U.S. patent application number 10/445690 was filed with the patent office on 2003-11-13 for state-variable control system.
Invention is credited to LaBudde, Edward V., Queeney, Paul J. JR..
Application Number | 20030211620 10/445690 |
Document ID | / |
Family ID | 26833028 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030211620 |
Kind Code |
A1 |
LaBudde, Edward V. ; et
al. |
November 13, 2003 |
State-variable control system
Abstract
The invention relates to a state-variable feedback control
system for monitoring and optimally controlling the operation of a
microfluidic aspirate dispense-system. A steady state operating
pressure is determined from the fluid, flow and/or operational
characteristics of the system. Measurements from one or more
pressure sensors are part of the control strategy to derive
information for active feedback control and/or to achieve the
desired operating pressure. Advantageously, the control system adds
to the versatility of the aspirate-dispense system, for example, by
permitting rapid dispensing of drops of different size. The control
system also desirably facilitates efficient, repeatable and
accurate performance and reduces wastage of valuable reagents or
fluid.
Inventors: |
LaBudde, Edward V.;
(Westlake Village, CA) ; Queeney, Paul J. JR.;
(Irvine, CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
26833028 |
Appl. No.: |
10/445690 |
Filed: |
May 27, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10445690 |
May 27, 2003 |
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09575395 |
May 22, 2000 |
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6589791 |
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60135131 |
May 20, 1999 |
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Current U.S.
Class: |
436/50 ;
436/180 |
Current CPC
Class: |
Y10T 436/115831
20150115; Y10T 436/12 20150115; Y10T 436/2575 20150115; B01L
2200/146 20130101; B01L 2400/0633 20130101; Y10T 137/0352 20150401;
B01L 3/0265 20130101; B01L 3/0268 20130101; B01L 2400/0478
20130101; B01L 2400/0487 20130101; G01N 35/1016 20130101 |
Class at
Publication: |
436/50 ;
436/180 |
International
Class: |
G01N 035/02 |
Claims
What is claimed is:
1. A method of active feedback control to monitor and control the
operation of a liquid reagent delivery system for transferring
predetermined quantities of reagent from a reagent source to
predetermined locations on or in a target, comprising: providing a
finite state machine controller for controlling the operation of a
positive displacement pump and a solenoid-actuated dispenser
hydraulically arranged in series with said positive displacement
pump and for controlling relative motion between said reagent
source, said target and said dispenser; determining a steady state
dispense pressure based on the fluid dynamical characteristic
equations of said delivery system; operating said positive
displacement pump to substantially overcome elastic compliance
within said delivery system and build pressure within said delivery
system to said steady state dispense pressure; operating said
positive displacement pump and opening and closing said dispenser
at a predetermined frequency to dispense said reagent in the form
of one or more droplets; providing relative motion between said
dispenser and said target to dispense said droplets at said
predetermined locations on or in said target; monitoring said
pressure within said delivery system using a pressure sensor and
comparing it to said steady state dispense pressure; and actively
adjusting said pressure in response to any substantial deviation of
said pressure from said steady state dispense pressure to generally
maintain said steady state pressure within said delivery
system.
2. The method of claim 1, wherein said method further comprises
aspirating said reagent from said reagent source into said
dispenser.
3. The method of claim 1, wherein said method further comprises
estimating and monitoring the Reynolds number.
4. The method of claim 1, wherein said method further comprises
estimating and monitoring the Weber number.
5. The method of claim 1, wherein said method further comprises
estimating and monitoring the elastic compliance.
6. The method of claim 1, wherein said method further comprises
estimating the density (.rho.) of said reagent.
7. The method of claim 1, wherein said method further comprises
estimating the viscosity (.mu.) of said reagent.
8. The method of claim 1, wherein said method further comprises
adjusting said steady state dispense pressure to dispense droplets
of varying size.
9. The method of claim 1, wherein said method further comprises
adjusting said steady state dispense pressure to dispense droplets
of varying exit velocity.
10. The method of claim 9, wherein said method further comprises
estimating and monitoring the exit velocity.
11. A method of providing active feedback control during the
operation of a fluid handling system for transferring fluid from
one or more sources to one or more targets, said fluid handling
system comprising a dispenser hydraulically coupled to a direct
current fluid source, an apparatus for providing relative motion
between said dispenser and said one or more sources and said one or
more targets, and a finite state machine controller for controlling
the operation of said fluid handling system, said method
comprising: computing or estimating a generally steady state
dispense pressure from the fluid and/or flow diagnostics of said
system; stabilizing system pressure within said system to set it at
said steady state dispense pressure by actuating said direct
current fluid source and/or said dispenser; operating said direct
current fluid source, said dispenser and said apparatus to deposit
predetermined quantities of said fluid at predetermined locations;
monitoring said system pressure using one or more pressure sensors;
and adjusting said system pressure and/or operational parameters in
response to any substantial deviations of said system pressure from
said steady state dispense pressure to provide accurate
dispensing.
12. The method of claim 11, wherein depositing said quantities of
said fluid comprises dispensing said quantities in the form of one
or more droplets.
13. The method of claim 12, wherein said method further comprises
selecting a dispense volume for each of said droplets.
14. The method of claim 11, wherein computing or estimating a
generally steady state dispense pressure comprises estimating the
fluid density (.rho.).
15. The method of claim 14, wherein computing or estimating a
generally steady state dispense pressure comprises estimating the
fluid viscosity (.mu.).
16. The method of claim 15, wherein the density and viscosity are
estimated using transient pressure measurements.
17. The method of claim 11, wherein computing or estimating a
generally steady state dispense pressure comprises: estimating the
capillary flow resistance (Rc) and orifice flow resistance (Ro) of
a nozzle through which said fluid is dispensed; and calculating the
fluid pressure drop through the nozzle during generally steady
state dispensing to estimate the steady state dispense pressure
(Pss) using the relationship: Pss=QRc+(QRo).sup.2 where, Q is the
flow rate.
18. The method of claim 11, wherein computing or estimating a
generally steady state dispense pressure comprises: estimating the
resistance to fluid flow through the system by perturbing or
modulating the flow rate about the desired flow rate to estimate
the capillary flow resistance (Rc) and orifice flow resistance
(Ro); and calculating the fluid pressure drop during dispensing to
estimate the steady state dispense pressure (Pss) using the
relationship: Pss=QRc+(QRo).sup.2 where, Q is the flow rate.
19. The method of claim 11, wherein said method further comprises
adjusting said steady state dispense pressure to dispense fluid
droplets of varying size and/or exit velocity.
20. The method of claim 11, wherein said method further comprises
aspirating said fluid from said one or more sources.
21. The method of claim 11, wherein said method further comprises
estimating and monitoring the Reynolds number.
22. The method of claim 11, wherein said method further comprises
estimating and monitoring the Weber number.
23. The method of claim 11, wherein said method further comprises
estimating and monitoring compliance within said system.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 09/575,395, filed May 22, 2000, which claims the benefit of
U.S. Provisional Application No. 60/135,131, filed May 20, 1999,
the entirety of each one of which is hereby incorporated by
reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to the aspiration
and dispensing of microfluidic quantities of fluid and, in
particular, to a feedback control system for controlling and
monitoring the operation of aspirate-dispense systems to provide
optimal, efficient and versatile operation and performance.
[0004] 2. Description of the Related Art
[0005] There is an ongoing effort, both public and private, to
spell out the entire human genetic code by determining the
structure of all 100,000 or so human genes. Also, simultaneously,
there is a venture to use this genetic information for a wide
variety of genomic applications. These include, for example, the
creation of microarrays of DNA material on substrates to create an
array of spots on microscope slides or biochip devices. These
arrays can be used to read a particular human's genetic blueprint.
The arrays decode the genetic differences that make one person
chubbier, happier or more likely to get heart disease than another.
Such arrays could detect mutations, or changes in an individual's
chemical or genetic make-up, that might reveal something about a
disease or a treatment strategy.
[0006] One typical way of forming DNA microarrays utilizes an
aspirate-dispense methodology. An aspirate-dispense system
aspirates ("sucks") reagent(s) from a source of single strands of
known DNA and dispenses ("spits") them on one or more targets to
form one or more DNA arrays. Typically, an unknown sample of DNA is
broken into pieces and tagged with a fluorescent molecule. These
pieces are poured onto the array(s); each piece binds only to its
matching known DNA "zipper" on the array(s). The handling of the
unknown DNA sample may also utilize an aspirate and/or dispense
system. The perfect matches shine the brightest when the
fluorescent DNA binds to them. Usually, a laser is used to scan the
array(s) for bright, perfect matches and a computer ascertains or
assembles the DNA sequence of the unknown sample.
[0007] Microfluidic aspirate-dispense technology also has a wide
variety of other research and non-research related applications in
the biodiagnostics, pharmaceutical, agrochemical and material
sciences industries. Aspirate-dispense systems are utilized in drug
discovery, high throughput screening, live cell dispensing,
combinatorial chemistry and test strip fabrication among others.
These systems may be used for compound reformatting, wherein
compounds are transferred from one plate source, typically a 96
microwell plate, into another higher density plate such as a 384 or
1536 microwell plate. Compound reformatting entails aspirating
sample from the source plate and dispensing into the target plate.
In these and other applications it is desirable, and sometimes
crucial, that the aspirate-dispense system operate efficiently,
accurately and with minimal wastage of valuable reagents.
[0008] Conventional aspirate-dispense technologies and methods are
well known in the art, for example, as disclosed in U.S. Pat. No.
5,743,960, incorporated herein by reference. These typically use
pick-and-place ("suck-and-spit") fluid handling systems, whereby a
quantity of fluid is aspirated from a source and dispensed onto a
target for testing or further processing. But to efficiently and
accurately perform aspirate and dispense operations when dealing
with microfluidic quantities, less than 1 microliter (i L), of
fluid can be a very difficult task. The complexity of this task is
further exacerbated when frequent transitions between aspirate and
dispense functions are required. Many applications, such as DNA
microarraying, can involve a large number of such transitions.
[0009] Conventional aspirate-dispense technology, when applied at
these microfluidic levels, can suffer from unrepeatable,
inconsistent and slow performance, and also result in wastage of
valuable reagent. This is especially true at start-up and during
transient or intermittent operations. Moreover, conventional
aspirate-dispense systems can be limited in their adaptability, for
example, in providing a sufficiently quick response to changes in
the desired fluid output.
[0010] Therefore, there is a need for improved technology and
methodology that provides for efficient, repeatable, accurate and
versatile aspirate-dispense operations when handling and
transferring fluids in microfluidic quantities, while reducing
wastage of such fluids.
SUMMARY OF THE INVENTION
[0011] The present invention overcomes some or all of the above
limitations by providing a state-variable feedback control system
for monitoring and optimally controlling the operation of a
microfluidic aspirate dispense-system. A steady state operating
pressure is determined from the fluid, flow and/or operational
characteristics of the system. Measurements from one or more
pressure sensors are part of the control strategy to derive
information for active feedback control and/or to achieve the
desired operating pressure. Advantageously, the control system adds
to the versatility of the aspirate-dispense system, for example, by
permitting rapid dispensing of drops of different size. The control
system also desirably facilitates efficient, repeatable and
accurate performance and reduces wastage of valuable reagents or
fluid.
[0012] In accordance with one embodiment, the invention provides a
method of actively controlling a fluid delivery system. The fluid
delivery system generally comprises a dispenser hydraulically
arranged in series with a direct current fluid source. The method
comprises the step of determining a steady state dispense pressure
based on the fluid dynamical characteristic equations of the
system. The direct current fluid source is operated to cause the
steady state dispense pressure to exist within the system. The
dispenser and the direct current fluid source are then actuated to
dispense precise and/or predetermined quantities of a fluid onto a
target.
[0013] For purposes of summarizing the invention and the advantages
achieved over the prior art, certain objects and advantages of the
invention have been described herein above. Of course, it is to be
understood that not necessarily all such objects or advantages may
be achieved in accordance with any particular embodiment of the
invention. Thus, for example, those skilled in the art will
recognize that the invention may be embodied or carried out in a
manner that achieves or optimizes one advantage or group of
advantages as taught herein without necessarily achieving other
objects or advantages as may be taught or suggested herein.
[0014] All of these embodiments are intended to be within the scope
of the invention herein disclosed. These and other embodiments of
the present invention will become readily apparent to those skilled
in the art from the following detailed description of the preferred
embodiments having reference to the attached figures, the invention
not being limited to any particular preferred embodiment(s)
disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a simplified schematic illustration of a
microfluidic aspirate-dispense system or apparatus having features
in accordance with one preferred embodiment of the present
invention.
[0016] FIG. 2 is cross-sectional detail view of a syringe pump for
use in the system of FIG. 1.
[0017] FIG. 3 is a schematic illustration of a solenoid valve
dispenser for use in the system of FIG. 1.
[0018] FIG. 4 is a simplified fluid circuit schematic of the
positive displacement system of FIG. 1.
[0019] FIG. 5 is a simplified electrical circuit analogue
representation of the fluid circuit schematic of FIG. 4.
