Patent Publication Number: US-9895490-B2

Title: Occlusion detection for a fluid infusion device

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application Ser. No. 13/866,240, filed Apr. 19, 2013. U.S. patent application Ser. No. 13/866,240 is a divisional of U.S. patent application Ser. No. 13/225,118, filed Sep. 2, 2011, which issued as U.S. Pat. No. 8,469,942. U.S. patent application Ser. No. 13/225,118 is a continuation-in-part of U.S. patent application Ser. No. 12/976,591, filed Dec. 22, 2010, and a continuation-in-part of U.S. patent application Ser. No. 12/976,619, filed Dec. 22, 2010, which issued as U.S. Pat. No. 8,197,444. 
    
    
     TECHNICAL FIELD 
     Embodiments of the subject matter described herein relate generally to medical devices. More particularly, embodiments of the subject matter relate to fluid infusion devices such as personal insulin infusion pumps. 
     BACKGROUND 
     Portable medical devices are useful for patients that have conditions that must be monitored on a continuous or frequent basis. For example, diabetics are usually required to modify and monitor their daily lifestyle to keep their blood glucose (BG) in balance. Individuals with Type 1 diabetes and some individuals with Type 2 diabetes use insulin to control their BG levels. To do so, diabetics routinely keep strict schedules, including ingesting timely nutritious meals, partaking in exercise, monitoring BG levels daily, and adjusting and administering insulin dosages accordingly. 
     The prior art includes a number of fluid infusion devices and insulin pump systems that are designed to deliver accurate and measured doses of insulin via infusion sets (an infusion set delivers the insulin through a small diameter tube that terminates at, e.g., a cannula inserted under the patient&#39;s skin). In lieu of a syringe, the patient can simply activate the insulin pump to administer an insulin bolus as needed, for example, in response to the patient&#39;s high BG level. 
     A typical infusion pump includes a housing, which encloses a pump drive system, a fluid containment assembly, an electronics system, and a power supply. The pump drive system typically includes a small motor (DC, stepper, solenoid, or other varieties) and drive train components such as gears, screws, and levers that convert rotational motor motion to a translational displacement of a stopper in a reservoir. The fluid containment assembly typically includes the reservoir with the stopper, tubing, and a catheter or infusion set to create a fluid path for carrying medication from the reservoir to the body of a user. The electronics system regulates power from the power supply to the motor. The electronics system may include programmable controls to operate the motor continuously or at periodic intervals to obtain a closely controlled and accurate delivery of the medication over an extended period. 
     Some fluid infusion devices use sensors and alarm features designed to detect and indicate certain operating conditions, such as non-delivery of the medication to the patient due to a fluid path occlusion. In this regard, a force sensor can be used in a fluid infusion device to detect when the force applied to the fluid reservoir stopper reaches a set point. The force sensor in such a fluid infusion device could be positioned at the end of the drive motor assembly that actuates a rotatable lead screw, which in turn advances the stopper of the reservoir. With such an arrangement, the force applied to the force sensor by the drive motor assembly is proportional to the pressure applied to the medication as a result of power supplied to the drive system to advance the stopper. Thus, when a certain force threshold (a set point corresponding to an occlusion condition) is reached, the fluid infusion device is triggered to generate an alarm to warn the user. 
     Early detection of an occlusion condition is helpful, because an occlusion can result in “under-dosing,” particularly if the drive system continues to receive commands to deliver medication when the fluid path is blocked. Accordingly, proper operation of the force sensor is important for purposes of occlusion detection, and it is desirable to have some diagnostic capability related to the health of the force sensor. 
     Existing force-based occlusion detection techniques typically rely on a fixed threshold or set point that is indicative of an occlusion condition. A threshold value is selected based on system tolerances. To avoid frequent false alarms, however, it is necessary to set the threshold value above the maximum expected force, based on the interacting system components. Because the threshold value is set at the maximum expected force, if a patient has a particular pump system with a nominal delivery force, it may take slightly longer to reach the threshold force. Accordingly, it is desirable to have an occlusion detection technique that does not solely rely on a fixed occlusion detection threshold force. 
     Some fluid infusion devices use replaceable fluid reservoirs that are secured in the housing of the device and actuated by a drive assembly. One form of infusion pump utilizes a threaded cap to seat and secure the fluid reservoir in the housing of the pump. The user unscrews the threaded cap to remove an empty reservoir, replaces the old reservoir with a new reservoir, and reinstalls the threaded cap to secure the new reservoir in place. During use, the threaded cap might be dislodged (especially if the fluid infusion device is a portable unit that is worn by the patient), resulting in an unseated or improperly installed reservoir. For example, if the user participates in certain physical activities (e.g., sports, hiking, or rigorous exercise), then the cap might be unintentionally loosened by physical rotation. As another example, if the user is in a crowded environment (e.g., a concert, a nightclub, or a full elevator), then the cap might be inadvertently unscrewed through contact with another person or an object. For this reason, it is desirable to have a reservoir presence and/or seating detection technique for a fluid infusion pump. 
     BRIEF SUMMARY 
     A method of operating a fluid infusion device is provided. The fluid infusion device includes a drive motor assembly and a force sensor associated with the drive motor assembly. The method activates a rewind operation of the drive motor assembly and determines a rewind force imparted to the force sensor during the rewind operation. The method initiates corrective action for the fluid infusion device when the rewind force is less than a lower threshold force or greater than an upper threshold force. 
     Also provided is an exemplary embodiment of a device for delivering fluid to a user. The device includes: a housing; a drive motor assembly in the housing to regulate delivery of fluid by actuating a piston of a fluid reservoir; a force sensor associated with the drive motor assembly to generate output levels in response to force imparted thereto; and an electronics module coupled to the force sensor to process the output levels to determine operating health of the force sensor. 
     Another embodiment of a method of operating a fluid infusion device is also provided. The fluid infusion device includes a drive motor assembly and a force sensor associated with the drive motor assembly. The method involves determining a measure of actuation force imparted to the force sensor during a fluid delivery action of the drive motor assembly, and comparing the measure of actuation force against a range of valid values that represents normally expected measures of actuation forces. When the measure of actuation force is outside the range of valid values, the method initiates corrective action for the fluid infusion device. 
     A method of determining a seating status of a fluid reservoir in the reservoir cavity of a fluid infusion device is also provided. The fluid infusion device includes a drive motor assembly, a force sensor associated with the drive motor assembly, and a reservoir cavity that accommodates fluid reservoirs. The method begins by confirming initial seating of the fluid reservoir in the reservoir cavity. The method continues by determining a measure of actuation force imparted to the force sensor during a fluid delivery action of the drive motor assembly, and comparing the measure of actuation force to an amount of force that is less than normally expected actuation forces of the fluid infusion device, where the amount of force is indicative of an unseated state of the fluid reservoir. The method continues by initiating corrective action for the fluid infusion device when the measure of actuation force is less than the amount of force. 
     A device for delivering fluid to a user is also provided. The device includes: a housing; a reservoir cavity within the housing to accommodate fluid reservoirs; a drive motor assembly in the housing to regulate delivery of fluid by actuating a piston of a fluid reservoir; a force sensor associated with the drive motor assembly to generate output levels in response to force imparted thereto; and an electronics module coupled to the force sensor to process the output levels to determine a seating status of the fluid reservoir in the reservoir cavity. 
     Another embodiment of a method of determining a seating status of a fluid reservoir in the reservoir cavity of a fluid infusion device is provided. The method obtains baseline actuation force imparted to a force sensor, after initial seating and priming of the fluid reservoir. The method continues by determining a measured actuation force imparted to the force sensor, the measured actuation force corresponding to a designated delivery stroke of the drive motor assembly. The method also generates indicia of an unseated reservoir condition when the measured actuation force is less than the baseline actuation force by at least a predetermined amount of force. 
     Also provided is a method of determining a seating status of a fluid reservoir in a fluid infusion device having a drive motor assembly that actuates the fluid reservoir using discrete delivery pulses. The method obtains measures of actuation force imparted to the force sensor for a number of consecutive fluid delivery pulses, and calculates a pulse-to-pulse difference between consecutive fluid delivery pulses, the pulse-to-pulse difference based on respective measures of actuation force for the consecutive fluid delivery pulses. The method continues by initiating corrective action for the fluid infusion device when the pulse-to-pulse difference is greater than a threshold force value. 
     Another embodiment of a method of determining a seating status of a fluid reservoir in a fluid infusion device is provided. The infusion device has a drive motor assembly that actuates the fluid reservoir using discrete delivery pulses, and the method involves: maintaining a count that is indicative of the seating status; storing an adaptive reference force value that corresponds to a previously recorded measure of actuation force imparted to the force sensor during a previous fluid delivery pulse; obtaining a current measure of actuation force imparted to the force sensor for a current fluid delivery pulse; changing the count when the current measure of actuation force is less than the difference between the adaptive reference force value and a threshold force value, resulting in an updated count; and generating a seating status alert when the updated count satisfies predetermined alert criteria. 
     An exemplary embodiment of a fluid infusion device includes a drive motor assembly and a force sensor associated with the drive motor assembly. A method of detecting occlusions in a fluid path of the fluid infusion device determines a plurality of force measurements for a fluid delivery action of the drive motor assembly, the plurality of force measurements indicating measures of actuation force imparted to the force sensor for the fluid delivery action. The method continues by determining a plurality of quantity measurements for the fluid delivery action, each of the plurality of quantity measurements being determined relative to a reference quantity measurement. An occlusion is indicated when a first slope of force-versus-quantity for a large sample of the plurality of quantity measurements is greater than or equal to a first threshold slope value. An occlusion is also indicated when: (a) a second slope of force-versus-quantity for a small sample of the plurality of quantity measurements is greater than or equal to a second threshold slope value; and (b) a current one of the plurality of force measurements is greater than or equal to a threshold force value. 
     An exemplary embodiment of a method of detecting occlusions in a fluid path of a fluid infusion device is also provided. The fluid infusion device has a drive motor assembly that actuates a piston of a fluid reservoir to deliver fluid from the fluid reservoir. The method begins by initiating a fluid delivery action to deliver an amount of fluid from the fluid reservoir. The method continues by determining, for each of a plurality of measurement points associated with the fluid delivery action, a respective measurement of a quantity that is indicative of pressure in the fluid path, along with a respective delivered volume measurement relative to a reference volume measurement. A first slope is calculated based upon a current measurement of the quantity, a previous measurement of the quantity, a current delivered volume measurement corresponding to the current measurement of the quantity, and a previous delivered volume measurement corresponding to the previous measurement of the quantity. A second slope is calculated based upon the current measurement of the quantity, an intervening measurement of the quantity that is determined after determining the previous measurement of the quantity, the current delivered volume measurement, and an intervening delivered volume measurement corresponding to the intervening measurement of the quantity. The method continues by indicating whether an occlusion has occurred by using the first slope and the second slope. 
     Also provided is an exemplary embodiment of a method of detecting occlusions in a fluid path of a fluid infusion device having a drive motor assembly that actuates a fluid reservoir by applying actuation force to a piston of the fluid reservoir. The method involves initiating a fluid delivery action intended to deliver an amount of fluid from the fluid reservoir, and determining measures of fluid pressure in the fluid path for the fluid delivery action. The method indicates an occlusion when a first rate of change of the measures of fluid pressure is greater than or equal to first threshold value, the first rate of change based upon a first measurement window. The method also indicates an occlusion when: (a) a second rate of change of the measures of fluid pressure is greater than or equal to a second threshold value, the second rate of change based upon a second measurement window; and (b) a current measure of fluid pressure is greater than or equal to a threshold force value. 
     A device for delivering fluid to a user is also provided. The device includes a housing, a reservoir cavity within the housing to accommodate fluid reservoirs, a drive motor assembly in the housing to regulate delivery of fluid by actuating a piston of a fluid reservoir, a force sensor associated with the drive motor assembly to generate output levels in response to force imparted thereto, and an electronics module coupled to the force sensor. The electronics module processes the output levels to detect occlusions in a fluid path of the device for a fluid delivery action. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the subject matter may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. 
         FIG. 1  is a schematic representation of an embodiment of a fluid infusion device; 
         FIG. 2  is an exploded perspective view of the fluid infusion device shown in  FIG. 1 ; 
         FIG. 3  is a cross sectional view of the fluid infusion device shown in  FIG. 1 , corresponding to a cross section taken longitudinally through the drive motor assembly and the fluid reservoir; 
         FIG. 4  is a schematic block diagram representation of an embodiment of a fluid infusion device; 
         FIG. 5  is a flow chart that illustrates an embodiment of a process associated with the operation of a fluid infusion device; 
         FIG. 6  is a flow chart that illustrates an embodiment of a rewind force calibration process for a fluid infusion device; 
         FIG. 7  is a flow chart that illustrates another embodiment of a process associated with the operation of a fluid infusion device; 
         FIG. 8  is a flow chart that illustrates an embodiment of a process that checks the seating status of a fluid reservoir of a fluid infusion device; 
         FIG. 9  is a flow chart that illustrates another embodiment of a process that checks the seating status of a fluid reservoir of a fluid infusion device; 
         FIG. 10  is a flow chart that illustrates yet another embodiment of a process that checks the seating status of a fluid reservoir of a fluid infusion device; 
         FIG. 11  is a graph that illustrates measures of actuation forces for a properly seated fluid reservoir; 
         FIG. 12  is a graph that illustrates measures of actuation forces for a fluid reservoir that becomes unseated; 
         FIG. 13  is a flow chart that illustrates an embodiment of an occlusion detection process for a fluid infusion device; 
         FIG. 14  is a flow chart that illustrates an exemplary embodiment of a multi-mode occlusion detection process; 
         FIG. 15  is a graph that depicts an exemplary plot of fluid reservoir actuation force versus volume of fluid delivered from a fluid infusion device; and 
         FIG. 16  is a flow chart that illustrates another exemplary embodiment of a multi-mode occlusion detection process. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Techniques and technologies may be described herein in terms of functional and/or logical block components, and with reference to symbolic representations of operations, processing tasks, and functions that may be performed by various computing components or devices. It should be appreciated that the various block components shown in the figures may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment of a system or a component may employ various integrated circuit components, e.g., memory elements, digital signal processing elements, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. 
