Abstract:
An electronic system that monitors a trailer hitch assembly having a plate hitch with a throat for receiving a kingpin of a trailer and a locking mechanism for locking the kingpin within the throat. The electronic system determines whether the trailer hitch assemblies is properly coupled to the trailer, and comprises a first magnet creating a first magnetic flux, and a first Hall-effect sensor for sensing the position of the kingpin of the trailer relative to the throat of the hitch plate by measuring the first magnetic flux. The system further comprises a control circuit operably coupled with the first Hall-effect sensor in determining whether the first magnetic flux indicates a proper location of the kingpin of the trailer relative to the throat of the hitch plate. The electronic system discriminates between the kingpin and foreign materials, thereby assuring proper locking of the locking mechanism.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/654,283, filed Feb. 18, 2005, entitled FIFTH WHEEL SENSOR ASSEMBLY, which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention is directed to an electronic system for monitoring the coupling of a trailer to a trailer hitch assembly that is mounted on a truck chassis, and in particular is directed to an electronic system that indicates whether the trailer is properly coupled to the trailer hitch assembly by discriminating between components of the trailer, components of the hitch assembly and foreign materials.  
         [0003]     Electronic coupling control systems for a vehicle trailer hitch assembly are described in each of U.S. Pat. No. 5,861,802, entitled “FIFTH WHEEL HITCH COUPLING CONTROL SYSTEM” to Hungerink et al. and U.S. Pat. Nos. 6,285,278 and 6,452,485, each entitled “ELECTRONIC SYSTEM FOR MONITORING A FIFTH WHEEL HITCH,” to Schutt et al. U.S. Pat. Nos. 5,861,802; 6,285,278 and 6,452,485 are each assigned to the assignee of the present invention and are hereby incorporated by reference in their entirety. Each of these patents disclose an electronic coupling control system that includes a trailer sensor for sensing when a trailer is located proximate the hitch assembly, a kingpin sensor for sensing the presence of a trailer kingpin in a hitch plate throat, and a lock sensor for sensing when the locking mechanism is locked in a secured position. These patents further disclose an indicator located within the vehicle for providing trailer hitch assembly coupling status information to a driver of a vehicle. A control circuit is coupled to the trailer sensor, the kingpin sensor, the lock sensor and the indicator. The sensors are utilized by the control circuit to inform a driver when a trailer is in close proximity to the trailer hitch assembly, when the trailer kingpin is positioned in the hitch throat and when the locking mechanism is in a locked position.  
         [0004]     The electronic coupling control system is also capable of performing various self-diagnostic routines to ensure proper operation of the system when the vehicle ignition is turned on.  
         [0005]     Heretofore, systems like those described above typically incorporate contact-type sensors susceptible to degradation from the stringent environment within which these sensors are utilized, including degradation from normal use, extreme use such as experienced during some coupling operations, and the inclusion of foreign solids within the environment, such as grease, water and ice each laden with ferromagnetic materials. These ferromagnetic materials as laden within the grease, etc., can be the cause of “false-positive” readings as conveyed to the operator, or readings that falsely indicate proper alignment of the kingpin with respect to the throat of the hitch plate. Improper alignment of the kingpin with the throat of the hitch plate may potentially result in dropping a trailer from the associated vehicle either at a shipping dock, or potentially on a public roadway, with significant damage to the associated equipment and surrounding property, and further personal injury or worse.  
         [0006]     An electronic system for monitoring the receiving of a kingpin of a trailer within a throat of a hitch plate is desired that accurately and reliably differentiates between the kingpin and foreign materials, thereby eliminating “false-positive” readings of proper alignment to the operator of an associated vehicle.  
       SUMMARY OF THE INVENTION  
       [0007]     One aspect of the present invention is to provide an electronic system for monitoring a trailer hitch assembly having a hitch plate and a throat for receiving a kingpin of a trailer and a locking mechanism for locking the kingpin within the throat, the electronic system determining whether the trailer hitch assembly is properly coupled to the trailer and comprising a first magnet creating a first magnetic flux, and a first Hall-effect sensor for sensing the position of the kingpin of the trailer relative to the throat of the hitch plate by measuring the first magnetic flux. The electronic system further comprises a control circuit operably coupled with the first Hall-effect sensor and determining whether the first magnetic flux indicates a proper location of the kingpin of the trailer relative to the throat of the hitch plate.  
