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BACKGROUND OF THE INVENTION
The use of infrared gas analyzers is becoming of increasing importance in
several different fields. For example, in the medical field, infrared gas
analyzers may be used to monitor concentration of gases in the blood or in
a sample of expired air. In conjunction with pollution control, infrared
gas analyzers may be used to monitor air pollutants deleterious to the
environment.
A particular method for monitoring CO.sub.2 in the blood is by means of a
transcutaneous measurement at the surface of the skin. Typically, the
outermost layer of a small area of skin on a patient is removed, e.g., by
repeated application of a strip of adhesive tape. A small pill box type
device may then be sealed onto the skin so that the atmosphere in the box
can come into pressure equilibrium with the body fluids in the skin. The
concentration of CO.sub.2 in the pill box may then be measured by a
suitable infrared gas analyzer which is preferably included within the
pill box.
An effective transcutaneous CO.sub.2 measurement of the kind described
above requires that the infrared gas analyzer be capable of providing a
very rapid response time while utilizing only a small sample volume.
Furthermore, the detector output should be insensitive to extraneous
variables such as the intensity of any light sources employed, detector
bias, and aging effects. Additionally, in medical applications of this
kind the detector will be exposed to various sterilizing agents as well as
contaminants present in the body fluids. The detector output should
therefore be insensitive to contamination from these sources.
SUMMARY OF THE INVENTION
In accordance with one of the illustrated preferred embodiments, the
present invention provides an infrared gas analyzer which is particularly
suited for use in a transcutaneous CO.sub.2 measurement device. The
analyzer uses a single infrared source and a single detector illuminated
through a single interference filter. Positioned in the optical path
between the source and the detector is a rotating wheel containing two
reference cells and a sample cell. The two reference cells are each
enclosed between a pair of window surfaces. One of the reference cells is
filled with a gas containing a standard quantity of CO.sub.2, which may be
mixed with an inert gas. The other reference cell contains no CO.sub.2. In
accordance with one embodiment of the invention, the sample gas is
circulated in a region surrounding the rotating wheel. The sample cell is
specially constructed to be an "open" cell directly accessible to the
circulating sample gas. For example, the specially constructed sample cell
may comprise a cell-like opening of the same volume as the reference
cells, but having a pair of sapphire windows closely spaced to each other
on only one side of the cell. Thus, a sample gas admitted to the region
surrounding the rotating wheel will freely flow also into this sample cell
region.
In operation, the wheel is rotated to sequentially present the two
reference cells and the sample cell in the optical path between the
infrared source and detector. Three signal outputs are thereby generated.
The detected signal outputs during the intervals when the two reference
cells are in the optical path provide two standard readings, e.g., zero
and full-scale output readings. During the interval when the sample cell
is in the optical path the CO.sub.2 in the sample gas absorbs some
radiation so that the signal amplitude is a function of the partial
pressure of CO.sub.2 in the sample. As will be described in more detail
below, these three signals may be utilized to produce an associated output
signal which is indicative of the partial pressure of CO.sub.2 in the
sample, and is also essentially independent of variations in the source
intensity, detector efficiency, or contamination of the optical windows by
contaminants in the sample.
In accordance with another of the illustrated embodiments of the invention,
an analyzer is provided in which the sample cell is an enclosed cell
isolated from the rotating reference cells. This embodiment of the
invention is particularly suited to applications in which there is a high
probability of contaminating fluids being present in the gas, e.g., in
measurements of CO.sub.2 in air expired from the lungs. The isolated
sample cell can be easily cleaned or replaced.
In the preferred embodiments of the invention, the rotating wheel is driven
by the interaction of a number of permanent magnets positioned in the
wheel with electromagnetic coils mounted in the fixed housing. The
electromagnetic coils are activated sequentially in response to
optoelectronic signals generated from a number of optical timing marks
positioned around the periphery of the wheel; very precise rotational
frequency is thereby maintained. The timing marks may also be used to
control external signal-processing circuitry.
DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a detector in which the sample gas in the sample cell is
a portion of the sample gas circulating in a cavity surrounding two
reference cells.
