Patent Publication Number: US-6656148-B2

Title: Infusion pump with a sealed drive mechanism and improved method of occlusion detection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of copending U.S. application Ser. No. 09/335,999, filed Jun. 18, 1999 now U.S. Pat. No. 6,423,035 entitled: “Infusion Pump With A Sealed Drive Mechanism And Improved Method Of Occlusion Detection.” 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an infusion pump for controlled delivery of a pharmaceutical product to a subject, and more specifically to an infusion pump having a sealed drive mechanism and improved method of occlusion detection for determining the presence of obstructions in the infusion path. 
     Infusion pumps provide a significant lifestyle benefit for individuals requiring multiple deliveries of volumetrically proportioned medication to their body over a period of time. Infusion pumps reliably dispense the required medication to the patient through an infusion path established between the patient and the pump. The infusion path is a conduit secured to the pump at one end and secured intravenously or subcutaneously to a patient on the other. The operation of the infusion pump is controlled by a processor. The processor controls the delivery of periodic dosages of medication to a patient at predetermined times. Thus, a patient is able to rely on the infusion pump for delivering the required dosage of medication intravenously or subcutaneously over a period of time. In this way, the patient need not interrupt life activities for repeated manual delivery of required medication. 
     As is known, infusion pumps often employ a piston-type drive mechanism for urging the contents of a pharmaceutical cartridge or “syringe” internal to the pump along the infusion path to the subject. Piston-type infusion pumps are susceptible to an occlusion in the infusion path. Additionally, piston-type infusion pumps include complicated drive assemblies which require periodic maintenance and/or user adjustment which further degrades the reliability of the device. 
     Most piston-type infusion pumps have an exposed lead-screw drive assembly that is manipulated by the user to reset the device each time a new syringe is inserted in the device. Because the lead screw is a precision mechanical assembly that drives a plunger through the syringe to infuse pharmaceutical product along an infusion path, dirt and debris in the exposed lead screw can cause the screw thread to either wear-down or lock-up at its point of engagement with a mated drive, either of which can cause a pump failure. Some manufacturers suggest periodic cleaning of the lead screw, while the other manufacturers have equipped their devices with disposable lead screws and nut assemblies to prevent such malfunctions. Installation of these parts in some pumps requires partial disassembly of the device, further complicating syringe installation. Furthermore, many piston type infusion pumps are used with syringe plungers manufactured with “O”-rings. The installation of the syringe will often break the plunger seal about the O-ring and cause medication to leak through the plunger into the pump, possibly damaging electrical components, but also causing medication not being delivered properly to the patients through infusion set electronics. Moreover, there is a need for an infusion pump with sealed and inaccessible electronics so the pump does not become damaged due to accidental or deliberate submersion in water, and a sealed drive mechanism to prevent damage to the lead screw. 
     Often piston-type infusion pumps also do not show the amount of medication remaining in a syringe. Some manufacturers use a transparent window to visually inspect whether a syringe requires replacement. If a patient is not diligent about making such visual checks, he runs the risk of running out of medication. Other pumps indirectly determine the amount of remaining medication and, therefore, are subject to inaccuracies. Thus, there is a need for an infusion pump that directly reports the amount of remaining medication. 
     While some infusion pumps are designed to subtract delivery volumes from a fixed full or a fixed half syringe volume, the amount of medication in the syringe must be manually entered into the device at the outset by the patient upon installation of the syringe, although it may actually be neither full nor half full initially. This requirement is still a further complication of the syringe installation process. 
     Regarding occlusion detection, when an occlusion occurs anywhere along the infusion path of a piston-type pump, medication is not delivered to the patient even though the piston moves to deliver the medication. As can be appreciated, the existence of an occlusion will prevent the infusion pump from delivering medication to a patient until the occlusion is detected and cleared from the infusion path. Thus, the rapid detection of occlusions along the infusion path is key to reliable operation of a pump. 
     Presently, a piston-type infusion pump is desired which provides an improved method of occlusion detection, the pump including a simplified and reliable piston-type drive mechanism. 
