United States Patent 3731679

A portable infusion system uses a disposable piston type syringe as a positive displacement pump. The syringe piston is reciprocally driven by a bidirectional DC motor under control of a battery powered circuit. Different selectable rates of pumping are maintained by controlling the width of bidirectional DC pulses coupled to the DC motor and by monitoring the motor back EMF during the off-time of the pulses. A disposable two-way valve connects the syringe pump with a fluid source and a catheter. Safety circuits protect against deleterious conditions such as the passage of an air bubble or an over-pressure condition.

Wilhelmson, Jack L. (Fenton, MO)
Weichselbaum, Theodore E. (St. Louis, MO)
Braun, Vernon F. (Berkeley, MO)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
128/DIG.1, 128/DIG.12, 128/DIG.13, 417/45, 417/411, 604/123, 604/152
International Classes:
A61M5/145; A61M5/168; A61M5/172; F04B9/02; F04B17/03; F04B49/06; H02P7/29; (IPC1-7): A61M5/00
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US Patent References:

Foreign References:
Other References:

Blum et al. "A Method of Continuous Arterial Infusion", Surgery, 1948, pp. 30-35..
Primary Examiner:
Truluck, Dalton L.
We claim

1. In a portable infusion system for transferring fluid from a fluid source to a catheter, a positive displacement pumping system having disposable parts, comprising: a self-contained source of DC power; disposable syringe means having a barrel and a piston movable in said barrel along a path to move fluid within the barrel, said barrel having a fluid passage opening; disposable valve means having a pump port in fluid communication with said passage opening, input port means connected with said fluid source for passing fluid to said barrel, and output port means for connection with said catheter for passing fluid to said catheter; base means removably holding said syringe means and said valve means to allow disposal after use with a patient; a holding member removably connected to said piston and mounted on said base for reciprocal movement along said path; motor means energized by said DC power source for reciprocating said holding member and said piston, said motor means including gear means connected to said holding member to effect reciprocal movement of said holding member in response to rotation of said gear means, and a DC motor for rotating said gear means, means for selecting one of a plurality of rates of pumping including generator means for generating DC drive pulses having different duty cycles selectable to effect different rates of pumping, circuit means for coupling said DC drive pulses to said DC motor, and voltage variation compensation means including means responsive to decreasing voltage from said DC power source for increasing the duty cycle of at least some of said DC drive pulses.

2. The system of claim 1 wherein said DC motor is a bidirectional DC motor having an armature shaft rotatable in opposite directions when opposite polarity current is coupled to the DC motor, said gear means converting rotational movement of said armature shaft into translational movement which drives said holding member, and circuit means for periodically reversing the polarity of current passed from said DC power source to said DC motor.

3. The system of claim 1 wherein said valve means includes sensing means for detecting the presence of an air bubble, and safety means responsive to detection of an air bubble for terminating the operation of said motor means.

4. The system of claim 1 including over-pressure means for detecting when the pressure of fluid passed to said catheter exceeds a predetermined maximum, and indicator means actuated when said predetermined maximum is exceeded for providing an alarm indication.

5. In an infusion system for transferring fluid to catheter means, a pumping system comprising a source of fluid capable of passing electricity, a valve assembly having a fluid channel with an outlet adapted for connection with the catheter means for directing the flow of said fluid to the catheter means, fluid pump means including a chamber connected in fluid communication with said fluid source and said fluid channel, and drive means in said chamber actuable to effect movement of fluid from said fluid source and pump fluid through said fluid channel to the catheter means, unidirectional valve means connected in fluid communication with said fluid channel to permit fluid flow from said fluid channel to the catheter and prevent fluid flow from the catheter to said fluid channel, said fluid channel including a fluid passageway having a fluid opening contiguous with said fluid channel for admitting fluid into said passageway a distance determined by the pressure of the fluid in said fluid channel, and a pair of electrode means located within said fluid channel and having extensions connectable with an external circuit, at least one of said pair of electrode means being located in said passageway a predetermined distance from said fluid opening so that the presence of fluid at said last named electrode means indicates a predetermined fluid pressure condition.

6. The pumping system of claim 5 further including third electrode means in said channel adjacent the other of said pair of electrodes, and circuit means responsive to the presence of an air bubble between said third and other electrode means to provide a signal indicative thereof.

7. The pumping system of claim 5 wherein said fluid passageway has a closed end opposite said fluid opening for trapping a compressible gas between said closed end and the fluid admitted trough said fluid opening, said last named electrode means being surrounded by said compressible gas when the pressure of fluid in said fluid channel means is less than said predetermined pressure condition.

