Abstract:
An inductive proximity sensor comprises: a housing including an opening on one side; an inductive core including two legs, each core leg including a foot portion; an inductive coil wound bobbin disposed about each leg of the core, each bobbin including an integral ledge cantilevered from an inside surface of the bobbin for forming a pocket under the integral ledge at a bottom of the bobbin for containing the foot portion of the corresponding core leg; a thruster element including a top surface and two legs, each thruster leg disposed into a respective bobbin resting on the integral ledge thereof, the core, wound bobbins and thruster element disposed in the housing; a spring element disposed on the top surface of the thruster element at the housing opening; and a housing cover disposed over the housing opening for compressing the spring element against the thruster element which renders the core and wound bobbins in a fixed relationship to each other in the housing. Also disclosed is a method of assembling the proximity sensor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention is directed to inductive proximity sensors in general, and more particularly, to an inductive proximity sensor with induction coils in fixed relationship with an inductive core by compression to avoid: (1) the use of adhesives to secure the induction coils to the inductive core; and (2) adjustment or calibration to achieve the desired inductive output, and method of assembling the same.  
         [0002]     Inductive proximity sensors typically comprise a core, which may be “C” or Omega shaped, for example, fabricated from a highly-permeable metal, with two inductive coils on bobbins placed over each leg of the core. The two coils are typically wound around their respective bobbins in opposite directions (one wound clockwise and the other wound counter-clockwise) and electrically connected in series. The series connected coils of the sensor are generally driven by an AC voltage at a desired frequency. The generated coil current, which may be monitored by a current sensing device, is commonly used as an inductive output of the sensor. Generally, the inductive output changes value when a target to be sensed moves from a near to a far position with respect to a sensing face of the sensor, and vice versa. There should be a sufficient change in value of inductive output over the span of operating conditions in order to be able to distinguish between the near and far target positions.  
         [0003]     The inductive output of the sensor is determined by several factors, including core material, core geometry, number of turns of coil wire, coil (bobbin) geometry, operating frequency and voltage, coil resistance, sensor housing material, and the relative position of the coils to the core, for example. The significant characteristics of all of the materials used in the sensor as well as the assembly process is controlled so that the assembled sensor may exhibit an established standard inductance (within tolerances). Usually, in order to meet an inductive specification, the sensor assembly or transducer is “calibrated”. The calibration may be accomplished in several ways. Three of the most common methods are: 1) move one of the coils along its core leg until the desired inductance is achieved and then secure the bobbin to the core leg with an adhesive, which may be an epoxy, for example; 2) add or remove turns of wire from one or both of the coils until the desired inductance is achieved; and 3) add an adjustable permeable shunt to the assembly which will magnetically interact with the core thereby effecting an adjustment to the transducer inductance. All of these calibration methods include manual intervention by the assembler of the sensor. It would be advantageous, from a manufacturing perspective, to successfully assemble the sensor without the need for calibration.  
         [0004]     As noted above, it is currently common practice during calibration to secure the bobbin (on which the coil is wound) to the core leg using an epoxy adhesive. Eliminating this step from the assembly process would significantly increase the reliability of the sensor and reduce manufacturing costs.  
         [0005]     The present invention as will be described in greater detail herein below incorporates features including self aligning coils which will allow for the successful assembly of a proximity sensor without calibration, and thus, without the use of an adhesive to secure the coil bobbin to the leg of the core during assembly of the sensor.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with one aspect of the present invention, an inductive proximity sensor comprises: a housing including an opening on one side; an inductive core including two legs, each core leg including a foot portion; an inductive coil wound bobbin disposed about each leg of the core, each bobbin including an integral ledge cantilevered from an inside surface of the bobbin for forming a pocket under the integral ledge at a bottom of the bobbin for containing the foot portion of the corresponding core leg; a thruster element including a top surface and two legs, each thruster leg disposed into a respective bobbin resting on the integral ledge thereof, the core, wound bobbins and thruster element disposed in the housing; a spring element disposed on the top surface of the thruster element at the housing opening; and a compressive element disposed over the housing opening for compressing the spring element against the thruster element which renders the core and wound bobbins in a fixed relationship to each other in the housing.  