         [0008]     Another aspect of the present invention is to provide a hitching system that comprises a trailer hitch assembly having a hitch plate with a throat for receiving a kingpin of a trailer and a locking mechanism for locking the kingpin in the throat. The hitching system also includes a first magnet creating a first magnetic flux, and a first Hall-effect sensor for sensing the position of the kingpin of the trailer relative to the throat of the hitch plate by measuring the first magnetic flux. The hitching system further includes a control circuit operably coupled with the first Hall-effect sensor and determining whether the first magnetic flux indicates a proper location of the kingpin of the trailer relative to the throat of the hitch plate.  
         [0009]     A further aspect of the present invention is to provide an electronic system for monitoring a trailer hitch assembly having a hitch plate with a throat for receiving a kingpin of a trailer and a locking mechanism for locking the kingpin in the throat, wherein the electronic system determines whether the trailer hitch assembly is properly coupled to the trailer. The electronic system comprises a non-contact proximity sensor for sensing the position of a kingpin of a trailer relative to a throat of a hitch plate, a control circuit operably coupled with the proximity sensor and determining whether a kingpin of the trailer is properly located relative to a throat of a hitch plate, and a display device operably coupled to the control circuit and displaying coupling status to an operator of a vehicle.  
         [0010]     Yet another aspect of the present invention is to provide a hitching system that comprises a trailer hitch assembly having a hitch plate with a throat for receiving a kingpin of a trailer and a locking mechanism for locking the kingpin within the throat, and a non-contact proximity sensor for sensing the position of a kingpin of a trailer relative to the throat of the hitch plate. The hitching system further comprises a control circuit operably coupled with the proximity sensor and determining whether a kingpin of a trailer is properly located within the throat of the hitch plate, and a display device operably coupled to the control circuit and displaying coupling status to an operator of a vehicle.  
         [0011]     The present inventive electronic system for monitoring the kingpin of a trailer within a throat of a hitch plate accurately and reliably differentiates between the kingpin and foreign materials, thereby eliminating “false-positive” readings of proper alignment to the operator of an associated vehicle, and increases the safety associated therewith. The system further reduces manufacturing costs, increases system reliability, is more durable, is capable of a long operating life and is particularly well adapted for the proposed use.  
         [0012]     These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a side view of a truck/tractor including an electronic system embodying the present invention for monitoring a trailer hitch assembly;  
         [0014]      FIG. 2  is a bottom view of the trailer hitch assembly;  
         [0015]      FIG. 3  is a side view of the trailer hitch assembly;  
         [0016]      FIG. 4  is a partial cross-section side view of a the trailer hitch assembly;  
         [0017]      FIG. 5  is a perspective view of an output device;  
         [0018]      FIG. 6A  is a schematic view of a first embodiment of the sensor assembly, wherein the sensor assembly is at a zero-state;  
         [0019]      FIG. 6B  is a schematic view of the first embodiment of the sensor assembly, wherein the sensor assembly indicates the location of a kingpin within the throat of an associated hitch plate;  
         [0020]      FIG. 6C  is a schematic view of the first embodiment of the sensor assembly, wherein a ferromagnetic material is positioned within the throat of the hitch plate;  
         [0021]      FIG. 7  is an electrical schematic of the first embodiment of the sensor assembly;  
         [0022]      FIG. 8  is a theoretical plot of sensor output versus flux intensity;  
         [0023]      FIG. 9A  is a schematic view of a second embodiment of the sensor assembly, wherein the second embodiment of the sensor assembly is at a zero-state;  
         [0024]      FIG. 9B  is a schematic view of the second embodiment of the sensor assembly, wherein the second embodiment of the sensor assembly indicates the location of the kingpin within the throat of the hitch plate;  
         [0025]      FIG. 10  is an electrical schematic of the electrical circuit of the second embodiment of the sensor assembly;  
         [0026]      FIG. 11A  is a schematic view of a third embodiment of the sensor assembly, wherein the third embodiment of the sensor assembly is at a zero-state;  
