Patent Publication Number: US-2018035913-A1

Title: Position detection system and operation method of position detection system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of PCT International Application No. PCT/JP2016/081695 filed on Oct. 26, 2016 which claims the benefit of priority from Japanese Patent Application No. 2015-236127 filed on Dec. 2, 2015, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to a position detection system and an operation method of the position detection system. 
     In recent years, a capsule medical apparatus introduced into a subject to acquire various types of information about the subject or to administer a drug to the subject has been developed. As an example, a capsule endoscope formed in a size which may be introduced into the gastrointestinal tract of a subject is known. The capsule endoscope has an imaging function and a wireless communication function inside a capsule-shaped casing, and after being swallowed into the subject, the capsule endoscope performs imaging while moving inside the gastrointestinal tract, and wirelessly transmits sequentially image data of images of the inside of an organ of the subject. 
     A system for performing position detection using such a capsule medical apparatus as a detection target has also been developed. For example, JP 2008-132047 A discloses a position detection system which includes a capsule medical apparatus having therein a magnetic field generating coil which generates a magnetic field for position detection by receiving power supply, and detection coils which detect the magnetic field generated by the magnetic field generating coil outside a subject, and performs calculation for detecting a position of the capsule medical apparatus based on the strength of the magnetic field detected by the detection coils. 
     In addition, a system for guiding a capsule medical apparatus introduced into a subject by a magnetic field has been proposed. For example, JP 2006-68501 A discloses a magnetic guidance medical system for guiding a capsule medical apparatus by introducing a capsule medical apparatus having therein a permanent magnet into a subject, providing a magnetic field generation unit outside the subject, and causing the magnetic field generation unit to move so as to change the magnetic field acting on the permanent magnet in the capsule medical apparatus. 
     SUMMARY 
     A position detection system according to one aspect of the present disclosure may include: a detection target including a magnetic field generator configured to generate an alternating magnetic field for position detection and a permanent magnet provided therein, the detection target being adapted to be introduced into a subject; a plurality of detection coils arranged outside the subject, each of the detection coils detecting the alternating magnetic field and outputting a detection signal; a guidance magnetic field generator including a magnetic field generation source configured to generate a guidance magnetic field for guiding the detection target, and a driving mechanism configured to change at least one of a position and a posture of the magnetic field generation source, wherein at least a part of the guidance magnetic field generator is formed of a conductor that generates an interference magnetic field by an action of the alternating magnetic field; a guidance magnetic field controller configured to control an operation of the driving mechanism; and a position detector configured to calculate at least one of a position and a posture of the detection target by using: a plurality of the detection signals respectively output from the detection coils; and at least one of a position and a posture of the conductor. 
     The above and other features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description of presently preferred embodiments of the disclosure, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an outline of a position detection system according to a first embodiment of the present disclosure; 
         FIG. 2  is a schematic diagram illustrating an example of an internal structure of a capsule endoscope illustrated in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating a detailed configuration of the position detection system illustrated in  FIG. 1 ; 
         FIG. 4  is a schematic view illustrating a configuration example of a guidance magnetic field generating device illustrated in  FIG. 3 ; 
         FIG. 5  is a block diagram illustrating a configuration example of a magnet drive unit illustrated in  FIG. 4 ; 
         FIG. 6  is a flowchart illustrating a position detection method according to the first embodiment of the present disclosure; 
         FIG. 7  is a schematic view illustrating an example of a positional relationship among an external permanent magnet, a capsule endoscope, and a plurality of detection coils; 
         FIG. 8  is a schematic view illustrating an example of a positional relationship among an external permanent magnet, the capsule endoscope, and the plurality of detection coils; 
         FIG. 9  is a table illustrating an example of correction coefficients for calculating a correction value with a coordinate in the vertical direction of the external permanent magnet as an input value; 
         FIG. 10  is a set of graphs illustrating a relationship between the coordinate in the vertical direction of the external permanent magnet and coordinates in respective directions of the capsule endoscope before and after correction; 
         FIG. 11  is a schematic view illustrating a partial configuration of a position detection system according to a third embodiment of the present disclosure; and 
         FIG. 12  is a schematic view illustrating a partial configuration of a position detection system according to a fourth embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a position detection system and a position detection method according to embodiments of the present disclosure will be described with reference to the drawings. In the embodiments to be described below, as one form of a detection target of which a position and a posture are objects to be detected by the position detection system, a capsule endoscope is exemplified which is orally introduced into a subject and captures images of the inside of the gastrointestinal tract of the subject. However, the present disclosure is not limited by these embodiments. That is, the present disclosure may be applied to detection of positions and postures of various devices introduced into a subject, for example, a capsule endoscope which moves inside a lumen from the esophagus to the anus of the subject, a capsule medical apparatus which delivers a medicine or the like into the subject, a capsule medical apparatus which includes a pH sensor for measuring a pH in the subject. 
     In the following description, each figure only schematically illustrates a shape, a size, and a positional relationship to the extent that the contents of the present disclosure may be understood. Therefore, the present disclosure is not limited exclusively to the shape, the size, and the positional relationship exemplified in each figure. In the drawings, the same parts are denoted by the same reference signs. 
     First Embodiment 
       FIG. 1  is a schematic diagram illustrating an outline of a position detection system according to a first embodiment of the present disclosure. As illustrated in  FIG. 1 , a position detection system  1  according to the first embodiment is a system which detects a position of a capsule endoscope introduced into a subject  20  to capture an image of the inside of the subject  20 , as an example of a detection target. The position detection system  1  includes a capsule endoscope  10 , a bed  21 , a magnetic field detection device  30 , a guidance magnetic field generating device  40 , a guidance magnetic field control device  50 , a calculation device  60  (position detection calculation device), a receiving device  70 , and a display device  80 . On the bed  21 , the subject  20  is placed. The magnetic field detection device  30  detects a position-detecting magnetic field generated by the capsule endoscope  10 . The guidance magnetic field generating device  40  generates a magnetic field for guiding the capsule endoscope  10 . The guidance magnetic field control device  50  controls an operation of the guidance magnetic field generating device  40 . The calculation device  60  (position detection calculation device) performs a calculation process for detection of a position of the capsule endoscope  10  and the like based on a detection signal of the position-detecting magnetic field output from the magnetic field detection device  30 . The receiving device  70  receives a signal wirelessly transmitted from the capsule endoscope  10  via a receiving antenna  71  affixed to the body surface of the subject  20 . The display device  80  displays an image output from the calculation device  60  and positional information of the capsule endoscope  10 , and the like. 