     The present invention is directed to a piston-type infusion pump which includes an enclosed lead screw which can not be accessed without disassembling the pump. Thus, the engagement and disengagement of drive mechanism are achieved remotely, by latching and unlatching of the pump door, minimizing likely user error or abuse. The pharmaceutical syringe has a U-shaped plunger designed to link with the drive mechanism for simple installation. Additionally, the pump displays exact amount of medication (i.e., insulin) remaining in the cartridge at any time and utilizes an improved method of occlusion detection. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly stated, the present invention provides a piston-type infusion pump having a remotely engaged piston-type drive mechanism and improved method of occlusion detection. The internal components of the pump are sealed from the outside when a pharmaceutical syringe is installed, thus creating a watertight seal when the pump is in its operational mode. 
     The infusion pump is designed to remotely engage and disengage the lead screw of a drive mechanism by way of a latch stem, which is a part of a pump door latching mechanism. The pump door latch has a watertight rotary seal between the casing of the infusion pump and the latch stem. When the pump door latch is moved up to allow the pump door to open, it disengages the drive, so that the plunger of the syringe is free to move. When the pump door latch is pushed down to lock the pump door of the infusion pump, it engages the drive. When in the locked position the plunger is moved only through rotation of the lead screw. Thus, the engagement and disengagement of drive mechanism are achieved remotely, by latching and unlatching of the pump door, minimizing likely user error or abuse. 
     The infusion pump includes processing circuitry for controlling the drive mechanism to infuse medication to a patient, including a sensor to track the position of the syringe plunger. The sensor provides information that determines the volume of remaining insulin at any time in the pump. The infusion pump processing circuitry also includes a force sensor and circuitry for uniquely processing signals indicative of the presence of an occlusion along the infusion path. The occlusion detector operates with good accuracy at low volumes and delivery rates. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
     In the drawings: 
     FIG. 1 is top view of the infusion pump with the top wall of the casing removed to show the layout of the components in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a top view of the infusion pump shown in FIG. 1 with the pump door open; 
     FIG. 3 is a front view of the pump door and latch assembly of the infusion pump shown in FIG. 1 with the latch in the open position and the pump door open for loading a syringe; 
     FIG. 4 is a front view of the infusion pump with the latch in the closed position and the pump door closed with the pump housing a syringe; 
     FIG. 5 is a side view of the infusion pump with the pump door open; 
     FIG. 6 is a side view of the latch stem assembly with the latch in the closed position and the pump door closed, with watertight seals in accordance with a preferred embodiment of the invention; 
     FIG. 7 is a front view of the latch stem assembly with the latch in the open position and the pump door open; 
     FIG. 8 is a front view of the lead screw and slide assembly in the engaged position in accordance with the preferred embodiment of FIG. 6; 
     FIG. 9 is a front view of the lead screw and slide assembly in the disengaged position in accordance with the preferred embodiment of FIG. 6; 
     FIG. 10 is a top view of the lead screw and slide assembly engaging a syringe in the infusion pump in accordance with the preferred embodiment of FIG. 6; 
     FIG. 11 is a side perspective view of the syringe and plunger assembly; 
     FIGS. 12 a  and  12   b  are block diagrams of the circuitry of the infusion pump in accordance with the preferred embodiment of FIG. 10; 
     FIG. 13 is a flow chart of a preferred system failure detection method employed by the circuitry of FIGS. 12 a  and  12   b;    
     FIG. 14 is a graph of force v. time showing the sampling of force data prior to the initiation of a delivery cycle; 
     FIGS. 15-19 are flow charts showing individual system failure detection methods in accordance with the preferred method of FIG.  13 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain terminology is used in the following description for convenience only and is not limiting. The words “right,” “left,” “lower” and “upper” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the infusion pump and designated parts thereof. The terminology includes the words specifically mentioned above, derivatives thereof and words of similar import. 
     The term “Bolus” as used herein refers to a dosage of medication which is large with respect to typical dosage levels. For example, when infusing insulin to a patient over a period of time, a bolus is typically delivered to a patient before or during a meal to compensate for the increased amount of insulin required to balance glucose produced by food, or when the blood glucose is high. “Basal” as used herein refers to the essential dosage of medication which must be delivered to a patient repeatedly over a period of time to maintain normal biological function. 
