TECHNICAL FIELD

The present invention relates, generally, to reciprocating pumps, and more particularly, relates to a magnetic direct drive reciprocating pump apparatus for liquid chromatography chemical analysis.

BACKGROUND ART

In recent years, significant advances have been made in liquid chromatography chemical analysis. Injecting sample fluid at a precise and reproducible flow rate is fundamental to any separation technique. Typical in these applications, rotating motor-type pumps are employed to inject or pump the sample solution into the column. While these rotating motor designs provide adequate flow rate precision, they necessitate the use of expensive mechanical linkage assemblies to push and pull the piston in and out of the piston chamber to effect pumping. Hence, these linkage assemblies increase costs, require multiple moving parts which inherently increase friction bearing surfaces, as well as potentially create greater maintenance problems.

One alternative to rotating motor-type pumps is the application of linear solenoid motor assemblies employing solenoid coils which generate magnetic fields. These magnetic fields cooperate with permeable slugs protruding or extending though the coil to

agnetically urge the slug in the direction of the central axis of the coil. A piston member, coupled to the permeable slug, is thus driven into the piston chamber at a substantially constant rate to pump the sample fluid therethrough.

In order to pull the piston member back out of the chamber during the refilling stroke, usually, at least one return spring is employed to act on the piston member in the direction opposite of the solenoid magnetic field. Typical of these patented solenoid- type electromagnetic pumps may be found in U.S Patent Nos. : 4,838,771; 4,352,645; 4,252,505; 4,080,552; 4,021,152; 3,804,558; 3,514,228 and 2,806,432.

While these electromagnetic pumps are adequate in some applications, the spring augmented solenoid pumps cause several operational problems. For example, the coils of the return spring may rub against a piston shaft or other exposed surfaces during operation to substantially increase friction and severely hamper operation thereof. Further, the spring force acting on the piston must be overcome by the electromagnetic force driving the piston in the opposite direction during the during the push or flush stroke. This force imbalance tends to cause erratic fluid flow so that the pump flow rate is neither substantially constant nor smooth. Depending upon the weight of the piston/slug assembly, the refill or pull stroke may be too fast such that an internal cushion or bumper is necessary to absorb and cushion contact as the spring fully withdraws the piston from the chamber.

Further, piston installation misalignment where the piston member is slightly skewed from the chamber longitudinal axis may cause the piston to rub against various components during operation. Such adverse

contact includes rubbing against the back-up washers, as well as side loading against the solenoid and seals.

Moreover, even when a smooth flow rate is initially provided in these pump arrangements, such precision is generally only attainable for a few minutes or, occasionally, a few hours. Eventually, the various frictional forces between the sliding components (e.g., the main and back-up seals, back-up washers and springs) cause erratic fluid flow of the pump.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide a solenoidal electromagnetic direct drive pump apparatus for liquid chromatography chemical analysis.

Another object of the present invention is to provide an electromagnetic pump apparatus with reduced operating frictional forces.

It is a further object of the present invention to provide an electromagnetic pump apparatus which is capable of providing a substantially constant flow rate.

Still another object of the present invention is to provide an electromagnetic pump apparatus with increased efficiency and longer operational life.

Yet another object of the present invention is to provide an electromagnetic pump apparatus enabling hydraulic pressure measurement thereof.

It is a further object of the present invention to provide an electromagnetic pump apparatus which is durable, compact, easy to maintain, has a minimum

number of components, and is easy to use by unskilled personnel.

In accordance with the foregoing objects, the present invention provides a magnetic direct drive pump apparatus including a pump head defining a chamber, and an elongated guide shaft having a piston member disposed in the chamber for reciprocal movement thereof along a guide axis of the guide shaft . An annular magnet is included having a central opening and a central axis generally co-axial with the guide axis of the guide shaft. The annular magnet is formed to internally generate a first magnetic field which is aligned along the central axis. A drive magnet is coupled to the guide shaft at a position through the annular magnet central opening. Further, the drive magnet internally generates an independent drive magnetic field which is aligned to cooperate with the first magnetic field. At least one of the annular magnet and the drive magnet is selectively capable of reversing the polarity of the respective magnetic field to one of attract and repulse the other magnet for controlled reciprocal movement of the piston member in and out of the chamber free of additional external forces driving the piston member.

Preferably, the annular magnet is provided solenoid coil is included which generates a bi-directional electromagnetic field in response to the respective direction of current flow therethrough. In contrast, the drive magnet is preferably provided by a permanent magnet.