       FIG. 2  is a schematic diagram illustrating an example of an internal structure of the capsule endoscope  10  illustrated in  FIG. 1 . As illustrated in  FIG. 2 , the capsule endoscope  10  includes a casing  100 , an imaging unit  11 , a control unit  12 , a transmitting unit  13 , a magnetic field generation unit  14 , a power supply unit  15 , and a permanent magnet  16 . The casing  100  is capsule-shaped and formed in a size easy to introduce into the subject  20 . The imaging unit  11  is accommodated in the casing  100  and captures an image of the inside of the subject  20  to acquire an imaging signal. The control unit  12  controls an operation of each unit of the capsule endoscope  10  including the imaging unit  11 , and performs a predetermined signal process to the imaging signal acquired by the imaging unit  11 . The transmitting unit  13  wirelessly transmits the imaging signal which has been subjected to the signal process. The magnetic field generation unit  14  generates an alternating magnetic field as a position-detecting magnetic field of the capsule endoscope  10 . The power supply unit  15  supplies power to each unit of the capsule endoscope  10 . 
     The casing  100  is an outer casing formed in a size which may be introduced into an organ of the subject  20 . The casing  100  has a cylindrical casing  101  having a cylindrical shape and two dome-shaped casings  102  and  103  having a dome shape and respectively closing open ends on both sides of the cylindrical casing  101 . The cylindrical casing  101  is formed of a colored member which is substantially opaque to visible light. The dome-shaped casing  102  provided on a side of the imaging unit  11  is formed of an optical member which is transparent to light of a predetermined wavelength band such as visible light. The casing  100  includes the imaging unit  11 , the control unit  12 , the transmitting unit  13 , the magnetic field generation unit  14 , the power supply unit  15 , and the permanent magnet  16  liquid-tightly. In  FIG. 2 , the imaging unit  11  is provided on a side of the dome-shaped casing  102  only, but the imaging unit  11  may be further provided on a side of the dome-shaped casing  103 . In that case, the dome-shaped casing  103  is also formed of a transparent optical member. 
     The imaging unit  11  includes illumination units  111 , an optical system  112 , and an imaging element  113 . Each illumination unit  111  has a light source such as an LED, and emits illumination light having a predetermined color component (for example, white light) in a region including an imaging view field of the imaging element  113  to illuminate the inside of the subject  20  through the dome-shaped casing  102 . The optical system  112  has one or a plurality of lenses, and condenses light from the subject  20  onto a light-receiving surface of the imaging element  113  to form an image. The imaging element  113  has an image sensor such as a CMOS or a CCD, converts the light received on the light-receiving surface into an electrical signal, and outputs the electrical signal as an imaging signal. 
     The control unit  12  operates the imaging unit  11  at a predetermined imaging cycle and causes each illumination unit  111  to emit light in synchronization with the imaging cycle. In addition, the control unit  12  performs a predetermined signal process including A/D conversion on the imaging signal generated by the imaging unit  11  to generate image data. 
     The transmitting unit  13  includes a transmitting antenna. The transmitting unit  13  sequentially acquires the image data subjected to the signal process by the control unit  12  and related information to perform a modulation process, and wirelessly transmits sequentially the modulated signal to the outside via the transmitting antenna. 
     The magnetic field generation unit  14  includes a magnetic field generating coil  141  which generates a magnetic field by a flow of a current and a capacitor  142  which is connected in parallel with the magnetic field generating coil  141  and forms a resonance circuit together with the magnetic field generating coil  141 . The magnetic field generation unit  14  receives power supply from the power supply unit  15  and generates an alternating magnetic field of a predetermined frequency as a position-detecting magnetic field. 
     The power supply unit  15  includes a power storage unit such as a button battery or a capacitor, and a switch unit such as a magnetic switch or an optical switch. When the power supply unit  15  is configured to have a magnetic switch, switching between ON and OFF states of power is performed by a magnetic field applied from the outside, and in the ON state, the power of the power storage unit is appropriately supplied to each component (the imaging unit  11 , the control unit  12 , and the transmitting unit  13 ) of the capsule endoscope  10 , and in the OFF state, the supply is stopped. 
     The permanent magnet  16  is provided to enable the capsule endoscope  10  to be guided by a magnetic field applied from the outside. The permanent magnet  16  is fixedly disposed inside the casing  100  so that a magnetization direction intersects a major axis La of the casing  100 . In the case illustrated in  FIG. 2 , the magnetization direction (an arrow M 1  in  FIG. 2 ) of the permanent magnet  16  is orthogonal to the major axis La. 
       FIG. 3  is a diagram illustrating a detailed configuration of the position detection system  1  illustrated in  FIG. 1 . The magnetic field detection device  30  illustrated in  FIG. 3  includes a coil unit  31 , and a signal processor  32 . In the coil unit  31 , a plurality of detection coils C 1  to C 12  is arranged. The signal processor  32  processes detection signals respectively output from the detection coils C 1  to C 12 . 
     The detection coils C n  (n=1 to 12) are each obtained by winding a wire in a spiral shape, and its size is, for example, about 30 to 40 mm in opening diameter and about 5 mm in height. The detection coils C n  are arranged on a main surface of a flat panel  33  formed of a nonmetallic material such as resin. In each of the detection coils C n , a current corresponding to a change in the magnetic field at an arrangement position thereof is generated and output to the signal processor  32 . In this sense, the current generated in each of the detection coils C n  is nothing less than the detection signal. 
     The arrangement position and the number of the detection coils in the coil unit  31  are determined depending on a detection target region when detecting the capsule endoscope  10  in the subject  20  to be examined on the bed  21 . The detection target region is set in advance depending on conditions such as a movable range of the capsule endoscope  10  and the strength of the position-detecting magnetic field generated by the capsule endoscope  10  in the subject  20  examined on the bed  21 . For example, in the case illustrated in  FIG. 1 , a detection target region R is set as a three-dimensional region including a part of a region above the bed  21 . 
     The signal processor  32  includes a plurality of signal processing channels Ch 1  to Ch 12 , the signal processing channels Ch 1  to Ch 12  corresponding to the detection coils C 1  to C 12 , respectively. The signal processing channels Ch n  each include an amplification unit  321 , an A/D converter (A/D)  322 , and an FFT processor (FFT)  323 . The amplification unit  321  amplifies a detection signal output from each of the detection coils C n . The A/D converter (A/D)  322  digitally converts the amplified detection signal. The FFT processor (FFT)  323  performs a fast Fourier transform process on the digitally converted detection signal and outputs the detection signal to the calculation device  60 . 
     The guidance magnetic field generating device  40  is disposed on an opposite side of the detection target region R for the capsule endoscope  10  with respect to the coil unit  31 , that is, on a lower region side of the coil unit  31 , and generates a guidance magnetic field for changing at least one of a position and a posture of the capsule endoscope  10  which has been introduced into the subject  20  on the bed  21 . Here, the posture of the capsule endoscope  10  is represented by an elevation angle which is an angle with respect to a horizontal plane of the major axis La (see  FIG. 2 ) of the capsule endoscope  10  with respect to the horizontal plane (XY plane) and a traverse angle (azimuth) of the major axis La rotating about an axis in a vertical direction (Z direction) from a predetermined reference position. 
       FIG. 4  is a schematic view illustrating a configuration example of the guidance magnetic field generating device  40 . As illustrated in  FIG. 4 , the guidance magnetic field generating device  40  includes a permanent magnet (hereinafter referred to as an external permanent magnet)  41 , a support member  42 , and a magnet drive unit  43 . The external permanent magnet  41  serves as a magnetic field generation source which generates the guidance magnetic field for the capsule endoscope  10 . The support member  42  supports the external permanent magnet  41 . The magnet drive unit  43  changes at least one of a position and a posture of the external permanent magnet  41  via the support member  42 . 