     Referring to FIG. 1, a piston-type infusion pump  5  in accordance with the present invention is shown for delivering medication  24  to a patient along an infusion path  14 . The infusion pump  5  includes a sealed pump casing  7 , processing circuitry  200 , power cells  70 , force sensor  16 , LED  207 , optical linear sensor  208 , motor  10 , lead screw  15 , half nut  18 , slide  22 , syringe  12 , gear train  28 , and infusion path  14 . 
     In operation, processing circuitry  200 , powered by power cells  70 , controls the operation of the infusion pump  5 . The motor  10  is incrementally engaged to infuse medication to a patient at predetermined intervals. Upon engagement, the motor  10  causes the lead screw  15  to rotate by means of the gear train  28 . When the half nut  18  is engaged with the lead screw  15  and the lead screw  15  is driven by the motor  10 , the slide  22  traverses the slide rail  72  (see FIGS. 8-9) pushing the plunger  20  of the syringe  12 . This causes delivery of medication  24  at the distal end  26  of the syringe  12 . An infusion path  14  to deliver the medication  24  is connected by the connector  27  to the dispensing tip  25  of the syringe  12  to provide fluidic communication between the infusion pump  5  and a patient. 
     Referring now to FIGS. 2-5, the casing of the infusion pump  5  is shown. The pump casing  7  is preferably formed of a thermoplastic material and preferably made watertight by sealing any openings in the pump casing  7 . The watertight pump casing  7  of FIGS. 2-5 preferably prevents damage to any components contained inside it. The pump casing  7  supports LCD display  30 , keypad  42 , priming button  44 , battery door  40 , exterior infusion port  46 , hinge  38 , pump door latch  48 , and pump door  36 . 
     LCD display  30  is a menu driven graphic display. In this embodiment the display items are listed vertically allowing the patient to scroll through menus until finding the desired item to access status data or to program the infusion pump  5 . Other display configurations of the LCD display  30  are possible in other embodiments, as well as the ability to display other information on the LCD display  30 . 
     FIGS. 2-5 show the syringe loading process. In FIGS. 2 and 3, the pump door  36  is opened to expose the interior infusion port  50  and the battery door  40 . The syringe  12  is installed through interior infusion port  50 , interior to the pump casing  7  of the infusion pump  5 , creating a watertight seal. In FIGS. 2 and 3, the pump door latch  48  has been rotated away from the pump casing  7  in order to release the pump door  36  so it may pivot open at hinge  38 . The release of pump door  36  enables the patient to rotate the pump door  36  about the hinge  38  thereby exposing interior infusion port  50  as shown in FIG.  3 . Interior infusion port  50  is provided to receive a syringe  12  containing a supply of pharmaceutical product to be infused along the infusion path  14  by the infusion pump  5  as best shown in FIG.  10 . The battery door  40  is removed for replacing the power cells  70 . The battery door  40  preferably snaps into place and preferably includes a seal to maintain the watertight properties of the pump casing  7 . 
     The syringe  12  ideally includes a plunger  20  and a plunger stem  21  for engagement with the slide  22  upon installation in the interior infusion port  50  (see FIG.  1 ). The syringe  12  is installed in the infusion pump  5  by rotating the pump door latch  48  away from the pump door  36  of the infusion pump  5  as shown in FIGS. 3 and 5. The pump door  36  is then rotated away from the infusion pump  5  about the hinge  38 , exposing the interior infusion port  50 , which receives the syringe  12 . The pump door  36  closes in the opposite manner of opening, and the pump door latch  48  is rotated back into place, locking the pump door  36  as shown in FIG.  4 . 
