Infusion pump assembly having a reverse rotation prevention system and method for operating the same

A system and method for preventing an undesirable reverse rotation of an infusion pump assembly. The infusion pump motor includes a roller assembly, a coil, a rotor, a rotor position sensor, and a controller. The rotor is in communication with the coil, and the rotor is rotationally coupled to a roller assembly. If the pump is in not pumping, then the system monitors for reverse rotation of the pump, and if reverse rotation of the pump is detected, energizes the coil in a manner that holds the rotor in place. The system may also activate an indicator if reverse rotation is detected.

TECHNICAL FIELD OF INVENTION

The invention relates to a system and method for preventing reverse rotation of an infusion pump assembly having a coil and a rotor arranged to form a motor that rotates a roller assembly.

BACKGROUND

An infusion pump infuses fluids, such as medications or nutrients, into a patient's body. Infusion pumps administer an injection every minute, or injections when requested by the patient. An infusion schedule typically consists of periods of desired flow during which the motor driving the pump is energized, and periods when the desired flow rate is zero. During zero flow periods, the motor driving the pump is de-energized. De-energizing the motor when the flow rate is zero saves energy, a particularly beneficial feature when the infusion pump is operating on battery power.

Many infusion pumps use a peristaltic type pump where a roller assembly moves a roller to progressively compress a tube through which the fluid flows. When the motor is de-energized, it is undesirable for the roller assembly to rotate backward as this may siphon blood out of the patient. Normally, the stiction of the roller assembly and related motor are sufficient to prevent reverse rotation of the pump. However, some infusion methods, subcutaneous for example or other conditions, such as a pinched tube, may produce relatively high pressures, pressures sufficient to rotate the roller assembly in backward. Various mechanical mechanisms that prevent reverse pump rotation such as pawls and clutches are known. However if the pawl gets stuck, or the clutch does not engage, the roller assembly may rotate backward. Infusion pumps used on human patients are certified to have no single point of failures. That is, no single cause of failure should cause the pump to silently fail to operate correctly. Mechanical devices are not well suited for detecting a failure such as reverse rotation and reporting that a failure has occurred.

What is desired is a way to detect reverse rotation of an infusion pump, and make energy efficient use of a motor to prevent reverse rotation. It is further desired that if reverse rotation is detected, an indicator is activated.

SUMMARY

In one aspect, this invention provides an infusion pump assembly having a reverse rotation prevention system. The infusion pump assembly includes a roller assembly configured to rotate forward to pump fluid at a flow rate. The assembly also includes a coil configured to generate a coil field in response to a coil voltage, and a rotor configured to generate a rotor field that cooperates with the coil field to urge the rotor toward a rotor position. The coil and rotor are arranged to cooperate and form a motor. The rotor field has a polarity that is dependent on the rotor position. The rotor is rotationally coupled to the roller assembly such that when the rotor is urged toward a rotor position the roller assembly is urged toward a corresponding roller assembly position. The rotor position is determined by a rotor position sensor arranged to output a rotor position signal indicative of the polarity. The infusion pump assembly also includes a controller adapted to receive an infusion schedule to determine the flow rate based on the infusion schedule, and receive the rotor position signal to determine the coil voltage based on the flow rate. The coil voltage is turned on to rotate the roller assembly forward when the flow rate is not zero, turned off when the flow rate is zero. Following the coil voltage being turned off, if the position signal indicates a change in the rotor position, then the coil voltage is turned on to hold the rotor in place and thereby prevent reverse rotation of the roller assembly.

In another aspect, this invention provides a method for operating an infusion pump infusion pump assembly having a reverse rotation prevention system. The infusion pump assembly includes a roller assembly that is rotated forward to pump fluid at a flow rate based on an infusion schedule, a coil that receives a coil voltage to generate a coil field, a rotor that generates a rotor field to cooperate with the coil field for urging the roller assembly to a roller assembly position. The method includes the steps of determining a flow rate base on the infusion schedule, and if the flow rate is not zero, the coil voltage is turn on to rotate the roller assembly forward. If the flow rate is zero, the coil voltage is turned off, and the roller assembly is monitored for movement. If a change in the roller assembly position is determined when the flow rate is zero, then a coil voltage effective to hold the rotor in place is output by the controller to prevent reverse rotation of the roller assembly.

