Abstract:
An ultrasonic probe is provided which may be employed in medical applications to radiate an ultrasonic wave for inspecting the interior of a patient&#39;s body noninvasively. The ultrasonic probe includes a scan mechanism which consists of a rotating mechanism and a swinging mechanism. The rotating mechanism is designed to rotate a cylindrical holder having installed thereon a piezoelectric element which emits an ultrasonic wave and receives the echo. The swinging mechanism is designed to swing a rotary base which supports the rotating mechanism to swing the cylindrical holder about an axis of rotation extending perpendicular to that of the piezoelectric element.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to an ultrasonic probe which is used in, for example, medical applications to radiate an ultrasonic wave for inspecting the interior of a patient&#39;s body noninvasively, and more particularly to an improved structure of a scan mechanism of an ultrasonic probe designed to rotate a piezoelectric element emitting the ultrasonic wave. 
     2. Background Art 
     Japanese Patent First Publication No. 5-337108 teaches a scan mechanism for an ultrasonic probe. The scan mechanism includes a rotor having a piezoelectric element installed thereon and two electric motors one of which rotates the piezoelectric element about a first axis and other of which swings the piezoelectric element about a second axis perpendicular to the first axis. 
     The scan mechanism is, however, complex and bulky in structure and encounters the drawback in that it is difficult to use the ultrasonic probe in a small cavity of a person&#39;s body, for example. 
     SUMMARY OF THE INVENTION 
     It is therefore a principal object of the present invention to avoid the disadvantages of the prior art. 
     It is another object of the present invention to provide a simple, lightweight, and inexpensive structure of a scan mechanism of an ultrasonic probe designed to rotate a piezoelectric element emitting an ultrasonic wave. 
     According to one aspect of the invention, there is provided an ultrasonic probe which comprises: (a) a piezoelectric element emitting an ultrasonic wave and receiving an echo thereof, the piezoelectric element converting the echo into an electric signal; (b) a holding member holding the piezoelectric element; (c) a rotating mechanism rotating the holding member about a first axis; (d) a rotary base supporting the rotating mechanism; (e) a chassis supporting the rotary base so as to allow the rotary base to swing about a second axis extending in a direction different from the first axis; and (f) a swinging mechanism swinging the rotary base. 
     In the preferred mode of the invention, the rotating mechanism includes a holding member angular position measuring encoder designed to measure an angular position of the holding member. 
     The holding member angular position measuring encoder may include a first encoder unit and a second encoder unit each of which is made of a magnetic member and a magnetoresistive element. The first encoder unit is designed to measure a change in angular position of the holding member by rotation about the first axis. The second encoder unit is designed to measure a preselected reference angular position of the holding member. 
     The rotating mechanism includes a holding member rotating electric motor rotating the holding member made of a rotary cylinder, a conductive cylinder arranged in alignment of a central axis with a central axis of the rotary cylinder, and a conductive brush installed on the rotary base in contact with the conductive cylinder. 
     The rotating mechanism may alternatively include a holding member rotating electric motor rotating the holding member made of a rotary cylinder, a first coil installed on the rotary cylinder, and a second coil installed on the rotary base so as to face the first coil. The first coil is responsive to a change in magnetic flux of the second coil to produce an electric signal for energizing the piezoelectric element. 
     The swinging mechanism includes a rotary base swinging electric motor, an motor angular position measuring encoder measuring an angular position of the rotary base swinging electric motor, and a gear train transmitting output torque of the rotary base swinging electric motor to the rotary base to swing the rotary base. 
     The swinging mechanism may alternatively include a voice coil motor and a rotary base angular position measuring encoder which measures an angular position of the rotary base. The voice coil motor is implemented by a magnet installed on one of the rotary base and the chassis and an electric coil installed on the other of the rotary base and the chassis. 
     A holding mechanism may also be provided which is designed to hold the rotary base on the chassis. 
     A second piezoelectric element may further be installed on the holding member which produces an ultrasonic wave having the same frequency of that produced by the piezoelectric element. 
     The second piezoelectric element may alternatively be designed to produce an ultrasonic wave having a frequency different from that produced by the piezoelectric element. 
     A magnetoresistive element may also be provided which is responsive to a change in magnetic flux produced by the magnetic member of the second encoder to provide a signal indicative of a neutral position of the holding member in swinging motion of the holding member. 