     Referring now to the preferred embodiment of FIGS. 6 and 7, there is preferably an elastomeric O-ring  52  assembled on the inside diameter of interior infusion port  50 . The O-ring  52  provides a seal between the pump casing  7  and the syringe  12 . When the syringe  12  is installed in the interior infusion port  50  of the pump casing  7 , the pump door  36  is closed and latched in place by rotating the pump door latch  48  over the pump door  36 . As pump door  36  is closed, the syringe  12  is pushed against the pump casing  7  as shown in FIG. 6, and the O-ring  52  is squeezed in the interior infusion port  50  creating a seal at the distal end  26  of the syringe  12 . The interior infusion port  50  is preferably the only opening in the pump casing  7 . Thus the inside of the pump casing  7  is preferably sealed from outside contaminants, creating a watertight seal. 
     Pump door latch  48  is fixedly mounted on latch stem  62  which extends along the interior length of the interior of pump casing  7 . The axis of latch stem  62  is parallel with the longitudinal axes of syringe  12  and lead screw  15 . The latch stem  62  is held in the wall of pump casing  7  by a rotary seal  54 . Seal  54  is held in place by bias spring  55  and washer  58 , which are held in place over latch stem  62  by collar  56 . Upon rotation of pump door latch  48  to secure the syringe  12  within interior infusion port  50 , the translation of the pump door latch  48  to the closed position causes a rotation of the latch stem  62 . Rotation of latch stem  62  moves the half nut  18  into engagement with the lead screw  15  as shown in FIGS. 8 and 9. 
     FIGS. 8 and 9 show the mechanism for the preferred embodiment of FIG. 6 that engages and disengages the half nut  18  to the lead screw  15  through rotation of the pump door latch  48  and latch stem  62 . The lead screw  15  is mounted on a carriage  64  (the carriage  64  mounted to the pump casing  7 ) so as to be parallel with latch stem  62 . The half nut  18  is attached to the slide  22 . In FIG. 8, when the pump door latch  48  is moved to lock the pump door  36 , it also rotates latch stem  62  to disengage cam  63  from nut lever  68 , and moves the half nut  18  to engage the lead screw  15 . The spring  66  is anchored to the half nut  18  and to the slide  22  as shown. Spring  66  holds the half nut  18  to the right side of the axis X when the cam  63  releases the nut lever  68 , thereby holding the half nut  18  in the engaged position. 
     Likewise, in FIG. 9, when the pump door latch  48  is rotated away from the pump door  36  to unlock the pump door  36 , it also turns the latch stem  62 . Cam  63  consequently pushes down on the nut lever  68  and, hence moves the half nut  18 , to the disengaged position, where it is held on the left side of the axis X by the spring  66 . This releases the slide  22  to move to any position along the slide rail  72  when pushed by the plunger stem  21  of syringe  12 . The lead screw  15 , latch stem  62 , the slide  22  and the half nut  18  are all preferably sealed internal components to the pump  5  and are not accessed by the user. 
     A buttress thread is preferably used for the lead screw  15  and the half nut  18 , since the lead screw  15  engages and pushes the half nut  18  only in one direction. The wear components e.g., the lead screw  15  and the half nut  18  are preferably coated with a low friction, wear resistant coating to prolong life and to reduce power required to drive the infusion pump  5 . 
     FIGS. 8 and 9 also show how the amount of medication  24  remaining in the syringe  12  can be determined by the processing circuitry  200  at any given time. A light source, LED  207  is mounted on the slide  22 . An optical linear sensor  208  is fixedly mounted to the carriage  64 . The position of the slide  22  can be accurately measured at any moment by the optical linear sensor  208  by determining location of the LED  207  relative to the sensor  208 . The position of the slide  22  also determines the position of the plunger  20 . Since the diameter and position of the syringe  12  are known, based on the position of the slide  22 , the amount of medication  24  remaining in the syringe  12  can also be determined by the processing circuitry  200  as described herein. 
     Referring now to FIG. 11, a syringe  12  for use with the infusion pump  5  is shown. The syringe  12  includes a plunger  20  which preferably includes a generally elongated, “cup-shaped” plunger stem  21  and a dispensing tip  25 . In FIG. 10 the plunger stem  21  contacts the slide  22  so that the slide  22  may push the plunger stem  21  when engaged by the half nut  18  and urged by the lead screw  15 . The plunger stem  21  is preferably cup-shaped so that the lead screw  15  may reside within the cavity created by the plunger stem  21 , without exerting any pressure on the plunger  20  or the plunger stem  21 . The slide  22  includes an aperture  23  (see FIGS.  8  and  9 ), larger than the diameter of the lead screw  15 , such that the lead screw  15  passes through the slide  22  to engage the half nut  18  and the gear train  28 . Upon installation of the syringe  12  in the interior infusion port  50  of the pump casing  7 , the plunger  20  normally advances the slide  22  axially away from the exterior infusion port  46  along the slide rail  72 . The slide  22  is free to move, because, when the pump door  36  is open, the half nut  18  is rotated out of engagement with the lead screw  15 . 
