Patent Publication Number: US-2022236081-A1

Title: Systems and methods for through wall locating

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 17/156,096, filed Jan. 22, 2021, the contents of which are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     Systems and methods are described for precisely locating a position through a wall. 
     BACKGROUND 
     Serious accidents may occur when construction or repair workers cutting or drilling through walls, including metal walls, fail to properly locate a target cutting location. Such workers may penetrate into hazardous areas such as fuel tanks, fuel lines, or electrical lines located on an opposite side of a wall they are cutting or drilling. 
     Such accidents are a common source of fires, for instance, in the ship repair and building industry where cutting torches applied from one side of a metal partition may inadvertently cut into hazardous areas on the opposite unseen side of the partition (such as a marine bulkhead). Systems and methods for precisely locating a safe place to cut so that damage is avoided on the opposite side of a partition have not previously been adequately developed due to problems with existing locating technologies. 
     Currently, workers often attempt to make precise measurements on both sides of the construction wall so that corresponding points on opposite sides of the wall are accurately located independently. Another common approach is to “tap” or use other acoustic energy (i.e., tones) and to have workers attempt to locate the source of the energy on the opposite side of the wall. These attempts are typically imprecise and unreliable. 
     Such problems are exacerbated when dealing with metal partitions or walls, and include difficulty with locating sounds or other types of energy through thick metal, such as structural plate steel. For instance, plate steel used in marine bulkheads dissipates acoustic energy from sharp taps or other sources so that precise location of corresponding points on opposite sides of walls are difficult. 
     In addition to the difficulty in locating hazards on opposite sides of walls, similar challenges exist in defining highly precise fiducial points from which to make measurements. Such marking is often impractical in many construction and repair settings. 
     Other existing approaches are to use permanent magnets to mark a location on one side of a wall, such that a magnetometer placed on the opposite side of the wall can detect the magnet&#39;s location. However, the high magnetic permeability of ferrous substances, such as structural steel, also tends to absorb, or shunt, magnetic fields, thereby attenuating and diffusing magnetic fields. This makes precise location difficult, even when using strong permanent magnets. 
     There is a need for systems and methods for the precise location of hazards on one side of a wall to be clearly revealed on the other side of that wall, so that workers can safely penetrate the wall surface. There is a further need for systems and methods for precisely locating fiducial points that can be used as references. 
     SUMMARY 
     The current disclosure provides systems and methods for overcoming limitations of previous approaches by employing a transmit/receiver pair that radiate and receive a series of strong, time varying electromagnetic pulses that can be used to precisely locate corresponding points on opposite sides of a wall. 
     Electromagnetic pulses produced on one side of a wall, emanating from a transmit coil create a sharp magnetic field gradient, resulting in a steeply sloping magnetic field, on the opposite side of the wall. A receiver with a sensitive pick-up coil can then discriminate small differences in magnetic field strength in order to locate the spot of peak magnetic strength indicating the position nearest the transmit coil on the opposite side of a wall. 
     The use of time varying magnetic pulses allows for optimizing a frequency of oscillations to maximize electromagnetic transmission through a metal wall. The pulses may be shaped and adjusted to further make those pulses easily audible to the human ear, once amplified. Such an approach allows for precise localization on opposite sides of a wall. Some embodiments may thereby produce localizations within an accuracy of 1-3 mm. 
     In some embodiments, the transmitter and receiver each include solenoid coils, and when a location is properly located, both solenoids are aligned with each other axially. In such an embodiment, slight variations in the position of the receiver coil with respect to the transmitter coil would produce large variations in signal strength, thereby achieving better accuracy. 
     In some embodiments, a transmitter unit is provided for transmitting a continuous waveform in a magnetic field, the continuous waveform being an unmodulated electromagnetic waveform. The transmitter unit typically comprises a transmitting solenoid for transmitting the continuous waveform, and an oscillator for generating the continuous waveform. 
     A receiver unit is also provided, the receiver unit having receiving circuitry, such as a receiving solenoid, for receiving the continuous waveform. The receiver unit also provides an output for outputting an indication of proximity between the transmitter unit and the receiver unit, where a characteristic of the indication of proximity depends on the strength of the magnetic field generated by the transmitter unit at the location of the receiving solenoid. 
     The receiver unit may further comprise an amplifier for amplifying the continuous waveform received at the receiving solenoid in proportion with an intensity of the waveform received at the receiving solenoid. 
     Typically, the transmitting and receiving solenoids are oriented perpendicular to a wall through which a location is being located during use. The receiving solenoid may be selected to be in resonance with the transmitting solenoid at a frequency of the continuous waveform. 
