Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1998-12-10
2001-04-03
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207260, C128S899000, C600S424000
Reexamination Certificate
active
06211666
ABSTRACT:
TECHNICAL FIELD
Various systems have been proposed for detecting the position and/or orientation of an object using magnetic or electromagnetic fields. These systems typically employ field transmitters, such as electromagnet coils, disposed at known locations in a fixed reference frame and a sensor, such as a coil or other transducer mounted to the object to be located. Each transmitter projects a field varying in space in a fixed frame of reference. The pattern of variation in space for each transmitter is different than the pattern for each other transmitter. For example the transmitters may be identical to one another but disposed at different locations or in different orientations. The field patterns of the transmitters are thus displaced or rotated relative to one another and relative to the fixed frame of reference. The sensor on the object includes one or more sensing elements for detecting the parameters of the field prevailing at the location of the object as, for example, the magnitude and/or direction of the field at the object or the magnitudes of individual components of the field at the object in one or more preselected local directions defined with reference to the sensor. The transmitters may be actuated in a predetermined sequence so that at any time only one transmitter is active and therefore the field prevailing at the object is only the field contributed by one transmitter, plus a background field due to the Earth's magnetic field and other environmental sources. Based upon the detected parameters of the fields from the individual transmitters, and the known pattern of variation of the field from each transmitter, a computer system calculates the position and orientation of the sensor, and hence the position of the object bearing the sensor, in the fixed frame of reference of the transmitters. In a variant of this system, the object to be located carries the transmitter or transmitters, whereas a plurality of sensing elements are disposed at various locations or orientations in the fixed frame of reference. The location and/or orientation of the object is deduced from signals representing the parameters of the field prevailing at the various sensors.
Systems of this general nature are disclosed in U.S. Pat. Nos. 4,849,692; 4,642,786; 4,710,708; 4,613,866 and 4,945,305. Systems according to this general design can be used to provide a three-dimensional spatial input capability for a computer. Another system of this nature is disclosed in International Patent publication WO 94/04938. In the '938 publication, the object to be located may be a medical endoscope. Such a system may include a sensor mounted on the tip of an endoscope, so that the location and/or orientation of the endoscope tip can be determined while the sensor is disposed inside the body of a medical patient. Other systems for locating medical instruments such as endoscopes and catheters based upon transmitted fields are disclosed in U.S. Pat. Nos. 5,042,486; 5,099,845; 5,211,165; 5,251,635; 5,253,647; 5,255,680; 5,265,610 and 5,391,199.
Typical sensing elements have limited operating ranges. For example, a sensing element such as a magnetoresistive or Hall-effect device adapted to provide an electrical signal representing the magnitude of a magnetic field component in a particular direction typically provides the most accurate signals when the field component magnitude lies within a relatively narrow range. As described in copending, commonly-assigned United States Patent Application U.S. Ser. No. 08/476,380 (now issued as U.S. Pat. No. 5,729,129 on Mar. 17, 1998) and in PCT Application PCT/US96/08411, entitled MAGNETIC LOCATION SYSTEM WITH ADAPTIVE FEEDBACK CONTROL, the disclosures of which are hereby incorporated by reference herein, a magnetic location system may be provided with feedback control to adjust the operation of the field transmitters responsive to the signals from the sensing elements. For example, a locating system may incorporate several multielement sensors, each including several different sensing elements for detecting field components in several different directions. Each sensor may be mounted to a different object. For example, one sensor may be mounted to a medical instrument, whereas another sensor may be mounted to the patient's body, so that the system can track the locations of the instrument and the body simultaneously. Where a first sensor is close to a first field transmitting coil, and far from a second field transmitting coil, and both coils are driven in the same way to produce fields of equal overall magnitude, the magnitudes of all of the magnetic field components detected by the sensing elements will be higher during operation of the first coil than during operation of the second coil. For a similar second sensor disposed adjacent the second coil and remote from the first coil, the signals will be higher during operation of the second coil.
According to preferred embodiments of the '380 application, actuation of the coils is controlled in response to the signals from the sensing elements to keep the field components at the sensing element within desired ranges. Some systems disclosed in the '380 application operate cyclically. During each cycle of operation, each coil is actuated during a plurality of separate sensing intervals. During each sensing interval, one sensor is operated and the other sensors are inactive. For example, where a system includes two sensors 1 and 2, and three coils denominated A,B and C, the cycle may include a first sensing interval A
1
in which coil A is activated and sensor
1
is operated to acquire a signal, followed by a second sensing interval in A
2
in which coil A is activated and sensor
1
is operated, and so on so that the oval cycle includes intervals A
1
,A
2
,B
1
,B
2
,C
1
,C
2
. During each cycle, the signal acquired from sensor
1
in the previous cycle is used to adjust the currents applied to the various coils in intervals A
1
,B
1
and C
1
, whereas the signal acquired from sensor
2
in the previous cycle is used to adjust the currents applied in intervals A
2
,B
2
,C
2
. In this way, the magnitude of the magnetic field impinging on sensor
1
during intervals A
1
,B
1
,C
1
will be maintained within the operating range of sensor
1
, whereas the magnitude of the magnetic field impinging on sensor
2
during intervals A
2
,B
2
,C
2
will be maintained within the operating range of sensor
2
. A control computer system keeps track of the currents used during each cycle, and hence the magnitudes of the magnetic fields applied by each coil. This information is factored into the equations used to derive position and orientation information for each sensor from the readings of the sensing elements. The actual current applied to each coil during each sensing interval will vary with the position of the sensor associated with such interval.
Where more sensors are used, more sensing intervals can be added to the cycle. The number of sensing intervals in the entire cycle may be equal to the product of the number of sensors and the number of coils. Also, in a system according to a further refinement also taught in the '380 application, separate sensing intervals, and separate coil current settings, may be provided for each sensing element in a multielement sensor. Thus, the magnetic field strength provided by each coil is adjusted separately for each sensing element, so that the field component in the direction associated with a particular sensing element will be within the desired range of that element. In such a system, the number of sensing intervals can be equal to the number of individual sensing elements times the number of coils.
The use of feedback control according to these embodiments of the '380 application provides significant improvements in accuracy, and allows the use of sensors which have limited operating range but which provide other significant advantages such as compactness. However, appreciable time is required to increase the current in each coil to the desired curr
Biosense Inc.
Capezzuto Louis J.
Snow Walter E.
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