     In operation, the patient primes the infusion pump  5  to remove air from the infusion path  14  by depressing the priming button  44  until the infusion path  14  is free from air bubbles. In priming mode, the motor  10  drives medication  24  along the infusion path  14  until the patient is satisfied that the infusion path  14  is clear of air. Once the infusion pump  5  is primed the device is ready for programmed operation for a basal rate or bolus operation depending on the patient&#39;s requirements. 
     In programmed operation, the motor  10  causes the lead screw  15  to rotate by means of the gear train  28 . When the half nut  18  engages the lead screw  15  and the lead screw  15  is driven by the motor  10 , the rotation of the lead screw  15  moves the half nut  18  and the slide  22  traverses the slide rail  72  pushing the plunger  20  of the syringe  12 . This causes delivery of medication  24  at the distal end  26  of the syringe  12 . An infusion path  14  is linked to exterior infusion port  46  to deliver the medication  24  to a patient. 
     Processing Circuitry 
     Referring now to FIGS. 12 a  and  12   b , a block diagram of processing circuitry  200  of the preferred embodiment of the infusion pump  5  is shown. Processing circuitry  200  includes: processor  220 , power section  205 , force sensor section  225 , position sensor section  245 , motor drive section  270 , as well as additional interface and signal conditioning circuitry described hereinafter. 
     Power section  205  preferably includes three sources of power for the infusion pump  5 , although other embodiments may utilize different power configurations. In the preferred embodiment of FIGS. 12 a  and  12   b  Vbatt 1  and Vbatt 2  (power cells  70  in FIG. 1) are each preferably 2 silver oxide batteries in series. Vlithium is a backup source when Vbatt 1  and Vbatt 2  are low or are being replaced. Power section  205  is connected to the processing circuitry  200  by diode  212 . Vlithium provides enough power to keep the system clock circuit  288  running. Vbatt 1  provides power to the 3.3 Volt DC-DC regulator  214 . The regulator  214  provides 3.3V to the processing circuitry  200  with the exception of the LCD module  218 . Vbatt 2  provides power to the motor drive  272  and the LCD module  218 . The processor  220  switches the 3.3V power to the various sub-systems as they require power. 
     In an alternative embodiment, Vlithium is not used, and instead the 32 KHz clock  288  is replaced by a lower power real time clock (RTC) circuit. The RTC is powered by the 3.3V regulator through a diode. When the batteries are low or they are being replaced the RTC will be powered from a charge stored in a capacitor. 
     A power converter  286  provides −3.3V and +5.2V as needed by various subsystems. 
     Processor and Support Circuits 
     The processor  220  is a microprocessor or microcontroller integrated with memory and peripherals. The processor  220  preferably operates at 8 MHz. This is supplied from a quartz crystal (not shown). The processor  220  monitors the force sensor section  225  and the position sensor section  245  and controls the motor drive section  270  in accordance with an instruction set as described below. The processor  220  also determines the battery voltages and will alarm the user when it is time to replace the batteries through the audible beeper  280  and/or LCD module  218 . A 32 KHz signal is also generated by system clock circuit  288  which is used when the processor  220  is placed in sleep mode. If the circuit power (3.3V from the regulator  214 ) drops below a threshold, the processor  220  will be placed in reset by under voltage circuit  300 . 
     Serial communications circuit  290  is an RS-232 port. The serial communications circuit  290  is provided for test purposes. 
     Keypad circuit  289  is an interface which allows the user, through keypad  42 , to program the infusion pump  5 , view status and history, deliver a bolus and turn on a back light for the LCD display  30 . 