     In some embodiments, both the transmitting and receiving solenoids are selected, along with corresponding capacitors, to be in resonance at the frequency of the continuous waveform, thereby increasing the strength of the magnetic field at the location of the receiving solenoid by way of resonant inductive coupling when the solenoids are substantially axially aligned. 
     In some embodiments, the continuous waveform is an ultra-low frequency or lower frequency waveform. In some embodiments, the waveform may be below 10 Hz, or at approximately 5 Hz. In some embodiments, the waveform is a square waveform. 
     In some embodiments, the output is an audio amplifier for amplifying the continuous waveform proportionally to the magnetic field strength at the receiving circuitry, such as the receiving solenoid, and outputting the amplified continuous waveform as audio. The characteristic of the indication of the proximity that at least partially depends on magnetic field strength may then be volume, frequency, sharpness, or pitch of the audio output. In some such embodiments, the continuous waveform may be a square waveform, and the volume and sharpness of the audio output may then vary based on location of the receiving circuitry. 
     In some embodiments, the transmitter unit may further comprise a permanent magnet for fixing the transmitter unit to a wall through which the system is being used to locate. 
     Also provided is a method for through wall locating. Such a method may comprise locating a transmitter unit adjacent a desired location on a first side of a wall, and transmitting a continuous electromagnetic waveform from the transmitter unit. 
     A user then locates a receiver unit adjacent the wall opposite the transmitter unit, where the receiver unit includes receiving circuitry and an output for outputting an indication of proximity. 
     The receiving circuitry then receives the continuous electromagnetic waveform output by the transmitter unit and generates an indication of proximity based at least partially on the strength of the magnetic field generated by the transmitter unit at the receiving circuitry. 
     The receiver unit continuously outputs the indication of proximity at the output while a user moves the receiver unit along the wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an embodiment of a system in accordance with this disclosure. 
         FIG. 2  shows flux coupling between adjacent solenoid coils with flux being transferred from an active solenoid to a passive solenoid. 
         FIG. 3A  shows the coupling of transmitting and receiving solenoid coils in the context of the system. 
         FIG. 3B  shows the coupling of  FIG. 3A  with an intervening wall attenuating flux through the wall. 
         FIG. 4A  shows the resulting magnetic flux in solenoid coils having different diameters. 
         FIG. 4B  shows the resulting magnetic flux in coupled solenoid coils when the coils are axially aligned. 
         FIG. 4C  shows the resulting magnetic flux in coupled solenoid coils when those coils are spaced apart. 
         FIG. 5A  shows the response to a square pulse at a receiving coil when transmission and receiving coils are misaligned. 
         FIG. 5B  shows the response to a square pulse at a receiving coil when transmission and receiving coils are aligned. 
         FIG. 6  is a flowchart illustrating a method in accordance with this disclosure. 
         FIG. 7  shows a second embodiment of a system in accordance with this disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The description of illustrative embodiments according to principles of the present invention is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description of embodiments of the invention disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation unless explicitly indicated as such. Terms such as “attached,” “affixed,” “connected,” “coupled,” “interconnected,” and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. Moreover, the features and benefits of the invention are illustrated by reference to the exemplified embodiments. Accordingly, the invention expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features; the scope of the invention being defined by the claims appended hereto. 
     This disclosure describes the best mode or modes of practicing the invention as presently contemplated. This description is not intended to be understood in a limiting sense, but provides an example of the invention presented solely for illustrative purposes by reference to the accompanying drawings to advise one of ordinary skill in the art of the advantages and construction of the invention. In the various views of the drawings, like reference characters designate like or similar parts. 
       FIG. 1  shows an embodiment of a system  100  in accordance with this disclosure. As shown, the system  100  includes a transmitter unit  110  and a receiver unit  120 . 
     The transmitter unit  110  typically transmits a continuous waveform as a magnetic field. The waveform may be transmitted by a transmitting solenoid  130  contained within the transmitter unit  110 . The transmitter unit  110  may further comprise a harmonic oscillator  140  for generating the continuous waveform to be transmitted and providing it to the transmitting solenoid  130  for transmission. 
     The receiver unit  120  may then comprise receiving circuitry, typically including a receiving solenoid  150 , which receives the continuous waveform transmitted by the transmitter unit  110 . The receiving solenoid  150  receives the continuous waveform through induction and generates a voltage from the magnetic field radiated by the transmitting solenoid  130 . 
     The receiver unit  120  may further comprise an output  160  for outputting an indication of a proximity between the transmitter unit  110  and the receiver unit  120 . The indication of proximity may include a characteristic depending on the strength of the magnetic field generated by the transmitter unit  110  at the location of the receiving solenoid  150 . 
