U.S. patent application number 12/280894 was filed with the patent office on 2010-03-11 for flow sensor calibrated by volume changes.
This patent application is currently assigned to FLUIDNET CORPORATION. Invention is credited to Jeffrey A. Carlisle, John M. Kirkman, Jr., Lawrence M. Kuba.
Application Number | 20100063765 12/280894 |
Document ID | / |
Family ID | 38509960 |
Filed Date | 2010-03-11 |
United States Patent
Application |
20100063765 |
Kind Code |
A1 |
Carlisle; Jeffrey A. ; et
al. |
March 11, 2010 |
Flow Sensor Calibrated by Volume Changes
Abstract
An infusion pump and method are provided which combine flow rate
measurements calibrated by accurate volume measurements over
time.
Inventors: |
Carlisle; Jeffrey A.;
(Stratham, NH) ; Kuba; Lawrence M.; (Nashua,
NH) ; Kirkman, Jr.; John M.; (Trumansburg,
NY) |
Correspondence
Address: |
SCOTT C. RAND, ESQ.;MCLANE, GRAF, RAULERSON & MIDDLETON, PA
900 ELM STREET, P.O. BOX 326
MANCHESTER
NH
03105-0326
US
|
Assignee: |
FLUIDNET CORPORATION
Portsmouth
NH
|
Family ID: |
38509960 |
Appl. No.: |
12/280894 |
Filed: |
February 27, 2007 |
PCT Filed: |
February 27, 2007 |
PCT NO: |
PCT/US07/04945 |
371 Date: |
August 27, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60777193 |
Feb 27, 2006 |
|
|
|
Current U.S.
Class: |
702/100 ;
73/1.16 |
Current CPC
Class: |
G01F 22/02 20130101;
A61M 5/16804 20130101; A61M 5/16886 20130101; A61M 5/1483 20130101;
A61M 2205/3306 20130101 |
Class at
Publication: |
702/100 ;
73/1.16 |
International
Class: |
G01F 25/00 20060101
G01F025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 23, 2007 |
US |
PCT/US2007/002039 |
Claims
1. An inline flow sensor, comprising: a conduit defining a flow
passageway between a fluid inlet and a fluid outlet; a ball member
movable within said flow passageway; a spring for urging said ball
member in a direction opposite a direction of fluid flow through
said conduit; and an optical sensor for determining a position of
said ball member within said conduit.
2. The inline flow sensor of claim 1, wherein said conduit
comprises: a housing defining a cavity having an axis and which is
tapered in the direction of said axis; and optionally, means within
said cavity for restricting movement of said ball member to
movement along said axis.
3. The inline flow sensor of claim 2, wherein said optical sensor
comprises: an optical transmitter positioned on a first side of
said housing; and an optical receiver positioned on a second side
of said housing opposite said first side.
4. The inline flow sensor of claim 3, wherein an optical path is
created through said ball member.
5. The inline flow sensor of claim 3, wherein said optical receiver
comprises a linear photosensor array extending in a direction
parallel to said axis.
6. The inline flow sensor of claim 5, wherein said photosensor
array is selected from the group consisting of a photodiode array,
a CCD array, and a CMOS digital detector array.
7. The inline flow sensor of claim 5, wherein said optical
transmitter comprises a linear LED array.
8. The inline flow sensor of claim 3, further comprising: said
optical transmitter comprises a light source having a light source
wavelength selected from a wavelength in the IR, visible, or UV
region; and said housing and ball member are formed of a material
that optically transmits light in said light source wavelength.
9. The inline flow sensor of claim 1, further comprising: means for
adjusting a preload force of said spring on said ball member.
10. A flow control system for a fluid delivery system of a type for
delivering a volume of fluid from a container, the fluid control
system comprising: an inline flow sensor fluidically coupled to an
output of the fluid delivery system for generating a signal
representative of a sensed flow rate of the fluid; means for
calculating volume of the liquid remaining in the container; and a
computer-based information handling system including: means for
monitoring the signal from the inline flow sensor; means for
calibrating said inline flow sensor using the signal from the
inline flow sensor and successive calculations of volume of the
liquid remaining in the container; and means for determining a flow
rate of the fluid using one or both of: the successive calculations
of volume of the liquid remaining in the container; and the signal
from the inline flow sensor.
11. A method of determining an absolute flow rate of a fluid to be
delivered in a fluid delivery system, comprising: sensing a
relative flow rate using an inline flow sensor fluidically coupled
to an output of the fluid delivery system for generating a signal
representative of the sensed relative flow rate of the fluid; and
determining the absolute flow rate of the fluid from the sensed
relative flow rate of the fluid.