     For the sake of brevity, conventional techniques related to infusion system operation, insulin pump and/or infusion set operation, blood glucose sensing and monitoring, force sensors, signal processing, and other functional aspects of the systems (and the individual operating components of the systems) may not be described in detail here. Examples of infusion pumps and/or related pump drive systems used to administer insulin and other medications may be of the type described in, but not limited to, U.S. Pat. Nos. 4,562,751; 4,678,408; 4,685,903; 5,080,653; 5,505,709; 5,097,122; 6,485,465; 6,554,798; 6,558,351; 6,659,980; 6,752,787; 6,817,990; 6,932,584; and 7,621,893; which are herein incorporated by reference. 
     The subject matter described here relates to a fluid infusion device of the type used to treat a medical condition of a patient. The infusion device is used for infusing fluid into the body of a user. The non-limiting examples described below relate to a medical device used to treat diabetes (more specifically, an insulin pump), although embodiments of the disclosed subject matter are not so limited. Accordingly, the infused fluid is insulin in certain embodiments. In alternative embodiments, however, many other fluids may be administered through infusion such as, but not limited to, disease treatments, drugs to treat pulmonary hypertension, iron chelation drugs, pain medications, anti-cancer treatments, medications, vitamins, hormones, or the like. 
     A methodology for monitoring the operational health of a sensor (e.g., a force sensor) is implemented by an exemplary embodiment of a fluid infusion device. The fluid infusion device monitors force measurements obtained from the force sensor during a motor rewind operation to determine whether or not the force sensor might be out of calibration, on the verge of failure, or the like. The force normally experienced by the force sensor during rewind operations should be zero or close to zero, due to the absence of a fluid reservoir in the fluid infusion device, and because the fluid infusion device is driving in rewind mode, i.e., away from the plunger of the fluid reservoir. Accordingly, the fluid infusion device can assume that a properly functioning force sensor will produce rewind force readings in the neighborhood of zero or close to zero. Thus, if a rewind force measurement significantly deviates from the assumed baseline value (or range of values), then the fluid infusion device can take appropriate corrective action. 
     Another methodology for monitoring the operational health of a force sensor obtains force readings during a fluid delivery operation and compares the force readings to determine whether or not the force sensor is operating as expected. This alternate methodology measures the forces associated with individual fluid delivery strokes or drive motor pulses. Under normal and typical operating conditions, these forces will be relatively stable during one fluid delivery operation, and the variation from one stroke to another will be slight (absent an external impact or shock suffered by the fluid infusion device). Thus, if the force sensor reading is out of the expected operating range during a fluid delivery operation, the fluid infusion device can take appropriate corrective action. For example, if the force sensor output during fluid delivery happens to be −0.5 pounds, then clearly there is a problem because in reality the measured force should not be a negative value. 
     A fluid infusion device may also have an occlusion detection feature that determines when the fluid delivery path is occluded. Occlusion detection techniques are usually based on sensor measurements (force, pressure, stress) that are influenced by the flow status of the fluid delivery path. An exemplary embodiment of a fluid infusion device as described here employs an adaptive occlusion detection technique that need not rely on a fixed occlusion detection force threshold. Instead, the adaptive occlusion detection technique evaluates the rate of change of a metric associated with force variations per units of fluid to be delivered. For example, the typical force variation for a fluid reservoir might result in a variation of about ±X pounds per unit (lb/U) over a set number of delivery strokes (or drive motor pulses). If, however, the fluid infusion device detects a significant increase in this metric during a fluid delivery operation (e.g., ±Y lb/U, where Y is significantly larger than X) over the same set number of delivery strokes, then the fluid infusion device can take appropriate corrective action. The values of X and Y can also be in units of lb/pulse or the like. An example of a corrective action might be, but not limited to, immediately indicate or warn of an occlusion or simply lower the set threshold value by a set constant or percentage and allow the pump to continue delivery for a set number of pulses or units to see if the pump recovers (recovery might occur in the case of a kinked cannula). The adaptive occlusion detection methodology allows the fluid infusion device to determine the existence of an occlusion much quicker, relative to a fixed threshold based methodology. Quicker occlusion detection is made possible because the fluid infusion device need not be operated until a high threshold force is reached; rather, occlusion can be detected earlier without having to wait for a high force condition. 
     An exemplary embodiment of a fluid infusion device may also be configured to determine whether or not a fluid reservoir is properly seated and installed. The presence (or lack thereof) of the fluid reservoir is determined based upon force sensor readings that are obtained after proper initial installation and seating of the fluid reservoir. In accordance with one embodiment, one or more force thresholds are used to determine whether or not the fluid reservoir is properly seated. If a measured force does not satisfy a force threshold that is indicative of proper reservoir seating, then the fluid infusion device can take corrective action. In accordance with another exemplary embodiment, the fluid infusion device measures and processes the forces associated with individual fluid delivery strokes or drive motor pulses to determine when the fluid reservoir has been dislodged, removed, or unseated. 
       FIG. 1  is a plan view of an exemplary embodiment of a fluid infusion device  100 .  FIG. 1  also shows an infusion set  102  coupled to the fluid infusion device  100 . The fluid infusion device  100  is designed to be carried or worn by the patient. The fluid infusion device  100  may leverage a number of conventional features, components, elements, and characteristics of existing fluid infusion devices. For example, the fluid infusion device  100  may incorporate some of the features, components, elements, and/or characteristics described in U.S. Pat. Nos. 6,485,465 and 7,621,893, the relevant content of which is incorporated by reference herein. 
     This embodiment shown in  FIG. 1  includes a user interface  104  that includes several buttons that can be activated by the user. These buttons can be used to administer a bolus of insulin, to change therapy settings, to change user preferences, to select display features, and the like. Although not required, the illustrated embodiment of the fluid infusion device  100  includes a display element  106 . The display element  106  can be used to present various types of information or data to the user, such as, without limitation: the current glucose level of the patient; the time; a graph or chart of the patient&#39;s glucose level versus time; device status indicators; etc. In some embodiments, the display element  106  is realized as a touch screen display element and, therefore, the display element  106  also serves as a user interface component. 
     The fluid infusion device  100  accommodates a fluid reservoir (hidden from view in  FIG. 1 ) for the fluid to be delivered to the user. A length of tubing  108  is the flow path that couples the fluid reservoir to the infusion set  102 . The tubing  108  extends from the fluid infusion device  100  to the infusion set  102 , which provides a fluid pathway with the body of the user. A removable cap or fitting  110  is suitably sized and configured to accommodate replacement of fluid reservoirs (which are typically disposable) as needed. In this regard, the fitting  110  is designed to accommodate the fluid path from the fluid reservoir to the tubing  108 . 
       FIG. 2  is an exploded perspective view of the fluid infusion device  100 . For the sake of brevity and simplicity,  FIG. 2  is a simplified depiction of the fluid infusion device  100  that does not include all of the elements, components, and features that would otherwise be present in a typical embodiment. It should be appreciated that a deployed implementation of the fluid infusion device  100  will include additional features, components, and elements that are not shown in the figures. 
     The embodiment of the fluid infusion device  100  illustrated in  FIG. 2  includes a housing  112  and a housing end cap  114  that is coupled to an end  116  of the housing  112  to enclose components within the housing  112 . These internal components include, without limitation: a battery tube subassembly  118 ; a sleeve  120 ; a slide  121 ; an electronics assembly  122 ; a drive motor assembly  124  having a drive screw  125 ; a force sensor  126 ; and a motor support cap  128 .  FIG. 2  also depicts some components that are located outside the housing  112 , namely, a keypad assembly  130  and a graphic keypad overlay  132  for the keypad assembly  130 . The keypad assembly  130  and the graphic keypad overlay  132  may be considered to be part of the user interface  104  of the fluid infusion device  100 . The outer edge of the motor support cap  128  is attached to the interior side of the housing  112 , and the motor support cap  128  contacts the force sensor  126  to remove assembly tolerances from the drive motor assembly  124 .  FIG. 2  also depicts an exemplary fluid reservoir  111 , which is inserted into a reservoir cavity defined within the housing  112 . The reservoir cavity is configured, sized, and shaped to accommodate fluid reservoirs, and the fluid reservoir  111  is maintained in the reservoir cavity using the fitting  110 . The electronics assembly  122  may include a suitably configured electronics module (not shown in  FIG. 2 ; see  FIG. 4  and related description below), which may include or cooperate with a power supply, at least one memory element, at least one processor, processing logic, and device software, firmware, and application programs. 
       FIG. 3  is a cross sectional view of the fluid infusion device  100 , corresponding to a cross section taken longitudinally through the drive motor assembly  124  and the fluid reservoir  111 .  FIG. 3  depicts the state of the fluid infusion device  100  after the fluid reservoir  111  has been inserted into the reservoir cavity  134  and after the fitting  110  has been secured to the housing  112  to hold the fluid reservoir  111  in place. While certain embodiments accommodate disposable, prefilled reservoirs, alternative embodiments may use refillable cartridges, syringes or the like. A cartridge can be prefilled with insulin (or other drug or fluid) and inserted into the housing  112 . Alternatively, a cartridge could be filled by the user using an appropriate adapter and/or any suitable refilling device. 
     When assembled as shown in  FIG. 3 , the drive motor assembly  124  is located in the housing  112 . The force sensor  126  is operatively associated with the drive motor assembly  124 . For this particular embodiment, the force sensor  126  is coupled to the drive motor assembly  124 , and it is located between a base end of the drive motor assembly  124  and the motor support cap  128 . In one implementation, the force sensor  126  is affixed to the base end of the drive motor assembly  124  such that the force sensor  126  reacts when it bears against the motor support cap  128 . In another implementation, the force sensor  126  is affixed to the housing end cap  114  such that the force sensor  126  reacts when the drive motor assembly  124  bears against the force sensor  126 . This configuration and arrangement of the drive motor assembly  124  and the force sensor  126  allows the force sensor  126  to react to forces imparted thereto by the drive motor assembly  124  and/or forces imparted to the drive motor assembly  124  via the fluid pressure of the fluid reservoir  111 . 
     The drive motor assembly  124  includes an electric motor  136  that is actuated and controlled by the electronics module of the fluid infusion device  100 . The motor  136  is preferably realized as a stepper motor that rotates in a stepwise or discrete manner corresponding to the desired number of fluid delivery strokes. Alternatively, the motor  136  could be a DC motor, a solenoid, or the like. The motor  136  may optionally include an encoder (not shown), which cooperates with the electronics module of the fluid infusion device  100  to monitor the number of motor rotations or portions thereof. This in turn can be used to accurately determine the position of the slide  121 , thus providing information relating to the amount of fluid dispensed from the fluid reservoir  111 . 
     The drive motor assembly  124  can be mounted in the housing  112  using an appropriate mounting feature, structure, or element. Alternatively, the mounting could be accomplished using a shaft bearing and leaf spring or other known compliance mountings. 
     The illustrated embodiment of the drive motor assembly  124  includes a drive member (such as the externally threaded drive gear or drive screw  125 ) that engages an internally threaded second drive member (such as the slide  121 ) having a coupler  142 . The coupler  142  may be attached to or integrated with the slide  121 , as depicted in  FIG. 2  and  FIG. 3 . The slide  121  is sized to fit within the housing of the fluid reservoir  111 , which enables the slide  121  to operatively cooperate with the fluid reservoir  111 . The fluid reservoir  111  includes a plunger or piston  144  with at least one sealing element or feature (e.g., one or more O-rings, integral raised ridges, or a washer) for forming a fluid and air tight seal with the inner wall of the fluid reservoir  111 . As mentioned previously, the fluid reservoir  111  is secured into the housing  112  with the fitting  110 , which also serves as the interface between the fluid reservoir  111  and the infusion set tubing  108 . For this embodiment, the piston  144  is in contact with a linear actuation member, such as the slide  121 . For example, the piston  144  may have a female portion  146  that receives the coupler  142  carried by the slide  121 . The female portion  146  is positioned at the end face of the piston  144 , and it is sized to receive and accommodate the coupler  142 . In certain embodiments, the female portion  146  includes a threaded cavity that engages external threads of the coupler  142 . 
     Referring to  FIG. 3 , rotation of the drive shaft of the motor  136  results in corresponding rotation of the drive screw  125 , which in turn drives the slide  121  via the threaded engagement. Thus, rotation of the drive screw  125  results in axial displacement of the slide  121  and, therefore, axial displacement of the coupler  142 . Such displacement of the coupler  142  moves the piston  144  (upward in  FIG. 3 ) to deliver a predetermined or commanded amount of medication or liquid from the fluid infusion device  100 . In this manner, the drive motor assembly  124  is configured to regulate delivery of fluid by actuating the piston  144  (under the control of the electronics module and/or control system of the fluid infusion device  100 ). As described above, if a stepper motor is employed, then the drive motor assembly  124  can regulate delivery of fluid from the fluid infusion device  100  in discrete actuation or delivery strokes. The fluid infusion device  100  can employ the sleeve  120  or an equivalent feature (such as an anti-rotation key) to inhibit rotation of the drive motor assembly  124 , which might otherwise result from torque generated by the motor  136 . In some embodiments, the drive shaft of the drive motor assembly  124 , the drive screw  125 , and the slide  121  are all coaxially centered within the longitudinal axis of travel of the piston  144 . In certain alternative embodiments, one or more of these components may be offset from the center of the axis of travel and yet remain aligned with the axis of travel, which extends along the length of the fluid reservoir  111 . 