FIG. 2 shows a front view of a wheel containing two reference cells and a
sample cell.
FIG. 3 is a graphical representation of the sequential signal outputs
related to the two sample cells and the reference cell.
FIG. 4 illustrates a partial view of an embodiment of the invention
utilizing a sample cell isolated from the region enclosing the reference
cells.
FIG. 5 illustrates a transducer including a temperature sensing and control
unit.
FIG. 6 is a partial view of a configuration of permanent magnets on a wheel
which are driven by a number of electromagnets to produce rotation of the
wheel.
FIGS. 7A and 7B illustrate several positions of the rotating wheel relative
to the electromagnets.
FIG. 8 shows an activation sequence for the electromagnets which provides
rotation to the wheel.
FIG. 9 is a circuit including a stepping pulse generator which drives the
electromagnets.
FIG. 10 illustrates an embodiment of the stepping pulse generator of FIG. 9
.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1 there is shown a transducer housing unit 11. Housing 11 may be
fabricated from any of a number of suitable materials such as plastic or
various metals; for example, devices have been built in which housing 11
is an aluminum unit of dimensions about 1 by 1 by 1. At the bottom of
housing 11 is a recessed portion or chamber 13 including an opening 15
into an inner chamber 17. A screen or mesh 19 is inserted in the recessed
slot 13 to stabilize the geometry of the skin-gas interface and provide
gas passages from the skin into opening 15. In accordance with procedures
known to those skilled in the art, a small area of the outer layer of a
patient's skin may be scraped or otherwise removed, whereupon the mesh 19
may be brought into intimate contact with the exposed area. Gases and
vapors in the blood and body tissue will then diffuse into chamber 17
until an equilibrium between the composition in the chamber and that in
the body is obtained. For some applications it may be desirable to utilize
a thin porous membrane 21 in recess 13 to prevent body fluids from
entering chamber 17, while freely admitting gases, including CO.sub.2 to
be measured, to equilibrate between chamber 17 and the body. To seal the
chamber from the outside environment the housing must be sealed against
the skin. Although various means of sealing may be used, it has been found
effective to achieve sealing with a double-sided adhesive tape layer 14
affixed to housing 11.
Included within housing 11 is a source of infrared radiation 23. Any source
of radiation which includes the known CO.sub.2 absorption band at 4.26
.mu.m would be a candidate for source 23. Preferably, however, a rugged
small well-defined source should be employed. One such source is disclosed
in copending U.S. Pat. application Ser. No. 404,845 now U.S. Pat. No.
3,875,413 entitled INFRARED RADIATION SOURCE, filed Oct. 9, 1973, by John
A. Bridgham and assigned to the same assignee as the present application.
An infrared radiation detector 25 is positioned opposite source 23. The
detector may be any of a number of infrared detectors available in the
art; e.g., a Model ATC 11 lead selenide detector available from
Opto-Electronics Incorporated in Petaluma, California. Interposed between
source 23 and detector 25 is an optical filter 27. This filter may be any
standard filter whose characteristics include a narrow bandpass region
around the carbon dioxide line at 4.26 .mu.m.
A cylindrical wheel 29 is mounted for rotation within cavity 17. A side
view of wheel 29 is illustrated in FIG. 2 including a line AA along which
is taken the cross-sectional view of FIG. 1. To provide best performance,
the wheel should be mounted for minimum rotational friction, e.g., by
using a pair of jeweled bearings (not shown).