8. In an infusion system for transferring fluid to catheter means, a pumping system comprising a source of fluid capable of conducting electricity, a syringe having a barrel and a reciprocal plunger slidable in said barrel for moving fluid into and out of said barrel, a valve assembly having a fluid source inlet connected to said source of fluid, a pressure fluid inlet connected to said syringe barrel and in fluid communication with said fluid source inlet, and a channel connected in fluid communication with said pressure fluid inlet and having an outlet connectable with the catheter, first valve means connected in fluid communication with said source of fluid to permit fluid flow from said source to said syringe barrel and to prevent return fluid flow from said syringe barrel to said fluid source, second valve means connected in fluid communication with said channel to permit fluid flow from said channel to the catheter and prevent return fluid flow from the catheter to said channel, and means for reciprocating said plunger to draw fluid from said source into said barrel through said pressure fluid inlet during movement thereof in one direction and supply pressurized fluid to said channel from said barrel during movement thereof in the opposite direction to transfer the fluid to the catheter, said valve assembly including a pressure chamber connected at one end in fluid communication with said channel and closed at the opposite end thereof, an electrode within said chamber, gas disposed in said chamber normally between said electrode and said one end to prevent contact between said fluid and said electrode, said gas being compressible to permit said fluid to contact said electrode upon the occurrence of fluid pressure in said channel of a predetermined value, and circuit means connected with said electrode for detecting the contact between the fluid and said electrode.

9. The infusion system of claim 8 wherein said circuit means includes a second electrode disposed in said channel in contact with the fluid therein.

10. The infusion system of claim 8 further including a pair of closely spaced electrodes in said channel and normally bridged by the fluid to normally provide a predetermined value of impedance between said pair of electrodes, and circuit means connected to said pair of electrodes and responsive to a change in said value of impedance upon the occurrence of the presence of an air bubble between said electrodes to interrupt the reciprocation of said plunger.

This invention relates to an improved pumping system and an improved control circuit, particularly adapted for use in an infusion system.

During typical blood transfusions and intravenous injections, a solution bottle is usually hung above a patient to allow gravity feed of fluid through disposable venoclysis tubing to a catheter inserted in the vein of the patient. Transportation of the patient is difficult because the solution bottle must always be located above the patient, requiring an attendant to hold the solution bottle. Even when the patient is located in a hospital, periodic monitoring of the process is required, utilizing valuable personnel time. Despite periodic monitoring, certain malfunctions can occur which may go unattended for lack of a suitable indication of the malfunction. For example, during an injection, it is possible for a needle to become displaced from its position in a vein and become lodged in a muscle.

In accordance with the present invention, a novel portable positive displacement pumping system replaces the gravity feed system typically used for transfusions and injections. As a result, the solution bottle may be located at any reasonable height with regard to the patient. A novel battery powered control circuit for the pump system includes a number of safety circuits which automatically monitor for deleterious conditions, such as passage of air bubbles or the dislodgement of the intravenous needle into a muscle, eliminating the requirement that an attendant periodically monitor the process. Sterile conditions are easily maintained because the positive displacement pumping system uses a disposable syringe and a disposable two-way valve which can be discarded after use with each patient and replaced with a new sterile syringe and valve.

Some attempts have been made to use disposable piston type syringes for pumping fluids at fixed locations. For example, it has been proposed to drive the piston of a syringe by an AC motor connected to an external AC line source. To control the rate of pumping, adjustment is made of the length of the drive stroke for the piston. Such apparatus is not usable in an infusion system, since air bubbles may be passed to the patient, and other serious malfunctions might occur which could not be automatically cured. Also, such apparatus does not permit priming of the syringe, nor is accurate control possible, as is essential in an infusion system.

The applicants' novel control circuit for driving the novel pumping system includes a unique DC motor drive which can be used to accurately drive loads other than pumps. The drive automatically compensates for variations in the load, long term aging of batteries for powering the control circuit, and detection of deleterious conditions associated with the driven load. Bidirectional motor movement is accomplished by a simple reversing circuit controlled by movement of the motor armature. The control circuit uses the known techniques of driving the DC motor by variable width pulses, and monitoring the back EMF across the motor during the off-time of the pulses to control the on-time width of the pulses. However, a pair of such circuits has heretofore been required when driving a motor in both forward and reverse directions. The applicants' control circuit accomplishes the same degree of control while substantially simplifying the circuit.

One object of this invention is the provision of an improved infusion system in which the sterile parts in contact with the fluid being pumped are disposable and readily replaceable with new sterile parts.