12. The method of claim 11, wherein determining the absolute flow
rate of the fluid comprises: calculating volume of the liquid to be
delivered; monitoring the signal from the inline flow sensor;
calibrating the inline flow sensor using the signal from the inline
flow sensor and one or both of: successive calculations of volume
of the liquid to be delivered; and a pressure decay of a
pressurized bladder bearing against the liquid to be delivered; and
determining the absolute flow rate of the fluid using one or more
of: the successive calculations of volume of the liquid to be
delivered; the signal from the inline flow sensor; and a pressure
decay of a pressurized bladder bearing against the liquid to be
delivered.
13. The method of claim 11, wherein determining the absolute flow
rate of the fluid comprises one or more of: comparing the sensed
relative flow rate of the fluid to previously stored sensed
relative flow rates for known absolute flow rates; and converting
the sensed relative flow rate of the fluid to an absolute flow rate
using an algorithmic formula mapping sensed relative flow rates to
absolute flow rates.
14. The method of claim 11, wherein the inline flow sensor
includes: a conduit defining a flow passageway; a ball member
movable within said flow passageway; a spring for urging said ball
member in a direction opposite a direction of fluid flow through
said conduit; and an optical sensor for determining a position of
said ball member within said conduit.
15. The method of claim 14, wherein determining the absolute flow
rate of the fluid comprises comparing known absolute flow rates for
ball member positions within said flow passageway.
16. The method of claim 15, wherein comparing known absolute flow
rates for ball member positions within said flow passageway
comprises looking up the known absolute flow rates in a look up
table.
17. The method of claim 11, further comprising: independently
calculating an absolute flow rate of the fluid as it exits a
primary fluid source of the fluid delivery system; and comparing
the independently calculated absolute flow rate to the absolute
flow rate determined using the inline sensor.
18. The method of claim 17, further comprising using differences
between said independently calculated absolute flow rate of the
fluid as it exits the primary fluid source and said absolute flow
rate determined using the sensed relative flow rate to identify one
or more of: fluid delivered from a secondary fluid source of the
fluid delivery system; and a quantity of gas leaving the fluid
delivery system.
19. The method of claim 17, further comprising: placing a flexible
bag containing the liquid to be infused within a rigid container of
known total volume and containing an inflatable bladder, said
inflatable bladder fluidically coupled to a pneumatic system for
inflating said bladder; using one or both of volume measurements in
said bladder over time and pressure measurements of said bladder
over time to calculate said independently calculated absolute flow
rate.
20. The method of claim 19, further comprising using differences
between said independently calculated absolute flow rate of the
fluid as it exits the primary fluid source and said absolute flow
rate determined using the sensed relative flow rate to identify one
or more of: fluid delivered from a secondary fluid source of the
fluid delivery system; a quantity of gas leaving the fluid delivery
system; and a leak in the pneumatic system.
21. The method of claim 19, wherein said independently calculated
absolute flow rate is calculated using a rate of pressure decay of
a volume of gas in the inflatable bladder.
Description
BACKGROUND
[0001] The present disclosure relates to fluid flow control
systems, such as intravenous infusion pumps, and more particularly
to feedback control infusion pumps with flow and volume
sensing.
[0002] In some situations, such as with intravenous infusions, the
flow rate range requirement is very broad, covering more than four
orders of magnitude of rate, e.g., from about 0.1 ml per hour to
about 6,000 ml per hour. If any sort of low flow rate sensitivity
is to be expected, then the response of a flow sensor should be
logarithmic or at least non-linear. The absolute calibration of
such a flow sensor would be very challenging, especially at low
flow rates. Even without the requirement for calibration, it is
difficult to find a commercially available flow sensor that
services this flow rate range.
[0003] Certain infusions have historically been managed by air
pressure delivery systems, most commonly found in the operating
room and in emergency situations. Prior art attempts have been made
to determine the flow rate via pressure monitoring and control. For
example, U.S. Pat. No. 5,207,645 to Ross et al. discloses
pressurizing an IV bag and monitoring pressure to infer flow rates.
However, the prior art systems lack independent flow sensing and,
therefore, do not offer enough information to provide accurate and
safe infusions. Under the best of circumstances, there is not
enough information in the pressure signal alone to provide the
accuracy needed for intravenous infusion therapy.
[0004] Furthermore, there are a number of likely failure modes that
would go undetected using this pressure signal alone. An infusion
pump must be able to respond to events in a relevant time frame.
International standards suggest that a maximum period of 20 seconds
can lapse before fluid delivery is considered "non-continuous." As
an example, for an infusion of 10 ml per hour, the system would
want to resolve 20 seconds of flow, which corresponds to 0.056 mL.
This volume represents one part in 180,000 of the total air volume.