In accordance with this embodiment of the invention, rotating wheel 29
includes three hollowed-out portions or cells. These three hollowed-out
volumes labeled 31, 33 and 35 in FIGS. 1 and 2 function as two separate
reference cells and a sample cell respectively. Reference cells 31 and 33
are each constructed with a pair of windows 37 and 39 which have good
transmission characteristics at a desired CO.sub.2 infrared absorption
band. A material such as sapphire is suitable. To facilitate operation,
reference cells 31 and 33 may comprise "snap in" cells which can be easily
removed from or inserted into wheel 29. According to the principles of the
invention, the sample cell 35 is constructed in an "inside out"
configuration. By this is meant that the gas to be sampled is not enclosed
in a volume between a pair of windows as are the gases in the reference
cells. Instead, a pair of windows 41 are positioned back-to-back, e.g., in
the center of the sample cell. Sample cell 35 is therefore an open cell in
direct communication with the region 17 into which the sample gas from the
body has been admitted. With the device so configured the sample gas
completely surrounds all three cells so that any contaminants present in
the sample will tend to contaminate windows of each of the three cells to
the same extent. Consequently, as will be described in more detail below,
any resulting changes in the transmission characteristics of the windows
will not be reflected in the measured value of the CO.sub.2
concentration. It is evident from the discussion above that windows 41 of
sample cell 35 may also be displaced from the geometric center of the
sample cell without altering the operation. For example, the two windows
may both be placed on one side of the sample volume. Alternately, it is
possible to include only one window in the sample cell. In that case, the
increased optical transmission through the sample cell will simply produce
an offset reading which may be compensated in the signal processing to be
described shortly. For different applications, particular configurations
within the spirit of the invention may be selected in accordance with ease
of manufacture, cost, and the particular signal processing means employed
in the device.
Understanding of the operation of the device may be facilitated by
reference to FIG. 3 and the following discussion. One of the reference
cells, e.g., cell 31, is filled with an inert gas containing a known
amount of CO.sub.2, e.g., no CO.sub.2. When this cell is in the optical
path between source 23 and detector 25 infrared absorption is a minimum
and the signal amplitude output a maximum. In FIG. 3 this maximum
amplitude is indicated by the letter Z (Z to indicate zero CO.sub.2). The
other reference cell labeled 33 is filled with a standard quantity of
CO.sub.2, e.g., a quantity which will yield a full-scale reading on a
measuring indicator. In practice, the CO.sub.2 may be mixed with an inert
gas. When reference cell 33 is in the optical path, the infrared
absorption is a maximum so that the signal amplitude is a minimum. In FIG.
3 this reference amplitude is designated by an F (F for fullscale
absorption). Now, as was described above, the "inside out" sample cell 35
includes a volume of the sample gas in communication with the gas present
in cavity 17, the optical path length through the sample cell being equal
to the path length through each of the two reference cells. When sample
cell 35 is in the optical path, there will be some infrared absorption,
the magnitude of which is dependent on the partial pressure of CO.sub.2 in
the sample gas. The relative signal amplitude output during this interval
will thus be reduced, for example, as indicated by the amplitude S (S for
sample) in FIG. 3. In operation then, if there is no CO.sub.2 present in
the sample the sample signal S will be equal to the reference signal Z. If
there is present in the sample an amount of CO.sub.2 equal to the standard
quantity of CO.sub.2 present in reference cell 33, the signal S will be
equal to the signal F. These relations will hold even though the sample
gas in the chamber 17 is present in the optical path, since the optical
path length through the "excess" sample is the same in all three cases.
In actual practice it is desirable to minimize effects of detector noise
and localized window irregularities by integrating the output signals over
a number of intervals when each cell is presented in the optical path.
This can most conveniently be accomplished by voltage to frequency
conversion methods known in the art. Other methods, e.g., analog
integration may also be utilized. For purposes of explanation it is
sufficient to note that the sumbols Z, S, and F may be taken to represent
the integrated signals from the two reference cells and sample cell
respectively. In order to determine the partial pressure of CO.sub.2 in
the sample an auxiliary amplitude is constructed which will be denoted by
A. In accordance with the invention the amplitude A is related to the
measured quantities S, Z and F by the relation A = S-Z/F-Z. It may be seen
immediately that as the concentration of CO.sub.2 varies from zero to the
standard value in the reference cell 33, the amplitude A varies from zero
to unity. Between these extreme values, the relationship between the
partial pressure of CO.sub.2 and the amplitude A is a nonlinear function
which is to be determined experimentally. However, it has been determined
experimentally that a unique and smoothly varying value of the partial
pressure of CO.sub.2 may be plotted as a function of A. Thus, the desired
CO.sub.2 measurement can be reliably obtained from the amplitude A. In
practice, if the amplitudes are integrated by a voltage-to-frequency
conversion, then several digital counters may be used to provide inputs to
an on-line computer which can compute the associated amplitude and
directly derive the CO.sub.2 concentration.