Another object of this invention is the provision of an improved control circuit for driving a DC motor through a bidirectional cycle of operation.

Yet another object of this invention is the provision of improved pump means driven by a DC motor and feedback means for modifying the operation of a control circuit in accordance with external conditions related to the operation of the pump.

Further objects and features of the invention will be apparent from the following specification, and from the drawings, in which:

FIG. 1 is a perspective illustration of an infusion system incorporating the pumping system of the present invention;

FIG. 2 is an exploded view of the pumping system, with the syringe pump being illustrated for clarity as located on the opposite side of the pump housing shown in FIG. 1;

FIG. 3 is a partly plan and partly sectional view of a disposable valve with embedded electrodes;

FIG. 4 is a sectional view taken along lines 4--4 of FIG. 3;

FIG. 5 is a plan view taken along lines 5--5 of FIG. 4; and

FIG. 6 is a schematic diagram of the control circuit for the pump system.

While an illustrative embodiment of the invention is shown in the drawings and will be described in detail herein, the invention is susceptible of embodiment in many different forms and it should be understood that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiment illustrated.


Turning to FIG. 1, a portable infusion system is illustrated for pumping fluids such as blood from a solution bottle 20 to a catheter 21 inserted into the vein of a patient. Fluid transfer is accomplished by a pumping apparatus 24 held by a caddy assembly 26 mounted to a rail 28 of a bed for the patient. The caddy 26 also removably holds the solution bottle 20, which can be located at any reasonable altitude with respect to the patient.

Solution bottle 20 is of conventional construction, and includes a cap 30 having an air valve 31 and an output port 32 for fluid transfer. Disposable venoclysis tubing 34 couples the port 32 to an input port 36 in a disposable two-way valve 40 which forms a part of the pump apparatus 24.

Pump apparatus 24 uses as a pump chamber a conventional disposable syringe 42 having a slidable piston 44 which can be reciprocated to pump fluid within a hollow syringe barrel coupled with the two-way valve 40 which includes an outlet or output port 46 connected by venoclysis tubing 50 with a conventional Y connector 52 for medication introduction. The output of the Y connector 52 is coupled by additional disposable venoclysis tubing 54 to the catheter 21.

The control circuit for pump apparatus 24, seen in detail in FIG. 3, is completely contained within the housing for the pump apparatus, and can be either externally or internally powered. During a back stroke, in which the syringe piston 44 is driven away from the valve 40, input port 36 admits fluids from solution bottle 20 into the syringe barrel. The valve in output port 46 is closed at this time. During a forward stroke, in which piston 44 is driven towards the valve 40, the input port 36 is closed and the output port 46 is opened, pumping the solution through venoclysis tubing 52 and 54 to the catheter 21.

The novel pumping apparatus 24 is seen in exploded view in FIG. 2. A sterile, positive displacement pump is economically formed by using a conventional disposable syringe 42 in combination with a unique disposable valve 40, to be described. Syringe 42 includes a gasket 70 fixedly mounted to the piston 44 for movement within a hollow barrel 72 which has a single fluid opening terminating in a needle connector 74. The syringe includes extending finger grip arms 76, which in the present invention are held by base means for the pump apparatus 24.

Syringe 42 and valve 40 are removably held by housing means in order to allow disposal after use with each patient and replacement with a new presteriled syringe and valve. A lower molded case 90 includes a pair of upstanding arms 92 each having a slot channel 94 which slidably receives one of the extensions 76 of the syringe. Lower case 90 also includes an upstanding post 100 having a concave surface 102 for holding the valve 40 when it is mated to the syringe 42, and for making electrical contact with electrodes embedded in the valve. A pair of female electrical sockets 106 in surface 102 receive air bubble detector electrodes, as will appear, and a female socket 108 (not illustrated in FIG. 2), which may be separate from case 90 or similarly molded in a portion thereof, receives an over-pressure detector electrode. The sockets 106 and 108 are connected by wires to the circuit of FIG. 3 which is contained within the hollow case 90.

The mechanical drive arrangement for piston 44 consists of a bidirectional DC motor 120 having an armature shaft 121 with an integral motor gear 122. The motor gear 122 meshes with an idler gear 126 rotatable about an idler shaft 128 rigidly attached to a pinion gear 130. The pinion gear 130 meshes with a drive gear 132 which is fixedly attached to the shaft of a jackscrew 134. A syringe cylinder carrier 140 includes a gripping head 142 having an opening therein for slidably receiving the head 45 of piston 44. The carrier 140 has an internally threaded central opening for engaging the threads of the jackscrew 134 to cause the carrier to act as a drive nut on the jackscrew.

