Motor unit for a fluid pump and method of operation

Abstract: An enteral nutrition pump system (10) operates in a cyclical manner with a period between cycles being selected in accordance with the desired fluid delivery rate. Each pump cycle may correspond to a single rotation of the rotor (18) or a fractional rotation of the rotor. Rotor rotation is sensed by monitoring of the AC component of current supplied to a DC motor driving the rotor.

Description: This invention relates to pumps for delivering medical fluids and particularly relates to peristaltic pumps for delivery of enteral nutrition fluids to a patient. In accordance with known techniques the delivery of enteral nutrition fluids to a patient can be accurately controlled as to volumetric delivery rate by the use of a delivery system which includes a motor unit and a disposable delivery set. Likewise similar systems may be used for pumping of other fluids for medical purposes, such as intravenous infusion, blood pumping or supply of measured volumes of liquid medication to pre-loaded syringes or other containers. In known systems for delivering enteral fluids the rate of fluid delivery is controlled by regulating the speed of a pump motor in accordance with the desired volume rate. Pump motor speed may be controlled, for example, by providing pulses to a stepper motor. Another system for providing variable rate fluid delivery bikes use of a peristaltic pump with variable tension on the pump tube in combination with a constant speed motor. In other known systems for pumping medical fluids there are provided means for monitoring rotation of the pump rotor, for example, by magnetic detection or by optical rotation detectors. In such systems the actual rotation rate of the motor is compared to the desired rotation rate for purposes of making corrections to the rotation rate of the motor. Alternately the motor may be operated to rotate the pump by a number of rotations corresponding to the desired volume. US-A-4498843 discloses a microprocessor-controlled peristaltic pump for use in a portable continuous insulin infusion device. The microprocessor controls the number of times the pump rotor turns during a cycle. It is an object of the present invention to provide a new and improved method for regulating the volumetric rate of fluid delivery in a medical fluid delivery system and to provide apparatus for carrying out the improved method. In accordance with the present invention there is provided a motor unit for a medical fluid delivery system for use with a disposable delivery set for pumping medical fluid, comprising, pump operating means, including a DC motor, for acting in cooperation with the delivery set having a pump tube to deliver a volume of the fluid during each operating cycle; pump control means for controlling the pump operating means to deliver the fluid at a desired volumetric rate, the pump control means including means responsive to signals indicating the desired rate for activating the pump operating means for one of the operating cycles and for repeating the activation at variable time intervals selected in accordance with the desired volumetric rate and means for sensing the condition of the pump operating means characterised in that the means for sensing the condition of the pump operating means comprises means for sensing the AC component of the current supplied to the DC motor. In accordance with a preferred embodiment of the invention the pump operating means is a pump rotor for operating in connection with a pump tube on the delivery set to form a peristaltic pump and the pump operating cycle comprises a selected angular rotation of the pump rotor. The medical fluid delivery system which comprises the novel motor unit and a disposable fluid delivery set carries out a novel method for controlling the rate of fluid delivery. The novel method is characterised in that the means for detecting the completion of an operating cycle comprising means for detecting the AC component of the DC current provided to the DC motor. In accordance with another aspect of the present invention the AC component of the DC motor current is detected and compared to a reference level in order to detect the current variation which results from the presence of a pump tube on the rotor. Accordingly, any mis-installation of the pump tube will be detected by the fluid delivery pump. For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjection with the accompanying drawings, in which: Figure 1 is a plan elevation view of an enteral fluid delivery system incorporating the present invention. Figure 2 (comprising Figures 2a, 2b and 2c) is a circuit diagram for the system of Figure 1. Figure 3 is a circuit diagram for a portion of a modified delivery system in accordance with the present invention. Figure 4 is a timing diagram illustrating signals utilized in the present invention. Figure 1 is an illustration of an enteral fluid delivery system incorporating a motor unit in accordance with the present invention. The enteral delivery system 10 includes a motor unit 12 and a disposable delivery set generally indicated as 14 which is arranged to be mounted on the motor unit. The motor unit 12 includes a housing 16, which in the illustrated embodiment includes a recess 17 within which a rotor 18 is mounted. Rotor 18 is driven by a conventional constant speed DC motor which drives shaft 42. The delivery set 14 includes a pump tube 20, made