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BACKGROUND OF THE INVENTION
The following invention relates to an apparatus for locating the position
of an object inside a body and more particularly to a medical system for
locating the position of a feeding tube inserted into the stomach of a
patient to verify its position and thus prevent injury to the patient.
Enteral feeding tubes are used in certain applications with patients who
are incapable of feeding themselves by conventional means. These tubes are
frequently inserted through the nasal cavity and then into the stomach for
nasal gastric feeding. The distal end of such tubes is perforated to allow
nutrients to be delivered to the proper location once the tube has been
positioned. The tubes are typically 36-45 inches in length and require
proper placement within the body to insure that the nutrients will be
released at the correct location and to avoid damaging any tissue.
Frequently, tube placement is verified by X-ray which requires that barium
sulfate or some other radio opaque material be incorporated within the
tube material. Also, reference markings on the tubes are used as general
guidelines for the length of tube to be inserted for nasal gastric or
transpyloric tube feeding.
The verification of tube placement is critical to safe use of these devices
and the methods above have not always been satisfactory for this purpose.
Tube markings are themselves only a rough approximation of the actual
location of the end of the tube. Using X-rays to locate the tube brings
the attendant problem of harm to the patient from repeated exposure to
X-rays and the expense of a radiologist to administer them.
In the past, other solutions have been proposed and an example is shown in
the McCormick U.S. Pat. No. 4,431,005. In the McCormick patent a feeding
tube includes a piece of metal near the distal end of the tube which is
detected by a probe that generates a small local magnetic field. The field
is disturbed by the magnetically permeable metal in the tube and the
sensitivity of the probe can be adjusted so that an alarm indication is
provided when the probe is directly adjacent the metal inside the tube.
This technique takes advantage of the fact that the magnetic field is not
greatly affected by the biological tissue separating the probe from the
metal in the tube. A problem with the aforementioned device, however, is
that it may be difficult to find the distal end of the tube with the
probe. The device emits a signal only when the probe is directly over the
metal in the tubing and the tiny piece of metal may be difficult to locate
as the tube is being inserted into the patient. There is no direction
finding feature that tells the user when the probe is in the vicinity of
the metal on the end of the tube but not yet exactly centered. The system
is also highly susceptible to other metal objects which may be in the
vicinity such as surgical instruments, needles and the like. Metal objects
are commonly used in such situations and tend to confuse a probe which is,
in essence, a metal detector.
What is needed, therefore, is a position verifying device which can locate
an enteral feeding tube inside body tissue with a high degree of precision
so that the position of the end of the tube within the body can be known.
SUMMARY OF THE PRESENT INVENTION
The following invention provides a system for detecting the position of an
object within a body or mass of material, which may be biological body
tissue, and includes a resonant circuit attached to an object, such as the
distal end of an enteral feeding tube, where the resonant circuit receives
inductive energy of a certain frequency and rings in response thereto.
Outside the body tissue a transmitter is provided for transmitting
inductive energy, and a receiver is provided for detecting the ringing of
the resonant circuit and providing an alarm for indicating the location of
the object containing the resonant circuit inside the body tissue.
The system includes a probe positioned outside the body, and the probe may
include the transmitter. The transmitter is in the form of a coil that
transmits electromagnetic energy and the receiver may include pairs of
coils that are wound in series opposing fashion so that the location of
the resonant circuit may be determined with reference to the proximity to
one or the other of the coil pairs. In this way, coil pairs that are
oriented for positions which may be labeled arbitrarily "up/down" and
"right/left" may be superimposed over each other to create a four quadrant
pattern. Circuitry is provided that causes an alarm such as a
light-emitting diode array positioned on the probe to indicate which of
the four quadrants is in closest proximity to the resonant circuit. A
further coil, which may be termed a sensing coil, may be provided which is
a receiver coil that determines if the resonant circuit is close enough to
either of the series opposed receiver coil pairs to trigger the alarm. A
second LED array and associated circuitry is also provided for giving an
indication of when the probe is exactly centered over the resonant circuit
object.
The probe itself may be a battery powered receiver/transmitter having a
substantially flat cylindrical search head in which the alarm LEDs are
arranged in four quadrants on the outer peripheral surface of the
cylindrical search head, and in a cluster near the center of the search
head. In this way, passing the search head over the body results in an
appropriate LED lighting up which indicates generally the direction of the
resonant circuit object relative to the search head. Moving the search
head in this direction will eventually result in the four centering LEDs
lighting up which indicates that the resonant circuit object has been
located and that the center of the search head, termed a "marker window,"
is directly over it.