The present invention further includes a method for measuring the hydraulic pressure of a pumped fluid by measuring the force applied to the piston assembly for movement thereof in the chamber. Subsequently,

depending upon the particular method, the hydraulic pressure or force is calculated by canceling either the seal stiction force between the seal and the piston assembly or the seal friction therebetween.

To cancel the effects of seal stiction forces, the method includes the steps of A) retaining the piston assembly at a first position along the path toward and away from the chamber in the presence of a first seal stiction formed between the seal and the piston assembly. This retainment is accomplished by applying a driving force at a first level to the piston assembly in a first direction opposite the hydraulic pressure force of the pumped fluid acting thereon in an opposite second direction. The next step includes B) incrementally increasing or decreasing the driving force from the first level to a second level at which the piston assembly just overcomes the first seal stiction. At this instance, the piston assembly breaks the first seal stiction, and measurably moves in either the first direction or the second direction along the path away from the first position. The direction of the movement depends upon the direction and magnitude of the force applied to the piston assembly, and the magnitude of the hydraulic pressure. The present invention further includes the steps of the C) measuring the driving force at the second level, and D) thereafter, retaining the piston assembly at a second position along the path in the presence of a second seal stiction formed between the seal and the piston assembly. This is accomplished by incrementally decreasing or increasing the driving force in the first direction or the second direction to a third level. After the second stop, the method includes the step of E) incrementally decreasing or increasing the driving force from the third level to a fourth level at which the piston assembly just overcomes the second seal

stiction. Again, the direction of the movement depends upon the direction and magnitude of the force applied to the piston assembly, and the magnitude of the hydraulic pressure. However, the direction of measurable movement will be in the second direction or the first direction, opposite the direction of travel in step B, along the path away from the second position. Finally, the present invention includes the step of F) measuring the driving force at the fourth level; and G) calculating the hydraulic pressure from the second level and the fourth level of driving force by canceling the opposite acting forces of the first seal stiction and the second seal stiction.

In the seal friction cancellation technique, the method includes the steps of: A) moving the piston assembly from a first position to a second position along the path in the chamber to attain a substantially constant first velocity of the piston assembly proximate the second position in the presence of a first seal friction between the seal and the piston assembly. This is accomplished by applying a continuous driving force to the piston assembly in a first direction along the path. The next steps includes B) measuring the driving force at a first level proximate the second position during movement of the piston assembly in step A at the substantially constant first velocity; and C) moving the piston assembly from proximate the second position to the first position along the path in the chamber to attain a substantially constant second velocity of the piston assembly proximate the first position. This constant second velocity occurs in the presence of a second seal friction between the seal and the piston assembly and is accomplished by applying the continuous driving force to the piston assembly in a second direction along the path opposite the first direction. The present invention further includes the

step of D) measuring the driving force at a second level proximate the first position during movement of the piston assembly in step C at the substantially constant second velocity; and finally E) calculating the hydraulic pressure from the first level and the second level of driving force by canceling the opposite acting forces of the first seal friction and the second seal friction.

In the preferred form of the present invention, each the seal stiction cancellation method and the seal friction cancellation method are performed on electromagnetically driven pump devices having solenoid coils magnetically driving the reciprocating piston assembly. Further, each pump device includes an independent drive magnet coupled to the piston assembly and having an independent, internally generated drive magnetic field aligned to cooperate with a bi¬ directional magnetic field of the solenoid coil.

To generate the driving force, a drive current is applied to the solenoid coil in one direction to drive the piston assembly along the path in the first direction, and in a reverse direction to drive the piston assembly along the path in the opposite second direction. The generated drive force applied to the piston assembly is proportionate to the drive current applied to the coil.

BRIEF DESCRIPTION OF THE DRAWING

The assembly of the present invention has other objects and features of advantage which will be more readily apparent from the following description of the best mode of carrying out the invention and the appended claims, when taken in conjunction with the accompanying drawing, in which:

FIGURES 1 and 2 are a schematic sequence of an electromagnetic direct drive pump apparatus constructed in accordance with the present invention, and illustrating the movement of the piston assembly and the corresponding polarities of the magnets.

FIGURE 3 is a schematic of an alternative embodiment of the electromagnetic direct drive pump apparatus of FIGURE 1 having a dual piston configuration.

DETAILED DESCRIPTION OF THE INVENTION While the present invention will be described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. It will be noted here that for a better understanding, like components are designated by like reference numerals throughout the various figures.

