Patent Publication Number: US-7898244-B2

Title: Electromagnetic sensor systems

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 11/325,143, entitled ELECTROMAGNETIC SENSOR SYSTEMS AND METHODS OF USE THEREOF, filed on Jan. 4, 2006, which is a continuation-in-part of International Application Ser. No. PCT/US2005/007363, filed Mar. 7, 2005, entitled INDUCTION SENSOR, both of which are incorporated by reference herein, and claims priority of U.S. Provisional Patent application Ser. No. 60/841,061, entitled INDUCTION LINEAR SENSOR SYSTEM, filed on Aug. 30, 2006, of U.S. Provisional Patent application Ser. No. 60/841,322, entitled HIGH TEMPERATURE INDUCTIVE SENSOR, filed on Aug. 31, 2006, and of U.S. Provisional Patent application Ser. No. 60/853,568, entitled BRAKE LINING THICKNESS SENSOR, filed on Oct. 23, 2006, all of which are also incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     These teachings relate to electro-mechanical measurement and control systems. 
     As digital electronic information processing has improved, the search has developed for digital signal sources to indicate physical parameters for measurement and system control. Interfaces have been developed that allow analog sensing devices to be used with digital controls. However, there remains a need for sensors that have digital output and integrate seamlessly with digital equipment. 
     When high-speed position measurement is made with conventional devices that employ a magnetic field there is a delay between the actual position and the indicated position. This delay is referred as measurement hysteresis. This measurement hysteresis is undesirable in practice. 
     Material considerations have discouraged the use of inductive proximity sensors at temperatures above 260° C. Conventionally used copper magnet wire may experience oxidation of the wire and degradation of its insulation at high temperature. Some assembly techniques have used materials that are unsuitable for exposure to high temperatures. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, the system of these teachings includes an oscillator circuit. In one instance, the sensing element is a variable reactance element. 
     For a better understanding of these teachings, together with other and further needs thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a schematic representation of an embodiment of the system of these teachings; 
         FIG. 1   b  is a schematic representation of another embodiment of the system of these teachings; 
         FIG. 1   c  is a schematic representation of yet another embodiment of the system of these teachings; 
         FIG. 1   d  is a schematic representation of a further embodiment of the system of these teachings; 
         FIG. 1   e  is a schematic representation of a yet a further embodiment of the system of these teachings; 
         FIGS. 2   a - 2   d  depict an embodiment of a sensing element of these teachings; 
         FIGS. 3   a - 3   d  depict an embodiment of a component of a physical structure of these teachings; 
         FIGS. 4   a - 4   c  show another embodiment of a component of a physical structure of these teachings; 
         FIGS. 5   a - 5   f  and  6   a - 6   c  show another embodiment of a sensing element of these teachings; and 
         FIG. 7  depicts a schematic representation of a further embodiment of the system of these teachings. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the system of these teachings is shown in  FIG. 1   a . The circuit shown in  FIG. 1   a  is a tuned oscillator circuit. The tuned oscillator circuit is comprised of an amplifier (U 1   a , U 1   b , and U 1   c ) and two reactive components, an inductor L 1  and a capacitor C 4 . L 1  and C 4  are in series connection with C 4  connected to ground (return) and L 1  connected to the output of the amplifier (U 1   a , U 1   b , and U 1   c ). The frequency of the oscillator is: 
     
       
         
           
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     In one embodiment, the inductor L 1  is a variable inductor and is the sensing component. In that embodiment, the capacitor C 4  is a fixed value capacitor (fixed capacitance). In another embodiment, the physical structure that comprises the inductor L 1  also exhibits variable capacitance (as, for example, but not limited to, the situation in which the electric and magnetic fields of physical structure are modified while performing a measurement). It should be noted that the conventional sources of DC and oscillator power are not shown in  FIG. 1   a  (or in the subsequent figures,  FIGS. 1   b ,  1   c ,  1   d  and  1   e ). The placement and configuration of such sources is conventional. 
     In one embodiment, the amplifiers) (having sections U 1   a , U 1   b , and U 1   c ) shown in  FIG. 1   a  is a high speed CMOS hex inverter. The resistor R 2  can be used to bias the input of the amplifier to compensate for the leakage current. The resistor R 3  and capacitor C 3  provide the feedback path. The oscillator is AC coupled by capacitor C 3  so that there is no DC voltage path through the oscillator. Resistor R 2  can influence the feedback path of the circuit. In one embodiment, the resistance value of R 2  can chosen so that when the variable reactance is obtained by placing a conducting non magnetic surface, such as, but not limited to, copper, in proximity to a magnetic field producing component, such of variation of reactance will cause will cause the frequency of the circuit to increase, while, when a ferromagnetic surface, such as, but not limited to, steel, is placed proximity to the magnetic field producing component, such a variation will cause the frequency to decrease. 