Further features and advantages of the invention will appear more clearly on a reading of the following detail description of the preferred embodiment of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings.

DETAILED DESCRIPTION

Referring toFIG. 1, in accordance with a preferred embodiment of this invention, an infusion pump assembly10having a reverse rotation prevention system is illustrated. Infusion pump assembly10is useful for infusing fluid based medications or nutrients into a patient. Reverse rotation prevention is desirable to prevent fluid or blood from being siphoned out the patient. Infusion pump assembly10includes a roller assembly20, also known as a peristaltic pump. Roller assembly20pumps fluid by progressively compressing a flexible tube22with a roller24against track26. Tube22is constrained within track26such that as roller24progresses along tube22, fluid is forced out of tube outlet28. Tube22is formed of an elastic material such that it returns to its original dimensions once roller24passes, whereupon fluid enters tube inlet30. Roller assembly20is illustrated showing three rollers arranged on a flange32so at least one roller is compressing tube20against track26for any flange32position. Alternately, two rollers may be used by extending track26so at least one roller is compressing tube22. Alternately, more than three rollers may be used. Using three rollers and track28having about a 180 degree arc provides an arrangement where tube22is readily inserted and removed by retracting track26. The features for retracting track26are not shown. Fluid is pumped out of outlet28when input shaft34is rotated in a forward direction indicated by arrow38. The flow rate of fluid out of outlet28is proportional to the rotational rate of roller assembly20, provided that the backpressure at outlet28is not sufficient to overcome the seal created by roller24compressing tube22against track26.

Roller assembly20has an input shaft34coupled to motor assembly40by way of mechanical coupling36illustrated as a dashed line. It is preferable that mechanical coupling36provide a reduction ratio such that each full rotation of roller assembly requires more than one full rotation of motor assembly40. A reduction ratio is advantageous because a smaller, faster rotating motor can be used to generate adequate torque and rotational speed for roller assembly20. Furthermore, as described in more detail below, since motor assembly40is used to detect reverse rotation a small degree of reverse rotation by roller assembly20causes a readily detected larger degree of reverse rotation at motor assembly40. An exemplary reduction ratio is 28:1, however it is understood that a wide range of reduction ratios could be employed to provide a usable infusion pump assembly10. Mechanical coupling36is preferably by way of a gear or set of gears. Gears provide a rigid, non-slip coupling so for each rotary movement of motor assembly40there is a corresponding rotary movement of roller assembly20.

Motor assembly40is preferably a brushless direct current (BLDC) motor. U.S. Pat. No. 7,053,583 to Hazelton shows a system and method for operating a brushless direct current motor, the disclosure of which is incorporated herein by reference. Motor assembly40has a coil42configured to generate a coil field in response to a coil voltage72. Coil voltage72may be a continuous voltage, or a pulse-width-modulated voltage, either applied by a full-wave motor drive as shown in Hazelton, or a half-wave drive where one terminal of coil42is fixedly connected to a first voltage potential, and the other terminal is switchably coupled to a second voltage potential. As used herein, turning off a coil voltage means the same as setting the coil voltage to zero, or not outputting a coil voltage. When a coil voltage is turned off, the coil will not have any current and so will not generate a coil field. Not outputting a coil voltage may be by open-circuiting one or both connections to coil42, or by shorting together both connections to coil42. Coil42is typically part of a stator assembly having more than one coil. As used herein, coil voltage may be refer to a single voltage such as coil voltage72, or may be a combination of voltages72,74, and76, where the combination is referred to as coil voltage70. As such, if coil voltage70is turned off, all of coil voltages72,74, and76are turned off. Alternately, if coil voltage70is turned on, one or two of the coils may have a voltage being applied, and no voltage is being applied to the remaining coil(s). Referring to Hazelton, it is understood that coil voltage70necessary for rotor44to hold in place is not the same coil voltage70necessary to rotate rotor44forward.