     An array of magnetoresistive elements may also be installed on the chassis. Each of the magnetoresistive elements is responsive to a change in magnetic flux produced by the magnetic member of the second encoder to provide a signal indicative thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
     FIG. 1 a cutaway perspective view which shows an ultrasonic probe according to the first embodiment of the invention; 
     FIG. 2 is a perspective view which shows a rotor motor installed in the ultrasonic probe of FIG. 1; 
     FIG.  3 ( a ) is a side view which shows a side of the rotor motor of FIG. 2; 
     FIG.  3 ( b ) is a side view which shows a side of the rotor motor opposed to FIG.  3 ( a ); 
     FIG. 4 is a sectional view which shows an internal structure of the rotor motor of FIG. 2; 
     FIG. 5 is an illustration which shows output signals from an encoder measuring an angular position of a piezoelectric element; 
     FIG. 6 is a perspective view which shows a rotor motor according to the second embodiment of the invention; 
     FIG. 7 is a perspective view which shows a rotor motor according to the third embodiment of the invention; 
     FIG.  8 ( a ) is a partially sectional view which shows a lock mechanism used in the rotor motor of FIG. 7; 
     FIG.  8 ( b ) is a partially sectional view which shows a side of the lock mechanism opposed to FIG.  8 ( a ); 
     FIG. 9 is a partially sectional view which shows a rotor motor according to the fourth embodiment of the invention; 
     FIG. 10 is a partially sectional view which shows a rotor motor according to the fifth embodiment of the invention; 
     FIG. 11 is an illustration which shows a measurement range of an ultrasonic probe of the fifth embodiment of the invention; 
     FIG. 12 is a perspective view which shows a rotor motor according to the sixth embodiment of the invention; 
     FIGS.  13 ( a ) and  13 ( b ) show waveforms of signals outputted from an encoder in the sixth embodiment of the invention; 
     FIG. 14 is a perspective view which shows a rotor motor according to the seventh embodiment of the invention; and 
     FIGS.  15 ( a ) and,  15 ( b ) and  15 ( c ) show waveforms of signals outputted from an encoder in the seventh embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like numbers refer to like parts in several views, particularly to FIG. 1, there is shown an ultrasonic probe  1  according to the first embodiment of the invention which may be employed in an ultrasonic diagnostic system designed to view the interior of a patient noninvasively. 
     The ultrasonic probe  1  includes a rotor motor  2 , an electric signal transmitting wire  3 , a drive shaft  4 , a second encoder  170  (a first encoder will be described later), a second electric motor  160  (a first electric motor will be described later), a motor bracket  15 , flexible joint  14 , a pipe  5 , a joint frame  6 , a housing  7 , a shaft holder  16 , a window  8 , and an oil seal  9 . 
     The rotor motor  2  works to scan an ultrasonic wave. The second electric motor  160  rotates the drive shaft  4 . The second encoder  170  measures a rotational angle of the drive shaft  4 . The motor bracket  15  supports the second electric motor  160 . The flexible joint  14  transmits the torque output of an output shaft of the second electric motor  160  to the drive shaft  4 . The pipe  5  retains the rotor motor  2  and covers the drive shaft  4 . The joint frame  6  supports the pipe  5 . The housing  7  is fixed on the joint frame  6 . The shaft holder  16  is fixed on the joint frame  6 . The window  8  covers the rotor motor  2  and outputs the ultrasonic wave therethrough. 
     The inside of the window  8  is filled with a coupling liquid  10  in which the degree of attenuation of the ultrasonic wave is low. The oil seal avoids leakage of the coupling liquid  10  from a clearance between the drive shaft  4  and the pipe  5 . For the brevity of illustration, the window  8 , the shaft holder  16 , the housing  7 , the joint frame  6 ,and the pipe  5  are partially cut away in FIG.  1 . 
     FIG. 2 shows the structure of the rotor motor  2 . For the convenience of illustration, a chassis  11  is partially cut away. 
