AU766731B2 - Drop detection method and apparatus - Google Patents

Description

y S&FRef: 254736D3

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Sherwood MPdi'. l mpa-y Address 1915 Olive Street- Sc.Lh s 4 s, se 9 Sof Applicant: St. I nir, u in 1-1-64-2 CH jol of Applicant: St. Louis Missouri 63103 16 12 CH-B o ScC,, Jnitd States of Amcrica L ft Actual Inventor(s): Address for Service: Invention Title: Denis Y Yerlikaya Randall Jeffrey Krohn Clarence Logan Walker Michael John Wilhelm Curtis Durrell Kinghorn Spruson Ferguson St Martins Tower,Level 31 Market Street Sydney NSW 2000 (CCN 3710000177) Drop Detection Method and Apparatus r r s The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845c DROP DETECTION METHOD AND APPARATUS TECHNICAL

FIELD

1. Field of the Invention The invention relates generally to drop detection in a medical liquid drop chamber and, more specifically, concerns a drop detection method and apparatus for use in an ambulatory or household environment.

2. Description of Related Art Medical drop chambers are used in various medical devices for metering and monitoring the flow rate of a fluid being administered to a patient. In a given drop chamber, each drop has a uniform volume of fluid. Therefore, by counting the number of drops! falling in a given time period, the flow rate can be calculated easily. Such drop chambers are used, for example, in gravity-driven or pump-driven infusion systems.

Devices are known in the art for automatically sensing the drops in a chamber. These may, for example, be connected to circuits that can compute and display the flow rate or to alarms that indicate when the flow rate is too high or too low. These drop detectors are often optical sensors that react to a drop breaking optical communication i: between a light source and a sensor. In a controlled environment, such as a hospital, few outside conditions affect the optical s&nsors. The ambient light is fairly uniform throughout the environment and the drop chamber is relatively immobile and usually kept upright.

However, in either an ambulatory or household :e environment, several factors that may affect the optical 30 sensors must be handled properly by the drop sensor to avoid false readings or alarms. These factors include widely varying ambient light conditions and excessive movement and tilting of the drop chamber, especially in ambulatory -2situations. False readings caused by these factors are a major reason for physicians' reluctance to use the ambulatory devices. It has therefore been a goal in the art that the drop detectors be capable of increased sensitivity to the drops, while being immune to the ambient light variation or movement and change in orientation of the chamber.

US Patent No. 4,720,636 to Benner, Jr. discloses a drop detection structure and detection circuitry that incudes two photodetectors, one for sensing a decrease in light caused by a drop passing in front of it, and another for detecting an increase in light caused when a drop passes nearby and reflects additional light. A drop would pass nearby, for example, if the chamber were tilted. However, in the event of a very high tilt angle, coherent drops are not always formed. The liquid may enter the chamber and immediately spread onto the interior surface of the chamber, rather than falling to the bottom of the chamber.

US Patent No. 4,718,896 to Arndt et al discloses a drop detector that includes an array of light emitter/sensor pairs arranged to detect drops falling at angles of up to degrees from the normal, vertical orientation. Tilt angles greater than 30 degrees are found in everyday use of the medical devices containing these detectors, rendering the detectors of this patent only partially effective.

DISCLOSURE OF THE INVENTION A need exists to provide an improved drop flow detector and method for detecting drops passing through a hollow drop chamber which substantially overcomes or ameliorates the above mentioned disadvantages.

There is accordingly provided a method for detecting drops passing through a substantially hollow drop chamber having an inlet end and an opposed outlet end and also 25 having an outer wall, said method comprising the steps of: passing light from at least one spaced apart light source through the drop chamber where each of the light sources has a main axis of illumination and each of the main axes of illumination are directed through the drop chamber; *4 reflecting light passed through the light chamber off an elliptical reflecting surface defining two focal points; :04: detecting the light emitted from the light sources after reflecting through the drop chamber and producing an electrical output signal related thereto; [R:\LIBE]03903.doc:edg blocking a DC component of the electrical output signal while transmitting frequency components at and above a predetermined threshold level, before said step of determining variation in the output signal; and determining variations in the output signal to determine that a drop has passed through the drop chamber.

There is further provided a drop flow detector comprising: to0 a substantially hollow drop chamber having an inlet end and an opposed outlet end, said drop chamber having an outer wall; means for detecting a drop passing from said inlet end to said outlet end, the drop passing either in a vertical direction or at an angle varied from the vertical direction including the drop moving along said outer wall of said drop chamber, said means for detecting a drop comprising: a reflective arrangement having a reflecting surface for reflecting light passed through the drop chamber, the reflective arrangement defining an ellipse having two focal points; a light source positioned at said first focal point, said light source directed through said drop chamber and toward said reflecting surface; and 25 means for detecting light emitted from said light source.

BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiments of the invention are described herein with reference to the appended drawings, wherein: 0 0000 0 0*

S

S

[R:\LIBE]03903.doc:edg FIG. 1 is a perspective view illustrating the manner in which an ambulatory patient could use a drop detection apparatus embodying the invention; FIG. 2 is a functional block diagram of a drop detection apparatus embodying the invention; FIG. 3 is a circuit schematic diagram showing a drop detection circuit according to invention; FIG. 4 is a perspective view of a portion of the infusion device, showing a mounting receptacle for a drop detector assembly.