Attention is now directed to FIGURE 1, where a magnetic direct drive pump apparatus, generally designated 10, is illustrated including a pump head 11 defining a chamber 13. The pump apparatus includes a piston assembly 15 having an elongated guide shaft 16 and a plunger or piston member 17 disposed in chamber 13 for reciprocal movement therein in the direction of a guide axis 18 of guide shaft 16. An annular magnet, generally designated 20, is included having a central opening 21 and a central axis 22 generally co-axial with the guide axis 18 of guide shaft 16. Annular magnet 20 is formed to internally generate a first magnetic field which is aligned along central axis 22. A drive magnet, generally designated 23, is coupled to

guide shaft 16 at a position through annular magnet central opening 21. Further, drive magnet 23 internally generates an independent drive magnetic field which is aligned to cooperate with the first magnetic field. At least one of the annular magnet 20 and the drive magnet 23 is selectively capable of reversing the polarity of the respective magnetic field to one of attract and repulse the other magnet for controlled reciprocal movement of piston member 17 in and out of the chamber 13 free of additional external forces driving piston member 17.

Accordingly, by providing a direct drive pump apparatus 10 having a drive magnet capable of generating its own magnetic field, as opposed to the mere magnetized permeable slug member of the prior art, several operational advantages are attainable. For example, movement of the piston member in both directions along the central axis can be effected by magnetic cooperation between the drive magnet and the annular magnet. The solenoids of conventional solenoid pumps, in contrast, are generally only capable of driving the pistons in a single direction (i.e., toward the central equilibrium position between the solenoid) , and require additional spring members to urge the pistons in the opposite direction (i.e., away from the center of the solenoid) . Moreover, as will be appreciated, the magnetic force applied to the piston is substantially constant relative displacement thereof through the annular magnet. This simplifies control of the piston assembly to enable more precision in either axial direction. Finally, the present invention eliminates the use of expensive rotary linkage assemblies, substantially reducing costs, to provide a more efficient and reliable single piston pump. ^•

In the preferred form, annular magnet 20 is provided by a solenoid coil which generates a bi-directional electromagnetic field in response to the respective direction of current flow therethrough (i.e., to reverse the polarity) . Further, drive magnet 23 is preferably provided by a permanent magnet member generating its own magnetic field, such as a neodymium iron boron magnet. It will be appreciated, however, that the annular magnet could be a permanent magnet while the drive magnet may be provided an electromagnet. Moreover, both magnets could be provided by electromagnets, having cooperating polarities, without departing from the true spirit an nature of the present invention.

As shown in FIGURE 1, piston assembly 15 is formed to reciprocate along the guide shaft axis 18 which is substantially co-axial with the central axis 22 of the annular coil 20. Hence, drive magnet 23 is also preferably positioned for reciprocating movement along the central axis 22 at an orientation generally positioned substantially central to the solenoid annulus. This positions the drive magnet well within the electromagnetic field of the solenoid coil to enable continuous interaction between the two magnetic fields {i.e., the electromagnetic field of the solenoid coil and that of the drive magnet) .

Briefly, drive magnet 23 is positioned between two opposing spider members 25, 25' which mount the drive magnet to the ends of guide shaft 16 and piston member 17, respectively. A lower distal end of piston member 17 is reciprocally positioned in the elongated chamber 13 of pump head 11. As piston member 17 reciprocates therein, sample fluid can be pumped from a first passage 26 in pump head 11 through chamber 13 and to second passage 27. Check valves 28, 28' (i.e., intake

valve 28 and exhaust valve 28') are mounted to head 11 in fluid communication with the respective passages 26, 27 and cooperatively enable flow of fluid through chamber 13 during operation of the pump assembly. These valves may be provided by mechanical or electro¬ mechanical valves commonly employed in the field. In the preferred embodiment, however, valves 28, 28' are gravity operated valves.

As set forth in FIGURES 1 and 2, an annular seal 30 is provided mounted to pump head 11 which slidably supports piston member 17 to deter contact with the chamber walls during reciprocating motion thereof. Annular seal 30 is preferably provided by TEFLON ^* for reduced friction, and further is formed to seal chamber 13 from the environment, while further preventing fluid flow therefrom. It will be appreciated that seal 30 could be fixedly mounted to piston member 17 such that the seal is in sliding contact with the inner walls forming pump chamber 13.