Temperature-induced change in pressure brought about by a normal
air conditioning cycle is far greater than this signal. The
measurement of pressure alone is not adequate for an intravenous
infusion device. No general-purpose full-range, infusion devices
using pressure-controlled delivery are known to be on the
market.
[0005] It would therefore be useful to develop a fluid delivery
device and a control system and method therefor that could control
the rate fluid flow based on a high resolution, highly responsive
fluid flow sensor that could be calibrated in situ by correlating
its signal with changes in fluid volume over time.
SUMMARY OF THE DISCLOSURE
[0006] In one aspect, a fluid control system that combines flow
rate and absolute volume calculations is provided. In further
aspects, a fluid delivery device and methods employing the same are
provided.
[0007] An advantage of the present disclosure resides in its
accuracy. The present disclosure offers two novel views of flow
control, one operating over many minutes, and the other operating
many times per second, which is analogous to having both a
speedometer and an odometer, so that a steady speed may be
maintained, while still making adjustments to arrive at one's
destination on time. The high degree of accuracy is advantageous
for infusion of vasoactive drugs, since the flow should be as
continuous as possible and the flow rate resolution should be high
enough to resolve a 1% change in requested flow rate. Likewise, for
long term doses (e.g., 12-72 hours), it is important to complete
the dose on time. Also, for longer-term infusions, the accuracy of
the system should not degrade over time. There are clinical and
operational problems with infusions that are completed too soon
(e.g., increased chance of clotting off the IV line) or too late
(e.g., unused medication, biohazard, etc.). For intermittent
infusions, the time the dose starts is important, so that the peak
serum levels of the intermittent medication occur as ordered. For
market acceptance, the device should perform well according to
worldwide testing standards, including the integrated flow volume
"trumpet curve."
[0008] An additional advantage of the present disclosure resides in
its simplicity. Instead of a complex electromechanical assembly of
gears, levers, motors, and switches, a sensor-based flow regulation
system can be relatively simple and low cost in its
construction.
[0009] In operation, a bag of fluid to be infused is set inside a
rigid container. The device automatically measures the volume of
liquid in the bag. A user input device is provided to allow the
user to input (1) how fast the fluid should flow; (2) when the
infusion should be complete; and/or (3) the amount of time the
infusions should take to finish.
[0010] In certain embodiments, the present disclosure is capable of
operating over an extended range of flow rates, e.g., from below 1
milliliter per hour to 6,000 milliliters per hour. At all flow
rates, the fluid flows continuously without the characteristic flow
disruptions of a typical motor driven large-volume pump.
[0011] Other benefits and advantages of the present disclosure will
become apparent to those skilled in the art upon a reading and
understanding of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention may take form in various components and
arrangements of components, and in various steps and arrangements
of steps. The drawings are only for purposes of illustrating
preferred embodiments and are not to be construed as limiting the
invention.
[0013] FIGS. 1 and 2 are perspective and side views, respectively,
of an infusion pump in accordance with an exemplary embodiment.
[0014] FIG. 3 is a functional block diagram showing the fluidic
connections of a volume measurement system according to an
exemplary embodiment.
[0015] FIG. 4 is a functional block diagram showing the control
elements of a volume measurement system according to an exemplary
embodiment.
[0016] FIG. 5 is a functional block diagram showing the sensing
elements of the system.
[0017] FIG. 6 is a flow chart of an exemplary method for
calculating the volume of liquid to be infused.
[0018] FIG. 7 is a perspective view of the exterior of the flow
sensor
[0019] FIG. 8 is an exploded view of the flow sensor.
[0020] FIG. 9 is a cross-sectional view of the flow sensor
[0021] FIG. 10 is a perspective view of the flow sensor
housing.
[0022] FIG. 11 is a graphical representation of force balancing in
the flow sensor.
[0023] FIG. 12 is a flow chart outlining an exemplary method of
calculating flow rate based on pressure decay.
[0024] FIG. 13 is a block diagram illustrating an exemplary system
having plural, independent methods for measuring flow rate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] Referring to the drawings, wherein like numerals reference
numerals are used to indicate like or analogous components
throughout the several views, FIGS. 1 and 2 depict an exemplary
volume and flow measurement system in accordance with an exemplary
embodiment of the present invention. The system includes a pressure
frame 10 that is of known total volume and contains within it an
air bladder 20 and a flexible bag 30 that contains within it a
liquid 40 to be infused.
[0026] Referring now to FIG. 3, the air bladder 20 is connected to
an air pump 50 via a bladder connection line 608, a bladder valve
106, and a bladder valve line 606. The air bladder 20 may be vented
to atmosphere via a bladder vent valve 108.
[0027] A calibration tank 60 of known volume is connected to the
air pump 50 via a tank connection line 604, a tank valve 102, and a
tank valve line 602. The tank 60 may be vented to atmosphere via a
tank vent valve 104.