     The rotor motor  2  includes the first electric motor  110 . The first electric motor  110  has an outer cylinder  111  on which magnetic members  121  and  122  are installed. The magnetic member  121  is, as will be described later in detail, magnetized to have a given magnetic pattern for measuring an angular change of the outer cylinder  111 . The magnetic member  122  is magnetized for determining a reference angular position of the outer cylinder  111 . The outer cylinder  111  has formed thereon a mount  118  for mounting a piezoelectric element  130  and installed thereon a conductive cylinder  150  in alignment of the center thereof with an axis of rotation of the outer cylinder  111 . The rotor motor  2  also includes a rotary base  140  which has formed therein a hole  141  for holding a central axis of the first electric motor  110  and installed thereon a magnetoresistive element  123  (i.e., a magnetroresistor) sensitive to a change in magnetic flux of the magnetic member  121  of the first encoder  120 , a magnetoresistive element  124  sensitive to a change in magnetic flux of the magnetic member  122  of the first encoder  120 , a conductive brush  151 , and a first spur gear  142 . On the chassis  11 , a second spur gear  143  and a first bevel gear  144 , as shown in FIGS.  3 ( a ) and  3 ( b ) are installed rotatably. The second spur gear  143  meshes with the first spur gear  142 . The first bevel gear  144  is arranged coaxially with the second spur gear  143  and meshes with a second bevel gear  145  coupled to the drive shaft  4 . The rotary base  140  has formed thereon arc-shaped protrusions or rails  146  and  147  which have a trapezoidal section and slidably engage grooves  12  and  13  formed in the chassis  11  to support the rotary base  140  so that it may swing. 
     Internally, the first electric motor  110 , as shown in a sectional view of FIG. 4, includes an electric coil  112 , permanent magnets  113  and  1   14 , a central shaft  115 , and bearings  116  and  117 . The electric coil  112  is attached to the central shaft  115 . The bearings  116  and  117  supports the outer cylinder  111  rotatably on the central shaft  115 . The permanent magnets  113  and  114  are installed in the outer cylinder  111 . 
     FIG. 5 shows the waveform of signals outputted by the first encoder  120 . The first encoder  120 , as described above, consists of a first encoder unit designed to measure an angular change of the outer cylinder  111  caused by rotation of the outer cylinder  111  and a second encoder unit designed to detecting the reference angular position of the outer cylinder  111 . The first encoder unit is made of the magnetic member  121  and the magnetoresistive element  123  which is sensitive to the magnetic pattern provided on the magnetic member  121  to produce electric signals, as discussed below. Similarly, the second encoder unit is made of the magnetic member  122  and the magnetoresistive element  124  which is magnetically responsive to the magnetic member  122  to produce an electric signal as discussed below. Specifically, the first encoder  120 , as clearly shown in the drawing, outputs three signals: angular signals  181  and  182  and a reference angular signal  183 . When the outer cylinder  111  is rotated about the central shaft  115 , it will cause the angular signals  181  and  182  to change in level cyclically as shown in the drawing. A phase difference between the angular signals  181  and  182  indicates the direction of rotation of the outer cylinder  111 . The number of pulses of each of the angular signals  181  and  182  represents a change in angular position of the outer cylinder  111 . The reference angular signal  183  is produced whenever the second encoder unit reaches a preselected reference angular position, that is, whenever the magnetic member  122  passes the magnetoresistive element  124 . The first encoder  120  resets the measured angular change of the outer cylinder  111  whenever the reference angular signal  183  is detected. In FIG. 5, the signals  181 ,  182 , and  183  are illustrated as having different signal levels for convenience, however, these signal levels are, in fact, identical with each other. 
     In operation, an electric signal produced by an ultrasonic diagnostic system (not shown) is inputted in the form of a pulse signal to the conductive brush  151  installed on the rotary base  140  of the rotor motor  2  through the wire  3 . The conductive brush  151  is in contact with the conductive cylinder  150  installed on the outer cylinder  111 , so that the electric signal inputted to the conductive brush  151  is transmitted to the conductive cylinder  150 . The conductive cylinder  150 , although not shown in the drawings, is electrically connected to the piezoelectric element  130 , so that the electric signal inputted to the conductive cylinder  150  is further transmitted to the piezoelectric element  130 . The piezoelectric element  130  converts the inputted electric signal into oscillations to produce an ultrasonic wave and radiates it to the outside. If there is a reflective object in a propagation path of the ultrasonic wave, the ultrasonic wave is returned to the piezoelectric element  130 . The piezoelectric element  130  converts the echo of the ultrasonic wave into an electric signal and outputs it to the ultrasonic diagnostic system through the conductive brush  151 , the conductive cylinder  150 , and the wire  130 . The ultrasonic diagnostic system converts the input signal into an image signal using known imaging techniques. 