         [0007]     In accordance with another aspect of the present invention, a method of assembling an inductive proximity sensor to maintain inductive coil wound bobbins in fixed relation to an inductive core within a housing comprises the steps of: configuring each of two inductive coil wound bobbins with an integral ledge cantilevered from an inside surface thereof for forming a pocket under the integral ledge at a bottom of each bobbin; disposing each inductive coil wound bobbin around a corresponding leg of the inductive core; containing a foot of each core leg into the pocket of the corresponding bobbin under the integral ledge thereof; inserting each of two legs of a thruster element into a corresponding bobbin, wherein each leg rests upon the ledge of the corresponding bobbin; disposing the inductive core, inductive coil wound bobbins, and thruster element into the housing; disposing a spring element on a top surface of the thruster element; and compressing the spring element against the top surface of the thruster element to render the core and wound bobbins in a fixed relationship to each other in the housing.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIGS. 1A and 1B  are cross-sectional profile and side views, respectively, of internal components of an exemplary proximity sensor assembly suitable for embodying the broad principles of the present invention.  
         [0009]      FIG. 1C  is a bottom view of the assembled internal components of the exemplary sensor assembly embodiment of  FIG. 1 .  
         [0010]      FIG. 2  is a cross-sectional view of the exemplary proximity sensor assembly of  FIG. 1  disposed in a sensor housing.  
         [0011]      FIG. 3  is an exploded, breakaway isometric view of the components of the exemplary proximity sensor embodiment of  FIG. 2 .  
         [0012]      FIG. 4  is an illustration of a proximity sensor suitable for use in another aspect of the present invention.  
         [0013]      FIG. 5  is an illustration of a non-intrusive proximity sensing configuration suitable for use in the other aspect of the present invention.  
         [0014]      FIGS. 6   a  and  6 B are illustrations depicting plan and side views, respectively, of a target for use in the other aspect of the present invention.  
         [0015]      FIGS. 7-9  are graphs of measured sensor inductance vs. drive frequency for different test conditions in accordance with the other aspect of the present invention.  
         [0016]      FIGS. 10-12  are graphs of measured sensor inductance vs. target gap for additional test conditions in accordance with the other aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     An exemplary embodiment of a proximity sensor assembly suitable for embodying the broad principles of the present invention will be described in connection with  FIGS. 1A, 1B ,  1 C,  2  and  3 . Referring to  FIGS. 1A, 1B ,  1 C,  2  and  3 , a pair of coil wound, square cross-sectional bobbins  10  and  12  are disposed over respective legs  14  and  16  of an inductive core  18 , which may be C-shaped, for example. Each bobbin is wound with a precise number of turns in order to meet the specified inductance of the proximity sensor. One bobbin, like  10 , for example, may be wound with the inductive coil clockwise and the other bobbin, like  12 , for example, may be wound with the inductive coil counter-clockwise. The windings of the two bobbins are connected in series and the unconnected inductive coil leads (not shown) are disposed in a housing  20  of the sensor assembly and in turn, connected to respective pins  22  and  24  which pass through a housing wall and become part of a connector  26  which is coupled externally to the wall housing  20 . The pins  22  and  24  of connector  26  provide an electrical coupling through the housing wall to the internal series connected inductive coils. Accordingly, an AC voltage at a desired frequency may be applied across the pins  22  and  24  to drive the inductive proximity sensor as will become better understood from the description found herein below.  