Motor assembly40also includes a rotor44configured to generate a rotor field. Rotor44is preferably a permanent magnet having an even number of distinct regions of alternating magnetic polarity uniformly spaced about the surface of rotor44. Rotor44is illustrated as having ten (10) distinct regions of alternating polarity fields. Motors having more than or fewer than ten (10) regions are known and may be adapted to operate suitable in an infusion pump assembly. Coil42and rotor44are arranged to form a motor. The rotor field generated by rotor44cooperates with the coil field generated by coil42to urge rotor44toward a rotor position. The polarity of the rotor field that interacts or cooperates with the coil field depends on the rotor position. The rotor field and coil field interact or cooperate by creating either a repelling force or attracting force based on the polarity of the coil voltage and the polarity of the rotor field. Rotor44has a rotor shaft46that is rotationally coupled to roller assembly20through mechanical coupling36, as described above. As such, when rotor44is urged toward a rotor position, roller assembly20is urged toward a corresponding roller assembly position.

Motor assembly40also includes a rotor position sensor48arranged to output a rotor position signal82indicative of the polarity of the rotor field interacting with the coil field. Rotor position sensor48is preferably a device comprising a Hall effect sensor. Alternately, rotor position sensor48may be an optical sensor detecting alternating light and dark areas painted on rotor44or present on an encoder wheel attached to rotor shaft46. Since the arrangement of alternating polarity regions is uniformly space about rotor44, when rotor position sensor48is arranged opposite coil42as illustrated, rotor position sensor48detecting one polarity of rotor field is an indication that the opposite polarity rotor field is interacting with coil42. As used herein, rotor position signal may be refer to a single signal such as rotor position signal82, or may be a combination of signals82,84, and86, where the combination is referred to as rotor position signal80.

Infusion pump assembly10includes a controller50. Controller50suitably includes a microprocessor, or the like, capable of inputting rotor position signal80. Controller50also has suitable power devices for outputting coil voltages70, and a memory for storing program instructions or data regarding medications. Controller50is adapted to receive an infusion schedule entered by a person operating or programming infusion pump assembly10through a keypad52. The entries made on keypad52and the operating status of infusion pump assembly10may be indicated on a display54. Controller50determines a flow rate based on the infusion schedule. The flow rate may be a continuous flow rate measured in units of volume per unit time, thereby requiring that roller assembly20be rotated at a constant fixed speed. Typically, the infusion schedule calls for the infusion pump assembly10to pump fluid on a periodic basis. The periodic infusion schedule may specify a dosage that may be delivered at some flow rate for a short time, a few seconds for example, and then wait for a longer period of time, minutes or hours for example, before another dosage is delivered. During this longer period of time infusion pump assembly10is put in a standby mode, such that the motor is de-energized. With the motor de-energized, the infusion pump assembly10is susceptible reverse rotation may occur and reverse rotation protection is advantageous.

When the infusion pump assembly10is operating and the flow rate is greater than zero, the controller50receives rotor position signal80and uses that signal to determine the appropriate coil voltage70to rotate roller assembly20forward. Varying the flow rate requires coil voltage70to be appropriately determined to rotate roller assembly20at a rotational rate necessary to provide the desired flow rate. If the flow rate is zero, controller50turns off coil voltage70. Following coil voltage70being turned off, controller50monitors rotor position signal80to detect if a change in the rotor position signal80occurs, thereby indicating a change in the rotor position. If a change is detected, then coil voltage70is turned on in such a way as to hold rotor44in place. Coil voltage70is selected so motor assembly40generates a detent torque curve that urges rotor44toward a detent position where rotor44will stay unless the detent torque is overcome. Holding rotor44in place prevents reverse rotation of roller assembly20.

Motor assembly40has three coils42,64, and68arranged about rotor44as indicate inFIG. 1. Such an arrangement is commonly called a brushless direct current motor. Brushless motors having four coils and multiples of three coils are also known.FIG. 1shows a three-pole brushless motor, each of the three phases is arbitrarily assigned as Phase A, Phase B, or Phase C. The number of coils and the number of rotor poles determine the relationship between the rotor position signal80and coil voltage70necessary for rotating the motor in a selected direction, or holding the motor in place. As such, it is advantageous to determine the direction of rotor movement when determining a coil voltage70to hold rotor44in place. Thus, controller50is further adapted to determine that the change of the rotor position is indicative of reverse rotation of roller assembly20.

Another embodiment of infusion pump assembly10includes an indicator56coupled to controller50. Indicator56is preferably an audible indicator. Alternately, indicator56may be a flashing light, an icon on a display, or a signal sent to another location such as a centralized nursing station. Activating indicator56when reverse rotation of roller assembly20is detected avoids a silent single point failure that may prevent an infusion pump from being certified for human use. Being able to detect reverse rotation cooperates with other features directed toward certifying infusion pump assembly10for human use. Reverse rotation may be an indication that flexible tube22extending beyond outlet28is pinched or otherwise obstructed.