[0020] FIG. 6A is a control block diagram representation of the
fluid circuit schematic of FIG. 4.
[0021] FIG. 6B is a simplified version of the control block diagram
of FIG. 6A.
[0022] FIG. 6C is a root-locus diagram of the fluid circuit
schematic of FIG. 4.
[0023] FIG. 7 is a graph illustrating non-steady state dispense
volumes versus steady state dispense, volumes and showing the
beneficial effects of pressure compensation prior to
dispensing.
[0024] FIG. 8 is a simplified top-level control system (in block
diagram format) schematically illustrating the operation of the
aspirate-dispense system of FIG. 1 and having features in
accordance with one preferred embodiment of the present
invention.
[0025] FIG. 9 is a simplified state diagram schematically
illustrating the operation of the aspirate-dispense system of FIG.
1 and having features in accordance with one preferred embodiment
of the present invention.
[0026] FIG. 10 is a simplified schematic of a finite state machine
controller or control system (in block diagram format) having
features in accordance with one preferred embodiment of the present
invention.
[0027] FIG. 11 is a simplified schematic of a state-variable fluid
controller or control system (in block diagram format) of the
finite state machine controller of FIG. 10 having features in
accordance with one preferred embodiment of the present
invention.
[0028] FIG. 12 is a detailed schematic of a fluid parameter
calculator (in block diagram format) of the fluid controller of
FIG. 11 having features in accordance with one preferred embodiment
of the present invention.
[0029] FIG. 13 is a simplified state diagram schematically
illustrating the operation of the fluid parameter calculator of
FIG. 12 having features in accordance with one preferred embodiment
of the present invention.
[0030] FIG. 14 is a detailed schematic of the state diagram of FIG.
13 having features in accordance with one preferred embodiment of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] FIG. 1 is a schematic drawing of a microfluidic
aspirate-dispense apparatus or system 10 having features in
accordance with one preferred embodiment of the present invention.
The aspirate-dispense system 10 generally comprises a dispenser 12
hydraulically arranged in series with a positive displacement
syringe pump 22 intermediate a reservoir 16. Preferably, and as
discussed in greater detail later herein, the aspirate-dispense
system 10 further comprises an automated feedback control system
200 to monitor and control the operation and performance of the
aspirate-dispense system 10. The control system includes a
controller 210 and one or more pressure sensors 50 (labeled 50a,
50b) to monitor the pressure within the aspirate-dispense system 10
and provide diagnostic information about various fluid and flow
parameters of the hydraulic system.
[0032] The dispenser 12 is used to aspirate a predetermined
quantity of fluid or reagent from a source or receptacle 29 and
dispense a predetermined quantity, in the form of droplets or a
spray pattern, of the source fluid onto or into a target 30. The
fluid source 29 can comprise a single-well receptacle, a multi-well
microtiter plate or other suitable fluid source. The target 30 can
comprise a glass slide, a substrate, a membrane, a multi-well
microtiter plate or other suitable destination to which fluid or
liquid is to be transferred.
[0033] The positive displacement pump 22 meters the volume and/or
flow rate of the reagent aspirated and, more critically, of the
reagent dispensed. The reservoir 16 contains a wash or system fluid
14, such as distilled water, which fills most of the
aspirate-dispense system 10. In some situations, where large
quantities of the same reagent are to be dispensed, the reservoir
16 and syringe pump 22 can be filled with the reagent and the
system 10 can be used purely for dispensing.
[0034] A robot arm may be used to maneuver the aspirate-dispense
system 10 or alternatively the aspirate-dispense system 10 and/or
its associated components may be mounted on movable X, X-Y or X-Y-Z
platforms. In one preferred embodiment, the source 29 and target 30
are mounted or seated on respective movable X, X-Y or X-Y-Z
platforms or tables 212, 214. The X, X-Y or X-Y-Z platforms or
carriages 212, 214 provide relative motion between the platforms
212, 214 and the dispenser 12.
[0035] Also, multiple aspirate-dispense systems 10 may be utilized
to form a line or array of dispensers 12. These multiple
aspirate-dispense systems can include one or more state-variable
control systems in accordance with the invention, as needed or
desired, to control the system operation. Moreover, the
state-variable control system of the invention can be used in
conjunction with a multi-channel system comprising a manifold
having a supply line or rail feeding into multiple independent
channels with each manifold channel being in fluid communication
with a respective dispenser.
[0036] The pump 22 is preferably a high-resolution, positive
displacement syringe pump hydraulically coupled to the dispenser
12. Alternatively, pump 22 may be any one of several varieties of
commercially available pumping devices for metering precise
quantities of liquid. A syringe-type pump 22, as shown in FIG. 1,
is preferred because of its convenience and commercial
availability. A wide variety of other direct current fluid source
means may be used, however, to achieve the benefits and advantages
as disclosed herein. These may include, without limitation, rotary
pumps, peristaltic pumps, squash-plate pumps, and the like, or an
electronically regulated fluid current source.
[0037] As illustrated in more detail in FIG. 2, the syringe pump 22
generally comprises a syringe housing 62 of a predetermined volume
and a plunger 64 which is sealed against the syringe housing by
O-rings or the like. The plunger 64 mechanically engages a plunger
shaft 66 having a lead screw portion 68 adapted to thread in and
out of a base support (not shown). Those skilled in the art will
readily appreciate that as the lead screw portion 68 of the plunger
shaft 66 is rotated the plunger 64 will be displaced axially,
forcing system fluid from the syringe housing 62 into the exit tube
70. Any number of suitable motors or mechanical actuators may be
used to drive the lead screw 68. Preferably, a stepper motor 26
(FIG. 1) or other incremental or continuous actuator device is used
so that the amount and/or flow rate of fluid or reagent can be
precisely regulated.
[0038] Referring to FIG. 1, the syringe pump 22 is connected to the
reservoir 16 and the dispenser 12 using tubing 23 provided with
luer-type fittings for connection to the syringe and dispenser.
Various shut-off valves 25 (labeled 25a, 25b) and check valves (not
shown) may also be used, as desired or needed, to direct the flow
of fluid 14 to and/or from the reservoir 16, syringe pump 22 and
dispenser 12. Typically, the valve 25b is in the open position. The
valve 25a is in the closed position and is opened to draw fluid 14
from the reservoir 16 into the syringe pump 22, as and when
needed.
[0039] The dispenser 12 (FIG. 1) may be any one of a number of
dispensers well known in the art for dispensing a liquid, such as a
solenoid valve dispenser, a piezoelectric dispenser, a fluid
impulse dispenser, a heat actuated dispenser or the like. In one
form of the present invention a solenoid dispenser 12,
schematically illustrated in FIG. 3, is preferred.
[0040] Referring to FIG. 3, the solenoid valve dispenser 12
generally comprises a solenoid-actuated drop-on-demand valve 20,
including a valve portion or cavity 34 and a solenoid actuator 32,
hydraulically coupled to a tube, capillary or tip 36 and a nozzle
38. The solenoid valve 20 is energized by one or more electrical
pulses 13 provided by a pulse generator 19. A detailed description
of one typical solenoid valve dispenser can be found in U.S. Pat.
No. 5,743,960, incorporated herein by reference.
[0041] Referring again to FIG. 1, the wash fluid reservoir 16 may
be any one of a number of suitable receptacles capable of allowing
the wash fluid 14, such as distilled water, to be siphoned into
pump 22. The reservoir may be pressurized, as desired, but is
preferably vented to the atmosphere, as shown, via a vent opening
15. The particular size and shape of the reservoir 16 is relatively
unimportant. A siphon tube 17 extends downward into the reservoir
16 to a desired depth sufficient to allow siphoning of wash fluid
14. Preferably, the siphon tube 17 extends as deep as possible into
the reservoir 16 without causing blockage of the lower inlet
portion of the tube 17. Optionally, the lower inlet portion of the
tube 17 may be cut at an angle or have other features as necessary
or desirable to provide consistent and reliable siphoning of wash
fluid 14.
[0042] The aspirate-dispense system 10 is preferably configured to
minimize the formation and accumulation of gaseous bubbles within
the fluid residing in the system 10, and particularly in the
dispenser 12 and feedline 23. For example, to minimize bubble
formation, the components of the aspirate-dispense system 10 can be
configured so that the fluid movements within the system avoid
sharp local pressure drops, and hence gaseous bubble precipitation.
Additionally, the components may be configured such that none or
few "dead spots" are encountered by the fluid, thereby discouraging
bubble accumulation within the system. These configurations can
utilize suitably tapered inner cavities or lumens within the valve
portion 34, tip 36 and/or nozzle 38 to provide relief from gaseous
bubble precipitation and/or "dead spots."
[0043] In one preferred embodiment, the aspirate-dispense system 10
(FIG. 1) further comprises a suitably configured bubble trap 220.
The bubble trap 220 is in fluid communication with the dispenser 12
and encourages the migration of gaseous bubbles to collect within
the trap 220. By opening an open-close valve 225 in fluid
communication with the bubble trap, the bubbles can be purged from
the system by expelling them via an exit line or tubing 223. The
exit tube 223 can lead to a waste position or to the reservoir
16.
[0044] The one or more pressure sensors 50 are provided at
appropriate locations on the aspirate-dispense system 10. In one
preferred embodiment, the pressure sensor 50a is situated at the
valve portion or cavity 34. Alternatively, or in addition, the
pressure sensor 50b can be placed intermediate the syringe pump 22
and the dispenser 12, such as on the feedline 23. In other
preferred embodiments, one or more pressure sensors can be
efficaciously placed at other suitable locations on the
aspirate-dispense system 10, as required or desired, giving due
consideration to the goals of providing suitably reliable system
pressure data, and/or of achieving one or more of the advantages
and benefits as taught or suggested herein.
[0045] Any one of a number of commercially available pressure
sensors may be used in conjunction with the invention. The pressure
sensors 50 are preferably differential pressure type devices.
Preferably, the full scale pressure limit of the pressure sensors
50 is about 68,950 Pa (10 psig). Preferably, the pressure sensors
50 have a resolution of about 0.01% maximum, an accuracy of about
1% maximum and a bandwidth of about 5 kHz minimum. The elastic
compliance at full scale pressure of the pressure sensors 50 should
preferably allow a volume flow less than about 3.times.10.sup.-10
m.sup.3. In other preferred embodiments, the pressure sensors 50
can be configured and designed in alternate manners with efficacy,
as required or desired, giving due consideration to the goals of
providing suitably reliable system pressure data, and/or of
achieving one or more of the advantages and benefits as taught or
suggested herein.
[0046] The controller 210 is a system finite state machine (FSM)
controller and generally comprises a host CPU or computer which
interfaces with some form of data memory. The host CPU serves as
the central controller and also the interface between the
controller 210 and the user. It allows the operator to input
dispensing and/or other data and to control, either independently
or simultaneously, each aspect of the aspirate-dispense system 10
(FIG. 1).
[0047] The host CPU or computer of the system controller 210 has a
slot or bus compatible to accept a plug-in circuit board. The
circuit board or "controller card" preferably includes an A/D
converter having a resolution of about 14 bits or more, an accuracy
of about 10 bits or more and a conversion speed of about 10 .mu.sec
(microseconds) or less. In other preferred embodiments, the
resolution, accuracy and/or conversion speed can be selected
otherwise with efficacy, as required or desired, giving due
consideration to the goals of providing suitably accurate and fast
data acquisition and control, and/or of achieving one or more of
the advantages and benefits as taught or suggested herein.
[0048] The controller card of the system controller 210 preferably
mounts or plugs into a computer bus providing data transfer and
communication of instructions. The host CPU or computer also
provides power to the controller card and further allows an
operator to access, program and control the functions of the
controller card. It is further contemplated that the host CPU or
computer contains suitable computer software compatible with the
host CPU (or computer) and the controller card which facilitates
operation of the system as described herein.
[0049] Preferably, a display device and data input means are
integral with the host CPU or computer thereby providing means to
input data into a memory or static RAM array located on the
controller card and to verify the same using the display device. As
is known by those of ordinary skill in the art, a keyboard, mouse,
trackball, light pen, capacitance touch screen, computer storage
media are all acceptable data input means. Likewise, a color video
monitor or screen provides a suitable display means.
[0050] Using a data entry device, such as a keyboard, an operator
or user may enter data into the host CPU or computer in the form of
a data array or graphical bit map to thereby instruct the
electronic controller and aspirate-dispense system of the desired
operation, parameters and characteristics. Conventional computer
software may facilitate the entry of the data array or bit map via
the host CPU to the memory of the controller card. The skilled
artisan will recognize that a wide variety of computer systems,
software and host CPUs may be efficaciously used with the present
invention.
[0051] As illustrated in FIG. 1, the system controller 210 is
interfaced with the dispenser 12, the pump stepper motor 26 and the
motion platforms 212, 214 and provides electrical signals having
frequency and duration to direct the operation of the
aspirate-dispense system 10. The system controller 210 further
receives pressure data from one or more of the pressure sensors 50.
As discussed in detail later, this pressure data is processed and
used to advantageously optimize the performance of the
aspirate-dispense system 10. The system controller 210 can also be
used to direct the opening and closing of any or all of the valves
25a, 25b and 225.