     In another embodiment, a transistor amplifier or operational amplifier can be used in place of hex inverter U 1   a,  U 1   b , and U 1   c . In one instance, two signals can be generated from the oscillator for use as output. One signal is a square wave and the other signal is a sine wave, both have the same frequency. 
     Another embodiment of the system of these teachings is shown in  FIG. 1   b . In  FIG. 1   b , Capacitors C 1  and C 2  replace capacitor C 4  of  FIG. 1   a . Capacitors C 1  and C 2  are connected in parallel and have an equivalent capacitance equal to the sum of the capacitance of capacitors C 1  and C 2 . In one instance, the two capacitors C 1  and C 2  provide temperature compensation in the circuit. In that instance, capacitor C 1  has a capacitance vs. temperature relation that is positive. Capacitor C 2  has a capacitance vs. temperature relation that is negative. By judicious choice of the capacitance values of C 1  and C 2  temperature induced frequency drift of the sensor output can be minimized. In another embodiment, the capacitance versus temperature relationship of each of the two capacitors C 1  and C 2  is selected such that a desired variation of the sensor output versus temperature can be obtained. In the embodiment shown in  FIG. 1   b , U 1   a , U 1   b , and U 1   c  are three sections of a 74HC04 hex inverter. 
     Yet another embodiment of the system of these teachings is shown in  FIG. 1   c . In  FIG. 1   c , the variable reactance L 1  of  FIG. 1   b  is replaced by a number of variable reactances L 2 , L 3 , L 4 , where each one of the variable reactances is connected in parallel to the other variable reactances. In the embodiment of the system of these teachings shown in  FIG. 1   c , variation of any one of the variable reactances L 2 , L 3 , L 4  will result in a change in the oscillator frequency. 
     Further embodiments of the system of these teachings are shown in  FIGS. 1   d  and  1   e . Referring to  FIG. 1   d , the embodiment shown therein includes a first oscillator circuit  15 , which in this embodiment is the circuit of  FIG. 1   a , and a second oscillator circuit  35 . The output of the first oscillator circuit  15  is a sensor output  25 . The second oscillator circuit  35  has a substantially fixed (other than due to temperature variations) inductor L 2 , a third capacitor C 5 , the inductor L 2  being separated from ground by the third capacitor C 5 , a fourth capacitor C 6  located in the feedback path from the connection between the inductor L 2  and the third capacitor C 5  to a second amplifier (U 2   d , U 2   e , U 2   f ). The output  45  of the second oscillator circuit  35  is a compensation signal output. The first oscillator circuit  15  is located in substantial proximity to the second oscillator circuit  35  and both the first oscillator circuit  15  and the second oscillator circuit  35  experience substantially a same temperature (in some instances, the variable reactance L 1  is located away from the rest of the first oscillator circuit  15 ). A computer subsystem (such as, but not limited to, a microprocessor subsystem in one instance) receives the sensor output  25  and the compensation signal output  45 . The computer subsystem includes one or more processors  55  and one or more computer readable media  65 . The computer readable media  65  has computer readable code embodied therein for causing the processors  55  to utilize the compensation signal output  45  in order to substantially compensate the sensor output  25  for temperature induced variations such as, but not limited to, temperature induced drift. The one or more processors  55  and are one or more computer readable media  65  are operatively connected by an interconnection component  67  (for example, a computer bus). The compensation signal output  45  and the sensor output  25  are also provided to the interconnection component  67 . 
     Depending on the nature of the compensation signal output  45  and the sensor output  25 , the signals are provided to the interconnection component  67  by different means. If the signals can be provided directly to the digital circuit, a direct connection to the interconnection component  67  is possible. In other instances, the compensation signal output  45  and the sensor output  25  are provided to the interconnection component  67  by interface circuits. The interface circuits are conventionally determined by the nature of the compensation signal output  45  and the sensor output  25  and the digital circuit. The nature of the compensation signal output  45  and the sensor output  25  is determined by a variety of factors including, but not limited to, signal amplitude and signal range. 
     In one instance, the third capacitor C 5  may be chosen with a capacitance versus temperature characteristics that results in a predetermined temperature variation of the compensation signal output  45 . In one instance, a substantially large (predetermined) change in output corresponds to to a change in temperature. In one instance, the amplifiers U 2   d,  U 2   e , U 2   f  are part of the same integrated circuit as the amplifiers U 2   a , U 2   b , U 2   c . (for example, a 74HC04). 