FIG. 5 is a perspective view of a drop chamber and drop detector assembly, showing the optical path coverage of the drop detector; FIG. 6 is a wave form diagram representing typical input and output of a portion of a circuit as in FIG. 3; FIG. 7 illustrates typical output waveforms of a circuit as in FIG. 3; FIG. 8 is a flow chart representing the drop discrimination process utilized in an apparatus embodying the invention; O. FIG. 9 is a flow chart representing the routine that determines if the drop counts are valid for the feeding profile; FIG. 10 is a flow chart representing the routine to variables generated elsewhere to determine whether to generate an error message; FIG. 11 is a flow chart representing the routine that implements the routines shown in FIGS. 9 and 10 during a pumping cycle; 30 FIG. 12 is a cross-sectional view of one embodiment of the invention; FIG. 13 is a cross-sectional view of another embodiment •of the invention; FIG. 14 is a cross-sectional view of another embodiment of the invention; FIG. 15 is a cross-sectional view of another embodiment of the invention; FIG. 16 is a perspective view of the light pipe of the embodiment shown in FIG. 15; and FIG. 17 is a perspective view of the embodiment of FIG.

14 with a reflective cone placed at the second focal point.

MODES FOR CARRYING OUT THE INVENTION In FIG. 1, a medical infusion device worn by a patient is generally designated by the reference numeral 10. The infusion device includes a pump for the enteral administration of fluids. It is to be understood that while the preferred embodiment is shown for a medical infusion device, the invention can be similarly used with any device making use of a drop chamber.

As can be seen in FIG. 1, the device is capable of being attached to the belt of a patient 11 in use, while the patient 11 is completely ambulatory. The device is thus subjected to significant tilting, jarring, and accelerations that must be accurately compensated for in -the internal mechanisms and circuits of the device The block diagram of FIG. 2 represents the electrical interaction of the major electronic and electromechanical components of the device 10 and shows signal connections.

A

keypad 12 allows operator input of device parameters, such 25 as fluid flow rate, which are sent to a microprocessor 14.

The microprocessor 14, in turn, provides information to the patient on a display'16 and controls a motor-driven pump 18.

Drop detector 20, described in detail below, has a drop chamber which is interposed in the fluid flow path between a fluid supply (not shown) and the pump 18. A sensor monitoring the drop chamber detects the flow of fluid through the drop chamber and sends corresponding signals to drop detection electronics 21. The electronics 21 filter unwanted components in the signals from the detector 20 and pass the remainder to the microprocessor 14. The microprocessor 14 also returns control signals to the electronics 21, as described below.

In operation, the pump 18 feeds fluid for the patient at a rate set into the device by means of the keypad 12 and maintained by the microprocessor 14. All of the fluid that the pump 18 feeds to the patient ii must pass through a drop chamber and no dripping occurs if the pump stops feeding fluid. Since the fluid can pass through the drop chamber only in the form of drops of fixed volume, the drop count is therefore a relatively accurate measure of the quantity of -fluid supplied to the patient. Accurate drop detection therefore permits accurate metering of fluid flow and accurate control of the pump by the microprocessor 14.

FIG. 5 illustrates the removable drop chamber 22, connected in series with and interrupting a delivery tube 24 that runs from a fluid source (not shown) to a patient (11 in FIG. Fluid enters the drop chamber 22 from the top portion 26 of the tube 24 as shown 'in FIG. 15 and exits the chamber 22 through the bottom portion 28. The drop chamber 20 22 is a sealed unit, except for the entrance and exit portions 26,28 of the tube 24, which penetrate the top and bottom of the chamber 22, respectively. The chamber 22 has a generally frusto-conical light-transmissive sidewall with the smaller diameter at its bottom. The top portion 26 of the tube extends partially into the chamber 22, creating a drop formation area 32. Fluid accumulates at this area 32, until it forms a complete drop, which then falls to the bottom of the chamber 22.

When the chamber 22 is tilted, as often happens when the infusion device is used in an ambulatory manner shown in FIG. l,:the drops will not fall to the bottom of the chamber 22, but will fall onto the side of the sloped sidewall 30 of the chamber 22. The tilt angle determines where the drop will hit the sidewall 30. At tilt angles above 70 degrees from vertical, the drops do not even fall, but tend to form a puddle on the sidewall 30. at position 34.

Drop detector 20 includes a yoke 36 (see FIG. which is mounted on device 10 and a drop chamber 22 (see FIG. which is removably received within yoke 36, thus supporting the drop chamber 22 in the infusion device 10. Yoke 36 has a passageway 36a, which receives drop chamber 22 in an upright position.

In a first preferred embodiment shown in Figure 12, two light sources 38, which are preferably infrared light emitting diodes, preferably Seimens SFH 485-2 IRLEDs, are mounted side-by-side, so as to face into passageway 36a and drop chamber 22. As shown in Fig. 12, diodes 38 have a illumination angle a which in the case of the Seimens SFH 485-2 IRLEDs is about 16 degrees on either side of the main axis of illumination 39. Again as shown in Fig. 12, diodes 38 are preferably directed toward each other so that the main axes of illumination 39 form an angle of 20 degrees.