of flexible plastic which surrounds rotor 18 and interacts with 3 rollers 36, 38 and 40 mounted on rotor 18 to form a peristaltic pump. Rotation of the rotor 18 in the direction indicated by the arrow in Figure 1 causes the rollers 36, 38 and 40 to interact with pump tube 20 and pump fluid through the tube at a rate which is determined by the rate rotation of rotor 18. Delivery set 14 includes an inlet tube 22, which is connected to a supply of enteral fluids, such as a fluid reservoir which may be mounted on an IV pole above the motor unit 12. The inlet tube 22 is connected to drip chamber 24 which is mounted to a recess on housing 16 and secured to one end of pump tube 20. The outlet end of pump tube 20 is provided with a mounting member 26 which is received in another recess on housing 16 to thereby secure the outlet end of tube 20. A fluid delivery tube 28 is connected to mounting member 26 and supplies fluid pumped by the system to an enteral feeding tube connected to a patient or to another medical fluid delivery system. The system illustrated in Figure 1 additionally includes a light source 30 and a light detector 32 for operation in connection with drip chamber 24 to detect the occurrence of drops in the drip chamber which pass between light source 30 and detector 32 in a manner which is known in the art. Mouting member 26 includes magnetized material, the presence of which can be detected by magnetic field detector 34. The motor unit 12 includes control buttons 51 through 58 for operating the unit to turn it on or off, to set the dose or volume rate of fluid delivery by the pump, to interrupt operation of the pump and to increase or decrease the designated fluid volume or volume rate. A four digit alphanumeric segment display 59 is provided for indicating the selected fluid delivery rate or delivered volume and for providing alarm messages or codes. Light emitting diode 60 and 61 are provided for indicating that the unit is plugged into AC power or indicating that the volume setting has been cleared. An enunciator 62 is provided for signalling an audible alarm to indicate, for example, that the pump has completed delivering a designated volume of fluid. Housing 16 is provided with a magnetic field sensor 50 which is arranged adjacent and behind rotor 18 in order to detect the magnetic field provided by magnets 44, 46 and 48 which are mounted on rotor 18. The presence of the magnets 44, 46 and 48 is detected as the magnets pass sensor 50 during rotation of rotor 18. Figure 2 is a schematic diagram of the circuits in the pump motor unit 12 of Figure 1. The schematic representations of the various components of Figure 1 have been given same reference numerals in Figure 2. The motor unit operates under the control of a microcomputer 64 which is provided with a control program which is set forth in Appendix I. A programmable interval timer 68 is provided for operating and initiating microcomputer 64. A clock 66, operating at 2 Mhz, provides clock pulses to the system. The various controls of the unit, 51 through 58, are provided as input signals which ground various input terminals of the microcomputer to thereby signal the operators input instructions. The alphanumeric display 59 is driven by the microcomputer as is LED indicator 61. Additional inputs to the microcomputer are provided by the magnetic field sensors 34 and 50 which sense respectively the magnetized mounting member 26 and the magnets 44, 46 and 48 on rotor 18. Likewise the drop detector 30, 32 is connected to provide input signals to the microcomputer. An AC power rectifier 72 is provided for AC operation and battery charging. Portable DC operation is available using battery 74. The AC circuit is arranged to charge the DC battery when the unit is connected to AC power. A low battery and dead battery detector circuit 70 is provided to signal the microcomputer that the battery needs recharging. The microcomputer provides an output motor signal which is coupled by transistor 80 to switching transistors 82, 84. Transistor 82 turns on the power supply to motor voltage regulator 88 when the motor is to be operated and transistor 84 short circuits the motor to lock it into position when the motor signal is no longer present. Switching transistor 78, which is provided with a power signal by transistor 76, operates to supply current to the motor system and the other electronic systems by voltage regulator 86 when power is turned on. The motor 90 is provided with a safety circuit 92 which provides a short circuit when the motor is operated for an excess period of time. The short circuit causes fuse 94 to open thereby disabling the set when continuous motor operation occurs, to avoid providing excess enteral fluid to a patient. Unlike conventional enteral nutrition systems the system 10 of the present invention is designed to provide an intermittent motor operation with the periodicity of the intermittent operation being regulated to adjust to the desired rate of fluid delivery. The operation of the system of the present invention is therefore cyclical and will be explained with respect to timing diagrams of Figure 4. Graph A of Figure 4 illustrates the motor voltage of the enteral fluid delivery system 10. The motor voltage is turned on and operated for a time period G which is regulated by detecting the rotation of rotor 18, in the case of Graph A for one complete revolution. With reference to Figure 1 it may be seen that