Another embodiment of infusion pump assembly10includes a primary power source58for supplying power to infusion pump assembly10, and a secondary power source60for supplying power to infusion pump assembly10when primary power source58is not supplying power to infusion pump assembly10. Primary power source58is suitably a wall outlet providing 110 VAC power for infusion pump assembly10. Secondary power source60is suitably a battery. In the event that building power fails, or infusion pump assembly10is inadvertently unplugged, secondary power source60will help maintain the desired infusion schedule until primary power source58is reactivated. If infusion pump assembly10is equipped with an indicator, then indicator56may also be activated when secondary power source60is supplying power to infusion pump assembly10. Activating indicator56when secondary power source60is supplying power avoids a silent single point failure, thereby cooperating with other features directed toward certifying infusion pump assembly10for human use.

The arrangement of coil42, a second coil62, and a third coil64illustrated inFIG. 1is sometimes referred to as a stator or stator assembly. The physical support for the coils is not shown, but a variety of suitable supports is known.FIG. 1illustrates the coils as being arranged to project coil fields radially inward toward the rotor44. An alternate arrangement is the coils oriented to project field coils radially outward, and the rotor44is a cylindrical permanent magnet arranged to surround such a stator and project the rotor field radially inward.

The stator assembly may also support the rotor position sensor48, a second rotor position sensor66, and a third rotor position sensor68. The three rotary position sensors48,66, and68are preferably Hall effect sensors arranged to determine the polarity of the rotor fields near each sensor. Rotor position sensors48,66, and68each output a polarity signal82,84, and86respectively, collectively known as rotor position signal80. It is advantageous to include the rotary position sensor as part of the stator assembly so the rotary position sensor can directly detect the polarity of the rotor field. Alternately, the rotary position sensors may be located away from the rotor44, and determine polarity indirectly based on an encoder attached to rotor shaft46.

FIG. 2is a flow chart200illustrating a method for operating infusion pump infusion pump assembly10shown inFIG. 1. At step210an infusion schedule is input by an operator, such as a nurse or doctor, and received by controller50. At step220, the controller determines the flow rate based on the infusion schedule. An infusion schedule will normally specify a dose to be delivered on a periodic basis, every hour for example. For example, the dose may require the pump to operate at some flow rate for a few seconds, followed by zero flow rate for the remainder of the hour. At step230a determination is made as to the flow rate value. If the flow rate is not zero, the coil voltage is turned on or output by controller50in a manner effective to rotate roller assembly20forward and pump fluid at the flow rate. If the flow rate is zero, coil voltage70is turned off so roller assembly20does not pump any fluid. At step250, a rotor position signal output by the rotor position sensor48is monitored or input by controller50to determine if the rotor position is changing. If the rotor position does not change, the NO logic path returns to step220where the flow rate is again determined. If the rotor position does change, then at step260the coil voltage is turned on in a manner effective to hold rotor44in place. At step270, the indicator56is activated to notify an operator that reverse rotation has been detected.

In another embodiment, the method may include activating a secondary power source58when a primary power source60is not supplying power. Secondary power source is suitably a battery and helps the pump infusion pump assembly10maintain the programmed infusion schedule. An indicator may also be activated if the secondary power source is supplying power. Such an action would be beneficial if for example, the primary power source58was a wall plug, and the primary power source had been inadvertently unplugged.

In another embodiment having three Hall effects sensor for indicating rotor position and rotor polarity, the method may include the step of determining a change in the roller assembly position by monitoring signals from three Hall effect sensors arranged about the rotor.

Therefore, an infusion pump assembly having a reverse rotation prevention system and method for operating the same is provided. The reverse rotation prevention system monitors the motor assembly that drives the pump for reverse rotation when the flow rate is zero, and if reverse rotation is detected, energizes coils in the motor assembly is such a was as to hold the motor and pump in place. This motor based reverse rotation prevention system can be used alone, or in conjunction with mechanical reverse rotation prevention or another sensor arrangement to provide redundancy to guard against single point failures. The motor based system readily detects reverse rotation and so can be used to activate an indicator. Furthermore, by turning off the motor when the flow rate is zero, energy is conserved so battery life is extended.