Diodes 38 are positioned in yoke 36 apart from each other opposite a detector 40 so that the main axes of iilumination 39 intersect near detector 40 as will be described in detail hereafter. As shown in Fig. 12, this positioning of diodes 38 combined with the.width of illumination by diodes 38 represented by the angle a illuminates virtually the entire cross-section of the drop chamber 22. This arrangement also provides for an overlap of illumination by diodes 38 in the center area of drop chamber 22, the area most likely to have a drop falling through it when the device 10 is not tilted significantly. The significance of this overlapping illumination pattern' will be described hereafter in connection with the description of the detectors In an alternate embodiment, the main axes of illumination 39 may be positioned to intersect at the central axis of drop chamber 22.* This alignment creates more- of an overlap of the beams of diodes 38 than in the alignment described above but produces a smaller.crosssectional area of illumination within drop chamber 22 than the just described alignment due to the increased overlap cf the beams.

In a further alternate embodiment, instead of usinc two diodes 38, three or more diodes could be placed around yoke 36 to illuminate drop chamber 22. In particular, as shown in Fig. 13, three diodes 38 could be arranged so that the main axis of illumination 39 of the middle diode is directed along the diameter of the drop detector chamber 22 from one side of yoke 36 to the other. Two additional diodes 38 are located on either side of the center diode 38 so that each of their most center directed lines of 50% illumination intersect the main axis of illumination 39 of the center diode 38 at detector 40. In this way, virtually the entire cross-sectional area of drop detection chamber 22 is illuminated. More particularly, the outer fringes of the drop chamber 22 are more brightly illuminated.

Mounted within the yoke 36 on the opposite side of the drop chamber 22 from the IRLEDs 38 is a detector 40. In the preferred embodiment, detector 40 is a rectangular 20 photodiode, preferably a Vactec VTS 3092 photodiode, measuring .6 by .1 inches (1.52 by .25 cm). It is mounted with its length parallel to the horizontal plane. The result of having two IRLEDs 38 opposite a single photodiode 40 is to create a triangular optical path 41, as viewed from above, that can be broken by a drop passing through any .portion of the horizontal cross section of the chamber 22 (as shown in FIG. If a drop contacts the sidewall 30 of the chamber 22 and then slides down the wall 30, regardless of which side it travels on, the drop will pass through the optical path between the two IRLEDs 38 and the photodiode Because the yoke 36 that holds the drop chamber 22 and the photodiode 40 is not sealed (as the drop chamber 22 and tube 24 are removable), ambient light is constantly detected by the photodiode 40, as well as light from.the IRLEDs 38.

This will be discussed in greater detail below.

In an alternate embodiment, a single detector 40 may be replaced by a series of two or more detectors located circumferentially on yoke 36 on the opposite side of drop chamber 22 from diodes 38.

In another alternate embodiment, the rectangular photodiode detector 40 may be replaced by an infrared sensitive film such as fluorocarbon PVDF, such as that manufactured under the trademark

KYNAR.

In a further alternative embodiment, instead of aligning multiple detectors 40 or a fluorocarbon PVDF film detector 40 in a plane on the opposite side of drop detection chamber 22 from diodes 38, detector(s) 40 may be located in yoke 36 on a curve around drop detection chamber 22. One preferred curve is where detector(s) 40 are located an equal distance from the outer edge of drop detection chamber 22. Another curve is where detector(s) are equidistant from a central point near or between light sources 38. In a variation on this embodiment, diodes 38 and detectors 40 may be interspersed on yoke 36 around the outer surface of drop detection chamber 22.

In all the embodiments of various numbers and arrangements of diodes 38 and detectors 40, the objective is to illuminate the largest percentage of drop detection chamber 22 and to produce the most detectable signal at detector 40 resulting from the blockage of light emitted by diode 38 by the drop either falling through or moving along the edge of drop detection chamber 22. Because light from diodes 38 spreads out by the angle a from the main axes of illumination 39, there is a space S between diodes 38 and spaces S' outside of the angle a of 50% intensity where only light emitted from diodes 38 having an intensity less than 50% of the maximum illumination value will be present.

Diodes 38 should be located around- yoke 36 so that the area of spaces S and S' within the drop detection chamber 22 are minimized. This may be accomplished by moving diodes 38 away from the outer edge of drop deteztion chamber 22.

Also, although the illumination in spaces S and S' are less than 50% of the maximum illumination, there is still illumination in this area. If a drop falls near or within space S it may still be detected by detector 40 because the drop is relatively near to diode 38 thereby causing a relatively large shadow on detector 40 compared to a drop falling closer to detector 40. Further, because the crosssectional area of the drop occupies a large percentage of the cross-sectional area of the drop detection chamber 22, a drop falling near the diodes 38, including a drop falling in or near space S, will likely also have a portion of the drop within the 50% of maximum illumination area of illumination.

Because of this, the drop will likely block enough light from diode 38 to be detected by detector 40 and its corresponding circuitry.