during one complete revolution, represented by motor voltage period G, three magnets 44, 46 and 48 all pass magnetic field detector 50 and are sensed thereby. Curve B in Figure 4 illustrates the output signal from the rotor sensing magnetic field detector 50 which occurs during the cycle of operation indicated by motor voltage G. During an initial period of approximately .45 seconds designated F in Figure 4 the operation of the rotor sensing is inhibiting by software in microcomputer 64, so that the initial on period J of magnetic field detector 50 is not responded to by the program. Thereafter, during one complete revolution of the rotor, the signal from detector 50 goes to zero as each magnet is encountered by detector 50. Upon detection of the third magnet, at the end of period G, the motor voltage is turned off. In accordance with the preferred embodiment of the present invention the unit repeats the cyclical operation a time period I after initiation of the first operation. The time period H during which there is provided no motor voltage is permitted to be variable, since it depends on the actual time taken for rotation of the rotor and the selected interval I. The interval I is selected according to the rate of fluid delivery to be provided by the set which is set by the operator. In one embodiment of the invention period I varies from 13.5 seconds corresponding to a delivery rate of 100 milliliters per hour to 4.5 seconds corresponding to a fluid delivery rate if 300 milliliters per hour. Motor operation period G takes approximately 4 seconds but may vary according to mechanical conditions of the motor and pump tube. Graph E in Figure 4 shows an alternate timing arrangement wherein the motor cycle consists of a single one-third of a rotation of the rotor 18. In accordance with the operation method of Graph E the motor current period G' is ended by the detection of the first of the three circumferentially arranged magnets by magnetic field sensor 50. Again the timing I between each operating cycle of the motor is varied in order to control volumetric fluid rate delivered by the pump. In the same embodiment as previously discussed a fluid rate of 1 to 100 milliliters per hour can be delivered using a cycle interval I which ranges from 450 to 4.50 seconds. According to the present invention as an alternate to providing magnets on rotor 18 for purposes of detecting completion of a motor cycle, the motor current is monitored for purposes of determining the rotational position of the rotor 18. Figure 3 is a schematic diagram of a circuit wherein there is provided a motor current monitoring circuit 96 which includes a low resistance resistor in series with the motor the voltage across which is AC coupled to an AC amplifier 92 for purposes of monitoring the AC component of the DC motor current. Graph C of Figure 4 illustrates a typical motor current for the operating cycle of Graph A of Figure 4. The motor current initially rises to a high level for purposes of overcoming the starting resistance and accelerating rotor 18 to its normal velocity. Thereafter the motor current drops but reaches periodic peaks corresponding to the resistance of rollers 36, 38 and 40 as they stretch pump tube 20 to its furthest position. The peak periods of motor current, which are illustrated as negative going pulses in the digitized signal of curve D, which is the output at point 96 of the circuit of Figure 3, may be used for purposes of detecting rotor position and may be used also for assuring that the pump tube 20 is properly mounted to rotor 18. Because of the initially high rotor current, which results from starting up the rotor, the current sensing is software inhibited for time period K of approximately .25 seconds prior to initiating the threshold detection which results in the pulses of curve D. Each of the pulses illustrated in curve D, which are negative going, have a positive going pulse which occurs a time period L prior to the end of a motor cycle, there being three such pulses during one rotation of the rotor. Accordingly, the curve D signal can be used for purposes of detecting and monitoring rotation position of rotor 18, and thereby indicating to the motor control circuit the completion of an operating cycle. As an alternate to providing delay L after the end of the curve D pulses, the motor cycle may be arranged to end at the end of the pulse, providing a different rotor position between cycles. The motor current monitoring previously described can additionally be used in cases wherein the motor voltage is provided only for a one-third rotation of the rotor as discussed with respect to curve E. An additional use of the motor current monitoring circuit, which provides the signal of curve D of Figure 4 is to provide assurance to the system that the pump tube 20 has been properly mounted on rotor 18. Accordingly at the initiation of motor current and after a delay period K a flag can be set by the microprocessor which is cleared by the negative going pulse of curve D to indicate proper pump tube positioning. The flag would be reset at the start of each operating cycle or may also be reset on the occurrence of the one-third rotation of the rotor sensing current shown by curve B. If the flag is not cleared by the negative going pulse of curve D there is an indication that either there is no pump tube or that the pump tube has been improperly mounted and an alarm signal can be initiated.