The insensitivity of the present measurement to a large number of spurious
factors may now be simply shown. For example, if a common term is added to
each of the amplitudes S, Z, and F the amplitude A will be unchanged.
Thus, the measurement will not be influenced by offset voltages in the
detector or in any electronic signal processing used in conjunction with
the detector. It is also evident that if the amplitudes S, Z, and F are
all multiplied by a common factor the overall amplitude A remains
unchanged. Therefore, the measurement is independent of such variables as
source intensity, detector responsivity, amplifier gain, and optical
attenuation in the system. In particular, any contaminants from the body
or sterilizing agents which are present in the sample gas will tend to
contaminate the windows of each of the three cells to about the same
extent and for most contaminants this will effectively multiply each of
the variables S, Z and F by a common factor as described above. The
measurement will therefore be insensitive to such contamination as well.
In FIG. 4 there is illustrated an embodiment of the detector which is
suitable for applications in which contaminants in the sample would be
likely to gum-up the accurate operation of the rotating cylinder 29. This
might occur if the CO.sub.2 analyzer were to be employed in measuring the
CO.sub.2 content of air expired from the lungs of a very sick patient. For
such applications, it is desirable to isolate the sample gas to be
analyzed from the rotating cylinder. FIG. 4 shows a configuration
including an enclosed sample cell 38 containing a sample gas to be
analyzed. In this configuration, rotating wheel 29 again includes two
reference cells 31 and 33 containing no CO.sub.2 and a standard amount of
CO.sub.2 respectively. The remaining position on the wheel is now occupied
by a solid mass to provide a zero reading period during which no direct
radiation reaches the detector. When reference cell 33 is interposed
between the sample and the detector, the CO.sub.2 in the reference cell
may ideally be seen as absorbing all of the radiation at the CO.sub.2
absorption band; variations in the CO.sub.2 concentration in the sample
thus do not affect the reference reading. However, when reference cell 31
is interposed between the sample and the detector, variations in CO.sub.2
concentration strongly influence the reference reading. The differences
between each of these two readings and the zero (dark) reading may be used
in conjunction with an initial null reading to produce a measure of
CO.sub.2 concentration in the sample. The initial null reading may be
obtained when no CO.sub.2 is present in sample cell 38. The optics
associated with this embodiment may be identical to those described above
in connection with FIG. 1. Thus, by using snap-in cells in wheel 29 and
modifying the shape of housing 11, a detector may be provided which is
readily adaptable for operation in either of the modes of FIGS. 1 or 4.
For either mode of operation, the observed infrared absorptions vary in
response to the sample CO.sub.2 density in a known consistent manner.
However, these absorptions also vary with temperature, and the conversion
of density to partial pressure requires a knowledge of the gas
temperature. It is therefore desirable to sense and control the transducer
temperature. FIG. 5 illustrates a transducer including a temperature
sensing and control unit.
Embedded in the transducer is a temperature sensing device 34, which may
be, e.g., a thermistor. To complete the unit, a heating element 36 is
positioned in thermal contact with housing 11. This element may be, e.g.,
a standard power transistor or an electric heating coil. By means of
control circuitry (not shown) heating element 36 is responsive to
temperature sensed by sensor 34. Precise control of the transducer
temperature is thus provided.
It is evident from the above description that operation of the analyzer
requires a drive mechanism which can accurately rotate the wheel within
the hermetically sealed chamber. Generally, the conventional method of
driving a rotor in a sealed chamber by means of an external motor and
drive shaft through a seal is unsatisfactory for the present application;
to overcome the seal friction a motor too large for the present
application would be required. In accordance with the present invention, a
specialized drive system is provided which utilizes minimum space and
provides excellent long and short term speed stability.
The mechanical construction of the drive system is shown in FIGS. 6 and 7.
In FIG. 6 a schematic outline of housing 11 and rotor 29 is shown.