     The transmitter unit  110  may include a current source  170  or amplifier paired with the oscillator  140  in order to drive the transmitting solenoid  130  and output the continuous waveform. The current source  170  or amplifier would then amplify voltage and current supplied to the transmitting solenoid  130 , such that the solenoid radiates an alternating current (AC) magnetic field. The continuous waveform is typically a periodic unmodulated electromagnetic waveform, such that the continuous waveform does not act as a carrier signal for carrying data. The waveform may be a square wave, and may be an ultra-low frequency or lower frequency waveform as discussed in more detail below. For example, the waveform may be a square waveform having a frequency of 5 Hz. 
     The transmitter unit  110  may further include a permanent magnet  180 , which may be used to locate the transmitter unit adjacent a desired location on a first side of a wall  190 . 
     The receiver unit  120  may include an amplifier  200 , which may be a pre-amp, for amplifying the continuous waveform received from the transmitting solenoid  130  by way of the receiving solenoid  150 . The amplifier  200  then amplifies the continuous waveform in proportion with an intensity of the waveform received at the receiving solenoid  150 . The intensity of the waveform is a function of the strength of the magnetic field generated by the transmitter unit  110  at the location of the receiving solenoid  150 . 
     As shown in  FIG. 1 , when in use, the transmitter unit  110  is located adjacent a desired location on a wall  190 , where it may be fixed using a permanent magnet  180 . For example, the wall may be structural steel or plate steel, or some other ferrous material, such that the permanent magnet may be used to fix the transmitter unit  110  for use in measurement. Typically, the desired location is either a specific location which the user would like to use as a fiducial point, such that an identical point opposite the wall should be identified, or the desired location is simply a location that is safe to cut or drill from the opposite side. 
     The receiver unit  120  may then be brought close to the wall  190  from the opposite side while the transmitter unit  110  is fixed by way of the permanent magnet  180 , such that the continuous waveform output by the transmitting solenoid  130  of the transmitter unit is received at the receiving solenoid  150  of the receiver unit  120 . 
     As discussed in more detail below, when the transmitter unit  110  and the receiver unit  120  are brought closer to each other through the wall  190  and are brought more closely into alignment, the magnetic field generated by the transmitter unit  110  at the location of the receiving circuitry  150  is increased. This is then reflected in the indication of proximity as a characteristic of that indication. 
     The amplifier  200  receives the continuous waveform from the receiving solenoid  150  as a voltage generated from the magnetic field and amplifies it in proportion with an intensity of the waveform received at the receiving solenoid. In some embodiments, the output  160  includes an audio amplifier which further amplifies the continuous waveform proportionally to the magnetic field strength at the receiving circuitry  150  and outputs the amplified continuous waveform as audio by way of speakers or earphones, where the characteristic of the indication of the proximity is volume, frequency, sharpness, or pitch of the audio output. 
     Accordingly, in the example of the square waveform above having a nominal frequency of 5 Hz, the square waveform would be received at the receiving solenoid  150 , amplified by the amplifier  200  and then fed into an audio amplifier  160 . The square waveform would then be output as audio proportional to the magnetic field strength associated with the square waveform at the receiving solenoid  150 . As such, when output through a speaker or headphones, the user would hear a 5 Hz beating sound, similar to the thump of helicopter rotors. 
     Because the magnetic field strength would change when the receiver unit  120  is moved, the changing field strength would increase and decrease in intensity as the user moves the receiver unit  120  closer to or farther away from the point at which the transmitter unit  110  is located on the opposite side of the wall  190 . Accordingly, when the user moves the receiver unit  120  closer to the transmitter unit  110 , the volume of the audio at the output  160  increases. The periodic nature of the waveform translated to audio makes it much easier for the human ear to detect than a static, field intensity related signal, such as that which would be generated by a permanent magnet. 
     When in use, the transmitting solenoid  130  and the receiving solenoid  150  are both typically oriented perpendicular to the wall  190  through which the location is being located. This allows for the receiving solenoid  150  to resonate effectively with the transmitting solenoid  130 . The transmitting solenoid  130  is typically located within the transmitter unit  120  such that it comes as close as possible to abutting the wall  190 . Similarly, the receiver unit  120  is typically packaged such that when in use, the receiving solenoid  150  may be placed as close as possible to abutting the wall  190  from the opposite side. 
     In the embodiment, shown, the transmitting solenoid  130  and the receiving solenoid  150  may be selected so as to enhance the transmission of the continuous waveform when the solenoids are close to each other and in alignment and to increase the sharpness of their drop off when taken out of alignment. Accordingly, the transmitting solenoid  130  and receiving solenoid  150  are each paired with resonant capacitors  210 ,  220  to form LC circuits. In order to bring the coil into resonance at the frequency of the desired continuous waveform, the values for the solenoid transmitter  130  inductance L and the capacitance of the corresponding capacitor  210  are selected to approximate the results of the resonance formula: 
     