[0052] The skilled artisan will recognize that the hydraulic
coupling between the pump 22 and the dispenser 12 of the
aspirate-dispense system 10 provides for the situation where the
input from the pump 22 exactly equals the output from the dispenser
12 under steady state conditions. Therefore, the positive
displacement system uniquely determines the output volume of the
system while the operational dynamics of the dispenser 12 serve to
transform the output volume into ejected drop(s) having size,
frequency and velocity.
[0053] It has been discovered, however, that within the
aspirate-dispense system 10 there exists an elastic compliance
partly due to the compliance in the delivery tubing and other
connectors and components, and partly due to gaseous air bubbles
that may have precipitated from air or other gases dissolved in the
system and/or source fluid. As a result of this elastic compliance,
initial efforts to dispense small quantities of fluid resulted in
gradually overcoming the system compliance and not in dispensing
fluid or reagent. Once this elastic compliance was overcome, a
steady state pressure was found to exist and complete dispensing
occurred thereafter. To understand this phenomenon and the features
and advantages of the present invention, it is helpful to first
discuss the theoretical predicted behavior and theoretical flow
models relating to the positive displacement dispensing and
aspirating system 10 of FIG. 1.
[0054] Theory of Operation for Positive Displacement
Dispensing/Aspirating
[0055] The models included herein depict the basic fluid mechanical
theory of operation of the positive displacement dispense/aspirate
system of FIG. 1. Of course, the models may also apply to other
direct current fluid source dispensing devices for dispensing small
quantities of fluid. These models examine the design and operation
of the dispensing system from a mathematical, physical, circuit and
block diagram perspective representation, with each perspective
being equivalent but offering a distinct view of the system.
[0056] FIG. 4 is a simplified fluid circuit schematic drawing of
the aspirate-dispense system or apparatus 10 of FIG. 1. The
dispense system 10 generally comprises a dispenser 12 and a
positive displacement syringe pump 22 driven by a stepper motor 26.
The syringe pump 22 is hydraulically coupled to the dispenser 12
via a feedline 23. The dispenser 12 includes a drop-on-demand valve
20, such as a solenoid-actuated valve with a solenoid actuator 32
and a valve portion 34. The valve 20 is coupled to a tube or tip 36
and a drop-forming nozzle 38. The positive displacement pump 22
meters the volume and/or flow rate of the reagent or fluid
dispensed. The dispenser 12 is selectively operated to provide
individual droplets or a spray pattern of reagent, as desired, at
the predetermined incremental quantity or metered flow rate. The
dispenser 12 may also be operated in an aspirate mode to "suck"
reagent or other liquids from a fluid source.
[0057] As noted above, the positive displacement pump 22 is placed
in series with the dispenser 12 (FIGS. 1 and 4) and has the benefit
of forcing the dispenser 12 to admit and eject a quantity and/or
flow rate of reagent as determined (under steady state conditions)
solely by the positive displacement pump 22. In essence, the
syringe pump 22 acts as a forcing function for the entire system,
ensuring that the desired flow rate is maintained regardless of the
duty cycle, frequency or other operating parameters of the
dispensing valve, such as the solenoid-actuated valve 20. This is
certainly true for steady state operation, as discussed in more
detail below. However, for non-steady state operation, it has been
discovered that the elastic capacitance of the feedline and
precipitated gaseous bubbles in the system can cause transient
changes in dispensing pressure and system behavior.
[0058] A major part of the hydraulic compressibility or compliance
within the system 10 (FIGS. 1 and 4) is due to precipitated air.
The nominal solubility of air in liquids is in the range of about
2%. Even a small amount of this air converted to bubbles within the
hydraulic system will dominate the compliance of the system 10.
Thus, the dissolved air represents an important variable in
determining the compliance or elastic capacitance, C, and hence
determining the actuations of the drop-on-demand valve 20 and
syringe pump 22 to bring the system to the desired predetermined
and/or steady state pressure conditions (as discussed in greater
detail herein below).
[0059] Preferably, the reagents or fluids used with the
aspirate-dispense system 10 of the present invention are degassed,
by using known surfactants. This reduces the influence of
precipitated air in the system, and hence simplifies valve and pump
actuations, and improved repeatability of the actuations to achieve
the desired pressure conditions. Also, as indicated above, various
components of the aspirate-dispense system 10 can be configured to
reduce bubble precipitation and accumulation. Moreover, in one
embodiment of the invention, the bubble trap 220 (FIG. 1) is
provided. Nevertheless, despite whatever measures are taken, there
will be at least some elastic compliance in the system which can
cause transient variations in performance. These are discussed in
more detail below.
[0060] In fluid flow analysis, it is typical to represent the fluid
circuit in terms of an equivalent electrical circuit because the
visualization of the solution to the various flow and pressure
equations is more apparent. The electrical circuit components used
in this analysis include flow resistance (R), elastic capacitance
(C) and inertial inductance (L). As is known in the art, the
electrical equivalent of hydraulic pressure, P, is voltage and the
electrical equivalent of flow or flow rate, Q, is current. The
following defines the basic mathematical characteristics of the
components.
[0061] Resistance
[0062] Flow resistance, R, is modeled as a resistor in the
equivalent circuit and can be mathematically represented by the
following: 1 P Q = R ( 1 )
[0063] In the case of fluid flow, the resistance is usually
nonlinear because of orifice constrictions which give rise to
quadratic flow equations. This is further elaborated below. In the
present analysis it is assumed that laminar flow conditions are
present and that fluid flows through a circular cross section.
There are two types of flow resistance: capillary and orifice.
Capillary flow resistance applies to flow through sections of tubes
and pipes. Orifice flow resistance applies to constrictions or
changes in flow direction. Capillary resistance can be represented
by the following: 2 Q = A u _ ( 2 ) R c = L c A c ( 3 ) = 8 r c 2 (
4 )
[0064] where, R.sub.c is the capillary flow resistance, Q is the
flow rate, A.sub.c is the cross-sectional area, {overscore (u)} is
the mean velocity of flow, .OMEGA. is the flow resistivity, L.sub.c
is the capillary length, .mu. is the viscosity, and r.sub.c is the
radius of the circular capillary.
[0065] Orifice resistance is represented as: 3 Q = P R o ( 5 ) R o
= 2 A o C d ( 6 )
[0066] where, R.sub.o is the orifice flow resistance, .rho. is the
fluid density, A.sub.o is the cross-sectional area, and C.sub.d is
the discharge coefficient.
[0067] For a nozzle, the orifice constriction occurs at the
entrance to the nozzle and the nozzle is modeled as a capillary
(straight tube). This results in two resistances, orifice and
capillary, in series. In general, the pressure and flow
relationships in a system composed of a number of orifices and
capillaries can be defined under these conditions as:
.DELTA.P=.SIGMA.R.sub.0.sup.2Q.sup.2+.SIGMA.R.sub.cQ (7)
[0068] where .DELTA.P is the pressure drop, the quadratic term
R.sub.o.sup.2Q.sup.2 is due to the orifice resistance, which
depends on the fluid density, and the linear term R.sub.cQ is due
to the capillary resistance, which depends on the fluid viscosity.
This suggests that for a given geometry it may be possible to
measure these fluid properties (density and viscosity) by
performing regression fits to pressure and flow data. In order to
model the resistance, all the orifices and capillaries of the
system need to be identified.
[0069] Inductance
[0070] In laminar fluid flow through capillaries, the fluid
velocity profile is parabolic with zero velocity at the capillary
wall and the maximum velocity at the center. The mean velocity
{overscore (u)} is one half the maximum velocity. Since the fluid
has mass and inertia, there is a time constant associated with the
buildup of flow in the tube. This is modeled as an inductance in
series with the resistance. The derivation of the inertial time
constant, .tau., is illustrated in Modeling Axisymmetric Flows, S.
Middleman, Academic Press, 1995, Page 99, incorporated herein by
reference. The time constant, .tau., can be defined as: 4 = L R c =
r c 2 a 1 2 ( 8 )
[0071] where L is the inductance and a.sub.1=2.403. Thus, the
inertial inductance can easily be computed from the time constant,
.tau., and the capillary flow resistance, R.sub.c.
[0072] Capacitance
[0073] The walls of the feedline, any precipitated gaseous bubbles
in the fluid, and (to a very limited extent) the fluid itself are
all elastic (compressible). This phenomenon gives rise to an
elastic capacitance, where energy can be stored by virtue of the
compression of the fluid and bubbles and/or the expansion of the
fedline walls. The magnitude of the capacitance, C, can be found
from the following equations:
Z.sub.a=.rho.C.sub.s (9)
[0074] 5 Z ratio = Z a L ( 10 ) C = L ( Z ratio R c ) 2 ( 11 )
[0075] where, Z.sub.a is the acoustic impedance and C.sub.s is the
speed of sound. The speed of sound, C.sub.s, accounts for the
effects of fluid bulk modulus, wall elasticity, and elastic effects
of any gas in the system. In the present modeling, the feedline is
the major contributor to the elastic capacitance.
[0076] Physical Fluid Circuit Representation
[0077] The overall fluid circuit schematic construction of the
positive displacement system 10 (FIG. 1) is shown in FIG. 4. As
discussed before, the system 10 generally includes a stepper motor
26, a syringe pump 22, a feedline 23, and a drop-on-demand valve
20, with a solenoid actuator 32 and a valve portion or cavity 34
coupled to a tip 36 and a nozzle 38.
[0078] The syringe pump 22 (FIGS. 1 and 4) of the system acts as a
fluid current source and forces a given volume per step into the
system. The force available from the stepper motor 26 (FIGS. 1 and
4) is essentially infinite, due to the large gear ratio to the
syringe input. The input is impeded from the forces feeding back
from the system. Since volume, V, is the integral of the flow
rate:
V=.intg.Qdt (12)
[0079] and the flow rate, Q, is modeled as current, the syringe
pump is therefore a current source rather than a pressure (voltage)
source. Since any impedance in series with a current source has no
effect on the flow rate, this has the beneficial effect of removing
the influence of the impedance of the feed line (resistance and
inductance) on the flow rate. Advantageously, this solves a major
problem that would be present if a pressure source were used as the
driving function. For a pressure source, the feedline impedance
would offer a changing and/or unpredictable resistance to flow and
could give rise to hydraulic hammer pressure pulses and varying
pressure drops across the feedline which could affect the flow rate
through the dispense system, and hence the fluid output. By
utilizing a current source, such as the syringe pump, the effect of
changes in fluid impedance is substantially negligible or none on
the flow rate, and thus accurate and repeatable fluid volumes can
be readily dispensed.
[0080] Electrical Circuit Analogue Representation
[0081] A simplified electrical circuit analogue representation 40
of the positive displacement aspirate-dispense system fluid circuit
schematic 10 (FIG. 4) is shown in FIG. 5. The syringe pump 22
forces a total flow rate of Q.sub.t into the system. The flow is
comprised of Q.sub.c and Q.sub.n. Q.sub.c is the flow that is
driven into the elastic capacitance C.sub.t of the system and
Q.sub.n is the flow rate that is output from the nozzle 38 of the
system. The inductance L.sub.t and resistance R.sub.t are the
totals of all elements within the valve 20, tip 36, nozzle 38 and
feedline 23. The valve resistance R.sub.v varies with the actuation
displacement of the valve 20 during operation from forces applied
by the solenoid actuator 32. When the valve 20 is closed, the valve
resistance R.sub.v is infinite. The pressure in the feedline 23 is
P.sub.f and the pressure at the nozzle 38 is P.sub.n.
[0082] Block Diagram Representation
[0083] A block diagram or control system representation 42 of the
positive displacement dispense aspirate-dispense system 10 fluid
circuit schematic (FIG. 4) is shown in FIG. 6A. This is perhaps the
best way to see why the output fluid volume is synchronized to the
syringe input. As can be seen from FIG. 6A, this block diagram
model 42 represents a feedback loop, in which the difference
between Q.sub.t and Q.sub.n drives the flow into the elastic
capacitance, Q.sub.c. If the flow out of the nozzle 38 is not
exactly the same as the flow input, Q.sub.t, then the pressure in
the feedline 23, P.sub.f, will change. The feedback loop forces the
value of P.sub.f to be whatever is necessary, at steady state, to
maintain the output flow rate, Q.sub.n, to equal the input flow
rate, Q.sub.t. This is true regardless of the value of R.sub.t. The
inductive time constant is .tau. (in FIG. 6A) and the Laplacian
Operator is s=j.omega..
[0084] The value of feedline pressure, P.sub.f, will increase when
the valve 20 (FIGS. 3 and 4) is closed (Q.sub.n=0), since all the
input flow will go into the elastic capacitance as Q.sub.c. The use
of a time constant in the block diagram 42 (FIG. 6A) simplifies the
mathematical calculations when the valve has infinite resistance.
Qualitatively similar results will be obtained if the block diagram
42 (FIG. 6A) is modeled in a form including the unreduced Laplacian
formula for inductance (L) instead of the simplified time constant
(.tau.).