When DC motor 120 is energized by voltage of predetermined polarity, the two-stage spur reduction gears rotate jackscrew 134 and cause the carrier 140 and attached cylinder 44 to be driven in a forward stroke. Carrier 140 includes a protrusion 150 with a permanent magnet which extends downward for magnetically actuating a sealed forward stroke limit switch 152 and a sealed reverse stroke limit switch 154, mounted to a circuit board 156 which contains the circuit of FIG. 3. The carrier 140 is driven in a forward stroke direction until protrusion 150 is directly over limit switch 152, at which time the circuit of FIG. 3 reverses the polarity of voltage to DC motor 120 in order to rotate armature 121 in a reverse direction. The carrier 140 and cylinder 44 are now longitudinally moved through a back stroke until the protrusion 150 is directly over limit switch 154, at which time the circuit of FIG. 3 again reverses the polarity of voltage to DC motor 120. While magnetically actuated proximity switches are preferred, a mechanical switch arrangement could alternately be used, actuated by mechanical engagement with protrusion 150.

Power for the DC motor 120 and the control circuit is obtained from a self-contained DC power source, as a pair of series connected DC batteries 160. Desirably, batteries 160 are rechargeable, sealed nickel-cadmium batteries which allow the pump apparatus to be powered either from an external AC source, or internally powered in order to allow the unit to be completely portable. If the unit is constructed for portable use only, the batteries 160 may be conventional 1.5 volt "D" size. The DC batteries 160 are housed within a battery retainer cylinder 162 molded in lower case 90. Electrical connection is made through a battery contact spring 164 and a contact on a battery retainer cap 165 which threads into the battery retainer cylinder wall to allow replacement of the batteries when necessary.

An upper case 170 mates with the lower case 90 to enclose the drive train assembly and the batteries 160. Case 170 includes a window 172 through which indicia on a thumbwheel knob 174 may be observed in order to allow operator selection of different rates of pumping fluid. Desirably, the indicia on wheel 174 directly indicate pump rate, such as one liter of fluid per one, two, three, etc., hours. A different range of pump rates may be provided by replacing syringe 42 with a syringe of different capacity, and knob 174 may be so marked with alternate indicia. A syringe prime switch 176 allows an operator to override the setting selected by wheel 174 in order to rapidly reciprocate the piston 44 when first priming the syringe 42 to eliminate air bubbles. During the time the switch 176 is actuated, the air bubble protector circuit is disabled.

Disposable Valve Assembly

The disposable valve assembly 40 is illustrated in detail in FIGS. 3-5. The assembly is economically formed by using a pair of identically manufactured valve units 190 mated in opposite fluid flow directions with a central fluid channel unit 192 so that one valve unit 190 forms input port 36 and the other valve unit 190 forms output port 46.

Each valve unit 190 includes a fluid input port having a tapered conical wall 194 which directs fluid to a check valve 195 formed of flexible, resilient material such as rubber. Check valve 195 is formed by a hollow center portion with an integral tapering nose 196 terminating in a rectangular slit opening 197 which passes fluid to an output port defined by a tubular wall 200 which also serves to anchor the hollow center portion of the check valve. The check valve 195 is of conventional construction and allows fluid flow in a direction from the input port defined by conical wall 194 to the output port defined by the tubular wall 200, but collapses to block fluid flow in an opposite direction.

The central fluid channel unit 192 includes an input fluid channel 202 into which is inserted the output port of the valve unit 190 which forms input port 36. Opposite input fluid channel 202 is an output fluid port or channel 203 having a conical wall which receives the tapered syringe connector 94. Contiguous with fluid channels 202 and 203 are an output fluid channel 204 and a closed fluid channel 206. Channel 204 terminates in a neck portion 208 of reduced diameter which mates with the input port of the check valve 190 which serves as the output port 46 for passing fluid flow to the catheter.

To detect the presence of an air bubble in the fluid channel, a pair of metal rods or electrodes 212 extend through the wall of the valve assembly and into the fluid channel 204. The electrodes 212 are spaced apart approximately 0.25 inches, and are placed ahead of the output functioning check valve 195. When fluid of 0.001 percent salinity or higher is present between the electrodes 212, the fluid completes a resistance path of sufficiently low impedance to allow the circuit of FIG. 6 to continue to operate. When an air bubble of predetermined size passes the electrodes, the impedance rises and breaks the circuit to cause the forward stroke of the pump to terminate.