     As mentioned above, certain embodiments of the fluid infusion device  100  accommodate removable and replaceable fluid reservoirs. When the slide  121  and, therefore, the piston  144  of the fluid reservoir  111  are in their fully extended positions, the piston  144  has forced most, if not all, of the fluid out of the fluid reservoir  111 . After the piston  144  has reached the end of its travel path, indicating that the fluid reservoir  111  has been depleted, the fluid reservoir  111  may be removed such that the female portion  146  of the piston  144  disengages from the coupler  142  of the slide  121 . After the empty (or otherwise used) fluid reservoir  111  is removed, the electronics module or control system of the fluid infusion device  100  initiates a rewind operation during which the motor  136  rotates in the reverse direction to rewind the slide  121  back to its fully retracted position. Thereafter, a new or refilled fluid reservoir  111  can be installed, seated, and primed for use. In this regard, an embodiment provides for advancement of the slide  121  upon the insertion of a fluid reservoir  111  into the housing  112 . The slide  121  advances until its coupler  142  comes into contact with the piston  144  of the fluid reservoir  111 . In alternative embodiments having a threaded piston engagement, the slide  121  advances until the threads of the coupler  142  engage the threads in the female portion  146  of the piston  144 . When the threads engage in this fashion, they need not do so by twisting. Rather, they may ratchet over one another. In operation, the force sensor  126  may be used to determine when the slide  121  contacts the piston  144 , when the coupler  142  is properly seated in the female portion  146 , and/or when the fluid reservoir  111  has been primed and is ready to deliver measured doses of fluid. 
     Although the illustrated embodiment employs a coaxial or inline drive system, alternative configurations could be utilized. For example, a drive system that uses a lead screw, a drive nut, and actuation arms (of the type described in U.S. Pat. No. 6,485,465) may be employed, with the force sensor  126  positioned in an appropriate location. In various embodiments, the drive train might include one or more lead screws, cams, ratchets, jacks, pulleys, pawls, clamps, gears, nuts, slides, bearings, levers, beams, stoppers, plungers, sliders, brackets, guides, bearings, supports, bellows, caps, diaphragms, bags, heaters, or the like. Moreover, although the illustrated embodiment employs a sensor positioned at the end of the fluid drive train, other arrangements could be deployed. For example, a sensor could be placed at or near the front end of the fluid drive train. 
     In particular embodiments, the force sensor  126  is used to detect when the slide  121  contacts the piston  144 . Thus, after the fluid reservoir  111  is placed into the fluid infusion device  100 , the motor  136  is activated to move the slide  121  toward the fluid reservoir  111  to engage the piston  144 . In this regard, when a shoulder region  150  (see  FIG. 3 ) of the slide  121  first contacts the piston  144 , the electronics module detects an increase in force imparted to the force sensor  126 . The measured force continues to increase as the motor  136  continues to drive forward, in response to the fluid resistance in the fluid reservoir  111 . When the slide  121  is properly seated with the piston  144 , the measured force increases to the seating threshold level. During the seating operation, if the measured force exceeds this seating threshold, the motor  136  is stopped until further commands are issued. The seating threshold is generally about 1.5 pounds. In alternative embodiments, higher or lower seating thresholds may be used depending on the force required to mate the slide  121  with the piston  144 , the force required to urge fluid from the fluid reservoir  111 , the speed of the motor  136 , the accuracy and resolution of the force sensor  126 , or the like. 
     It should be appreciated that other force thresholds can be used for other purposes. During priming of fluid reservoirs, for example, a threshold of about 4.0 pounds is used. In some embodiments, levels greater than about 5.0 pounds are used to detect shock loads that may be damaging to the fluid infusion device  100 . 
     The force sensor  126  is configured to react in response to force imparted thereto. In this regard, electrical, mechanical, magnetic, and/or other measurable or detectable characteristics of the force sensor  126  vary in accordance with the amount of force applied to the force sensor  126 . In practice, the force sensor  126  might implement or otherwise leverage known sensor technologies, such as the sensor technology described in U.S. Pat. No. 6,485,465. As shown in  FIG. 2 , the force sensor  126  includes at least one electrical lead  154  that is electrically coupled to the electronics module (or controller) of the fluid infusion device  100 . Alternatively, the force sensor  126  could use wireless data communication technology to provide force-related data to the electronics module. In certain implementations, the force sensor  126  is suitably configured to indicate or generate a plurality of different output levels that can be monitored and/or determined by the electronics module. In practice, the output levels obtained from the force sensor  126  are initially conveyed as analog voltages or analog currents, and the electronics module includes an analog-to-digital converter that transforms a sampled analog voltage into a digital representation. Conversion of sensor voltage into the digital domain is desirable for ease of processing, comparison to threshold values, and the like. 
     In particular embodiments, the force sensor  126  is realized as an electromechanical component having at least one variable resistance that changes as the force applied to the force sensor  126  changes. In alternative embodiments, the force sensor  126  is a capacitive sensor, a piezoresistive sensor, a piezoelectric sensor, a magnetic sensor, an optical sensor, a potentiometer, a micro-machined sensor, a linear transducer, an encoder, a strain gauge, or the like, and the detectable parameter or characteristic might be compression, shear, tension, displacement, distance, rotation, torque, force, pressure, or the like. In practice, changing characteristics of the force sensor  126  are associated with output signal characteristics that are responsive to a physical parameter to be measured. Moreover, the range and resolution of the monitored output signal provides for the desired number of output levels (e.g., different states, values, quantities, signals, magnitudes, frequencies, steps, or the like) across the range of measurement. For example, the force sensor  126  might generate a low or zero value when the applied force is relatively low, a high or maximum value when the applied force is relatively high, and intermediate values when the applied force is within the detectable range. 
     In certain exemplary embodiments, the electronics module of the fluid infusion device  100  maintains a constant supply voltage across the force sensor  126 , and the monitored output signal of the force sensor  126  is a signal current that passes through a resistive material of the force sensor  126 . Thus, the signal current varies with the amount of force applied to the force sensor  126  because the resistance of the force sensor  126  varies with force and the supply voltage across the force sensor  126  is constant. The electronics module converts the monitored signal current into a signal voltage, which is then used as an indication of the force imparted to the force sensor  126  (which may be caused by the drive motor assembly  124 , by fluid pressure in the fluid reservoir  111 , by impact experienced by the fluid infusion device  100 , etc.). In alternative embodiments, a constant supply current is used and the signal voltage across the force sensor  126  varies with force (fluid pressure). 
     In certain embodiments, sensor measurements are taken prior to commanding the drive system to deliver fluid, and soon after the drive system has stopped delivering fluid. In alternative embodiments, sensor data is collected on a continuous basis at a particular sampling rate (for example, 10.0 Hz, 3.0 Hz, once every 10 seconds, once a minute, once every five minutes, or the like). In further alternative embodiments, the sensor data is only collected prior to commanding the drive system to deliver fluid. In still further alternative embodiments, sensor data is collected during fluid delivery (during delivery strokes and/or between delivery strokes). 
     In practice, the force sensor  126  and associated electronics are designed to measure forces between about zero pounds and about five pounds with a desired resolution of about 0.01 pounds. In preferred embodiments, the force sensor  126  and associated electronics provide a relatively linear voltage output in response to forces applied to the force sensor  126  by one or more drive train components. In alternative embodiments, the range and resolution of the force sensor  126  might vary from that specified above. Furthermore, the sensor range and/or resolution may vary in accordance with the concentration of the fluid being delivered, the diameter of the fluid reservoir  111 , the diameter of the fluid path, the nominal range of force experienced during normal operation of the drive motor assembly  124 , the amount of sensor noise, the algorithms applied to detect trends from sensor measurements, or the like. Moreover, the fluid infusion device  100  and the force sensor  126  should be suitably configured to survive shock levels that result in much higher forces being applied to the force sensor  126  than the intended sensor measurement range. 
     As mentioned previously, the fluid infusion device  100  is suitably configured to support a number of techniques, processes, and methodologies that utilize the force sensor  126 . In practice, the fluid infusion device  100  includes an electronics module, processing logic, software applications, and/or other features that are used to carry out the various operating processes described here. In this regard,  FIG. 4  is a schematic block diagram representation of an embodiment of the fluid infusion device  100 .  FIG. 4  depicts some previously-described elements of the fluid infusion device  100  as functional blocks or modules, namely, the display element  106 ; the user interface  104 ; the drive motor assembly  124 ; and the force sensor  126 .  FIG. 4  also depicts the fluid reservoir  111  and the infusion set  102  in block format. This particular embodiment of the fluid infusion device  100  also includes, without limitation: a suitable amount of memory  160 ; an electronics module  162  (which may include or cooperate with one or more processors, processing modules, controllers, state machines, or the like); a power supply  164  such as a battery or a battery pack; and other infusion pump hardware, software, and applications  166 . The elements of the fluid infusion device  100  may be coupled together via an interconnection architecture  168  or arrangement that facilitates transfer of data, commands, power, etc. 
     The display element  106  represents the primary graphical interface of the fluid infusion device  100 . The display element  106  may leverage known plasma, liquid crystal display (LCD), thin film transistor (TFT), and/or other display technologies. The actual size, resolution, and operating specifications of the display element  106  can be selected to suit the needs of the particular application. Notably, the display element  106  may include or be realized as a touch screen display element that can accommodate touch screen techniques and technologies. In practice, the display element  106  may be driven by a suitable display driver to enable the fluid infusion device  100  to display physiological patient data, status information, clock information, alarms, alerts, and/or other information and data received or processed by the fluid infusion device  100 . 
     The user interface  104  may include a variety of items such as, without limitation: a keypad, keys, buttons, a keyboard, switches, knobs (which may be rotary or push/rotary), a touchpad, a microphone suitably adapted to receive voice commands, a joystick, a pointing device, an alphanumeric character entry device or touch element, a trackball, a motion sensor, a lever, a slider bar, a virtual writing tablet, or any device, component, or function that enables the user to select options, input information, or otherwise control the operation of the fluid infusion device  100 . In this context, the user interface  104  may cooperate with or include a touch screen display element  106 . The user interface  104  allows a user to control the delivery of fluid via the infusion set  102 . 
     The electronics module  162  may include or be implemented with a general purpose processor, a content addressable memory, a digital signal processor, an application specific integrated circuit, a field programmable gate array, any suitable programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination designed to perform the functions described here. A processor device may be realized as a microprocessor, a controller, a microcontroller, or a state machine. Moreover, a processor device may be implemented as a combination of computing devices, e.g., a combination of a digital signal processor and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a digital signal processor core, or any other such configuration. 
     The electronics module  162  may include one processor device or a plurality of cooperating processor devices. Moreover, a functional or logical module/component of the fluid infusion device  100  might be realized by, implemented with, and/or controlled by processing logic maintained by or included with the electronics module  162 . For example, the display element  106 , the user interface  104 , the drive motor assembly  124 , and/or the infusion pump hardware, software, and applications  166  (or portions thereof) may be implemented in or controlled by the electronics module  162 . 
     The memory  160  may be realized as RAM memory, flash memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. In this regard, the memory  160  can be coupled to the electronics module  162  such that the electronics module  162  can read information from, and write information to, the memory  160 . In the alternative, the memory  160  may be integral to the electronics module  162 . As an example, a processor of the electronics module  162  and the memory  160  may reside in an ASIC. In practice, a functional or logical module/component of the fluid infusion device  100  might be realized using program code that is maintained in the memory  160 . Moreover, the memory  160  can be used to store data utilized to support the operation of the fluid infusion device  100 , including, without limitation, sensor data, force measurements, force thresholds, alert/alarm history, and the like (as will become apparent from the following description). 
     The infusion pump hardware, software, and applications  166  are utilized to carry out fluid infusion features, operations, and functionality. Thus, the infusion pump hardware, software, and applications  166  may include or cooperate with the infusion set  102  and/or the fluid reservoir  111  (as described above). It should be appreciated that the infusion pump hardware, software, and applications  166  may leverage known techniques to carry out conventional infusion pump functions and operations, and such known aspects will not be described in detail here. 
     A fluid infusion device can support one or more features or operations that enhance its fluid infusion functionality and/or enhance the user experience of the fluid infusion device. The following sections include descriptions of various processes and methods that may be performed by a fluid infusion device. The various tasks performed in connection with a given process may be performed by software, hardware, firmware, or any combination thereof. For illustrative purposes, a process might be described with reference to elements mentioned above in connection with  FIGS. 1-4 . In practice, portions of a given process may be performed by different elements of the described system, e.g., a sensor, a drive motor assembly, an electronics module, a processor, or the like. It should be appreciated that a described process may include any number of additional or alternative tasks, the tasks included in a particular flow chart need not be performed in the illustrated order, an embodiment of a described process may omit one or more of the illustrated tasks, and a given process may be incorporated into a more comprehensive procedure or process having additional functionality not described in detail herein. 