[0028] The liquid 40 is fluidically coupled to an output 500 via a
liquid drain line 610, going through a fluid flow resistor 400 and
through an output line 612. The liquid 40 may be, for example, a
medication fluid, intravenous solution, or the like, and the output
500 may be, for example, a patient or subject in need thereof. An
inline flow sensor 900 is provided in the line 612, as described in
greater detail below.
[0029] The tank 60 is connected to a tank pressure sensor 204 and a
tank temperature sensor 304. The bladder 20 is connected to a
bladder pressure sensor 202 and a bladder temperature sensor
302.
[0030] Referring now to FIG. 4, an electronic module includes a
processing unit 700 such as a microprocessor, microcontroller,
controller, embedded controller, or the like, and is preferably a
low cost, high performance processor designed for consumer
applications such as MP3 players, cell phones, and so forth. More
preferably, the processor 700 is a modern digital signal processor
(DSP) chip that offers low cost and high performance. Such
processors are advantageous in that they support the use of a 4th
generation programming environment that may substantially reduce
software development cost. It also provides an ideal environment
for verification and validation of design. It will be recognized
that the control logic of the present development may be
implemented in hardware, software, firmware, or any combination
thereof, and that any dedicated or programmable processing unit may
be employed. Alternately, the processing unit 700 may be a finite
state machine, e.g., which may be realized by a programmable logic
device (PLD), field programmable gate array (FPGA), field
programmable object array (FPOA), or the like. Well-known internal
components for processor 700, such as power supplies,
analog-to-digital converters, clock circuitry, etc, are not shown
in FIG. 4 for simplicity, and would be understood by persons
skilled in the art. Advantageously, the processing module 700 may
employ a commercially available embedded controller, such as the
BLACKFIN.RTM. family of microprocessors available from Analog
Devices, Inc., of Norwood, Mass.
[0031] With continued reference to FIG. 4, the processing unit 700
controls the air pump 50 via a pump control line 750. The processor
700 controls the tank vent valve 104 via a tank vent valve control
line 704. The processor 700 controls the tank valve 102 via a tank
valve control line 702. The processor 700 controls the bladder vent
valve 108 via a bladder vent valve control line 708. The processor
700 controls the bladder valve 106 via a bladder valve control line
706.
[0032] With reference now to FIG. 5, the processor 700 can measure
pressure and temperature from the bladder 20 and tank 60. The
processor 700 reads the pressure in the tank 60 via a tank pressure
sensor 204, which is coupled to the via tank pressure line 724. The
processor 700 reads the pressure in the bladder 20 via a bladder
pressure sensor 202, which is coupled to the processor 700 via a
tank pressure line 722. The processor 700 reads temperature of the
gas in the tank 60 via a tank temperature sensor 304, which is
coupled to the processor 700 via a tank temperature line 714. The
processor 700 reads the temperature of the gas in the bladder 20
via a bladder temperature sensor 302, which is coupled to the
processor 700 via a bladder temperature line 712. The processing
system 700 may receive flow rate data from the inline flow sensor
900 via data line 710.
Volume Measurement
[0033] Ultimately, the objective of volume measurement is to know
the quantity of liquid 40 remaining in an infusion and how that
quantity changes over time.
[0034] The pressure frame 10 defines a rigid container of known
volume, V.sub.frame. This volume is known by design and is easily
verified by displacement methods. Within the pressure frame 10,
there is the air bladder 20, which has a nominal capacity greater
than the volume V.sub.frame. When expanded, the bladder must
conform to the geometry of the rigid container and its contents.
The volume of liquid 40 to be infused, V.sub.tbi, is equal to
V.sub.frame, less the fixed and known volume of the bladder 20
itself, V.sub.blad, less any incompressible materials of the bag
30, V.sub.blag, and less the volume of gas in bladder 20,
V.sub.gas. Once the value V.sub.gas is computed, then V.sub.tbi may
be computed as follows:
V.sub.tbi=V.sub.frame-V.sub.blad-V.sub.bag-V.sub.gas
[0035] With the following method, at any given point in time, the
volume of air contained in the bladder, V.sub.gas, can be measured
and V.sub.tbi can be subsequently computed.
[0036] For purposes of economy and flexibility, the pump 50 may be
an imprecise air pump, such as that of a rolling diaphragm variety,
although other types of pumps are also contemplated. The output of
such a pump may vary significantly with changes in back pressure,
temperature, age of the device, power supply variation, etc. One
advantage of the device and method disclosed herein is that they
allow an imprecise pump to be used in a precision application, by
calibrating the pump in situ.
[0037] FIG. 6 shows the steps leading to computation of V.sub.tbi.