The objective of detecting a drop falling through a drop detection chamber that may be tilted at an angle from vertical may also be accomplished through another species of..

the invention as described in the following. These species makes use of the property of ellipses that light emitted at one focal point of an ellipse with a reflective inner :surface will be focused on at the second focal point.

As shown in Fig. 14, one embodiment of this species of drop detection device uses an elliptical reflector assembly 130 to scan a wide area within the drop chamber 22 for passing drops with the use of a single light source 38 and a single light detector 40. A light source 38 is located at a focal point A of an ellipse B (shown in dotted outline). An elliptical reflector 132 having a reflective surface 134 S 30 along the outline of the ellipse B is placed around a portion of the periphery of the ellipse. The reflector 132 preferably includes only a portion of the ellipse, although *a reflector 132 encompassing the entire surface of the ellipse may be used. In the most preferred embodiment, the inner surface 134 extends from a point E on the surface of ellipse B intersecting a line C drawn through focal points A w 11 and D, to a point F on the surface of ellipse B midway between focal points A and D. The exact location of the termination of the reflective surface 134 is not critical so long as most light emitted from light source 38 and passing' through drop detector 22 is likely to impinge on reflective surface 134 so that it will be focused at focal point D.

A detector 40 is placed at focal point D. The drop chamber 22 may be placed anywhere within the inside of the ellipse between the light source 38 and the reflective inner surface 134. Experience has shown that if the drop chamber 22 is placed relatively close to the light source 38, the shadow caused by the drops falling through drop chamber 22 will be larger and thus more detectable at detector 40 than if drop chamber 22 is placed relatively further away from the light source 38.

As can be seen with reference to Fig. 14, light emitted from light source 38 will pass-through drop detector 22 and then be reflected off the elliptical inner reflective surface 134 which will focus the light at focus D on light detector 40. Even though there will be some refractions and reflections of light on the surface of the drop chamber 22 and also as a result of interaction of light from light source 38 with the falling drops, the majority of the light emitted through the drop chamber 22 will be approximately focused at the light detector 40. As a result, a drop falling through the drop chamber 22 will cause a measurable decrease in light detected by the detector In an alternate embodiment, a light pipe insert may be used in place of the elliptical reflector 132. Light pipe 136 (FIG. 16) is preferably made of a solid transparent material such as plastic or glass. However, light pipe 136 may be hollow with transparent walls, as will be described hereafter, made of glass, plastic or similar material. In this embodiment, shown in Fig. 15, light pipe 136 has an alternate outer elliptical surface 138. The shape of elliptical surface 138 corresponds to the surface of an ellipse B' (shown in dotted outline) having focal points A' and D' Light pipe 136 has an inner surface 140 approximately in contact with the drop chamber 22. Ideally, inner surface 140 is curved with a radius of curvature extending from light source 38, the radius being just long enough to avoid touching drop chamber 22. Inner surface 140 has this curvature so that light from light source 38, in the absence of refraction by drop chamber 22, will strike inner surface at a right angle. Because inner surface 140 is relatively close to drop chamber 22, even light from light source 38 which is refracted by drop chamber 22 will likely strike inner surface 140 at an angle very near a right angle.

Because light from light source 38 strikes inner surface 140 at near a right angle, most of this light will enter light pipe 136 and not be reflected-at inner surface 140.

Light pipe 136 has an outer surface 142 defined by a line C' extending between the focal points A' and D' and beyond the focal point D. A further outer surface 144 :i 20 connects the outer elliptical surface 138 with the inner surface 140.

The outer elliptical surface 138 is coated with a reflective material so that it forms a reflective surface toward the inside of ellipse The entire surface 142 is preferably coated with a mat surface to minimize stray or S unintentional reflections off these surfaces due to the fact that a incomplete ellipse is being used. In addition, because surface 142 has a mat surface, light leaving light *_oe source 38 at a very acute angle to line when reflected 30 off of surface 138 will not be internally reflected at surface 142, but will instead be diffused into detector ee- as will be described. Surface 144 is also preferably coated •with a reflective surface to keep diffracted light within the light pipe 136. Inner surface 140 is transparent so that the light emitted frcm light source 138 and passing through the drop chamber 22 will pass substantially unimpeded into the light pipe 136.

As before, a light source 38 is placed at a focal point A' of the ellipse A light detector 40 is located on outer surface 142 at focal point Light from light source 38 passes through drop chamber 22 and into light pipe 136. There, the light reflects off of the reflective coating on surface 138 and is focused at focal point D' on detector A block 146 may be used to position and retain light pipe 136 in position with respect to drop chamber 22 and light source 38.

Although the description of this.species is described as containing only one light source 38 and one light detector 40, a cluster of light sources positioned at focal point A,A' or a cluster of light detectors positioned at focal point together or in combination with a single detector or light source, respectively, may be used as desired.

In the two embodiments just described, as well as the other embodiments described, the light emitted from the light source 38 will likely be in a cone shape expanding away from the light source 38. Ideally the light source 38 is positioned in yoke 36 so that the central axis of this 25 light "cone" will be perpendicular to the elongated axis of the drop chamber 22. Since Figures 12-15 show cross- S•sectional views of embodiments of the invention through the drop chamber 22, the central axis of the light "cone" will be in the plane of the drawing.