[0085] The block diagram model 42 (FIG. 6A) indicates that the
system has the potential for damped oscillations in flow. The
elastic capacitance is an integrator and the inertial time
constant, .tau., in the loop can give rise to the possibility of
underdamped oscillations in transient flow. These oscillations may
show up in pressure readings in the feedline 23 (FIGS. 1 and 4).
The magnitude of the oscillations is dependent on the damping,
which, in turn, is dependent on the flow resistance and the
resonate frequency of the system.
[0086] The closed-loop transfer function of the control system 42
(FIG. 6A) may be generally stated as follows: 6 W ( s ) = G ( s ) 1
+ G ( s ) H ( s ) ( 13 )
[0087] where:
[0088] W(s)=transfer function of the system expressed in the
Laplace domain;
[0089] G(s)=forward transfer function; and
[0090] H(s)=feedback transfer function.
[0091] The forward transfer function G through blocks or control
elements 54, 56, 58 (FIG. 6A) may be expressed as follows: 7 G ( s
) = 1 Cs 1 R t 1 s + 1 = ( 1 R t C ) 1 s ( s + 1 ) ( 14 )
[0092] By using equation (14), the control block diagram 42 (FIG.
6A) can also be represented by a simplified equivalent block
diagram 60 (FIG. 6B) with a block element 61 (FIG. 6B). The control
or block element 61 (FIG. 6B) incorporates the reduced forward
transfer function of equation (14). The feedback transfer function
H for the block diagram 42 (FIG. 6A) may be expressed as
follows:
H(s)=1 (15)
[0093] Substituting equations (14) and (15) in equation (13), the
unreduced closed-loop transfer function is expressed as: 8 W ( s )
= G ( s ) 1 + G ( s ) H ( s ) = Q n Q t = ( 1 R t C ) 1 s ( s + 1 )
1 + ( 1 R t C ) 1 s ( s + 1 ) ( 16 )
[0094] Equation (16) can be simplified to yield the closed-loop
transfer function in a reduced form, as shown below by equation
(17): 9 W ( s ) = Q n Q t = 1 1 + ( R t C ) s ( s + 1 ) ( 17 )
[0095] The characteristic equation of the control system 42 is
defined by setting the denominator of equation (16) equal to zero
and is given by: 10 1 + ( 1 R t C ) 1 s ( s + 1 ) = 0 ( 18 )
[0096] The zeros and poles of the characteristic equation can be
determined by the expression: 11 K Z ( s ) P ( s ) = G ( s ) H ( s
) = ( 1 R t C ) 1 s ( s + 1 ) ( 19 )
[0097] where, K is the gain and Z(s) and P(s) are polynomials which
yield the zeros and poles. The above characteristic equation (18)
has no zeros (n.sub.z=0) and two poles (n.sub.p=2) P.sub.1=0 and
P.sub.2=-1/.tau., where n.sub.z is the number of zeros and n.sub.p
is the number of poles. Also, the gain K of the system can be
defined as: 12 K = 1 R t C ( 20 )
[0098] The characteristic equation (18) can be manipulated to give
a quadratic equation (21): 13 s 2 + ( 1 ) s + K = 0 ( 21 )
[0099] where K is the gain as defined above by the expression (20).
Since equation (20) is a quadratic equation it has two roots which
can be expressed as: 14 s r = - 1 2 [ 1 1 - 4 2 K ] ( 22 )
[0100] These roots s.sub.r determine the stability characteristics
of the control system 42 (FIG. 6A). The nature of the roots s.sub.r
is dependent on the magnitude of the gain K=1/(R.sub.tC.tau.), or
more specifically on the magnitude of the parameter
(4.tau..sup.2K=4.tau./R.sub.tC). Note that since the time constant
(.tau.), the resistance (R.sub.t), and the capacitance (C) are all
positive real numbers, the parameter (4.tau..sup.2K) is also a
positive real number. The only exception to this is when the valve
20 (FIGS. 3 and 4) is closed, and hence the resistance R.sub.t is
infinite which results in K=0, so that (4.tau..sup.2K)=0.
[0101] For the case of 0<(4.tau..sup.2K).ltoreq.1, it is easily
deduced that the characteristic equation (18) or (21) has two real
roots s.sub.r<0. This indicates that the control system 42 (FIG.
6A) is unconditionally stable for
0<(4.tau..sup.2K).ltoreq.1.
[0102] For the case of (4.tau..sup.2K)>1, it is easily deduced
that the characteristic equation (18) or (21) has two real complex
conjugate roots s.sub.r which have negative real parts. This
indicates that the control system 42 (FIG. 6A) is unconditionally
stable for (4.tau..sup.2K)>1.
[0103] For the case of (4.tau./R.sub.tC)=0, that is when the valve
20 (FIGS. 3 and 4) is closed and the resistance R.sub.t is infinity
(K=0), it is easily deduced that the characteristic equation (18)
or (21) has two real roots s.sub.r=0 and s.sub.r<0. This
indicates that the control system 42 (FIG. 6A) is limitedly stable
for (4.tau..sup.2K)=0 or K=0.
[0104] The above stability analysis shows that the control block
representation 42 (FIG. 6A) of the fluid circuit schematic 10 (FIG.
4) of the positive displacement aspirate-dispense system 10 (FIG.
1) is always stable. This is true as the parameter (4.tau..sup.2K),
or alternatively the gain K, is varied from zero to infinity.
[0105] Another popular technique for studying the stability
characteristics of a control system involves sketching a root locus
diagram of the roots of the characteristic equation as any single
parameter, such as the gain K, is varied from zero to infinity. A
discussion of the root locus method can be found in most control
theory texts, for example, Introduction to Control System Analysis
and Design, Hale, F. J., Prentice-Hall, Inc., 1973, Pages 137-164,
incorporated herein by reference.
[0106] FIG. 6C shows a sketch of a root locus diagram 72 for the
control system representation 42 (FIG. 6A). The root locus diagram
72 is plotted in the s-plane and includes a real axis 74, Re(s), an
imaginary axis 76, Im(s), and a sketch of the root locus 78.
[0107] Typically, the determination of the root locus relies on a
knowledge of the zeros and poles of the control system. As
indicated above, the characteristic equation (18) of the control
block diagram 42 (FIG. 6A) has no zeros (n.sub.z=0) and two poles
(n.sub.p=2). Thus, the root locus 78 (FIG. 6C) will have two
branches and two zeros at infinity. On the real axis 74 (FIG. 6C),
the root locus will exist only between the two poles P.sub.1=0 and
P.sub.2=-1/.tau.. Since there are two infinite zeros, there will be
two asymptotes to the locus branches at angles given by: 15 k = ( 2
k + 1 ) 180 n p - n z k = 0 , 1 ( 23 )
[0108] so that, .theta..sub.k=90.degree., 270.degree.. The cg or
intersection of the asymptotes and the real axis 74 (FIG. 6C) is
given by: 16 c g = poles - zeros n p - n z ( 24 )
[0109] so that, cg=-1/2.tau.. Since there are only two poles
P.sub.1 and P.sub.2 on the real axis the breakaway point between
the two poles, P.sub.1=0 and P.sub.2=-1/.tau., is halfway between
the poles, that is, at s=-1/2.tau.. Also, since two branches are
leaving the breakaway point, the angles at breakaway are
.+-.90.degree.. This completes the sketch of the root locus 78 as
shown in FIG. 6C.
[0110] The root locus 78 (FIG. 6C) begins at the poles P.sub.1=0
and P.sub.2=-1/.tau. with the gain K being equal to zero. The root
locus 78 (FIG. 6C) then travels along the negative segment of the
real axis 74 (FIG. 6C) while the value of K is incremented and
converges at the breakaway point at s=-1/2.tau.. At the breakaway
point the root locus 78 (FIG. 6C) branches, parallel to the
imaginary axis 76 (FIG. 6C), towards the zeros at infinity with the
gain K being further incremented until it reaches infinity.
[0111] It will be appreciated that the root locus 78 (FIG. 6C)
represents all values of s in the Laplace domain for which the
characteristic equation (18) is satisfied as the gain K is varied
from zero to infinity. From the root locus diagram 72 (FIG. 6C) it
may be observed that all of the roots (except the root at the pole
P.sub.1=0) lie on the left side of the imaginary axis 76 in the
s-plane. This indicates that the system is unconditionally stable
for all possible values of the gain K>0 and the system is
limitedly stable when the gain K=0. Thus, the control system
representation 42 (FIG. 6A) of the fluid circuit schematic 10 (FIG.
4), and hence of the positive displacement aspirate-dispense system
10 (FIG. 1), demonstrates stability for all values of K. This
concurs with the above stability analysis based on the solution for
the roots of the characteristic equation (18) or (20).
[0112] It was demonstrated above that providing a positive
displacement pump 22 in series with a dispenser 12 (FIG. 1) has the
benefit of forcing the dispenser 12 to admit and eject a quantity
and/or flow rate of reagent as determined solely by the positive
displacement pump 22 for steady state operation. In essence, the
syringe pump 22 acts as a forcing function for the entire system,
ensuring that the desired flow rate is maintained regardless of the
duty cycle, frequency or other operating parameters of the
dispensing valve, such as the solenoid-actuated valve 20 (FIG. 3).
With such configuration and at steady state operation one does not
really care what the pressure in the system is because it adjusts
automatically to provide the desired flow rate by virtue of having
a positive displacement or direct current fluid source as a forcing
function for the entire system.
[0113] However, this does not address the situation of latent
and/or transient pressure variations, such as associated with
initial start-up of each dispense and aspirate function. In
particular, it has been discovered that the pressure in the system
is of critical concern for non-steady state operation involving
aspirating or dispensing of microfluidic quantities, typically less
than about 50 microliters (.mu.L), of liquid reagents or other
fluids. Specifically, for an aspirate function it has been
discovered that a system pressure close to or below zero is most
preferred, while for a dispense function it has been discovered
that a finite and positive predetermined steady state pressure is
most preferred.
[0114] The transitions between various modes (aspirate, dispense,
purge/wash) and/or flow rates or other operating parameters can
result in pressure transients and/or undesirable latent pressure
conditions within the positive displacement dispense/aspirate
system. Purge and wash functions usually entail active dispensing
in a non-target position. In some cases, when the same reagent is
to be aspirated again, several aspirate-dispense cycles can be
performed before executing a purge or wash function. Also,
sometimes a purge function may have to be performed during a
dispense function, for example, to alleviate clogging due to the
precipitation of gaseous bubbles within the system and/or source
fluid. Moreover, the accumulation of these bubbles can change the
system compliance over time, and hence the desired optimum
dispensing pressure.
[0115] For example, line 910 in FIG. 7 illustrates transient
dispense effects caused by initial start-up of a dispensing system
10 (FIG. 1) in which no pressure compensation scheme is utilized.
The x-axis 903 represents the dispense number or number of
dispenses and the y-axis 902 represents the dispense volume, in
nanoliters (nL) of each droplet or droplets dispensed. Line 914 in
FIG. 7 represents the target dispense volume of 100 nL.
[0116] As can be seen by the data of FIG. 7, the non-pressure
compensated (non-steady state) dispensed volume represented by line
910 is substantially smaller than the target dispense volume of 100
nL (line 914) since the system pressure at start-up is
substantially lower than the desired steady state and/or
predetermined pressure. The non-pressure compensated dispense
volume (line 910) can be lower by a factor of about ten compared to
the target dispense volume (line 914). Moreover, even after 23
dispenses (see FIG. 7) the dispensed volume (line 910) is still
below the target volume (line 914).
[0117] Line 912 represents a series of about 100 nL dispenses
performed in accordance with one preferred method of the present
invention, wherein an optimized pressurizing (300 steps of the
syringe plunger 64--shown in FIG. 2) is performed prior to
dispensing. The pressure compensation scheme provides dispense
volumes (line 912) which are in substantially close conformity with
the target dispense Volume (line 914) of 100 nL.
Under-pressurization (200 steps of the syringe plunger 64), as
illustrated by line 916, can result in dispense volumes that are
undesirably less than the target dispense volume 914. Similarly, as
illustrated by line 918, over-pressurization (400 steps of the
syringe plunger 64) can result in dispense volumes that are
undesirably more than the target dispense volume 914.
[0118] Automated Feedback Control System
[0119] The above discussion highlights the desirability of
controlling the hydraulic pressure within a microfluidic
aspirate-dispense system. The state-variable control system 200
(FIG. 1) of the present invention causes a steady state pressure to
exist within a fluid delivery, transfer or handling system, such as
the positive-displacement aspirate-dispense system 10 (FIG. 1),
prior to initiating dispensing operations. The initial positive
pressure overcomes the system's elastic compliance and thereby
achieves a steady state pressure condition prior to dispensing.
Advantageously, this assures that the fluid displaced by the
syringe pump 22 (FIG. 1) will be completely transferred as output
to the system nozzle, such as the nozzle 38 (FIG. 3).
[0120] The manner in which the active feedback control system 200
(FIG. 1) of the present invention monitors and handles the
operation of the aspirate-dispense system 10 (FIG. 1), including
the syringe pump 22 and dispenser 12, is discussed in detail now.