         [0027]      FIG. 11B  is a schematic view of the third embodiment of the sensor assembly, wherein the sensor assembly indicates the location of a component different than the kingpin in close proximity to the sensor assembly;  
         [0028]      FIG. 11C  is a schematic view of the third embodiment of the sensor assembly, wherein the sensor assembly indicates the location of a kingpin within the throat of the hitch plate;  
         [0029]      FIG. 12  is an electrical schematic of the electrical circuit of the third embodiment of the sensor assembly;  
         [0030]      FIG. 13A  a schematic view of a fourth embodiment of the sensor assembly, wherein a Hall-effect type sensor and a biased magnet are positioned across a portion of the throat of the hitch plate from one another;  
         [0031]      FIG. 13B  is a schematic view of the fourth embodiment of the sensor assembly, wherein the a kingpin is positioned within the throat of the hitch plate;  
         [0032]      FIG. 14  is an electrical schematic of the electrical circuit of the fourth embodiment of the sensor assembly;  
         [0033]      FIG. 15A  is a schematic view of a fifth embodiment of the sensor assembly, wherein the sensor assembly is at a zero-state;  
         [0034]      FIG. 15B  is a schematic view of the fifth embodiment of the sensor assembly, wherein the sensor assembly indicates the location of a component different than the kingpin;  
         [0035]      FIG. 15C  is a schematic view of the fifth embodiment of the sensor assembly, wherein the sensor assembly indicates the location of the kingpin within the throat of the hitch plate;  
         [0036]      FIG. 16  is an electrical schematic of the electrical circuit of the fifth embodiment of the sensor assembly; and  
         [0037]      FIG. 17  is a schematic view of an electrical circuit of a sixth embodiment of the sensor assembly that includes an inductive proximity sensor. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]     For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in  FIG. 1 . However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise.  
         [0039]     The reference numeral  10  ( FIGS. 1-3 ) generally designates an electronic monitoring and sensing system incorporated within a truck/tractor  12  which includes a trailer hitch assembly having a base  16  securely mounted to a chassis  18 , a trailer hitch plate  20  pivotally mounted on the base  16  on a transverse axis, and a locking mechanism  22  for locking a conventional trailer kingpin in place. The electronic system  10  preferably includes a non-contact kingpin sensor assembly  24  mounted to the hitch assembly  14 , a tilt sensor assembly  25 , a lock sensor  27 , and an output device  26  mounted in the cab of the tractor  12 . The tilt sensor assembly  25  and the lock sensor assembly  27  are described in U.S. Pat. Nos. 5,861,802; 6,285,278; and 6,452,485. The sensor assemblies  24 ,  25 ,  27  are coupled to the output device  26  by a multi-conductor cable  28 . In a preferred embodiment, the non-contact kingpin proximity sensor  24  includes an inductive-type sensor, however, other proximity sensors may be utilized, including Hall-effect type sensors, and the like, as discussed below.  
         [0040]     In the illustrated example, the sensor assembly  24  is mounted to the hitch plate  20  near the throat  30  formed in the hitch plate  20 , into which a trailer kingpin  32  is positioned and locked.  FIG. 4  provides an upside-down side view and partial cross section illustrating the location of the trailer kingpin  32  when properly disposed within the throat  30 . As constructed, the sensor assembly  24  outputs a detection signal when the kingpin  32  is disposed within the throat  30 . The calibration of the sensor assembly  24  prevents it from indicating that the kingpin  32  is present when a misaligned coupling occurs, which prevents the locking mechanism  22  from securing the kingpin  32  (i.e., the trailer) to the hitch plate assembly  14 , or further from providing “false-positives” or untrue readings of a proper coupling, as discussed below. The locking mechanism  22  of the hitch plate assembly  14  is biased by a compression spring to automatically lock-in and secure the trailer kingpin  32  as soon as the trailer kingpin  32  enters the hitch throat  30 . Those of ordinary skill in the art will appreciate that the present invention may be used in connection with any type of locking mechanism. It should further be noted that the present invention may be applied to tractor hitch assemblies having other constructions and is not limited to particular mounting locations as shown for the embodiments of the sensor assembly  24  described herein.  