Shown as step 802, the first step is to find an optimum amount of
air mass, N.sub.pump, to add to the bladder 20 to effect a
significant pressure change, for example, on the order of about
10%. If the amount of air mass added to the bladder is too small,
then the pressure change will not be measurable with accuracy. If
the amount of the air mass is too great, then pressure in the
bladder will increase more than necessary and energy will be
wasted.
[0038] The initial pressure in the bladder 20, .sub.P.sub.bladder1,
is measured using the bladder pressure sensor 202. The tank valve
102 is set to a closed state via the tank control valve line 702
from the processor 700. The bladder valve 106 is set to an open
state via the tank control valve line 706 from the processor 700.
The pump 50 is activated by the processor 700 via the pump control
line 750 for a period of time, S.sub.test, nominally, for example,
about 250 milliseconds. A new measurement of the pressure in the
bladder 20 is made, P.sub.bladder2. Based on the percent of
pressure change from this pumping action, a new pump activation
time, S.sub.pump, will be computed. This calculation needs no
precision; it is only intended to find an amount of pumping that
provides a significant change in pressure, P.sub.deltatarget, in
the bladder 20, for example, on the order of about 10%.
S pump = S test * P deltatarget ( P bladder 2 - P bladder 1 ) / P
bladder 1 ##EQU00001##
[0039] In step 804, the pump 50 or the tank vent valve 104 are
activated to increase or decrease, respectively, the pressure,
P.sub.tank, in the tank 60, so that it approximately equals the
pressure, P.sub.bladder, in the bladder 20. The combination of
valve and pump settings required for such adjustments are shown in
the table below:
TABLE-US-00001 Bladder Tank Pump Valve Bladder Vent Valve Tank Vent
10 106 Valve 108 102 Valve 104 Increase P.sub.bladder ON OPEN
CLOSED CLOSED CLOSED Decrease OFF CLOSED OPEN CLOSED CLOSED
P.sub.bladder Increase P.sub.tank ON CLOSED CLOSED OPEN CLOSED
Decrease P.sub.tank OFF CLOSED CLOSED CLOSED OPEN
[0040] Adjustments made in step 804 can be made iteratively until
P.sub.tank is roughly equal to P.sub.bladder, for example, within
about 5% of the relative pressure measured in P.sub.bladder. This
does not need to be a precise process. Following the adjustment,
the pressure in tank 60, P.sub.tank2, is recorded.
[0041] In step 806, the system is configured to increase the
pressure in tank 60, as shown in the above table. The pump 50 is
activated for a time period equal to S.sub.pump. After a delay of
approximately five seconds, the pressure in the tank 60 is
measured, P.sub.tank3. This delay is to reduce the effect of an
adiabatic response from the increase in pressure in the tank
60.
[0042] In step 808, the system is configured to increase the
pressure in the bladder 20, as shown in the above table. The pump
50 is activated for a period equal to S.sub.pump. After a delay of
approximately five seconds, the pressure in the bladder 20 is
measured, P.sub.bladder3. This delay is to reduce the effect of an
adiabatic response from the increase in pressure in the bladder
20.
[0043] Because the initial pressures in the bladder 20 and the tank
60 were approximately equal, the quantity of air mass injected into
the tank 60 in step 806 and into the bladder 20 in step 808 will be
roughly equal, even though the pump 50 need not be a precise
metering device.
[0044] We take advantage of several simplifications. First, the
ambient temperature for sequential steps 806 and 808 is unchanged.
Second, the atmospheric pressure during sequential steps 806 and
808 is unchanged. These conditions simplify the ideal gas law
formula and allow the use of gauge pressure measurements, rather
than absolute pressure.
[0045] In step 810, the volume of gas in the bladder 20, V.sub.gas,
can be calculated with a reduced form of PV=nRT:
V gas = V tank * ( P tank 3 - P tank 2 ) ( P bladder 3 - P bladder
2 ) ##EQU00002##
[0046] As examples of this calculation, if the pressure change were
the same in the bladder 20 and the tank 60, then V.sub.gas would be
equal to V.sub.tank. If the pressure change in the bladder 20 were
20% as large as that in the tank 60, then V.sub.gas would be 5
times greater than V.sub.tank.
[0047] Step 812 derives the value for V.sub.tbi from V.sub.gas,
using known values for V.sub.frame, V.sub.blad, and V.sub.bag and
using the calculated value of V.sub.gas, from step 810.
V.sub.tbi=V.sub.frame-V.sub.blad-V.sub.bag-V.sub.gas
[0048] The valves 102, 106, 104, and 108 can be configured in many
ways, including multiple function valves and or manifolds that
toggle between distinct states. The depiction herein is made for
functional simplicity, not necessarily economy or energy
efficiency.