It will be appreciated that the control system of the present
invention may efficaciously be utilized with other liquid delivery
systems, direct current fluid sources and dispensers.
[0121] Top Level Control and Underlying Physics
[0122] FIG. 8 is a simplified top-level state-variable control
system 100 (in block diagram format) which schematically
illustrates the control and operation of a liquid delivery system,
such as the positive displacement aspirate-dispense system 10 shown
in FIG. 1, and has features in accordance with one preferred
embodiment of the present invention. For clarity and convenience,
the blocks of FIG. 8 are labeled with the same reference numerals
as those used for any corresponding system elements or components
of FIGS. 1-3.
[0123] The control system/diagram 100 (FIG. 8) shows block elements
for the stepper motor 26, the syringe pump 22, the fluid reservoir
16, the valve cavity 34, the pressure sensor(s) 50, the tip 36, the
nozzle 38, the bubble trap 220, the fluid source 29, the target 30,
a waste position 31 and the system finite state machine (FSM)
controller 210. The control system/diagram 100 also includes a
stepper motor switch SMSW, a syringe valve switch SVSW, an actuator
valve switch AVSW and a trap valve switch TVSW.
[0124] As schematically illustrated in FIG. 8, the controller 210
operates and controls the syringe pump 22 via actuations of the
stepper motor switch SMSW and the syringe valve switch SVSW (this
is represented by arrow A1 in FIG. 8). The controller 210 also
operates and controls the drop-on-demand valve 20 via actuations of
the actuator valve switch AVSW (this is represented by arrow A2 in
FIG. 8). The controller 210 further operates and controls the
bubble trap 220 via actuations of the trap valve switch TVSW (this
is represented by arrow A3 in FIG. 8).
[0125] The controller 210 receives pressure data from the pressure
sensor(s) 50 (this is represented by arrow A4 in FIG. 8). As
discussed in greater detail later herein, the pressure data is used
to derive various fluid and/or flow diagnostics. The controller
also receives data input by the operator (this is represented by
arrow A5 in FIG. 8).
[0126] The controller 210 controls the relative X, X-Y or X-Y-Z
motion between various components or associated components of the
aspirate-dispense system 10 (FIG. 1) such as between the source 29,
target 30 and the drop-forming nozzle 38. This can be achieved by
utilizing stepper motors and the like with the X, X-Y or X-Y-Z
tables 212, 214 (FIG. 1) and is schematically represented by the
block element 102 in FIG. 8. The resulting relative motion is
represented by the block element 104 in FIG. 8.
[0127] FIG. 9 is a simplified top-level state diagram 110 which is
associated with the top-level control system/diagram 100 of FIG. 8
and schematically illustrates one preferred sequence, combination
or cycle of steps or acts for controlling the operation of the
aspirate-dispense system 10 (FIG. 1). The user or operator provides
the inputs (as listed below) and the system hardware parameters (as
listed below) are set, if needed, in step 112 and the system 10 is
started. These inputs are used by the controller 210 to set,
compute and/or estimate the operational parameters such as the
valve actuation frequency, the valve on time, the stepper rate and
flow rate(s), as needed or desired. These operational parameters
can also be set, computed, estimated or adjusted at a later time.
In step 114, the syringe pump 22 is filled with system fluid such
as distilled water or other solvents. The system 10 now needs a
fluid which is to be transferred. Prior to aspiration of the fluid,
in step 116 the system pressure is stabilized or set to a
predetermined and/or steady state aspirate pressure. Once the
desired aspirate pressure is achieved, the fluid is aspirated in
step 118. After aspiration, if the fluid parameters such as density
and viscosity are not known then the system compliance is
estimated, determined and/or computed in step 119. This value of
the compliance in conjunction with other known parameters is used
to estimate, determine and/or compute fluid parameters (such as,
flow resistances, density and viscosity). If the density and
viscosity of the fluid are already known, steps 119 and 120 can be
skipped.
[0128] Prior to dispensing of the fluid onto the desired target, in
step 122 the system pressure is stabilized or set to a
predetermined and/or steady state dispense pressure. Once the
dispense pressure is stabilized, the system 10 is then used to
dispense droplets in step 124. During dispensing in step 124, the
compliance of the system may increase to an undesirable level or
limit due to bubble precipitation and accumulation. In this case,
the bubbles are dumped or purged in step 126. Once the bubbles have
been expelled, in step 122 the system pressure is again stabilized
or set to a predetermined and/or steady state dispense pressure and
dispensing commences in step 124.
[0129] If more fluid is needed to continue dispensing, the pressure
is again stabilized or set to a predetermined and/or steady state
aspirate pressure for aspirating in step 116. Once the desired
aspirating pressure is achieved, the fluid is aspirated in step
118. After aspiration, if needed, the relevant parameters are
estimated, computed and/or determined in steps 119 and 120. In step
122, the system pressure is stabilized or set to a predetermined
and/or steady state dispense pressure for dispensing. Once the
pressure is stabilized to the desired dispensing pressure, the
system 10 is then used to dispense droplets in step 124.
[0130] Once the dispense cycle is complete, any remaining source
fluid can be purged or flushed from the system 10 in step 128. Once
the purge is complete, the system 10 is ready to start again at
step 112, if needed. TABLE 1 below is a mode matrix of the
top-level control and is useful in summarizing and clarifying the
various operations as illustrated by the control system 100 of FIG.
8 and the state diagram 110 of FIG. 9.
1TABLE 1 Mode Matrix of the Top-Level System of FIG. 8 Stepper
Syringe Actuator Motor Valve Trap Valve Valve X, X-Y or Switch
Switch Switch Switch X-Y-Z MODE (SMSW) (SVSW) (TVSW) (AVSW)
Positioner Fill Syringe Reverse Fill Off Off X Dispense Forward
Operate Off Single or Stationary Dot/Droplet Multiple over Target
Pulse Dispense Line Forward Operate Off Multiple Motion or Spaced
Pulse over Target Dots/Droplets Aspirate Reverse Operate Off
Multiple Source Pulse or 100% Duty Cycle Hold Stop Operate Off Off
X Purge Forward Operate Off Multiple Waste Pulse or 100% Duty Cycle
Stabilize Reverse Operate Off Off X Aspirate Pressure Dump Forward
Operate Vent Off X Bubbles Calculate X X Off X X Fluid Parameters
Stabilize Forward Operate Off Off X Dispense Pressure
[0131] Fill Syringe Mode
[0132] The "Fill Syringe" mode is used to draw system fluid 14
(FIG. 1) from the reservoir 16 into the syringe pump 22. The
open-close valve 25a is opened and the stepper motor 26 is operated
in the reverse direction to draw a predetermined quantity of system
fluid 14. Once the desired quantity of system fluid 14 has been
collected, the valve 25a is closed. Preferably, and to expedite the
filling, the stepper motor 26 is operated at the maximum allowable
speed, though alternatively slower speeds may be efficaciously
utilized.
[0133] Dispense and Calculate Fluid Parameters Modes
[0134] During dispense modes the actuator valve 20 (FIG. 1) and the
stepper motor 26 are provided pulse width and frequency commands
from the system controller 210. The stepper motor 26 is operated in
the forward direction with the open-close valve 25a in the closed
position. One or more droplets can be dispensed at one location
before proceeding to the next. The target 30 can comprise a glass
slide, substrate, membrane or microtiter plate and the like.
Typically, in the dispense line mode the drops are dispensed closer
to one another as compared to the dispense dot/droplet mode. The
following is a list of user inputs and hardware parameters that are
provided to the system controller:
[0135] User Inputs:
[0136] Desired Droplet Size: Vd in liters
[0137] Fluid Density: .rho. in kg/m.sup.3 (if known)
[0138] Fluid Viscosity: .mu. in Pa-sec (if known)
[0139] Fluid Surface Tension: .sigma. in N/m
[0140] System Hardware Parameters or Inputs (provided by user):
[0141] Motion Inputs or Parameters or Pattern, including:
[0142] Distance Between Drops: X_drop in meters (depends on
application)
[0143] Translator or Table Velocity: U_xy in m/sec (can be
varied)
[0144] Stepper volume per step: Vstep in liters
[0145] Stepper maximum step rate: Fstepmax in Hz
[0146] Valve minimum on time: Tv_min in sec
[0147] Nozzle diameter: D_nom in meters
[0148] Nozzle length: L_nom in meters
[0149] Nozzle Discharge Coefficient: Cd
[0150] Fluid Angle with Target or Substrate: .theta. in radians (if
needed)
[0151] Valve Control Equations
[0152] The valve control is based on meeting a target,
predetermined or preselected Weber number in the nozzle 38 so that
proper drop detachment occurs. This is done as follows: 17 Qnom_Nwe
= 2 Nwe_max / ( D_nom / 2 ) 3 / 2 2 ( 25 ) Qest = Vd 1000 Tv_min (
26 )
Vd_min=1000Qnom.sub.--NweTv_min (27)
if, Vd<Vd_min:Q=Qest, otherwise:Q=Qnom.sub.--Nwe (28)
[0153] 18 Tv_est = Vd 1000 Q ( 29 )
if, Tv.sub.--est.ltoreq.Tv_min:Tv=Tv_min,otherwise:Tv=Tv.sub.--est
(30)
[0154] 19 Fvalve = U_x y X_drop ( 31 )
[0155] where, Qnom_Nwe is the nominal nozzle flow rate based on the
target Weber number Nwe_max, Qest is the estimate of maximum nozzle
flow rate for a given droplet volume Vd and the hardware parameter
Tv_min, Vd.sub.13 min is the minimum drop size based on Qnom_Nwe
and Tv_min, Q is the nozzle flow rate, Tv_est is an estimate of the
valve open or on time based on Q and the desired drop size Vd, Tv
(pulse width) is the valve open or on time, and Fvalve is the valve
open-close frequency. The target or preselected Weber number at the
nozzle 38 is achieved unless the user requests too small of a drop
size. A lower limit for the drop size can be determined empirically
for a given production set up.
[0156] Stepper Control Equations
[0157] The stepper control is based on meeting the input volume
requirements as follows: 20 Fstep = Vd Fvalve V step ( 32 )
[0158] where, Fstep is the stepper step rate or frequency.
[0159] Nozzle Flow Parameter Estimates
[0160] 1. Flow Resistance Parameters
[0161] The nozzle pressure or pressure drop at normal dispensing
operating conditions can be estimated from the following: 21 Rc = 8
L_nom ( D_nom 2 ) ( 33 ) Ro = 2 C d ( D_nom 2 ) 2 ( 34 )
Ps_cap=QRc (35)
Ps_orf =(QRo).sup.2 (36)
Ps.sub.--in=Ps_cap+Ps_orf (37)
[0162] where, Rc is the nozzle capillary flow resistance, Ro is the
nozzle orifice flow resistance, Ps_cap is the pressure drop due to
Rc, Ps_orf is the pressure drop due to Ro, Ps_in is the nozzle
pressure drop, and Q is the nozzle flow rate. Since the bulk of the
pressure drop during dispensing is through the system nozzle 38
(FIG. 3) due to the nozzle 38 being the major contributor to flow
resistance, Ps_in is an estimate of the desired dispensing steady
state pressure Pss.
[0163] 2. Nozzle Parameters from Steady State Pressure
Measurements
[0164] An estimate of the steady state pressure can also be
obtained by estimating the nozzle capillary and orifice flow
resistances by utilizing pressure measurements from the sensor(s)
50 during dispensing. The capillary flow resistance and the orifice
flow resistance can be estimated by making two measurements of the
system pressure at two flow rates during steady state dispensing
from the following: 22 Rc_est = PlQh 2 - PhQl 2 QhQl ( Qh - Ql ) (
38 ) Ro_est = PhQl - PlQh QhQl ( Qh - Ql ) ( 39 )
[0165] where, Ql is the low flow rate, Qh is the high flow rate, Pl
is the pressure measurement at Ql, Ph is the pressure measurement
at Qh, Rc_est is the estimate of the capillary flow resistance and
Ro_est is the estimate of the orifice flow resistance. Note that
Rc_est and Ro_est can desirably include contributions from the flow
resistances of the tip 36, valve 20 and/or other resistances in the
fluid flow path, though as indicated above these resistances are
expected to be small compared to the nozzle flow resistance.
[0166] The two pressure measurements, Pl and Ph, can be made can be
made during steady state on-line dispensing by modulating or
perturbing the flow rate about the operating point by a small
amount, for example, about .+-.5%. Optionally, a calibration mode
can be used off-line to make the pressure measurements. Once
estimates of the capillary flow resistance, Rc_est, and the orifice
flow resistance, Ro_est, have been determined, these can be used as
follows to obtain an estimate of the pressure drop which can be
estimated as a steady state pressure Pss:
Pss=QRc_est+(QRo_est).sup.2 (40)
[0167] Advantageously, the above estimates of the capillary flow
resistance, Rc_est, and the orifice flow resistance, Ro_est, permit
the density and viscosity of the fluid to be computed by using: 23
_est = Rc_est ( D_nom 2 ) 4 8 L_nom ( 41 ) _est = 2 ( Cd D_nom 2 4
Ro_est ) 2 ( 42 )
[0168] where, .rho._est is the estimated fluid viscosity and
.mu._test is the estimated fluid density.