         [0041]      FIG. 5  illustrates an exemplary output device  26 . A multiple conductor cable  28  couples the sensor assembly  24  to the output device  26 . The internal components (i.e., the control circuitry) of the output device  26  are further shown and described in U.S. Pat. No. 6,285,278, which is incorporated by reference herein in its entirety. The output device  26  includes a display panel  34  for providing coupling status information to the driver/operator of the tractor  12 . In a preferred embodiment, the display panel  34  includes an “unlocked” icon  36 , a “locked” icon  38 , a “fifth wheel” icon  40  and seven-segment display  42 . In the embodiment, the display  42  provides an error code indicating possible sources of a coupling malfunction, again as further described in U.S. Pat. No. 6,285,278. Preferably, a red light diode (LED) is provided behind the “unlocked” icon  36 . Further, a yellow, a red, and green LED are provided behind the “fifth wheel” icon  40  and a green LED is provided behind the “lock” icon  38 . One of ordinary skill in the art will appreciate that the individual LEDs could be replaced by an LED array capable of providing multiple colors. While output device  26  as shown only indicates visual indicators, one of ordinary skill in the art will readily appreciate that and audio output may be provided. For example, by adding a speaker and appropriate voice processing circuitry, the output device  26  may provide voice output to instruct a driver as to possible causes of a coupling malfunction. Additionally, a warning buzzer may be activated in addition to, or as an alternative, providing an unlocked icon  36 .  
         [0042]     In a first embodiment, the sensor assembly  24  ( FIG. 6A ) includes an analog Hall-type sensor  44  with an integrated circuit, a biasing magnet  46  having a magnetic axis  47  and producing a magnetic flux  48 , and a threshold adjustment and a switching circuit  50 . The Hall-effect sensor  44  is sensitive to magnetic flux in a direction perpendicular to the larger dimension thereof. As best illustrated in  FIG. 6A , the biasing magnet  46  provides a base or zero level flux  48  when the kingpin  32  is not properly located within the throat  30 . The strength of the bias magnet  46  and the dynamic range of the Hall device within the sensor  44  determine the effective range of the sensor  44 . As illustrated in  FIG. 6B , with the kingpin  32  moved in a direction as illustrated and represented by directional arrow  52  and positioned proximate the sensor assembly  24 , the flux  48  of the magnet  46  as read by the Hall sensor  44  is greater in strength due to the proximity of the ferromagnetic material comprising the kingpin  32 . A positive signal is then generated indicating proper location of the kingpin  32  within the throat  30  of the hitch plate  20 . As illustrated in  FIG. 6C , a foreign material, such as grease, water, ice, and the like containing shavings or particles of a ferromagnetic material, commonly referred to as swarf, does not provide an adequate amount of flux  48 , per proper calibration of the adjustment and switching circuit  50 , in order to indicate a positive and proper location of the kingpin  32  within the throat  30 .  
         [0043]     A schematic view of the sensor assembly  24  is illustrated in  FIG. 7  and includes a power supply  54  and a ground  56  each coupled to the Hall sensor  44 , and the switching circuit  50 . The switching circuit  50  includes a comparator circuit  58 , a hysteresis feedback loop  60 , an analog or digital potentiometer  62  for adjusting the threshold sensitivity of the sensor assembly  24 , and a signal conditioner  64  for conditioning the output signal  66  for the desired switching. A theoretical plot of the sensor output for optimizing the switching is illustrated in  FIG. 8 , wherein output is plotted versus the flux intensity.  
         [0044]     The reference numeral  24   a  ( FIG. 9A ) generally designates another embodiment of the present invention, having a first Hall effect sensor  68 , a second Hall sensor  70 , a third Hall sensor  72 , a first bias magnet  74  having a magnetic axis  73  and creating a first magnetic flux  75 , a second bias magnet  76  having a magnetic axis  79  and creating a second magnetic flux  77 , and a switching circuit  78 . Each Hall sensor  68 ,  70 ,  72  is sensitive to the magnetic flux in a direction perpendicular to the larger dimension thereof. In the illustrated example, the first Hall sensor  68  is sensitive to the flux flowing between the magnets  74 ,  76  and along the magnetic axis  73 ,  79  thereof, while the second Hall sensor  70  and the third Hall sensor  72  are sensitive to magnetic fields that are perpendicular to the magnetic axis  73 ,  79  of the magnets  74 ,  76 . As illustrated in  FIG. 9B , the flux  75 ,  77  created by the magnets  74 ,  76  is drawn off through the second Hall sensor  70  and third Hall sensor  72  by a proper positioning of kingpin  32  within the throat  30 .  