     Referring to  FIG. 1   e , the first oscillator circuit  15  in the embodiment shown therein is the same circuit as the circuit of  FIG. 1   b . The two capacitors C 1  and C 2  can be selected such that a desired temperature variation of the sensor output  25  is obtained. In one instance, these teachings not being limited only to that instance, the first oscillator circuit  15  and the second oscillator circuit  35  are configured such that the (frequency) sensor output  25  and compensation signal output  45  increase with increasing temperature in a predetermined manner (in one instance, in substantially the same manner). In one possible embodiment, the variable reactance L 1  is obtained in a manner that the sensor output  25  (frequency) increases or decreases upon measurement In that embodiment, a predetermined difference between the sensor output  25  (frequency) and the compensation signal output  45  (frequency) will indicate a predetermined measurement. 
     It should be noted that embodiments of the sensor circuits shown in  FIGS. 1   d  and  1   e  in which the variable reactance L 1  is located at a distance from the rest of the first oscillator circuit  15  (such as the embodiment shown in  FIG. 7  below) are within the scope of these teachings. 
     During measurement, when utilizing the embodiments shown in  FIGS. 1   d  and  1   e , a sensor output is obtained from a first oscillator circuit including a variable reactance, a compensation signal output is obtained from a second oscillator circuit having a substantially fixed inductor, and the compensation signal output is utilized in order to substantially compensate the sensor output for temperature variations. An exemplary embodiment is presented hereinabove. Other embodiments in which at least the temperature variation of the second oscillator circuit  35  is substantially predetermined (or designed to be substantially predetermined) are also possible. 
     Embodiments in which the temperature variation of the second oscillator circuit  35  (and the first oscillator circuit  15  in some instances) is predetermined by calibration or predetermined by design are both within the scope of these teachings. 
     In one embodiment the variable reactance is obtained from a physical structure. In one instance the physical structure includes a substantially linear element (substantially linear as used herein refers to material properties such as a substantially conducting element or a material having a substantially linear permeability; see, for example, the number of structures  9  in  FIG. 3   b ) and a sensing element for generating electromagnetic fields (see, for example, the sensing element  4  in  FIG. 3   c ). The substantially linear element and the sensing element can move with respect to each other and the spatial relationship between the substantially linear element and the sensing element determines the variable reactance. Embodiments in which the substantially linear element and a sensing element move in a direction substantially parallel to a central axis of the sensing element or move in a direction substantially perpendicular to the central axis of the sensing element are within the scope of these teachings. 
     One embodiment of the variable reluctance element (L 1  in  FIG. 1   a , L 2 , L 3  or L 4  in  FIG. 1   c ) is shown in  FIGS. 2   a - 2   d,    FIGS. 3   a - 3   d  and  FIGS. 4   a - 4   c . Referring to  FIGS. 2   a - 2   d , the sensing element shown therein includes a first end portion, a second end portion and a central portion (labeled  3  in  FIG. 2   a ) disposed on a base portion ( 1 ,  FIGS. 2   a - 2   d ) and a coil  2 , capable of carrying an electrical current, disposed (or wound) around the central portion  3 . The first end portion, the second end portion, the central portion and the base portion are comprised of a substantially magnetic material (a substantially magnetic material, as used herein, refers to a ferromagnetic or a ferrimagnetic material).  FIG. 2   a  and  FIG. 2   b  are two elevations of a sensing element (ferrite core)  4  in a structure conventionally referred to as an E core. The structure shown therein has width dimension “A” and length dimension “B”. When the coil  2  is energized by an electrical current, an electromagnetic field is projected from the faces of the end portions and the central portion  3 . In one instance, the core shown therein is comprised out of a ferrite material.  FIG. 2   c  and  FIG. 2   d  show two elevations of the ferrite core with electrical coil  2  assembled into a sensing element  4 . 
     In one embodiment, an exemplary instance of which is shown in  FIGS. 3   a - 3   d , the substantially linear element includes a number of substructures, where each substructure ( 5 ,  FIG. 3   a ) is comprised of a substantially conductive material and a substrate  6 , on which the substructures are disposed. In one instance the substrate  6  is comprised of a substantially magnetic material. Each substructure has a characteristic dimension representative of height above the substrate  6  (in the embodiment shown in  FIGS. 3   a - 3   d , the characteristic dimension is the thickness of the foil). Each substructure  5  is disposed at a predetermined distance from the adjacent substructure (forming a linear array of substructures). 