         [0018]     Each bobbin  10  and  12  includes an integral ledge  30  and  32 , respectively, which creates a pocket in the bottom of each bobbin into which the foot of the respective leg  14  and  16  may be contained. Each ledge  30  and  32  is configured in the respective bobbin to rest on top of the foot of the respective core leg  14  and  16  as shown in  FIG. 1A and 2 . Once the core  18  and coil wound bobbins  10  and  12  are assembled and disposed in the housing  20  through an opening  33 , a thruster component  34  is added to the assembly. In the present embodiment, the thruster  34  is “pi” shaped comprising legs  36  and  38  which are disposed respectively through openings  40  and  44  on top of the C shaped core  18  and into the bobbins  10  and  12 , respectively. The bottoms of the legs  36  and  38  rest respectively on top of the ledges  30  and  32  within the bobbins  10  and  12 .  
         [0019]     A spring element  50 , which may be a wavy leaf spring, for example, is disposed on top of the “pi” shaped thruster  34  and positioned in place by integral guides  52  which protrude from each side of the top of the thruster  34 . The spring element  50  may extend over almost the entire length of the top surface of the thruster  34 . After the proximity sensor is assembled in the housing  20 , a cover plate  60  is affixed over the housing opening  33 , thereby compressing the leaf spring  50  atop the thruster  34  which in turn, applies a downward force on the thruster legs  36  and  38 . Each thruster leg  36  and  38 , in turn, forces the respective integral bobbin ledge  30  and  32  into the respective foot of the C shaped inductive core  34 . This compression ultimately presses each foot of the core  34  onto the internal surface of the sensing wall or face  62  of the sensor housing  20 . It is understood that the cover plate  60  is used in the present embodiment only by way of example and that a compressive element other than the cover plate may be used just as well.  
         [0020]     The spring element  50  may be designed to withstand the number of g forces called for by a design specification. In the present embodiment, the force, exerted by the leaf spring  50 , represents 500 to 1,000 g&#39;s (gravitational constants) of preload. In addition, the cover plate  60  may be forced down upon the leaf spring  50  and into an indented rim  64  around the housing opening  33  with a press ram, and tack welded into place, for example. Thereafter, the press ram may be removed and the welding of the cover plate  60  to the rim  64  of the housing opening  33  may be completed. In the present example, the cover plate  60  is flush mounted to the housing wall around the opening  33  affording a permanent sealed chamber within the housing  20  containing the compressed assembly of components of the proximity sensor. This compressed assembly provides self-aligned coils and prevents the sensor components from separating under high shock conditions.  
         [0021]     As noted above, each bobbin  10  and  12  includes a square pocket in the bottom thereof to contain the foot of the respective leg  14  and  16  of the C shaped core  34 . As the foot of each core leg  14  and  16  fits into a square pocket under the respective ledge  30  and  32  in the bottom of each bobbin  10  and  12 , each bobbin  10  and  12  is rotationally constrained under compression. The depth of the square pocket in the bottom of each bobbin  10  and  12  together with the thickness of the material of the core  34  determines the relative position of the inductive coil/bobbin  10  and  12  to the inductive core leg  14  and  16 , respectively.  
         [0022]     The housing  20  may include L-shaped, winged extensions  66  and  68  at each side thereof. Each side of each L-shaped extension  66  and  68  includes an opening  72  and  74  through which a screw or bolt may be inserted for mounting the sensor assembly to a barrier (not shown). Accordingly, the sensor assembly may be mounted and secured to the barrier depending on the movement of the target to the sensing face  62 .  
         [0023]     Since the dimensions of the sensor components may be controlled to a high degree of precision, then the relative positions of the bobbin and core leg may be likewise controlled to a high level of precision, i.e. self-aligned. The magnetic properties of the core material, the number of turns of the induction coils, the sensor housing material properties, the core geometry, and all of the aforementioned specifications may be likewise controlled to a high degree of precision. These features will allow the proximity sensor to be successfully fabricated and assembled without in-process calibration and without securing adhesives which will also significantly enhance the survivability of the proximity sensor, particularly in high shock environments.  
         [0024]     Normally, inductive proximity sensors similar in design to the sensor described herein above can not operate effectively to monitor the relative position of a target of permeable or magnetic material through barriers of conductive material, like aluminum and copper, for example, due primarily to the operating frequencies thereof. In accordance with another aspect of the present invention, applicant has discovered a method of operating a non-intrusive proximity sensor for monitoring the relative position of a target of permeable or magnetic material through barriers of conductive material. This aspect of the present invention will now be described in connection with  FIGS. 4-12 .  