Flow Rate Measurements
[0049] FIG. 3 shows the presence of an in-line flow sensor 900 in
the output line 612. FIG. 7 shows the external features of an
exemplary inline flow sensor 900. Fluid enters an inlet port 901
and exits an outlet port 902, defining a flow path therebetween. An
optical sending unit 921 passes light through a proximal housing
932 in general and through optical ribs 933a and 933b. A light
pattern is read by an optical sensing array 922. A distal body 931
houses an adjustment mechanism 910 that can be turned by the
activation of an adjustment gear 909. The proximal housing 932 and
the distal body 931 may be secured via one or more fasteners, such
as threaded fasteners 934.
[0050] The exploded view of FIG. 8 shows the internal parts of the
inline sensor 900. A first O-ring 945 and a second O-ring 944 are
shown in the illustrated preferred embodiment to create a
fluid-tight assembly, although other bonding or sealing methods are
also contemplated. A compression spring 941, e.g., a cylindrical or
conical helical spring includes a first, fixed end 962 received
within an axial bore 963 of the adjustment mechanism 910. A second
end 961 of the spring 941 bears against a sensor ball 942, which is
received within the flow path. The spring 941 applies a force to
the sensor ball 942, urging the ball in the direction opposite to
the direction of fluid flow. Alternative elements providing this
spring function may be, for example, a resilient band, a resilient
or compressible material such as a foam structure, and so forth.
The adjustment mechanism 910 may be threaded into an axial opening
964 in the distal body 931, e.g., via external helical threads
formed on the distal body 931 which are complimentary and mating
with internal helical threads within the opening 964.
Alternatively, the spring fixed end 962 may be fixed in position
and non-adjustable in such embodiments, the position of the spring
fixed end 962 is set by the design in a fixed position of spring
pre-load. Rotation of the adjustment mechanism 910 relative to the
distal body 931 axially advances or retracts the adjustment
mechanism 910, depending on the direction of rotation, and thus
causes axial movement of the fixed end 951 of the compression
spring 941 to alter the force preload on the sensor ball 942.
Alternately, the spring member may be a leaf spring having a first
end which is fixed and a second end which is deflectable in the
axial or flow direction and which bears against the ball member
942.
[0051] The interior of the proximal housing 932 is shown in FIG.
10. The sensor ball 942 (see, e.g., FIG. 9) axially slides within a
cavity 955 defined by the assembly. Optional interior ribs 951 and
interior flats 952 may be dimensioned in close tolerance to the
sensor ball 942 to allow the sensor ball 942 to travel freely
within the cavity 955 while remaining centered in the cavity 955.
Tapered walls 953 are fabricated with a draft angle, such that the
gap around the sensor ball 942 changes as sensor ball 942 is
positioned in different positions within cavity 955.
[0052] Referring to the section view of FIG. 9, assume that fluid
pressure at inlet port 901 is greater than the fluid pressure at
outlet port 902, such that fluid flows from higher pressure to
lower pressure. Fluid flow will push on the sensor ball 942 against
the urging of the spring 941. Depending on the gap between the
sensor ball 942 and the proximal housing 932 and depending on the
rate of fluid flow, a force will be exerted upon the sensor ball
942. The compression spring 941 is in contact with the sensor ball
942 such that a spring force is generated to the extent that
compression spring 941 is compressed. Fluid traverses beyond sensor
ball 942 and exits the assembly via outlet port 902. The gap
through which fluid flows between the sensor ball 942 and the
proximal housing 932 increases as the sensor ball 942 moves towards
compression spring 942.
[0053] The graph depicted in FIG. 11 shows the forces created by
fluid flow and an opposing spring force. As flow rate increases,
the force on the sensor ball 942 will increase, which pushes sensor
ball 942 in a manner that has two consequences. First, the gap
between sensor ball 942 and proximal housing 932 increases, so that
the force applied to sensor ball 942 is reduced due to a larger
effective area for fluid to travel around the sensor ball 942.
Secondly, the sensor ball 942 moves to increase the force applied
by the compression spring 941. The sensor ball 942 thus moves until
the force of the compression spring 941 is balanced by the force of
the fluid flow against the sensor ball 942. An exemplary
equilibrium point 851 is shown in FIG. 11, where the force of the
compression spring 941 is balanced with the force created by a flow
rate of 1 ml per hour and the sensor ball moves to a displacement
approximately 0.07 inches (0.18 cm) away from a seated position of
sensor ball 942.
[0054] In operation, the light source 921, which may be, for
example, an LED array, transmits light through the first optical
rib 933a and into the cavity 955. The light incident upon the ball
942 is transmitted through the ball 942 and through the second
optical rib 933b to form a light intensity pattern on the
photosensor array 922. The photosensor array 922 may be, for
example, a charge-coupled device (CCD) array, photodiode array,
complimentary metal oxide semiconductor (CMOS) digital detector
array, or the like.