[0169] 3. Nozzle Parameters from Transient Pressure
Measurements
[0170] Prior to steady state dispensing, transient pressure
measurements utilizing the pressure sensor(s) 50 can be used to the
estimate the capillary and orifice flow resistances. Again,
desirably these flow resistances can include contributions from the
flow resistances of the tip 36, valve 20 and/or other resistances
in the fluid flow path, though as indicated above they are expected
to be small compared to the nozzle flow resistance. The transient
approach is generally accurate when the initial pressure is within
about 30-50% of steady state value because a linearized
approximation of the differential equations is used. The linearized
pressure equations for an initial pressure of Po at the time that
pulsed dispensing operation begins and decays to the steady state
value of Pss can be approximated by: 24 P ( t ) = Pss + ( Po - Pss
) - t FvalveTv ( 43 ) = C ( Rc + 2 Ro 2 Qstep FvalveTv ) ( 44 )
Pss=Ro.sup.2Qnozzle.sup.2+RcQnozzle (45)
[0171] 25 Qnozzle = Qstep FvalveTv ( 46 )
[0172] where, P(t) is the instantaneous pressure as a function of
time t, .tau. is the system time constant, C is the elastic
capacitance, Qstep is the flow rate provided by the stepper motor
26, and Qnozzle is the flow rate through the nozzle 38. The elastic
capacitance, C, can be estimated from pressure and volume changes
with the drop-on-demand valve 20 closed, as is discussed below.
Note that (FvalveTv) is a scaling factor since the drop-on-demand
valve 20 is not open all the time in pulsed dispensing operation.
If the valve 20 is open continuously, this scaling factor reverts
to 1 since the nozzle flow rate, Qnozzle, and the stepper flow
rate, Qstep, are the same.
[0173] The above equations (43) to (46) can be manipulated to give:
26 = t1 ln ( Po - Pss ) - ln ( p1 - Pss ) FvalveTv ( 47 ) Rc =
Fvalve Qstep ( 2 PssTv - Qstep FvalveC_Est ) ( 48 ) Ro = Fvalve
Qstep ( Qstep C_EstFvalve - PssTv ) Tv ( 49 )
[0174] where, Po is the measured initial pressure prior to pulsed
dispensing at time to, Pss is the measured steady state pressure
after a substantially long time tss, and Pl is the measured
pressure during decay at an intermediate time t1. These pressures
can be measured using the pressure sensor(s) 50. Several
measurements of pressure/time can be made and the results averaged
to reduce noise. In this manner estimates of the nozzle capillary
flow resistance, Rc, and nozzle orifice flow resistance, Ro, are
obtained. Note that Rc and Ro can desirably include contributions
from the flow resistances of the tip 36, valve 20 and/or other
resistances in the fluid flow path, though as indicated above these
resistances are expected to be small compared to the nozzle flow
resistance. Once estimates of the capillary flow resistance, Rc,
and the orifice flow resistance, Ro, have been determined, these
can be used as follows to obtain an estimate of the pressure drop
which can be estimated as a steady state pressure Pss:
Pss=QRc+(QRo).sup.2 (50)
[0175] Advantageously, the above estimates of the capillary flow
resistance, Rc, and the orifice flow resistance, Ro, permit the
density .rho. and viscosity .mu. of the fluid to be computed by
using: 27 = ( D_nom 2 ) 4 Rc 8 L_nom ( 51 ) = 2 ( Cd ( D - nom 2 )
2 Ro ) 2 ( 52 )
[0176] Aspirate Mode
[0177] The "aspirate mode" is used to draw fluid from the source 29
into the system via the nozzle 38. The stepper motor 26 is operated
in the reverse direction with the nozzle 38 dipped in the source
fluid. The aspiration is preferably performed at a predetermined
and/or steady state system pressure. Preferably, the aspirate
pressure is at or less than zero, slightly negative or reduced
relative to the ambient pressure level.
[0178] Preferably, the valve 20 is open continuously during
aspiration, that is, a 100% duty cycle is utilized. Advantageously,
since the system pressure is at or close to zero, predetermined
small volumes of source fluid can be substantially accurately
aspirated by metering the displacement of the syringe pump 22.
Also, by preferably utilizing an optimally slow motion of the
syringe pump plunger 64 (FIG. 2), via the stepper motor 26, while
having the valve 20 fully open, the reduced/negative aspirate
system pressure is kept close to zero so that the flow of source
fluid into the nozzle 38 and tip 36 is maintained generally
laminar. Moreover, utilizing a 100% valve duty cycle, during
aspiration, further assists in maintaining a generally laminar flow
of source fluid into the nozzle 38 and tip 36. Thus, turbulent
mixing of source fluid with system fluid 14 (FIG. 1) is
minimized.
[0179] In other preferred embodiments, the valve frequency, on time
and/or duty cycle and the stepper motor speed can be selected in
alternate manners with efficacy, as required or desired, giving due
consideration to the goals of effectively aspirating source fluid,
and/or of achieving one or more of the advantages and benefits as
taught or suggested herein.
[0180] Hold Mode
[0181] "Hold mode" is a standby condition used while the system is
waiting because of various reasons with the stepper motor 26
stopped. This may happen, for example, when the system is awaiting
further instructions from the operator.
[0182] Purge Mode
[0183] The "purge mode" is used to flush the system. For example,
this may be done at the termination of a dispense cycle to remove
any residual aspirated fluid in the system prior to the aspiration
of a new source fluid. During purging, the nozzle 38 is placed over
a waste position or receptacle 31 (FIG. 8).
[0184] Normal Pressure Purge
[0185] In one preferred embodiment, the normal dispensing operating
pressure is used while purging the system and the stepper motor 26
is operated in the forward direction. A larger than normal drop
size can be selected by the user for the purge mode, for example,
ten times the usual droplet size and the system operated in a
droplet dispensing mode. Alternatively, a 100% valve duty cycle may
be utilized, that is, the valve 20 is continuously open during the
purging. The total amount of volume to be purged is also
selected.
[0186] High Pressure Purge
[0187] In another preferred embodiment, the system is operated at a
higher pressure than that used for normal dispensing operations.
This can be accomplished by setting the stepper rate to two to
three or more times the normal dispensing rate, thereby raising the
pressure. The valve 20 may be pulsed or a 100% valve duty cycle
utilized. The total amount of volume to be purged can also be
selected.
[0188] Dump Bubbles Mode
[0189] When the elastic compliance within the system exceeds a
certain level, the unwanted gaseous bubbles are expelled from the
system by opening the bubble trap valve 225 (FIG. 1) preferably
with the valve 20 closed. The bubble-infested fluid can be dumped
into a waste position 31 (FIG. 8) or it can be returned to the
reservoir 16.
[0190] Pressure Bleed
[0191] In one preferred embodiment, the trap valve 225 is opened
and the pressure is left to bleed down to zero or ambient
conditions by itself, thus reducing the fluid loss.
[0192] Pressure Pump
[0193] In another preferred embodiment, the trap valve 225 is
opened and the syringe pump 22 (or stepper motor 26) is operated in
the forward direction until a desired or predetermined volume of
fluid is pumped out of the system. Alternatively, the system
pressure is raised by operating the syringe pump 22 (or stepper
motor 26) in the forward direction with the valve 225 initially
closed. The valve 225 is then opened and the syringe pump 22
operated in the forward direction until a desired or predetermined
volume of fluid is pumped out of the system. Alternatively, after
the system has been pressurized, the pressure may be allowed to
bleed down to zero or ambient conditions by itself. The pressurized
bubble dump or purge technique is faster than the pressure bleed
method and is generally more reliable in ensuring that all or most
of the bubbles are expelled from the system.
[0194] Stabilize Pressure Mode
[0195] As indicated above, aspirate and dispense operations are
preferably performed at a predetermined and/or steady state
pressure. Thus, after purge, bubble dump, aspirate and dispense
functions the pressure may need to be adjusted to the predetermined
and/or steady state value prior to proceeding with aspiration or
dispensing.
[0196] Set Pressure at Operating Point
[0197] Preferably, a pre-dispense pressure adjustment, compensation
or correction involves displacing the syringe pump plunger 64 (FIG.
2), via the stepper motor 26, while maintaining the valve 20 in a
closed position. The amount of plunger displacement can be computed
from the elastic compliance and the steady state pressure. The
steady state pressure, typically between 2000 to 6000 Pascals (Pa),
can be estimated, as discussed above, from flow resistances and/or
prior steady state or transient pressure measurements. The steady
state pressure can also be estimated from previously formulated
parametric tables or charts based on parameters such as the desired
drop size and/or flow rate and nozzle dimensions, among other
parameters. The elastic capacitance, C, can be estimated from: 28
C_Est = V P ( 53 )
[0198] where, V is the change in volume as determined by the
displacement of the syringe pump plunger 64 and P is the change in
pressure as measured by the pressure sensor(s) 50, with the valve
20 closed. Thus, the volume displacement, V, of the syringe pump
plunger 64, as provided by the steps of the stepper motor 26,
required to achieve steady state pressure conditions, Pss, can be
estimated by using:
.DELTA.V=C.sub.--Est(P-Pss) (54)
[0199] where, P in equation (54) is the instantaneous pressure as
measured by the pressure sensor(s) 50. By constantly or
periodically monitoring the pressure, P, as the syringe pump
plunger 64 is moved a continuous or periodic and updated
measurement of the elastic compliance, C_Est, can be iteratively
used in equation (54) until the system pressure converges to the
steady state value.
[0200] Equation (54) can be similarly used to estimate the plunger
displacement to provide pressure adjustment or compensation prior
to an aspirate function. The plunger 64 is displaced to adjust or
reduce the system pressure with the valve 20 in the closed
position. In this case, and as discussed before, the desired
aspirating pressure will typically be less than zero, slightly
negative or less than the ambient pressure.
[0201] Advantageously, this technique of setting the aspirate and
dispense pressure at the desired operating point does not waste
valuable fluid or reagent since the valve 20 is closed. Moreover,
the pressure adjustment can desirably be implemented quickly, since
the plunger volume displacement has already been determined. This
adds to optimizing the performance of the aspirate-dispense system
10.
[0202] Dispense Drops at Waste Container or Position
[0203] This involves dispensing a number of drops in a waste
position while actuating the valve 20 and stepper motor 26 until
the system pressure reaches or decays to a steady state value.
Though this results in some wastage of fluid, the advantages are
that the pressure/time data can be used to estimate the nozzle flow
resistances (using equations (47) to (49)), the fluid viscosity and
density (using equations (51) and (52)), and the elastic
capacitance using the basic differential equation of the elastic
compliance: 29 P t = Q C ( 55 )
[0204] where, Q is the nozzle or stepper flow rate and C is the
elastic compliance.
[0205] Error Flags (Warnings)
[0206] The operation of the system is monitored and various error
flags and warnings are provided to alert the system controller or
the operator of possible undesirable operating conditions.
[0207] Flow Numbers
[0208] The nozzle or droplet exit velocity Unom is given by the
following: 30 Unom = 4 Q D_nom 2 ( 56 )
[0209] The Reynolds number Re is given by the following: 31 Re =
D_nomUnom ( 57 )
[0210] The Weber number We is given by the following: 32 We =
D_nomUnom 2 ( 58 )
[0211] The Error Flags can include the following:
[0212] Laminar Flow Check
[0213] If Re>2000:
[0214] "Reynolds Number Too High; Turbulent Flow May Result"
[0215] Drop Detachment Check
[0216] If We<1:
[0217] "Weber Number Too Low; Unreliable Drop Detachment May
Result"
[0218] High Pressure Check
[0219] System pressure P is or will be greater than the feed line
burst pressure Pmax (for example, P>60,000 Pa):
[0220] "Operating Pressure Too High; Feed lines May Fail"
[0221] Splash Check
[0222] Unom>Usplash:
[0223] "Nozzle Velocity Too High; Splashing May Result"
[0224] Usplash is the splash velocity and can be estimated by the
following equation (59): 33 Usplash = - 3 CDK cos + 2 C 9 K 2 2 + 3
C 3 DK - 12 CD + 12 DK C D + C 4 K D
[0225] where, C defines the maximum diameter of the drop on the
surface when a splash occurs relative to the spherical drop
diameter before impact, K is the total energy multiple required for
a splash to occur, D is nozzle diameter and .alpha. is a constant
that expresses the uncertainty in the viscous power loss (see
Modeling Axisymmetric Flows: Dynamics of Films, Jets and Drops,
Stanley Middleman, 1995, Academic Press, Page 185, incorporated by
reference herein).
