CA1184821A

CA1184821A - Stepping motor control procedure for achieving variable rate, quasi-continuous fluid infusion

Info

Publication number: CA1184821A
Application number: CA000413946A
Authority: CA
Inventors: Edward G. Solomon
Original assignee: Oximetrix Inc
Current assignee: Oximetrix Inc
Priority date: 1981-10-22
Filing date: 1982-10-21
Publication date: 1985-04-02
Also published as: AU8925682A; GB2107800B; GB2107800A; US4474309A; FR2515273A1; DE3239190A1; JPS5881046A; FR2515273B1; AU552309B2; NL8204000A; DE3239190C2

Abstract

STEPPING MOTOR CONTROL PROCEDURE FOR
VARIABLE RATE, QUASI-CONTINUOUS
INTRAVENOUS ADMINISTRATION

Abstract A control procedure for governing the incremental step operation of a pump stepping motor (74) employed in conjunction with a parenteral fluid metering device (2) is disclosed. The control procedure enables fluid to be pumped from the metering device (2) in non-pulsatile, essentially continuous fashion by suitably adjusting the stepping motor speed to compensate for the interruption of fluid flow which occurs during the refill phase (Pr) of each pumping cycle (C). Adjustment of the stepping motor speed also compensates for non-linearities otherwise introduced into the fluid flow as a result of the camming mechanism (76,78) utilized to convert rotary stepping motor movement into rectilinear pumping movement. Operating power for the stepping motor is conserved by drawing rela-tively large amounts of current through the stepping motor (74) only at the outset of each incremental step while drawing relatively smaller amounts of current through the stepping motor (74) during the remaining interval between incremental steps.

Description

4~
STEPPING MOTOR CONTROL PROCEDURE F'OR
ACHIEVING VARIABLE RATE, QUASI-CONTINUOUS

2 FLUI D INFUS ION
,

3 Technical Field

4 The present invention is directed to a means and method for administering fluids to patients intravenously 6 or intraarterially and more particularly concerns a control 7 mechanism and a procedure such that carefully metered 8 portions of fluid are delivered in quasi-continuous fashion 9 to patients.
Backqround Art 11 Considerable attention has been focused in recent 12 years on intravenous and intraarterial delivery of fluids 13 to patien-ts. Precision control over the rate at which 14 such paren-teral delivery occurs is of critical import-ance, inasmuch as improper administration of fluids can 16 retard the recovery of patients or, in extreme situations, 17 lead to further injury or even death. Early parenteral 18 delivery systems relied on gravity flow to transfer fluid 19 from a fluid container or reservoir to the patient.
Attempts to accurately regulate gravity flow, however, 21 proved difficult because the pressure forcing the fluid 22 between the reservoir and the patient decreased as the 23 fluid level within the reservoir dropped during the delivery 24 operation. Thus, delivery rates in gravity-flow systems tended to vary in an unacceptable manner.
26 More recent parenteral dellvery systems have employed 27 pump motors in an effort to increase fluid delivery rate 28 accuracy. Often, the pump motors comprise stepping motors 29 which drive plunger or piston-like fluid pumps in response to suitahle stepping motor control procedures. These procedures ,,~

are highly compatible with the precision control requirements of parenteral administration because they provide the neces-sary degree of accuracy and are capable of implementation through reliable and efficient microprocessor programming techniques. U.S. Patent Nos. 4,037,598 issued to Georgi on July 26, 1977; 3,994,294 issued to Knute on November 30, 1976;
3,985,133 issued to Jenkins et al on October 12, 1976 and 3,736,930 issued to Georgi on June 5, 1973 all disclose intra-venous delivery systems wherein stepping motors are utilized in conjunction with camming mechanisms and pumping structures to achieve accurate delivery rate control. In spite of the advantages offered by prior art systems, however, certain improvements in the fluid delivery characteristics of existing delivery systems can be made. For example, none of the step-ping motor control procedures associated with the stepping motors of the aforementioned patents compensate for non-linearities introduced into the fluid delivery rate as a result of interaction between the camming mechanisms, the stepping motors, and the pumping structures. Nor are efforts made to minimize the pulsatile discontinuities in fluid flow brought about bv the interruption of fluid delivery during those portions of each pumping cycle devoted to refilling the pumping structures. Consequently, the prior art fails to provide a parenteral administration system capable of pumping precise amounts of fluid at linear rates in essentailly continuous fashion.
Disclosure of The Invention It is therefore the object of the present invention to provide a control procedure for governlng the operation of a pump motor in a parenteral fluid delivery system.