     The guidance magnetic field generating device  40  is at least partially formed of a conductor. Generally, there exists the position-detecting magnetic field generated by the capsule endoscope  10  in a region where the guidance magnetic field generating device  40  is disposed. Consequently, by the position-detecting magnetic field changing with time, an eddy current flows through the conductor included in the guidance magnetic field generating device  40  and a new magnetic field (interference magnetic field) is generated. Therefore, the conductor included in the guidance magnetic field generating device  40  is a generation source of the interference magnetic field with respect to the position-detecting magnetic field. Since the conductor included in the guidance magnetic field generating device  40  moves and rotates under the control of the guidance magnetic field control device  50 , the interference magnetic field also changes with time. 
     The external permanent magnet  41  is achieved by a bar magnet having a rectangular parallelepiped shape, for example. In that case, in an initial state, the external permanent magnet  41  is disposed so that one plane PL of four planes parallel to a magnetization direction thereof is parallel to the horizontal plane (see  FIG. 4 ). Although the material of the external permanent magnet  41  is not particularly limited, for example, a metal magnet such as a neodymium magnet may be used. When a metal magnet is used as the external permanent magnet  41 , the external permanent magnet  41  itself is a generation source of the interference magnetic field. Since the guidance magnetic field generated by the external permanent magnet  41  is stationary, the guidance magnetic field may be separated from the position-detecting magnetic field which is an alternating magnetic field. 
     The material of the support member  42  is not particularly limited, but when the support member  42  is formed of a conductor such as metal, the support member  42  may also be a generation source of the interference magnetic field. 
     The magnet drive unit  43  is a driving mechanism which changes a position and a posture of the external permanent magnet  41  via the support member  42 . The magnet drive unit  43  includes a motor or the like which translates or rotates the external permanent magnet  41 . Since a metal member is used in a commonly used motor, the magnet drive unit  43  may also be a generation source of the interference magnetic field with respect to the position-detecting magnetic field. When the support member  42  is formed of metal and the magnet drive unit  43  is covered by the support member  42  as seen from all the detection coils C 1  to C 12  as illustrated in  FIG. 3 , there is no need to consider the magnet drive unit  43  as a generation source of the interference magnetic field. 
       FIG. 5  is a block diagram illustrating a configuration example of the magnet drive unit  43 . The magnet drive unit  43  includes a planar position changing unit  431 , a vertical position changing unit  432 , an elevation angle changing unit  433 , and a traverse angle changing unit  434 . The planar position changing unit  431  translates the external permanent magnet  41  in the horizontal plane. The vertical position changing unit  432  translates the external permanent magnet  41  in the vertical direction. The elevation angle changing unit  433  changes the elevation angle of the external permanent magnet  41  by rotating the external permanent magnet  41  about an axis which passes a center of the external permanent magnet  41 , is orthogonal to the magnetization direction of the external permanent magnet  41  and is parallel to the horizontal plane. The traverse angle changing unit  434  changes the traverse angle of the external permanent magnet  41  by rotating the external permanent magnet  41  with respect to an axis in the vertical direction which passes the center of the external permanent magnet  41 . Hereinafter, a rotation axis (an axis a illustrated in  FIG. 4 ) used when the elevation angle changing unit  433  changes the elevation angle of the external permanent magnet  41  is referred to as a central axis a, and a rotation axis (an axis b illustrated in  FIG. 4 ) used when the traverse angle changing unit  434  changes the traverse angle of the external permanent magnet  41  is referred to as a vertical axis b. 
     Through the operation of the magnet drive unit  43  described above, the external permanent magnet  41  and the support member  42  have five degrees of freedom: translation in a three-dimensional space, rotation about the central axis a, and rotation about the vertical axis b. 
     The guidance magnetic field control device  50  controls the guidance magnetic field generating device  40  in order to achieve guidance desired by a user with respect to the capsule endoscope  10 . As illustrated in  FIG. 3 , the guidance magnetic field control device  50  includes an operation input unit  51 , a control signal generation unit  52 , and a control signal output unit  53 . The operation input unit  51  is used by the user when guiding the capsule endoscope  10  introduced into the subject  20 . The control signal generation unit  52  generates a control signal for the magnet drive unit  43  (driving mechanism) based on the operation to the operation input unit  51 . The control signal output unit  53  outputs the control signal to the magnet drive unit  43  and the calculation device  60 . 
     The operation input unit  51  includes an input device such as a joystick, an operation console including various buttons and switches, and a keyboard, and inputs, to the control signal generation unit  52 , a signal corresponding to an operation performed from the outside. Specifically, the operation input unit  51  inputs, to the control signal generation unit  52 , an operation signal for changing at least one of the position and the posture of the capsule endoscope  10  introduced into the subject  20  depending on an operation performed by the user. 
     The control signal generation unit  52  generates a control signal for controlling the magnet drive unit  43  of the guidance magnetic field generating device  40  depending on the operation signal input from the operation input unit  51 . 
     The control signal output unit  53  outputs this control signal to the guidance magnetic field generating device  40  and to the calculation device  60 . 
     When guiding the capsule endoscope  10 , the magnet drive unit  43  is operated under the control of the guidance magnetic field control device  50 , and thereby the external permanent magnet  41  is translated via the support member  42  in each of the horizontal plane and the vertical direction, and the elevation angle and the traverse angle are changed. The position and the posture of the capsule endoscope  10  change following the movement of the external permanent magnet  41 . 
     The calculation device  60  executes a calculation process for calculating the position and the posture of the capsule endoscope  10  based on detection signals of the position-detecting magnetic field output from the signal processor  32  and a calculation process for generating an image of the inside of the subject  20  based on the received signal received by the receiving device  70 . The calculation device  60  includes, as illustrated in  FIG. 3 , a position calculator  601 , a correction value acquisition unit  602 , a position correction unit  603 , a storage unit  604 , an image processor  605 , and an output unit  606 . The position calculator  601  calculates at least one of the position and the posture of the capsule endoscope  10  based on the position-detecting magnetic field generated by the capsule endoscope  10 . The correction value acquisition unit  602  acquires correction values for correcting at least one of the position and the posture of the capsule endoscope  10 . The position correction unit  603  corrects at least one of the position and the posture of the capsule endoscope  10  calculated by the position calculator  601 . The storage unit  604  stores various types of information used in the position detection system  1 . The image processor  605  performs a predetermined image process to the received signal received by the receiving device  70 , thereby generating image data of an image of the inside of the subject  20  captured by the capsule endoscope  10 . The output unit  606  outputs the image of the inside of the subject  20  and various types of information such as the position and the posture of the capsule endoscope  10  to the display device  80 . 
     The position calculator  601  acquires each of the detection signals of the position-detecting magnetic field generated by the capsule endoscope  10  from a plurality of channels (Ch 1  to Ch 12  in  FIG. 3 ) of the signal processor  32 , and calculates the position and the posture of the capsule endoscope  10  based on these detection signals. 
     The correction value acquisition unit  602  acquires position information of the capsule endoscope  10  calculated immediately beforehand by the position correction unit  603  from the storage unit  604 , acquires a control signal for the guidance magnetic field generating device  40  from the guidance magnetic field control device  50 , and acquires, based on the position information and the control signal, correction values for correcting at least one of the position and the posture of the capsule endoscope  10  calculated by the position calculator  601  from a lookup table (LUT) described later. 
     The position correction unit  603  corrects the position and the posture of the capsule endoscope  10  calculated by the position calculator  601  by using the correction values acquired by the correction value acquisition unit  602 , thereby calculating at least one of the corrected position and posture of the capsule endoscope  10 . 