       
         
           
             
               
                 f 
                 r 
               
               = 
               
                 1 
                 
                   2 
                   ⁢ 
                   π 
                   ⁢ 
                   
                     
                       L 
                       ⁢ 
                       C 
                     
                   
                 
               
             
             ⁢ 
             Hz 
           
         
       
     
     Where f r  is the frequency at which resonance is sought, in this case 5 Hz. The receiving solenoid  150  and capacitor  220  are similarly selected to be in resonance at the desired frequency. By selecting both LC circuits to be in resonance, during use, the strength of the magnetic field generated by the transmitter unit at the location of the receiving circuitry may then be increased by resonance inductive coupling when the transmitting solenoid  130  and the receiving solenoid  150  are substantially axially aligned. 
     The transmitter solenoid  130  typically has a solid core which may be formed from a high permeability mu metal. In such embodiments, the metal core may have mu&gt;50,000 at 5 HZ, thereby increasing its inductance. 
       FIG. 2  shows flux coupling between adjacent solenoid coils  230 ,  250  with flux being transferred from an active solenoid to a passive solenoid. As shown, even where the transmitter solenoid  230  is active and the receiving solenoid  250  is a passive metal core solenoid, the magnetic fields of the solenoids will couple. In the example shown, the flux density is shown using grayscale, and an increase in flux density in the metal core of the receiving solenoid  150  is generated by the magnetic field of the active transmitter solenoid  130  so long as the solenoids are in or near alignment. This increased flux, visible even in a passive receiving solenoid  150 , relative to the surrounding medium (having a permeability of 1 in the image shown), supports transfer of the continuous waveform from the transmitter solenoid  130  to the receiving solenoid  150 . 
       FIG. 3A  shows the coupling of transmitting  130  and receiving  150  solenoid coils in the context of the system  100 .  FIG. 3B  shows the coupling of  FIG. 3A  with an intervening wall  190  attenuating flux through the wall. As shown, the flux coupling effect, which was visible in the context of the passive receiving solenoid of  FIG. 2 , is more dramatic in the context of an active receiving solenoid  150 . 
     This flux coupling by way of the magnetic field generated by the transmitter unit  110  allows for the transmission of the continuous waveform as discussed above. As shown in  FIG. 3A , when coupled without any intervening wall, the transmitting solenoid  130  and the receiving solenoid  150  fully couple to form a single magnetic field. 
     As shown in  FIG. 3B , when a ferrous metal wall  190  is present, that wall acts as a magnetic shield, concentrating magnet lines of flux within it. This effect reduces the magnetic field detectable on the opposite side of the wall  190 . In the configuration discussed above in reference to  FIG. 1 , the transmitting solenoid  130  acts like a primary winding of a transformer while the receiving solenoid  150 , in the receiver unit  120 , acts as the secondary winding of the hypothetical transformer. 
     Because the ferrous metal wall  190  blocks or effectively shunts a significant portion of the magnetic field generated in the transmitting solenoid  130 , only a small portion of the original transmitted magnetic field is available to generate a voltage in the receiving solenoid  150 . The reduction in field strength, as well as resulting signal strength, due to the ferrous metal wall  190 , makes it difficult for a receiver unit  120  to detect magnetic fields produced by the transmitter unit  110 , unless the transmitting  130  and receiving  150  solenoids are axially aligned in order to support flux coupling. This results in a dramatic drop off in the strength of the magnetic field when the solenoids  130 ,  150  are taken out of alignment. 