[0226] Valve Frequency Check
[0227] If Fvalve>1/Tv:
[0228] "Valve Frequency Too High; Cannot Produce Correct Drop
Size"
[0229] Stepper Resolution Check
[0230] If Fstep>KFvalve, where K=2 or more:
[0231] "Stepper Volume Per Step 1s Too High; Incorrect Drop Size
May Result"
[0232] Stepper Rate Check
[0233] If Fstep>Fstepmax:
[0234] "Stepper Frequency Too High; Syringe Can Not Supply Required
Flow Rate"
[0235] Elastic Compliance
[0236] If C>Cmax (upper limit of allowable capacitance):
[0237] "Elastic Compliance Too Large; Time To Vent Bubbles"
[0238] Note that the elastic capacitance can be estimated from
equations (53) and/or (55). The elastic capacitance and/or changes
in elastic capacitance can also be inferred from observing the
measured pressure and/or pressure profiles during pulsed
dispensing. The maximum difference in the instantaneous pressure
measurements (.DELTA.Pmax) during pulsed operation will decrease as
the system capacitance increases and this may be used to infer or
estimate the elastic capacitance and/or changes in the elastic
capacitance.
[0239] Synchronization
[0240] The actuations of the stepper motor 26, translator 214 (and
translator 212) and the valve 20 are synchronized to avoid errors.
The synchronization can utilize coincident start and stop or
predetermined phase lags or leads.
[0241] Stepper-Valve Timing
[0242] Improper stepper motor-valve synchronization can cause
undesirable changes in pressure. These can be estimated from the
basic differential equation (55) of the elastic compliance. For a
nominal stepper flow rate of about 5.times.10.sup.-10 m.sup.3/sec
and an elastic compliance of about 1.times.10.sup.-14 m.sup.5/N,
the pressure slew rate will be about 50,000 Pa/sec. For a pressure
change or error of about 250 Pa, the timing error is about 5
milliseconds. Note that higher values of the elastic compliance aid
in reducing the pressure slew rate, however, it takes longer for
the error to settle with higher values of compliance.
Alternatively, stepper-valve actuations can utilize predetermined
phase lags or leads, as needed or desired, giving due consideration
to the goals of achieving one or more of the benefits and
advantages as taught or suggested herein
[0243] Valve-Translator Timing
[0244] The translator is preferably allowed a time lead in order to
allow for it to accelerate up to the operating velocity. Once the
translator reaches the desired velocity, the valve can be actuated.
An error in synchronizing the valve-translator will result in a
position error at the dispensing location. If a position error of
10% is tolerable on a dot pitch of 2-3 mm with a maximum velocity
of 1 m/sec, then the maximum timing error between the valve and the
translator should be less than 0.25 milliseconds. Phase lags and/or
leads can be provided, as needed or desired, for example, to
compensate for the probable or anticipated trajectory of the
droplets.
[0245] State Finite System Machine (FSM) Controller
[0246] FIG. 10 is a simplified schematic of a finite state machine
controller or control system 210 (in block diagram format) having
features in accordance with one preferred embodiment of the present
invention. The controller 210 generally comprises a fluid
controller or control system 130, the X, X-Y or X-Y-Z motion
controller 102 and a user interface 134 (as discussed above).
[0247] The controller 210 receives inputs from a user 132 via the
user interface 134. The inputs including the system hardware
parameters are provided, communicated or transferred to the fluid
controller 130. These inputs (and parameters) include the desired
drop volume, the fluid density (if known), the fluid viscosity (if
known), the fluid surface tension, the motion parameters or pattern
including the distance between drops and the carriage velocity, the
stepper volume per step, the stepper maximum step rate, the valve
minimum on time, the nominal nozzle diameter, the nominal nozzle
length, the nozzle discharge coefficient and the fluid angle with
the target or substrate.
[0248] The user interface 134 also permits the user or operator 132
to select the mode of operation (aspirate mode, dispense mode and
other modes, as described above) which is communicated to the
controller 130. One or more of the pressure sensors 50 provide,
communicate or transfer pressure data to the controller 130.
[0249] The controller 130 processes the information it receives and
uses the processed information or output to control the operation
of the syringe pump 22 (via the stepper motor 26), the
drop-on-demand valve 20 and the bubble trap 220. This is done by
providing electrical signals having length and duration to the pump
stepper motor 26, the drop-on-demand valve 20, the syringe valve
25a (FIG. 1), and the bubble trap valve 225 (FIG. 1).
[0250] The operating parameters provided to the stepper motor 26 by
the controller 130 include the stepper operating step rate and mode
of operation. The operating parameters provided by the controller
130 to the drop-on-demand valve 20 include the valve on or open
time, the valve open frequency and/or the valve duty cycle.
Open-close commands are provided by the controller 130 to the
syringe valve 25a and the bubble trap valve 225.
[0251] The motion parameters or pattern including the distance
between drops and the carriage velocity are also provided,
communicated or transferred to the X, X-Y or X-Y-Z motion
controller 102. The mode of operation (aspirate mode, dispense mode
and other modes, as described above) is also communicated to the
motion controller 130. As indicated above, the controller 102
provides relative motion between the system nozzle 38 and the fluid
source 29, fluid target 30 and waste position 31, and/or other
components or associated components of the aspirate-dispense system
10, as needed or desired.
[0252] State-Variable Fluid Controller
[0253] FIG. 11 is a schematic (in block diagram format) of the
state-variable fluid controller or control system 130 having
features in accordance with one preferred embodiment of the present
invention. The control system 130 generally comprises a control
block or system element 140 for calculating operating parameters, a
control block or system element 142 for calculating or estimating
fluid parameters (density and viscosity) and the system elastic
capacitance, a control block or system element 144 for providing an
estimate of the target and/or desired aspirate system pressure, a
control block or system element 146 for providing an estimate of
the target and/or desired dispense system pressure, a control block
or system element 148 for providing an estimate of the target
and/or desired dispense system pressure prior to or during the
bubble dump/purge function.
[0254] The state-variable fluid controller 130 also comprises a
subtracter system element 152 for comparing and computing the
difference between the desired and/or target pressure and the
actual system pressure as measured by the pressure sensor(s) 50.
The subtract system element 152 feeds the pressure difference
(.DELTA.P) output into a control block or system element (syringe
pump message formatter) 156 either directly or via a control block
or system element 154. The control block or system element 154
computes the volume change or displacement (.DELTA.V) that should
be provided by the stepper motor 26 (or syringe pump 22). The
control block or system element 156 then processes the pressure or
volume input data and provides the stepper motor 26 with stepper
frequency and stepper size commands to effectuate the pressure
adjustment, correction or compensation. A further control block or
system element (aspirate valve table) 158 is provided to control
the on-time and/or duty cycle of the valve 20 during
aspiration.
[0255] The control block or system element 140 receives input data
including hardware parameters from the user. These inputs (and
parameters) include the desired drop volume, the fluid density (if
known), the fluid viscosity (if known), the fluid surface tension,
the motion parameters or pattern including the distance between
drops and the carriage velocity, the stepper volume per step, the
stepper maximum step rate, the valve minimum on time, the nominal
nozzle diameter, the nominal nozzle length, the nozzle discharge
coefficient, and the fluid angle with the target or substrate.
[0256] If the fluid density and viscosity are not known, they can
be estimated by the fluid parameter control block or system element
142, as discussed below, and provided to the control block or
system element 140. If the fluid surface tension is not known, an
off-line calibration can be used to estimate the surface tension,
as discussed below, and provided to the control block or system
element 140.
[0257] The control block or system element 140 further receives
system pressure data as measured by the pressure sensor(s) 50. The
system operator also communicates the mode of operation (aspirate,
dispense, bubble dump and other modes, as discussed above) to the
control block or system element 140 and/or to the control block or
system elements 142, 144, 146, 148. If needed or desired, other
data, if and when available, such as the flow resistances, flow
rates and elastic capacitance can be provided to the control block
or system element 140.
[0258] The control block or system element 140 processes the input
data and controls the operation of the drop-on-demand valve 20 and
the stepper motor 26 (and hence the syringe pump 22). The valve and
stepper control equations (25) to (32) are used, and the control
block or system element 140 provides command signals to control the
valve on or open time, the valve open frequency and/or the valve
duty cycle, and the stepper frequency or step rate and the stepper
step size.
[0259] The control block or system element 140 also controls the
operation of the bubble trap 220 via the valve 225 (FIG. 1) by
opening the valve 225 when bubbles within the system are to be
expelled. Furthermore, the control block or system element 140
continuously or periodically monitors and/or computes the Reynolds
number, the Weber number, the nozzle velocity, the system pressure,
the valve frequency, the stepper resolution, the stepper rate and
the system elastic compliance and provides the system controller or
user with warning signals or error flags (as discussed above) of
any undesirable operational conditions. This can be done by
providing an alert message via the user interface and/or by an
audible alarm, among other ways.
[0260] TABLE 2 below is a top-level mode matrix of the fluid
controller 130 and summarizes and clarifies some of the various
operations as illustrated in FIG. 11 via the positions of the
switches S1, S2, S3, S4, S5, S6 and S7. (The "adj pres" switch
position in TABLE 2 and FIG. 11 refers to adjust pressure; the
"purge" switch position in TABLE 2 refers to the "bubble purge
pressure.sub.table (+/-)" switch position in FIG. 11.)
2TABLE 2 Mode Matrix of the Fluid Controller of FIG. 11 MODE S1 S2
S3 S4 S5 S6 S7 Hold normal normal X operate operate off X (normal)
(normal) Aspirate X X X adj pres adj pres X X Dispense X X X X X X
X Bubble X X purge adj pres adj pres off .DELTA.V Purge or Dump
[0261] Aspirate
[0262] TABLE 3 below is a mode matrix illustrating the aspirate
sub-operations of the fluid controller 130 and summarizes and
clarifies some of the various operations as illustrated in FIG. 11
via the positions of the switches S1, S2, S3, S4, S5, S6 and S7.
(The "adj pres" switch position in TABLE 3 and FIG. 11 refers to
adjust pressure; the "aspirate" switch position in TABLE 3 refers
to the "aspirate pressure .sub.table(+/-)" switch position in FIG.
11.)
3TABLE 3 Aspirate Submodes of the Fluid Controller of FIG. 11 MODE
S1 S2 S3 S4 S5 S6 S7 Initialize X X aspirate adj pres adj pres off
.DELTA.V (Stabilize) Aspirate Pressure Input Fluid X X aspirate adj
pres adj pres aspirate .DELTA.P (Aspirate)
[0263] Prior to aspiration of source fluid the system pressure is
initialized, stabilized or set to a predetermined and/or
steady-state aspirate pressure. The desired aspirate pressure is
preferably less than zero, slightly negative or reduced relative to
the ambient pressure value. The control block or system element 144
(aspirate pressure table) provides this aspirate pressure. The
aspirate pressure can depend on the hardware parameters (for
example, the nozzle and tip dimensions), the fluid parameters (for
example, density and viscosity) and other parameters such as
operational parameters. Some or all of these parameters can be
provided to the control block or system element 144 which then
outputs the aspirate pressure value. Alternatively, or in addition,
the aspirate pressure may be predetermined for a given production
set-up.
[0264] With the valve 20 closed, the subtract system element 152
computes the difference between the pressure output value from the
control block or system element 144 and the system pressure as
measured by the pressure sensor(s) 50. The pressure difference
.DELTA.P is provided to the control block or system element 154
which continuously or periodically computes the needed volume
displacement of the syringe pump 22 using equation (54) or
.DELTA.V=C_Est.DELTA.P and provides this information to the control
block or system element 156. The control block or system element
156 in turn communicates stepper frequency (and stepper size)
commands to operate the stepper motor 26 to achieve the desired
aspirate pressure. The elastic capacitance C_Est can be iteratively
calculated during the pressure adjustment, correction or
compensation using updated pressure measurements which desirably
provides feedback control. Alternatively, the value of the elastic
capacitance C_Est can be provided from the control block or system
element 142.
[0265] Once the desired predetermined and/or steady state aspirate
pressure is reached, the system is ready to proceed with aspiration
of source fluid. Advantageously, the control system of the present
invention allows the desired aspirate pressure to be reached
quickly and without or with minimal wastage or leakage of any
remaining source or system fluid from the system nozzle 38 since
the valve 20 is closed during the pressure adjustment procedure.
Desirably, this adds to the optimizing and efficiency of the
operation of the aspirate-dispense system of the present
invention.
[0266] During aspiration, the nozzle 38 is dipped in the fluid
source 29 and the stepper motor 26 (and syringe pump 22) are
operated in reverse or decremented to suck a precise and/or
predetermined quantity of source fluid. The control block or system
element 156 communicates stepper frequency (and stepper step size)
commands to the stepper motor 26. The valve 20 is provided with
commands from the control block or system element (aspirate valve
table) 158 to operate the valve 20 at a predetermined valve on or
open time, valve open frequency and/or duty cycle. In one preferred
embodiment, a 100% valve duty cycle is utilized, though in other
embodiments alternate duty cycles, on times and/or open frequencies
can be utilized with equivalent efficacy, as required or desired,
giving due consideration to the goals of achieving one or more of
the advantages or benefits as taught or suggested herein.