     In one exemplary embodiment, these teachings not be limited only to that exemplary embodiment, each substructure is a rectangle of metal foil (a substructure having a substantially planar rectangular surface and another substantially planar surface, the two surfaces being disposed at a predetermined distance from each other) that has width dimension “C” and length dimension “D”. The metal foil is made of non magnetic electrically conducting material, such as, but not limited to, copper or aluminum and may, in an exemplary embodiment, have thickness of 0.003 inches. Width dimension “C” is, in one exemplary instance, the same or greater than dimension “A” of sensing element  4  shown in  FIG. 2   c . Length dimension “D” may, in an exemplary instance, be equal to length dimension “B” of sensing element  4 . Length dimension “D” may also be greater than length dimension “B” in which case the increased length dimension “D” may compensate for lateral misalignment of sensor  4  in its travel.  FIG. 3   b  shows an array of sensor target elements in relation to each other for use in one embodiment of these teachings. In general the elements  5  are arranged to make a uniform pattern (in one instance, similar to teeth of a rack gear). The distance “E” between each element  5  may be equal to or greater than dimension “C”.  FIG. 3   c  and  FIG. 3   d  show the substructures  5  in relation to substrate  6  and sensing element  4  as used in one embodiment of these teachings. Elements  5  are fixed to substrate  6  in the pattern shown in  FIG. 2   b . In one instance, the substrate  6  is a substantially magnetic material as steel, iron, or ferrite. Sensing element  4 , in one instance, is positioned so that face of sensing element  4  is substantially parallel to the substructures  5  on substrate  6 . The sensing element  4 , in one embodiment, moves in a direction substantially perpendicular to dimension “D” of substructures  5 , as indicated in  FIG. 3   d , in such a manner as to traverse over the substructures  5 . 
     In another embodiment, shown in  FIGS. 4   a - 4   c , the substantially linear element includes a first laminate layer  7  having a substantially adhesive surface. The first laminate layer  7  is disposed over the substructures  5 . The substantially adhesive surface is adjacent to the substructures  5 . The substantially linear element also includes a second laminate layer  8  having two substantially adhesive surfaces. The substructures  5  are disposed over one of the two substantially adhesive surfaces. Another one of the two substantially adhesive surfaces is disposed over the substrate  6 . In one exemplary embodiment, not a limitation of these teachings, the first laminate layer  7  may be a self-adhesive flexible polyester tape with glass filament reinforcements. The second laminate layer  8  may be self-adhesive tape with adhesive on both surfaces (for example, double-sided adhesive tape). One adhesive surface is applied to the substructures. The other adhesive surface is applied to a substrate  6 , which in one embodiment is comprised of a substantially magnetic material. 
     In an exemplary application of the circuit of these teachings shown in  FIG. 1   a  or  1   b , and the sensing element and substantially linear element shown in  FIGS. 3   a - 3   d , the frequency output of the circuit of  FIG. 1   a  or  1   b  can be provided as input to a microprocessor. In one instance, the output is utilized by the microprocessor in order to count each time the frequency attain some value in the midpoint of the range of frequencies. In one exemplary embodiment (not a limitation of these teachings), if the frequency attains a value of 74 kHz when the face of the sensing element  4  is adjacent to a substructure  5  and the frequency attains a value of 66 kHz when the face of the sensing element  4  is adjacent to a region of the substrate  6  between two substructures  5 , then a count or pulse (indicative that the sensing element  4  is at a location near the edge of the substructure  5 ) can be generated when the frequency attains a value of 69 kHz. The counts can be summed and from knowledge of the distance between the substructures, an indication of distance can be determined from the sum. 
     In order to better illustrate the present teachings, another exemplary embodiment is disclosed hereinbelow. In the exemplary embodiment, not a limitation of these teachings, a coil of 170 turns of 34 AWG wire is assembled into a ferrite core of dimension ⅛ inch width and ½ inch length in order to form the sensing element  4 . Copper elements comprising the substructures  5  have dimension ⅛ inch×½ inch each and each element has thickness of 0.003 inch. The glass re enforced polyester lamination  9  has thickness 0.012 inches and is attached to a mild steel substrate  6 . 
     In yet another embodiment of the system of these teachings, in order to operate above a predetermined temperature, the sensing element is comprised of a material having a Curie temperature above the predetermined temperature. In one instance, the coil  2  of  FIG. 2   d  is encapsulated in a ceramic material. In another instance, after placing the encapsulated coil around the center portion of the core (as in  FIG. 2   d ), the sensing element  4  is encapsulated in a ceramic material. 