         [0045]     A schematic of the sensor assembly  24   a  is shown in  FIG. 10 , wherein the sensor assembly  24   a  includes a power supply  80  operably coupled to each of the Hall sensors  68 ,  70 ,  72 , and ground lines  82  for the same, and the switching circuit  78 . In the illustrated example, the switching circuit  78  includes a threshold adjustment circuit  84 , a first to second Hall sensor comparator  86 , a first to third Hall sensor comparator  88 , a window and comparator  90  to compare the differences between the outputs of the comparators  86 ,  88 , and an output signal conditioner  92  providing an output signal  94 .  
         [0046]     The reference numeral  24   b  ( FIG. 11A ) represents another embodiment of the sensor assembly. In the illustrated example, the sensor assembly  24   b  includes a first Hall sensor  96 , a second Hall sensor  98  oriented perpendicularly to the first Hall sensor  96 , a bias magnet  100  having a magnetic axis  101  and creating a magnetic flux  103 , and a thresholding and switching circuit  102 . As illustrated, the Hall sensors  96 ,  98  are oriented such that the first Hall sensor  96  is sensitive to flux along the magnetic axis  101 , while the second Hall sensor  98  is sensitive to flux perpendicular to the magnet axis  101 , thereby making the sensor assembly  24   b  more sensitive to monitoring objects located along the magnetic axis  101  of the magnet, as well as perpendicular thereto. For example, as illustrated in  FIG. 11B , the sensor assembly  24   b  may be calibrated to take into account magnetic flux as caused by components of the hitch assembly  14 , such as hitch plate  20 . In this arrangement, the second Hall sensor  98  is sensitive to flux  104  flowing in a perpendicular direction to the magnetic axis  101 , which is used to precisely adjust the sensitivity of the overall sensor assembly  24   b . As illustrated in  FIG. 11C , proper placement of the kingpin  32  within the throat  30  causes an increase in the flux  106  flowing through the first Hall sensor  96 , which is compared to the steady state reading developed from the orientation as illustrated in  FIG. 11B .  
         [0047]      FIG. 12  illustrates the circuitry of the sensor assembly  24   b  and includes a power supply  105  and a ground  106  to each of the first Hall sensor  96  and the second Hall sensor  98 , and the switching circuit  102 . The switching circuit  102  includes a comparator  108  for comparing the outputs of the first Hall sensor  96  and the second Hall sensor  98 , a feedback loop  110 , and an output signal conditioner  112  producing an output signal  114 .  
         [0048]     The reference numeral  24   c  ( FIG. 13A ) represents yet another alternative embodiment of the sensor assembly that includes a Hall switch  116  and a bias magnet  118  having a magnetic axis  121  and creating a magnetic flux  122 . In the illustrated example, the Hall switch  116  is mounted on one side of a horseshoe-shaped member  120  preferably constructed of a soft iron or other highly permeable material, thereby attracting magnetic flux on account of low magnetic resistance thereof. The magnet  118  is mounted to an opposite side of the member  120  and is positioned so as to direct the flux  122  created thereby in the direction of the Hall switch  116 . Due to the spacing across the ends of the member  120 , a relative small amount of the flux  122  is encountered by the Hall switch  116  when the kingpin  32  is not present within the throat  30 . As illustrated in  FIG. 13B , the flux  122  is increased with the presence of the kingpin  32  within the throat  30 . It should be noted that in the illustrated example of the sensor assembly  24 C the build up or addition of swarf material  52  within the throat  30  and about the kingpin  32  actually assists in the trip of the Hall switch  116  by filling any air gaps located between the kingpin  32  and the side edges of the throat  30 . It should further be noted that the arrangement of the sensor assembly  24   c  has the advantage of not requiring an analog output to properly function.  
         [0049]      FIG. 14  is a schematic view of the sensor assembly  24   c  that includes the Hall switch  116  having a power supply  124  and a ground  126 , and a switching circuit  128 . The switching circuit  128  includes a comparator  130 , a feedback loop  132 , a threshold adjustment circuit  134  and an output signal conditioner  136  providing an output signal  138 .  