The system is a pulsed system and as such the transmitter coil may also be
used as the receiver sensing coil. The transmitter/sensing coil may
include a pair of series aiding coils which encircle respective receiver
coil pairs. The transmitter coil is pulsed to transmit energy and then it
is turned off. After a dead band in the system's cycle to allow transients
to die out, the receiver coils and the transmitter coil (now functioning
as a sensing coil) are activated and signals on these coils are sampled
over separate and discrete time windows. The signals are then demodulated
and compared to determine relative amplitude and polarity. The appropriate
LEDs are then activated in response to the demodulated coil signals thus
indicating the location of the target.
Unambiguous and omnidirectional detection of the target is provided through
the use of a target having three independent mutually orthogonal coils
with appropriate capacitors to provide minimal cross coupling. Two coils
are wound in mutually orthogonal planes about a small bobbin which also
houses the tiny capacitors, and a third coil is wound as a solenoid coil
about the outside of the bobbin. In this way the target can be detected
regardless of its spatial orientation inside the body.
Metal detection and noise resulting from the natural conductivity of
biological tissue is avoided by the use of alternate transmit and receive
windows with dead time provided between windows to allow most metallic
target responses and signals resulting from tissue capacitance or
conductivity to decay to near zero. This maximizes reception of the
resonant target ringing and provides ample noise rejection.
If desired, a multiple target system may be used with resonant circuit
targets having at least two differing frequencies and a search probe that
can toggle between the two or more different frequencies to provide an
even greater degree of directional orientation for an elongate object such
as an enteral feeding tube with multiple targets attached thereto.
A principal object of this invention is to provide a system for verifying
the position of an object inside a body of biological tissue or other mass
with a high degree of precision.
A further object of this invention is to provide an object locating system
for locating an object within a body of material which will discriminate
between the object and other nearby metal objects.
A still further object of this invention is to provide a probe having the
ability to search along the surface of a body or mass of material
containing a resonant circuit target to find and precisely locate the
position of the target.
A still further object of this invention is to provide a search head for
locating an object having inherent noise rejection capability which
focuses the detection circuitry on the object to be located and rejects
extraneous noise.
A still further object of this invention is to provide a resonant target
locator system that is a pulsed, multiplexed system that provides an
improved degree of noise rejection over similar systems that operate on a
continuous wave principle.
The foregoing and other objectives, features and advantages of the
invention will be more readily understood upon consideration of the
following detailed description of the invention, taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a probe including a search head that
operates according to the present invention.
FIG. 1A is a top view of a control panel located on the probe of FIG. 1.
FIG. 2 is an exploded perspective view of an enteral feeding tube
containing the resonant target objects to be located by the probe of FIG.
1.
FIG. 2A is an exploded perspective view of a tubular spacing member with
weights to be inserted therein.
FIG. 2B is a partial perspective view of a second embodiment of an enteral
feeding tube employing the invention.
FIG. 2C is a partial exploded perspective view of an alternate embodiment
of the resonant circuit targets to be affixed to an enteral feeding tube.
FIG. 2D is a partial perspective view of a feeding tube with resonant
circuit targets attached thereto.
FIG. 2E is a partial perspective view of an alternate embodiment to the
tube structure of FIGS. 2C and 2D.
FIG. 3 is a top view of the up/down receiver coils and one of the
transmitter/sensing coils located on an upper circuit board in the search
head shown in the probe in FIG. 1.
FIG. 4 is a top view of the left/right receiver coils and the other of the
transmitter/sensing coils located on a bottom circuit board in the search
head on the probe of FIG. 1.
FIG. 5 is a partially cutaway side view of a resonant circuit target as
used in the enteral feeding tube shown in FIG. 2.
FIG. 5A is an end view, partially cutaway, of the resonant circuit target
of FIG. 5.
FIG. 5B is an end cutaway view taken along line 5--5 of FIG. 5.
FIG. 6 is a schematic block diagram of the transmit/receive and signal
processing circuits housed in the probe of FIG. 1.