Drive magnet 23, as mentioned, is preferably provided by a permanent magnet generating an internal magnetic field which is aligned to cooperate with the directional electromagnetic field of coil 20. FIGURE 1, illustrates that the permanent drive magnet is oriented to position its Positive pole (PJ and Negative pole (N _p ) co-axially along central axis 22. While FIGURE 1 illustrates the drive magnet Positive pole (P _p ) at the bottom of the drive magnet, while the Negative pole (N _p ) is positioned at an upper end thereof, it will be appreciated that the polarities may be switched without departing from the true spirit and nature of the present invention.

Solenoid coil 20 collectively generates a bi¬ directional electromagnetic field depending upon the

-In¬ direction of current flow therethrough. To drive piston assembly 15 in the direction of arrow 31 in FIGURE 1 (during the push or flush stroke) , the current must flow through coil in the proper direction to generate a Positive pole (P _c ) at a bottom portion of solenoid coil 20, while the Negative pole (N _c ) is positioned at a top portion of the coil. Accordingly, the Positive pole (P _P ) of drive magnet 23 is repelled by the lower Positive pole (P _c ) of solenoid coil 20. Similarly, the Negative pole (N _p ) of drive magnet 23 is repelled by the upper Negative pole (N _c ) of solenoid coil 20, while simultaneously being attracted to the lower Positive pole (P _c ) of solenoid coil 20. Collectively, this combination magnetically drives the piston assembly in the direction of arrow 31 to flush fluid from chamber 13.

In contrast, to drive the piston assembly 15 in the opposite direction (arrow 32 in FIGURE 2) during the pull or fill stroke, the current flow through coil 20 is reversed. This generates a Negative pole (P _c ) at the bottom portion of solenoid coil 20, while the Positive pole (P _c ) is positioned at the top portion of the coil. Hence, the Positive pole (P _p ) of drive magnet 23 is attracted to the lower Negative pole (N _c ) of solenoid coil 20. Likewise, the Negative pole (N _p ) of drive magnet 23 is attracted to the upper Positive pole (P _c ) of solenoid coil 20, while further simultaneously repelled by the lower Negative pole (N _c ) of solenoid coil 20. Collectively, this combination magnetically drives the piston assembly in the direction of arrow 32 to fill chamber 13 with fluid.

Accordingly, the dual magnet approach of the present invention enables a more controlled movement of the piston assembly in either direction along the central axis 22. This is due to the fact that the magnetic

force, generated by the independent, internally generated magnetic fields of the two magnets, is proportional to the current flow through the coil. Hence, employing the magnetic equation Force= Constant x Current x Strength of Permanent Magnet, the magnetic force induced on the piston assembly is generally constant through a substantial portion of the interaction between the magnets along the central axis. In essence, the magnetic force induced on the piston assembly is generally constant during reciprocal displacement of the piston member along the central axis. This substantially simplifies controlled movement of the piston assembly.

By comparison, in conventional solenoid magnetic pumps, the electromagnetic force induced on the permeable slug is proportional to the square of the current flow through the coil. In addition, the magnetic force acting on the slug substantially increases as the slug moves toward the stationary magnetic structure of the solenoid, and substantially decreases as the slug moves away from the stationary magnetic structure of the coil. Hence, controlling the precise movement of the piston assembly, in this arrangement, is much more difficult and less precise than the present invention. As a result, the fluid flow rate of the pump apparatus

10 is more erratic.

To further stabilize reciprocal movement of piston assembly 15, a guide member 33 is included as a second guide bearing support (FIGURES 1 and 2) for piston assembly 15. Guide member 33 is provided by a slip- sleeve member defining a bore 35 formed and dimensioned for sliding receipt of the guide shaft 16 therethrough. This provides sliding support and alignment of the guide shaft, and hence piston member 17, during reciprocal movement thereof.

In the preferred form of the present invention, to further reduce friction and side loading of the piston assembly against the bearings, seals and solenoid, the piston assembly is vertically oriented to reciprocate along a substantially vertical guide axis 18. This orientation reduces friction, or more importantly, variations in friction to enhance smoother reciprocating operation, and hence, fluid flow.

