Abstract:
A wireless electromagnetic tracking system using a nonlinear passive transponder is provided. The transponder employs a coil connected in parallel with a diode. The transponder emits a response signal when an excitation signal is incident upon the coil of the transponder. Inclusion of the diode in the transponder circuit introduces nonlinear characteristics into the waveform of the response signal emitted by the transponder. The nonlinear characteristics can be varied by changing the capacitance level of the transponder circuit. The nonlinear characteristics of the response signal can be used to discern the response signal from the excitation signal when both signals are received at a receiver. The nonlinear characteristics can also be utilized in a system of encoding data that is to be transmitted from a transponder to a receiver.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable.  
       REFERENCE TO MATERIALS ON COMPACT DISC  
       [0003]     Not Applicable.  
       BACKGROUND OF THE INVENTION  
       [0004]     The present invention generally relates to an electromagnetic tracking system. More particularly, the present invention relates to an electromagnetic tracking system accommodating a transponder that emits a signal where a portion of the emitted signal contains an additional frequency not found in the excitation signal.  
         [0005]     Many medical procedures involve a medical instrument, such as a drill, a catheter, scalpel, scope, shunt or other tool. In some cases, a medical imaging or video system may be used to provide positioning information for the instrument. However, medical practitioners often do not have the use of such medical imaging systems when performing medical procedures. For example, the use of medical imaging systems for instrument tracking may be limited for health and safety reasons (e.g., radiation dosage concerns), financial limitations, physical space restrictions, and other concerns.  
         [0006]     To compensate for limitations on the use of medical imaging systems, tracking systems that require only limited use of a medical imaging system may be employed. For example, a tracking system that provides position information of the medical instrument with respect to the patient or a reference coordinate system may be used. In such a system, an x-ray of an immobilized patient may be taken and a coordinate system may then be overlaid onto the x-ray image. The resulting x-ray and coordinate system may then be used to provide a map of the patient&#39;s anatomy. Subsequently, a medical practitioner may use the tracking system to ascertain the position of a medical instrument with respect to the coordinate system overlaid onto the x-ray image when the medical instrument is not within the practitioner&#39;s line of sight.  
         [0007]     The tracking system allows the medical practitioner to visualize the patient&#39;s anatomy and track the position and orientation of the instrument. The tracking system may be used to determine when the instrument is positioned in a desired location and allow the medical practitioner to locate and operate on a desired or injured area while avoiding other structures. The tracking system may also be used to verify that the instruments have been removed from the patient at the end of a medical procedure. Increased precision in locating medical instruments within a patient provides for a less invasive medical procedure by facilitating improved control over smaller instruments with less impact on the patient. Improved control and precision with smaller instruments may also reduce risks associated with more invasive procedures like open surgery.  
         [0008]     Tracking systems may also be used to track the position of items other than medical instruments in a variety of applications. For example, tracking technology may be used in forensic or security applications. Retail stores use tracking technology to prevent theft of merchandise. In such cases, a passive transponder is located on the merchandise and a transmitter is strategically located within the retail facility. The transmitter emits an excitation signal at a frequency that is designed to produce a response from a transponder. When merchandise carrying a transponder is located within the transmission range of the transmitter, the transponder produces a response signal that is picked up by a receiver. The receiver then determines the location of the transponder based upon characteristics of the response signal.  
         [0009]     Tracking systems are similarly used in virtual reality systems or simulators. A transponder or transponders are located on a person or object. A transmitter emits an excitation signal and a transponder produces a response signal. The response signal is picked up by a receiver. The signal emitted by the transponder can then be used to monitor the position of a person or object in a simulated environment.  
         [0010]     Some existing electromagnetic tracking systems have a transmitter and receiver wired to a common device or box. In systems with the transmitter and receiver wired to a common device, the object being tracked is wired to the same device as the components performing the tracking. Thus, the range of motion of the object being tracked is limited.  
         [0011]     Wireless electromagnetic tracking systems allow for the object being tracked to be moved freely without being limited by connections with the transmitter or receiver. To reduce the bulk associated with attaching a battery or other power source to a transponder, passive transponders may employ a coil as a means of coupling with and receiving power from other devices.  
         [0012]     Typically, a transponder is located on or within a device in order to track its movement. In order to determine the transponder&#39;s location, a transmitter generates an excitation signal that is incident on the transponder. The incidence of the excitation signal on the transponder causes the transponder to emit a response signal. In systems with passive transponders, the response signal is typically emitted at the same frequency as the excitation signal.  
         [0013]     The response signal emitted by the transponder and the excitation signal emitted by the transmitter are incident upon a receiving coil. Typically, in a tracking system using a passive transponder the excitation signal is much larger than the response signal when both signals are received at the receiver. Because the response signal is emitted at the same frequency as the excitation signal and the response signal is much smaller than the excitation signal, it is difficult to accurately separate and measure the response signal.  
         [0014]     Thus, a tracking system that improves separation and measurement of the response signal would be highly desirable. Additionally, an improved tracking system that may transmit data from a transponder to a receiver would be highly desirable.  