FIG. 6A is a block schematic diagram of a continued portion of the circuits
shown in FIG. 6.
FIG. 7 is a timing waveform diagram showing the transmit/receive cycle of
the probe of FIG. 1.
FIG. 8 is a waveform timing diagram showing the system cycle of the probe
of FIG. 1.
FIG. 9 is a waveform timing diagram illustrating a timing interruption
feature of the circuit of FIGS. 6 and 6A.
DETAILED DESCRIPTION OF THE INVENTION
An external probe 10 includes a search head 12 and a handle portion 14. The
handle portion 14 includes a control panel 16 and a rocker switch 18. The
rocker switch 18 is a search mode selector switch that has three
positions. In the center position the system is constantly resetting and
is not in a search mode. In the rear position the probe 10 is in a search
mode as long as the switch 18 is held down, but if the switch 18 is
toggled to the forward position it locks, holding the probe in a search
mode until the switch is moved once again to the center position.
The search head 12 includes a center aperture 20 which serves as a target
marking window. The marking window 20 is surrounded by four centering LEDs
22 which serve as pinpoint indicators to indicate that the object to be
located is centered directly within the marking window 20. Directional
indicators for the directions up, down, right and left, 24a, 24b, 24c and
24d, respectively, are positioned about the periphery of the search head
12. These indicators light up to indicate the direction that the search
head is to be moved in order to locate the target object within the
marking window 20.
Referring now to FIG. 2, there is shown a tube suitable for insertion into
biological tissue. An enteral feeding tube 111 includes a tubular member
113 having first and second ends 112a, 112b. A longitudinally extending
lumen 114 extends through the length of tubing with at least one feeding
port 26 extending through a wall of the tubular member. A coupling member
28 joins the tubular member 113 to an auxiliary tubing member 30. The
auxiliary tube member 30 contains two resonant circuit targets 32, 34
which are maintained in a spaced-apart relationship by a spacing member
such as tubular spacing member 36. The structure and operation of the
resonant circuit targets 32, 34 are described with more particularity
below with reference to FIG. 5. A bolus plug 38 is attached to the
auxiliary tubing member to plug the end thereof and hold the targets and
tubular spacing member therein. An exploded view of the tubular spacing
member 36 is presented in FIG. 2A. At least one weight 37 is disposed
within the lumen of the tubular spacing member 36 for the purpose of
adding weight in the vicinity of the second end 112b of the feeding tube
111, thus facilitating the placement of the tube in biological tissue. In
an exemplary embodiment of the invention the number of weights is three,
and the weights are comprised of tungsten. However, it is understood that
any desired number of weights, of any suitable configuration and material
may be used in the practice of the invention. A means for connecting the
enteral feeding tube to a source of nutrition, such as a Y connector 115,
may be attached to the end 112a of the tubular member 113 which is distal
from the auxiliary tubing member 30. A stylet 116 may be provided for the
purpose of adding rigidity to the tubular member during the process of
inserting the enteral feeding tube into biological tissue. As used herein,
and in the claims, an enteral feeding tube is understood to include both
nasogastric and nasojejunal tubes as well as tubes which may be inserted
into the digestive tract via a surgically created opening such as a
gastrostomy or jejunostomy. The components of an enteral feeding tube
which will contact biological tissue and/or food should be medical grade
materials (such as silicone rubbers, polyurethanes and polyvinyl
chloride), which have been approved by the Food and Drug Administration of
the Federal Government of the United States of America for food contact
and for biocompatibility. The components of the enteral feeding tube may
be connected to one another using solvent bonding techniques which are
suitable for the selected materials. The enteral feeding tubes of the
invention may be placed using feeding tube placement techniques which are
well known in the medical field.
The structure of feeding tubes according to the present invention may be
varied to accommodate the use of special materials or to incorporate other
desirable features. In an alternate embodiment, as shown in FIG. 2B, the
feeding ports 26 in the tubing member 113 are deleted, and the coupling
member 28a which joins the tubing member to a bolus 110, has feeding ports
therein. Furthermore, while the resonant circuit targets 32, 34 of the
embodiment shown in FIG. 2 will not readily allow a liquid nutritional
product to flow through the structure of the target, it is considered to
be within the scope of the invention to employ resonant circuit targets
which are tubular in structure, such as printed on a film. Such film
targets 117, 118 could be placed external of a tubular member as shown in
FIGS. 2C and 2D. A bolus 110 could then be attached to one end of the
tubular member 11, with a lumen extending through the bolus to communicate
with the lumen of the tubular member and a feeding port 8 located at the
end of the bolus which is distal from the tubular member. The bolus 110
may be formed of a plastic matrix having tungsten particles dispersed
therein for the purpose of adding weight to facilitate placement of the
feeding tube in the digestive tract. The bolus may have at least one
additional feeding port 9 located along the length of the bolus as shown
in FIG. 2C, or alternatively, the bolus may have a single feeding port at
the end thereof as shown in FIG. 2D.