     The storage unit  604  includes a position information storage unit  607 , a LUT storage unit  608 , and an image data storage unit  609 . The position information storage unit  607  stores information indicating the corrected position and posture of the capsule endoscope  10  calculated by the position correction unit  603 . The LUT storage unit  608  stores a lookup table (LUT) having information regarding correction values for correcting the position and the posture of the capsule endoscope  10  stored therein. The image data storage unit  609  stores image data of an image generated by the image processor  605 . Hereinafter, information indicating the position and the posture of the capsule endoscope  10  is also referred to as position information. 
     The LUT storage unit  608  stores a lookup table in which at least one of the position and the posture of the capsule endoscope  10 , at least one of the position and the posture of the generation source of the interference magnetic field, as well as correction values for at least one of the position and the posture of the capsule endoscope  10  are associated with one another. The correction values here correspond to error in the position and the posture of the capsule endoscope  10  generated depending on a relationship between the relative position and posture of the capsule endoscope  10  and those of the generation source of the interference magnetic field. This lookup table is generated by actually measuring in advance or measuring through a simulation position detection results of the capsule endoscope  10  when the position and the posture of the capsule endoscope  10  and the position and the posture of the generation source of the interference magnetic field are changed, and is stored in the LUT storage unit  608 . 
     The storage unit  604  is achieved by using a ROM, a RAM, or the like. The storage unit  604  stores various control programs and various parameters for controlling each unit of the calculation device  60 , a position detection calculation program for the capsule endoscope  10 , an image processing program, and the like. 
     The calculation device  60  having the above configuration is configured, for example, by a computer such as a personal computer or a workstation including a general-purpose processor such as a CPU, a ROM, and a RAM. 
     The receiving device  70  selects, from a plurality of receiving antennas  71  to be affixed to the body surface of the subject  20  when the examination is performed by the capsule endoscope  10 , a receiving antenna  71  having the highest received strength with respect to a radio signal transmitted from the capsule endoscope  10 , and performs a demodulation process or the like to the radio signal received via the selected receiving antenna  71 , thereby acquiring an image signal and related information. 
     The display device  80  includes various types of display such as a liquid crystal display and an organic EL display and displays information of an in-vivo image of the subject  20  and the position and the posture of the capsule endoscope  10  on a screen thereof based on the position information and the image data generated in the calculation device  60 . 
     Next, a position detection method according to the first embodiment will be described.  FIG. 6  is a flowchart illustrating the position detection method performed by the position detection system  1 .  FIG. 7  is a schematic view illustrating a positional relationship among the capsule endoscope  10  illustrated in  FIG. 3 , the plurality of detection coils C 1  to C 12 , and the external permanent magnet  41 . An arrow M 2  in  FIG. 7  indicates a magnetization direction of the external permanent magnet  41 . 
     In the following description, in order to simplify the description, it is assumed that only the external permanent magnet  41  is the generation source of the interference magnetic field with respect to the position-detecting magnetic field generated by the capsule endoscope  10 , and that the influence of the support member  42  and the magnet drive unit  43  is negligible. In the position correction method described below, it is assumed that a process for correcting a position and a posture of the capsule endoscope  10  is performed. 
     First, in Step S 10 , the capsule endoscope  10  is turned on. As a result, power supply from the power supply unit  15  (see  FIG. 2 ) to each unit of the capsule endoscope  10  is started, the imaging unit  11  starts imaging, and the magnetic field generation unit  14  starts generating the position-detecting magnetic field. 
     In the subsequent Step S 11 , the capsule endoscope  10  is introduced into the subject  20 , and guidance to the capsule endoscope  10  is started. In detail, when the user operates the operation input unit  51  (see  FIG. 3 ), the operation input unit  51  inputs an operation signal corresponding to the input operation to the control signal generation unit  52 . In response to the operation signal, the control signal generation unit  52  generates a control signal for changing the position (x, y, z) and the posture (an elevation angle φ, a traverse angle θ) in the three-dimensional space of the external permanent magnet  41 . The control signal output unit  53  outputs the control signal to the magnet drive unit  43  and to the correction value acquisition unit  602  of the calculation device  60 . 
     In the subsequent Step S 12 , the position calculator  601  calculates the position and the posture of the capsule endoscope  10  based on multiple detection signals respectively output from multiple detection coils C n . Specifically, five values (x s (t i ), y s (t i ), z s (t i ), φ s (t i ), θ s (t i )) indicating the position and the posture of a capsule endoscope  10  at time t i  are calculated. Here, the suffix i at the time t i  represents the order of time of detection of the position-detecting magnetic field, and i=0, 1, 2, . . . . 
     In the subsequent Step S 13 , the correction value acquisition unit  602  acquires from the position information storage unit  607  latest corrected position and posture of the capsule endoscope  10  calculated immediately beforehand by the position correction unit  603 . That is, the latest position and posture stored in the position information storage unit  607  are acquired. Specifically, five values (x c (t i-1 ), y c (t i-1 ), z c (t i-1 ), φ c (t i-1 ), θ c (t i-1 )) indicating the corrected position and posture of the capsule endoscope  10  at time are calculated. When the corrected position and posture of the capsule endoscope  10  have not yet been calculated (that is, when i=0), the correction value acquisition unit  602  may acquire, as data corresponding to the latest corrected position and posture of the capsule endoscope  10 , the position and the posture before correction calculated in Step S 12 , or may acquire preset initial values from the storage unit  604 . 
     In the subsequent Step S 14 , based on the control signal output from the control signal output unit  53 , the correction value acquisition unit  602  acquires current position and posture of the generation source of the interference magnetic field with respect to the position-detecting magnetic field generated by the capsule endoscope  10 . Specifically, five values (x m (t i ), y m (t i ), z m (t i ), φ m (t i ), θ m (t i )) indicating the position and the posture of the external permanent magnet  41  at the time t i  are acquired. 
     In the subsequent Step S 15 , the correction value acquisition unit  602  acquires correction values for the position and the posture of the capsule endoscope  10  based on the corrected position and posture of the capsule endoscope  10  acquired in Step S 13  and the position and the posture of the generation source of the interference magnetic field acquired in Step S 14 . 
     In detail, the correction value acquisition unit  602  extracts correction values (Δx, Δy, Δz, Δφ, Δθ) from the lookup table stored in the LUT storage unit  608  employing, as input values, the position and the posture (x c (t i-1 ), y c (t i-1 ), z c (t i-1 ), φ c (t i-1 ), θ c (t i-1 )) of the capsule endoscope  10  and the position and the posture (x m (t i ), y m (t i ), z m (t i ), φ m (t i ), θ m (t i )) of the external permanent magnet  41 . When the calculation device  60  corrects either one of the position and the posture of the capsule endoscope  10 , the correction value acquisition unit  602  extracts only the correction values for the one to be corrected. 
     In the subsequent Step S 16 , the position correction unit  603  corrects the position and the posture of the capsule endoscope  10  calculated from the detection signals in Step S 12  by using the correction values acquired in Step S 15 . That is, as expressed by the following formula (1), by respectively subtracting the correction values (Δx, Δy, Δz, Δφ, Δθ) from values (x s  (t i ), y s (t i ), z s (t i ), φ s  (t i ), θ s (t i )) indicating the position and the posture of the capsule endoscope  10  calculated from the detection signals, the corrected position and posture (x c (t i ), y c (t i ), z c (t i ), φ c (t i ), θ c (t i )) of the capsule endoscope  10  at the time t i  are calculated. 
     