     Sensor Health Monitoring 
     For reasons presented above, the force sensor  126  in the fluid infusion device  100  is an important component that, at a minimum, is used to determine when a new fluid reservoir  111  is seated and when an occlusion has occurred. During use, the output and/or electromechanical characteristics of the force sensor  126  may drift over time. The drift can be attributed to the aging of mechanical components, impacts or shocks suffered by the fluid infusion device  100 , environmental exposure, etc. It is desirable to monitor sensor drift so that the fluid infusion device  100  can alert the patient if the sensor drift exceeds a tolerable amount. Notably, the force sensor  126  cannot be easily or conveniently calibrated, for example on a yearly basis, because it is not accessible. Consequently, the force sensor  126  should not divert from a calibration curve (which contemplates typical variations or drifting of the force sensor  126 ) over the life of the product. 
     The sensor health monitoring features of the fluid infusion device  100  can be utilized to determine and monitor the drift characteristics of the force sensor  126 . In accordance with one approach, it is assumed that the force sensor  126  experiences a consistent and relatively low load during rewind operations, which are performed before installing a new fluid reservoir  111 . During a calibration routine (which may be performed, for example, during manufacturing of the fluid infusion device  100 ), the device records force data collected during one or more rewind stages. The force data is used to generate a nominal rewind force value, which may represent an average of the collected values, the maximum collected value, the minimum collected value, or the like. This rewind force value is saved in the memory  160  of the fluid infusion device  100 . Thereafter, when deployed and operating, the fluid infusion device  100  performs a rewind force average check and compares the value to the saved rewind force. If the measured rewind force is within a specified range of the calibration rewind force value, then the fluid infusion device  100  continues to operate as usual. If, however, the measured rewind force drifts below a level that is not acceptable, then the fluid infusion device  100  can generate an alarm, an alert, or respond in a predetermined manner. The sensor health can be checked whenever a new fluid reservoir  111  is installed, typically every three days. 
       FIG. 5  is a flow chart that illustrates an embodiment of a process  200  associated with the operation of a fluid infusion device, such as the fluid infusion device  100  described above. The process  200  may begin with the activation of a rewind operation of the drive motor assembly (task  202 ). As explained above, the drive motor assembly rewinds after removing an old fluid reservoir and before installing a new fluid reservoir. Accordingly, task  202  could be automatically performed whenever a fluid reservoir is removed from the fluid infusion device. As another option, task  202  could be performed (either automatically or initiated by the user) at other times when the fluid infusion device  100  is not delivering fluid. In this regard, the slide  121  can be retracted from the piston  144  while leaving the piston  144  in place. The position of the slide  121  prior to retraction can be stored such that the slide  121  can be precisely returned to its former position to continue delivering fluid. In response to the rewind activation, the process  200  rewinds the drive motor assembly by an appropriate amount (task  204 ). To accommodate installation of a new fluid reservoir, task  204  rewinds the drive motor assembly until the slide is fully retracted. 
     The absence of a fluid reservoir during the rewind operation results in no loading on the drive motor assembly. For this reason, the process  200  determines a rewind force imparted to the force sensor during the rewind operation (task  206 ). Task  206  could obtain a single rewind force measurement at any time during the rewind operation, it could calculate an average rewind force based upon any number of rewind force measurements obtained during the rewind operation, or it could generate any rewind force value or metric that is based upon one or more individual rewind force measurements obtained during the rewind operation. For simplicity, this particular embodiment of the process  200  assumes that a single rewind force measurement is determined at task  206 . 
     The process  200  may continue by comparing the rewind force measurement to one or more threshold forces. For this example, the process  200  determines whether or not the rewind force measurement falls within a predetermined range, and initiates corrective action at the fluid infusion device when the rewind force measurement does not fall within that range. Thus, if the rewind force measurement is less than the lower threshold force (query task  208 ), then the fluid infusion device initiates and executes appropriate corrective action (task  210 ). Similarly, if the rewind force measurement is greater than the upper threshold force (query task  212 ), then the fluid infusion device initiates and executes appropriate corrective action (task  214 ). The corrective action taken by the fluid infusion device may include one or more of the following, without limitation: generating an alert or an alarm at the fluid infusion device; stopping or inhibiting fluid delivery; presenting instructions, a maintenance reminder, or a message to the user; or the like. In practice, an alert, alarm, or warning may include, without limitation: sounds; one or more synthesized voices; vibrations or other haptic feedback; displayed symbols or messages; lights; transmitted signals; Braille output; or the like. Other forms of corrective action include, without limitation: running a self test of the fluid infusion device; recalibrating the threshold forces; temporarily disabling the fluid infusion device; or the like. 
     A rewind force measurement that is less than the lower threshold force indicates that the measured force is less than the actual force applied to the force sensor. Consequently, it may be more difficult for the fluid infusion device to accurately and quickly detect the onset of an occlusion in the fluid path based on the force sensor output. On the other hand, a rewind force measurement that is greater than the upper threshold force indicates that the measured force is greater than the actual force applied to the force sensor. Consequently, the fluid infusion device might detect the onset of an occlusion too soon, or falsely detect an occlusion. Therefore, the type of corrective action taken at task  210  may be different than the type of corrective action taken at task  214 . In this regard, different alert characteristics (colors, volume, frequency, sounds), different message content or formats, and/or different combinations of the corrective actions described above could be initiated and executed at tasks  210 ,  214 . 
     It should be appreciated that the fluid infusion device processes the output levels associated with the force sensor to determine the current operating health of the force sensor. If the rewind force measurement falls within the specified range (i.e., it is greater than the lower threshold force and less than the upper threshold force), then the fluid infusion device can continue operating as usual. For example, the fluid infusion device can complete the rewind operation (task  216 , which may be performed before or during tasks  206 ,  208 ,  210 ,  212 ,  214 ) before placement of a new fluid reservoir into the fluid infusion device. After the new fluid reservoir is installed, the process  200  activates a reservoir seating operation (task  218 ) and uses the force sensor to detect when the new fluid reservoir is seated in the fluid infusion device. In this regard, the drive motor assembly is activated to advance the slide until a predetermined seating force has been detected (task  220 ). After detection of this seating force, the drive motor assembly can be further advanced by an appropriate amount during a priming operation, which is activated to prepare the fluid infusion device and the new fluid reservoir for normal operation. 
     As mentioned above, a rewind operation and the related force measurements could be performed before the fluid reservoir needs to be replaced. If the process  200  determines that the force sensor is operating as expected during such a rewind operation, then task  218  and task  220  are performed to return the slide  121  to its former position in contact with the piston  144 . Thereafter, fluid delivery may continue as though the rewind operation never took place. 
     During normal use, the fluid infusion device regulates the delivery of fluid from the fluid reservoir by controlling the movement of the drive motor assembly (task  222 ). The same force sensor used to determine the rewind force measurements can also be used to monitor for the onset of an occlusion (task  224 ). In particular, the force imparted to the force sensor (which is indicative of the pressure in the fluid reservoir) is determined and analyzed in accordance with one or more occlusion detection schemes to detect an occlusion of the infusion set or elsewhere in the fluid delivery path. 
     In certain embodiments, the measured rewind forces (see task  206 ) observed during operation are recorded in the memory of the device. The historical rewind force data may be used to detect trends and drifting in the measured rewind force, e.g., consistently decreasing, consistently increasing, random variation, or the like. Thus, even if a measured rewind force does not trigger corrective action, the rewind force values can be saved for diagnostic purposes, statistical evaluation of the device, and the like. 
     The preceding description of the process  200  illustrates how rewind force thresholds can be used to check the operating health of the force sensor. The threshold forces may be fixed or adaptive (to accommodate and compensate for drifting of the force sensor). In this regard,  FIG. 6  is a flow chart that illustrates an embodiment of a rewind force calibration process  300  for a fluid infusion device, such as the fluid infusion device  100 . In certain scenarios, the process  300  is performed at least once during manufacturing of the fluid infusion device, and the calibrated threshold forces are stored as fixed values for the life of the device. In other situations, the process  300  could be performed periodically (e.g., at the request of the user, every year, or in accordance with a maintenance schedule). The illustrated embodiment of the process  300  begins by performing a calibrating rewind operation (task  302 ). No fluid reservoir is present during calibration, and the drive motor assembly is controlled as necessary to execute a rewind operation. 
     During the calibrating rewind operation, the process  300  determines a calibration rewind force imparted to the force sensor (task  304 ). Task  304  could obtain a single rewind force measurement at any time during the calibrating rewind operation, it could calculate an average calibration rewind force based upon any number of rewind force measurements obtained during the calibrating rewind operation, or it could generate any calibration rewind force value or metric that is based upon one or more individual calibration rewind force measurements obtained during the calibrating rewind operation. For simplicity, this particular embodiment of the process  300  assumes that a single calibration rewind force measurement is determined at task  304 . If the calibration period is finished (query task  306 ), then the process  300  continues. If not, the process  300  performs another calibrating rewind operation and determines a respective calibration rewind force measurement for that operation. In other words, tasks  302 ,  304 ,  306  can be repeated any desired number of times, resulting in a plurality of calibration rewind forces that can be saved for subsequent analysis and processing. 
     Upon completion of the calibration period, the process  300  calculates a nominal rewind force from the plurality of calibration rewind forces (task  308 ). The nominal rewind force can be determined using any desired formula, algorithm, relationship, or equation. For the simple implementation described here, the nominal rewind force is calculated as an average of the plurality of calibration rewind forces. As mentioned previously, the nominal rewind force is ideally equal or equivalent to a load of zero pounds on the drive motor assembly. Accordingly, an acceptable nominal rewind force will typically be within the range of about −0.50 to +0.50 pounds (this range is merely exemplary, and an embodiment could utilize different upper and/or lower values). In this regard, the process  300  might check the calculated nominal rewind force to ensure that it falls within a predetermined range of acceptable values. Thus, if the nominal rewind force falls outside of that range, the process  300  could generate an alarm, an alert, or a warning for the user, and/or repeat the portion of the calibration routine associated with tasks  302 ,  304 ,  306 , and  308 . 
     Assuming that the calculated nominal rewind force is acceptable, it can be stored in a memory element of the fluid infusion device (task  310 ) for future reference if needed. For this particular embodiment, the process  300  derives the lower threshold rewind force and the upper threshold rewind force from the calculated nominal rewind force (task  312 ). For example, the lower threshold force might be calculated by subtracting a designated amount from the nominal rewind force, and the upper threshold force might be calculated by adding a designated amount to the nominal rewind force. Moreover, the manner in which these threshold forces are calculated could vary as a function of the nominal rewind force itself. For instance, one threshold calculation scheme could be used when the nominal rewind force is greater than zero, and a different threshold calculation scheme could be used when the nominal rewind force is less than zero. After the rewind force thresholds have been derived, they can be stored in a memory element of the fluid infusion device (task  314 ) for subsequent use as needed, e.g., during execution of the process  200 . In practice, these rewind force thresholds can be saved as fixed values that do not change during the operating life of the fluid infusion device. In this manner, the process  300  results in a specified range of rewind forces that is indicative of a healthy operating status of the force sensor. 
     In accordance with another approach, the operating health of the force sensor  126  is checked more frequently, namely, at times other than during a rewind operation. This alternate approach (which may be utilized in conjunction with the first approach described above) checks the health of the force sensor in an ongoing manner without having to wait until a rewind operation. More particularly, this technique checks the measured or detected force associated with the drive motor assembly at designated motor pulses (e.g., at every motor pulse). If the measured force is below a predetermined value, the fluid infusion device will take corrective action because this condition indicates that the force sensor has drifted beyond its limit in the negative direction. If the measured force is above a predetermined value, then the fluid infusion device assumes that the force sensor has drifted in the positive direction, which might result in false or early detection of an occlusion. 
       FIG. 7  is a flow chart that illustrates another embodiment of a process  400  associated with the operation of a fluid infusion device, such as the fluid infusion device  100  described above. The process  400  may begin any time after activating a fluid delivery operation of the fluid infusion device (task  402 ). During a fluid delivery operation, the drive motor assembly is used to actuate the fluid reservoir in a controlled manner. In certain implementations, fluid delivery operations are carried out in a stepwise manner such that fluid is administered in discrete fluid delivery strokes (or pulses) of the slide. In practice, a fluid delivery operation may involve multiple fluid delivery strokes; the exact number will depend on the desired amount of fluid to be delivered. Accordingly, the process  400  continues by performing the next fluid delivery action, pulse, or stroke with the drive motor assembly (task  404 ), and determining and saving the corresponding measure of actuation force imparted to the force sensor during that fluid delivery action (task  406 ). 
     Next, the process  400  compares the measure of actuation force to a range of valid values for the fluid infusion device (task  408 ). In practice, there will be a range of force sensor outputs or readings that correspond to or otherwise represent normally expected measures of actuation forces. For example, if the force sensor is operating as expected, then it might have a limited and predetermined analog output range, which in turn corresponds to a limited and predetermined range of encoded digital values. If, however, the force sensor is damaged, is beginning to fail, or is otherwise operating in an unexpected or unusual manner, then the resulting analog output and encoded digital values could be outside of the normally expected range. Measured values of actuation force that are outside of the normally expected range are therefore treated as invalid or undefined measures. 