Another alternative embodiment is presented in FIG. 2E wherein a tubular
member 113 has no bolus associated therewith, but does have at least two
tubular shaped resonant circuit targets 117, 118 disposed near one end
thereof.
While feeding tubes have been presented as exemplary embodiments of the
invention, it is understood to be within the scope of the invention to use
these types of structures in other types of tubes such as tubes used in
parental feeding, catheterizations, etc., which are intended for insertion
into the body of a mammal.
The distal target 34 and the proximal target 32 are both resonant circuit
targets and as such include coil and capacitor combinations that resonate
at particular frequencies. The resonant frequency of the distal target 34
is designed to be double the resonant frequency of the proximal target 32
and the probe 10 has provision for searching at both frequencies.
Turning to FIG. 1A, the control panel for the probe includes a target
select switch which toggles between the resonant frequency of the distal
target 34 and that of the proximal target 32. Each target has an indicator
which may be an LED labeled "prox" and "dist," respectively. The front
panel also includes a power on/off switch and a low battery indicator.
The search head 12 includes a pair of parallel planar circuit boards shown
in dashed outline as boards 15 and 17, respectively, that contain the
transmit and receive coils shown in more detail in FIGS. 3 and 4. FIG. 3
shows the configuration of the coils on the top printed circuit board 17
which include a circular transmitter/sensing coil 40, and a pair of
up/down receiver coils 42a and 42b which are positioned inside the
transmitter/sensing coil 40. Each of the receiver coils 42a and 42b are
D-shaped coils that are connected in series-opposed fashion. The "D" shape
maximizes the use of the available area inside the sensing coil. The coils
40, 42a and 42b all lie in a common plane on the top circuit board 17.
Directly underneath the board 17 is circuit board 15 which includes a
second transmitter/sensing coil 41 and left/right coils 44a and 44b. The
coils 44a and 44b are likewise connected in series-opposed fashion while
the coils 41 and 40 are connected in series-aiding fashion. It should be
noted that although boards 15 and 17 are shown as separate circuit boards,
they could be considered to be the top and bottom planar surfaces of a
single board.
The series opposing connection for the coils 42a, 42b, 44a and 44b is for
the purpose of creating a null current condition when the target object is
equidistant from the two coils. When the target lies along an axis that
bisects either of the coil pairs along the flat portion of the "D," the
currents generated in that coil pair will be equal and of opposite
polarity, thus creating a null. If a null condition exists for both coil
pairs, the target is centered. The sensing coil pair 40, 41 generates a
current that determines whether the return signal strength of the target
is great enough to be counted as a valid signal, thus providing inherent
noise rejection by rejecting invalid targets. The series opposing
connection of the coils also serves to reduce noise resulting from
electromagnetic interference because for each such signal induced in one
coil, an equal and opposite is induced in the other coil. Thus, sources of
EMI do not have an adverse effect on the system. The series opposing
connection, together with the symmetrical positioning of the transmitter
coils, prevents large currents from being induced into the receiver coils.
The target object to be used with the tube placement verifier system is
shown in FIGS. 5, 5a and 5b. Each target, whether it is the distal target
34 or the proximal target 32 in the system, is a multicoiled target which
includes a bobbin 50 formed by a pair of mutually perpendicular cross
pieces 52 and 54 (refer to FIG. 5b). A pair of mutually perpendicular
coils 56 and 58 are wound lengthwise around the intersections of the cross
pieces 52 and 54. A third coil 59 is wound as a solenoid coil having as
its central axis the longitudinal axis of the bobbin 50. Each of the coils
have capacitors 57 which are held within the body of the bobbin 50. During
manufacture the bobbin 50 includes a projecting member 55 which is used to
hold the bobbin in a fixture (not shown) where it can be wound. Once the
bobbin has been wound, the projecting member 55 can be broken off.