         [0025]     A sketch of a proximity sensor  70  suitable for use in the operating method of the present invention is shown in  FIG. 4 . The proximity sensor  70  may be similar in design to the sensor assembly described herein above and for this reason, common reference numerals will be used for sensor components previously described. For example, the proximity sensor  70  may have a housing  20 , a sensing face  62  and an electrical connector  26  as described above.  FIG. 5  is a sketch illustrating an application of a non-intrusive application of the proximity sensor  70 . Referring to  FIG. 5 , the proximity sensor  70  is secured in place with the sensing face  62  thereof in juxtaposition with one side of a conductive barrier  72 , which may be a wall or bulkhead of an aircraft, for example. A target  74  is positionable relative to the proximity sensor  70  on the other side of the conductive plate  72  so that the sensor  70  may monitor the relative position of the target  74  through the conductive plate  72 , i.e. non-intrusively. In addition, a drive and signal processing unit  76  is electrically coupled across the pins  22  and  24  of the connector  26 , which pins are electrically coupled to the inductive coils of the sensor  70  as described herein above.  
         [0026]     The electronic drive and signal processing unit  76  being used for proximity sensing in the present embodiment may be digital signal processor (DSP) or microprocessor based. The unit  76  may be programmed to output a drive signal via a digital-to-analog (D/A) converter to the sensor  70  (i.e. across pins  22  and  24 ) at virtually any frequency. Also, it is understood that in an aircraft system, for example, there may be a large number of different proximity sensors all configured for corresponding different non-intrusive proximity sensing applications, such as through conductive barriers of pressure bulkheads, door casings, aircraft skins, composite panels containing a conductive mesh for EMI protection, etc. The present embodiment would also permit operation of proximity sensors in close proximity to conductive side metals such as sensor or target mounting brackets, for example. Accordingly, the different proximity sensors in the system may be all coupled to a common processing unit  76 . Such an architecture would allow the common processing unit  76  to be programmed to excite each different individual sensor  70  at a frequency appropriate for its installation configuration. A mechanical actuation distance  78  (sensor/target gap variation during actuation) and the thickness and composition of the material of the barrier  72  would determine the appropriate drive frequency for that particular installation.  
         [0027]     In the present embodiment, the target  74  may be a conventional plate of permeable or magnetic material, like a ferrous material, for example, as shown in the plan and side views of  FIGS. 6A and 6B , respectively. The length L, width W and thickness T of the target plate  72  for the present example are 1.50 inches, 0.75 inches and 0.10 inches, respectively.  
         [0028]     Tests were conducted on the proximity sensing configuration of  FIG. 5  which illustrates an orientation of the proximity sensor  70 , the conductive barrier  72 , and the standard ferrous target  74  which is positionable relative to the sensor  70 . The sensor  70  is interfaced with the processing unit  76  which may include a programmable LCR bridge capable of measuring the inductance or inductive output of the sensor  70  at various drive frequencies.  FIG. 7  is a graph of measured sensor inductance vs. drive frequency for two test conditions using an 0.062 inch thick aluminum conductive barrier  72 . In  FIG. 7 , line  80  exemplifies the test results of the proximity sensor inductance with the conductive barrier  72  in place and no target  74  present (i.e. indicative of a “far” position); and line  82  exemplifies the test results of the proximity sensor inductance with the conductive barrier  72  in place and the target  74  present (i.e. indicative of a “near” position). Note that at or below drive frequencies of approximately 350 Hz, the measured inductances of the two conditions  80  and  82  are sufficiently distinguishable for proximity sensing operation.  