~84~

1 It is another object of the present invention 2 to provide a motor control procedure for use with a parent-3 eral fluid pump motor and camming mechanism wherein essen-4 tially linear fluid delivery rates are achieved despite the non-linearities inherent in the conversion of rotary 6 motor movement to rectilinear pumping movement.
7 It is yet another object of the present invention 8 to provide a pump motor control procedure for use with 9 a parenteral fluid pump motor and camming mechanis~ wherein the speed of the motor is varied in accordance with the 11 angular position of the camming mechanism in order to 12 compensate for non-linearities which occur in the conver-13 sion of rotary motor movement to rectilinear pumping 14 movement.
The foregoing objects are achieved by providing 16 a pump controller mechanism that includes a stepping 17 motor connected to a plunger through a camming means 18 with the camming means driving the plunger through a 19 plurality of rectilinear distances as the motor rotates the camming means through a plurality of e~uiangular 21 distances. The rectilinear distances at the ends of 22 the pumping stroke are shorter than those at the center 23 of the stroke, and hence a control means is provided 24 to drive the motor at a higher rate at the ends of the stroke to accommodate this and achieve a generally uniform 26 flow.
27 In another aspect of the present invention, the 28 time re~uired for refilling the pumping chamber is also 29 taken into account by initially driving the stepping motor at a high rate for a time sufficient to make up I the time required for filling so that the motor can return 2 to the preset rate which is the desired delivery rate 3 for the entire pumpin~ cycle.
4 Brief Description of the Drawings These and other objects of the present invention 6 can best be understood by examining the following Brief 7 Description of The Drawings and Best Mode For Carrying 8 Out The Inventionl in which:
g Figure 1 is a perspective view illustrating the parenteral delivery system of the present invention;
11 Figure 2 is a cross-sectional view of the pumping 12 cassette, valve stepping motor and main stepping motor 13 employed in the parenteral delivery system of Figure l;
14 Figure 3 schematically depicts a motor controller for governing the operation of the valve stepping motor 16 and main stepping motor of Figure 2, whereby the amount 17 of current drawn through both the valve stepping motor 18 and main stepping motor is alternatively switched between 19 high and low values in order to conserve energy;
Figure 4 is a graph representing the interrelated 21 operating sequences of the valve stepping motor and the 22 main stepping motor;
23 Figure 5 is a graph illustrating the changes in 24 fluid delivery rate which occur over a single pumping cycle when the control procedure of the present invention 26 is implemented in the flui.d delivery system of Figures 27 1, 2 and 3;

''~' Figure 6 is a flow-chart illustrating one method for achieving the fluid delivery rates graphically depicted in Figure 5;
Figure 7 provides a perspective view of the mechanical interface between the cam connected to the main stepping motor and the plunger which reciprocates to furnish a rectilinear pump-ing force in response to the rotation of the cam; and Figure 8 is a graphic illustration of the camming curve associated with the cam of Figure 7.
Best Mode For Carrying Out The Invention One type of parenteral fluid metering device for delivering controlled amounts of fluid to a patient is schematically illustrated in Figures 1 and 2. Basic and improved embodiments of the fluid metering device are disclosed in co-pending Canadian Application Serial Nos. 383,019 and 402,120; respectively filed on July 31, 1981 and May 3, 1982.
Both of these co-pending applications are assigned to the assignee of the present invention. For the sake of convenience, only the improved fluid metering device of application Serial No.
402,120 will be described in any detail, although it is under-stood that the control procedure of the present invention can be suitably modified for use with the fluid metering device of application Serial No. 383,019. Referring first to Figure 1 the fluid metering device 2 is shown positioned within a metering device control unit 4. An in-flow conduit 6 on fluid metering device 2 is connected to a container of fluid 8 by means of conventional tubing 10. Tubing 12, extending from out-flow conduit 14 of the fluid metering device 2, transfers precise amounts of fluid to the patient being -treated in res-ponse to actuation of a stepping motor and camming mechanism (not shown in Figure 1) housed in control unit 4.
Turning to Figure 2, the construction of the fluid metering device 2, as well as the stepping motor and camming mechanism, is shown in greater detail. Fluid metering device 2 includes a hollow cassette structure 16 having a pumping chamber 18 disposed therein. A resilient -diaphragm 20 is secured across the top of pumping chamber 18.
An inlet port 22 at one end of passageway 24 formed in gas retention conduit 26 permits fluid to pass from a gas reten-tion chamber 28 into pumping chamber 18. Gas retention cham-ber 28 in turn fluidically communicates with in-flow conduit 6 through an intermediate passageway 30. A valve actuator 32 operatively connected to valve stepping motor 34 via a cam and shaft mechanism 36, 38 controls the admission of fluid into pumping chamber 18 by displacing a portion 40 of resi-lient diaphragm 20 positioned above inlet port 22. Current is supplied to valve stepping motor 34 by a power control cen-ter 42 whi.ch may be alternatively connected to an AC power supply 44 or a battery supply 46. Valve stepping motor 34 is driven through a series of incremental steps in response to commands received from motor controller 48, whereupon valve actuator 32 reciprocates to move the diaphragm portion 40 between an open position, as indicated by solid lines in Figure 2, and a sealing engagement with a valve seat 50 formed around the peri-phery of inlet port 22, as indicated in phantom in Figure 2.
A blasing means such as spring 52 seated on hollow boss 54 formed in control unit 4 provides the necessary force for urging valve actuator 32 into positive contact with the camming surface 56 of cam 36.