     If the current measure of actuation force is not invalid as defined by the particular settings and configuration of the fluid infusion device (query task  410 ), then the process  400  may return to task  404  to perform the next fluid delivery action. This enables the fluid infusion device to monitor the operating integrity of the force sensor during fluid delivery and in an ongoing and dynamic manner If, however, the current measure of actuation force is outside the range of valid values, then the process may generate a flag or otherwise record an invalid force event for the current measure of actuation force (task  412 ). The recorded event may include information such as, for example, the delivery time of the current stroke, the current measure of actuation force, the current position of the slide or drive motor, or the like. Data associated with the recorded event can be saved for subsequent analysis, for reporting purposes, for troubleshooting or diagnostic purposes, etc. For instance, this embodiment of the process  400  continues by analyzing a set of invalid force events to determine a course of action for the fluid infusion device (task  414 ). In this regard, the set of invalid force events may represent a designated number of past invalid force events, including the current invalid force event, collected over a particular period of time, collected for a designated number of delivery strokes, or the like. In certain situations, the set of invalid force events may correspond to only one event, which could be sufficient to trigger an alarm or an alert if deemed necessary. This allows the process  400  to initiate corrective action based on a single actuation stroke or pulse, based on an average measure of multiple pulses, based on detected patterns of measured forces, or the like. Task  414  may be performed to reduce the likelihood of false alerts or false alarms associated with the operating health of the force sensor. In this regard, the process  400  may be used to detect a positive drift and/or a negative drift in the force sensor, which may result in the lack of timely alerts. Thus, the detection of only one invalid force event during an extended period of time or over the course of many delivery strokes can be disregarded without triggering an alert. 
     The process  400  could utilize any type of criteria that influences whether or not a single invalid force event or a set of invalid force events will cause the fluid infusion device to respond. For example, the criteria may dictate that at least a threshold number of invalid force events corresponding to consecutive fluid delivery actions must be recorded before a user alert is generated. As another example, the criteria may dictate that at least a threshold number of invalid force events must be recorded within a designated period of time (such as 60 minutes) before any corrective action is taken. The criteria may be chosen such that transient conditions that might influence the operation of the force sensor (e.g., handling of the device, bumping or dropping the device, driving over a pothole, etc.) do not trigger an alarm. Rather, the criteria may be selected such that the fluid infusion device is given the opportunity to recover and settle from such transient events. 
     Accordingly, if certain designated criteria is satisfied (query task  416 ), then the process  400  can initiate and execute appropriate corrective action at the fluid infusion device (task  418 ). If the criteria has not been satisfied, then the process  400  may return to task  404  to perform the next fluid delivery action. Thus, some form of corrective or remedial action can be taken in response to the recording of one or more invalid force events, where the recorded events are indicative of poor operating health of the force sensor. Task  418  may initiate and execute any of the corrective actions described above for tasks  210  and  214  of the process  200 . 
     The process  400  represents a simplified embodiment that analyzes actuation forces and checks for valid measures of actuation force from one delivery stroke or pulse of the drive motor assembly to another. Alternative embodiments, however, could implement a more complex scheme that calculates and considers any suitable parameter, measure of force, force-related metric, or the like. For example, rather than compare the actuation forces to a range of valid measures per se, the fluid infusion device could instead calculate any appropriate parameter from the measured actuation force, where the parameter is somehow indicative of the operating health of the force sensor, and then compare the value of that parameter to certain predetermined performance criterion for the force sensor. If the parameter does not satisfy the performance criterion, then the fluid infusion device can take corrective or remedial action. 
     In some embodiments, the fluid infusion device  100  is suitably configured to check the operating condition of the force sensor  126  using a known and calibrated force applied to the force sensor  126 . Although not always required, this example assumes that the force sensor  126  is tested after removing the fluid reservoir  111  and during a time when fluid need not be dispensed. Imparting a known nonzero calibration force to the force sensor  126  can be accomplished using any suitable component, device, fixture, or equipment. In accordance with one exemplary embodiment, the fitting  110  is replaced with a calibration fitting that is provided with a precisely calibrated spring or other element that provides a known force at a specified amount of deflection. After installing the calibration fitting, the slide  121  is advanced by a designated amount, which can be controlled by monitoring encoder counts or other metrics related to the operation of the drive motor  136 . When the slide  121  has advanced by the designated amount, the force element (e.g., the spring) is expected to impart the calibrating force to the slide  121 , which in turn imparts the calibrating force to the force sensor  126 . 
     When the slide  121  has reached the calibration position, the corresponding measure of actuation force is recorded and compared to a value associated with the expected calibrating force. If the recorded value is different than the expected calibration value by more than a stated amount, then the fluid infusion device  100  (and/or the user) can assume that the force sensor  126  is defective or otherwise not functioning according to specification. It should be appreciated that the calibration force should fall within the normal measuring range of the force sensor  126 . 
     Reservoir Seating (Presence) Monitoring 
     The force sensor  126  in the fluid infusion device  100  may also be utilized to monitor the presence and seating status of the fluid reservoir  111 . In this regard, the electronics module  162  of the fluid infusion device  100  can be utilized to process the output levels of the force sensor  126  to determine the seating status of the fluid reservoir  111  in the reservoir cavity  134  in an ongoing manner. The fluid infusion device  100  can alert the user when the fluid reservoir  111  has been accidentally removed or inadvertently dislodged. The fitting  110  might be inadvertently rotated or loosened during physical activity (e.g., while the user is playing a sport or exercising), which in turn might result in removal or dislodging of the fluid reservoir  111 . When this happens, proper coupling between the piston  144  of the fluid reservoir  111  and the coupler  142  of the slide  121  could be lost. For safe measure, the fluid infusion device  100  notifies the user shortly after the fluid reservoir  111  is partially removed, completely removed, or dislodged by more than a predetermined amount. 
     The fluid infusion device  100  uses the force sensor  126  to determine the seating status of the fluid reservoir  111 . Accordingly, no additional components, sensors, or assembly time is needed to implement this feature. In certain embodiments, the fluid infusion device  100  uses a scheme that adaptively tracks the forces of delivery strokes. The fluid infusion device  100  records the force of a delivery stroke. If the measured force remains the same or increases from stroke-to-stroke, then the fluid infusion device  100  assumes that a fluid reservoir is in place and is properly seated. Moreover, slight variations in the detected force can be disregarded to contemplate normal and expected force variations that typically occur along the travel path of a fluid reservoir. However, a characterized drop in force that is greater than a certain amount indicates that (1) the fluid reservoir  111  has been removed or dislodged or (2) the fluid infusion device  100  may have been dropped or bumped, temporarily disturbing the force sensor  126 . For the second scenario, design engineers can characterize how many pulses (delivery strokes) and/or how much time is typically needed to allow the fluid infusion device  100  to recover from a disturbing impact or force, assuming that the fluid reservoir  111  remains present and properly seated. If the measured force does not return to a nominal value after a designated number of strokes or a predetermined amount of time, the fluid infusion device  100  can conclude that the fluid reservoir  111  has been removed or disturbed and, in turn, generate an alert or a warning message for the user. 
     Notably, the reservoir presence scheme is adaptive in nature, and it takes into account variations such as sensor drift, reservoir frictional force variation, and minor shocks. The reservoir presence methodology actively monitors for an ongoing drop in force greater than a set value. This approach is desirable because it accommodates variations (such as sensor drift) that might be introduced over the life of the fluid infusion device  100 , variations from one reservoir to another, and variations (such as fluid pressure) that might occur during use of a single reservoir. For example, if frictional force increases due to reservoir dynamics, the fluid infusion device  100  will adapt and set the increased force measure as a new baseline value. Similarly, if the frictional force is decreasing due to reservoir dynamics, the fluid infusion device  100  will adapt the lower force measure as the new baseline value (as long as the rate of decrease and/or the force variation is less than a specified amount). This feature accommodates variation in frictional force without false alarms. Accordingly, the fluid infusion device  100  adaptively resets the baseline force value as long as the rates of change are within normal limits. 
       FIG. 8  is a flow chart that illustrates an embodiment of a process  500  that checks the seating status of a fluid reservoir of a fluid infusion device, such as the fluid infusion device  100  described above. The process  500  employs a simple force threshold for purposes of determining the seating status of the fluid reservoir. In this regard, the process  500  may begin by obtaining, calculating, or accessing the force threshold (task  502 ), which represents an amount of force that is indicative of a dislodged, removed, displaced, or otherwise unseated state of the fluid reservoir. The force threshold may be adaptively updated in response to certain operating conditions, it may be one of a plurality of different available threshold values, it may be a predetermined and fixed value, or the like. For this example, the force threshold is a pre-stored value that is fixed during the operating lifespan of the fluid infusion device. In other embodiments (described below), the force threshold can be calculated in a dynamic manner to contemplate typical variations such as drifting of the force sensor. 
     Under typical operating conditions for exemplary embodiments, the normally expected force imparted to the force sensor when the fluid reservoir is properly seated is less than about 1.5 pounds. Moreover, under typical operating conditions for exemplary embodiments, the normally expected actuation force imparted to the force sensor during a fluid delivery stroke is less than about 1.5 pounds. In certain implementations, the force threshold used by the process  500  is calculated as a function of the nominal seating force and/or as a function of the nominal actuation force, and the force threshold is stored in a memory element of the fluid infusion device. For example, the force threshold might be calculated to be less than the average fluid delivery actuation force by a given amount, such as a percentage of the average fluid delivery actuation force. As another example, the force threshold is calculated such that it is a predefined amount of force less than the nominal expected actuation force. In practice, regardless of the manner in which it is calculated, the force threshold will typically be less than about 0.5 pounds. 
     In practice, the process  500  can be initiated whenever a fluid reservoir is installed into the fluid infusion device. Accordingly, the process  500  may confirm the initial seating of the fluid reservoir in the reservoir cavity (task  504 ). Task  504  may additionally (or alternatively) confirm when the fluid infusion device has performed a priming operation, which typically occurs after installation of a fluid reservoir. After confirming that the fluid reservoir has been properly seated and/or otherwise properly installed, the fluid infusion device will eventually activate a fluid delivery operation, which in turn actuates the fluid reservoir with the drive motor assembly (task  506 ). As described in more detail above, actuation of the fluid reservoir causes an amount of force to be imparted to the force sensor. Accordingly, the process  500  determines a measure of actuation force imparted to the force sensor during the fluid delivery operation (task  508 ). Task  508  could obtain a single actuation force measurement at any time during the fluid delivery operation, it could calculate an average actuation force based upon any number of actuation force measurements obtained during the fluid delivery operation (e.g., a plurality of actuation forces corresponding to a plurality of consecutive delivery strokes), or it could generate any actuation force value or metric that is based upon one or more individual actuation force measurements obtained during the fluid delivery operation. For simplicity, this particular embodiment of the process  500  assumes that a single actuation force measurement is determined at task  508 . For this example, task  508  determines the measure of actuation force during the fluid delivery stroke itself. Alternatively (or additionally), task  508  could determine the measure of actuation force between fluid delivery strokes, and/or after a final fluid delivery stroke. 
     The process  500  may continue by comparing the actuation force measurement to one or more threshold forces. For this example, the process  500  compares the measure of actuation force to an amount of force (i.e., the force threshold obtained at task  502 ) that is less than the normally expected actuation forces of the fluid infusion device. Accordingly, the process  500  checks whether or not the measure of actuation force (F A ) is less than the force threshold (F T ) at query task  510 . If the measure of actuation force is not less than the force threshold, then the process  500  assumes that the fluid reservoir is still in place and remains properly seated. Accordingly, the process  500  returns to task  506  to continue monitoring actuation force for the current fluid delivery operation (and for subsequent fluid delivery operations). If, however, query task  510  determines that the measure of actuation force is less than the force threshold and, therefore, that the measure of actuation force is indicative of an unseated state, then the fluid infusion device initiates and executes appropriate corrective action (task  512 ). The corrective action taken by the fluid infusion device may include, without limitation, one or more of the actions described above for the process  200  (see  FIG. 5 ). It should be appreciated that the process  500  may consider any number of events (i.e., determinations that the measure of actuation force is less than the force threshold) and analyze a set of events to determine whether or not to initiate the corrective action. In this regard, the process  500  may determine whether a set of events satisfies certain predetermined criteria before taking corrective action, as described above with reference to tasks  412 ,  414 , and  416  of the process  400 . 
     The process  500  is one exemplary embodiment that utilizes a fixed force threshold value to determine whether or not the fluid reservoir is seated. In contrast,  FIG. 9  is a flow chart that illustrates another embodiment of a process  600  that employs an adaptive scheme for checking the seating status of the fluid reservoir. The process  600  can be performed whenever a fluid reservoir is installed into the fluid infusion device. Thus, the process  600  may begin by confirming the initial seating and priming of the fluid reservoir (task  602 ), and activating a fluid delivery operation (task  604 ), as described above for the process  500 . After initial seating of the fluid reservoir, the process obtains a baseline actuation force imparted to the force sensor (task  606 ). The baseline actuation force may correspond to a measure of actuation force that is obtained during the priming operation or shortly thereafter, or it may correspond to a measure of actuation force that is obtained during the first fluid delivery operation for a new fluid reservoir. The process  600  assumes that the baseline actuation force is measured while the fluid reservoir is properly seated. Accordingly, the baseline actuation force can be stored in a memory element of the fluid infusion device for later use. 
     Eventually, the process  600  determines a measured actuation force imparted to the force sensor, where the measured actuation force corresponds to a designated delivery stroke of the drive motor assembly (task  608 ). Task  608  could obtain a single actuation force measurement at any time during the fluid delivery operation, it could calculate an average actuation force based upon any number of actuation force measurements obtained during the fluid delivery operation (e.g., a plurality of actuation forces corresponding to a plurality of consecutive delivery strokes), or it could generate any actuation force value or metric that is based upon one or more individual actuation force measurements obtained during the fluid delivery operation. For simplicity, this particular embodiment of the process  600  assumes that a single actuation force measurement is determined at task  608 . As mentioned previously, the actuation force could be determined during the fluid delivery stroke itself, between fluid delivery strokes, or after a final fluid delivery stroke. 
     The process  600  may continue by comparing the actuation force measurement to a measure of force that is influenced by the baseline actuation force (F B ). For this example, the process  600  checks (query task  610 ) whether the measured actuation force (F A ) is less than the baseline actuation force by some predetermined amount (F T ), which may be a fixed, adaptive, or dynamic threshold, as explained above for the process  500 . In other words, query task  610  determines whether F A &lt;F B −F T . If query task  610  determines that the measure of actuation force is indicative of an unseated or dislodged fluid reservoir, then the fluid infusion device initiates and executes appropriate corrective action (task  612 ), e.g., generates an alert or some indicia of an unseated fluid reservoir. Alternatively or additionally, the corrective action taken by the fluid infusion device may include, without limitation, one or more of the actions described above for the process  200  (see  FIG. 5 ). It should be appreciated that the process  600  may consider any number of events (i.e., individual determinations that F A &lt;F B −F T ) and analyze a set of events to determine whether or not to initiate the corrective action. In this regard, the process  600  may determine whether a set of events satisfies certain predetermined criteria before performing task  612  (see the above description of tasks  412 ,  414 , and  416  of the process  400 ). 
     If the measure of actuation force is indicative of a properly seated fluid reservoir, then the process  600  updates the baseline actuation force as a function of the measured actuation force (task  614 ). For this particular embodiment, task  614  updates the baseline actuation force by saving the measured actuation force for use as the next baseline actuation force. In practice, the fluid infusion device could implement a maximum and/or a minimum allowable baseline actuation force to ensure that the process  600  maintains a realistic baseline value. If for some reason the measured actuation force falls outside of the stated range of baseline force values, the fluid infusion device could generate an alert or take appropriate action. In this regard, a maximum or minimum value could serve as a confirmation or check of the operating health of the force sensor (see the Sensor Health Monitoring section of this description). Referring back to task  614 , after updating the baseline actuation force, the process  600  returns to task  708  to continue monitoring actuation force for the current fluid delivery operation (and for subsequent fluid delivery operations). Thus, the baseline actuation force is adjusted and updated in an ongoing manner while the process  600  monitors the seating status of the fluid reservoir. This adaptive approach enables the process  600  to consider and compensate for slight variations in measured actuation forces. 
       FIG. 10  is a flow chart that illustrates yet another embodiment of a process  700  that checks the seating status of a fluid reservoir. The process  700  uses a threshold force value that corresponds to the maximum expected variation in actuation force for the fluid reservoir. This threshold force value may be empirically determined, calculated based on known operating parameters of the fluid infusion device, or generated and dynamically updated during operation of the fluid infusion device. In certain embodiments, the threshold force value is determined and stored as a fixed value during the operating lifespan of the fluid infusion device. For example, and without limitation, the typical fluid delivery force for a fluid reservoir might be about 1.4 pounds between initial piston seating and depletion of the fluid reservoir, and the threshold force value might be about 0.2 pounds. 
     The process  700  obtains the measures of actuation force imparted to the force sensor for consecutive fluid delivery pulses (in practice, a measure of actuation force is recorded for each fluid delivery pulse). The process  700  calculates a pulse-to-pulse difference between consecutive fluid delivery pulses, where the pulse-to-pulse difference is based on respective measures of actuation force for the consecutive fluid delivery pulses. If the pulse-to-pulse difference is greater than the designated threshold force value, then the fluid infusion device initiates corrective action in some manner. In other words, if the difference in actuation force between the last fluid delivery pulse and the current fluid delivery pulse is more than the normally expected force variation, then the fluid infusion device assumes that the detected condition is indicative of a dislodged or removed fluid reservoir, or a transient state caused by an impact or sudden acceleration to the fluid infusion device. Note that in order to assume a removed or dislodged reservoir, the difference in force from one pulse to the next must be negative, i.e., the measured force has decreased rather than increased. 
     The process  700  may include some tasks that are similar or identical to counterpart tasks found in the process  600 , and such tasks will not be redundantly described here. The process  700  assumes that the fluid reservoir has already been properly seated for fluid delivery. This embodiment of the process  700  begins by initializing and maintaining a count (task  702 ) that is indicative of the seating status of the fluid reservoir. The count may be initialized at any suitable value, although this example assumes that the count is initialized at a value of zero. The process  700  also sets the value of PULSE 1  to “True” (task  702 ). The value of PULSE 1  is True only for the first actuation pulse following installation of a fluid reservoir. For all subsequent delivery pulses, the value of PULSE 1  is False. The fluid infusion device is primed (task  704 ) such that fluid is introduced into the fluid pathway and through the infusion set tubing. During priming, the force imparted to the force sensor typically spikes (to about 1.4 pounds) when the slide initially contacts the plunger of the reservoir. After priming, however, the force decays and settles to a lower value (typically around 0.5 pounds). 
     The process  700  activates a fluid delivery operation (task  706 ) to actuate the fluid reservoir, and performs the next fluid delivery action, stroke, or pulse (task  708 ). As explained in more detail below, tasks  706  and  708  are usually performed after the priming operation is complete and after the force has settled to its nominal value (of about 0.5 pounds). If, however, the user commands a fluid delivery operation prematurely while the force is still decaying, then the force imparted to the force sensor at that time may be above the nominal value. The process  700  contemplates this scenario, as described below. 
     The process  700  continues by determining or obtaining a current measure of actuation force imparted to the force sensor for the current fluid delivery pulse (task  710 ), as described previously. The current measure of actuation force (F) is recorded or saved in an appropriate manner. If PULSE 1  is True (query task  712 ), meaning that the pulse actuated at task  708  is the first pulse for the installed reservoir, then the process  700  continues by setting the value of PULSE 1  to “False” (task  714 ), and by comparing the current measure of actuation force (F) to a reference upper threshold force value that applies to initial pulses (RUT 1 ), as indicated by query task  716 . The value of RUT 1  is selected to account for situations where the initial pulse is commanded prematurely, i.e., before the force has settled to its nominal value after priming. In other words, RUT 1  is selected to contemplate the possibility of unusually high force measurements associated with the first pulse commanded for a newly installed reservoir. For this particular example (where the normally expected nominal actuation force for the fluid reservoir is about 0.4 pounds), the value of RUT 1  may be chosen to be about 0.5 pounds, without limitation. 
     If the current measure of actuation force is less than RUT 1  (query task  716 ), then the process  700  stores the current measure of actuation force (task  718 ) for use as an adaptive reference force value (F R ), which is used for comparison purposes against subsequent force measurements. Execution of task  718  indicates that the actuation force associated with the initial delivery pulse does not represent a force measured shortly after priming, while the fluid in the delivery path is still settling to its nominal state. On the other hand, if the current measure of actuation force (F) is not less than RUT 1  (query task  716 ), then the process  700  stores the value of RUT 1  as the current value of F R  (task  720 ). Execution of task  720  indicates that the actuation force associated with the initial delivery pulse may have been sampled during a time when the fluid in the delivery path is still settling and, therefore, the measured force is still decaying from the relatively high value (e.g., about 1.4 pounds). Consequently, under this scenario the process  700  uses RUT 1  as the current adaptive reference force value. 
     After the value of F R  has been set (task  718  or task  720 ), the process  700  waits for the next fluid delivery pulse (task  722 ). When it is time to perform the next fluid delivery pulse, the process  700  returns to task  708  and continues as described above. In contrast to that described above, however, the value of PULSE 1  is False for the second and all further delivery pulses. Referring again to query task  712 , if PULSE 1  is not True (i.e., PULSE 1 =False), then the process  700  may continue by comparing the current measure of actuation force to the difference between the adaptive reference force value (F R ) and a threshold force value (query task  724 ). For this example, the adaptive reference force value (F R ) may correspond to a past measure of actuation force that was recorded for a previous fluid delivery pulse, or it may correspond to RUT 1 , as described above. In particular embodiments, F R  might represent the measure of actuation force for the immediately preceding fluid delivery pulse (indeed, under normal operating conditions where the fluid reservoir remains properly seated, F R  will be adaptively updated to reflect the most recent measure of actuation force). In other words, the previous fluid delivery pulse associated with F R  and the current fluid delivery pulse will typically be consecutive fluid delivery pulses. Thus, the process  700  stores, maintains, and updates the adaptive reference force value as needed during operation of the fluid delivery device. 
     In practice, the process  700  might generate or store an initial value of F R  whenever a new fluid reservoir is installed, at the beginning of a fluid delivery operation, at designated times, at the request of the user, or at other appropriate times. This example assumes that F R  is initialized as described above in response to the first fluid delivery pulse of a fluid delivery operation. Thus, under typical and normal operating conditions the first measure of actuation force will be used as F R  for the immediately following fluid delivery pulse. If, however, the first measure of actuation force exceeds the initial upper threshold value, then the initial upper threshold value RUT 1  will instead be used as F R  for the next fluid delivery pulse. 
     Referring again to query task  724 , if the current measure of actuation force (F) is not less than the difference between the adaptive reference force value (F R ) and the threshold force value (F T ), then the process  700  assumes that the fluid reservoir remains properly seated. Accordingly, the process  700  resets the count to its initial value, e.g., zero (task  726 ). The illustrated embodiment of the process  700  continues by comparing the current measure of actuation force (F) to an upper threshold force value (RUT), as indicated by query task  728 . This upper threshold value is selected such that it is indicative of the expected maximum actuation force for the fluid reservoir. For this particular example (where the normally expected actuation force for the fluid reservoir is about 1.4 pounds), the upper threshold force value may be chosen to be about 1.8 pounds, without limitation. If the current measure of actuation force is less than the upper threshold force value, the process  700  stores the current measure of actuation force for use as F R  with the next iteration (task  730 ). In this regard, F R  can be adaptively and dynamically updated in an ongoing manner during the fluid delivery operation. After updating F R , the process  700  waits until it is time to perform the next fluid delivery pulse (task  722 ). Referring back to query task  728 , if the current measure of actuation force is not less than the upper threshold force value, then the process  700  leaves F R  unchanged and waits for the next fluid delivery pulse (task  722 ). In other words, the previous value of F R  is retained for the next processing iteration. When it is time to perform the next fluid delivery pulse, the process  700  returns to task  708  and continues as described above. 
     Referring again to query task  724 , if F&lt;F R −F T , then the process  700  initiates some form of corrective action, which is triggered by the detection of an abnormal or unexpected measure of actuation force. In this regard, the process  700  may place the fluid infusion device into a flagged state, perform additional checks, and/or perform additional data analysis to determine whether or not to execute corrective action, issue an alert, sound an alarm, generate a user message, etc. This particular example continues by obtaining one or more additional force readings (task  732 ) and calculating an average measure of actuation force (F AVE ) based on the additional force readings. In practice, F AVE  may be calculated from the current measure of actuation force and any or all of the additional force readings. In certain embodiments, task  732  collects four additional force readings, and F AVE  is calculated as a weighted average of the current measure of actuation force and the four repeated measures of actuation force imparted to the force sensor. 
     Although the process  700  could use the current measure of actuation force itself as a trigger value, the illustrated embodiment instead uses F AVE  as the trigger value. In other words, the process  700  checks whether F AVE &lt;F R −F T  (query task  734 ). If F AVE  is not less than the difference between the adaptive reference force value (F R ) and the threshold force value (F T ), then the process  700  assumes that the fluid reservoir remains properly seated, resets the count to its initial value (task  726 ), and continues from task  726  as described above. If, however, F AVE &lt;F R −F T , then the process  700  changes the count by a designated amount to obtain an updated count (task  736 ). Depending upon the embodiment and the initial count value, task  736  may increase or decrease the count. In certain embodiments, task  736  changes the count by an amount that is influenced or dictated by a volume of fluid to be delivered by a subsequent fluid delivery pulse, e.g., the next fluid delivery pulse. Alternatively, task  736  might change the count by an amount that is influenced or dictated by a volume of fluid delivered by the current fluid delivery pulse, or by a previous fluid delivery pulse. In accordance with one non-limiting example, task  736  increases the count by one when the next fluid delivery pulse corresponds to a relatively low volume of fluid (e.g., 0.025 units), increases the count by two when the next fluid delivery pulse corresponds to a relatively intermediate volume of fluid (e.g., 0.050 units), and increases the count by six when the next fluid delivery pulse corresponds to a relatively high volume of fluid (e.g., 0.200 units). 
     The above methodology for task  736  accounts for “slack” that is created in the drive system when the fluid infusion device is dropped or when the reservoir is dislodged. For example, assume that the slack represents a separation between the plunger of the reservoir and the tip of the actuating slide, and assume that it takes six pulses (each corresponding to a delivery of 0.025 Units), equivalent to 0.15 Units, to remove the slack. Therefore, the counter limit or threshold will be set to six. If after six “counts” the slack is not removed, i.e., the force is still low, the process  700  will trigger an alarm. If the device delivers fluid in 0.025 Unit pulses, it will take six pulses and, therefore, the count is incremented by one. On the other hand, if the fluid delivery is in 0.05 Unit increments, then the counter is incremented by two; if the fluid delivery is in 0.2 Unit pulses, the counter increments by six. Accordingly, the limit or threshold is met after six 0.025 Unit pulses, after three 0.05 Unit pulses, or after only one 0.2 Unit pulse. This allows the fluid infusion device to deliver various pulses and increment correctly. 