     The use of flux coupling to increase the effective magnetic field at the receiver unit  120  results in a signal that is strongest when the transmitter unit  110  and receiver unit  120  are well aligned, and a rapid fall off of signal strength as the transmitting  130  and receiving  150  coils are moved out of alignment. This allows for precise localization of the receiver unit  120 , as small displacements from perfect or near perfect alignment will produce very noticeable and dramatic differences in signal strength. In the system  100  of  FIG. 1 , this results in a quick drop off of the intensity of acoustic signals delivered to the ears of a human operator. 
       FIG. 4A  shows the resulting magnetic flux in solenoid coils  400 ,  410  having different diameters. As shown, a magnetic field generated by a transmitting coil drops off as a distance from the coil increases. However, that drop off is faster when observed in a cross section perpendicular to an axis of the solenoid than in a cross section parallel to that axis. Accordingly, the gradient shown is much more spread out when viewed axially than when viewed perpendicularly. This results in a relatively sharp magnetic field extending from the lateral ends of the solenoids. The sharpened nature of this field means that the magnetic field will drop off quickly when a receiver unit is not properly aligned with a transmitter unit. 
     Further, as shown in  FIG. 4A , the axially extending magnetic field is sharper when the solenoid  410  has a smaller diameter. This increased fall off of field strength results in increased sensitivity as the diameter of the solenoid decreases. As such, the solenoids used in the transmitter and receiver discussed above may be provided with small diameters. 
       FIG. 4B  shows the resulting magnetic flux in coupled solenoid coils  400 ,  410  when the coils are axially aligned. As shown, the increased axial length of the solenoid results in a further sharpened magnetic field at the ends of the coupled solenoids. 
       FIG. 4C  shows the resulting magnetic flux in coupled solenoid coils  420 ,  430  when those coils are spaced apart. As shown, the increased flux in the receiving coil  430  that results from coupling is retained even when the coils are spaced apart. However, when the coils are moved out of alignment, the magnetic field at the receiving coil  430  is substantially reduced. Further, as shown in  FIG. 4C , field strength is greater between the two solenoids  420 ,  430 , due to the flux coupling discussed above, than at the lateral axial ends of the magnetic field. This field strength is at a maximum, regardless of the space between the solenoids  420 ,  430 , when the solenoids are aligned. 
     As discussed above, in the example of a square wave output at 5 Hz, a user will hear a beating sound that increases and decreases in intensity when the user moves the receiver unit  120  closer to and farther away from the location of the transmitter unit  120  on the opposite side of the wall. When the peak sound is perceived, the user knows that the transmitting  110  and receiver unit  120  are precisely aligned, because inductive coupling of transmitting  130  and receiving solenoids  150  is at a maximum when the solenoids are aligned. 
       FIG. 5A  shows the response to a square pulse at a receiving coil when transmission and receiving coils are misaligned by approximately two inches.  FIG. 5B  shows the response to a square pulse at a receiving coil when transmission and receiving coils are aligned. As shown,  FIG. 5B , which shows the oscilloscope readout at a receiving coil, shows a response having a higher amplitude than the response shown for the misaligned configuration of  FIG. 5A . Further, the slopes of the waveform received in  FIG. 5B  are steeper than for the misaligned configuration of  FIG. 5A . 
     Accordingly, as shown, as the receiving coil is brought into alignment with the transmitting coil, the volume output at the audio amplifier  160  in the context of the system  100  discussed above is increased significantly. Further, in the case of the square pulse shown, the sound becomes sharper as the coils come into alignment. As such, when the solenoids are aligned, users hear loud, sharp, clicking noises. In contrast, when the solenoids are off axis, the pulse is softer, due to the shallower slope, and the click is also quieter. 
     It is further noted that magnetic field strength typically falls off at a rate of approximately the cube of the distance from the magnetic source. Therefore, small misalignments will, as shown, result in dramatic fall offs in magnetic field strength, which are then reflected in the output discussed. 