     During the radiation, the ultrasonic wave is scanned over  360 ° to produce an ultrasonic tomogram (i.e., an ultrasonogram) of the object. The scanning is achieved by energizing the electric coil  112  of the first electric motor  110  to rotate the outer cylinder  111  about the central shaft  115  (i.e., the x-axis in FIG.  2 ), thereby rotating the piezoelectric element  130 . The angular position of the outer cylinder  111  is measured by the first encoder  120  to provide an angular position signal to the ultrasonic diagnostic system as indicating the position of a scanned portion of the object on the ultrasonic tomogram. 
     When it is required to shift the radiation of the ultrasonic wave (i.e., the scanned area of the object) in a direction perpendicular to the rotation of the outer cylinder  111 , the second electric motor  160  is actuated to provide torque to the second bevel gear  145  through the drive shaft  4 . The torque is subsequently transmitted to the first bevel gear  144  and to the first spur bear  142  through the second spur gear  143 . The first spur gear  142  is installed on the rotary base  140 , so that the rotary base  140  swings about the x-axis extending, as shown in FIG. 2, perpendicular to the y-axis along guide paths defined by slidable engagement of the rails  146  and  147  with the grooves  12  and  13  formed in the chassis  11 . The angular position of the drive shaft  4  is measured by the second encoder  170  to provide an angular position signal to the ultrasonic diagnostic system as indicating the angular position of the rotary base  140  on the guide path. 
     As apparent from the above discussion, the first embodiment of the invention provides a simple and small-sized structure of the rotor motor  2  to enable the ultrasonic probe  1  to be constructed which is capable of producing an ultrasonic monogram of an object and measures the angular position of the head of the probe  1  with high and confidence levels. 
     FIG. 6 shows a rotor motor  2 A according to the second embodiment of the invention. The same reference numbers as employed in the first embodiment refer to the same parts, and explanation thereof in detail will be omitted here. 
     The rotor motor  2 A is different from the rotor motor  2  of the first embodiment only in that the outer cylinder  111  has installed thereon the first coil  200  connected to the piezoelectric element  130 , and the rotary base  140  has installed thereon the second coil  201  facing the first coil  200  with a given gap. 
     In operation, the ultrasonic diagnostic system provides a pulse signal of 80V to 200V to the second coil  201  through the wire  3 . This causes the magnetic flux of the second coil  201  to be changed suddenly, thereby resulting in production of voltage at the first coil  200 . The first coil  200  is, as described above, connected to the piezoelectric element  130 , so that the voltage produced at the first coil  200  is applied to the piezoelectric element  130 , thereby causing pulses to be radiated in the form of an ultrasonic wave. When the echo of the ultrasonic wave reaches the piezoelectric element  130 , the piezoelectric element  130  converts the input into an electric signal in the form of a pulse signal outputs it to the first coil  200 , so that the magnetic flux thereof is changed suddenly, thereby causing the second coil  201  to produce the voltage. The voltage is transmitted to the ultrasonic diagnostic system through the wire  3  and converted into an image signal. 
     The rotor motor  2 A of this embodiment is, as apparent from the above, designed to establish signal transmission between a rotary member and a stationary member without mechanical sliding motion, thereby reducing the wear of parts of the ultrasonic probe. 
     FIG. 7 shows a rotor motor  2 B according to the third embodiment of the invention. The same reference numbers as employed in the first embodiment refer to the same parts, and explanation thereof in detail will be omitted here. 