To measure and monitor displacement of piston assembly 15 relative solenoid coil 20 and pump head 11, a displacement sensor 36 is included cooperating with and responsive to reciprocating movement of guide shaft 16. One such sensor is a variable cylindrical capacitor positioned just past the guide shaft distal end, as shown in FIGURES 1 and 2, which is formed and dimensioned for sliding receipt of guide shaft 16 therein. As the end of guide shaft 16 reciprocates in and out of the preferably brass-sleeve capacitor, the capacitance between the shaft and the sleeve vary linearly with respect to the depth of insertion. A simple circuit is employed to translate capacitance to voltage upon which the real displacement can be calculated. It will be understood that other displacement sensors may be employed as well such as linear potentiometers and photo detectors.

Coupled between the displacement sensor and the solenoid coil is a real-time servo control loop or mechanism (not shown) . This mechanism is employed to sense the displacement and position of piston assembly 15 and, thus, adjust the current applied to the solenoid coil. The solenoid coil is thus controlled during each stroke such that the piston assembly displacement vs. time curve matches the desired profile. Hence, the direct drive pump apparatus 10 of the present invention enables measurement of the

starting and ending position of each stroke with substantial repeatability. This arrangement, furthermore, is capable of automatically overriding changes or variations in friction and pressure in the system.

For example, when a particular stroke is shortened due to a sudden increase in friction, the servo control mechanism can adjust the coil current so that the next stroke is increased by an amount compensating for the deficit. Accordingly, long term variations can be effectively compensated so that the pump apparatus 10 operates at substantially constant flow rates as great a 10 ml/min, with higher efficiency.

In an alternative embodiment of the present invention, a dual piston configuration may be included which is particularly suitable for use as a dual analytical pump apparatus. As shown in FIGURE 3, guide shaft 16 includes a second opposite piston member 17' positioned on and opposite side of solenoid coil 20. Piston member 17' reciprocates in chamber 13' of a second pump head 11' . When one piston member pushes the pumped fluid from the corresponding chamber, the second opposite piston member simultaneously pulls the pumped fluid into the corresponding chamber.

Accordingly, not only is the flow rate effectively doubled, a smoother flow is acheived. Although there will be a short pause at the end of each stroke, due to the change of direction of the magnetic force, the frequency of the pulses will be doubled, and thus smoother. Hence, a lower capacity pulse damper may be employed, or perhaps even eliminated.

In this dual configuration, while a displacement sensor is not illustrated in FIGURE 3, one may be provided

- 16 - between either piston member and the solenoid. Further, a guide member may be provided as well.

In addition, dual, single piston pump apparatus may be employed which would eliminate many of the constraints inherent in the current rotary single cam motors. For instance, since the proper stroke overlap is dependent upon hydraulic pressure, a dual, single piston pump apparatus of the present invention can accurately control the overlap to eliminate the pressure pulses at the crossover. Further, during the refill stroke, the piston speed can be varied to facilitate proportioning accuracy. This is especially true since the pressure feedback of the servo control mechanism of the present invention is more efficient than for a rotary-motor pump since gear lash hysteresis is eliminated. Further, the movement and acceleration of the drive magnet is very high resulting in near instantaneous response.

In another aspect of the present invention, a method for measuring the hydraulic pressure of the pumped fluid through the piston assembly is provided without the application of pressure transducers. By measuring the drive force applied to the pump assembly during selected segments of the piston stroke, either the seal stiction forces or the seal friction forces can be effectively canceled in an empirical equation. This equation enables calculation of the hydraulic force, and hence pressure, acting on the piston.

These cancellation techniques are best performed by orienting the reciprocating piston assembly 15 vertically to substantially eliminate all the side loading frictional forces acting the bearings and the piston assembly. Accordingly, by systematically removing the substantially constant gravitational force

or weight of the piston assembly, the only other variable force, other than hydraulic, is either the seal friction (when the piston assembly is moving at a substantially constant velocity in the chamber) or seal stiction (when the piston assembly is fixed relative the chamber) . Seal friction, or seal stiction forces, may widely vary and are unpredictable during the life of the seal since the seal materials often coat the piston member, and occasionally slough off.

Briefly, as will be described below, in each cancellation technique, the direction and magnitude of the force applied to piston assembly 15 for movement thereof or to initiate movement thereof along chamber 13 is dependent upon the magnitude of the hydraulic pressure urging piston assembly 15 out of chamber 13.

In the "seal stiction" cancellation technique, a method is provided for calculating the hydraulic pressure of the pumped fluid in the pump assembly by measuring the drive force applied to the piston assembly to initiate movement thereof in the chamber. By measuring the respective drive force to initiate movement of the piston assembly in both directions along the chamber, the hydraulic force is calculated by subsequently canceling out the seal stiction forces (which will be in opposite directions) between the seal and the piston assembly, and further by subtracting the weight of the piston assembly therefrom.