The three resonant circuits thus formed (each of the coils is connected to
one of the capacitors 57) have substantially identical response
characteristics. Since the coils 58, 56 and 59 are mutually orthogonal in
three dimensions, there is little or no cross coupling between the
adjacent resonant circuits regardless of the orientation of the bolus tube
30 where the targets are housed. At any given time, at least one of the
resonant circuit coils is likely to be in a plane that permits close
coupling between the resonant circuit and the coils in the search head 12.
Although the solenoid coil 59 is different in configuration from the two
other coils 58 and 56, it provides ample response for the probe 10 if its
central axis is pointed toward the planes of the receiver coils on circuit
boards 15 and 17.
Although coil 59 is a solenoid and has less area, it does have more turns
than coils 58 and 56. If the wire gauge is properly selected, the
resistance may be made approximately the same along with the Q and the
inductance. In such a case the current induced from the solenoid coil 59
into the receiver and sensing coils turns out to be nearly the same as for
coils 56 and 58 because of the higher number of turns.
Three coils on the bobbin 50 are necessary since position ambiguity may
result when any single or double axis coil is positioned at an angle
greater than about 221/2.degree. away from the plane of the search head
12. The three coil system solves this problem by providing at least one
coil that will respond to the search head regardless of the position of
the bolus tube 30 inside biological tissue.
Although only a single target is shown in FIGS. 5, 5a and 5b, both of the
targets 34 and 32 are constructed in similar fashion with the resonant
frequencies of each of the three coils on the target being the only
difference. In the preferred embodiment one target has a frequency that is
double the resonant frequency of the other target. For all practical
purposes the 2:1 resonant frequency ratio insures that the distal target
will not be picked up by the search head when the proximal target is
selected for identification and vice-versa. The 2:1 frequency ratio gives
better rejection because the synchronous demodulator, as will be explained
below, rejects any even harmonics or subharmonics generated by the
resonant targets. The use of two targets provides the search head 12 with
the means of detecting the alignment of the feeding tube 11 because the
relative locations of the two targets define a straight line and thus
provides some indication of the alignment of at least the distal end of
the tube.
A schematic block diagram of the system electronics is shown in FIGS. 6 and
6a. Referring first to FIG. 6, the probe 10 is powered by a power
management circuit 60 and a switching voltage regulator 62. Power
management is provided by four rechargeable batteries and the regulator,
which provides .+-.8 volts and .+-.7.5 volts DC is a switching regulator
that includes a SYNC input. The purpose of this input is to insure that
the regulator 62 is not switching during the "read" windows for the
up/down and right/left coils as will be explained below.
Timing is provided by a crystal oscillator 64 which is connected to a
divide by four circuit 66. The output of the divide by four circuit 66 is
coupled to a frequency select circuit 68 which in turn drives a four phase
clock 70. A random state generator 72 is coupled to the frequency select
circuit 68 for dithering the frequency select circuit 68 to affect the
duty cycle of the frequency select output in a random fashion. The random
state generator 72 feeds a pulse RND2 to the frequency select circuit 68
at random times to interrupt the system timing in such a way as to prevent
two systems operating in close proximity from being coherent for any
length of time. This effectively reduces interference between two adjacent
systems by a factor of about 10:1 and allows the systems to operate closer
together by a factor of about 3:1.
The frequency select circuit 68 selects one of two operating frequencies
for the system in response to the push button located on the front panel
as shown in FIG. 1a, depending upon whether the system is to search for
the distal target 34 or the proximal target 32. The 4FI and 4FI signals
are provided to a four phase clock 70 which has five outputs, each of
which is 1/4 the frequency of 4FI and 4FI. These outputs of the four phase
clock 70 are square waves but differ in phase. These are designated
.phi.1, .phi.2, .phi.3, .phi.4, and their complements. The frequency of
these signals is equal to the frequency that is applied to the transmitter
coil. This frequency will be either 211 kHz or 422 kHz. A .phi.1B signal
is also applied to the demodulator. This signal is a buffered signal that
is required to prevent feedback from the demodulator from affecting the
true .phi.1 signal.