An outlet port 58 is formed in pumping chamber 18 opposite inlet port 22. Outlet port 58 communicates with out-let conduit 14 through an intermediate passageway 60. A
ball check 62 is mounted between outlet port 58 and inter-mediate passageway 60. A biasing means such as spring 64urges the ball check into sealing engagement with a valve seat 66 formed around the periphery of outlet port 58. A
projection 68 formed on resilient diaphragm 20 opposite ball check 64 displaces the ball check from valve seat 66 during pump priming operations. A manual latch valve 70 is used to move projection 68 into contact with the ball check.
Motive power for pumping fluid through the cassette 16 of 1uid metering device 2 is supplied by a plunger 72 operatively connected to a main stepping motor 74 via a cam and shaft mechanism 76, 78. Main stepping motor 74 also re-ceives current from power control center 42 under the command of motor controller 48. One end 80 of plunger 72 contacts resilient diaphragm 20 while the remaining end 82 is urged into positive contact with the camming surface 84 of ca~ 7~ by a biasing means such as spring 86 seated on hollow boss 88 of control unit 4. The incremental or step rotation of main stepping motor 74, and hence of cam 76, drives plunger 72 in a reciprocal fashion between a fully retracted position, indi-cated by solid lines in Figure 2, and a fully extended posi-tion indicated in phantom at 90 in Figure 2. Resilient dia-phragm 20 flexes in response to the reciprocal motion of plunger 72 to periodically vary the volume of pumping chamber 18, thereby providing the pumping action necessary to drive a metered amount of fluid from the pumping chamber into the fluid outflow conduit 14.

~34~

The motor controller 48 which governs the operation of valve stepping motor 34 and main stepping motor 74 is schematically illus-trated in Figure 3. ~lotor controller 48 includes a microprocessor 92 connected by a data link 94 to a read-only-memory 96. Suitable control procedures for the valve stepping motor 34 and the main step-ping motor 74 are stored in read-only-memory 96 and supplied to microprocessor 92 on demand. The microprocessor in turn directs a pair of octal latches 98, lOO to drive the valve stepping motor and the main stepping motor respectively through their incremental s~eps in accordance with the control procedures stored in the read-only-memory. Sensing means 102 and 104 provide a count of the number of steps taken by the valve stepping motor and the main stepping motor, enabling microprocessor 92 to determine the incremental step position of each. Pump run-away prevention circuit 106 is connected to sensing means 102 and monitors the speed of main stepping motor 74 to prevent the occurrence of potentially dangerous fluid over- or under-delivery conditions. Finally, multiplexer 108 is connected to various data sensors such as the plunger pressure transducer (not shown) disclosed in aforementioned Serial No. 402,120. A/D converter 110 converts the signals from multiplexer 108 into a form useable by microprocessor 92, feeding the signals so converted to the microprocessor via data link 111 .
Octal latch 100, under the direction of microprocessor 92 in Figure 3, is connected to high and low current drivers 112, 113 via leads 11 - 18. The high current drivers 112 comprise a set of transistor drivers 114, 116, 118 and 120 respectively connected in series with the motor windings Wl, W2, W3 and W4 of main stepping motor 74. The low current drivers 113 comprise a simi.lar set of transistor drivers 122, 124, 126 and 128 respectively Gonnected in series with windings Wl - W4 through a set of resistors 130, 132, 134 and 136. For each incremental step taken by main stepping motor 74, octal latch 100 generates a combination of control signals along leads 11, 12, 13 and 14

5 to switch high current drivers 114-120 on. Thereafter, a relatively large current is drawn from the power control center 42 through the main stepping motor windings Wl - W4 and drivers 114-120 to provide sufficient energy for rotating the main stepping motor from one incremental step position to the next.
It should here be noted that the energy required by main stepping motor 74 in effecting the downward stroke of plunger 72 (not shown in Figure 3) is not constant throughout each incremental step of the main stepping motor; but rather varies as a function of the elastic characteristics of resilient diaphragm 20 (also not shown in Figure 3). That is, during the interval between each incremental step, the resilient diaphragm attempts to return to its original or undeformed position, and in so doing exerts a force against plunger 72 tending to move the plunger back to a retracted position. At the beginning of the next incremental step, a relatively greater amount of energy must be expended to overcome the force exerted by the diaphragm before plunger 72 can continue its downward movement. Once an incremental step has been completed, main stepping motor 7~ again moves into a holding or "quiescent" mode and less energy is required to maintain the plunger and diaphragm at the displaced position associated with that incremental step.
The circuitry of Figure 3 compensates for the differing energy requirements of the main stepping motor holding mode and the main stepping motor stepping mode by adjusting the current drawn through the main stepping motor between steps, using the _ g _ set of low current drivers 122, 124, 126 and 128 connected in series with main stepping motor windings Wl - W4 through the set of resistors 130, 132, 134 and 136. A short time after the main stepping motor rotates to its next incremental step positior. in response to the switching on of high current drivers 114-120, octal latch 100 turns the high current drivers off while simultaneously supplying a combination of control signals along leads 15, 16, 17 and 18 to turn low current drivers 122-128 on.
Current is subsequently drawn from power control center 42 through windings Wl - W4 and continues to pass through resistors 130-136 to the low current drivers. The presence of resistors 130-136, of course, reduces the amount of current drawn from power control center 42 to a level sufficient to meet the requirements of main stepping motor 74 during the main stepping motor holding mode. Thus, the net amount of energy expended in operating main stepping motor 74 is conserved. Where high incremental step rates occur, such as where high fluid delivery rates have been selected, it is possible for the interval between incremental steps to be shorter than the pre-programrned switching interval between the main stepping motor stepping and holding modes. In the latter case, octal latch 100 never has the opportunity to switch the low current drivers on and the main stepping motor will not draw any low holding current between its incremental steps.
An analogous arrangement o~ high current drivers and low current drivers, shown generally at 138 and 140 in Figure 3, are controlled by octal latch 98 to energize valve stepping motor 34 and drive valve actuator 32 against portion 40 of resilient diaphragm 20. The smaller size of the valve stepping motor relative to main stepping rnotor 74 permits the overall current level in both the stepping and the holding modes of the valve stepping motor to be reduced.
The pumping operation of fluicl metering device 2 will now be described. Returning to Figure 2, it can be seen that incoming fluid transmitted by tubing 10 to fluid in-flow conduit 6 passes into gas retention chamber 28, whereupon any gases otherwise present in the fluid are prevented from reaching pumping chamber 18 by the presence of gas retention conduit 26. Liquid free of gas bubbles next travels from the gas retention chamber 28 1~ through passageway 24 to inlet port 22. During the refill phase of each pumping cycle, valve stepping motor 34 operates to re-ciprocate valve actuator 32 upwardly, allowing fluid free of gas bubbles to pass through the inlet port into pumping chamber 18.
Shortly thereafter, plunger 72 is reciprocated upwardly by main stepping motor 74 to increase the volume and reduce the pressure within pumping chamber 18, aiding the flow of fluid through the inlet port. The spring-loaded ball check 62 seated against valve seat 66 effectively closes off outlet port 58 while valve actuator 32 is in the open position. Accordingly, no fluid can leak into fluid out-flow conduit 14 during the refill phase of the pumping cycle and precise control over the amount of fluid to be pumped from pumping chamber 18 is maintained. After a brief interval in the open position, valve actuator 32 is moved to a closed position~ Plunger 72 i5 then reciprocated downwardly as described hereinabove to decrease the volume within pumping chamber 18.
As the volume within the pumping chamber decreases, the pressure within the pumping chamber increases to overcome the bias exerted by spring 64 against ball check 62 and 2 precise amount of metered fluid is pumped from pumping chamber 18 through out-flow conduit 14 and tubing 12 to the patient. The fluid pressure necessary to open ball check 62 is determined in large part by the spring constant of spring 64.
Turning to Figure 4, the interrelationship between the reciprocal movements of valve actuator 32 and plunger 72 can be graphically seenO The pumping cycle is initiated at time to~ with plunger 72 in a fully extended position, i.e., at the bot-tom of its stroke. Valve stepping motor 34 begins to step through a series of positions which serve to retract valve actuator 32 and open inlet port 22. The energization of main stepping motor 74 is programmed into motor controller 48 such that plunger 72 also begins retracting at time tl part-way through the valve actuator retraction sequence. At time t2 shortly after time tl, valve actuator 32 reaches a fully open or retracted position to enable the flow of fluid from fluid lS in-flow conduit 6 to pumping chamber 18 and valve stepping motor 34 is de~energized by a signal from motor controller `48.
Meanwhile, main stepping motor 74 continues stepping through a series of positions to retract plunger 72. At time t3 plunger 72 reaches the fully retracted position at the top of its stroke and main stepping motor 74 is de-energized by a second signal from motor controller 48. Valve actuator 32 remains fully retracted for an additional interval until time t4. This additional interval allows the resilient diaphragm 20 to return to a relatively relaxed or undeformed state following the retraction of plunger 72. Hence, the difference between time t4 and time t3 should be greater than the relaxation time associated with the particular material used in fabricating resilient diaphragm 20.
At time ta/ valve stepping motor 34 is re-energized by a control signal from motor controller 48 and begins stepping - 12 ~

through various incremental steps until time t5 when valve actuator 32 reaches a fully extended position to seal off inlet port 22. Once the valve actuator is in its fully extended position, valve stepping motor 34 is again de-energized to await the beginning of -the next pumping cycle. Simultaneous with the sealing of inlet port 22 at time t5, main stepping motor 74 is re-energized and shifts through several incremental steps, driving plunger 72 in a downward direction to provide the pressure necessary for overcoming the bias on ball check 620 ~t time t6, the ball check unseats to open outlet port 58. Fluid is subse-quently discharged from pumping chamber 18 through the outlet port until time t7, when plunger 72 reaches its fully extended position at the bottom of the plunger stro~e to complete the pumping cycle. The period of time from to to t6 is characterized as the pumping cycle refill phase, i.e., that portion of the pumping cycle dedicated to refilling pumping chamber 18 with fluid in preparation for delivery of the fluid to the patient, while the period of time between t6 and t7 is characterized as the pumping cycle delivery phase. The pumping cycle refill phase in turn includes the interval from time t5 to time t6 given over to pressurizing pumping chamber 18, which interval is designated the pumping cycle pressurizing phase.
Unless otherwise corrected, the interruption of fluid flow which occurs during the refill phase of the pumping cycle can lead to the formation of fluid pulses in the outflow conduit 14 of fluid metering device 2 and tubing 12. Such pulses are undesirable, particularly where the fluid involved is a rapidly metabolized drug capable of producing near instantaneous physio-logical reaction in the patient. Pulsatile fluid delivery can be significantly attenuated, however, and an essentially continuous flow of fluid between metering device 2 and the patient can be established, by making the time required for completing the refill phase as short as possible relative to the time required to carry out the fluid delivery phase. There is little room for compression of the refill phase itself because the time from to to t5 used for actually refilling pumping chamber 18 with fluid is substantially fixed by the geometry of the pumping chamber. Only the pressurizing portion of the refill phase can be reduced, through the expedient of driving main stepping motor 7~ at an increased speed between time t5 and t6.
The greater advantage of the present control procedure lies in adjusting the operation of the main stepping motor during the delivery phase in order to stretch out the downward stroke of plunger 72 as long as possible, subject to the constraint imposed by the necessity to pump enough fluid within a specific interval to meet the desired fluid delivery rate. This stretching oùt of the plunger downstroke may be accomplished, for example, by setting t7 equal to some constant divided by the desired fluid delivery rate. That is:
t7 = k/RATE, where k is a constant chosen on the basis of the pumping chamber geometry and RATE is a value representing the desired rate of fluid delivery. In this manner, the pulsatile effects of the discontinuity in fluid delivery associated with the refill phase are minimized over the entire pumping cycle.
Figure 5 graphically illustrates the rate of fluid delivery from fluid metering device 2 as a function of time.
Throughout most of the refill phase Pr of each pumping cycle C, valve actuator 32 is in an open position and fluid passes through inlet port 22 into pumping chamber 18. Plunger 72 is either retracting or completely retracted and ball check 62 is closed to prevent any fluid from leaving pumping cham-ber 18. Moreover, the passage of fluid from pumping chamber 18 continues to be blocked for a short interval near the end of the refill phase Pr, as plunger 72 begins its downstroke and the pressure in pumping chamber 18 builds to a value suf~i-cient to open ball check 62. This latter interval is the pressurizing phase Pp. Of course, the delivery rate during the entire refill phase Pr, including the pressurizing phase Pp, is zero as indicated at 142 in Figure 5. During the re-mainder or delivery phase Pd of the pumping cycle, plunger 72 is reciprocating toward a fully extended position to depress resilient diaphragm 20 and pump fluid from pumping chamber 18 lS past the now-open ball check 62. According to the teachings of the present invention, main stepping motor 74 is controlled such that plunger 72 moves through the downstroke at a con-stant velocity for most of the delivery phase Pd. Hence, as indicated at 14~ in Figure 5, the fluid delivery rate during delivery phase Pd is largely constant. In order to compensate for the fact that no fluid is delivered from pumping chamber 18 during the refill phase Pr, however, it is necessary for a short period after the initiation of the delivery phase Pd to provide an increased fluid delivery rate, designated 146 in Figure 5. This momentarily high delivery rate 146 occurs during a catch-up phase Pc immediately following the onset of the delivery phase Pd, and it can be seen upon reflection that the net effect of high delivery rate 146 is to keep the average delivery rate for the entire pwnping cycle as close as possi~le to the time-linear or constant delivery rate 144. The imple-mentation of a catch-up phase Pc, then, further contributes to the desired goal of providing an essentially continuous or time-linear flow of fluid between fluid metering device 2 and the patient.
One method for achieving the appropriate high delivery rate during the catch-up phase of the pumping cycle is out-lined in flow chart form in Figure 6. The Figure 6 method is -basically an accounting procedure, whereby a running tally is kept of the amount of fluid which should be delivered from pumping chamber 18 in order to maintain an essentially time~linear average delivery rate throughout the pumping cycle. The running tally is used at the end of the refill phase to drive the main stepping motor 74 at an in-creased speed until the actual deficit in fluid delivery caused by the refill phase is overcome. At this point, the main stepping motor is driven at a normal speed to provide the essentially constant fluid delivery rate indicated at 144 in Figure 5.
A VOLUME OWING register in microprocessor 92 is first designated to accumulate rate-representative signals RATE at predetermined intervals in the pumping cycle. In the pre-ferred embodiment o~ the present invention, the RATE signal is directly proportional to the desired fluid delivery rate.
As indicated at program block 148 of Figure 6, the control sequence implemented in motor controller 48 is interrupted every n seconds, where _ is a fractional value representing a time interval much smaller than the length of the refill phase. At each such interruption, the RATE signal is added to the VOLUME OWING register, indicated at program block 150 of Figure 6, and a determination is subsequently made as to ~ 16 -~8~

wllether the pumping cycle is in the refill phase or the de-livery phase. This determination, which is indicated at program block 152, can be based on the sensed incremental step position of rnain stepping motor 74. That is, if main step-ping motor 74 is at an incremental step position other thanthose incremental step positions associated with the delivery phase Pd of pumping cycle C, the pumping cycle is in a re-fill phase.
During the refill phase of the pumping cycle, the determination at program block 152 results in continued shiftiny of valve stepping motor 34 through its incremental steps, indicated at program block 154. The motor control process of Figure 6 then returns to the process starting point at program block 148 to await the next control sequence inter-ruption. In this manner, the RATE signal is added to theVOLUME OWING register every _ seconds and the amount in the VOLUME OWING register progressively increases or accumulates throughout the refill phase. When~ however, it is determined at program block 152 that the refill phase has ended and the fluid delivery phase has begun, a comparison, indicated at program block 156, is made between the accumulated amount in the VOLUME OWING register and a predetermined value STEP VOLUME.
STEP VOLUME represents the average volume of fluid discharged from pumping chamber 18 for each incremental step taken by main stepping motor 74 during the delivery phase of the pumping cycle. If the'amount in the VOLUME OWING register is greater than or equal to the value of STEP VOLUME, main stepping motor 74 is advanced one step, indicated at program block 158, and the amount in the VOLUME OWING register is decreased by the value of STEP VOLUME, as indicated at program block 160. The comparison at program block 156 is now repeated at n second intervals, advancing main stepping motor 74 through a series of incremental steps also at _ second intervals to discharge fluid from pumping chamber 18 at the high delivery rate 146 illustrated in Figure 5. This high delivery rate continues until the total volume of fluid so discharged compensates for the interruption in fluid flow which occurs during the refill phase, i.e., until the amount in the VOLUME OWING register is depleted to a value less than the value S~EP VOLUME re-presenting the volume of fluid discharged from pumping cham-ber 18 by a single main stepping motor step. At this point,.
the main stepping motor operation of program block 158 is bypassed and the amount in the VOLUME OWING register begins to accumulate until it exceeds the value of STEP VOLUME and the motor is once again advanced through an incremental step at program block 158. The latter situation, wherein the volume of 1uid discharged from pumping chamber 18 keeps pace with the amount accumulating in the VOLUME OWING register, occurs during that portion of the delivery phase charac-terized by the essentially constant fluid delivery rate 144of Figure 5.
An additional factor involved in maintaining a time-linear average deliver~ rate arises from the mechanical inter-relationship between cam 76 and plunger 72. Figure 7 provides a close up illustration of cam 75 including camming surface 84 and shaft 78 connecting cam 76 to main stepping mo-tor 74.
Main stepping motor 74 rotates through a series of steps in response to control signals received from motor controller 48.
Each of the steps is angularly displaced from the succeeding step by an equal amount. Hence, as main stepping motor 74 2~

moves from step to step, shaft 78 and cam 76 also undergo a series of equal anglllar displacements. In the preferred embodiment of the present invention, main s-tepping motor 74 has twenty-four steps per half-revolution, the twenty-four steps serving to drive plunger 72 from the fully retracted position through the downstroke to the fully extended posi-tion. The motor is next reversed and driven through the same twenty-four steps in reverse order to guide plunger 72 from the fully extended position through the upstroke back to the fully retracted position. For the sake of simplicity, only representative angular displacements 0x' ~y and ~z are shown in Figure 7. As previously indicated, all of these angular displacements are equal. However, due to the interrelation-ship between the shaft 78, camming surface 84 and plunger 72, equal angular displacements of cam 76 do not produce equal rectilinear movements of the plunger. Those angular dis-placements 0x which occur during the first few motor steps, when the lower portion 162 of camming surface 84 is in con-tact with plunger 72, produce less rectilinear movement of the plunger than do those angular displacements ~ which occur during intermediate motor steps when the middle portion 164 of camming surface 84 is in contact with the plunger.
The motor steps which produce angular displacements ~z near the mid-point of each motor revolution, when the upper portion 166 of camming surface 84 is in contact with plunger 72 at the end of the plunger downstroke, l:kewise result in relatively less rectilinear movement of the plunger per motor step.
The camming curve 168 of Figure 8 graphically illus-trates the relationship between motor steps and rectilinear plunger displacement during the plunger downstroke. In the preferred embodiment of the present invention, main stepping motor 7~ has twenty-four steps per half-revolution. There are three distinctly different camming phases evident from the camming curve 168. The first phase occurs at the outset of each pumping cycle, as main stepping motor 74 rotates through its initial steps to point a. It will be recalled that the rectilinear displacement of plunger 72 during this first phase is minimal. The second camming phase, between points a and b, is characterized by the rotation of main stepping motor 74 through its intermediate motor steps and the movement of plunger 72 on the downstroke is approximately linear, i e., plunger 72 is displaced by an equal amount for each angular displacement of cam 74. From point b until point c marking the fully extended plunger position at the twenty-fourth motor step, the rectilinear displacement of plunger 72 per step is again non-linear, progressively de-creasing until the fully extended plunger position is reached.
During the second camming phase, where the recti~
linear displacement of plunger 72 proceeds in approximately equal increments for each motor step, the volume of fluid expelled from pumping chamber 18 at each motor step is also approximately equal. On the other hand, the volume of fluid expelled from the pumping chamber per motor step during the first and third camming stages varies substantially between motor steps. It is therefore evident that the stepping speed of main stepping motor 74 must be varied from camming phase to camming phase if the volume expelled from pumping chamber 18 per unit of time is to remain constant throughout the pumping cycle. This is accomplished in the preferred embodiment of the present invention by programming motor controller 48 to increase the speed at which mai.n stepping motor 74 is driven through its incremental steps in the for-ward direction during the first and third camming phases.
Where main stepping motor 74 is characterized by twenty-four steps per half-revolution, the first camming phase occurs over approximately the first six motor steps, while the last four motor steps constitute the third camming phase. Motor controller 48 is accordingly designed to drive main stepping motor 74 at increased speeds whenever the main stepping motor is at steps one through six or twenty through twenty-four of the forward direction. It should also be noted with regard to the fluid metering device of Figure 2 that the first six steps of main stepping motor 74 coincide with the pres-surizing phase Pp of the pumping cycle C illustrated in Figure 5. Consequently, driving the main stepping motor at higher speeds during motor steps one through six to compensate for the effects of the first camming phase simultaneously shortens the refill phase Pr of the pumping cycle in the manner pre-viously contemplated. Upon leaving the high speed control sequence mode associated with the first camming phase, then, the motor controller ~.8 will move directly into the high speed control sequence mode associated with the pumping cycle catch-up phase Pc of Figure 5 before reaching the reduced speed control sequence mode normally associated ~5 with the second camming phase of Figure 8.
The entire stepping motor control procedure of the present invention may be carried out via either dedicated hardware or programmable hardware using appropriate software routines. One such software routine for accomplishing the stepping motor control procedure discussed in connection with Figures 3-~ is outlined as follows:

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FAGE 017 IIECH .SAIl F400 F400 FF~OCFAM FOF~ ~805 04730 0118~ 1.32 I SUENOUTINE SIli!;TP CDI~VEF:TS A SINAF;Y NUI'IE'EF TD ITS STEPFING t~UTDF
04750 0118O ~; CDDE EGUIVALENT
04770 0118EI~'~ 1532 A4 03 A hND ~'~00000011 5TPIP ALL EIUT 2 LSEI Df PDSITIOI~
04790 01190A 1535 D6 1539 A LDA STFTSL.X LOOK UP MOTUF~ PHASING Il~ STEP ThE:LE
04300 01191A 153a 81 04a20 01193A 1539 OA A STF'TE:L FCE, ~/10(~1 04330 Ollq9A 153A 09 ~ FCE. '0101 o4390 011'75A 153L: 05 A FCB '~.0110 _ _ . . . _ ~ 28 --8~