         [0029]      FIG. 8  is a graph of measured sensor inductance vs. drive frequency for another two test conditions using a 0.125 inch thick aluminum conductive barrier  72 . In  FIG. 8 , line  84  exemplifies the test results of the proximity sensor inductance with the conductive barrier  72  in place and no target  74  present (i.e. indicative of a “far” position); and line  86  exemplifies the test results of the proximity sensor inductance with the conductive barrier  72  in place and the target  74  present (i.e. indicative of a “near” position). Note that at or below drive frequencies of approximately 100 Hz, the measured inductances of the two conditions  84  and  86  are sufficiently distinguishable for proximity sensing operation.  
         [0030]      FIG. 9  is a graph of measured sensor inductance vs. drive frequency for yet another two test conditions using an 0.062 inch thick copper conductive barrier  72 . In  FIG. 9 , line  88  exemplifies the test results of the proximity sensor inductance with the conductive barrier  72  in place and no target  74  present (i.e. indicative of a “far” position); and line  90  exemplifies the test results of the proximity sensor inductance with the conductive barrier  72  in place and the target  74  present (i.e. indicative of a “near” position). Note that at or below drive frequencies of approximately 150 Hz, the measured inductances of the two conditions  84  and  86  are sufficiently distinguishable for proximity sensing operation.  
         [0031]     For the foregoing described tests, sensor inductance was measured with the standard target  74  located immediately adjacent to the conductive barrier  72  which is representative of the target in the “near” position in a typical mechanical actuation. The graphs of  FIGS. 10-12  represent three additional test conditions performed whereby the target  74  is incrementally moved away from the conductive barrier  72  which is representative of a typical mechanical actuation where the target moves from the “near” position to the “far” position. The graphs of  FIGS. 10-12  indicate the total available inductance bandwidth in a typical application at the selected drive frequency.  
         [0032]     Line  92  of  FIG. 10  exemplifies test results of sensor inductance vs. gap  78  at a drive frequency of 300 Hz using a 0.062 inch thick aluminum conductive barrier  72 . Note that the test results indicate an available inductance bandwidth of approximately 800 microHenries for a change in gap  78  from 0.062 to 0.35 inches. Line  94  of  FIG. 11  exemplifies test results of sensor inductance vs. gap  78  at a drive frequency of 100 Hz using a 0.125 inch thick aluminum conductive barrier  72 . Note that the test results indicate an available inductance bandwidth of approximately 630 microHenries for a change in gap  78  from 0.125 to 0.475 inches. Line  96  of  FIG. 12  exemplifies test results of sensor inductance vs. gap  78  at a drive frequency of 150 Hz using a 0.063 inch thick copper conductive barrier  72 . Note that the test results indicate an available inductance bandwidth of approximately 700 microHenries for a change in gap  78  from 0.062 to 0.400 inches.  
         [0033]     The above described test results demonstrate that, for drive frequencies less than 350 Hz (depending on the barrier material and its thickness), eddy current effects caused by non-intrusive proximity sensing through a conductive barrier may be reduced to a level where reliable proximity sensing can be achieved. From the particular set of tests it is observed that a minimum operating bandwidth for the proximity sensing is approximately 400 microHenries of inductance. This minimum allows for reasonable system tolerances and built in test (BITE) information. The operating inductance bandwidth may be determined by driving the proximity sensor at a selected frequency based on the material and thickness of the conductive barrier and measuring the sensor inductance with the conductive barrier in place and under the two conditions of: (1) no target on the opposing side of the barrier (i.e. “far” position); and (2) a standard sized permeable or magnetic target placed in close proximity to or against the opposing side of the conductive barrier (i.e. the side opposite the side on which the sensor is mounted) which is indicative of a “near” position. The measured inductance value without the target present (far position) is subtracted from the measured inductance value with the standard target present (near position). Note that the operating inductance bandwidth of the sensor is associated with the selected drive frequency.  
         [0034]     While the present invention has been described herein above in connection with one or more embodiments, it is understood that the description is merely by way of example with no intent on limiting the present invention in any way to any single embodiment. Rather, the present invention should be construed in breadth and broad scope in accordance with the recitation of the claims appended hereto.