     The LCD module  218  consists of LCD display  30  and graphics controller/driver and backlighting. The graphics controller/driver integrated circuit is controlled through a parallel interface (not shown) from the processor  220 . A 2 KHz clock signal is provided to the LCD module  218  by system clock circuit  288 ; Vbatt 2  provides the power to LCD module  218 . 
     A watchdog timer  284  is used to ensure that the pump motor  10  is stopped if the instruction set of processor  220  has lost control of the infusion pump  5  or if a diagnostic test fails. 
     The non-volatile memory  282  is preferably an EEPROM used to store user programmable variables and pump history data for use by the instruction set of processor  220 . 
     Force Sensor Section 
     Force sensor section  225  includes FSR circuit  230  (including force sensor  16 ), reference voltage circuit  227 , and amplifier circuit  229 . 
     A DC motor  10  of the infusion pump  5  drives the lead screw  15  that drives the plunger  20  of syringe  12  to deliver medication  24  to a patient along the infusion path  14 . An FSR (force sensitive resistor) circuit  230 , through force sensor  16 , is used to sense the force on the lead screw  15  prior to the initiation of the delivery cycle. If there is an occlusion (an obstruction in the infusion path  14 ) the force on force sensor  16  will increase and will be detected by the processor  220 . Similarly, if there is a leakage or absence of the syringe  12  within the infusion pump  5 , the force sensor  16  will reflect a low force value. The processor  220  will alarm the user through audible beeper  280  and/or LCD module  218 . 
     A 2.5V reference is supplied to the FSR circuit  230  and the output is amplified by amplifier circuit  229  before it is digitized by the processor  220 . The processor  220  monitors the force and applies an algorithm (as described herein) to detect if an occlusion has occurred. 
     Position Sensor 
     Position Sensor Section  245  includes linear sensor circuit  244 , and amplifier circuit  242 . 
     An optical linear sensor  208  of linear sensor circuit  244 , such as a linear sensor manufactured by Hamamatsu Corp., is used to track the motion of the plunger  20 . The main function of the optical linear sensor  208  is to provide information that determines the volume of medication  24  remaining in the infusion pump  5  at any time. The linear sensor signal is also used to monitor any gross inaccuracy in medication  24  delivery by calculating delivery volume between any two points of time. 
     The optical linear sensor  208  is attached to the infusion pump  5  in a known, fixed position. The LED  207  of linear sensor circuit  244  is attached to the slide  22  that is moved to push the plunger  20  to cause delivery of medication  24 . By knowing the position of the LED  207 , the processor  220  calculates the position of the plunger  20 . Since the syringe  12  is of a known diameter and is in a fixed position in the infusion pump  5 , the position of the plunger  20  is used to determine the volume of remaining medication  24  in the syringe  12  at any time. 
     The optical linear sensor  208  of linear sensor circuit  244  is preferably a two electrode photo-diode device that provides continuous position data of light spots traveling over its surface. The current at each electrode of the optical linear sensor  208  is inversely proportional to the distance of the light source from the electrode. When the LED  207  is pulsed on, the current from each electrode of the optical linear sensor  208  is inversely proportional to the distance of the LED  207 ; by using two electrodes, errors due to power fluctuations can be minimized. The electronic pulses on the LED  207  cause current to flow from each of the sensor&#39;s electrodes. The currents are fed into trans-impedance amplifiers  242  and the processor  220  reads and digitizes the amplified current and applies an algorithm to determine and monitor the position of the plunger  20 . 
     For example, in the preferred embodiment, the algorithm used to determine the medication  24  remaining (based on 300 units in a full syringe of U 100  concentration insulin): 
     
       
         Units= G   1 [ A−B]/[A+B]+[ 150 −K   1 ] 
       
     
     G 1 =[37/25.5778]*150 
     A=8 bit digitized value of sensor A output 
     B=8 bit digitized value of sensor B output 
     K 1 =offset value from calibration routine 
     
       
         or= G   1 [ A center-of-travel− B center-of-travel]/[ A center-of-travel+ B center-of-travel] 
       
     
     Motor Drive 
     Motor drive section  270  includes motor circuit  278 , encoder circuit  274 , and amplifier circuit  276 . 
