HK1161155B - Infusion pump and method of in situ measuring the diameter of an infusion tube - Google Patents

Description

Infusion pump and method for in situ measurement of diameter of infusion tube

Background

The field of the invention is infusion pumps and generally relates to systems, devices and methods for pumping or infusing volumes of medical fluid to a patient, typically via an intravenous line.

Infusion pumps are used to infuse drugs and liquids to patients, typically via intravenous lines. While some infusion pumps handle relatively large volumes, it may be more interesting to have a pump that is capable of delivering only a very small controlled volume of liquid. The drugs used may be of great importance, such as analgesics, anesthetics including opiates, anti-inflammatory agents, insulin, anti-spasmodics, antibiotics, chemotherapeutic agents, cardiovascular drugs, and the like. Many such drugs need to be administered continuously at very low doses so that a stable and reliable flow, e.g. 0.1ml per hour, is administered to the patient over a long period of time. If pulses are used, the dose rate, measured in nanoliters or microliters per pulse or bolus, can be measured. Whether a small volume pump or a larger volume pump is used, the accuracy of the pump can lead to successful results for the patient.

Some infusion pumps have a pumping chamber with inlet and outlet valves along the length of the tubing. The infusion fluid is received into a length of tubing within the plenum chamber through an open inlet valve, and the tubing is then closed to isolate the infusion fluid by closing the inlet valve at the inlet of the tubing. The outlet valve is then opened and the pumping mechanism compresses or otherwise massages the length of tubing of interest to pump or expel fluid from the pumping chamber to the patient. Since the inlet is blocked by the closed valve, liquid can only flow out through the outlet with the valve open. The outlet valve is then closed and the inlet valve and pumping mechanism are opened to allow additional fluid from the fluid source to enter the pumping chamber. The above process is referred to as a single pumping cycle or stroke.

The pumping mechanism may comprise a single pumping member that presses the tubing against a stationary block or platen. In this case, the pumping member or pressure plate may have a length substantially similar to the length between the inlet and outlet valves. Alternatively, the pumping mechanism may comprise a plurality of pumping fingers or members that sequentially compress the tube. In this case, the inlet and/or outlet valves may not be required, especially if there are enough pumping fingers such that at least one finger compresses the tube at any time.

The overall infusion accuracy depends on the accuracy of each pumping cycle. In other words, it is important to know exactly the volume of fluid pumped per pumping cycle to know the total infusion volume over time. The volume of each pumping cycle depends on the internal diameter of the tube. The problem arises from the difference in internal diameter from tube to tube. Such differences are caused by, among other things, manufacturing processes and tolerances. It would be advantageous to enable an infusion pump to determine or measure the internal diameter of a particular IV tube for a particular infusion. Based on this information, the pump can adjust the function of the pumping mechanism (speed and stroke length of the pumping mechanism) to ensure and maintain accuracy regardless of the difference in the inner diameters of the tubes.

In addition, the pump can use this information to avoid over-compressing the tubing (shortening the life of the tubing due to over-stress) and under-compressing the tubing (resulting in inaccuracies and inefficiencies).

Infusion pumps are used to accurately infuse medications and other fluids to a patient. By knowing exactly the internal dimensions of a particular tube used to dispense a particular liquid for a patient, the amount dispensed can be improved.

Disclosure of Invention

The present disclosure includes an infusion pump that can deliver a specified amount of a drug, such as insulin or morphine, to a patient. The pump delivers exactly the correct medication in the indicated amount, ensuring that the patient achieves the best possible effect. The pump operates using tubing, particularly contact length tubing, to deliver medication from a source such as an intravenous ("IV") container through the contact length so that the medication does not contact air, thereby avoiding patient contact with contaminants. Typically, the tube is prepared by extruding a plastic material from a die. The dimensions of the resulting tube, such as the inner or outer diameter, may vary by as much as three percent or four. The pump of the present invention overcomes this problem by determining the actual dimensions of the contacting portions of the tubes during use.

The infusion pump operates using at least one sensor that measures the distance between the physical restraints that hold the tubing in the contact area. Physical restraints may include a stationary surface or platen and a moving surface or platen between which the tube compresses and decompresses. In particular, a method and corresponding system comprises the steps of: loading the tube into the clamp; compressing the tube between opposing surfaces of the clamp; receiving a signal indicative of compression of the tube while compressing; receiving a signal indicative of a contact length of the tube against at least one of the opposing surfaces; and calculating the diameter and thickness of the tube.