The four-phase clock 70 is coupled to a counter 74 that develops six
frequency divided signals from the .phi.1 output of the clock 70. The
signals are F/8, F/16, F/32, F/64, F/128, and F/256. These are frequency
divided signals that have various purposes as Will be explained below. The
F/16 and F/32 outputs of the counter 74 are coupled to a read window
circuit 76. The purpose of the read window 76 is to develop the
positive-going 25% duty cycle 9T/8 or READ pulse which defines the
receiver on-time. The read window 76 also develops signals 5T/8 and 13T/8
which are coupled to an integrator window 78 whose function will be
explained below. T is the period of the transmitter frequency. All of the
delay times are referenced to the F/32 negative-going transition.
The F/32 signal from the counter 74 and the .phi.2 signal from the four
phase clock 70 are coupled to a delay circuit 80. The delay circuit 80
develops XMS and XMS. XMS is identical to F/32 except that it is delayed
by 1/4T. This circuit also develops PCAL which is used for certain
calibration operations not discussed herein. The principal purpose of the
delay circuit 80 is to provide a 1/4T delay in XMSC, which is the signal
used to turn the coil driver on and off, relative to F/32 in order to make
the coil driver (driving in phase with .phi.1) start and end the
transmitter on-time with 1/4 cycle periods of a single polarity rather
than starting and ending with 1/2 cycle periods. This helps prevent high
peak currents in the transmitter coil (coils 40 and 41) after transmitter
turn on, and it helps prevent high residual currents in the transmitter
coil system at the end of the transmitter on-time. The reason for this is
that the current in the coil, which lags the voltage turn-on or turn-off,
will rise to zero or decay to zero during the 1/4 cycle positive pulse.
This zero current switching feature is very important in order to
eliminate current from the transmitter coil during the times that the read
windows are opening and the coils 40, 41 are being used as sensing
receiver coils.
The read window 76 has outputs F/16 and 0T which are connected to sync
logic circuit 82. These signals, in conjunction with XMS from the delay
circuit 80 and F/8 from the counter 74, are used to develop the SYNC
signal which forms an input to the regulator 62 and the random state
generator 72.
The output of the delay labeled XMS is coupled to the preamp controller
which also receives inputs T/2 and 5T/8 to generate the PC and PC signals.
PC is approximately a square wave with frequency F having a rising edge
aligned with the rising edge of T/2 and having a falling edge aligned with
the rising edge of XMS. During normal operation the PC output is used to
control the on-time of the preamp MUX section. The preamp MUX is allowed
to be "on" whenever PC is low. This means it turns on at the rising edge
of T/2 and remains on until the beginning of the transmitter on-time when
XMS goes high. The PC and PC outputs are both used to control injection of
certain compensatory signals into the preamp MUX section when it turns on.
The preamp controller 84 also develops PC2 which has a falling edge
aligned with the rising edge of 5T/8. This output is used to control
switching in the preamp section.
Three outputs of the counter 74 which are F/64, F/128 and F/256 are coupled
to a MUX logic circuit 86. This circuit logically combines these three
inputs to develop three positive-going outputs called SENS, LR and UD. One
and only one of these outputs is in its high state at any given time.
These three outputs define the SENS, LR and UD periods which are the
windows for reading signals on the sensing coil 40 and 41, the left/right
receiver coils 44a, 44b and the up/down receiver coils 42a, 42b. These
three signals are coupled to reader logic circuit 88 and preamp logic
circuit 90.
The three outputs of the MUX logic circuit 86 are signals that define eight
periods of a system cycle but they do not identify the specific times
during each of the eight periods when the receiver should be reading each
of three input coil systems. The function of the reader logic circuit is
to logically combine SENS, LR and UD with another input, 9T/8, the read
pulse to generate SENS READ, LR READ and UD READ which are positive-going
pulses that control the receiver. The reader logic section also develops
an LR REF output by combining the LR READ input with the REF input. The LR
REF output is identical to the REF input except that it occurs only during
LR periods. The LR REF output serves as an input to the preamp section
where it turns on autozeroing feedback loops.