In accordance with the present invention, the method includes the steps of A) retaining piston assembly 15 at a first position along the path toward and away from the chamber in the presence of a first seal stiction formed between seal 30 and piston assembly 15. This retainment is accomplished by applying a driving force at a first level to the piston assembly in a first

direction (arrow 31 in FIGURE 1) opposite the hydraulic pressure force of the pumped fluid acting thereon in an opposite second direction (arrow 32 in FIGURE 2) . The next step includes B) incrementally increasing or decreasing the driving force from the first level to a second level at which piston assembly 15 just overcomes the first seal stiction. At this instance, the piston assembly breaks the first seal stiction formed between seal 30 and piston assembly 15, and measurably moves in either the first direction or the second direction along the path away from the first position. As mentioned above, the direction of the movement depends upon the direction and magnitude of the force applied to the piston assembly, and the magnitude of the hydraulic pressure.

The present invention further includes the steps of the

C) measuring the driving force at the second level, and

D) thereafter, retaining piston assembly 15 at a second position along the path in the presence of a second seal stiction formed between the seal and the piston assembly. This is accomplished by incrementally decreasing or increasing the driving force in the first direction or the second direction to a third level. After the second stop, the method includes the step of E) incrementally decreasing or increasing the driving force from the third level to a fourth level at which the piston assembly just overcomes the second seal stiction. Again, the direction of the movement depends upon the direction and magnitude of the force applied to the piston assembly, and the magnitude of the hydraulic pressure. However, the direction of measurable movement will be in the second direction or the first direction, opposite the direction of travel in step B, along the path away from the second position. Finally, the present invention includes the step of F) measuring the driving force at the fourth

level; and G) calculating the hydraulic pressure from the second level and the fourth level of driving force by canceling the opposite acting forces of the first seal stiction and the second seal stiction.

As mentioned, the forces acting on the piston assembly to initiate movement thereof in the chamber from a fixed position are the hydraulic forces (H) , the drive force (D) , the gravitational force (i.e., the weight (W) ) and the seal stiction forces (S _s ) . In simplified form, the hydraulic force is essentially equal to the sum of the weight of the piston assembly, the drive force necessary to break free of the first seal stiction (in the "seal stiction" method) and the seal stiction force itself.

Accordingly, by measuring the drive force at the instant the piston assembly measurably breaks free from a first seal stiction from a first fixed position in one direction (or initially in the opposite second direction) , and then measuring the drive force at the instant the piston assembly measurably breaks free from a second seal stiction from a second fixed position in the opposite second direction (or the one direction) , the seal stiction forces will be in opposite directions. Hence, assuming the seal stiction forces (S _s )are substantially equal and opposite, by adding the two force equations together, the respective seal stiction forces (S _s ) can be canceled out . This enables the sum of the hydraulic forces to be calculated through the equation H,, + H ₂ = C(D, _. + D ₂ + 2W) , where C is an empirical constant to be determined using conventional calibration techniques.

In the preferred embodiment of the present invention, the seal stiction cancellation method is performed on an electromagnetically driven pump device such as the

apparatus above-described. Accordingly, the driving force applied to the piston assembly is generated by the drive current flowing through the solenoid coil in one direction to drive the piston assembly along the path in the first direction, and in a reverse direction to drive the piston assembly along the path in the opposite second direction. It will be understood that the generated drive force applied to the piston assembly is proportionate to the drive current flowing through the coil. Hence, incrementally increasing or decreasing the drive current incrementally increases or decreases the drive force applied to the piston assembly proportionately.

The "seal stiction" cancellation method of the present invention may be commenced by initiating movement of piston assembly 15 from the fixed first position in either the first direction (arrow 31 in FIGURE 1) or the second direction (arrow 32 in FIGURE 2) . In the first instance, the driving force initially urges piston member 17 into chamber 13 in the first direction

31 along axis 18. Subsequently, after the drive force of pump device or apparatus 10 has caused piston assembly 15 to stop relative chamber 13 (i.e., step D) at the second position, the driving force will cause piston member 17 to reverse direction and back out of chamber 13. Similarly, in the second instance, the driving force initially urges piston member 17 out of chamber 13 in the second direction 32 along axis 18. Subsequently, after the driving force has caused piston assembly 15 to stop at the second position, the driving force will cause piston member 17 to reverse direction and move back into chamber 13. In either instance, however, the seal stiction forces will be substantially equal and opposite in direction which enables cancellation thereof.