~ s indicated previously, the stepping motor control ~rocedure of the present invention may, with suitable modi-fication, be employed to operate the fluid metering device of co-pending application Serial No. 383,019. I.ndeed, the major distinction between the fluid metering device illustrated in Figure 2 and the fluid metering device of application Serial No. 383,019 is the use in the latter device of a pressure actuated valve mechanism, as opposed to a cam and stepping motor actuated valve mechanism, for opening and closing the inlet port to the pumping chamber. H.ence~ th.e only sub-stantial modification necessary to render the stepping motor control procedure of the present invention compatible with the fluid metering device of application Serial Mo. 383~()19 involves eliminating that portion of the stepping motor control procedure directed to the valve stepping motor operating sequences.
It is also to be understood that additional modi-fications to the stepping motor control procedure disclosed in Figures 4 through 9 may be made by those skilled in the art without departing from the spirit and scope of the present invention. All such.modifications and variations are, of course, considered to be within the purview of the appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A pump controller mechanism that drives a pump for metering intravenous fluids to a patient at a prede-termined rate in a relatively continuous, nonpulsatile fashion, said mechanism comprising a stepping motor, having a rotatable output shaft, a plunger for displacing fluid in a pumping chamber, rotatable camming means driven by the output shaft and operatively connected to the plunger for driving the plunger in incremental linear movements downwardly for expelling fluid from the chamber and upwardly for permitting the refilling of the chamber, said camming means being arranged to be driven through a plurality of incremental equiangular movements providing a linear output drive for the plunger wherein the incremental linear movements of the plunger at the beginning and end of the pumping stroke are substantially shorter in length than those during the rest of the stroke, said incremental linear movements in said rest of the stroke all being of generally the same length, and control means for driving said motor so that the camming means is driven at a high rate of speed through those equiangular movements which produce shorter incre-mental linear movements of the plunger just prior to the completion of its downward movement, during the upward movement of the plunger while the pumping chamber is refilled with fluid, and during the initial downward movement of the plunger and so that the camming means is driven at a lower rate of speed during the remainder of its movement so that the average rate of delivery from the pump over the entire pumping cycle generally equals the delivery rate provided by the lower rate of speed, said high rate of speed accommodating both the time required for refilling the chamber and the shorter incremental movements at the beginning and end of the pumping stroke.

2. A positive displacement intravenous administration pump controller comprising a stepping motor having an incrementally rotatable drive shaft, a plunger operatively connected to said drive shaft for displacing fluid in an intravenous administration pump in a manner such that the plunger is moved in one direction during a filling portion of the operating cycle and in the opposite direc-tion during the pumping portion of the operating cycle, means for driving said drive shaft of the motor through a plurality of equiangular distances, and control circuitry for controlling the rate at which said driving means is operated, said control circuitry including means for operating said driving means at a predetermined high rate during the filling portion and a part of the pumping portion of the operating cycle, and means for operating said driving means at a second rate during the remainder of the pumping portion, said second rate causing the fluid to be delivered at a desired preset rate over the operating cycle, and means for controlling the time that the high rate is maintained during the pumping portion of the cycle so that the average rate over the entire operating cycle equals the desired preset rate, said last named means including a storing means, means for periodically updating the storing means throughout the operating cycle by an incremental value determined by the desired preset rate, means for comparing at the periodic rate the total value in the storing means during the pumping portion of the operating cycle with a second value equivalent to the amount of fluid to be delivered during one incremental movement of the drive shaft, and means for activating said motor to cause it to rotate the drive shaft through one of said equiangular distances when the value in the storing means is equal to or greater than said second value.

3. A method for precise metering of fluids from a metering device, the metering device including a pumping means for providing reciprocal motion to pump fluid from the metering device, a motor means which serves as a source of rotational movement and a camming means which converts the rotational movement of the motor means into the reciprocal motion of the pumping means by rotating with the motor means to move the pumping means through greater and lesser rectilinear displacements as a func-tion of the angular displacement of the motor means, said method comprising the steps of:
operating the motor means at relatively faster rotational speeds during those period when the camming means moves the pumping means through rectilinear displace-ments of lesser magnitude; and operating the motor means at relatively slower rotational speeds during those periods when the camming means moves the piston means through rectilinear displace-ments of greater magnitude.

4. The method as set forth in claim 3, wherein said motor means includes a stepping motor which operates in incremental steps to rotate the camming means through equal angular displacements, each angular displacement of the camming means producing a greater or lesser rectilinear displacement of the pumping means as a function of the total angular displacement of the motor means, said step of operating the motor means at said relatively faster rotational speeds including the further step of shifting the stepping motor through incremental steps at relatively shorter intervals during those period when the angular displacements of the camming means produce lesser rectilin-ear displacements of the pumping means and said step of operating the motor means at said relatively slower rotational speeds includes the further step of shifting the stepping motor through incremental steps at relatively longer intervals during those periods when the angular displacements of the camming means produce greater rectilinear displacements of the pumping means.