     Square waves provide a broad spectrum stimulus containing energy at the fundamental and all odd harmonics. This makes it effective for allowing the human ear, which is sensitive to spectral content, to discriminate different positions. Accordingly, clicks having broadband content in the form of a square wave convey more precise location information to the brain than do pure tones. 
     As such, by providing the continuous waveform in the system  100  in the form of a square pulse, the user can listen to the sound change both in terms of intensity and sharpness. 
     While the embodiment of  FIG. 1  is discussed in the context of a 5 Hz square continuous waveform, additional waveforms are contemplated as well. It is noted the ideal frequency may vary across different materials. Accordingly, 5 Hz may be ideal for most hardened steel materials used in solid metal walls, such as in marine bulkheads. However, ferrous metals may exhibit different efficiencies as transformer cores at different frequencies, which is why allows chosen for transformer cores are selected to have peak response and minimum losses at specific frequencies. 
     Accordingly, in some embodiments, the frequency of the continuous waveform may be modified or selected on the basis of the particular material through which a point is being located. In such embodiments, the resonating capacitors  210 ,  220  paired with the solenoids  130 ,  150  may be adjustable, such that when the frequency is modified, the capacitance of the capacitors are similarly modified so as to maintain the LC circuit of the solenoid and capacitors in resonance at the newly selected frequency. 
     While the frequency of the continuous waveform may be modified to increase propagation through particular materials, it is further noted that low frequencies, generally in the ultra-low frequency or lower range, and typically below 10 Hz, are more easily heard and discernable as individual sounds by users. Accordingly, the 5 Hz waveform discussed would be heard by a user as a beating sound, while a higher frequency waveform may not be heard at all, or may be heard only as a hum. The lower frequency sound may make it easier to discern changes in characteristics of the sound, such as volume, frequency, sharpness, or pitch, thereby making it easier for a user to determine if they are approaching a local maximum during use of the system  100  described. 
     Further, while a square waveform has advantages, as discussed above, different waveform shapes are contemplated as well. For example, the waveform may be sawtooth, sinusoidal, or more complex. 
       FIG. 6  is a flowchart illustrating a method for using the system  100  discussed above in accordance with this disclosure. The method may be used for identifying a particular location on a second side of a metal wall  190  corresponding to a desired location on a first side of the wall. The wall  190  may be, for example, a ferrous metal wall. This may be to identify a particular fiducial point for use as a reference, or it may be to identify a location on the wall that is safe for drilling or cutting. 
     A user of the system  100  initially locates ( 600 ) the transmitter unit  110  at a desired location on the first side of the wall  190 . This may be by fixing the transmitted unit  110  to the wall by way of a permanent magnet  180  integrated into a housing of the transmitter unit  110 . 
     The user then transmits ( 610 ) a continuous electromagnetic waveform from the transmitter unit  110 . As discussed above, the continuous electromagnetic waveform may be a square wave transmitted at approximately 5 Hz, and may be transmitted by way of a transmitting solenoid  130  located directly adjacent the wall  190  when the transmitter unit  110  is fixed thereto. 
     While the continuous electromagnetic waveform is being transmitted by the transmitter unit  110 , thereby generating a magnetic field, the user locates ( 620 ) the receiver unit  120  adjacent the wall  190  opposite the transmitter unit  110 . This initial locating is typically a guess made by the user as to the approximate location of the transmitter unit  110  opposite the wall  190 . The receiver unit  120  typically comprises receiving circuitry, such as a receiving solenoid  150 , and an output  160 , such as an audio amplifier and speaker or earphones for outputting some indication of proximity. 