30 However, because the light "cone" is expanding as it moves away from light source 38, much of the light emitted from light source 38 will be emitted above and below the S" plane of the drawings of Figures 12-15. The detectors heretofore described have been designed primarily to detect light at either a single point or along an axis parallel to the plane of the drawings cf Figures 12-15. With these detectors, some of the light emitted from light source 38 will not impinge on detectors 40 and will therefore not be detected. Further, even light emitted from the light source originally in the plane of the drawings of Figures 12-15 may be refracted or reflected into paths diverging from this plane while passing through the walls of drop chamber 22 or while interacting with the drops themselves.

One solution to this problem is to expand the size of detector 40. For example, instead of a horizontal detector 40 as used in the embodiments of Figures 12 and 13, a detector 40 having a large vertical dimension in addition to the horizontal dimension could be used to detect light moving in paths divergent to the plane of the drawings.

In the embodiments shown in Figures 14 and 15, the detector 40 could have a substantial dimension along the axis passingthrough the element labeled 40 and perpendicular to the plane of the drawing while still maintaining its small cross-sectional area at the focus D,D' of the ellipse B,B, respectively (FIG. 16). With such a :i 20 detector 40, light which is divergent to the plane of the '.:drawing would still be focused along this elongated detector and would therefore be detected.

Another solution to the problem of detecting light emitted from light source 38 or diffracted by interaction with drop chamber 22 or drops therein, is to extend light i pipe 136 pbove and below the plane of the drawing in Figure S15, terminate both ends 148 of light pipe 136 with a planar surface parallel to the plane of the drawing and coat these .eo. ends 148 with a reflective material (FIG. 16). In this way, light diverging from the plane of the drawing will pass through:inner surface 140 into light pipe 136. The light will be reflected off the reflective coating on outer surface 138 in a direction toward one of the ends of light pipe 136. The light will be reflected off the reflective coating on the ends of light pipe 136 back toward the plane of the drawing and toward detector 40 where it may be detected. This embodime- c keeps the light within the light pipe where it has a greater chance of being detected than if the light were allowed to pass out of the light pipe through the ends of the light pine.

As stated above, a possible problem with the embodiments shown in Figures 14 and 15, is that light impinging on inner surface 134 (FIG. 14) or outer su .rface 138 (FIG. 15) at a very acute angle to line C or respectively, will be reflected to detector 40 also at a very acute angle. Because detector 40 will typically have a flat surface for detecting, and in the embodiment of Figure because of the possibility of internal reflections of f o f surface 142, much light reflected toward detector 40 at a very acute angle will be difficult to detect.

To solve this problem, a reflective cone 150 (FIG. 17) may be used having its central axis perpendicular to the plane of the drawings of Figures 14 and .15. and centered on what is label ed detector 40. In this embodiment, detector is moved to be located above the apex 15.2 of the cone and directed toward the cone 150. With this reflective cone, 0@S@ light approaching focus D or D' at an acute angle to line C 0or respectively, as well as all other light approaching 0* @0 focal points D,D' will be reflected of f of the reflective surface of the cone 150 into detector 40. Virtually all the S 25 light impinging on detector 40 will strike the typically *s flat surface of detector 40 at a nearly perpendicular angle 0000 which increases the probability that the light will be detected.

In the embodiment shown in Figure 14, the reflective 30 cone 150 as described above could be placed at focal point

D

with detector 40 positioned by yoke 36 above and directed toward the apex of the .reflective cone. In the embodiment Figure 15, a cone shaped recess could be cut into light pipe 136, oriented as described above. The surface of the cone should then be coated with a reflective coating so that light reflected off of outer surface 138 will then be reflected off of this reflective coating into detector Again, in this embodimen, detector 40 would be located above and directed toward the apex of the reflective cone.

Diodes 38 and detecter 40 are referably offset from the top of the drop chamber 22 by a sufficient distance to allow drop formation fro the drop formation area t 32.

With a high tilt angle of the drop chamber 22 and varying ambient light conditions, the changes in light actually caused by drops can-be relatively small and difficult to detect with the photodiode 40. To compensate for these conditions, the photodiode 40 is preferably connected to a drop detection circuit 41, schematically illustrated in FIG. 3. The drop detection circuit filters out any unwanted portions of the signal from the photodiode 40 and amplifies the remainder of the signal, which is presumably caused by drop flow. The microprocessor 14 processes the output signal from the circuit 41 to determine Kif proper flow is occurring and control pump 18 and display 16 accordingly.

20 The drop detection circuit.41 shown in FIG. 3 includes a driver circuit 42 that powers the two IRLEDs 38 and preferably provides a constant current supply to the IRLEDs 38 to maintain constant optical output. Any variation in S the optical output would add unwanted signals to the photodiode 40, so constant optical output is important.

A

detector circuit 44 receives electrical signals from the photodiode 40 and converts them to a signal indicating whether or not a drop is flowing.

3 The detector circuit 44 includes an operational amplifier 46, which amplifies the signal from the photodiode after which it is applied to a low pass filter 47.