The preamp logic circuit 90 is connected to the outputs of the MUX logic
circuit 86 and the PC output of the preamp controller 84. The function of
the preamp logic circuit 90 is to generate a digital code that selects
ground potential, the sensing coil 40, 41, the left/right coils 44a, 44b
or the up/down coils 42a, 42b, respectively. The particular output code is
based upon whichever one of the SENS, LR or UD signals is high unless PC
is also high. If PC is high, ground potential is selected. Because of the
timing of the PC signal, ground potential is selected from the time the
transmitter turns on until shortly after the transmitter turns off when PC
is set low by the rising edge of T/2. The read window begins 5T/8 later
(refer to FIG. 7) as a result of the 9T/8 pulse from the read window 76.
This delay insures a dead zone between transmitter turn-off and the
beginning of the read window to allow transients (which can result from
nearby metal objects, body tissue conductivity, or electrostatic effects
in body tissue) to decay prior to activating the receiver coils. This
brief dead zone therefore serves to eliminate a great deal of noise that
would otherwise interfere with the reception of signals.
The sync logic section 82 puts out a negative-going SYNC pulse at a
frequency of F/32. This pulse controls the regulator 62 in such a way as
to guarantee that the output transistor of the regulator 62 is always on
(and therefore not switching) during the read period. This prevents
switching transients in the regulator from being picked up by the coils
and thus causing offsets and noise in the system. (The output transistor
is forced "on" rather than "off" because a forced off state results in a
regulator duty cycle that is incorrect for achieving the desired amount of
voltage step-up.)
The 5T/8 and the 13T/8 pulses developed by the read window 76 are combined
by the integrator window 78 to develop a timing pulse IW. The IW pulse
activates a feed-back loop in the coil driver circuit 92 that makes fine
adjustments in the duty cycle of the transmitter to further eliminate
residual current flowing in the transmitter coils during the read period.
Referring now to FIG. 6a the coil driver 92 is the circuit that drives the
transmitter coil 40, 41 in a balanced fashion. That is, both ends of the
coil are driven, but they are driven to opposite polarities at any given
time so that the average potential of the coil always remains near zero.
The driver attempts to apply square wave voltages to the two ends of the
coil system 40, 41 but the actual voltage applied is somewhat distorted
due to interactions of the driver output impedance with the coil current
flow and energy storage. The .phi.1 input provides the square wave timing.
The XMSC input turns the coil driver 92 off when it is high and on when it
is low. The timing of XMSC relative to .phi.1 is such that the coil driver
92 is turned on and off about midway between .phi.1 transitions. The delay
circuit 80 provides a 1/4T delay in XMSC relative to F/32 in order to make
the coil driver start and end the transmitter on-time with 1/4 cycle
periods of a single polarity rather than starting and ending with half
cycle periods. This helps prevent high peak currents in the transmitter
coil near the start of the transmitter on-time and helps prevent high
residual currents in the transmitter coil at the end of the transmitter
on-time. The coil driver 92 is "on" for 16 .phi.1 cycles and then off for
16 .phi.1 cycles. When the coil driver 92 is off, the transmitter coil
system acts as a sensing coil input to the receiver section. The two ends
of the transmitter coil system are connected as input SENSCOIL+ and
SENSCOIL- to the preamp MUX circuit 94.
If any residual current flows in the transmitter coil when the coil driver
92 turns off, there will be an exponential decay of the coil voltage that
can extend into the read period. If amplified by the preamp section, this
voltage can overdrive the preamp and interfere with target readings. A
feedback loop operates during the IW pulse which occurs shortly after the
coil driver is turned off. The preamp output PO+ is fed back to the coil
driver 92 to provide information about the residual current magnitude.
This allows the duty cycle of the coil driver to be adjusted in order to
set the residual current to a minimum.
The preamp MUX circuit 94 contains a dual four channel multiplexer which
responds to a two-line, two-digit code to direct it as to which of four
possible differential inputs are to be selected at any given time.
Although it is not shown in the drawing of FIG. 6a, the up/down coil
system 42a, 42b and right/left coil system 44a, 44b are connected to the
preamp MUX circuit 94, along with inputs from the coil driver, SENSCOIL+
and SENSCOIL-. The logic code appears as inputs A and B and represent four
possible digital combinations to select either ground, the SENSCOIL inputs
and left/right, or up/down. Ground potential is selected during the
transmitter on-time and for a short time thereafter (until the T/2 pulse
occurs). After this, the appropriate coil selection is made according to
the type of period occurring. The readings that are taken appear at the
outputs labeled PI+ and PI-.
Switching transients appear in the multiplexer that cause | | |