     The rotary base  140 , like the first embodiment, has formed thereon the arc-shaped rails  146  and  147  which engage, as shown in FIGS.  8 ( a ) and  8 ( b ), the grooves  12  and  13  formed in the chassis  11  slidably. The rotary base  140  has a magnet  210  installed thereon. The chassis  11  has installed thereon an electric coil  211  which forms a voice coil motor together with the magnet  210 . The rotary base  140  has disposed thereon a magnetic member  222  magnetized to have a given magnetic pattern. The chassis  11  has disposed thereon a magnetoresistive element  224  which is sensitive to a change in magnetic field produced by the magnetic member  222 . The rotary base  140  has also installed thereon a magnetic member (not shown) which is identical with the magnetic member  222 . The chassis  11  has also installed thereon a magnetoresistive element (not shown) which is identical with the magnetoresistive element  224  and which measures a preselected neutral position of the rotary base  140  in the swing thereof in the same manner as that of the first encoder  120  in the first embodiment. These magnetic members and magnetoresistive elements form a swing angle measuring encoder. The rotary base  140  has, as shown in FIGS.  8 ( a ) and  8 ( b ), formed therein a hole  148  into which a lever  301  is inserted to hold he rotary base  140  in the chassis  11  fixedly. The lever  301  is urged by a coil spring  302  into constant engagement with the hole  148  and moved out of the engagement by a solenoid  300 . 
     In operation, when the solenoid  300  is energized, it will cause the lever  301  to be attracted downward, as viewed in FIGS.  8 ( a ) and  8 ( b ), into disengagement from the hole  148 , thereby allowing the rotary base  140  to rotate. 
     Subsequently, when the coil  211  disposed within a magnetic field produced by the magnet  210  is energized, it produces a magnetic force. The coil  211  is fixed on the chassis  11 , so that a reactive force is produced which swings the rotary base  140  along the guide , paths defined by the rails  146  and  147  formed on the rotary base  140  and the grooves  12  and  13  formed in the chassis  11 . The angular position of the rotary base  140  changed by the swing is measured by the swing angle measuring encoder. When it is required to hold the rotary base  140  from swinging, the solenoid  300  is reenergized to urge the lever  301  into engagement with the hole  148  in the rotary base  140  with aid of the spring load of the coil spring  302  to lock the rotary base  140 . 
     The structure of the third embodiment is designed to swing the rotary base  140  without use of a gear train as employed in the second embodiment, thereby minimizing the wear of parts working to swing the rotary base  140 . Further, when not in use, the lever  301  is fitted within the hole  148  formed in the rotary base  140  to lock the rotary base  140 , thereby avoiding undesirable movement of the rotary base  140  during transportation of the ultrasonic probe, for example. 
     The electric coil  211  and the magnet  210  may alternatively be disposed on the chassis  11  and the rotary base  140 , respectively. 
     FIG. 9 shows a rotor motor  2 C according to the fourth embodiment of the invention which is different from the rotor motor  2  of the first embodiment only in that two piezoelectric elements  130  and  131  are installed on the outer cylinder  111  which produce oscillations at the same frequency. Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     The rotor motor  2 C may also use more than two piezoelectric elements for speeding up the acquisition of ultrasonograms. 
     FIG. 10 shows a rotor motor  2 D according to the fifth embodiment of the invention which is different from the rotor motor  2  of the first embodiment only in that two piezoelectric elements  130  and  132  are installed on the outer cylinder  111  which produce oscillations at different frequencies. Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     When the ultrasonic wave passes through an object, as the frequency of the ultrasonic wave increases, the resolution becomes fine, but the degree of attenuation increases. Therefore, the use of ultrasonic wave having a higher frequency, enables acquisition of a finer ultrasonogram, but the distance to an object which allows an ultrasonogram to be formed decreases. Conversely, the use of ultrasonic wave having a lower frequency results in an increase in distance to an object which allows an ultrasonogram to be formed, but the resolution of the ultrasonogram decreases. The rotor motor  2 D of this embodiment, as described above, uses the piezoelectric elements  130  and  132  producing oscillations having different frequencies and is designed to, as shown in FIG. 11, rotate the outer cylinder  111  in a direction as indicated by an arrow  133  to scan an angular area  134  using the higher frequency piezoelectric element  130  and an angular area  135  using the lower frequency piezoelectric element  132 . Specifically, an ultrasonogram of a nearby portion of an object is derived by use of an ultrasonic wave having a higher frequency, thereby increasing the resolution of the ultrasonogram, while an ultrasonogram of a distance portion of the object is derived by use of an ultrasonic wave having a lower frequency, thereby increasing the distance the ultrasonic wave travels. 
     The rotor motor  2 D may also use more than two piezoelectric elements producing oscillations having different frequencies. 