         [0050]     The reference numeral  24   d  ( FIG. 15A ) represents another embodiment of the sensor assembly. The sensor assembly  24   d  includes a first Hall-effect sensor  140 , a bias magnet  142  having a magnetic axis  143  creating a flux  146 , and a switching circuit  144 . In the illustrated example, the first Hall sensor  140  is oriented perpendicularly to and is offset from the axis of the magnet  142 . As best illustrated in  FIG. 15A , the flux  146  flows generally along the axis of the magnet  142 . As illustrated in  FIG. 15B , the flux  146  is pulled perpendicularly to the axis of the magnet  142  towards a component of the trailer hitch assembly  14 , such as the hitch plate  20 , thereby allowing for adjustment and fine tuning of the overall sensor assembly  24   d  and allowing the adjustments thereof to take into account the presence of components other than the kingpin  32 . The sensor assembly  24   d  further includes a pair of shields each containing an amount of ferromagnetic material that aids in directing the flux  146  about the sides of the sensor  24   d , thereby creating or acting as a shield to any other external medal components or swarf in the monitored area. As best illustrated in  FIG. 13C , a larger amount of the flux  146  is directed along the axis of the magnet  142  and toward the kingpin  32  away from the Hall sensor  140  when the kingpin  32  is properly located within the throat  30 . As illustrated, the sensor assembly  24   d  allows the setting of a trip point to incorporate a threshold to ignore detrimental amounts of swarf causing a “false-positive” reading of the sensor assembly  24   d.    
         [0051]      FIG. 16  is a schematic view of the sensor assembly  24   d  including a power supply  150  and a ground  152  to the Hall sensor  140 , and the switching circuit  144 . The switching circuit  144  includes a comparator  154 , a feedback loop  156 , a threshold circuit  158  and an output signal conditioner  160  providing an output signal  162 .  
         [0052]     The electronic monitoring system  10  preferably includes an induction proximity sensor  164  ( FIG. 17 ) in place of the Hall-effect sensor arrangements as disclosed herein. As is known in the art, inductive proximity sensors function by sensing a change to the properties of a related inductor. The properties of the inductor will change if a ferrous or conductive material is placed within a space-sensing region within the inductors magnetic field that may extend outwardly of the inductor. Typically, inductor sensors utilize an oscillating (AC) signal within the inductor to sense inductor property changes, with the frequency of the oscillations changing the importance of the various properties of the inductor. As illustrated, the inductor sensor of the induction proximity sensor  164  senses the amount of loss on the inductor when a ferromagnetic material is in close proximity thereto. In the present example, the operating frequency of the inductive proximity sensor is preferably less than 50 kHz, 20 kHz nominal, thereby reducing false-positives as caused by ferromagnetic material laden swarf. A specific advantage of the induction proximity sensor  164  is the elimination of a bias magnet as required with Hall-effect and reed switch sensors.  
         [0053]      FIG. 17  is a schematic view of the inductive proximity sensor  164  that comprises a tank circuit  166  including a sense coil  168  and a series combination of a pair of capacitors  170 . The oscillation frequency of the tank circuit  166  is preferably less than or equal to 50 kHz (20 kHz nominal). The tank circuit  166  forms part of a Colpitts oscillator circuit  172  that includes a capacitor  174 , resistors,  176 ,  178 ,  180 ,  182 ,  184 ,  186 , a capacitor  188 , and a transistor  190 . The transistor  190  provides feedback energy needed to maintain the oscillation of the tank circuit  166 . The resistors  176 ,  178 ,  180  and the series combination of the resistors  182 ,  184 ,  186  set the DC operating point of the transistor  190 . The capacitor  188  is utilized to AC-bypass the resistor  186  to increase the AC gain of the transistor  190  independent of the DC bias. The components of the oscillator circuit minimize the amount of feedback required to maintain the oscillation. In the present example, losses induced by a conductive object proximate the sense coil  168  causes the oscillator to decrease in amplitude. Preferably, the resistor  184  is utilized to compensate for temperature-dependent circuit losses, and mainly the resistance of the copper winding utilized within the sense coil  168 . A transistor  192  and a resistor  194  provide a buffered output of the Colpitts oscillator  172 . A capacitor  196  blocks the DC level present at the emitter of the transistor  192 . A first diode  198  provides a new ground-based DC level, and a second diode  200  half-wave rectifies the AC output of the oscillator. A capacitor  202  and a resistor  204  filter the rectified AC to obtain a DC level proportional to the amplitude of the oscillator, that is compared to a DC level derived from a resistor  210  and a resistor  212 . A comparator  214  and a resistor  216  perform the comparison function and generate an output signal  218  for the inductive proximity sensor  164 .  
         [0054]     In the foregoing description, it will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims by their language expressly state otherwise.