The effective magnitude and direction of the driving force causing movement or retainment of piston assembly 15 relative chamber 13 is a function of the magnitude of the hydraulic pressure relative the gravitational forces and the seal stiction forces. For example, due to the vertical nature of the piston assembly, if the hydraulic pressure is relatively small, the gravitational forces and the seal stiction forces urged upon the piston assembly will have greater impact on the direction and magnitude of the driving force required to move the piston assembly along the path than the hydraulic force. Accordingly, to move the piston assembly into the chamber in the first direction

(arrow 31 in FIGURE 1) , the mere weight of the piston assembly urging the same into the chamber may be sufficient to overcome the hydraulic force urging the piston assembly back of chamber in the second direction

(arrow 32 in FIGURE 2) . Hence, to retain piston assembly at a fixed position relative chamber 13 or to move piston member 17 out of chamber 13, the drive force generally must be directed in the second direction combining the drive force with the hydraulic force to oppose the weight of the piston assembly. Moreover, depending upon the weight of piston assembly relative the hydraulic force, to extend piston member 17 back into chamber 13, the drive force may only need to be reduced, still directed in the second direction, from the drive force level required to retain the piston assembly relative chamber 13. In other instances, to move piston assembly 15 into chamber 13, the drive force may have to be reversed in direction (i.e., in the first direction) .

In contrast, should the hydraulic force urging piston member 17 out of chamber 13 be substantially greater than in the previous example, the direction of drive forces urged upon the piston assembly to effect similar

movements thereof may need to be reversed. For instance, to retain piston assembly at a fixed position relative chamber 13 or to move the same into chamber 13, the drive force generally must be directed in the first direction, combining the drive force with the weight of the piston assembly to oppose the greater hydraulic force urging the piston member out of the chamber. Moreover, again, depending upon the weight of piston assembly relative the hydraulic force, to push piston member 17 back out chamber 13, the drive force may only need to be reduced, still directed in the first direction, from the drive force level required to retain the piston assembly relative chamber 13. In other instances, to move piston assembly 15 into chamber 13, the drive force may have to be reversed in direction (i.e., in the second direction) .

Regardless of the magnitude of the hydraulic force and the direction of movement of the piston assembly, the present invention can be employed to cancel the seal stiction forces in an effort to determine the sum of the hydraulic forces measured during movement in the first direction and the second direction. During the measurement of the hydraulic forces to determine the hydraulic pressure, it will be understood that the intake valve 28 will be closed while the exhaust valve 28' will be opened for the duration thereof. Further, while the hydraulic pressure may vary during substantial movement of the piston assembly along the path of chamber 13, the pressure variation resulting from the relatively short displacement of the piston assembly during the seal stiction measurement technique is insignificant.

In accordance with the present invention, the calculating step further includes the step of multiplying the sum of the second current and the first

current by the empirical constant (C) to convert the current summation into the hydraulic pressure. Another step includes subtracting the weight of the piston assembly from the hydraulic force value .

The method may further include the step of extending the piston member of the piston assembly beyond the typical stopping point of a normal stroke in an effort to clear debris built up on the seal. This tends to slough off the debris from the spot where the initial backward stiction measurement was made to prevent the debris from interfering with one of the strokes.

In the "seal friction" cancellation technique, another method for calculating the hydraulic pressure of the pumped fluid in the pump assembly is provided by measuring the drive force applied to the piston assembly to move the same along the chamber at a substantially constant velocity. By measuring the respective drive force to move the piston assembly in both directions along the chamber (i.e., the first direction and the second direction) , the hydraulic force is calculated by subsequently canceling out the seal friction forces (which will be in opposite directions) between the seal and the piston assembly, and further by subtracting the weight of the piston assembly therefrom.