Embedded in rotor 29 are three permanent magnets, two of which are
illustrated in FIG. 6 and numbered 40A and 40B. In FIG. 7A all three
permanent magnets numbered 40A, 40B, and 40C are shown. The magnets are
positioned symmetrically (i.e., adjacent magnets are separated by an
angular displacement of 120.degree.). The magnetic axes of the three
magnets are each positioned parallel to the rotor axis, and the poles
(indicated by N and S in FIG. 6) are all aligned in the same direction.
Within housing 11 but external to chamber 17 are positioned four
electromagnets. Two of these numbered 41 and 42 are illustrated in FIG. 6,
while FIG. 7A shows all four magnets, labeled 41, 42, 43, and 44,
respectively. FIG. 6 also includes an electro-optical timing system 46,
which may include e.g., a solid state light source and detector which
reflects and detects light signals from a series of reflecting timing
marks spaced around the periphery of rotor 29. Electrical signals
generated by the timing system are used to control the drive speed, as
will be discussed in detail below.
Operation of the drive may be understood by reference to FIGS. 7A and 7B.
FIG. 7A illustrates an "equilibrium" position of the rotor in which the
magnetic forces exerted on permanent magnets 40A-C by electromagnets 41-44
are balanced to produce a zero net torque on the rotor. In particular,
electromagnet 43 is energized so as to attract magnet 40B while
electromagnet 44 attracts magnet 40C; both of the electromagnets 41 and 42
are energized to repel magnet 40A, producing a net zero torque.
Now, by reversing the polarity of the electromagnets in a particular
sequence, the rotor can be rotated on-twelfth revolution clockwise to the
position illustrated in FIG. 7B. By means of control circuitry to be
described below, the polarity of electromagnet 44 is reversed so that it
repels permanent magnet 40C while the polarity of electromagnet 42 is also
reversed so as to attract permanent magnet 40A. The rotor will therefore
rotate one-twelfth revolution clockwise. For continuous rotation the
electromagnets are continually switched in pairs, each successive
switching advancing the rotor one-twelfth revolution. FIG. 8 is a
graphical representation of the switching sequence, the first stage of
which was just described. A "+" indicates polarity attracting the
permanent magnets, while a "-" indicates a repelling polarity. In FIG. 8,
wheel position number 1 corresponds to the equilibrium position
illustrated in FIG. 2A while position 2 is that illustrated in FIG. 7B.
In FIG. 9 there is illustrated schematically one simple circuit arrangement
for generating the currents to be passed through coils 41-44 to achieve
the switching sequence described above. Coils 41 and 43 are connected in
series with a standard D-type flip-flop 45. The coils are connected with
their polarities reversed to achieve the synchronized switching
illustrated in FIG. 8. Similarly, coils 42 and 44 are connected in series
with a D-type flip-flop 49. Two operational amplifiers 47 and 51 are
interposed between the flip-flops and the coils in order to provide a
plus/minus current variation rather than a zero-to-plus swing which would
otherwise be present at the output of the flip-flops. A negative voltage
labeled -V.sub.0 provides the required offset. Flip-flops 45 and 49 are
interconnected in a four state sequencer arrangement which is driven by a
source of pulses from a pulse generator 53. A more detailed description of
a suitable pulse generator will be given below. It will be apparent to
those skilled in the art that various other arrangements of logic
circuitry may also be used to generate the switching sequence of FIG. 8.
For the very accurate rotation of rotor 29 required in some applications,
there are several difficulties which must be overcome. First, it can be
noted that while a single source of evenly timed stepping pulses will
suffice to keep the motor running at constant frequency in a continuous
run mode, in order to start the motor and gradually bring it up to speed,
the pulses should begin at a low rate and be gradually increased in
frequency. Secondly, in applications such as the present in which there is
little frictional damping of the rotor, there is likely to be present a
low frequency instability of the rotor speed. Both of these problems are
resolved in the present application by utilizing as a stepping pulse
generator (53 in FIG. 9) a device such as that illustrated in FIG. 10. The
pulse generator provides accurately timed pulses in the running mode
including provision for automatic starting and restarting in the case of
accidental shut-down. Additionally, the device provides for a smooth
transition between starting and running modes and for critical damping of
the rotor to overcome the instability mentioned above.