Another method and corresponding system comprises the steps of: loading the tube into the clamp; compressing the tube between opposing surfaces of the clamp; generating and receiving a signal indicative of a distance between opposing surfaces while compressing; generating and receiving a signal indicative of a contact length of the tubing against at least one of the opposing surfaces; and calculating the inner diameter and thickness of the tube.

The above-described methods and systems are particularly applicable to infusion pumps. The infusion pump comprises: a tube gripping section having a movable portion and a stationary portion; a first sensor mounted on one of the movable part or the stationary part for detecting a distance between the movable part and the stationary part; at least one second sensor for detecting a contact length of the pipe with at least one of the movable part and the stationary part; an inlet valve; an outlet valve; and a spool having a spool static portion and a spool active portion configured to compress a length of tubing between the spool static portion and the spool active portion, wherein the spool active portion moves toward and away from the spool static portion to operate the infusion pump.

Alternatively, the infusion pump comprises: a housing; a tube gripping section having a movable portion and a stationary portion and mounted on the housing; a first sensor mounted on one of the movable part or the stationary part for detecting a distance between the movable part and the stationary part; at least one second sensor for detecting a contact length of the pipe with at least one of the movable part and the stationary part; and a positive displacement pump for manipulating the tube to accurately deliver the drug.

It is therefore an advantage of the present invention to provide a system and method for compensating for tubing manufacturing degradation in determining the volume of medical fluid pumped through a tubing pump.

Another advantage of the present invention is to provide a system and method for compensating for tubing loading degradation when determining the volume of drug fluid pumped through a tubing pump.

Additional features and advantages will be described herein, and will be apparent from, the following detailed description and the figures.

Drawings

FIG. 1 is a front view of a spool-type infusion pump having the tubing measurement system and method of the present disclosure.

FIG. 2 is a schematic flow chart showing in a general manner the algorithm used by the pump controller to make the tubing measurements of the present disclosure.

Fig. 3A, 3B and 4A to 4C are elevational cross-sectional views of the tube contacting portion of the spool-type infusion pump of fig. 1, showing the tube compressed within the clamp.

Figures 5A and 5B are elevational cross-sectional views illustrating one embodiment of an apparatus and method for measuring compression of a tube.

Fig. 6 is a perspective view showing a planar sensor array for use in the embodiment of fig. 5A and 5B.

Fig. 7A and 7B are elevational cross-sectional views illustrating one embodiment of an apparatus and method for measuring the distance between two platens.

Fig. 8A and 8B are elevation cross-sectional views illustrating one embodiment of an apparatus and method for measuring the distance between two platens and the contact length of a tube with an upper platen.

Fig. 9 to 11 are graphical views showing sensor readings obtained from the above described apparatus and method.

Figures 12A through 12D are various views illustrating one system and method for correcting tube misalignment.

FIG. 13 illustrates an alternative cam driven pump embodiment of the tube measurement apparatus and method of the present disclosure.

Detailed Description

Referring now to the drawings, and in particular to FIG. 1, one embodiment of a spool-type infusion pump 50 of the present disclosure is shown. Infusion pump 50 includes tube 16, inlet valve 52, outlet valve 53, and spool portion 10 having upper movable platen 12 and lower stationary platen 14. The valves 52 and 53 and the spool valve portion 10 are actuated by linear actuators 54a to 54c, respectively. Pump controller 100 may work with other processors of infusion pump 50, such as a monitoring processor and a safety processor (not shown), controlling pump 50 and its linear actuators 54 a-54 c.

To pump fluid, actuator 54a opens inlet valve 52. Actuator 54b closes outlet valve 53 and actuator 54c retracts pressure plate 12 to allow tube 16 to open to receive liquid medication, for example, by gravity. Next, actuators 54a and 54b reverse the state of valves 52 and 53, respectively, and actuator 54c pushes platen 12 toward platen 14 to compress tubing 51, thereby displacing the volume of fluid that just fills tubing 51 between platens 12 and 14.