To detect an over-pressure condition, as is caused when the catheter becomes lodged in a muscle, the closed fluid channel 206 forms a pressure detector. A cap 217 closes the end of fluid channel 206, trapping air between the cap 217 and the fluid which enters the channel 206. A single metal rod or electrode 220 is embedded through the wall of the valve assembly and into the fluid channel 206. When an over-pressure condition occurs, the pressure of the fluid within central channel unit 192 further compresses the trapped air and allows fluid to further enter the closed channel 206 until it contacts the electrode 220, thereby completing a circuit through the fluid to one of the electrodes 212 in order to indicate an over-pressure condition.

Desirably, electrodes 212 and 220 are an integral part of the valve assembly 40, rather than a part of the syringe 42. As a result, a conventional disposable syringe of low cost may be used as the pump. The valve assembly itself may be economically molded of plastic, except for the pair of check valves 190 which may be molded of rubber. The externally extending ends of the metal electrodes 212 and 220 are directly inserted in the female sockets 106 and 108, respectively, as previously described.

Control Circuit

The control circuit for the pump assembly is illustrated in detail in FIG. 5. DC power is provided between a DC potential line 248 and a source of reference potential or ground 250. When external 115 volt AC is available, a plug 256 may be inserted into the external AC source so as to couple 115 volt AC to a stepdown transformer 258. The transformer is connected through a full wave diode rectifier to a line 260 connectable through a socket with line 248. The rechargeable batteries 160 form a filter capacitor for the full wave rectified AC voltage, reducing the ripple of the voltage on DC line 248. If desired, an additional filter capacitor 262 may be provided. The stepdown transformer 258 and full wave rectifier may be housed within the plug 256, and connected through a two-line cord to the socket receptacle on the pump assembly. When the pump assembly is to be used independent of the external AC source, the line plug is simply removed from the receptacle on the pump assembly, allowing the previously recharged batteries 160 to thereafter power the control circuit.

DC motor 120 is a shunt wound permanent magnet motor which rotates in a forward direction when current flows from a terminal 260 to a terminal 262, and rotates in a reverse direction when current flows from terminal 262 to terminal 260. As will appear, the motor is driven by pulses having a less than 100 percent duty cycle. During the off-time of the pulses, the motor 120 acts as a generator or tachometer, and the back EMF across the terminals is sensed and stored in order to control the duty cycle of the drive pulses.

An electronic reversing switch, including transistors 265, 266, 267, 268, 269, and 270 forms a double-pole, double-throw switch. Transistors 265 and 268 are synchronously driven conductive to pass current in a forward direction through motor 120. Alternatively, transistors 266 and 267 may be synchronously driven conductive to complete a reverse current path for motor 120 to drive the motor through its reverse or back stroke. When transistors 265 and 268 are on, current passes from a positive line 275 through transistor 265 to terminal 260 of motor 120, through motor 120 and out terminal 262 to transistor 268, and thence to ground 250. When the forward limit of travel is reached, as indicated by the permanent magnet on protrusion 150 actuating limit switch 154, a reversing switch driver, to be described, turns transistors 265 and 268 off and transistors 266 and 267 on. Current then flows from the positive line 275 through transistor 266 to terminal 262, and thence through motor 120 and out terminal 260 to transistor 267 and thence to ground 250.

The reversing switch driver, consisting of transistors 280, 281, 282, and 283, acts as a regenerative bistable switch useful to obtain the heavy drive capability which is necessary when using low supply voltage, such as 3.0 volts from the pair of batteries 160. Transistors 280 and 283 drive each other into saturation when magnetic protrusion 150 actuates switch 152 at the end of a back stroke, grounding the base of transistor 282. Alternatively, transistors 282 and 281 drive each other into saturation when magnetic protrusion 150 actuates the switch 154, grounding the base of transistor 283 at the forward stroke limit of travel.

When transistor 281 saturates, current flows from its emitter to base and through a resistor 290 to the base of transistor 267 to provide drive for the reversing switch. At the same time, the voltage at the collector of transistor 281 rises to the potential of line 275, back biasing transistors 269 and 265. Transistor 282 is also saturated at this time, causing current to flow through the emitter-base of transistor 266, through a resistor 292 and via a line 293 to the collector of transistor 282 and thence to ground 250. This provides drive for the other half of the reversing switch. Since the collector voltage of transistor 282 is at approximately ground potential, no current flows through a resistor 295 to transistor 270, nor transistor 268. When the opposite stable state of the bistable is set by magnetic protrusion 150, transistors 280 and 283 act similar to the above described operation for transistors 281 and 282, providing drive for transistors 265 and 269, and transistors 268 and 270, as will be explained with reference to the bubble detector circuit.