Hence, this method includes the step of A) moving the piston assembly 15 from a first position to a second position along the path in chamber 13 to attain a substantially constant first velocity of piston assembly 15 proximate the second position in the presence of a first seal friction between seal 30 and the piston assembly 15. This is accomplished by applying a continuous driving force to the piston assembly in a first direction along the path. The next

steps includes B) measuring the driving force at a first level proximate the second position during movement of piston assembly 15 in step A at the substantially constant first velocity; and C) moving the piston assembly from proximate the second position to the first position along the path in chamber 13. In accordance with the present invention, proximate the first position, the velocity of the piston assembly should be a substantially constant second velocity. This constant second velocity occurs in the presence of a second seal friction between seal 30 and piston assembly 15 and is accomplished by applying the continuous driving force to piston assembly 15 in a second direction along the path opposite the first direction. The present invention further includes the step of D) measuring the driving force at a second level proximate the first position during movement of piston assembly 15 in step C at the substantially constant second velocity. Finally, the method includes the step of E) calculating the hydraulic pressure from the first level and the second level of driving force by canceling the opposite acting friction forces of the first seal friction and the second seal friction.

Similar to the "seal stiction" force cancellation technique, the direction and magnitude of the force applied to piston assembly 15 for movement thereof along chamber 13 is a function of the magnitude of the hydraulic pressure relative the gravitational forces and the seal friction forces. The forces acting on the piston assembly during reciprocating movement thereof in the chamber at a substantially constant velocity are the hydraulic forces (H) , the drive force (D) , the gravitational force (i.e., the weight (W) ) and the seal friction forces (S _F ) . In simplified form, the hydraulic force is essentially equal to the sum of the weight of the piston assembly, the drive force

necessary to move the piston assembly at a substantially constant velocity along the path, and the seal friction force itself.

Accordingly, by measuring the drive force at the instant the piston assembly moves at a substantially constant velocity in one direction (or initially in the opposite second direction) , and then measuring the drive force at the instant the piston assembly moves at a substantially constant velocity in the opposite second direction (or the one direction) , the seal friction forces (S _F ) will be in opposite directions. Hence, assuming the seal friction forces (S _F ) are substantially equal and opposite, by adding the two force equations together, the respective seal friction forces can be canceled out. This enables the sum of the hydraulic forces to be calculated through the equation H _x + H = C (O + ₂ D + 2W) , where C is an empirical constant to be determined using conventional calibration techniques.

Again, similar to the "seal stiction" force cancellation technique, in the preferred embodiment of the present invention, the seal friction cancellation method is performed on an electromagnetically driven pump device such as the apparatus above-described. Accordingly, the driving force applied to the piston assembly is generated by the drive current flowing through the solenoid coil in one direction to drive the piston assembly along the path in the first direction, and in a reverse direction to drive the piston assembly along the path in the opposite second direction. It will be understood that the generated drive force applied to the piston assembly is proportionate to the drive current flowing through the coil. Hence, incrementally increasing or decreasing the drive

current incrementally increases or decreases the drive force applied to the piston assembly proportionately.

The "seal friction" cancellation method of the present invention may be initially performed by movement of piston assembly 15 at a substantially constant velocity from either the first position to the second position in the chamber, or from the second position to the first position. In either instance, however, the seal friction forces will be substantially equal and opposite in direction which enables cancellation thereof. During the measurement of the hydraulic forces to determine the hydraulic pressure, it will again be understood that the intake valve 28 will be closed while the exhaust valve 28' will be opened for the duration thereof. Further, while the hydraulic pressure may vary during substantial movement of the piston assembly along the path of chamber 13, the pressure variation resulting from the relatively short displacement (about .01 inch) of the piston assembly during the seal stiction measurement technique is insignificant.

In accordance with the present invention, the calculating step further includes the step of multiplying the sum of the second current and the first current by the empirical constant (C) to convert the current summation into the hydraulic pressure. Another step includes subtracting the weight of the piston assembly from the hydraulic force value.

Step A is accomplished by applying the drive current at a substantially constant first quantity which corresponds to generating a driving force at the first level. Further, this embodiment of the present invention includes the step of : after step B and before step C, retaining piston assembly 15 in chamber 13

proximate the second position to enable the a direction change of the piston assembly either from the first direction of movement to the second direction, or from the second direction of movement to the first direction. The retaining step is accomplished by incrementally decreasing or increasing the driving force in the first direction or the second direction from the first level to a third level. This is caused by incrementally increasing or decreasing the drive current from the first quantity to a third quantity. The third quantity of the current through the solenoid coil corresponds to the generation of the third level of the driving force.

Step C is accomplished by incrementally increasing or decreasing the driving force in the second direction or the first direction from the third level to the second level, which of course is accomplished by incrementally increasing or decreasing the drive current from the third quantity to a second quantity.

Finally, step E is accomplished by calculating the hydraulic force acting on the piston assembly from the first quantity of drive current and the second quantity of drive current.