Note: Descriptions are shown in the official language in which they were submitted.
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y
I
MAGNETIC DIRECT DRIVE RECIPROCATING PUMP
APPARATUS AND METHOD WITH INTEGRAL PRESSURE SENSING
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 solutian 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
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magnetically 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 o-f 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.
a
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
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contact includes rubbing against the back-up washers,
as well as side leading 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
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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. Tn 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,
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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 may, for
example, be 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. The method further 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
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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.
Another broad aspect provides a method of measuring
hydraulic pressure of a pumped fluid in a chamber of a pump
device having a reciprocating piston assembly in fluid
communication with the pumped fluid in said chamber, and
movable along a path toward and away from said chamber in
sliding sealed contact with a seal for said chamber.
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 include 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
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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
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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
h
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.
F3RIEF 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:
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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
,r
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.
DETAIDED 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 directionof 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
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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
l0 and out of the chamber 13 free of additional external
forces driving piston member 17.
Accordingly, by providing a direct drive pump apparatus
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.
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In the preferred form, annular magnet 20 is provided by
a solenoid coil which generates a bi-c7.irectional -
electromagnetic field in response to the respective
direction of current flow therethrough (i.e., to
i
reverse the polarity). Further, drive magnet 23 is
preferably provided by a permanent magnet member
generating its own magneticf.ield, 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
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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
3
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 (Pp) and
Negative pole (Np) co-axially along central axis 22.
While FIGURE 1 illustrates the drive magnet Positive
pole (Pp) at the bottom of the drive magnet, while the
Negative pole (Np) 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
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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~) at a bottom portion of
solenoid coil 20, while the Negative pole (N~) is
positioned at a top portion of the coil. Accordingly,
the Positive pole (Pp) of drive magnet 23 is repelled
by the lower Positive pole (P~) of solenoid coil 20.
Similarly, the Negative pole (Np) of drive magnet 23 is
repelled by the upper Negative pole (N~) of solenoid
coil 20, while simultaneously being attracted to the
lower Positive pole (P~) 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 l5 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~) at
the bottom portion of solenoid coil 20, while the
Positive pole (P~) is positioned at the top portion of
the coil. Hence, the Positive pole (PP) of drive
magnet 23 is attracted to the lower Negative pole (N~)
of solenoid coil 20. Likewise, the Negative pole (Np)
of drive magnet 23 is attracted to the upper Positive
pole (P~) of solenoid coil 20, while further
simultaneously repelled by the lower Negative pole (N~)
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
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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.
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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
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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.
f
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 I6
includes a second opposite piston member l7' 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
" 30 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
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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
ZO 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
suction 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
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or weight of the piston assembly, the only other
variable force, otherthan hydraulic, is~either the
seal friction (when the piston assembly is moving at a
substantially constant velocity in the chamber) or seal
suction (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 dough 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 suction 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 suction
- 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
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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 for-ce 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
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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 suction.
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 (SS) . 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
suction 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 suction 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
(SS)are substantially equal and opposite, by adding the
two force equations together, the respective seal
suction forces (Ss) can be canceled out . This enables
the sum of the hydraulic forces to be calculated
through the equation Hl + Hz = C (D1 + DZ + 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
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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 37 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 25 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.
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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
l
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
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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
suction 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
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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.
Tn 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
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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 ofthe 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 (Sg) . In simplified form, the -
hydraulic force is essentially equal to the sum of the
weight of the piston assembly, the drive force
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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 (SF) will be in opposite directions.
Hence, assuming the seal friction forces (SF) 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~ + ~I - C 4D +Z 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
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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 .O1 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
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proximate the second position to enable the a direction
change of the piston _ assembly either from the ~ f 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
J
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
IO 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.