During the forward stroke, transistor 270 is driven on by pulses having approximately a 25 percent duty cycle. For one circuit which was constructed, the drive pulses for minimum motor speed had a four millisecond on-time out of a sixteen millisecond interval, producing a sixty hertz frequency. The duty cycle during the forward stroke is adjustable, as will appear, and is controlled by a forward stroke control.

The reverse stroke always occurs at maximum speed since transistors 266 and 267 are fully saturated during reverse motor movement. As the DC voltage from batteries 160 slowly drops with age and use, lesser voltage is passed through the reverse stroke transistors 266 and 267 to the DC motor 120, resulting in a decreased speed of movement. As will appear, a battery voltage variation compensation circuit is responsive to decreased battery voltage to decrease the off-time of the pulses controlled by the forward stroke control, thus increasing speed in the forward stroke in order to maintain the selected rate of pumping.

The forward stroke control includes transistors 300, 301, 302, 303, and 304, connected basically as an unsymmetrical astable multivibrator. To allow selection of different rates of pumping, thumbwheel knob 174 is connected to the wiper 310 of multi-position switches 312. Wiper 310 is connected to any one of a plurality of resistors 315 each having a different resistance value. A master OFF switch 316 when actuated connects the wiper 310 to DC line 248, via prime switch 176. When the thumbwheel 174 is rotated to cause the wiper 310 of switch 312 to contact one particular resistor 315, a path is formed from DC line 248, through actuated switch 316 and unactuated switch 176 to wiper 310, and thence through the selected resistor 315 to the emitter of transistor 300. The collector of transistor 300 is connected through a capacitor 317 and thence through the collector-emitter of transistor 301 to ground 250. The duty cycle of the pulse coupled to transistor 270 is determined by the capacitance of capacitor 317, the selected value of resistor 315, and the voltage at the base of transistor 300 (from the velocity feedback circuit as will appear).

The on-time of the duty cycle is determined by the time period transistors 301 and 303 are saturated and transistors 302 and 304 are turned off. Transistor 300 acts as a controlled current source that discharges capacitor 317 during the time it holds transistor 304 turned off. When transistor 301 turns on, transistor 303 is turned on by current flowing from its base and through a resistor 320 and conducting transistor 301 to ground 250. Transistor 303 drives transistor 270 of the reversing switch driver through a resistor 322. Thus, the on-time of the duty cycle which controls the forward stroke of the motor is determined by saturation of transistor 303.

The off-time of the duty cycle is controlled by saturation of transistor 304, at which time transistors 301 and 303 are turned off. This off-time is determined by the capacitance value of a capacitor 325, the voltage to which the capacitor 325 is allowed to charge during the prior on-time, and the resistance values of a pair of series connected resistors 327 and 328. The allowable voltage to which capacitor 325 is allowed to charge is set by the battery voltage variation compensation circuit, to be explained.

The detailed operation of the forward stroke control circuit is as follows. Assume transistor 301 has just turned on with capacitor 317 fully charged and capacitor 325 fully discharged. When transistor 301 saturates, the negative terminal of capacitor 317 has a negative voltage equal to the supply potential. For this example, it will be assumed that the supply potential from batteries 160 is at maximum potential, or 3.0 volts. Current now flows from the +3.0 volt supply and through switches 316, 176 and 310 to the selected resistor 315 and thence through transistor 300 to discharge capacitor 317. When the negative terminal of capacitor 317 reaches 1.2 volts (the base-emitter drop of transistors 302 and 304), transistors 302 and 304 are turned on, turning transistor 301 off and recharging capacitor 317 to supply voltage through a resistor 330. Capacitor 325 discharges through the series resistors 327 and 328 until the base-emitter voltage of transistor 301 is reached, at which time transistor 301 turns on and the cycle is repeated.

During the forward stroke, the pulse coupled to the DC motor has an approximately 25 percent off-time at the maximum infusion rate selectable by switch 310. Due to mechanical inertia, the motor continues to turn and generates a back EMF proportional to the angular velocity of the armature. This voltage is sensed by a velocity feedback circuit and stored in order to control transistor 300 and adjust the on-time of the pulses to compensate for variations in load. Thus, various fluids and syringes may be used without effecting to any significant extent the calibration of thumbwheel knob 174.