Referring now to FIG. 10, the fundamental generator of the stepping pulses
is a monostable multivibrator 55 which is set to generate a pulse of about
40 .mu.s width. This pulse is simultaneously applied to the "clear" input
terminals of two standard J-K flip-flops 57 and 59. Understanding of the
detailed description which follows will be facilitated by noting one
overall concept: the frequency of the pulses generated by multivibrator 55
is controlled at all times by one of two sources; when the rotating wheel
is accelerating up to speed the pulses are responsive to a signal from the
wheel via flip-flop 57. When the rotor is operating in a constant speed
mode, the pulses are responsive to a signal from a preset timer via
flip-flop 59. In more detail, the control scheme is as follows:
multivibrator 55 will output a pulse only when activated at both terminals
A and B, these two signals being the outputs of flip-flops 59 and 57
respectively. Thus, if two activating transitions arrive at terminals A
and B in sequence, multivibrator 55 will effectively be triggered by the
later-arriving transition, whether that activating transition be from
flip-flop 57 or from flip-flop 59. Flip-flop 57 is itself triggered by a
series of pulses generated by the rotating wheel through the optical
system 45 described above (see FIG. 6). The frequency of these pulses from
flip-flop 57 is therefore dependent on the rotor speed. Flip-flop 59 is
triggered by a timer 61 which generates pulses of any desired time
duration. The pulse width of the timer should preferably correspond to
1/12 the required period of a single rotor revolution. During the
acceleration period of the rotor, the duration between these timer pulses
will always be shorter than the interval between pulses from optical
system 63. Therefore, flip-flop 59 will be clocked before flip-flop 57 and
multivibrator 55 will effectively trigger on the later-received pulses
from flip-flop 57, responsive to signals received from the rotating wheel.
Thus, during the acceleration period the stepping pulses are increased in
frequency only as the angular velocity is increased. Conversely, however,
as the motor attempts to accelerate to a speed greater than that for which
the timer is preset, the timer pulses will fall behind the pulses from
optical system 63, so that flip-flop 57 will be triggered first and
flip-flop 59 triggered later. Multivibrator 55 will therefore not put out
a stepping pulse until it receives the later pulse from flip-flop 59. This
retardation of the stepping pulse prevents further acceleration. The rotor
will then run in a synchronous mode controlled by timer 61. To insure a
smooth transition between the acceleration mode and the synchronous mode,
timer 61 is reset from the stepping pulse itself so that the phase between
the timer and the motor is locked prior to the transition.
The additional problem of lack of mechanical damping of the rotor possibly
causing instability is compensated for by a feedback signal from flip-flop
57 via network 65 (e.g., an RC network) to provide negative feedback to
the control input of timer 61. More particularly, the output of flip-flop
57 is a rectangular wave at the stepping pulse frequency with a duty cycle
proportional to the phase between the timer pulses and those received from
the rotating wheel. Filter network 65 passes those frequency components of
this signal that are associated with the frequency of phase instability
(typically a few Hz) while blocking d.c. and the stepping pulse
frequencies. If a phase transient is introduced (as might occur if the
transducer were suddenly rotated in the course of normal handling) the
pulse rate of timer 61 will be altered momentarily by the feedback signal
to reduce the phase error. By suitably choosing the pass band attenuation
of network 65 critical damping of the rotor drive system may be achieved.
Finally it is important that pulses be available to initially start the
motor and at any other time that the motor might accidentally stop due to
mechanical shock, momentary power failure, etc. This capacity is provided
by a monostable multivibrator 67 set to generate pulses at a predetermined
interval, for example, 0.5 second. If for any reason the stepping pulses
fail for longer than about 0.5 second, the multivibrator 67 will output a
pulse to set flip-flop 57 and switch flip-flop 59 into a toggle mode.
Thus, so long as power is supplied to multivibrator 67, it is assured that
multivibrator 55 will be triggered and pulses generated to start or
restart the system.
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