As described in detail below, sensors 18,19 (e.g., sensor pairs) are embedded within the movable platen 12 and the stationary platen 14. The transmitter 18 may be attached to the movable platen 12 and the receiver 19 to the stationary platen 14. In use, when the valve plunger movable platen 12 closes the tube 51 to pump fluid to be infused into the patient, the transmitter 18 and receiver 19 respectively send and receive signals and detect the distance between the pair, as will be discussed in detail below. At the same time, a sensor array 24 of a plurality of sensors detects the length of the tube segments in contact with platens 12 and 14, as discussed below. In this way, the sensors 18,19 and sensor array 24 detect and measure the compression distance and contact length of the tubing and send these data to the controller 100 to calculate the exact volume of solution actually pumped. This sensing may be repeated for each pumping stroke. The pump controller 100 then sums the accurately determined volumes to adjust the frequency and/or distance of movement of the movable platen 12 to ensure accuracy.

Referring now to FIG. 2, a high level flow chart illustrates one embodiment of an algorithm or process flow diagram performed by the controller 100 for various embodiments described herein. The first step 101 of the process is to load a tube into the spool valve portion 10. After loading, the tube is compressed between two opposing surfaces having sensors that measure the distance between the two surfaces and the length of contact between the tube and at least one of the surfaces in step 102. While the test is being performed, the controller 100 monitors the sensor for a change in signal output in step 103. When there is little change in the signal, the movement of the surface may be stopped, as shown in step 104. Next, the signal is recorded in step 105 and the controller 100 performs calculations to determine the contact length of the tube, the thickness of the tube, and the inner and outer diameters of the tube, as shown in step 106. The controller 100 then calculates the actual pumped liquid volume using the actual dimensions described above and adjusts future pumping (e.g., stroke frequency) so that the actual total volume of fluid pumped is equal to the target total volume.

Referring now to fig. 3A and 3B, the spool portion 10 of the infusion pump 50 of fig. 1 is shown in greater detail and includes a lower stationary platen 14 and an upper movable platen 12 that work in conjunction with the tubing 16. In the illustrated embodiment, lower platen 14 is parallel to upper movable platen 12. The tube 16 is typically PVC, but may be made of polyEthylene, polypropylene, another medically acceptable plastic, or a combination thereof. In FIG. 3A, prior to compression, the tube 16 has a tube thickness t and an outer radius R when initially disposed within the spool portion 10₀。

As shown in fig. 3B, when movable platen 12 is closed, tube 16 is compressed. In fig. 3A and 3B, d is the distance between upper platen 12 and lower platen 14, r is the radius of the continuously varying tube curve, and the point where the tube separates from the platens defines the tangential contact distance of the tube with the platens. The length l of the tube defines this contact distance with upper platen 12 and lower platen 14, and wherein the edges of l define the tangential release points, as indicated by the arrows on the left and right sides of tube 16.

The formulas shown in fig. 3A and 3B will now be explained. The continuous profile of the sides of tube 16 extends from the contact length of the tube with lower platen 14 and upper platen 12, which are parallel to each other. The newly formed curve is thus a semi-circle with a radius r, which is equal to half the distance d between the platens. Each newly formed semi-circle has a curve length equal to pi r and the total length of the two semi-circles or ends is 2 pi r. The following implications are expressed in conjunction with the formula shown in FIG. 3A: the total external curve length of the tube 16, which is equal to the circumference 2 pi R of the tube 16 in its circular shape, is constant during compression₀. When the tube is compressed to the position shown in the right-hand portion of fig. 1, the total outer curve length is equal to the length of the two newly formed semi-circles, 2 pi r (or pi d) + twice the contact length l. The actual radius R of the pipe is required to be solved₀Then, R is known₀Is d/2+ l/pi. In fig. 3A, tube 16 is just tangential to platens 12 and 14. In this case, the distance d between the two pressure plates is exactly the outer diameter 2R of the tube 16₀. Thus, the contact length is zero, and twice this length, 2l, is also zero. The circumference of the tube is thus pi times the measured distance d, i.e. the diameter, and is equal to 2 pi R, or in this case 2 pi R₀. The area in the plane of the tube is pi times the square of the inner radius and the volume is calculated by multiplying this area by the length of the tube or the length of the liquid propelled by the infusion pump.