     The output of an integrated DC motor  10  of motor circuit  278 , encoder and gear reducers are used to drive the lead screw  15  that moves the plunger  20  of the infusion pump  5 . The motor  10  is driven by a PWM (pulse width modulated) signal which is provided by the processor  220 . 
     The motor  10  of motor circuit  278  is a closed loop velocity control with the feedback signal being the back-EMF of the motor which is amplified by the amplifier circuit  276 . The closed loop control algorithm is a proportional type of control. The motor  10  is commanded to a constant speed and the processor  220  counts the pulses from the encoder circuit  274 . When the motor  10  has moved the required number of pulses, the PWM signal from the processor  220  is turned off and a brake is applied. On the next maneuver the processor  220  will compensate for any undershoot or overshoot of the previous maneuver. For a basal delivery the motor  10  will move the lead screw  15  every 3 minutes. For the minimum required basal rate of 0.1 units/hr (1 microliter/hr) this means 0.005 units or 0.05 microliters is delivered every 3 minutes. 
     Referring now to FIG. 13 (and the respective step numbers), a method of detecting an occlusion or leakage in the infusion path  14  of the infusion pump  5  is shown. FIGS. 15-19 show the individual methods of occlusion and leakage detection which the infusion pump  5  utilizes. When an occlusion or leakage occurs anywhere in the infusion path  14 , medication  24  is not properly delivered to the patient. The extra volume of medication not delivered to the patient must occupy space within the infusion path  14  or the syringe  12 . The infusion path  14  and syringe  12  are preferably made of semi-rigid or semi-flexible plastic. An increase in medication volume causes an increase in pressure within the medication fluid which can be monitored as force incident on the force sensor  16  at the end of the lead screw  15 . As such, where medication  24  is frequently delivered (e.g., basal delivery every 3 minutes), this pressure increases with each delivery if the infusion path  14  is and remains occluded. Thus, the force sensor  16  located at the end of the lead screw  15  will encounter increased force prior to every delivery cycle of medication if there is an occlusion present in the infusion path  14 . 
     At the outset of a delivery cycle, step  1  in FIG. 13, the processor  220  reads the signal of the force sensor circuit  230  which indicates the amount of force (FN) incident to the force sensor  16 . The signal FN is passed to the processor  220  and is then compared to a reference value in step  2 , FMAX, in this example, 2 volts. This process (see FIG. 15) will detect an occlusion in the system, since in all cases the FN reading should be less than the reference value. The reference value FMAX is stored in the non-volatile memory  282  of the processing circuitry  200 . If the signal FN is greater than this value, an occlusion is present and the audible beeper  280  is sounded in step  6 . Otherwise the process moves to step  3 . 
     In step  3  of FIG. 13 (see also FIG.  16 ), the force signal FN is compared to a minimum threshold reference value to determine whether or not a syringe  12  having medication  24  is properly loaded in the infusion pump  5 . This minimum reference value, FLEAK, is also stored in the memory  282  of the processing circuitry  200 . If the signal FN at the force sensor  16  does not exceed the minimum threshold, the alarm is sounded in step  6 . Such a condition also indicates possible leakage in the infusion pump  5  or in the infusion path  14  or the absence altogether of a syringe  12  in the infusion pump  5 . 
     For the lowest basal rate the force signal FN does not always increase under occlusion conditions. This is because, at such a low basal rate, the increase in force due to occlusion is sometimes less than the variation in the force signal FN due to the drive mechanism turning the lead screw  15 . Thus, a particular method is required to detect an occlusion if an extremely low basal rate is being used. In step  4  of FIG. 13 (see also FIG.  17 ), the processor  220  determines the present basal rate. If the rate is less than the predetermined threshold, BMIN, the process proceeds to step  7 . 