     After obtaining the updated count, the process  700  checks whether the updated count satisfies certain predetermined alert criteria (query task  738 ). The alert criteria for the illustrated embodiment is simply a threshold count value or a limit, such as twelve or any appropriate number. Thus, if the updated count is greater than the limit, then the process  700  assumes that the fluid reservoir has been dislodged, loosened, or unseated. Consequently, the process  700  initiates and executes appropriate corrective action (task  740 ), e.g., generates a seating status alert or some indicia of an unseated fluid reservoir. Alternatively or additionally, the corrective action taken by the fluid infusion device may include, without limitation, one or more of the actions described above for the process  200  (see  FIG. 5 ). If, however, query task  738  determines that the updated count does not satisfy the stated alert criteria, then the process  700  does not actually implement or execute any corrective action at this time. Rather, the process  700  may lead back to task  722  to wait for the next fluid delivery pulse, as described above. Notably, corrective action is executed by the illustrated embodiment of the process  700  only when the count exceeds the threshold limit. Moreover, the manner in which the count is reset by the process  700  ensures that unexpectedly low actuation force readings must be recorded for consecutive fluid delivery pulses before the fluid delivery device actually issues a warning or an alert. These aspects of the process  700  reduce nuisance alerts and allow the fluid infusion device to recover from transient conditions that might cause a temporary drop in the measured actuation force (e.g., dropping or bumping the fluid infusion device, mishandling the fluid infusion device, or the like). In certain embodiments, if at any point when the force is low and the count is being updated the force increases (i.e., the slide contacts the plunger of the reservoir), the counter is again reset to zero. 
       FIG. 11  is a graph that illustrates measures of actuation forces for a properly seated fluid reservoir. The plot  800  represents actuation force versus time (or, equivalently, fluid delivery pulses). The initial portion  802  of the plot  800  corresponds to the seating of the fluid reservoir. In practice, the device stops the seating process when a force of about 1.4 pounds is reached. Momentum of the motor results in some additional actuation, resulting in a peak of about 1.8 pounds. After seating and priming, however, the nominal force typically settles to about 0.5 pounds. Accordingly, the label P 1  represents the first fluid delivery pulse following the seating/priming procedure. The dashed line  804  schematically represents the threshold force value corresponding to the allowable drop in actuation force over any two consecutive fluid delivery pulses. Notably, even though the plot  800  fluctuates somewhat, the actuation force values do not violate the limit defined by the threshold force value. In other words, the plot  800  is indicative of a properly seated fluid reservoir under normal and expected operating conditions. 
     In contrast,  FIG. 12  is a graph that illustrates measures of actuation forces for a fluid reservoir that becomes unseated. The plot  850  begins at a point after initial seating of the fluid reservoir. The initial segment  852  of the plot  850  is indicative of a properly seated fluid reservoir (assuming that the threshold force value is 0.15 pounds). The plot  850  experiences a temporary drop  854  that spans two fluid delivery pulses. For this example, the temporary drop  854  would trigger the counting mechanism described above with reference to the process  700 . However, the plot  850  recovers after the temporary drop  854  and, therefore, the count would be reset and no alert would be generated. At the fluid delivery pulse  856 , the plot  850  exhibits a significant and “permanent” drop. At this point, the counting mechanism would be activated. Notably, the drop in measured actuation force does not recover even after ten consecutive fluid delivery pulses. Consequently, the count continues to increase and, for this example, the upper count limit is eventually reached. At that time, the fluid infusion device would generate an appropriate alert, alarm, or warning message, as described previously. 
     Adaptive Occlusion Detection 
     The force sensor  126  and/or other sensors, measurement devices, or components in the fluid infusion device  100  may also be used for purposes of occlusion detection. Most conventional occlusion detection schemes function by triggering an occlusion alarm when a certain preset threshold force is reached. For example, if the typical actuation force for a fluid reservoir is about one pound and the occlusion threshold is three pounds, a detected force that exceeds three pounds will initiate an alert or an alarm. In fluid infusion devices that are occluded, the force might increase by only fractions of a pound per unit of fluid (e.g., insulin) desired to be delivered. As a result, there can potentially be long wait times before an occlusion is actually confirmed. 
     In contrast, the technique described here is adaptive in nature, and occlusions can be determined prior to reaching the preset threshold by evaluating consecutive rates of change (slopes) of force. For example, assume that the typical force variation in a reservoir is only about 0.02 pound per unit (lb/U) over three or four delivery strokes or pulses. If an occlusion is present, the detected force might increase at a calculated rate of 0.30 lb/U over four consecutive pulses. If the fluid infusion device detects an occlusion, then an alarm might be generated and/or the occlusion force threshold might be adjusted downward by a certain amount. For example, if the normal occlusion threshold is three pounds, a detected occlusion condition might result in a downward adjustment of the occlusion threshold by thirty percent, resulting in an adjusted occlusion threshold of about two pounds. The adaptive approach enables the fluid infusion device to quickly detect an occlusion, relative to traditional methods that only use a static force threshold. 
       FIG. 13  is a flow chart that illustrates an embodiment of an occlusion detection process  900  for a fluid infusion device, such as the fluid infusion device  100  described above. This embodiment of the process  900  employs an occlusion force threshold, which is consistent with traditional methodologies. Thus, the process  900  may begin by obtaining, calculating, or retrieving the occlusion force threshold (task  902 ). In typical implementations this occlusion force threshold is about 2.4 pounds, although the actual amount may vary from one embodiment to another. 
     In practice, the process  900  can be performed whenever a fluid reservoir is installed into the fluid infusion device. Accordingly, the process  900  may confirm the initial seating and/or priming of the fluid reservoir (task  904 ), and then activate a fluid delivery operation, which in turn actuates the fluid reservoir with the drive motor assembly (task  906 ). As described in more detail above, actuation of the fluid reservoir causes an amount of force to be imparted to the force sensor. Accordingly, the process  900  determines and saves actuation forces for a designated number of delivery strokes (task  910 ). This particular embodiment determines and saves actuation forces for a plurality of consecutive delivery strokes and then determines an average actuation force for the plurality of delivery strokes (task  912 ). 
     The process  900  may continue by comparing the average actuation force (F AVE ) to the occlusion force threshold (F T ) obtained at task  902 . If the average actuation force is greater than the occlusion force threshold (query task  914 ), then the fluid infusion device initiates and executes appropriate corrective action (task  916 ). The corrective action taken by the fluid infusion device may include, without limitation, one or more of the actions described above for the process  200  (see  FIG. 5 ). If, however, query task  914  determines that the average actuation force is not greater than the occlusion force threshold, then the process  900  continues. 
     The process may continue by analyzing the actuation force and the amount of fluid (typically expressed as a number or fraction of units) that is desired to be administered for delivery (task  918 ). As mentioned above, the process  900  analyzes the rate of change of a metric corresponding to the amount of detected force per unit of fluid (as commanded by the fluid infusion device). In this regard, the process  900  may compute this metric (e.g., in lb/U) in an ongoing manner during the fluid delivery operation. Moreover, the process  900  calculates the rate of change of this metric in an ongoing manner during the fluid delivery operation. Consequently, if the fluid infusion device determines that the rate of change exceeds a predetermined maximum value (query task  920 ), then the process  900  leads to task  916  and initiates appropriate corrective action. If, however, query task  920  determines that the rate of change does not exceed the maximum value, then the process  900  assumes that the fluid delivery path is not occluded. Accordingly, the process  900  returns to task  910  to continue monitoring actuation forces for the current fluid delivery operation (and for subsequent fluid delivery operations). The maximum tolerable rate of change may be a fixed value or it may be adaptive in nature. In typical embodiments, the maximum rate of change value will be about 1.0 lb/unit, although different values could be used depending upon the embodiment. 
     It should be appreciated that the process  900  may be practiced in conjunction with conventional occlusion detection schemes if so desired. For example, one or more of the occlusion detection approaches described in U.S. Pat. Nos. 6,485,465 and 7,621,893 (or modified versions thereof) could be employed with the process  900 . 
     Multi-Metric Occlusion Detection 
     The force sensor  126  in the fluid infusion device  100  may also be used to support a multi-metric occlusion detection scheme. The multi-metric occlusion detection approach may be used with or without a traditional reservoir force threshold approach. One benefit of the multi-metric technique is that it accommodates drifting or other changes of the force sensor, which might occur over the lifespan of the fluid infusion device  100 . In practice, therefore, the multi-metric technique can tolerate drifting of the force sensor without generating false alarms, while still accurately detecting the presence of an occlusion. 
     The exemplary approach presented here employs a rate of change methodology incorporating two different conditions to take advantage of the rate of change in force with respect to fluid delivery. If either of the two conditions is satisfied, the fluid infusion device  100  indicates an occlusion and takes appropriate action. The rate of change approach is desirable because it does not depend solely on the system to settle to a force level. Moreover, the approach described here does not rely on the starting frictional force of a reservoir. In practice, different reservoirs might have different nominal actuation forces; for example, reservoir A may have running force of 1.0 pound, and reservoir B may have a running force of 1.8 pounds. Based on a simple threshold force alone, it will take more time for reservoir A to reach occlusion than reservoir B. Using the rate of change approach presented here, as long as the threshold rate of change is reached, occlusion will be detected independent of starting frictional force value. 
     The first monitoring mode uses one rate of change of reservoir force as a triggering parameter, where the rate of change is determined for a relatively large delivery window. For example, assume that the measured rate of change in force over fluid delivery in a three-unit (3 U) window is not to exceed 0.375 lb/unit. If, over any three-unit window, the rate of change exceeds 0.375 lb/unit, the fluid infusion device  100  alarms. It is desirable to have a predetermined measurement window (3 U in this case) because during normal delivery there might be some sudden changes in force. In this regard, a reservoir might experience a running force of 1.0 pound and suddenly increase to 1.3 pounds over a delivery window of 0.5 units, and continue to deliver at 1.3 pounds. This scenario would not be indicative of an occlusion; rather, this would be indicative of a normal variation in force. If the measurement window of three units is not utilized, this scenario could have led to a false occlusion alarm. 
     In practice, the measurement window and the rate of change threshold should be large enough to avoid false positives. However, most of the time these parameters can be more restrictive. Accordingly, the exemplary embodiment described here utilizes the second monitoring mode, which employs a different rate of change threshold and a smaller measurement window, along with a threshold force value that serves as an “early” indication of a possible occlusion. For the exemplary embodiment described here, if a reservoir is known to be operating at actuation forces beyond its normal range (for example above 1.4 pounds), then the second condition will be considered. For example, if the measured actuation force is 1.8 pounds, the fluid infusion device  100  recognizes that the nominal operating load has been exceeded, but the fluid infusion device  100  does not determine the presence of an occlusion based solely on this high force reading (high forces can be caused by an overly sensitive force sensor, among other reasons). If, however, the rate of change of the force also exceeds a predetermined threshold value, then the fluid infusion device  100  alarms. In this scenario, the rate of change and the measurement window can be selected for higher detection sensitivity because the fluid infusion device  100  is already operating at a higher than normal reservoir actuation force. The second condition is based on the observation that a reservoir that is over the force limit and also experiencing an increasing rate of change in force is most likely to be occluded. 
     The multi-rate approach presented here allows the fluid infusion device  100  to tolerate actuation forces that are slightly above the nominal reservoir force range, yet not generate an alarm unless one of the conditions (with different measurement windows and different rates of change of force) is met. This approach allows the fluid infusion device  100  to experience a slightly higher force due to mechanical issues, normal lifespan variation, and/or an oversensitive force sensor, while continuing to function within safe and predictable limits. 
       FIG. 14  is a flow chart that illustrates an exemplary embodiment of a multi-mode occlusion detection process  1000  for an infusion device, such as the fluid infusion device  100 . The process  1000  may be performed in connection with any fluid delivery operation or action, such as the delivery of therapy, the priming of the fluid path, the initializing of a new fluid reservoir, or the like. Accordingly, the process  1000  may begin by initiating or activating a fluid delivery action (task  1002 ) to deliver a designated or commanded amount of fluid from the fluid reservoir. The fluid delivery action actuates the fluid reservoir with the drive motor assembly, as described previously. The fluid delivery action may be stepwise or continuous, depending upon the particular embodiment. This example assumes that the fluid reservoir is actuated in a stepwise or pulsed manner, with a series of discrete actuation strokes that collectively result in the desired amount of fluid delivered from the reservoir. 
     During the fluid delivery action the process  1000  determines, obtains, or samples one or more measures of actuation force, using the force sensor (task  1003 ). Each measure of force corresponds to an output level recorded from the force sensor. In practice, therefore, a measure of force (also referred to here as a “force measurement”) may be a voltage, a current, a capacitance, a resistance, a digital value, an analog value, or any detectable characteristic of the force sensor that somehow indicates the actuation of pressure in the fluid reservoir. The exemplary embodiment determines a plurality of force measurements during the commanded fluid delivery action, and each force measurement also has an associated quantity metric or measurement (e.g., a delivered volume of fluid relative to a reference volume or other reference point). Alternatively (or additionally), each force measurement might be associated with a corresponding time, delivery stroke, or other measurable parameter that can be used to calculate a rate of change of the actuation force. 
     The process  1000  detects an occlusion in one of two different modes associated with different measurement windows. As used here, a “measurement window” may refer to one or more of the following, without limitation: a period of time; a delivery time relative to a reference time; a measure of quantity (e.g., a delivered volume, mass, or weight); an amount of travel or distance associated with actuation of the piston of the fluid reservoir; a number, a count, or other parameter associated with turns or rotation of the drive motor assembly; a measured amount of fluid flow; or the like. For the exemplary embodiment described here, the measurement windows represent a volume of fluid dispensed from the fluid reservoir. Moreover, each measurement window is identified relative to a reference volume measurement. 