To measure the diameter of the tube using this method, the tube can be compressed to any position in theory. As shown in equation box 2 of FIG. 3B, the contact length l₁、l₂Taken from two different distances d₁And d₂To (3). Contact length l₁And l₂Is proportional to the difference in the platen distance d during the compression step. Using these corresponding values, the change in contact length l can be determined using the change in platen distance d. Furthermore, when measuring the diameter of a pipe using this method, multiple tests can be performed by compressing the pipe to many different locations, and then using the average of all calculated values to obtain a more accurate value.

Fig. 4A to 4C show a typical situation in which, when the movable pump platen 12 is closed, the tube 16 is compressed and the tube 16 is squeezed against the lower stationary platen 14. The distance d between platens 12 and 14 is measured with an ultrasonic sensor (sensor pair 18 and 19) having, in this embodiment, a transmitter 18 at the top platen 12 and a receiver 19 at the bottom platen 14. Many infusion pumps already include an ultrasonic sensor that acts as an air-in-line (AIR-IN-LINE) sensor. Such often pre-existing sensors may be used as sensors 18 and 19 of infusion pump 50 (fig. 1). Other embodiments may use capacitive sensors, linear sensors such as linear variable differential transformers ("LVDTs") or other distance measuring sensors. As shown in FIG. 4B, as upper platen 12 is lowered, tube 16 flattens to a distance d.

As shown in fig. 4C, when movable platen 12 is fully lowered, tube 16 compresses such that platens 12,14 are separated only by tube 16 itself and distance d is twice the thickness t of the tube wall. The controller 100 may utilize the sensors 18,19 to see that the distance d is no longer changing and determine that the tube 16 is fully compressed as shown in fig. 4C. Even before full compression, the distance sensors 18,19 still show that the distance d changes very slowly when the platens 12 and 14 are very close to each other, separated only by the thickness of the tube itself, and no air or liquid is present in the tube. As described in connection with fig. 1, the actuator 54c applies a force to the upper platen 12 to close the platen against the stationary platen 14. A force sensor may also or alternatively be provided and observed for an increase in force to signal the full compression of fig. 4C. The power or current draw of the actuator 54c may also or alternatively be monitored to observe an increased value of current draw and indicate whether compression is complete.

As shown in fig. 5A and 5B, there are further alternatives to determine the distance traveled or the amount of tube compression. As described above, the spool portion 10 includes a movable upper or platen 12, a bottom platen 14, and a tube 16. Here, however, platen 12 is moved by a motor 20 (e.g., linear) having an encoder 22. The motor may include a lead screw, ball screw, jackscrew, or the like, for converting rotational motion into translational motion to move platen 12 against tube 16. The motor 20 and encoder 22 are connected to the controller 100 as shown above, which provides position information that is available to the controller 100 and converted to velocity information. The controller 100 controls the motor 20 and records data from the encoder 22 that is related to the position of the shaft of the motor 20 and translates the position or change in position into an accurate calculation of the change in the translational position of the platen 12. The travel and position of platen 12 can be determined at any time, starting from a known position, using information from the encoder and tracking the recorded distance d over a number of discrete time periods during compression of tube 16.

Fig. 5A, 5B and 6 show the sensor array 24. The sensor array 24 includes at least two sensors 26 separated by a distance l. As shown in FIG. 6, there may be one or more columns or rows of at least two sensors 26, each row or column separated by a distance 1, which may be the same or different. For many applications a single row of two sensors 26 may be sufficient. Two sensors 26 detect the presence of the tube 16 between the sensors 26. When the tube 16 no longer contacts the sensor 26, the sensor 26 does not indicate that a tube is present. This may occur when there are only tangential points of contact between the tube and the platens 12 and 14. The distance l between the sensors 26 may be set to a length of: when the distance d between the platens was 2t, the tube edge just touched the sensor. For a given tube configuration having a known inner diameter, outer diameter and wall thickness, when there is a known amount of tube compression, the tube length reaches a length l, and the tube thickness is about 2t, platens 12 and 14 will be spaced apart by a distance d. To set the distance l, a measurement can be made and the calibration data points used to measure the length l of the tube contacting the platen and the sensor when the platens are separated by a distance d and the tube 16 contacting the platen has a thickness of 2 t.