[0055] The optical transmitter may include one or more light source
elements having a wavelength, for example, in the infrared (IR),
visible, or ultraviolet (UV) region and the housing and ball member
may be formed of a material that optically transmits light of the
light source wavelength. The light source may be an array of light
elements, such as LEDs, or laser, etc. The light source may be
segmented along the axis or may be a continuous, e.g., scanned or
otherwise optically formed beam. The light source may illuminate
the detector array along its length simultaneously or by
sequentially scanning along its length. The refractive effect of a
transparent ball member may have a focusing effect on the light
passing therethrough that may be detected by the photosensor array.
Alternatively, a nontransmissive ball may be employed and the ball
position may be determined by detecting the position of a shadow
cast by the ball on the photosensor array. In still further
embodiment, the ball member may have reflective surface and the
optical sensor array may be positioned to detect light reflected
from the surface of said ball.
[0056] The output of the photosensor array 922 may be passed via
the data line 710 to the processing system 700, which may include a
position-detection module or circuitry wherein the axial position
of the ball 942 within the channel 955 is determined. The axial
position of the ball 922 may in turn be used to determine a flow
rate and/or calibrate or correlate ball 922 positions with known
flow rates calculated by other means such as plural volume
measurements made using the method outlined in FIG. 6 over time, or
using the pressure decay method outlined in FIG. 12 and described
below.
[0057] In certain embodiments, the known flow rates corresponding
to axial ball positions may be stored in a memory of the processing
system 700, for example, in a table, database, or the like. In such
embodiments, when an axial position of the ball 922 is measured,
the measured position of the ball may be compared with the table of
known flow rates and the flow rate corresponding to the measured
axial position is then determined. In other embodiments,
calibration measurements of axial ball position and known flow
rates may be used to derive an algorithmic formula for mapping a
measured axial ball position to a corresponding flow rate. In such
other embodiments, when an axial position of the ball 922 is
measured, the derived algorithmic formula may then be used to
determine the flow rate.
[0058] While the present disclosure provides a currently preferred
implementation of the system herein, it will be recognized that
alternate inline flow sensors are also contemplated.
Flow Rate Calculation
[0059] Once the fluid volume has been computed, then multiple
measurements made over time will yield knowledge of fluid flow
rate, which is, by definition, fluid volume changing over time.
Repeated measurements of volume over time provided more and more
resolution of average flow rate. The average flow rate and the
volume of liquid 40 remaining to be infused can be used to estimate
the time at which the fluid volume will be delivered. If the
infusion is to be completed within some specified period of time,
any error between the specified time and the estimated time can be
calculated and the flow rate can be adjusted accordingly.
[0060] There are situations where the short-term flow rate is of
interest. Rather than make repeated volume measurements over a
short period of time, there is an alternative approach. Once the
gas volume in bladder 20 is known, then the observation of pressure
decay in the bladder can be converted directly to a flow rate. It
is important to know that the measurement of pressure decay, by
itself, is not adequate to compute flow rate. For example, if the
pressure were decaying at a rate of 10% per hour, this information
cannot be converted into flow rate, unless the starting gas volume
is known. As an example, if V.sub.gas has been measured to be 500
ml and the absolute pressure is decaying at a rate of 5% per hour,
then the flow rate is 5% of 500 ml per hour or 25 ml per hour. The
knowledge of the initial volume is critical to compute fluid flow
rate.
[0061] The measurement of pressure decay is a simple procedure of
observing the time the absolute pressure of P.sub.bladder to drop
by a small, but significant, amount, preferably for example about
2%. Because the processor 700 is capable of measuring times from
microseconds to years, this measurement carries a very wide dynamic
range. By observing a 2% drop, the change in pressure is well above
the noise floor of the pressure measurement system.
[0062] A flow chart outlining an exemplary process 1000 for
calculating flow rate by monitoring the rate of pressure decay in
the bladder 20 is shown in FIG. 12. At step 1004, the volume of gas
in the bladder 20 is calculated as detailed above. At step 1008,
the pressure in the bladder 20, P.sub.bladder is measured using the
sensor 202 at time T1, which is recorded in step 1012. The pressure
in the bladder 20 is measured again at step 1016 and the time T2 is
recorded at step 1020. The change in pressure, .DELTA.P, between
the time T1 and the time T2 is calculated in step 1024 as
P.sub.bladder-P.sub.bladder2 and the change in time, .DELTA.T, is
calculated as T2-T1 at step 1028. At step 1032, it is determined
whether .DELTA.P is greater than some predetermined or prespecified
threshold value, e.g., about 2% with respect to P.sub.bladder1. If
.DELTA.P has not reached the threshold value at step 1032, the
process returns to step 1016 and continues as described above. If
.DELTA.P has reached the threshold value at step 1032, the rate of
pressure decay is calculated as .DELTA.P/.DELTA.T at step 1036. The
flow rate is then calculated as
.DELTA.P/.DELTA.T.times.V.sub.gas-P.sub.bladder1 at step 1040.