During the forward stroke, transistor 265 is on, connecting terminal 260 to the supply voltage at line 275. During the off portion of the forward stroke pulse, transistor 270 is off, blocking transistor 268 and disconnecting ground 250 from the motor terminal 262. The back EMF across the motor terminal is now coupled through a resistor 335 and a pair of series connected diodes 336 and 337 to a capacitor 340 connected to ground 250. The capacitor 340 charges to a potential that is the sum of the supply voltage and the voltage generated by the motor.

During the on-time of the forward stroke control, transistor 270 is driven into conduction, driving transistor 268 into conduction and hence connecting motor terminal 262 to approximately ground potential, back biasing the diodes 336 and 337. The voltage charge across capacitor 340 is now used to control the base drive of transistors 300, establishing an on-time duration proportional to the voltage across the capacitor. A resistor 342 allows the voltage across capacitor 340 to slowly leak off. Since the emitter of transistor 300 is referenced to the DC supply voltage, the current through transistor 300 is dependent solely on the back EMF across the DC motor, eliminating the effect of supply voltage variations.

The control circuit also includes a number of special circuits described in the following sections.

Bubble Detector

The bubble detector circuit includes the bubble detector electrodes 212 and transistors 350 and 351. When fluids having a conductivity equal to a salinity of 0.001 percent or greater are present between electrodes 212 which are spaced 0.25 inches apart, the resistance therebetween is on the order of 200 kilohms or lower. This causes current to flow from the supply line 275, through the emitter-base of transistor 350, through a resistor 352, as 10 kilohms, to one electrode 212 and thence through the fluid to the other electrode 212 to charge a capacitor 353, as 1.0 microfarads. Capacitor 353 is discharged by the forward stroke control circuit through a diode 355. The time constants are chosen such that capacitor 353 is never charged to more than 0.1 volts unless the forward stroke control circuit fails. If the forward stroke control circuit fails in such a way that the forward stroke would be at full supply voltage across the motor 120, capacitor 353 charges to supply voltage and turns transistor 350 off. This terminates operation. Thus, the patient is protected from excessive infusion rates which otherwise might be caused by failure of critical parts in the circuit. The current passing through the fluid is on the order of 10 microamps or less thereby creating no hazard of electrolysis or other hazard to the patient.

The current that charges capacitor 353 causes a current of at least 200 times magnitude to flow from the supply, through the emitter-collector of transistor 350, through a resistor 357 and into the base of transistor 351. This forward biases transistor 351, creating a path to ground through the transistor 351 and a resistor 358 connected to the base of transistor 269, thereby allowing drive for transistors 269 and 265 to flow when the transistors 269 and 265 are turned on by the reversing switch driver circuit. When an air bubble or cavity is present between the electrodes 212, the current path is broken and transistor 351 is biased off. Therefore, the motor 120 stops on the forward stroke. Prime switch 176 in the forward stroke control circuit is used to override this shut-off during syringe priming.

The combination of the bubble detector circuit and the placement of the electrodes 212 and 220 in the two-way valve assembly 40 creates a fail safe apparatus which detects air leaks caused by a defect in the pump assembly itself. Referring to FIG. 4, the electrodes 212 are located in the pressure side of the fluid channel, between the pair of check valves 195. Should the metal electrodes 212 not be completely surrounded by the plastic material forming the wall of the valve channel, as might occur due to dropping of the valve assembly, for example, an air passageway or void would be created which would allow air to seep from the atmosphere into the fluid channel 204. If the electrodes 212 were located in input port 36 upstream of the check valve 195, the electrode located furthest downstream could leak air during a back stroke operation. If the bubble should pass the check valve 195, it would escape detection by the bubble detector circuit.

To prevent such an occurrence, the electrodes 212 are located in a region which has high pressure during a forward stroke. During the forward stroke, the pressure in channel 204 is in excess of atmospheric pressure, therefore an air passageway adjacent either electrode 212 merely causes fluid to seep out of the channel 204, but does not create an air bubble within the channel. During the back stroke, a low pressure region is formed in fluid channel 204, allowing air to seep from the atmosphere into the channel 204. Regardless of the electrode 212 which leaks air, the bubble will travel upstream towards the pump port 203, so that the bubble will again have to pass the electrodes 212 during the forward stroke. This allows the air bubble to be detected in the same manner as if the bubble had been drawn in from the fluid supply.

The bubble detector control circuit serves the dual purposes of providing a safety device to prevent accidental passage of an air bubble, and also automatically shuts off the pumping apparatus when all the fluid in the solution bottle is used up. At the end of the supply of fluid, air is introduced into the solution bottle and is pumped to the valve assembly 40. When the air reaches the point where the two sensing electrodes 212 are placed, the current path is broken and motor operation is terminated, turning off the pumping system.