       
         
           
             
               
                 
                   
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     In the subsequent Step S 17 , the position correction unit  603  causes the position information storage unit  607  to store the corrected position and posture of the capsule endoscope  10 . 
     In the subsequent Step S 18 , the calculation device  60  determines whether to end a position detection calculation for the capsule endoscope  10 . Specifically, the calculation device  60  determines to end the position detection calculation when transmission of the wireless signal from the capsule endoscope  10  has been stopped, a case where a predetermined period of time has passed since the capsule endoscope  10  was turned on, or a case where an operation to end the operation of the calculation device  60  has been performed. 
     When the position detection calculation is not ended (Step S 18 : No), the process moves to Step S 12 . On the other hand, when the position detection calculation is ended (Step S 18 : Yes), the process is ended. 
     As described above, according to the first embodiment of the present disclosure, by forming, at least partially, the guidance magnetic field generating device  40  with a conductor, this conductor may be used as a generation source of a known interference magnetic field with respect to the position-detecting magnetic field. Therefore, even when the position and the posture of the generation source of the interference magnetic field change with time, by removing the influence of the interference magnetic field generated by the conductor included in the guidance magnetic field generating device  40  through calculation based on the position and the posture of the generation source and the position and the posture of the capsule endoscope  10 , accuracy of detecting the position and the posture of the capsule endoscope  10  may be improved. 
     Modification 
     Next, a modification of the first embodiment of the present disclosure will be described. In the first embodiment described above, the correction value acquisition unit  602  acquires the correction values with reference to the lookup table stored in the LUT storage unit  608 , but the correction values may be calculated by using functions produced in advance. 
     In detail, by employing a position and a posture of the capsule endoscope  10  as well as a position and a posture of the generation source of the interference magnetic field (external permanent magnet  41  and the like) as variables (input values), functions which give correction values to the position and the posture of the capsule endoscope  10  are produced in advance and stored in the storage unit  604 . As expressed by the following formula (2), the correction values (Δx, Δy, Δz, Δφ, Δθ) are respectively given by functions (f x , f y , f z , f φ , f θ ) employing, as variables, coordinates (x c , y c , z c ), an elevation angle φ c , and a traverse angle θ c  of the capsule endoscope  10 , as well as coordinates (x m , y m , z m ), an elevation angle φ m , and a traverse angle θ m  of the generation source of the interference magnetic field. 
     