The sensors 26 of the array 24 are able to detect the presence of a tube when it is pressed against the pressure plates 12 and 14 of the spool valve portion 10. For example, small compact pressure sensors, capacitive sensors, or inductive sensors may be used. Sensor 26 detects the presence of a flattened section of tube 16 as described above. When the tube is in only tangential contact with the platen, the sensor 26 will cease detecting the presence of the tube 16. For example, when tube 16 contacts pressure sensor 26, the pressure sensor will display a rapid rise or sharp rise in pressure. When the pressure is removed, the pressure drop and pressure signal will be equally rapid. The capacitive sensor 26 will operate in a similar manner to quickly detect tube material, particularly wet material, when the tube is in proximity to a capacitive sensor, such as two capacitive sensors 26 spaced apart by a known distance l.

In addition to the sensor array 24 shown above, there are other methods that may be used to detect contact length and tube compression. Fig. 7A and 7B show the spool portion 10 also having a stationary platen 14, a movable platen 12, and a length of infusion pump tubing 16. In the illustrated embodiment, a capacitive sensor 30 is mounted on the top or movable platen 12, while a target 32 is mounted on the lower, stationary platen 14. When the top platen 12 is lowered into position to squeeze the tube 16, the capacitive sensor 30 detects the target 32 on the bottom platen 14. Calibrating the pump 50 (fig. 1) using the sensor 30 enables accurate detection using a capacitive sensor. It should be appreciated that other proximity sensors, such as inductive and ultrasonic sensors, may be used in this configuration. Such proximity sensors are small and unobtrusive (unobtrusive) with respect to the operation of the infusion pump 50 (fig. 1). Target 32 likewise does not protrude. For example, target 32 may simply be a metallic bead or block molded into platen 14 or other portion of the body of infusion pump 50 located in movable platen 12.

Fig. 8A and 8B illustrate another apparatus and method for determining contact length and distance. Here again, the slide valve portion 10 includes a top platen 12 and a bottom platen 14 that receive a circular plastic tube 16. The top platen 12 is equipped with two types of sensors, namely a proximity sensor 34 and two microswitches 36. The bottom platen 14 carries a matching sensing object 38 for the proximity sensor 34. When the proximity sensor 34 is, for example, a capacitive sensor, the sensing object 38 is a target, such as a thin metal plate or a conductive region, adapted to be detected by the capacitive sensor. When the bottom platen 14 is metal, inductive or capacitive sensors can sense the platen 14 itself without a separate target.

Top platen 12 also includes two micro-switches 36. The micro-switch is a small limit switch that is triggered when the contact portion of the tube 16 is either close (tube closed) or away (tube open) from the bottom surface of the top platen 12. Thus, the operation of the micro-switches 36 is similar to the sensor array 24 described above, with the distance between the micro-switches used as the distance/of the sensor array. Alternatively, a linear variable differential transformer ("LVDT"), sometimes referred to as a linear voltage displacement transformer, may be used to determine the distance d between platens 12 and 14.

The sample readings of the various sensors described in connection with the above figures are discussed in connection with fig. 9-11. In FIG. 9, a proximity sensor is used to sense the approach of the movable platen 12 toward the stationary platen 14. This proximity may be non-linear and shows a somewhat non-linear signal. However, when the two platens are very close together, the signal tends to vary very little. That is, the distance change decreases non-linearly to distance 24, and it remains stable between 2t and the imaginary zero distance. Thus, the controller 100 may be configured to observe the distance change or Δ d going to zero to determine that the tube 16 is fully compressed. Thus, in one embodiment, the distance 2t is inferred when the proximity sensor signal strength is stabilized within a certain amount or percentage. This mode is applicable to, for example, capacitive sensors, inductive sensors, ultrasonic sensors.

In fig. 10, the reading of the pressure sensor is disclosed. In this embodiment, the pressure sensor, for example as part of the sensor array 24, reads a zero value, which rises to a very small value at the tangent point d-2 r (fig. 3B) as shown. As the platen 12 continues to close, the pressure rises as the tube is compressed from d 2r to d 2t until a very rapid rise in pressure occurs when the distance reaches 2t (the tube is flattened).

Figure 11 shows the readings of the micro-switch which will be turned on or off as required when the tube compresses the contacts. In the illustrated case, the switch is normally on, and when the pressure plate 12 is closed, the switch remains on and issues a steady signal until the tangent point (d ═ 2r) is reached, the switch turns off. The switch then remains closed between d 2r and d 2t, even at the point 2t, until the pressure plate 12 is opened and the microswitch is reset.

The present disclosure also includes situations where the tube 16 is not located exactly in the center of the spool portion 10. For example, FIG. 12A shows upper platen 12 and lower platen 14 with tube 16 offset to the left by a distance Δ d. Sensors 26a and 26b (e.g., pressure sensors) will notice this offset. Fig. 12B shows that the array 24 of two pressure switches 26a and 26B separated by a distance l acquires pressure at different times. In this case, the tube 16 is biased to the left, and the first pressure sensor 26a on the left first detects the pressure at a time and distance different from the pressure detected by the sensor 26b on the right. In fig. 12C, when two pin-type microswitches as shown in fig. 8A and 8B are used, the left switch 36 is tripped by the tube before the right switch 36 is tripped. In this case, the true pipe contact length would be equal to l + Δ l, where l is the distance between the two sensors (fig. 12A) and Δ l is the extra contact length due to the time difference between the pressure detected by the first sensor on the left and the pressure detected by the second sensor on the right, as shown in fig. 12D. The extra length of the tube contacting the platen is Δ l. The difference Δ d in the platen distance can be measured from the time difference between when the first and second pressure sensors detect a sharp rise in pressure. Then, Δ l is calculated using Δ D, as shown in fig. 12D. Using the total tube contact length l + Δ l and the distance between the platens, the tube diameter can be calculated. Of course, if there is a delay caused by the offset, the distance change Δ d will be reversed from the contact length change Δ l.

Referring now to fig. 13, there is shown a tube diameter detection of the present disclosure using a linear peristaltic pump 60 for alternative operation. Infusion pump 60 includes a motor 61, a drive shaft 62, and a plurality of cam plates 63 for compressing a pump rod 64 against a tube 65. The actuator 64 is pressed against the stationary part 66, thereby continuously squeezing infusate between the rods. Infusion pump 60 also includes an additional cam plate 67. In this embodiment, the pump controller 100 controls the motor 61, disengages the cam plate 67, and receives a signal from the proximity sensor 69. The cam plate 67 includes a proximity sensor 69 at the command of the controller of the infusion pump. As the cam plate is pushed forward, the proximity sensor 69 senses a target 70 within the stationary portion 68, which target 70 may be part of the stationary portion 66, or may be different. The stationary part 68 comprises a length sensor 71 for sensing the contact length of the tube 65 against the stationary part 68. The microprocessor controller 100 receives signals from the sensors 69 and 71 and controls the motor 61, cam plate 67 and other parts of the infusion pump.

The microcontroller 100 also has memory or has access to memory for a computer program on a computer readable medium to save the above formula and calculate the contact length and diameter of the tube as described above. From these readings and calculations, the controller 100 calculates the volume of drug or infusion fluid that has been delivered to the patient.

It should be understood that various changes and modifications to the preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.

Claims (23)

1. A method of measuring a pipe (16), the method comprising:

loading a tube (16) into the clamp (10);

compressing the tube (16) between opposing surfaces (12,14) of the clamp (10);

receiving a signal indicative of compression of the tube (16) while compressed, wherein the signal indicative of compression of the tube (16) is indicative of a distance between the opposing surfaces (12,14) of the clamp (10);

receiving a signal indicative of a contact length of the tube (16) against at least one of the opposing surfaces (12, 14); and

calculating the inner diameter and thickness of the tube (16).

2. The method of claim 1, further comprising monitoring the signal indicative of the compression of the tube (16) or the signal indicative of the contact length.

3. The method according to either one of claims 1 and 2, wherein the opposing surfaces (12,14) include a stationary surface and an active surface, and the signal indicative of the compression of the tube (16) is received from a sensor disposed on the stationary or active one of the opposing surfaces (12, 14).

4. The method according to either one of claims 1 and 2, wherein the signal indicative of the compression of the tube (16) is generated by an LVDT, an ultrasonic sensor (18,19), an air-in sensor or a capacitive sensor (30).

5. The method according to either one of claims 1 and 2, wherein the signal indicative of contact length is generated by a sensor (26).

6. The method of claim 5, wherein the sensor (26) comprises a pressure sensor (26a,26b), a sensor array (24), a switch array (36), or a capacitive sensor (30).

7. The method according to either one of claims 1 and 2, wherein the opposing surfaces (12,14) are part of an infusion pump (50).

8. The method according to either one of claims 1 and 2, wherein the calculating step calculates the diameter of the tube (16) using two semicircles and the length of contact of the tube (16) with the opposing surfaces (12, 14).

9. The method according to either one of claims 1 and 2, the method comprising: -generating and receiving said signal indicative of the compression of said tube (16); and generating and receiving the signal indicative of the contact length of the tube.

10. The method according to either one of claims 1 and 2, wherein the calculating step includes calculating an inner diameter of the tube (16).

11. The method of any of claims 1 and 2, further comprising calculating a volume of liquid infusate using the calculated inner diameter.

12. The method according to either one of claims 1 and 2, wherein the signal indicative of the compression of the tube (16) is a signal indicative of a position of a shaft of a motor (20) compressing the tube (16).

13. The method according to either one of claims 1 and 2, wherein receiving the signal indicative of the compression of the tube (16) comprises: using readings from the encoder (22), the travel and position of the platen (12) over a plurality of discrete time periods during compression of the tube (16) is tracked.

14. An infusion pump (50), the infusion pump (50) comprising:

an inlet valve (52);

an outlet valve (53);

a spool (10), said spool (10) comprising a spool stationary portion (14) and a spool active portion (12), and configured to squeeze a length of tubing (16) between said spool stationary portion (14) and said spool active portion (12), wherein said spool active portion (12) moves toward and away from said spool stationary portion (14) to operate said infusion pump (50);

a first sensor (18,19), the first sensor (18,19) being mounted on one of the movable part (12) or the stationary part (14) for detecting a distance between the movable part (12) and the stationary part (14);

at least one second sensor (26), said at least one second sensor (26) being adapted to detect a length of contact of said tube (16) with at least one of said movable part (12) and said stationary part (14); and

a controller (100), the controller (100) adapted to calculate an inner diameter and a thickness of the tube (16).

15. The infusion pump (50) according to claim 14, wherein the first sensor comprises an LVDT, an ultrasound sensor (18,19), an air-in sensor or a capacitive sensor (30).

16. The infusion pump (50) according to either one of claims 14 and 15, wherein the second sensor (26) includes a pressure sensor (26a,26b), a sensor array (24), a switch array (36), or a capacitive sensor (30).

17. The infusion pump (50) according to either one of claims 14 and 15, further including a target (32) mounted on the other of the movable portion (12) and the stationary portion (14).

18. An infusion pump (60), the infusion pump (60) comprising:

a positive displacement pump for manipulating a tube (65) to accurately deliver medication;

a first sensor (69), the first sensor (69) mounted on one of a moving portion (67) or a stationary portion (68) of the positive displacement pump for detecting a distance between the moving portion (67) and the stationary portion (68);

at least one second sensor (71), said at least one second sensor (71) being adapted to detect a length of contact of said tube (65) with at least one of said movable part (67) and said stationary part (68); and

a controller (100), said controller (100) being adapted to calculate an inner diameter and a thickness of said tube (65) at least as a function of said distance between said movable portion (67) and said stationary portion (68) of said tube (65).

19. The infusion pump (60) of claim 18, wherein the positive displacement pump is a spool pump or a peristaltic pump.

20. The infusion pump (60) according to either one of claims 18 and 19, wherein the first sensor (69) is an air entrainment sensor or a proximity sensor.

21. The infusion pump (60) according to either one of claims 18 and 19, wherein the at least one second sensor (71) includes two spaced apart sensors (26) or a microswitch (36).

22. The infusion pump (60) according to either one of claims 18 and 19, wherein the controller (100) executes a computer program on a computer readable disk to calculate the length, the thickness, and the inner diameter of the tube (65).

23. The infusion pump (60) of claim 18, wherein the infusion pump (60) is a linear peristaltic pump and the movable portion (67) is one of a plurality of cam plates.