Flow Rate Correlations
[0063] The relationship and purpose of having two independent
measurement methods for determining flow rate is best described by
referring to FIG. 13.
[0064] One purpose of the two measurement systems is to calibrate
flow measurement 865 with repeated values over time from primary
volume measurement 861. Flow measurement 865, e.g., as determined
as described above by way of reference to the flow sensor 900, is a
measurement of flow rate or the first derivative of fluid quantity
with respect to time. If one were to integrate the value of flow
measurement 865 over time, the result would be a quantity of fluid.
Any errors in this signal would accumulate, providing decreasing
volume accuracy over time.
[0065] In contrast, an integral signal, such as that from primary
volume measurement 861, e.g., calculated using the volume
measurement method as described herein, has a fixed error that does
not accumulate over time. In fact, as a percentage, the error
obtained with an integral signal will decrease over time. As an
analogy, if one were to attempt to reach a certain distance in a
determined period of time, the use of a speedometer alone would
lead to an obvious and significant error. Using this analogy, if
one were to use integral measurements, such as those provided by an
odometer and a clock, the resultant accuracy would be high.
[0066] Flow measurement 865, as described above, operates over a
very wide flow rate range and cannot, in any practical way, be
calibrated in advance to accommodate manufacturing variances and
other environmental factors such as fluid viscosity. For any given
fluid flow rate, the signal from flow measurement can be measured
and correlated with repeated measurements over time from primary
volume measurement 861. For example, if the measurement from flow
measurement 865 was observed to a value "x" over a period of ten
minutes and a measurement made by primary volume measurement 861 at
the beginning of this period was 100 mL and a subsequent
measurement made by primary volume measurement 861 at the end of
this period was 90 mL, a correlation could be made between flow
signal "x" and a flow rate of 10 mL per 10 minutes, or, 60 mL per
hour. Flow rate calibration data may be maintained in memory,
preferably a nonvolatile memory, of the processing system 700.
[0067] Another purpose of the dual measurement system is to
distinguish between two sources of fluid directed to the same
output. For purposes of distinguishing the source of fluid, assume
that flow measurement 865 has been calibrated at various flow rates
as described above. If a secondary fluid source 862 is connected to
the system, as shown in FIG. 13, and has a fluid driving pressure
greater than the fluid within the subsystem for primary volume
measurement 861, then the fluid from secondary fluid source 862
will flow towards flow measurement 865 and will block any fluid
flow coming from primary volume measurement 861 by the operation of
a one way check valve 863. In this case, the signal from primary
volume measurement 861 will be unchanging over time. In this
circumstance, the non-zero signal from flow measurement 865 will
represent fluid flow from the secondary fluid source 862.
Alternatively, the flow signal 865 may be integrated to provide an
estimate of volume delivered over any period of time. The
measurement of volume delivered from secondary fluid source is, in
the instance of an intravenous infusion system, an important
clinical measurement.
[0068] Yet another purpose of the dual measurement system is to
detect a condition where gas is expressed from the primary infusion
liquid. If a quantity of air leaves the system by way of an in-line
air elimination filter 864, then an increased pressure drop will be
observed. By itself, this increased pressure drop would indicate
that the fluid flow rate increased proportionally. If air were to
escape the system from air elimination filter 864, the signal from
flow measurement 865 would remain unchanged, providing an
indication that the pressure drop should be interpreted as an
escape of air, not an increased in fluid flow. In this
circumstance, without flow measurement 865, the pressure signal
would be interpreted incorrectly.
[0069] Yet another purpose of the dual measurement system is to
detect a condition where a leak in the pneumatic system exists. If
an air leak occurs in the system, a pressure drop will be observed.
By itself, this pressure drop would indicate that fluid is flowing
from the system. If air were leaking, the signal from flow
measurement 865 would be zero, providing an indication that the
pressure drop should be interpreted as a leak of air, not as fluid
flow. In this circumstance, without flow measurement 865, the
pressure signal would be interpreted incorrectly.
[0070] The invention has been described with reference to the
preferred embodiments. Modifications and alterations will occur to
others upon a reading and understanding of the preceding detailed
description. It is intended that the invention be construed as
including all such modifications and alterations insofar as they
come within the scope of the appended claims or the equivalents
thereof.
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