     As shown in step  7  of FIG. 13, for a low basal rate, the force signal value FN is compared to a stored minimum value, FMIN. If the force signal FN is greater than FMIN the process proceeds to step  12 . In step  12  the amount by which FN exceeds FMIN is determined. If the amount that FN exceeds FMIN (FN−FMIN) is greater than a predetermined threshold, FINC (here 0.1 volts), an occlusion has been detected, and the process proceeds to step  6  to sound the audible beeper  280 ; if FN does not exceed FMIN by the predetermined threshold FINC (i.e., FN−FMIN is less than 0.01), the process proceeds to step  11  and delivers medicine  24  along the infusion path  14  by incrementally moving the plunger  20 . 
     In step  7  of FIG. 13, if FN is less than a stored minimum value FMIN, step  9  stores the current value of FN as the new FMIN in the memory  282  and the process proceeds to step  11  to delivers medication  24  along the infusion path  14  by incrementally moving the plunger  20 . 
     If step  4  determines that the basal rate is greater than a predetermined rate, the process proceeds to step  8  (see also FIG.  18 ). In step  8 , FN is compared to the force signal from the previous delivery cycle, FN(n−1), stored in the memory  282 . If in step  8  FN is greater than FN(n−1), a counter is incremented in step  13  to record an instance of increasing pressure from FN(n−1) to FN. If in step  14  the counter shows increasing pressure for a predetermined number (greater than 1) of cycles, an occlusion is declared and the beeper  280  is sounded in step  6 . Otherwise the process proceeds to step  11  and delivers medication  24  along the infusion path  14  by incrementally moving the plunger  20 . 
     If the signal FN in step  8  does not exceed FN(n−1), the counter is reset to zero in step  10  and the process proceeds to step  11  to deliver medication  24  along the infusion path  14  by incrementally moving the plunger  20 . 
     FIG. 19 reflects an occlusion detection method whereby step  8  compares FN to the force signal from the previous delivery cycle, FN(n−1). If the FN is greater than FN(n−1) by a predetermined maximum amount, FLIMIT, the beeper is sounded in step  6  without incrementing or checking the counter in step  13 . If FN is not greater than FN(n−1) by FLIMIT, the counter is reset to zero in step  10  and the process proceeds to step  11 . 
     Prior to initiating the delivery cycle over again, the processor proceeds to step  15  to wait until a predetermined time has passed. 
     Thus, the infusion pump  5  determines the presence of an occlusion in the infusion path  14  by processing force measurements from the force sensor  16  if one of the following occurs: 
     If the force measured during a delivery cycle, at a point immediately before the start of the subsequent delivery of medication, is greater than the force at the identical point in the previous cycle, and has been so for a predetermined number of delivery cycles, i.e.: 
     
       
           F   1 &lt; F   2 &lt; F   3 &lt; F   4 , . . .  FN ( n− 1)&lt; FN  for a predetermined number of cycles. 
       
     
     If the force measured during a delivery cycle, at a point immediately before the start of the subsequent delivery of medication, is greater than the force at the identical point in the previous cycle by an amount greater than a predetermined value, i.e.: 
     
       
           FN−FN ( n− 1)&gt; F LIMIT. 
       
     
     If, in situations using low basal rates, the difference in force value taken during a cycle, at a point immediately before the start of the subsequent delivery of medication, and the force value at an identical point from any previous cycle for low basal rates, is greater than a predetermined value, i.e.: 
     
       
         ( FN−F MIN)&gt;a predetermined value,  F INC. 
       
     
     If the force measured during a cycle, at a point immediately before the start of the subsequent delivery of medication is greater than a predetermined value, i.e.: 
     
       
           FN&gt; a predetermined value,  F MAX. 
       
     
     The force measurements are also used to detect if the syringe  12  is removed, or if the infusion path  14  is not connected to exterior infusion port  46 . In these cases the force measured will be close to zero and the infusion pump  5  will alarm the user to check the syringe  12  and infusion path  14  for possible leakage or other condition. 
     Referring now to FIG. 14, a graph of four sequential force signals during an occlusion condition are shown. The force v time relation of an occlusion is apparent from the figure, as the amount of force immediately preceding each delivery cycle is shown as higher than the preceding cycle, indicating the presence of an occlusion in the infusion path  14 . FIG. 14 represents the condition which the method of FIG. 18 detects. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention.