Abstract:
An infusion pump uses sensors and a computer program to detect tubing in a tube-loading section of the pump. The pump and the computer program also measures one or more of the tubing outer diameter, outer circumference, inner diameter, inner circumference, and tubing wall thickness. The infusion pump utilizes proximity sensors, such as an ultrasonic sensor, a capacitive sensor, or even an air-in-line sensor to detect proximity between two infusion pump surfaces, such as the surfaces that clamp the tubing in the pump, to determine thickness. At least one of the clamping surfaces is equipped with sensors to indicate contact length of the tubing with the surface. Using these measurements, the tubing wall thickness and inner diameter can be determined. Knowing the actual tubing inner diameter, increased volumetric accuracy is possible, up to a three or four percent improvement over present measurements.

Description:
BACKGROUND 
       [0001]    The field of the present disclosure invention is infusion pumps and relates generally to systems, apparatuses, and methods for pumping or infusing volumes of medical fluids to a patient, typically via an intravenous route. 
         [0002]    Infusion pumps are used to infuse drugs and liquids into patients, typically via intravenous lines. While some infusion pumps deal with relatively large volumes, there may be more interest in pumps with a capability of delivering only very small controlled volumes of liquid. The drugs used may be very important, such as analgesics, anesthetics including opiates, anti-inflammatory agents, insulin, anti-spasmodic drugs, antibiotics, chemotherapy agents, cardiovascular drugs, and the like. Many of these drugs are needed in very low doses on a continuous basis, so that the patient has a steady, reliable stream over a long period of time, such as 0.1 ml per hour. If pulses are used, the dosage rate may be measure in terms of nanoliters or microliters per pulse or bolus. Regardless of whether a small volume or larger volume pump is being used, the accuracy of the pump is to a successful outcome for the patient. 
         [0003]    Some infusion pumps have, along the length of tubing, a pumping chamber having an inlet valve and an outlet valve. The infusion fluid is admitted into a length of tubing in the pumping chamber through an open inlet valve and then isolated by occluding the tube by closing the inlet valve at an inlet of the tubing. Then, the outlet valve is opened and a pumping mechanism compresses or otherwise massages the length of tubing in question to pump or expel the fluid from the pumping chamber and towards the patient. Since the inlet is blocked by the closed valve, the liquid can only exit through the outlet, with an open valve. The outlet valve is then closed and the inlet valve and pumping mechanism opened to permit additional fluid to enter the pumping chamber from a fluid source. The above is something referred to as a singe pumping cycle or stroke. 
         [0004]    The pumping mechanism can comprise a single pumping member that compresses the tube against a stationary block or platen. In this case the pumping member or platen may have a length substantially similar to that between the inlet and outlet valves. Alternatively, the pumping mechanism may comprise a plurality of pumping fingers or members that compress the tube in sequence. In this instance, particularly if there are sufficient pumping fingers, such that at least one is compressing the tube at all times, there may be no need for an inlet and/or outlet valves. 
         [0005]    The accuracy of an overall infusion is dependent upon the accuracy of each pumping cycle. In other words, it is important to know with accuracy the volume of fluid pumped with each pumping cycle, to know over time the volume of the entire infusion. The volume of each pumping cycle is dependent upon the internal diameter of the tube. A problem arises due to the variability of internal diameters from tube to tube. This variability is due to, among other things, manufacturing processes and tolerances. It would be helpful for the infusion pump to be capable of determining, or measuring the internal diameter of the specific IV tube being utilized for a specific infusion. Based on this information, the pump could adjust the functionality of the pumping mechanism (speed and stroke length of the pumping mechanism) to ensure and maintain accuracy regardless of tubing inner diameter variability. 
         [0006]    Additionally, the pump can use this information to avoid overly compressing the tube (decreases tube life due to overstressing) and under compressing the tube (leads to inaccuracies and inefficiencies). 
         [0007]    Infusion pumps are used to accurately infuse medicines and other liquids to patients. The amount that is dispensed could be improved by an accurate knowledge of the inner dimensions of the particular tubing used to dispense a particular liquid to a patient. 
       SUMMARY 
       [0008]    The present disclosure includes an infusion pump that can deliver a prescribed amount of medicine, such as insulin or morphine, to a patient. The pump accurately delivers the prescribed amount of the proper medicine in order to insure the best possible outcome for the patient. The pump operates with tubing, and in particular with a contact length of the tubing, to convey the medicine from a source, such as an intravenous (“IV”) container, through the contact length, such that medicine does not contact air, risking exposure to contaminants to the patient. The tubing is made typically by extruding the plastic material through a die. The dimensions of the resulting tubing, such as the inner diameter or the outer diameter, can vary by as much as three or four percent. The present pump overcomes this problem by determining the actual dimensions of the contact portion of the tubing during use. 
         [0009]    The infusion pump operates with at least one sensor that measures the distance between physical constraints holding the tubing at the contact area. The physical constraints can include a stationary surface or platen and a moving surface or platen between which the tubing is compressed and decompressed. In particular, one method and corresponding system includes the steps of loading tubing into a fixture, compressing the tubing between opposed surfaces of the fixture, receiving a signal indicative of compression of the tubing while compressing, receiving a signal indicative of a contact length of the tubing against at least one of the opposed surfaces, and calculating a diameter and a thickness of the tubing. 
         [0010]    Another method and corresponding system includes steps of loading tubing into a fixture, compressing the tubing between opposed surfaces of the fixture, generating and receiving a signal indicative of a distance between the opposed surfaces while compressing, generating and receiving a signal indicative of a contact length of the tubing against at least one of the opposed surfaces, and calculating an inner diameter and a thickness of the tubing. 
         [0011]    The methods are systems as discussed are particularly well suited for an infusion pump. The infusion pump includes a tubing clamping section having a moveable portion and a stationary portion, a first sensor mounted on one of the movable portion or the stationary portion for detecting a distance between the movable portion and the stationary portion, at least one second sensor for detecting a contact length of tubing with at least one of the movable portion and the stationary portion, an inlet valve, an outlet valve, and a shuttle having a shuttle stationary portion and a shuttle moveable portion configured for squeezing a length of tubing between the shuttle stationary portion and the shuttle movable portion, wherein the shuttle moveable portion moves toward and away from the shuttle stationary portion to operate the infusion pump. 
         [0012]    The infusion pump alternatively includes a housing, a tubing clamping section having a movable portion and a stationary portion, the tubing clamping section mounted on the housing, a first sensor mounted on one of the movable portion or the stationary portion for detecting a distance between the movable portion and the stationary portion, at least one second sensor for detecting a contact length of tubing with at least one of the movable portion and the stationary portion, and a positive displacement pump for manipulating the tubing to accurately deliver a medicament. 
         [0013]    It is accordingly an advantage of the present disclosure to provide a system and method for compensating for tubing manufacturing variations in determining medical fluid volume pumped via a tubing pump. 
         [0014]    It is another advantage of the present disclosure to provide a system and method for compensating for tubing loading variations in determining medical fluid volume pumped via a tubing pump. 
         [0015]    Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0016]      FIG. 1  is an elevation view of a shuttle-type infusion pump having a tubing measurement system and method of the present disclosure. 
           [0017]      FIG. 2  is a schematic flow diagram illustrating, in a general way, an algorithum used by a pump controller to perform the tubing measurements of the present disclosure. 
           [0018]      FIGS. 3A ,  3 B and  4 A to  4 C are elevation section views of the tube contact portion of the shuttle-type pump infusion pump of  FIG. 1 , depicting the compression of tubing in a fixture. 
           [0019]      FIGS. 5A and 5B  are elevation section views depicting one embodiment of an apparatus and method for measuring tubing compression. 
           [0020]      FIG. 6  is a perspective view illustrating a planar sensor array for use in the embodiment of  FIGS. 5A and 5B . 
           [0021]      FIGS. 7A and 7B  are elevation section views depicting one embodiment of an apparatus and method for measuring a distance between two platens. 
           [0022]      FIGS. 8A and 8B  are elevation section views depicting one embodiment of an apparatus and method for measuring a distance between two platens and a contact length of the tubing with the upper platen. 
           [0023]      FIGS. 9 to 11  are graphical views depicting sensor readings taken in the above apparatus and methods. 
           [0024]      FIGS. 12A to 12D  are various views depicting one system and method for correcting tubing offsets. 
           [0025]      FIG. 13  depicts an alternative cam driven pump embodiment of the tubing measurement apparatus and method of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Referring now to the drawings and in particular to  FIG. 1 , an embodiment of a shuttle-type infusion pump  50  of the present disclosure is illustrated. Infusion pump  50  includes tubing  16 , an inlet valve  52 , an outlet valve  53 , and a shuttle portion  10  having an upper moving platen  12  and a lower stationary platen  14 . Valves  52  and  53  and the shuttle portion  10  are actuated by linear actuators  54   a  to  54   c , respectively. A pump controller  100 , which can operate with other processors of infusion pump  50 , such as a supervisory processor and a safety processor (not shown), controls pump  50  and its linear actuators  54   a  to  54   c.    
         [0027]    To pump fluid, actuator  54   a  opens inlet valve  52 . Actuator  54   b  closes outlet valve  53  and actuator  54   c  retracts platen  12 , allowing tubing  16  to open to receive a liquid medication, e.g., via gravity. Actuators  54   a  and  54   b  then cause the states of valves  52  and  53  to reverse, respectively, and actuator  54   c  pushes platen  12  towards platen  14  to compress tubing  51 , dispelling the volume of fluid that just filled tubing  51  between platens  12  and  14 . 
         [0028]    As discussed in detail below, a sensor  18 ,  19  (e.g., sensor pair) is imbedded into moving platen  12  and stationary platen  14 . A transmitter  18  can be attached to moving platen  12 , while a receiver  19  is in turn attached to stationary platen  14 . In use, as the shuttle moving platen  12  closes tubing  51  to pump the liquid 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 discussed below. At the same time, a sensor array  24 , which is comprised of multiple sensors, detects the length of the tubing segment in contact with platens  12  and  14 , also discussed below. In this manner, sensor  18 ,  19  and sensor array  24  detect and measure the tubing compression distance and contact length, which are sent to controller  100  to calculate an accurate volume of solution actually pumped. This sensing can be repeated for each pump stroke. Pump controller  100  then integrates the accurately determined volumes to adjust the frequency and/or distance of movement of moving platen  12  to ensure accuracy. 
         [0029]    Referring now to  FIG. 2 , a high-level flowchart shows one embodiment of an algorithm or process flow diagram that is performed by controller  100  for the multitude of embodiments discussed herein. A first step  101  in the process is to load the tubing into shuttle portion  10  discussed above. Once loaded, the tubing is compressed at step  102  between two opposed surfaces with sensors that measure a distance between the surfaces, and that measure a contact length between the tubing and at least one of the surfaces. While the testing is taking place, controller  100  monitors the sensors at step  103  for changes in signal output. Movement of the surfaces may be stopped, as seen at step  104 , when there is almost no change in the signal. The signal is then be recorded at step  105 , and controller  100  performs calculations to determine the contact length of the tubing, the thickness of the tubing, and its outer and inner diameters, as seen at step  106 . Controller  100  then calculates a volume of liquid actually pumped using the above actual dimensions and adjusts future pumping (e.g., stroke frequency) to make an actual overall volume equal a target overall volume of fluid pumped. 
         [0030]    Referring now to  FIGS. 3A and 3B , shuttle portion  10  of the infusion pump  50  of  FIG. 1  is shown in more detail and includes a lower platen, stationary platen  14  and an upper, moving platen  12 , which operate with tubing  16 . The lower platen  14  is parallel to the upper moving platen  12  in the illustrated embodiment. Tubing  16  is typically PVC, but may also be made from polyethylene, polypropylene, another medically-acceptable plastic, or a combination of these. In  FIG. 3A , prior to compression, tubing  16  has a tubing thickness t and an outer radius R 0  when placed initially into shuttle portion  10 . 
         [0031]    When the movable platen  12  is closed, as shown  FIG. 3B , tubing  16  is compressed. In both  FIGS. 3A and 3B , d is the distance between the upper and lower platens  12  and  14 , r is the radius of the continuously changing tubing curves, and wherein the points at which the tubing separates from the platens defines a tangential contact distance of the tubing with the platens. As shown by the arrows on the left and right side of tubing  16 , a length l of the tubing defines such contact distance with the upper and lower platens  12 ,  14 , and wherein the edges of length l define tangential release points. 
         [0032]    The equations shown in  FIGS. 3A and 3B  will now be explained. The continuously changing curves at the sides of tubing  16  extend from contact lengths of the tubing that contact lower and upper platens  14  and  12 , which are parallel to teach other. Thus, the newly formed curves are semi-circles with a radius r, which is equal to half the distance d between the platens. The curve length of each newly-formed semicircle is equal to πr, and the total length of both semicircles or ends is 2πr. The equations shown in connection with  FIG. 3A  express the following: the total outer curve length of the tubing  16  will not change during compressing, the total outer curve length equal to its circumference when the tubing  16  is in the shape of a circle equals 2πR 0 . When the tubing is compressed to a position as shown in the right-hand portion of  FIG. 1 , the total outer curve length equals the length of the two newly-formed semicircles, 2πr (or πd)+twice the contact length l. Solving for R 0 , the actual radius of the tubing, it is seen that R 0  is d/2+l/π. In  FIG. 3A , the tubing  16  is just tangent to the platens  12 ,  14 . In this case, the distance d between the two platens is exactly the outer diameter, 2R 0 , of tubing  16 . The contact length is thus zero and twice this length, 2l, is also zero. Therefore, the circumference of the tubing is n times measured distance d, the diameter, equal to 2πr, or in this instance, 2πR 0 . The area within a plane of the tubing is it times the inner radius squared, and the volume is calculated by multiplying by the length of tubing or the length of liquid that is propelled by the infusion pump. 
         [0033]    To measure the tubing diameter by this method, the tubing in theory can be compressed to any position. As shown in equation block  2  of  FIG. 3B , contact lengths l 1 , l 2  are taken at two different distances, d 1  and d 2 . The difference in contact lengths l 1  and l 2  is proportional to the difference in platen distance d during the step of compressing. Using these corresponding values, the change in contact length l can be determined by the change in platen distance d. In addition, when measuring tubing diameters with this method, multiple tests can be performed by compressing tubing to many different positions, and then using an average of all the calculations to arrive at a more accurate value. 
         [0034]      FIGS. 4A to 4C  depict a typical situation in which tubing  16  is compressed when moveable pump platen  12  is closed, squeezing tubing  16  against lower fixed platen  14 . The distance d between the platens  12 ,  14  is measured by an ultrasonic sensor (sensor pairs  18  and  19 ) in this embodiment with a transmitter  18  located with top platen  12  and a receiver  19  located with bottom platen  14 . Many infusion pumps already include an ultrasonic sensor, which is used as an air-in-line sensor. This often times pre-existing sensor can be used for sensor  18 ,  19  of infusion pump  50  ( FIG. 1 ). Other embodiments may use a capacitive sensor, linear transducers such as linear variable differential transformer (“LVDT”), or other distance-measuring sensor. As upper platen  12  is lowered, tubing  16  is flattened, as seen in  FIG. 4B , to a distance d. 
         [0035]    When moveable platen  12  is completely lowered, as shown in  FIG. 4C , tubing  16  compresses such that platens  12 ,  14  are separated by only the tubing  16  itself, and the distance d is twice the thickness t of the tubing wall. Controller  100  can use sensor  18 ,  19  to see that the distance d is no longer changing and determine that tube  16  is compressed fully as seen in  FIG. 4C . Even before full compression, distance sensor  18 ,  19  shows that the distance d, is changing very slowly as platens  12  and  14  closely approach each other, separated only by the thickness of the tubing itself, with no air or liquid within. As discussed with  FIG. 1 , actuator  54   c  applies a force to upper platen  12 , to close the platen against fixed platen  14 . A force sensor can also or alternatively be provided and look for an increase in force to signal the complete compression of  FIG. 4C . A power or current draw of actuator  54   c  can also or alternatively be monitored to look for an increase in current draw, indicating full compression. 
         [0036]    There are still further alternative ways to determine the distance traveled, or amount of tubing compression, as shown in  FIGS. 5A and 5B . As before, shuttle portion  10  includes a moving upper portion or platen  12 , a bottom platen  14 , and tubing  16 . Here however, platen  12  is propelled by a motor  20  (e.g., linear) with an encoder  22 . The motor may include a lead screw, a ball screw, jackscrew, and so forth, to convert rotational motion to translational motion, to move platen  12  against tubing  16 . Motor  20  and encoder  22  are connected to controller  100 , shown above, which provides positional information that controller  100  can use and also convert to rate information. Controller  100  controls motor  20  and records data from encoder  22 , regarding the shaft of position of motor  20  and converts that position, or change in position into an accurate calculation of the change in translational position of platen  12 . Beginning from a known position, the travel and position of the platen  12  can be determined at any time using the information from the encoder, tracking an recoding distance d over many, many discrete time segments during the compression of tubing  16 . 
         [0037]      FIGS. 5A ,  5 B and  6  show a sensor array  24 . 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, each column or row separated by a distance l, which may be the same or may be different. For many applications, a single row of two sensors  26  may be adequate. The two sensors  26  detect the presence of tubing  16  between the sensors  26 . When the tubing  16  no longer in contact with the sensors  26 , the sensors  26  will not indicate the presence of the tubing. This situation may occur the tubing has only tangential contact points with platens  12  and  14 . The distance l between the sensors  26  may be set at the length in which the edges of the tubing are just in contact with the sensors when the distance d between the platens is 2t. For a given tubing configuration, with known inner and outer diameters and wall thickness, the platens  12 ,  14 , will be separated by a distance d when there is a known amount of tubing compression, the tubing length reaches length l, and the tubing thickness is about 2t. To set the distance l, measurements may be taken and calibration data points used to measure the length of tubing l in contact with the platens and sensors when the platens are separated by a distance d and the tubing  16  in contact with the platens has thickness 2t. 
         [0038]    The sensors  26  of array  24  are capable of detecting the presence of the tubing when the tubing presses against platens  12  and  14  of shuttle portion  10 . For example, small compact pressure sensors, capacitive or inducting sensors may be used. Sensors  26  detect the presence of the flattened-out portion of tubing  16  shown above. Sensors  26  will cease to detect the presence of tubing  16  when the tubing is only tangentially contacting the platens. A pressure sensor  26 , for example, will show a rapid rise or run-up in pressure when tubing  16  contacts the sensor. When the pressure is removed, the fall-off of pressure and the pressure signal will be just as rapid. A capacitive sensor  26  will operate in a similar manner, with a rapid detection of tubing material, especially wet material, as the tubing approaches the capacitive sensors, e.g., two capacitive sensors  26  spaced apart a known distance l. 
         [0039]    Besides sensor array  24  depicted above, there are other approaches that may be used to detect contact length and tubing compression.  FIGS. 7A and 7B  depict shuttle portion  10  again having a stationary platen  14 , a moving platen  12 , and a length of infusion pump tubing  16 . In the illustrated embodiment, a capacitive sensor  30  is mounted on the top or moving platen  12 , and a target  32  is mounted on the lower, stationary platen  14 . When top platen  12  is lowered into place to squeeze tubing  16 , capacitive sensor  30  detects the target  32  on bottom platen  14 . Calibration of pump  50  ( FIG. 1 ) with sensor  30  makes accurate detection using a capacitive sensor possible. It will be recognized that in this configuration, other proximity sensors may be used, e.g., inductive and ultrasonic sensors. Such proximity sensors are small and unobtrusive relative to the operation of infusion pump  50  ( FIG. 1 ). Target  32  is equally unobtrusive. For example, target  32  may simply be a small bead or square of metal molded into platen  14  or other part of the body of infusion pump  50  located near moving platen  12 . 
         [0040]    Another apparatus and method for determining the contact length and distance is depicted in  FIGS. 8A and 8B . Here again, shuttle portion  10  includes a top platen  12  and bottom platen  14 , which receive round plastic tubing  16 . Top platen  12  is equipped with two types of sensors, a proximity sensor  34  and two microswitches  36 . Bottom platen  14  is equipped with a mating sensing object  38  for proximity sensor  34 . If proximity sensor  34  is a capacitive sensor, for example, sensing object  38  is a target suitable for being detected by a capacitive sensor, e.g., a thin metal plate or conductive area. If the bottom platen  14  is metal, inductive or capacitive sensors can sense platen  14  itself without a separate target. 
         [0041]    Top platen  12  also includes two microswitches  36 . The microswitches are small limit switches that are triggered as the contact portion of tubing  16  approaches (tube closing) or leaves (tube opening) the bottom surface of the top platen  12 . Thus, microswitches  36  operate similar to sensor array  24  discussed above, with the distance between the microswitches acting as the distance l of the sensor array. Further alternatively, a linear variable differential transformer (“LVDT”), sometimes also called a linear voltage displacement transformer, may also be used to determine distance d between platens  12  and  14 . 
         [0042]    Sample readings of the various sensors described in connection with the above figures are discussed with reference to  FIGS. 9 to 11 . In  FIG. 9 , a proximity sensor is used to sense the approach moveable platen  12  towards stationary platen  14 . The approach may not be linear, and a somewhat nonlinear signal is shown. However, the signal tends to change very little when the two platens are very close. That is, the distance changes non-linearly down to distance  24  and that remains constant between 2t and a fictitious zero distance. Controller  100  can therefore be configured to look for change or delta d to go to zero to determine that tubing  16  is compressed completely. Thus, in one embodiment, when the proximity sensor signal strength is constant to within a certain amount or percentage, the 2t distance is inferred. This pattern holds true for capacitive sensors, inductive sensors, ultrasound sensors, for example. 
         [0043]    In  FIG. 10 , a pressure sensor reading is disclosed. In this embodiment, a pressure sensor, for example as part of sensor array  24 , reads a zero value, rising to a very low value, as shown, at the point of tangency d=2r ( FIG. 3B ). When platen  12  continues to close, the pressure rises as the tubing is compressed from d=2r to d=2t until a very rapid rise occurs when the 2t distance (tubing compressed flat) is reached. 
         [0044]      FIG. 11  depicts readings for a microswitch, which will switch on or off as desired when the tubing compresses the contact. In the illustrated instance, the switch is normally on, and as platen  12  closes, the switch remains on, with a constant signal, until the point of tangency is reached (d=2r), at which point the switch is tripped off. The switch then remains off between d=2r and d=2t, even past the point of 2t, until platen  12  opens and the microswitch is reset. 
         [0045]    The present disclosure also covers a situation in which tubing  16  does not sit squarely in the center of shuttle portion  10 .  FIG. 12A , for example, depicts upper and lower platens  12 ,  14 , in which the tubing  16  is offset a distance Δd to the left. Sensors  26   a  and  26   b  (e.g., pressure sensors) will notice the offset.  FIG. 12B  shows that an array  24  of two pressure switches  26   a  and  26   b  separated by distance l picks up the pressure at different times. In this instance, tubing  16  is offset to the left and the first sensor, pressure sensor  26   a  on the left, detects the pressure first, at a different time and distance from the pressure detected by sensor  26   b  on the right. In  FIG. 12C , if two pin-type microswitches are used, as in  FIGS. 8A and 8B , switch  36  on the left is tripped by the tubing before switch  36  on the right is tripped. In this case, the real tubing contact length will be equal to l+Δl, where l is the distance between the two sensors ( FIG. 12A ), and Δl is the extra contact length caused by the time difference between the time when the first sensor on the left detects pressure and the time when the second sensor on the right detects pressure as shown in  FIG. 12D . The extra length of tubing in contact with the platen is Δl. The difference Δd in platen distance can be measured by the difference in time from when the first and second pressure sensors detect a sharp rise in pressure. Δl is then calculated using Δd, as shown in  FIG. 12D . With the total tubing contact length, l+Δl, and the distance between the platens, the tubing diameter can be calculated. Of course, if there is a time delay caused by the offset, the change in distance Δd will be inverse to the change in contact length Δl. 
         [0046]    Referring now to  FIG. 13 , the tubing diameter detection of the present disclosure is shown in alternative operation with a linear peristaltic pump  60 . Infusion pump  60  includes a motor  61 , a drive shaft  62 , and a plurality of cam plates  63  for pressing pump rods  64  against tubing  65 . The actuators  64  press against stationary portion  66 , successively squeezing infusate from rod to rod. Infusion pump  60  also includes an additional cam plate  67 . Pump controller  100  in this embodiment controls motor  61 , separate cam plate  67 , and receives signals from a proximity sensor  69 . Under the command of a controller of the infusion pump, cam plate  67  includes a proximity sensor  69 . When the cam plate is urged forward, proximity sensor  69  senses a target  70  in stationary portion  68 , which may a portion of stationary portion  66 , or may be different. Stationary portion  68  includes length sensor  71 , for sensing a contact length of tubing  65  against stationary portion  68 . Microprocessor controller  100  receives signals from the sensors  69 ,  71  and controls the motor  61 , the cam plate  67 , and the other portions of the infusion pump. 
         [0047]    Microcontroller  100  also has a memory, or has access to a memory, for a computer program on a computer-readable medium for storing the formulae discussed above and for calculating the contact length and the diameter of the tubing, as also discussed above. From these readings and calculations, controller  100  calculates a volume of a medication or infusate that has been delivered to a patient. 
         [0048]    It should be understood that various changes and modifications to the presently 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.