       BRIEF SUMMARY OF THE INVENTION  
       [0015]     A preferred embodiment of the present invention provides a wireless electromagnetic tracking system using a nonlinear passive transponder. The transponder employs a coil connected in parallel with a nonlinear device. The transponder emits a response signal when an excitation signal is incident upon the coil of the transponder. Inclusion of the nonlinear device in the transponder circuit introduces nonlinear characteristics into the waveform of the response signal emitted by the transponder. Characteristics of the nonlinear waveform may be varied by changing the capacitance level of the transponder circuit. The nonlinear characteristics of the response signal may be used to discern the response signal from the excitation signal when both signals are received at a receiver. The nonlinear characteristics may also be utilized in a system of encoding data that is to be transmitted from a transponder to a receiver. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  illustrates a nonlinear passive transponder in accordance with an embodiment of the present invention.  
         [0017]      FIG. 2  is a circuit diagram of the nonlinear passive transponder illustrated in  FIG. 1  in accordance with an embodiment of the present invention.  
         [0018]      FIG. 3  illustrates a nonlinear passive transponder in accordance with an embodiment of the present invention.  
         [0019]      FIG. 4  is a circuit diagram of the nonlinear passive transponder illustrated in  FIG. 3  in accordance with an embodiment of the present invention.  
         [0020]      FIG. 5  is a circuit diagram of the nonlinear passive transponder in accordance with an embodiment of the present invention.  
         [0021]      FIG. 6  is a circuit diagram of the nonlinear passive transponder in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]      FIG. 1  illustrates a nonlinear passive transponder  10  in accordance with an embodiment of the present invention. The transponder  10  includes a core  20 , the terminals  30 , a diode  40 , and a coil  50 . The coil  50  is wound around the core  20 . The core  20  has flanges on both ends to contain the build-up of the wire turns of coil  50 . The two ends of the wire of coil  50  are connected to two terminals  30  that are attached to one of the flanges of core  20 . The diode  40  is connected across the two terminals  30 . In an embodiment, the diode  40  is connected in parallel with the coil  50  as depicted in the circuit diagram of  FIG. 2 .  
         [0023]     In operation, the transponder  10  may be utilized in a tracking system (not shown). In a tracking system, a transmitter emits an excitation signal. The excitation signal may be one of various types of signals such as amplitude modulated, frequency modulated, phase modulated or continuous wave. The excitation signal emitted by the transmitter induces a signal in the coil  50  of transponder  10 . In response to the excitation signal emitted by the transmitter, the transponder  10  emits a response signal.  
         [0024]     Without the diode  40  connected to the transponder  10 , the response signal emitted by the transponder  10  would be emitted at the same frequency as the excitation signal emitted by the transmitter. With the diode  40  connected to the transponder  10  as illustrated in  FIGS. 1 and 2 , the diode  40  introduces nonlinear characteristics into the transponder depicted in  FIGS. 1 and 2 . Because of the nonlinear characteristics of the diode  40 , a portion of the response signal emitted by the transponder  10  contains an additional frequency or frequencies not found in the excitation signal emitted by the transmitter.  
         [0025]     The additional frequencies contained in the portion of the response signal allow a receiver that is receiving signals from both a transmitter and transponder  10  to more easily distinguish between the excitation signal emitted by the transmitter and the response signal emitted by the transponder  10 . Once the receiver has identified the response signal, the characteristics of the response signal may be used to calculate the position, orientation, and gain of the transponder.  
         [0026]     The additional frequencies contained in the portion of the response signal emitted by the transponder  10  can also be used to transmit data to a receiver. By connecting a controller to the transponder  10 , characteristics of the response signal emitted by the transponder  10  may be controlled. For example, the controller may electrically connect and disconnect the diode  40  from the transponder  10  by opening and closing a switch  70  as depicted in  FIG. 5 . Connecting and disconnecting the diode  40  from the transponder  10  by operating a switch  70  alters the waveform of the response signal emitted by the transponder  10 .  
         [0027]     Values may be assigned to various states of the response signal that result when components are switched in and out of the transponder circuit  10 . The values assigned to the various states may also depend upon the duration of time the response signal remains in a given state. The values assigned to the various states may be used in a system for encoding data that is intended to be transmitted from the transponder  10  to a receiver. For example, the state of the response signal when the diode is switched in the transponder circuit  10  may represent a “1” or “on” and the state of the response signal when the diode is switched out of the transponder circuit may represent a “0” or “off”. Thus, data may be transmitted by switching the diode in and out of the transponder circuit  10  and varying the response signal.  
         [0028]     The receiver may be connected to a system that detects and identifies fluctuations in the response signal emitted by the transponder  10 . Using the values assigned to the various states of the response signal, the system at the receiver end may translate the variations in the response signal into data such as the 1&#39;s and 0&#39;s mentioned previously.  
         [0029]     In an alternative embodiment, nonlinearity may be introduced into the response signal by replacing the diode  40  or both the diode  40  and switch  70  with another type of nonlinear device such as a transistor or a synchronous rectifier. The device may then be used to track particular items and transmit code similar to the embodiment depicted in  FIGS. 1, 2  and  5 .  
