconductor materials such as, but not limited to, silicon (Si), silicon nitride (SiN x ), indium phosphate (InP), gallium arsenide (GaAs), silicon-germanium (Si—Ge), silicon oxide (SiO 2 ), silicon carbide (SiC) and gallium nitride (GaN), germanium; metals or metallic alloys such as stainless steel, inconel, aluminum; ceramic materials such as quartz, sapphire (Al 2 O 3 ), or any other semiconductor material or metallic alloys known in the art to be suitable for micro-machining. Similarly, the cantilevers  14  and  68  may be constructed using materials such as but not limited to, silicon, silicon nitride, silicon-germanium, aluminum, gold, titanium, chromium, or using a dielectric material, or materials having high elasticity such as stainless steel. The diaphragm  26 ,  54  and  80  may comprise a thin membrane made of a semiconductor material such as silicon, silicon nitride, metals and metal alloys such as stainless steel, titanium, hastelloy, ceramics or other materials with desirable mechanical properties, such as high elasticity, fatigue resistance, etc. One example of a dielectric material that may be used is a polyimide film, such as KAPTON® that is commercially available from E. I. DuPont De Nemours and Company of Wilmington, Del.  
         [0036]     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.