Claims: A motor unit for a medical fluid delivery system for use with a disposable delivery set for pumping medical fluid, comprising, pump operating means, including a DC motor (90), for acting in cooperation with the delivery set (14) having a pump tube (20) to deliver a volume of the fluid during each operating cycle; pump control means for controlling the pump operating means to deliver the fluid at a desired volumetric rate, the pump control means including means responsive to signals indicating the desired rate for activating the pump operating means for one of the operating cycles and for repeating the activation at variable time intervals selected in accordance with the desired volumetric rate and means for sensing the condition of the pump operating means characterised in that the means for sensing the condition of the pump operating means comprises means for sensing the AC component of the current supplied to the DC motor (90). A motor unit as claimed in claim 1 wherein the pump operating means further comprises a rotor (18) for contacting a flexible tube (20) to form a peristaltic pump. A motor unit as claimed in claim 1 wherein the operating cycle comprises a selected angular rotation of the motor. A motor unit as claimed in claim 2 wherein the operating cycle comprises a selected angular rotation of the rotor (18). A motor unit as claimed in any one of the preceding claims which further comprises means for comparing the sensed AC component to a reference level thereby to detect current variations resulting from the presence or absence of the pump tube (20). A method for controlling the rate of fluid delivery in a medical fluid delivery system comprising a motor unit (12) including a DC motor and a disposable fluid delivery set (14) for mounting to the motor unit to form a pump, and wherein the motor unit acts on the fluid delivery set in repeated operating cycles to deliver a volume of fluid for each cycle, which method comprises, providing means for detecting the completion of the operating cycle; operating the motor unit until the completion of the operating cycle is detected; and repeating operation of the motor unit at time intervals selected in accordance with the desired rate of fluid delivery; characterised in that the means for detecting the completion of the operating cycle comprises means for detecting the AC component of the DC current provided to the DC motor. A method as claimed in claim 6 further comprising the step of counting the number of the operating cycles and interrupting the step of repeating operation of the motor unit when a number of operating cycles corresponding to a desired volume of fluid has been delivered by the fluid delivery system. A method as claimed in claim 6 wherein the step of providing means for detecting the completion of the operating cycle further comprises the step of forming digital pulses representative of the AC component of the DC current detected by the means for detecting the AC component. A method as claimed in claim 8 wherein the step of operating the motor unit includes the step of operating the motor unit until a selected number of the digital pulses have accumulated. A method as claimed in claim 6 further including the step of inhibiting the detection of the AC component for a selected time. A method as claimed in claim 6 further comprising the step of comparing the detected AC component to a reference level thereby to detect current variations resulting from the presence of a fluid delivery system mounted to the delivery unit.

Inventors: JACKSON EDWARD, PASQUALUCCI JOSEPH, JACKSON, EDWARD, PASQUALUCCI, JOSEPH
Applicants: SHERWOOD MEDICAL CO, SHERWOOD MEDICAL COMPANY
Application year: 1989
Main classification: A61M5
IPC codes: F04B43, A61M5, F04B49