Filter 47 is a switched capacitor low pass filter, preferably a National Semiconductor Corporation LMF60-100.

-i filters out any componets the signal above a nominal cut-off frequency that is ate-inec ry an input clock signal 48 from the microprocessor 14 of the infusion device The drop rate is directly proportional to the speed of the pump motor, which is constant and controlled by the microprocessor. The flow rate number of drops per unit time) is varied by starting and stopping the pump motor for different time periods. The microprocessor thus produces a filter clock signal 48 to control the cut-off frequency of the filter 47, based on this known speed and drop rate. In the preferred embodiment, the filter clock is at 3-20Hz, and filter 47 is designed to divide the filter clock by 100 to derive a cutoff frequency of 3.2 hz.

Connected in series between the photodiode 40 and low pass filter 47 is a capacitor Cl. This capacitor blocks the DC component of the voltage produced by the photodiode which is typically developed in response to the ambient Slight level. Only variable signals, such as those caused by drops, are passed to the filter 47. Some changes in ambient light may also produce signals that will pass through the 20 capacitor to the filter 47. However, the cutoff frequency determined by the microprocessor tends to limit the filter's passband narrowly to only signals produced by drops.

Blocking the DC component of the signal from photodiode 40 also allows the relatively weak signals from the photodiode to be amplified with a much higher gain than would normally be possible. If the gain of the operational amplifier 46 in filter 47 (approximately 70) were applied to the signals of conventional drop photodetectors, the amplifier 46 would saturate.

30 After amplification and filtering, the signals are passed through a differentiator circuit 49. The effect of this circuit 49 on the signals is illustrated in FIG. 6, wherein the upper waveform 6a represents typical output of the low pass filter 47, which is input to the circuit 49, and lower waveform. 6b represents the output of the circuit 49. As can be seen in FIG. 6, the circuit outputs a positive pulse in response to a negative slope of waveform 6a, preferably a slope greater than .3 volts per second which has been found to be a reliable indicator of drop flow. The duration of the positive pulse equals the duration of the negative slope of waveform 6a.

The signals are then passed through logic inverter and on to the microprocessor 14. For the microprocessor 14 to consider signals from the drop detection circuit 44 as representing a valid drop, there must be a rising edge, a falling edge and a subsequent minimal hold time, preferably milliseconds. As seen in FIG. 7, at least three different types of inputs from the drop detection circuit 44 to the microprocessor 14 will result in a valid drop being detected.

In waveform 7a of FIG. 7, a long positive pulse is followed by the necessary hold time. This can occur when the drop chamber 22 is tilted at a high angle and a drop slides down the side of the drop chamber 22 past the photodiode more slowly than if it had fallen to the bottom 20 of the chamber 22.

SIn waveform 7b of FIG. 7, a narrow positive pulse is followed by the requisite hold time. This represents a drop passing quickly past the photodiode, such as when the chamber 22 is in its proper vertical position.

In waveform 7c of FIG. 7, several narrow positive pulses are followed by the requisite hold time. This can represent any of various conditions, one of which is a drop bounding from excessive agitation of the infusion device FIG. 8 is a flow chart representing the process 30 performed by microprocessor 14 to determine if a valid drop has occurred, based upon signals such as those illustrated in FIG. 7. Processor 14 performs this routine every 1.36 msec. on an interrupt basis. The microprocessor 14 makes use of three sof-ware flacs to keep track of the transitions in the signal received frc- drop detection electronics 21.

The flag DROP ACX is raised upon the occurrence of a negative transition if not previously set. The second and third flags reflect past states cf DROP_DETECT, the bit in microprocessor 14 memory that shows the status of drop detection electronics 21. Flag LAST_DROP shows the status of DROPDETECT at the end of the previous iteration; flag THIS_DROP shows that status of DROP_DETECT at the end of the current iteration.

Referring to the flow chart of Fig. 8, the present routine is entered at block 110. Timer T_HOLD is tested at block 112 to determine if 50 msec has passed since the last transition of DROP_DETECT. If 50 msec has passed, the software tests LASTDROP in block 114 to get the status of DROP DETECT in the previous iteration, otherwise execution passes to block 120. If LASTDROP is low at block 114, DROPACK is tested in block 118, otherwise execution passes to block 120. If DROP_ACK is high at block 118, execution passes to block 120. This signifies that the drop has already been'acknowledged and counted by the microprocessor 14, as will be seen below.

20 Flags THIS_DROP and LASTDROP are compared at block 120. If they are not equal, a transition of DROPDETECT has occurred, and DROP_ACK and T_HOLD are reset at block 124, and execution passes to block 126; if they are equal, T HOLD is incremented at block 122, and execution continues at block 126.

At block 126, LAST_DROP is set equal to THIS DROP and then THIS_DROP is set equal to DROP_DETECT. The routine then ends at block 128.

It should be appreciated that, in operation, it will 30 require many passes through the process illustrated in FIG.

8 to detect the occurrence of a valid drop. For example, should waveform 7a of FIG. 7 be encountered, DROP ACK will be set to 1 at block 11 upon the occurrence of a negativegoing transition followed by a 50 :sec hold. Thereafter, blocks 112, 120, 122 and 126 are rerformed in repeated sequential passes until a positive transition is seen by block 120. In the next pass through the routine, blocks 112, 120, 122 or 124, and 126 are performed until THOLD exceeds 50 msec. At this point, a valid drop is detected, and DROPACK is set until the next transition of

DROPDETECT.

When a waveform such as waveform 7b in Fig. 27 is encountered, it is handled in precisely the same manner as just described, except that the negative transition is detected much sooner than it was with respect to waveform 7a.

Should a signal such as waveform 7c be encountered, the initial positive and negative-going transitions are handled in the same manner as they were from waveform 7a. Whenever a transition occurs in DROPDETECT as tested in block 120, THOLD and DROP_-ACK are reset in block 124. This action will continue until no transitions are detected within a msec window. The state of LASTDROP is then tested in block 114; if low, DROP_-ACK is tested in block 116. If*DROPACK is low, a valid drop is counted by the microprocessor 14 and DROP_-ACK is set in block 118 Variable drop_cnt is incremented in order to accumulate the number of drops in a pumping cycle.

The microprocessor has software embedded in it* that allows an error message to be generated f or the user. This error message warns the pump operator that an insufficient number of drops is passing through the drip chamber, due to a set occlusion or an empty formula container. Figures 9-11 :illustrate the embodiment of software that tracks the number of drops and generates an error for the operator when 3 0 certain requirements are not met.

Figure 9 describes the setting of parameters used to determine if the drop counts are valid for the given feeding profile. After the program is started, it passes to step 100. Step 100 sets variable max-f-err equaal to 4, which is the maximum number of consecutive pumping cycles (motor activations) that may occu-r with insufficient drop counts before generating an error message. From step 100, the program passes to step 102.

Step 102 tests variable flow for a value less than 100.

The flow variable contains the pumping rate in units of ml/hr. If flow is greater than or equal to 100, the program passes to step 108 where variable mindrop is set to 4.

From step 108, the routine terminates. The effect of this cycle is that the software must see 4 or more drops during a pumping cycle at flow rates of 100 ml/hr or greater for the cycle to be error-free. If flow is less than 100, control passes to step 104, where flow is again tested to see if the flow rate is lower than 6 ml/hr. If variable flow is less than 6, the program passes to step 110 where variable max_f_err is set to 3. This means that at flow rates less than 6 ml/hr, a flow error may be generated if two consecutive cycles have an insufficient number of drops.

If in step 104 flow is greater than or equal to 6 meaning that the flow.rate is greater than!or equal to 6 ml/hr, control passes to step.106. :At step 106, flow is 20 tested against the value of 11. If flow -is less than 11 meaning that the flow rate is less than 11 ml/hr, the program passes to step 112 where variable max_f_err is set to 3. Otherwise, the software passes from step 106 to step 114. Step 114 sets the variable min_drop equal to 2. The routine then terminates.

In summary, Figure 9 sets the variables max_f_err and min_drop depending on the value of flow. This is summarized below. The variable'flow contains the value of the flow rate in ml/hr.

FLOW MAX_FERR MIN DROP 3 2 6-10 3 2 11-95 4 2 100-400 4 4 Figure 10 describes the testing of variables used to determine whether or not to generate an error message. This routine is only accessed when an insufficient number of drops has been detected in a pumping cycle.

The routine starts in step 200, where bit drops_checked is tested. If this bit is set, control passes to the end of the routine, meaning that this routine has been executed at some previous time in the pumping cycle. If bit drops_checked is not set, control then passes to step 202, where variable drop_cnt is compared to variable min_drop.

Variable drop_cnt is incremented in the software described in Figure 8. This variable accumulates the number of valid drops detected in a pumping cycle. If drop_cnt is less than min_drop control passes to step 204. At step 204, variable f_errctr is incremented to accumulate the number of consecutive cycles with insufficient drop counts. Also at step 204, bit f_enow is set to indicate than an "'insufficient drop count has occurred in the present cycle.

From step 204, the program passes to step 206. If the 20 comparison in step 202 indicates that a sufficient number of drops has been seen for the cycle so that drop_cnt is greater than or equal to min_drop, the routine ends.

Step 206 compares the number of consecutive pumping cycles with insufficient drop counts to a threshold set previously in ferr_ctr. If variable ferr-ctr is greater than or equal to variable maxf_err, the program passes to step 208 where appropriate flags are set that later cause an error message to be displayed to the pump operator. If the comparison in step 206 is not true, the program passes to step 207 where bit drops_checked is set. After executing step 207, the routine ends.

Figure 11 describes the use of the software in Figures 9 and 10 during a pumping cycle. The routine in Figure 11 would start when the operator commands the pump to begin delivering formula. Step 300 executes the SET_ERROR_CONDITIONS routine as described in Figure 9 and then passes to step 302.

Step 302 resets three variables f_enow, dropcnt, and f_err_ctr and passes to step 304. Step 304 resets variables cycle_time (pumping cycle timer) and bit drops_checked.

