Stepping Motor Control Procedure for Achieving Variable Rate, Quasi-Continuous Fluid Infusion

The present invention is directed to a means and method for administering fluids to patients intravenously or intraarterially and more particularly concerns a mechanism and a control procedure such that carefully metered portions of fluid are delivered in quasi-continuous fashion to patients.

Considerable attention has been focused in recent years on intravenous and intraarterial delivery of fluids to patients. Precision control over the rate at which such parenteral delivery occurs is of critical importance, inasmuch as improper administration of fluids can retard the recovery of patients or, in extreme situations, lead to further injury or even death. Early parenteral delivery systems relied on gravity flow to transfer fluid from a fluid container or reservoir to the patient. Attempts to accurately regulate gravity flow, however, proved difficult because the pressure forcing the fluid between the reservoir and the patient decreased as the fluid level within the reservoir dropped during the delivery operation. Thus, delivery rates in gravity-flow systems tended to vary in an unacceptable manner.

More recent parenteral delivery systems have employed pump motors in an effort to increase fluid delivery rate accuracy. Often, the pump motors comprise stepping motors which drive plunger or piston-like fluid pumps in response to suitable stepping motor control procedures. These procedures are highly compatible with the precision control requirements of parenteral administration because they provide the necessary degree of accuracy and are capable of implementation through reliable and efficient microprocessor programming techniques. U.S. Pat. No. 4,037,598 issued to Georgi on Jul. 26, 1977; U.S. Pat. No. 3,994,294 issued to Knute on Nov. 30, 1976; U.S. Pat. No. 3,985,133 issued to Jenkins et al on Oct. 12, 1976 and U.S. Pat. No. 3,736,930 issued to Georgi on Jun. 5, 1973 all disclose intravenous 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 stepping 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 by 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.

It is therefore the object of the present invention to provide a control procedure for governing the operation of a pump motor in a parenteral fluid delivery system.

It is another object of the present invention to provide a motor control procedure for use with a parenteral fluid pump motor and camming mechnism wherein essentially linear fluid delivery rates are achieved despite the nonlinearities inherent in the conversion of rotary motor movement to rectilinear pumping movement.

It is yet another object of the present invention to provide a pump motor control procedure for use with a parenteral fluid pump motor and camming mechanism wherein the speed of the motor is varied in accordance with the angular position of the camming mechanism in order to compensate for non-linearities which occur in the conversion of rotary motor movement to rectilinear pumping movement.

The foregoing objects are achieved by providing a pump controller mechanism that includes a stepping motor connected to a plunger through a camming means with the camming means driving the plunger through a plurality of rectilinear distances as the motor rotates the camming means through a plurality of equiangular distances. The rectilinear distances at the ends of the pumping stroke are shorter than those at the center of the stroke, and hence a control means is provided to drive the motor at a higher rate at the ends of the stroke to accommodate this and achieve a generally uniform flow.

In another aspect of the present invention, the time required for refilling the pumping chamber is also taken into account by initially driving the stepping motor at a high rate for a time sufficient to make up the time required for filling so that the motor can return to the preset rate which is the desired delivery rate for the entire pumping cycle.

These and other objects of the present invention can best be understood by examining the following Brief Description Of The Drawings and Best Mode For Carrying Out The Invention, in which:

FIG. 1 is a perspective view illustrating the parenteral delivery system of the present invention;

FIG. 2 is a cross-sectional view of the pumping cassette, valve stepping motor and main stepping motor employed in the parenteral delivery system of FIG. 1;

FIG. 3 schematically depicts a motor controller for governing the operation of the valve stepping motor and main stepping motor of FIG. 2, whereby the amount of current drawn through both the valve stepping motor and main stepping motor is alternately switched between high and low values in order to conserve energy;

FIG. 4 is a graph representing the interrelated operating sequences of the valve stepping motor and the main stepping motor;

FIG. 5 is a graph illustrating the changes in fluid delivery rate which occur over a single pumping cycle when the control procedure of the present invention is implemented in the fluid delivery system of FIGS. 1, 2 and 3;

FIG. 6 is a flow-chart illustrating one method for achieving the fluid delivery rates graphically depicted in FIG. 5;

FIG. 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 pumping force in response to the rotation of the cam; and

FIG. 8 is a graphic illustration of the camming curve associated with the cam of FIG. 7.

One type of parenteral fluid metering device for delivering controlled amounts of fluid to a patient is schematically illustrated in FIGS. 1 and 2. Basic and improved embodiments of the fluid metering device are disclosed in co-pending application Ser. Nos. 174,666 now U.S. Pat. No. 4,336,800 and 278,954; respectively filed on Aug. 1, 1980 and Jun. 30, 1981. Both of these co-pending applications are assigned to the assignee of the present invention and both are incorporated herein by reference. For the sake of convenience, only the improved fluid metering device of application Ser. No. 278,954 will be described in any detail, although it is understood that the control procedure of the present invention can be suitably modified for use with the fluid metering device of application Ser. No. 174,666. Referring first to FIG. 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 response to actuation of a stepping motor and camming mechanism (not shown in FIG. 1) housed in control unit 4.

Turning to FIG. 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 retention chamber 28 into pumping chamber 18. Gas retention chamber 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 resilient diaphragm 20 positioned above inlet port 22. Current is supplied to valve stepping motor 34 by a power control center 42 which may be alternately 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 FIG. 2, and a sealing engagement with a valve seat 50 formed around the periphery of inlet port 22, as indicated in phantom in FIG. 2. A biasing 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 outlet conduit 14 through an intermediate passageway 60. A ball check 62 is mounted between outlet port 58 and intermediate passageway 60. A biasing means such as spring 64 urges 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 fluid 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 receives 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 cam 76 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, indicated by solid lines in FIG. 2, and a fully extended position indicated in phantom at 90 in FIG. 2. Resilient diaphragm 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.

The motor controller 48 which governs the operation of valve stepping motor 34 and main stepping motor 74 is schematically illustrated in FIG. 3. Motor 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 stepping 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, 100 to drive the valve stepping motor and the main stepping motor respectively through their incremental steps 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 occurance of potentially dangerous fluid over- or under-delivery conditions, as disclosed more fully in co-pending application Ser. No. 320,528, filed Nov. 12,1981 and incorporated herein by reference. Finally, multiplexer 108 is connected to various data sensors such as the plunger pressure transducer (not shown) disclosed in aforementioned Ser. No. 278,954. 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 FIG. 3, is connected to high and low current drivers 112, 113 via leads 1.sub.1 -1.sub.8. The high current drivers 112 comprise a set of transistor drivers 114, 116, 118 and 120 respectively connected in series with the motor windings W.sub.1, W.sub.2, W.sub.3 and W.sub.4 of main stepping motor 74. The low current drivers 113 comprise a similar set of transistor drivers 122, 124, 126 and 128 respectively connected in series with windings W.sub.1 -W.sub.4 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 1.sub.1, 1.sub.2, 1.sub.3 and 1.sub.4 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 W.sub.1 -W.sub.4 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 FIG. 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 FIG. 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 74 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 FIG. 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 set of low current drivers 122, 124, 126 and 128 connected in series with main stepping motor windings W.sub.1 -W.sub.4 through the set of resistors 130, 132, 134 and 136. A short time after the main stepping motor rotates to its next incremental step position 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 1.sub.5, 1.sub.6, 1.sub.7 and 1.sub.8 to turn low current drivers 122-128 on. Current is subsequently drawn from power control center 42 through windings W.sub.1 -W.sub.4 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-programmed 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 of high current drivers and low current drivers, shown generally at 138 and 140 in FIG. 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 motor 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 fluid metering device 2 will now be described. Returning to FIG. 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 through passageway 24 to inlet port 22. During the refill phase of each pumping cycle, valve stepping motor 34 operates to reciprocate 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 is 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 a 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 FIG. 4, the interrelationship between the reciprocal movements of valve actuator 32 and plunger 72 can be graphically seen. The pumping cycle is initiated at time t.sub.0, with plunger 72 in a fully extended position, i.e., at the bottom 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 t.sub.1 part-way through the valve actuator retraction sequence. At time t.sub.2 shortly after time t.sub.1, valve actuator 32 reaches a fully open or retracted position to enable the flow of fluid from fluid 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 t.sub.3 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 t.sub.4. 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 t.sub.4 and time t.sub.3 should be greater than the relaxation time associated with the particular material used in fabricating resilient diaphragm 20.

At time t.sub.4, valve stepping motor 34 is re-energized by a control signal from motor controller 48 and begins stepping through various incremental steps until time t.sub.5 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 t.sub.5, 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 62. At time t.sub.6, the ball check unseats to open outlet port 58. Fluid is subsequently discharged from pumping chamber 18 through the outlet port until time t.sub.7, when plunger 72 reaches its fully extended position at the bottom of the plunger stroke to complete the pumping cycle. The period of time from t.sub.0 to t.sub.6 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 t.sub.6 and t.sub.7 is characterized as the pumping cycle delivery phase. The pumping cycle refill phase in turn includes the interval from time t.sub.5 to time t.sub.6 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 physiological 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 t.sub.0 to t.sub.5 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 74 at an increased speed between time t.sub.5 and t.sub.6. 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 out of the plunger downstroke may be accomplished, for example, by setting t.sub.7 equal to some constant divided by the desired fluid delivery rate. That is:

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.

FIG. 5 graphically illustrates the rate of fluid delivery from fluid metering device 2 as a function of time. Throughout most of the refill phase P.sub.r 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 chamber 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 P.sub.r, as plunger 72 begins its downstroke and the pressure in pumping chamber 18 builds to a value sufficient to open ball check 62. This latter interval is the pressurizing phase P.sub.p. Of course, the delivery rate during the entire refill phase P.sub.r, including the pressurizing phase P.sub.p, is zero as indicated at 142 in FIG. 5. During the remainder or delivery phase P.sub.d 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 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 constant velocity for most of the delivery phase P.sub.d. Hence, as indicated at 144 in FIG. 5, the fluid delivery rate during delivery phase P.sub.d is largely constant. In order to compensate for the fact that no fluid is delivered from pumping chamber 18 during the refill phase P.sub.r, however, it is necessary for a short period after the initiation of the delivery phase P.sub.d to provide an increased fluid delivery rate, designated 146 in FIG. 5. This momentarily high delivery rate 146 occurs during a catch-up phase P.sub.c immediately following the onset of the delivery phase P.sub.d, 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 pumping cycle as close as possible to the time-linear or constant delivery rate 144. The implementation of a catch-up phase P.sub.c, 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 outlined in flow chart form in FIG. 6. The FIG. 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 increased 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 FIG. 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 preferred embodiment of the present invention, the RATE signal is directly proportional to the desired fluid delivery rate. As indicated at program block 148 of FIG. 6, the control sequence implemented in motor controller 48 is interrupted every n seconds, where n 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 FIG. 6, and a determination is subsequently made as to whether the pumping cycle is in the refill phase or the delivery phase. This determination, which is indicated at program block 152, can be based on the sensed incremental step position of main stepping motor 74. That is, if main stepping motor 74 is at an incremental step position other than those incremental step positions associated with the delivery phase P.sub.d of pumping cycle C, the pumping cycle is in a refill phase.

During the refill phase of the pumping cycle, the determination at program block 152 results in continued shifting of valve stepping motor 34 through its incremental steps, indicated at program block 154. The motor control process of FIG. 6 then returns to the process starting point at program block 148 to await the next control sequence interruption. In this manner, the RATE signal is added to the VOLUME OWING register every n 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 n second intervals to discharge fluid from pumping chamber 18 at the high delivery rate 146 illustrated in FIG. 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 STEP VOLUME representing the volume of fluid discharged from pumping chamber 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 fluid discharged from pumping chamber 18 keeps pace with the amount accumulating in the VOLUME OWING register, occurs during that portion of the delivery phase characterized by the essentially constant fluid delivery rate 144 of FIG. 5.

An additional factor involved in maintaining a timelinear average delivery rate arises from the mechanical interrelationship between cam 76 and plunger 72. FIG. 7 provides a close up illustration of cam 76 including camming surface 84 and shaft 78 connecting cam 76 to main stepping motor 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 moves from step to step, shaft 78 and cam 76 also undergo a series of equal angular displacements. In the preferred embodiment of the present invention, main stepping 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 position. 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 .phi..sub.x, .phi..sub.y and .phi..sub.z are shown in FIG. 7. As previously indicated, all of these angular displacements are equal. However, due to the interrelationship 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 displacements .phi..sub.x which occur during the first few motor steps, when the lower portion 162 of camming surface 84 is in contact with plunger 72, produce less rectilinear movement of the plunger than do those angular displacements .phi. .sub.y 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 .phi..sub.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, likewise result in relatively less rectilinear movement of the plunger per motor step.

The camming curve 168 of FIG. 8 graphically illustrates the relationship between motor steps and rectilinear plunger displacement during the plunger downstroke. In the preferred embodiment of the present invention, main stepping motor 74 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 decreasing until the fully extended plunger position is reached.

During the second camming phase, where the rectilinear 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 main stepping motor 74 is driven through its incremental steps in the forward 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 FIG. 2 that the first six steps of main stepping motor 74 coincide with the pressurizing phase P.sub.p of the pumping cycle C illustrated in FIG. 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 P.sub.r of the pumping cycle in the manner previously contemplated. Upon leaving the high speed control sequence mode associated with the first camming phase, then, the motor controller 48 will move directly into the high speed control sequence mode associated with the pumping cycle catch-up phase P.sub.c of FIG. 5 before reaching the reduced speed control sequence mode normally associated with the second camming phase of FIG. 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 FIGS. 3-8 is outlined as follows: ##SPC1##

As indicated previously, the stepping motor control procedure of the present invention may, with suitable modification, be employed to operate the fluid metering device of co-pending application Ser. No. 174,666. Indeed, the major distinction between the fluid metering device illustrated in FIG. 2 and the fluid metering device of application Ser. No. 174,666 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. Hence, the only substantial modification necessary to render the stepping motor control procedure of the present invention compatible with the fluid metering device of application Ser. No. 174,666 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 modifications to the stepping motor control procedure disclosed in FIGS. 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.