       FIG. 15  is a graph that depicts an exemplary plot  1050  of fluid reservoir actuation force versus volume of fluid (units) delivered from a fluid infusion device.  FIG. 15  illustrates the concept of measurement windows. For example, a measurement window  1052  is defined such that it corresponds to a delivered volume of 3.0 units. This measurement window  1052  may be defined by the current state of the fluid infusion device and a previous state of the fluid reservoir. For this example, the current state of the fluid infusion device corresponds to the point where 5.0 units have been delivered, and the previous state corresponds to the point where 2.0 units have been delivered, i.e., the state when the fluid reservoir had 3.0 more units of fluid. Each measurement window may correspond to a moving window that tracks the real-time status of the fluid infusion device and the real-time status of the fluid reservoir. Thus, as the fluid reservoir progresses from its “full” state to its “empty” state, a measurement window can move to contemplate any two moving endpoints that define the predetermined volume (for the exemplary embodiment, one of the two endpoints corresponds to the current state, the current force measurement, and the current volume measurement). 
     The exemplary embodiment of the process  1000  employs two different measurement windows, e.g., a large window and a small window. For ease of understanding,  FIG. 15  depicts two measurement windows relative to a point at which 15.0 units have been delivered from the fluid reservoir. The large (3.0 units) window  1054  and the small (1.0 unit) window  1056  are both measured relative to the current measurement point of 15.0 units. Referring again to  FIG. 14 , if the rate of change of the measure of force (based on the large window) is greater than or equal to a first threshold value (query task  1004 ), then the process  1000  indicates an occlusion (task  1006 ) and initiates and executes appropriate corrective action (task  1008 ). The corrective action taken by the fluid infusion device may include, without limitation, one or more of the actions described above for the process  200  (see  FIG. 5 ). If the rate of change is less than the first threshold value (the “No” branch of query task  1004 ), then the process  1000  may proceed to a query task  1010 , which compares the rate of change of the measure of force (based on the small window) to a second threshold value. More specifically, the process  1000  indicates an occlusion (task  1006 ) if: (a) the rate of change of the measure of force (based on the small window) is greater than or equal to the second threshold value; and (b) the current force measurement is greater than or equal to a threshold force value. For this particular embodiment, the first rate of change threshold value is greater than the second rate of change threshold value. 
     If the first rate of change is less than the first threshold value and the second rate of change is less than the second threshold value (the “No” branch of query task  1010 ), then the process  1000  may check to determine whether more delivery strokes or pulses are needed for the current fluid delivery operation (query task  1012 ). If more delivery stokes are required, then the process  1000  waits for and executes the next fluid delivery pulse and returns to task  1003  to continue as described above. If there are no additional delivery strokes required, then the process  1000  ends. 
     The above description of the process  1000  assumes that the small and large windows are fixed in size. In certain embodiments, however, the small and/or large window size could vary as a function of the current force measurement. For example, as the current measured force increases, the small window size may decrease. Similarly, the first threshold slope value and/or the threshold slope value could vary as a function of the current force measurement if so desired. 
     The process  1000  represents a simplified and generalized version of a multi-mode occlusion detection technique. In practice, a fluid infusion device could carry out an occlusion detection process having more detailed steps and specific operating parameters. In this regard,  FIG. 16  is a flow chart that illustrates another exemplary embodiment of a multi-mode occlusion detection process  1100 . The following description utilizes certain parameters and settings for an exemplary implementation. These parameters and settings are not intended to limit or otherwise restrict the scope or application of the described subject matter in any way. The parameters, settings, and variables used in the following description of the process  1100  are:
         F 0 =initial force measurement   F TH   _   0 =4.0 pounds=initial force limit or threshold   F TH   _   D =4.0 pounds=gross delivery force limit or threshold   i=index variable   F(i)=current force measurement   F TH =1.8 pounds=threshold force value   U(i)=delivered volume measurement, relative to a reference volume   W SM =1.5 unit=small volume window   W LG =3.0 units=large volume window   S SM =calculated slope for the small window   S LG =calculated slope for the large window   S TH   _   SM =0.220 lb/U=threshold slope value for the small window   S TH   _   LG =0.380 lb/U=threshold slope value for the large window       

     The process  1100  may be performed in connection with any fluid delivery operation or action and, as such, the process  1100  may begin by initiating a fluid delivery action (task  1102 ) to deliver a designated or commanded amount of fluid from the fluid reservoir. At this point, fluid need not be actually delivered. Rather, the process  1100  is initiated when the system intends to deliver fluid to check whether or not an occlusion is present before actually delivering fluid. The process  1100  determines, measures, or obtains a plurality of force measurements during the fluid delivery operation of the drive motor assembly, including an initial force measurement (F 0 ) and at least one subsequent force measurement. The illustrated embodiment performs an initial gross occlusion check before or at the beginning of each fluid delivery action. To this end, the process  1100  determines the initial force measurement (task  1104 ), which indicates an initial measure of actuation force imparted to the force sensor, and compares the initial force measurement (F 0 ) to the initial force limit (F TH   _   0 ) (query task  1106 ). If F 0 &gt;F TH   _   0 , the fluid infusion device indicates an occlusion and initiates and executes appropriate corrective action (task  1108 ), as described previously. For this example, the value of F TH   _   0  (4.0 pounds) is intentionally chosen to be much higher than any normal operating actuation force and to be greater than the force threshold (F TH ) that is used during fluid delivery. The process  1100  assumes that any measured force above F TH   _   0  is by definition caused by an occlusion and, therefore, the “Yes” branch of query task  1106  leads directly to the task  1108  for immediate corrective action. 
     If F 0  is less than or equal to the initial force limit, then the process  1100  determines the current force measurement and saves the value as F(i) (task  1110 ). The value of F(i) may be calculated in any desired manner. This particular example determines F(i) by averaging a number of force measurements that are sampled while the drive motor assembly is stationary, e.g., immediately before or immediately after a delivery pulse. More specifically, the process  1100  may acquire ten force readings, discard the maximum and minimum readings, and compute the average of the remaining eight readings to arrive at F(i). 
     The process  1100  also determines a plurality of quantity measurements during the fluid delivery action, where each quantity measurement is determined relative to a reference quantity measurement, and where each quantity measurement corresponds to a respective one of the force measurements. For this embodiment, each quantity measurement represents or indicates a volume of fluid dispensed from the fluid reservoir, relative to a reference volume, a reference time, or any appropriate reference point or marker. In practice, the reference volume may be defined to be any initial value (such as zero units) when a fluid reservoir is installed in the fluid infusion device, after completion of a priming function for a newly installed fluid reservoir, immediately following the completion of a fluid delivery operation, or the like. Thus, if the current fluid delivery operation is intended to deliver 0.5 units and the reference volume is 0.0 units, then the recorded volume measurements should range between 0.0 units and 0.5 units. Referring again to  FIG. 16 , the process  1100  determines the current delivered volume measurement and saves the value as U(i) (task  1112 ). Accordingly, each saved force measurement will have a corresponding saved volume measurement with the same index (i) value. 
     The process  1100  analyzes the force and volume measurements over two different measurement windows, as described previously. Accordingly, the process  1100  must collect enough measurement points before determining whether an occlusion has occurred. In this regard, the process  1100  may compare the current delivered volume measurement U(i) to the predetermined small and large volume windows (task  1114 ). If U(i)&lt;W SM  (which is 1.5 unit for this example), then the process  1100  may skip to a query task  1124 . The process  1100  skips to query task  1124  at this time because additional measurement points are needed before the intervening tasks can be properly executed. 
     If U(i)≧W LG  (which is 3.0 units for this example), then the process  1100  assumes that enough measurement points have been collected, and continues by calculating a slope (rate of change) for a large sample of volume measurements (task  1116 ). In other words, the process calculates a slope (S LG ) that is based on the large volume window. The value of S LG  may be calculated using any suitable formula, algorithm, technique, or methodology. This particular example calculates S LG  based on the two “endpoints” of the large volume window. More specifically, S LG  is calculated based upon the current force measurement F(i), the current volume measurement U(i), a previous force measurement taken 3.0 units “in the past”, and a previous volume measurement taken 3.0 units “in the past”. Referring again to the scenario depicted in  FIG. 15 , the slope for the large window  1054  would be computed based on the forces measured at 15.0 units and at 12.0 units. The simple linear slope calculation employed by the process  1100  disregards measurement points taken between the two endpoints. Alternate embodiments may, of course, consider any number of intervening measurement points if so desired. 
     The process  1100  may continue by comparing the calculated value of S LG  to the stated value of S TH   _   LG  (which is 0.380 lb/U for this example). If S LG ≧0.380 lb/U (the “Yes” branch of query task  1118 ), then the process  1100  indicates an occlusion and initiates and executes appropriate corrective action (task  1108 ). In practice, this condition is indicative of a gradually increasing slope over a relatively large measurement window, even though a threshold actuation force has not been reached. Accordingly, the fluid infusion device can still generate an appropriate alarm without relying on a threshold trigger. If, however, S LG &lt;0.380 lb/U (the “No” branch of query task  1118 ), then the process  1100  may continue by calculating a slope (S SM ) that is based on the small volume window (task  1120 ). It should be appreciated that task  1120  need not be performed in response to the determination made during query task  1118 . Indeed, task  1116  and task  1120  could be performed in parallel and in an ongoing and dynamic manner during the fluid delivery operation regardless of the results of the various query tasks included in the process  1100 . 
     Referring again to task  1120 , the value of S SM  may be calculated using any suitable formula, algorithm, technique, or methodology. This particular example calculates S SM  based on the two “endpoints” of the small volume window. More specifically, S SM  is calculated based upon F(i), U(i), an intervening force measurement taken 1.5 unit “in the past”, and an intervening volume measurement taken 1.5 unit “in the past”. The intervening force measurement and the intervening volume measurement are “intervening” in the sense that they correspond to a state of the fluid infusion device that occurred at a time between the current state and the “previous” state, where the “previous” force measurement and the “previous” volume measurement are associated with the endpoint of the large measurement window as described above. Referring again to the scenario depicted in  FIG. 15 , the slope for the small window  1056  would be computed based on the forces measured at 15.0 units and at 13.5 units. 
     The process  1100  may continue by comparing the calculated value of S SM  to the stated value of S TH   _   SM  (which is 0.220 lb/U for this example). For this example, the process  1100  indicates an occlusion when both: (a) S SM ≧0.220 lb/U; and (b) F(i)≧F TH  (the “Yes” branch of query task  1122 ). Indication of an occlusion at this time also causes the process  1100  to initiate and execute appropriate corrective action (task  1108 ). In practice, this condition is indicative of an increasing slope over a relatively small measurement window, combined with the actuation force exceeding a stated threshold level (1.8 pounds for this example). Accordingly, the fluid infusion device can utilize a sensitive slope criteria if the actuation force is already above a certain threshold value. Moreover, the different sized measurement windows (small volume versus large volume, short delivery time versus long delivery time, shorter piston travel versus longer piston travel, etc.) facilitate accurate occlusion detection while reducing the likelihood of false alarms. 
     If S SM &lt;0.220 lb/U (the “No” branch of query task  1122 ), then the process  1100  may continue by checking whether more delivery strokes or pulses are needed for the current fluid delivery operation (query task  1124 ). If there are no additional delivery strokes required, then the process  1100  ends. If more delivery stokes are required, then the process  1100  may begin or continue the programmed delivery stroke (task  1126 ) to actuate the fluid reservoir by the desired amount. In connection with the next delivery stroke or pulse, the process  1100  may perform another gross occlusion check (query task  1128 ). To this end, the process  1100  compares the current force measurement to the gross delivery force limit (F TH   _   D ), which is 4.0 pounds for this example. If F&gt;F TH   _   D , the fluid infusion device indicates an occlusion and initiates and executes appropriate corrective action (task  1108 ), as described previously. Otherwise, the process  1100  increments the index (i) at task  1130 , and continues in the manner described above, beginning at task  1110 . Thus, the process  1100  continues to monitor for an occlusion using both of the slope conditions, while updating and moving the two measurement windows dynamically as the fluid reservoir delivers additional fluid. The small and large volume windows employ the current measurement point as one endpoint, and previous measurement points as the other endpoints (e.g., 1.5 unit in the past and 3.0 units in the past). These measurement windows dynamically shift as the fluid reservoir is actuated such that the fluid infusion device samples and processes the measurement points that fall within the boundaries of the windows. Referring to  FIG. 15 , the measurement window would shift to the right as the fluid reservoir is actuated. 
     The occlusion detection techniques described above assume that actuation force is the metric or measurement quantity used to determine whether or not an occlusion in the fluid path has occurred. In such embodiments, a force sensor is utilized to obtain the measurements. In alternate embodiments, the fluid infusion device may obtain and process measurements of any quantity (or quantities) that is indicative of pressure in the fluid delivery path for purposes of occlusion detection. In this regard, actuation force associated with the fluid reservoir is an indirect way of measuring fluid pressure, where increasing fluid pressure is indicative of a potential occlusion. 
     Depending upon the particular implementation, the measured or detected quantity might be associated with one or more of the following, without limitation: reservoir actuation force (as described in detail above); fluid pressure in the fluid path of the device; a flow rate of fluid in the fluid path, based on time, number of delivery strokes or pulses, etc.; electric current of the drive motor, which relates to the load on the drive motor, which in turn relates to the fluid pressure in the reservoir; torque of the drive motor, which relates to the load on the drive motor, which in turn relates to the fluid pressure in the reservoir; or the like. In practice, therefore, the fluid infusion device may obtain measurement information or data from one or more sources in lieu of or in addition to a force sensor. For example, the occlusion detection schemes presented above may process measurements obtained from one or more of the following, without limitation: a pressure sensor; a flow meter; a torque meter; an electrical circuit that measures motor current; or any appropriate sensor, detector, or circuit that is dynamically responsive to the fluid pressure in the fluid path or reservoir of the fluid infusion device. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.