From step 304, the program passes to step 306. Step 306 tests cycle_time to see if the pumping cycle has gone on for approximately 38 seconds. If this is true, the program passes to step 308 where the routine check_underflow is executed as described in Figure 10 so that the drop counts for the cycle are tested. From step 308, control passes to step 310. If step 306 is not true, control also passes to step 310.

Step 310 again tests cycle_time against another variable CALC_TIME. CALC_TIME is set in another piece of code, and is simply used to control the length of the pumping cycle, and therefore the delivery rate. The pumping cycle is through when cycletime equals CALCTIME. If Svariable CYCLETIME shows that the pump is not at the end of :20 the pumping cycle so that CYCLE_TIME does not equal CALL_TIME, control passes to step 312. If the pumping cycle is through, control passes from step 310 to step 314, where the routine check_underflow (Figure 10) is executed. From step 314, the program proceeds to step 316, where bit fe now is tested. If this bit is true, an insufficient o number of drops has passed through the drop chamber 22 in the current cycle and therefore variable f_errctr is not reset in step 318. instead, the program passes from step 316 directly to step 320. Variable ferrctr is only reset in step 318 when a sufficient number of drops have passed through the drop chamber 22 in the current pumping cycle, as indicated by bit f_e now not being set in the test in step 316. From step 318, the program passes to step 320. Step 320 resets bit fe_now before control passes back to step 304.

If an error condition is generated because of an insufficient number of drops or some other condition, the test in step 312 will break the execution of the loop described by steps 306, 310, 312, and control will pass to step 322. If no error condition is detected at step 312, the program passes to step 306. Step 322 generally stops the pumping cycles and allows the software to proceed to an error handler. This portion of software then ends.

From the above description of the preferred embodiments, it can be seen that the effect of movement and tilting- of the drop chamber 22 on the output of the detection electronics is eliminated, while the effect of changes in ambient light are minimized. As a result, a drop chamber 22 may be accurately monitored in an ambulatory and changing environment.

While the disclosed embodiment of the invention is fully capable of achieving the results desired, it is to be understood that this embodiment has been shown and described S: for purposes of illustration only and not for purposes of 20 limitation. Moreover,;.those skilled in the art will appreciate that many additions, modifications and substitutions are possible without departing from the scope and spirit of the invention as defined by the accompanying claims.

Claims (8)

1. A method for detecting drops passing through a substantially hollow drop chamber having an inlet end and an opposed outlet end and also having an outer wall, said method comprising the steps of: passing light from at least one spaced apart light source through the drop chamber where each of the light sources has a main axis of illumination and each of the main axes of illumination are directed through the drop chamber; reflecting light passed through the light chamber off an elliptical reflecting surface defining two focal points; detecting the light emitted from the light sources after reflecting through the drop chamber and producing an electrical output signal related thereto; blocking a DC component of the electrical output signal while transmitting frequency components at and above a predetermined threshold level, before said step of determining variation in the output signal; and :o :....determining variations in the output signal to determine that a drop has passed through the drop chamber.

2. The method as claimed in claim 1, wherein a first light source is positioned at a first focal point of the ellipse defined by the reflective arrangement.

3. The method as claimed in claim 2 wherein said first focal point is relatively farther from said reflecting surface and said second focal point. S 30

4. The method as claimed in claim 1, wherein the elliptical reflecting surface is provided by a reflector.

The method as claimed in claim 1, wherein the elliptical reflecting surface is provided by a light pipe made of a material transparent to light and having an [R:\LIBE]03903.doc:edg -26- outer surface of a reflective coating to reflect light incident upon the reflective coating.

6. A drop flow detector comprising: a substantially hollow drop chamber having an inlet end and an opposed outlet end, said drop chamber having an outer wall; means for detecting a drop passing from said inlet end to said outlet end, the drop passing either in a vertical direction or at an angle varied from the vertical direction including the drop moving along said outer wall of said drop chamber, said means for detecting a drop comprising: a reflective arrangement having a reflecting surface for reflecting light passed through the drop chamber, the reflective arrangement defining an ellipse having two focal points; a light source positioned at said first focal point, said light source directed through said drop chamber and toward said reflecting surface; and means for detecting light emitted from said light source.

7. A drop flow detector as claimed in claim 6, wherein the reflective arrangement comprises: a reflector having an elliptical reflecting surface, said elliptical S* reflecting surface defined by a portion of the surface of an ellipse having a first focal point and a second focal point, said first focal point being relatively farther S. .from said reflecting surface than said second focal point; :i

8. A drop flow detector as claimed in claim 6, wherein the reflective arrangement 0 0 comprises: a light pipe made of a material transparent to light, said light pipe having an outer surface defined by a portion of the surface of an ellipse having a [R:\LIBE]03903.doc:edg -27- first focal point and a second focal point, said outer surface having a reflective coating so that light from within said light pipe striking said outer surface is reflected back into said light pipe, said first focal point being relatively farther from said reflecting outer surface than said second focal point, said light pipe placed near said drop chamber. DATED this first Day of August, 2003 Sherwood Services AG Patent Attorneys for the Applicant SPRUSON FERGUSON e* ee* [R:\LIBE]03903.doc:edg