     FIG. 12 shows a rotor motor  2 E according to the sixth embodiment of the invention which is different from the rotor motor  2  of the first embodiment only in that a magnetroresistive element  400  is installed on the chassis  111  which is magnetically responsive to the magnetic member  122  to detect a neutral position of the outer cylinder  111  in swing of the rotary base  140  about the y-axis. Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     FIG.  13 ( a ) shows the waveform of an output signal of the magnetoresistive element  400  during a 360° rotation of the outer cylinder  111  when the rotary base  140  is located at a swing angle of zero (i.e., the neutral position). FIG.  13 ( b ) shows the waveform of an output signal of the magnetoresistive element  400  during a 360° rotation of the outer cylinder  111  when the rotary base  140  is swung along the guide paths, as described in the first embodiment. When the swing angle of the rotary base  140  is zero (0° ), that is, when the rotary base  140  is in the neutral position, a complete rotation of the outer cylinder  111  causes the magnetic member  122  to pass the magnetoresistive element  400  disposed on the chassis  11  one time, thereby resulting in, as shown in FIG.  13 ( a ), a peak output  410  of the magnetoresistive element  400 . As the magnetic member  122  approaches the magnetoresistive element  400 , the value of the peak  410  becomes great. When the rotary base  140  is swung from the neutral position, the magnetic member  122  moves away from the magnetoresisitve element  400 , so that no peak is, as shown in FIG.  13 ( b ), produced. The measurement of the neutral position of the rotary base  140  is, thus, achieved by swinging the rotary base  140  and monitoring the value of the peak of an output of the magnetoresistive element  400 . 
     FIG. 14 shows a rotor motor  2 F according to the seventh embodiment of the invention which is a modification of the sixth embodiment in which three magnetoresistive elements  401 ,  402 , and  403  are so installed on the chassis  11  as to face the magnetic member  122  of the first encoder  120  used to detect the reference angular position of the outer cylinder  111 . Other arrangements are identical with those in the sixth embodiment, and explanation thereof in detail will be omitted here. 
     FIG.  15 ( a ) shows waveforms of output signals of the magnetoresistive elements  401 ,  402 , and  403  during a 360° rotation of the outer cylinder  111  when the rotary base  140  is located at a swing angle of zero (i.e., the neutral position).  401 A indicates the output of the magnetoresistive element  401 .  402 A indicates the output of the magentoresistive element  402 .  403 A indicates the output of the magnetoresistive element  403 . FIG.  15 ( b ) shows waveforms of output signals of the magnetoresistive elements  401 ,  402 , and  403  during a 360° rotation of the outer cylinder  111  when the rotary base  140  is swung toward the magnetoresistive element  401  from the neutral position.  401 B indicates the output of the magnetoresistive element  401 .  402 B indicates the output of the magentoresistive element  402 .  403 B indicates the output of the magnetoresistive element  403 . FIG.  15 ( c ) shows waveforms of output signals of the magnetoresistive elements  401 ,  402 , and  403  during a 360° rotation of the outer cylinder  111  when the rotary base  140  is swung toward the magnetoresistive element  403  from the neutral position.  401 C indicates the output of the magnetoresistive element  401 .  402 C indicates the output of the magentoresistive element  402 .  403 C indicates the output of the magnetoresistive element  403 . 
     In operation, the direction of rotation of the outer cylinder  111  is determined by monitoring the levels of peaks of the outputs from the magnetoresistive elements  401 ,  402 , and  403 . Specifically, when the level of the peak of the output from the magnetoresistive element  401  is, as indicated by  401 B in FIG.  15 ( b ), greater than those of the magentoresistive elements  402  and  403 , it is determined that the rotary base  140  being swinging close to the magnetoresistive element  401  from the neutral position. Alternatively, when the level of the peak of the output from the magnetoresistive element  403  is, as indicated by  403 C in FIG.  15 ( c ), greater than those of the magentoresistive elements  401  and  402 , it is determined that the rotary base  140  being swinging close to the magnetoresistive element  403  from the neutral position. When the level of the peak of the output from the magnetoresistive element  402  is, as indicated by  401 A in FIG.  15 ( a ), the greatest of the three, and the levels of the outputs of the magnetoresistive elements  401  and  403  are equal to each other, it is determined that the rotary base  140  is in the neutral position. 
     While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.