     The user then receives ( 630 ) at the receiving solenoid  150  of the receiver unit  120 , the continuous electromagnetic waveform output by the transmitter unit  110 , and generates ( 640 ) some indication of proximity based at least partially on the strength of the magnetic field generated by the transmitter unit  110 , in the form of the continuous electromagnetic waveform. The indication of proximity is then output ( 650 ) to a user by way of the output  160  of the receiver unit  120 . 
     Typically, the output ( 650 ) of the indication of proximity would be by way of the audio amplifier and speaker or earphones such that the user can listen to the indication. This may be by directly converting the received continuous electromagnetic waveform to a voltage at the receiving solenoid  150  and converting it directly to audio by way of a pre-amp  200  and/or an audio amplifier at the output  160 . This direct conversion would be by amplification proportional to the magnetic field at the receiving solenoid  150 , and would thereby increase in intensity, and therefore volume when that field strength increased and decrease in volume when the field strength decreases. 
     Accordingly, some characteristic of the indication of proximity, in this case audible volume, would be based at least partially on the magnetic field strength at the receiving solenoid. The user would then move ( 660 ) the receiver unit  120  along the wall while the receiver unit continuously generates and outputs ( 640 ,  650 ) the indication of proximity to the user. 
     The user would then continue to move ( 660 ) the receiver unit  120  until a local maximum is found ( 670 ) for the characteristic of the indication of proximity, in this case volume. Such a local maximum would indicate the location of the transmitter unit  110  opposite the wall  190 . 
       FIG. 7  shows a second embodiment of a system  700  in accordance with this disclosure. The system  700  shown shares many components with the system  100  discussed above with respect to  FIG. 1 . Therefore, the system  700  generally provides a transmitter unit  710  and a receiver unit  720 . 
     The transmitter unit  710  transmits a continuous waveform as a magnetic field by way of a transmitting solenoid  730 , typically paired with a resonance capacitor  810 . The transmitter unit  710  may further comprise a harmonic oscillator  740  for generating the continuous waveform and providing it to the transmitting solenoid  730  for transmission by way of a current source or amplifier  770 . 
     The receiver unit  720  may then have receiving circuitry, such as a receiving solenoid  750  paired with a resonating capacitor  820  to receiving the continuous waveform. The receiving solenoid  750  then converts the continuous waveform, received in the form of a magnetic field, into a voltage and provides it to an amplifier  800 , such as a pre-amp, which amplifies the voltage from the receiving solenoid  750  in proportion to the strength of the magnetic field. 
     In the embodiment shown, the output  160  of the system  100  discussed in reference to  FIG. 1  is replaced by a digital output comprising an analog to digital converter  760  which feeds the converted signal to processing circuitry  830 . The converted signal continues to be proportional to the magnetic field strength at the receiving solenoid  750  and is then may be output as audio at speakers or earphones  840 , or may be further processed to display a metric to a user at a digital display  850 . 
     While the present invention has been described at some length and with some particularity with respect to the several described embodiments, it is not intended that it should be limited to any such particulars or embodiments or any particular embodiment, but it is to be construed with references to the appended claims so as to provide the broadest possible interpretation of such claims in view of the prior art and, therefore, to effectively encompass the intended scope of the invention. Furthermore, the foregoing describes the invention in terms of embodiments foreseen by the inventor for which an enabling description was available, notwithstanding that insubstantial modifications of the invention, not presently foreseen, may nonetheless represent equivalents thereto.