     An embodiment of an encapsulated sensing element is shown in  FIGS. 5   a - 5   f  and  6   a - 6   c . Referring to  FIGS. 5   a - 5   f , a core is shown therein having a first side portion  12 , a second side portion  12 , a center portion  14  and a base portion  16 . The base portion  16  has a surface with a substantially circular area. Each of the side portion  12  are disposed along a portion of the half-circumference of the substantially circular area and the center portion  14  has a cross-sectional area that is smaller than the substantially circular area of the base portion  16 . The coil  20  is disposed around the center portion  14  and between the center portion  14  and the side portions  12 . The wire comprising the coil  20  is encapsulated in a ceramic material. Leads  30  allow providing an electrical current to the coil  20 . 
     In one exemplary embodiment, these teachings not being limited only to the exemplary embodiment, the coil  20  is made by winding aluminum magnet wire that has been anodized. The anodized surface on the wire provides electrical insulation for the wire. As the wire is wound onto a form to make the coil, ceramic cement is applied to the wire. The cement is allowed to cure and then the coil is removed from the winding tool. The cement used may be one of a number of conventional cements used for encapsulating or joining electrical heating elements and electrical lighting elements. (For example, these teachings not being limited only to this example, one source for the cement is Sauereisen.) 
     The material of the core  10  is chosen to have a Curie temperature above the temperature at which the sensing element will operate. In an exemplary embodiment (not a limitation of these teachings), the core  10  is a ferrite core. Many types of soft ferrite are conventionally available. For any particular sensing element, a ferrite material is chosen that has at least a predetermined magnetic permeability at the temperature at which the sensing element will operate. For a high temperature sensing element, a material is chosen with a Curie temperature above the temperature at which the sensor will operate. There are various NiZn ferrite materials that have a Curie temperature above 320° C. These ferrite materials can be used to make inductive proximity sensors that will operate at these high temperatures. (In one exemplary embodiment, these teachings not being limited to only that embodiment, one source for the ferrite material is Ferroxcube). 
     In the embodiment shown in  FIGS. 5   e  and  5   f , the coil  20  is assembled onto/into the core  10  of  FIG. 5   a . The assembled sensing element is then encapsulated in a ceramic material.  FIGS. 6   a - 6   c  show and embodiment of the encapsulated sensing elements corresponding to the sensing element of  FIGS. 5   a - 5   f . The encapsulated sensing element includes the sensing elements  40  that has been encapsulated in a shell  80  of ceramic cement. Coil leads  30  may be joined to sensor leads  50  by crimping, welding, or brazing connections  60 . The shell  80  encloses the sensing element  40  of  FIGS. 5   e  and  5   f  and wire connections  60  and substantially prevents air (ambient gas) from coming in contact with the contents of the shell  80 . In one instance, not a limitation of these teachings, shell  80  is formed by suspending the sensing element  40  with attached connections  60  to sensor leads  50  in a mold and filling the mold cavity with ceramic cement. 
       FIG. 7  shows the embodiment shown in  FIG. 1   b  of the sensor circuit of the present teachings with the encapsulated sensing element  70  of  FIG. 6   c . It should be noted that these teachings are not limited only to the embodiment shown in  FIG. 1   b . Other embodiments, for example, such as those shown in  FIG. 1   a  or  1   c , are also within the scope of these teachings. In the embodiment shown in  FIG. 7 , the sensing element  70  is separated from the rest of the sensor circuit by cable  90  so as to prevent the high temperature that affects the sensing element  70  from affecting the other components in the circuit. 
     While exemplary embodiments including specific materials have been disclosed hereinabove, it should be noted that these teachings are not limited to only those embodiments and all our exemplary embodiments are also within the scope of these teachings. 
     Not desiring to be bound by theory, the embodiments described above are not limited by the description of the physical mechanisms detailed above. 
     Elements and components described herein may be further divided into additional components or joined together to form fewer components for performing the same functions. 
     Each computer program within the scope of the claims below may be implemented in any programming language, such as assembly language, machine language, a high-level procedural programming language, or an object-oriented programming language. The programming language may be a compiled or interpreted programming language. 
     Each computer program may be implemented in a computer program product tangibly embodied in a computer-readable storage device for execution by a computer processor. Method steps of the invention may be performed by a computer processor executing a program tangibly embodied on a computer-readable medium to perform functions of the invention by operating on input and generating output. 
     Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CDROM, any other optical medium, punched cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, or any other medium from which a computer can read. From a technological standpoint, a signal or carrier wave (such as used for Internet distribution of software) encoded with functional descriptive material is similar to a computer-readable medium encoded with functional descriptive material, in that they both create a functional interrelationship with a computer. In other words, a computer is able to execute the encoded functions, regardless of whether the format is a disk or a signal. 
     Although these teachings have been described with respect to various embodiments, it should be realized these teachings are also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.