Battery Voltage Variation Compensation

This circuit, consisting of transistors 370 and 371, is responsive to decreases in the battery voltage to decrease the off-time of the pulses in the forward stroke control. As previously described, the back stroke is not controlled and will vary in speed with voltage variations. The time lost on the back stroke is gained by speeding up the motor on the forward stroke.

Transistor 371 acts as an ideal diode, establishing a reference voltage of approximately 0.6 volts at its collector, which is coupled in series through a resistor 376 and a resistor 377 to a line coupled through switch 316 with the positive potential line 248. In shunt with resistors 376 and 377 and transistor 371 is a resistor 380 in series with the emitter of transistor 370, and a resistor 382 in series between the collector of transistor 370 and ground 250. The collector of transistor 370 is directly coupled to the junction between transistor 304 and capacitor 325. As the battery supply voltage lowers, the current through resistor 380 changes linearly. This causes the collector voltage of transistor 370 to rise linearly at a rate established by the ratio of resistor 380 to resistor 382 and a resistor 384 in series between the collectors of transistors 303 and 304. The voltage at the base of transistor 370 also varies, but at reduced ratio.

For the particular motor driven mechanism which was constructed, the circuit constants were chosen so that the voltage on the collector of transistor 370 and hence also transistor 304 lowered as the battery voltage lowered by a ratio of 1.5, that is, 0.1 volt battery variation produced 0.15 volts less charge on capacitor 325. While this ratio produced the correct compensation, other ratios may be utilized for other loads driven by the motor. The range of the battery voltage compensation circuit is such that battery voltages down to approximately 2.0 volts may be tolerated, representing a decrease of 33 percent from the full battery voltage of 3.0 volts.

For the circuit constants disclosed above, a battery supply voltage of less than 2.0 volts indicates that the batteries must be replaced or recharged in order to maintain the calibrated accuracy of the pumping apparatus. A low battery indicator circuit is formed by integrated circuit NOT gates 390 and 391 for energizing a low battery indicator lamp 393. When the supply voltage is above 2.0 volts, a divider formed by resistors 395 and 396 in series between ground 250 and the supply line via switch 316 and line 248 produces a voltage above 0.8 volts at the junction between resistors 395 and 396 which causes NOT gate 390 to saturate, turning NOT gate 391 off and thus maintaining the lamp 393 off. When the supply voltage drops to 2.0 volts, gate 390 turns off, causing gate 391 to turn on and hence energize the lamp indicator 393. The indicator lamp 393 is desirably located beneath a window in the upper case of the pump assembly so as to be visible by an operator.

Over-Pressure Detector

This circuit is formed by integrated circuit gates 400, 401 and a transistor 372. Gates 400 and 401 are connected to form a bistable multivibrator. During normal operation (no over-pressure condition), gate 401 is on and transistor 372 is off. To insure this state, a capacitor 405 is made five times as large as a capacitor 406. When the control circuit is first energized, the capacitor 405 holds one input of gate 400 low long enough to set the bistable with gate 401 saturated and gate 400 off.

When fluid reaches electrode 220, indicating an over-pressure condition, a circuit path is formed from one input of gate 400 to the supply voltage line 275 via transistor 350 and the electrode 212 connected through resistor 352 to the base thereof, saturating gate 400 and turning gate 401 off. This turns transistor 372 on, turning off transistor 351 which in turn opens the bias path for transistors 269 and 265. This stops the system on the forward stroke. The over-pressure detector circuit may be reset by turning the control circuit off and back on, causing capacitor 405 to again saturate gate 401.

Bubble and Over-Pressure Indicator

This circuit consists of transistors 310 and 411 which control energization of a visual indicator, such as a light emitting diode (LED) 413. Desirably, a light emitting diode is used rather than an incandescent lamp due to its low power consumption. When an air bubble or an over-pressure condition is detected by the circuits previously described, transistor 351 is turned off. This in turn biases off transistor 410, ungrounding a junction formed between a resistor 415 and a diode 416 connected in series between the anode of LED 413 and the base of transistor 411. The transistor 411 is thus forward biased, creating a current path for the LED 413 to ground through a resistor 420 and the collector-emitter junction of the conducting transistor 411. The LED 413 is located adjacent a jewel lens mounted in case 170 in order to give a visual indication of a circuit shut-off caused by the detection of an air bubble or an over-pressure condition.

For some applications, it may be desirable to include less than the number of individual circuits described above, or to include various combinations thereof, as will be apparent to one skilled in the art.