       
         
           
             
               
                 
                   
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     In that case, in Step S 15  of  FIG. 6 , the correction value acquisition unit  602  assigns the corrected position and posture of the capsule endoscope  10  acquired in Step S 13  and the position and the posture of the generation source of the interference magnetic field acquired in Step S 14  to the above functions, thereby calculating and outputting the correction values of the position and the posture of the capsule endoscope  10 . 
     Second Embodiment 
     Next, a second embodiment of the present disclosure will be described.  FIG. 7  is a schematic view illustrating an example of a positional relationship among an external permanent magnet, a capsule endoscope, and a plurality of detection coils. 
     In the second embodiment, the number of input values employed when acquiring correction values is reduced in comparison with that in the first embodiment by using a relative positional relationship between a capsule endoscope  10  and a generation source of an interference magnetic field and symmetry of a shape of the generation source of the interference magnetic field. 
     A specific description will be given below. For example, when the capsule endoscope  10  is floating in liquid inside a subject  20  (see  FIG. 1 ), as illustrated in  FIG. 7 , the capsule endoscope  10  is usually constrained by a guidance magnetic field vertically above an external permanent magnet  41 , and moves following translational motion in a horizontal plane of the external permanent magnet  41 . That is, the coordinates (x c , y c ) in the horizontal plane of the capsule endoscope  10  become substantially equal to the coordinates (x m , y m ) in the horizontal plane of the external permanent magnet  41 , and in the horizontal plane, error in the position due to the influence of the interference magnetic field hardly occurs. Therefore, in that case, the coordinates (x c , y c ) of the capsule endoscope  10  and the coordinates (x m , y m ) of the external permanent magnet  41  may be excluded from the input values employed when the correction value acquisition unit  602  acquires the correction values. In other words, the coordinates (x c , y c ) of the capsule endoscope  10  and the coordinates (x m , y m ) of the external permanent magnet  41  may be excluded from the lookup table or the input values in the functions for acquiring the correction values by the correction value acquisition unit  602 . 
     Furthermore, in that case, the capsule endoscope  10  rotates following the rotation of the external permanent magnet  41  about the vertical axis b. That is, the traverse angle θ c  of the capsule endoscope  10  becomes substantially equal to the traverse angle θ m  of the external permanent magnet  41 , and error in the traverse angle direction due to the influence of the interference magnetic field hardly occurs. Therefore, the traverse angle θ c  of the capsule endoscope  10  and the traverse angle θ m  of the external permanent magnet  41  may also be excluded from the input values employed when the correction value acquisition unit  602  acquires the correction values. In other words, the traverse angle θ c  of the capsule endoscope  10  and the traverse angle θ m  of the external permanent magnet  41  may also be excluded from the lookup table or the input values in the functions for acquiring the correction values by the correction value acquisition unit  602 . 
     Therefore, in that case, as expressed by the following formulas (3a) to (3e), the correction value acquisition unit  602  acquires the correction values of the position and the posture of the capsule endoscope  10  by employing, as input values, the coordinates z c , z m  in a vertical direction of the capsule endoscope  10  and the external permanent magnet  41 , as well as the elevation angles φ c , φ m  of the capsule endoscope  10  and the external permanent magnet  41  only. 
       Δ x=f   x ( z   c ,φ c   ,z   m ,φ m )  (3a)
 
       Δ y=f   y ( z   c ,φ c   ,z   m ,φ m )  (3b)
 
       Δ z=f   z ( z   c ,φ c   ,z   m ,φ m )  (3c)
 
       Δφ =f   φ ( z   c ,φ c   ,z   m ,φ m )  (3d)
 
       Δθ =f   θ ( z   c ,φ c   ,z   m ,φ m )  (3e)
 
     When the calculation device  60  corrects either one of the position and the posture of the capsule endoscope  10 , the correction value acquisition unit  602  extracts only the correction values for the one to be corrected. 
     As described above, according to the second embodiment of the present disclosure, in addition to a similar effect to that in the first embodiment, it is possible to reduce the number of input values used when acquiring the correction values and to reduce a calculation load by using the relative positional relationship between the capsule endoscope  10  and the generation source of the interference magnetic field, and the symmetry of the shape of the generation source of the interference magnetic field. 
     Modification 
     Next, a modification of the second embodiment of the present disclosure will be described. A case is considered where, as illustrated in  FIG. 8 , an external permanent magnet  44  is used which has a rotationally symmetric shape about an axis orthogonal to a magnetization direction. In  FIG. 8 , the external permanent magnet  44  has a columnar shape. An arrow M 3  in  FIG. 8  indicates the magnetization direction of the external permanent magnet  44 . In that case, due to the symmetry of the shape of the external permanent magnet  44 , the influence of the interference magnetic field on a position-detecting magnetic field does not change even if the external permanent magnet  44  is rotated about the central axis a of rotational symmetry thereof. Therefore, the elevation angle φ c  of the capsule endoscope  10  and the elevation angle φ m  of the external permanent magnet  44  may be excluded from the input values employed when the correction value acquisition unit  602  acquires the correction values. In other words, the elevation angle φ c  of the capsule endoscope  10  and the elevation angle φ m  of the external permanent magnet  44  may also be excluded from the lookup table or the input values in the functions for acquiring the correction values by the correction value acquisition unit  602 . 
     Therefore, the correction value Δz of the position of the capsule endoscope  10  may be acquired as a correction value only with the coordinates z c , z m  in the vertical direction of the capsule endoscope  10  and the generation source of the interference magnetic field. Furthermore, when the capsule endoscope  10  is floating in liquid in the subject  20  (see  FIG. 1 ), the coordinate z c  in the vertical direction of the capsule endoscope  10  is determined by gravity, buoyancy, and a magnetic attracting force depending on the distance from the external permanent magnet  44 , which act on the capsule endoscope  10 . Therefore, in that case, as expressed by the following formulas (4a) to (4e), the correction value acquisition unit  602  acquires the correction values of the position and the posture of the capsule endoscope  10  by employing only the coordinate z m  in the vertical direction of the external permanent magnet  44  as an input value. 
       Δ x=f   x ( z   m )  (4a)
 
       Δ y=f   y ( z   m )  (4b)
 
       Δ z=f   z ( z   m )  (4c)
 
       Δφ =f   φ ( z   m )  (4d)
 
       Δθ =f   θ ( z   m )  (4e)
 
     As an example, the following formula (5) indicates a correction formula used when the correction value acquisition unit  602  corrects the position of the capsule endoscope  10  when the coordinate z m  in the vertical direction of the external permanent magnet  44  as the generation source of the interference magnetic field is employed as an input value. 
     
       
         
           
             
               
                 
                   
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     The left-hand side of the formula (5) indicates the position in the three-dimensional space of the capsule endoscope  10  after the correction. The first term on the right-hand side of the formula (5) indicates the position in the three-dimensional space of the capsule endoscope  10  before the correction, that is, the position calculated from the detection signals output from the plurality of detection coils C n . The second term on the right-hand side of the formula (5) indicates correction values (Δx, Δy, Δz) employing the coordinate z m  in the vertical direction of the external permanent magnet  44  as an input value (variable). 
     Of the second term on the right-hand side of the formula (5), a matrix of three rows and seven columns is a matrix indicating correction coefficients. Values of respective elements k xj , k yj , k zj  (j=0, 1, 2, 3, 4, 5, 6) of this matrix indicating the correction coefficients are illustrated in  FIG. 9 . Each value illustrated in  FIG. 9  was obtained by a simulation. Of the second term on the right-hand side of the formula (5), the column vector of seven rows and one column is a base vector of a seven-dimensional space configured by using the coordinate z m . The correction value acquisition unit  602  calculates the correction values (Δx, Δy, Δz) of the position of the capsule endoscope  10  by performing a calculation with which a matrix indicating the correction coefficients is operated on this column vector. 
     (a) to (c) of  FIG. 10  are graphs illustrating a relationship between the coordinate z m  in the vertical direction of the external permanent magnet  44  and coordinates (x s , y s , z s ) of the capsule endoscope  10  before the correction, as well as the coordinate z m  in the vertical direction of the external permanent magnet  44  and the coordinates (x c , y c , z c ) of the capsule endoscope  10  after the correction for each of X-, Y-, and Z-directions. As illustrated in (a) to (c) of  FIG. 10 , it may be seen that, as the coordinate z m  in the vertical direction of the external permanent magnet  44  increases, that is, as the external permanent magnet  44  approaches the capsule endoscope  10 , the influence of the interference magnetic field increases, and error in the position of the capsule endoscope  10  before the correction increases. 
     Third Embodiment 
     Next, a third embodiment of the present disclosure will be described.  FIG. 11  is a schematic view illustrating a partial configuration of a position detection system according to the third embodiment of the present disclosure. The configuration of the position detection system according to the third embodiment is similar to that in the first embodiment as a whole (see  FIGS. 1 to 3 ), and the shape of a support member which supports an external permanent magnet  41  is different from that in the first embodiment. 
     As illustrated in  FIG. 11 , a guidance magnetic field generating device  40 A according to the third embodiment includes a support member  45  which is capable of translating in a three-dimensional space and supports the external permanent magnet  41  rotatably about a central axis a and a vertical axis b. In  FIG. 11 , a rotation mechanism which rotates the external permanent magnet  41  in the support member  45  is omitted. 
     The support member  45  includes a disc-shaped plate material  451  and a frame  452  fixed to the plate material  451 . The frame  452  has a plurality of (four in  FIG. 11 ) support columns  453 , each of the support columns  453  extending along a vertical direction, and an annular member  454  supported above the plate material  451  by these support columns  453 . The whole support member  45  including the plate material  451  and the frame  452  is formed to be rotationally symmetric about a central axis in the vertical direction. In the case illustrated in  FIG. 11 , this central axis coincides with the vertical axis b. 
     The plate material  451  and the frame  452  are formed of a conductor such as metal. Therefore, the support member  45  may be a generation source of an interference magnetic field. 
     Since the frame  452  on an upper surface and side surfaces of the support member  45  does not cover the periphery of the external permanent magnet  41 , a guidance magnetic field generated by the external permanent magnet  41  is not shielded by the support member  45 , and is generated also in the detection target region R (see  FIG. 1 ). Therefore, by translating the external permanent magnet  41  in the three-dimensional space via the support member  45  and by rotating the external permanent magnet  41  inside the support member  45 , the capsule endoscope  10  may be guided by the guidance magnetic field. 
     The annular member  454  of the frame  452  is arranged so as to be located close to detection coils C n  in comparison with the external permanent magnet  41 . Accordingly, for the position-detecting magnetic field at the position of the detection coils C n , the influence of the interference magnetic field by the frame  452  is dominant. Therefore, even if the external permanent magnet  41  rotates about the central axis a or the vertical axis b inside the frame  452 , the rotation of the external permanent magnet  41  hardly affects detection signals output by the detection coils C n . In that case, it is possible to exclude the elevation angle φ m  and the traverse angle θ m  of the external permanent magnet  41  from the input values employed when the correction value acquisition unit  602  acquires the correction values. In other words, the elevation angle φ m  and the traverse angle θ m  of the external permanent magnet  41  may be excluded from a lookup table or the input values in functions for acquiring the correction values by the correction value acquisition unit  602 . 
     As a result, in a case of using the support member  45 , it is possible to reduce the lookup table or the input values in the functions used by the correction value acquisition unit  602  when acquiring the correction values in Step S 15  of  FIG. 6  to five values (x c , y c , z c , φ c , θ c ) which indicate the position and the posture of the capsule endoscope  10 , and to three values (x m , y m , z m ) which indicate the position of the external permanent magnet  41  (the support member  45 ). 
     As described above, according to the third embodiment of the present disclosure, since the support member  45  formed of a conductor serving as a generation source of the interference magnetic field is intentionally disposed as a support member for supporting the external permanent magnet  41 , and the external permanent magnet  41  is rotated inside the support member, it is possible to reduce the number of input values used when acquiring the correction values and to reduce a calculation load. 
     Modification 
     Next, a modification of the third embodiment of the present disclosure will be described. As described above, when the capsule endoscope  10  is floating in liquid in a subject  20  (see  FIG. 1 ), the capsule endoscope  10  is usually constrained by the guidance magnetic field vertically above the external permanent magnet  41 , and moves following translational motion of the external permanent magnet  41  in the horizontal plane. In that case, in the horizontal plane, the coordinates (x c , y c ) of the capsule endoscope  10  and the coordinates (x m , y m ) of the external permanent magnet  41  become substantially equal, and error in the position due to the influence of the interference magnetic field hardly occurs. Therefore, in comparison with the third embodiment, the coordinates (x m , y m ) in the horizontal plane of the external permanent magnet  41  (the support member  45 ) may be further excluded from the input values employed when acquiring the correction values. That is, the input values regarding the external permanent magnet  41  may be reduced to be only the coordinate z m  in the vertical direction. 
     Fourth Embodiment 
     Next, a fourth embodiment of the present disclosure will be described.  FIG. 12  is a schematic view illustrating a partial configuration of a position detection system according to the fourth embodiment of the present disclosure. The configuration of the position detection system according to the fourth embodiment is similar to that in the first embodiment as a whole (see  FIGS. 1 to 3 ), and the shape of a support member which supports an external permanent magnet  41  is different from that in the first embodiment. 
     As illustrated in  FIG. 12 , a guidance magnetic field generating device  40 B according to the fourth embodiment includes a support member  46  which is capable of translating in a horizontal plane and supports the external permanent magnet  41  rotatably about a central axis a and a vertical axis b and translatably in a vertical direction. In  FIG. 12 , a rotating mechanism which rotates the external permanent magnet  41  and a moving mechanism which moves the external permanent magnet  41  in the vertical direction in the support member  46  are omitted. 
     As with the case of the support member  45  illustrated in  FIG. 11 , the support member  46  includes a disc-shaped plate material  461  and a frame  462  fixed to the plate material  461 , and is formed to be rotationally symmetric about a central axis in the vertical direction. In the case illustrated in  FIG. 12 , this central axis coincides with the vertical axis b. The frame  462  has a plurality of (four in  FIG. 12 ) support columns  463 , each of the support columns  463  extending along the vertical direction, and an annular member  464  supported above the plate material  461  by these support columns  463 . The length of each of the support columns  463  is longer than that of the support columns  453  illustrated in  FIG. 11 , and the external permanent magnet  41  may move in the vertical direction within a range of the length of the support columns  463 . The annular member  464  is arranged so as to be located close to detection coils C n  in comparison with the external permanent magnet  41 . 
     The plate material  461  and the frame  462  are formed of a conductor such as metal. Therefore, the support member  46  may be a generation source of an interference magnetic field. 
     In the fourth embodiment, the support member  46  is translated only in the horizontal plane while the height in the vertical direction is fixed. Consequently, the annular member  464 , which is the generation source of the interference magnetic field which has a dominant influence on a position-detecting magnetic field at the positions of the plurality of detection coils C n , has a constant height. Therefore, even if the external permanent magnet  41  moves in the vertical direction, or rotates about the central axis a or the vertical axis b inside the support member  46 , the movement and the rotation of the external permanent magnet  41  hardly affect detection signals output from the plurality of detection coils C n . In that case, the correction value acquisition unit  602  may exclude the coordinate z m  in the vertical direction, the elevation angle φ m , and the traverse angle θ m  of the external permanent magnet  41  from the input values employed when acquiring the correction values. In other words, the coordinate z m  in the vertical direction, the elevation angle φ m , and the traverse angle θ m  of the external permanent magnet  41  may be excluded from the lookup table or variables in the functions for acquiring the correction values by the correction value acquisition unit  602 . 
     As a result, in a case of using the support member  46 , it is possible to reduce the lookup table or the input values in the functions used when acquiring the correction values in Step S 15  of  FIG. 6  to five values (x c , y c , z c , φ c , θ c ) which indicate the position and the posture of the capsule endoscope  10  and to two values (x m , y m ) which indicate the position of the external permanent magnet  41  (the support member  46 ) in the horizontal plane. 
     As described above, according to the fourth embodiment of the present disclosure, the support member  46  formed of a conductor serving as a generation source of the interference magnetic field is intentionally disposed as a support member for supporting the external permanent magnet  41 , and the external permanent magnet  41  is rotated and moved in the vertical direction inside the support member, and thereby it is possible to further reduce the number of input values used when acquiring the correction values and to reduce a calculation load. 
     Modification 
     Next, a modification of the fourth embodiment of the present disclosure will be described. As described above, when the capsule endoscope  10  is floating in liquid in a subject  20  (see  FIG. 1 ), the capsule endoscope  10  is usually constrained by a guidance magnetic field vertically above the external permanent magnet  41 , and moves following translational motion of the external permanent magnet  41  in the horizontal plane. Consequently, error in the position due to the influence of the interference magnetic field hardly occurs in the horizontal plane. Therefore, in comparison with the fourth embodiment, the coordinates (x m , y m ) in the horizontal plane of the external permanent magnet  41  (the support member  46 ) may be further excluded from the input values employed when acquiring the correction values. That is, it is possible to acquire the correction values only with the position and the posture of the capsule endoscope  10 . 
     The first to fourth embodiments of the present disclosure described above and the variations thereof are merely examples for carrying out the present disclosure, and the present disclosure is not limited thereto. In addition, the present disclosure may make various disclosures by appropriately combining a plurality of constituent elements disclosed in the above-mentioned first to fourth embodiments and the variations thereof. It is obvious from the above description that the present disclosure may be variously modified in accordance with specifications and the like, and that various other embodiments are possible within the scope of the present disclosure. 
     According to the present disclosure, since a guidance magnetic field generating device is at least partially formed of a conductor, and at least one of a position and a posture of a detection target is calculated using at least one of a position and a posture of the conductor, it is possible to accurately detect the position and the posture of the detection target based on a position-detecting magnetic field generated by the detection target even when a position or a posture of a generation source of an interference magnetic field changes. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the disclosure in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.