         [0030]      FIG. 3  illustrates a nonlinear passive transponder  100  in accordance with an embodiment of the present invention. The transponder  100  includes a core  120 , the terminals  130 , a diode  140 , a coil  150 , and a capacitor  160 . The coil  150  is wound around a core  120 . The core  120  has flanges on both ends to contain the build-up of the wire turns of coil  150 . The two ends of the wire of coil  150  are connected to two terminals  130  that are attached to one of the flanges of core  120 . A diode  140  is connected across the two terminals  130 . A capacitor  160  is also connected across the two terminals  160 . In an embodiment, the diode  140 , the capacitor  160  and the coil  150  are connected in parallel as depicted in the circuit diagram of  FIG. 4 .  
         [0031]     In operation, the transponder  100  is similar in operation to the transponder  10  of  FIG. 1 . That is, the transponder  100  may be utilized in a tracking system (not shown). An excitation signal emitted by a transmitter induces a signal in the coil  150  of transponder  100 . In response to the excitation signal emitted by the transmitter, the transponder  100  emits a response signal.  
         [0032]     Without the diode  140  connected to the transponder  100 , the response signal emitted by the transponder  100  is emitted at the same frequency as the excitation signal emitted by the transmitter. With the diode  140  connected to the transponder  100  as illustrated in  FIGS. 3 and 4 , the diode  140  introduces nonlinear characteristics into the transponder  100  depicted in  FIGS. 3 and 4 . Because of the nonlinear characteristics of the diode  140 , a portion of the response signal emitted by the transponder  100  contains an additional frequency or frequencies not found in the excitation signal emitted by the transmitter.  
         [0033]     The additional frequencies contained in a portion of the response signal allow a receiver that is receiving signals from both a transmitter and transponder  100  to more easily distinguish between the excitation signal emitted by the transmitter and the response signal emitted by the transponder  100 . Once the receiver has identified the response signal, characteristics of the response signal may be used to calculate a position, orientation, and gain of the transponder.  
         [0034]     The additional frequencies contained in a portion of the response signal emitted by the transponder  100  may also be used to transmit data to a receiver. By connecting a controller to the transponder  100 , characteristics of the response signal emitted by the transponder  100  may be controlled. For example, the controller may electrically connect and disconnect the diode  140  or the capacitor  160  from the transponder  100  by opening and closing switches  170 ,  180  as depicted in  FIG. 6 . Connecting and disconnecting the diode  140  or the capacitor  160  from the transponder  100  by operating switches  170 ,  180  may alter the waveform of the response signal emitted by the transponder  100 .  
         [0035]     Varying the level of capacitance of the capacitor  160  modifies characteristics of the additional frequencies present in a portion of the response signal emitted by the transponder  100 . For example, voltage and current values at various harmonic levels for a given transponder configuration will vary as the capacitance of the capacitor  160  is varied. These changes in harmonic levels and other waveform characteristics can be used to distinguish between various transponders  100  having a capacitor  160  with different levels of capacitance attached to them. Being able to distinguish one transponder from another transponder may then allow a tracking system to track and identify the different devices to which the transponders are attached.  
         [0036]     Values may be assigned to the various states of the response signal that result when components are switched in and out of the transponder circuit  100 . The values assigned to the various states may also depend upon the duration of time the response signal remains in a given state. The values assigned to the various states may be used in a system for encoding data that is intended to be transmitted from the transponder  100  to a receiver. For example, the state of the response signal when the diode  140  is switched in the transponder circuit  100  may represent a “1” or “on” and the state of the response signal when the diode  140  is switched out of the transponder circuit may represent “0” or “off”. Additionally, states of the response signal when the capacitor  160  is switched in or out, alone or in combination with switching of the diode  140 , may represent assigned values such as “0”, “1”, “2”, “3”, etc. Thus, data may be transmitted by switching the diode  140  and/or capacitor  160  out of the transponder circuit  100  and varying the response signal.  
         [0037]     The receiver may be connected to a system that detects and identifies fluctuations in the response signal emitted by the transponder  100 . Thus, electrically switching the diode  140  or the capacitor  160  in and out of the transponder circuit  100  may be used to transmit encoded data from a transponder  100  to a receiver. Using the values assigned to the various states of the response signal, the system at the receiver end can translate variations in the response signal into data such as the 1&#39;s and 0&#39;s mentioned previously.  
         [0038]     In an alternative embodiment, nonlinearity may be introduced into the response signal by replacing the diode  140  or both the diode  140  and switch  180  with another type of nonlinear device such as a transistor or a synchronous rectifier. The device may then be used to track particular items and transmit code similar to the embodiment depicted in  FIGS. 3, 4  and  6 .  
         [0039]     While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. For example, the diode  40 ,  140  may be replaced with another type of switching or nonlinear device such as a transistor or a synchronous rectifier for introducing nonlinearity into the response signal. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.