[0267] Advantageously, during aspiration the subtract system
element 152 computes or compares the difference between the desired
predetermined and/or steady state aspirate pressure and the system
pressure as measured by the pressure sensor(s) 50, and communicates
this information to the control block or system element 156. If
this pressure difference .DELTA.P exceeds a certain tolerance
limit, the control block or system element 156 adjusts the stepper
frequency (and/or stepper step size) accordingly to maintain the
system pressure substantially the same as the desired aspirate
pressure. Desirably, this feedback control (loop) ensures that the
aspiration is performed at substantially the desired predetermined
and/or steady state pressure.
[0268] Dispense
[0269] TABLE 4 below is a mode matrix illustrating the dispense
sub-operations of the fluid controller 130 and summarizes and
clarifies some of the various operations as illustrated in FIG. 11
via the positions of the switches S1, S2, S3, S4, S5, S6 and S7.
(The "adj pres" switch position in TABLE 4 and FIG. 11 refers to
adjust pressure; the "calc" switch position in TABLE 4 refers to
the "syringe init pressure .sub.calc(+/-)" switch position in FIG.
11; the "init" switch position in TABLE 4 refers to the "syringe
init pressure .sub.table(+/-)" switch position in FIG. 11)
4TABLE 4 Dispense Submodes of the Fluid Controller of FIG. 11 MODE
S1 S2 S3 S4 S5 S6 S7 Calculate calcu- calcu- calc adj pres adj pres
off .DELTA.P C_Est late late Calculate calcu- calcu- X normal
normal operate X Fluid late late Parameters Initialize X X init adj
pres adj pres off .DELTA.V (Stabilize) Dispense Pressure Output
normal normal X normal normal operate X Drops/Fluid (Dispense)
[0270] The "calculate C_Est" and "calculate fluid parameters" modes
are used when the fluid parameters (density and viscosity) are
unknown and need to be estimated experimentally. The "calculate
C_Est" and "calculate fluid parameters" modes or operations are
discussed in greater detail later herein.
[0271] Prior to dispensing of fluid onto or into the target 30, the
system pressure is initialized, stabilized or set to a
predetermined and/or steady-state dispense pressure. The control
block or system element (syringe pump pressure table) 146 provides
this dispense pressure. The control block or system element 146 can
determine this desired dispense pressure in a number of ways. The
desired drop volume and the system hardware parameters (including
nozzle dimensions) are communicated to the control block or system
element 146. Other parameters such as fluid parameters (density,
viscosity), system parameters (flow resistances, elastic
capacitance), flow parameters (flow rates, fluid exit velocity
through the nozzle) and operational parameters (valve on time and
frequency and/or valve duty cycle, stepper frequency and step size)
can also affect the selection of the desired dispense pressure.
Some or all of these parameters, as available or estimated, can be
communicated to the control block or system element 146 to
facilitate in the proper selection of the dispense pressure.
[0272] The desired steady state dispense pressure can be estimated,
as discussed above, from flow resistances and/or prior steady state
pressure measurements or transient pressure measurements. The
steady state pressure can also be estimated from previously
formulated parametric tables or charts based on some or all of the
above fluid, system, flow and operational parameters. The control
block or system element 146 can also utilize regression analysis
techniques to estimate the optimum dispense pressure.
Alternatively, or in addition, the dispense pressure may be
predetermined for a given production set-up.
[0273] With the valve 20 closed, the control block or system
element 146 communicates the desired dispense pressure to the
subtracter 152. The subtract system element 152 computes the
difference between the pressure output value from the control block
or system element 146 and the system pressure as measured by the
pressure sensor(s) 50. The pressure difference .DELTA.P is provided
to the control block or system element 154 which continuously or
periodically computes the needed volume displacement of the syringe
pump 22 using equation (54) or .DELTA.V=C_Est.DELTA.P and provides
this information to the control block or system element 156. The
control block or system element 156 in turn communicates stepper
frequency (and stepper size) commands to operate the stepper motor
26 to achieve the desired dispense pressure. The elastic
capacitance C_Est can be iteratively calculated during the pressure
adjustment, correction or compensation using updated pressure
measurements which desirably provides feedback control.
Alternatively, the value of the elastic capacitance C_Est can be
provided from the control block or system element 142.
[0274] Once the desired predetermined and/or steady state dispense
pressure is reached, the system is ready to proceed with dispensing
onto or into the target 30. Advantageously, the control system of
the present invention allows the desired dispense pressure to be
reached quickly and without or with minimal wastage or leakage of
any source and system fluid from the system nozzle 38 since the
valve 20 is closed during the pressure adjustment procedure.
Desirably, this adds to the optimizing and efficiency of the
operation of the aspirate-dispense system of the present
invention.
[0275] During dispensing, the nozzle 38 is positioned over the
target 30 and the stepper motor 26 (and syringe pump 22) are
incremented or operated in the forward direction to meter precise
and/or predetermined quantities or volumes and/or flow rates of
fluid via the nozzle 38 onto or into the target 30. The control
block or system element 140 communicates stepper frequency (and
stepper step size) commands to the stepper motor 26. The valve 20
is also provided with commands from the control block or system
element 140 to operate the valve 20 at a predetermined valve on or
open time, valve open frequency and/or duty cycle. Relative motion
may be provided between the target 30 and the nozzle 38, as needed
or desired, by the motion controller 102.
[0276] During dispensing, the pressure is monitored by the pressure
sensor(s) 50 and communicated to the control block or system
element 140. Changes in the measured pressure and/or pressure
profile during pulsed dispensing operation can indicate a change in
the system elastic capacitance due to gaseous bubble precipitation
and accumulation. These changes in the measured pressure and/or
pressure profile can be used to estimate and monitor the elastic
compliance. Alternatively, or in addition, the differential
equation (55) may be used to estimate the elastic compliance. Also,
dispensing may be temporarily halted and the elastic compliance
estimated using equation (53).
[0277] The updated value of the elastic compliance is communicated
to the control block or system element 146 and an updated estimate
obtained for the desired dispense pressure. The system pressure can
then be adjusted to the desired dispense pressure by adjusting the
actuations of the valve 20 and or stepper motor 26 without halting
dispensing. Alternatively, dispensing may be temporarily halted,
for example, if a significant pressure adjustment is needed, and
the system pressure stabilized to the predetermined and/or steady
state pressure as described above via the control block or system
elements 152, 154, 156. Moreover, the system pressure is adjusted,
as needed or desired, between dispense cycles. This feedback
control desirably adds to the efficiency of the aspirate-dispense
system of the present invention. A similar approach may be utilized
if transient behavior is observed for other fluid, flow or system
parameters.
[0278] The determination of the desired optimum dispense steady
state pressure by the control system of the present invention
advantageously results in accurate and repeatable performance, for
example, in substantially consistently achieving the desired
output, droplet size, flow rate and/or droplet exit velocity. The
adjustment to the operating pressure to compensate for variations
in the fluid, flow and/or operational characteristics of the system
further enhances the repeatability and accuracy of the system.
[0279] Advantageously, the control system of the present invention
adds to the versatility of the system and allows the rapid
dispensing of droplets of different size. The control system can
quickly adjust the system pressure to achieve the desired
predetermined and/or steady state dispense pressure to change the
ejected droplet size. Alternatively, or in addition, other system
and operational parameters can be quickly adjusted to facilitate
the rapid dispensing of droplets of different size. This can have
various applications, for example, in performing a dilution series
across one or more microtiter plates. The adaptability in quickly
dispensing microfluidic droplets of different size can also be used
in ink jet printing to produce high resolution halftone printed
images.
[0280] The control system of the present invention also permits
efficient operation to achieve varying desired flow or output
characteristics. For instance, the system pressure and/or other
system and operational parameters can be quickly adjusted to
achieve a desired droplet velocity while maintaining the same
droplet size. Typically, a lower output fluid velocity is preferred
when dispensing on a glass slide to avoid splashing compared to
dispensing in a microtiter plate wherein splashing is generally not
a concern. Thus, the control system permits reliable operation at
multiple system pressures to produce substantially the same fluid
output and, in effect, can operate at forced, quasi or pseudo
steady state and non-steady state pressures. This further adds to
the versatility and adaptability of the control system of the
present invention.
[0281] Bubble Purge or Dump
[0282] As discussed above, the bubble purge or dump mode can be
performed in a number of ways. In one embodiment of the invention,
prior to a bubble purge or dump procedure, the system pressure is
initialized, stabilized or set to a predetermined pressure. The
control block or system element (bubble purge table) 148 provides
this predetermined pressure. This predetermined pressure can be
dependent on the volume of fluid that is to be removed and/or the
speed of the procedure.
[0283] With the valves 20 and 225 closed, the control block or
system element 148 communicates the desired predetermined pressure
to the subtracter 152. The subtract system element 152 computes the
difference between the pressure output value from the control block
or system element 148 and the system pressure as measured by the
pressure sensor(s) 50. The pressure difference .DELTA.P is provided
to the control block or system element 154 which continuously or
periodically computes the needed volume displacement of the syringe
pump 22 using equation (54) or .DELTA.V=C_Est.DELTA.P and provides
this information to the control block or system element 156. The
control block or system element 156 in turn communicates stepper
frequency (and stepper size) commands to operate the stepper motor
26 to achieve the desired predetermined pressure. The elastic
capacitance C_Est can be iteratively calculated during the pressure
adjustment, correction or compensation using updated pressure
measurements which desirably provides feedback control.
Alternatively, the value of the elastic capacitance C_Est can be
provided from the control block or system element 142.
[0284] Once the desired predetermined pressure is reached, the
bubble trap valve 225 (FIG. 1) is opened and the fluid containing
bubbles is expelled from the system. During this bubble expulsion,
the stepper motor 26 can be operated in the forward direction to
expedite the bubble removal procedure. Alternatively, the system
pressure can be allowed to bleed down to zero or ambient pressure
conditions by itself via the open bubble trap valve 225.
[0285] The high pressure bubble purge can also be performed via the
system nozzle 38 with the valve 20 continuously open or pulsed.
Similar pressurization procedures, as described for the bubble
purge or dump mode, can be also be used to purge or flush the
system of any residual or remaining aspirated fluid (or other
fluid) through the system nozzle 38.
[0286] Calculate C_Est and Fluid Parameters
[0287] Density and Viscosity
[0288] Referring to FIGS. 12 and 13, if the fluid density and/or
fluid viscosity are not known, the control block or system element
142 calculates or estimates the density and/or viscosity. As
discussed above, this can be done from steady state pressure
measurements (using equations (38)-(39) and (41)-(42)) or from
transient pressure measurements (using equations (47)-(49) and
(51)-(52)). These estimates of the density and viscosity are
communicated to the control block or system element 140 (and
146).
[0289] FIG. 12 is a schematic of the fluid parameter calculator 142
(in block diagram format) which utilizes transient pressure
measurements to estimate or determine the fluid density and/or
viscosity and has features in accordance with one preferred
embodiment of the present invention. The fluid parameter calculator
142 generally comprises a control block or system element 160 for
calculating or estimating the system elastic capacitance, a control
block or system element 162 for calculating or estimating the flow
resistances, a control block or system element 164 for computing or
estimating the fluid density and/or viscosity, and an initializer
control block or system element 166.
[0290] If the system elastic compliance C_Est is not known, it has
to be estimated or determined prior to computing the density and
viscosity. TABLE 5 below is a mode matrix illustrating the
Calculate C_Est dispense submode (represented by the control block
or system element 160 in FIG. 12) and summarizes and clarifies some
of the various operations as illustrated in FIG. 12 via the
positions of the switches SW1, SW2, SW3, SW4, SW5, SW6, SW7, SW8,
SW9 and SW10. Note that all inputs in TABLE 5 are sample and hold,
and the values are read when the switches open. (The "init" switch
position in TABLE 5 refers to corresponding "init density" and
"init viscosity" switch positions in FIG. 12 which respectively
represent the initial or user input density and viscosity.)
5TABLE 5 Mode Matrix of the Calculate C_Est Dispense Submode of
FIG. 12 MODE SW1 SW2 SW3 SW4 SW5 SW6 SW7 SW8 SW9 SW10 Start closed
closed closed init init closed closed closed closed Po Calcu-
lations Measure closed closed closed init init open closed open
closed PV2 PV1 Measure closed closed closed init init open open
open open Po PV2
[0291] In the Calculate C_Est dispense submode, the initial system
pressure Po=PV1 is measured by the pressure sensor(s) 50 and
communicated to the control block or system element 160. A
corresponding reference volume V1 is also communicated to the
control block or system element 160. The valve 20 is closed during
the Calculate C_Est dispense submode.
[0292] A predetermined pressure PV2 that is to be achieved within
the system is then communicated to the subtract system element 152.
The subtract system element 152 computes the difference between the
pressure output value from the control block or system element 142
and the system pressure as measured by the pressure sensor(s) 50.
The pressure difference .DELTA.P is provided to the control block
or system element 154 which computes the volume displacement of the
syringe pump 22 to achieve PV2 using equation (54) or
.DELTA.V=C_Est.DELTA.P, based on a guess or estimate of C_Est, and
provides this information to the control block or system element
156. The control block or system element 156 in turn communicates
stepper frequency (and stepper size) commands to operate the
stepper motor 26 to achieve the pressure PV2. The elastic
capacitance C_Est is iteratively calculated during the pressure
adjustment procedure by the control block or system element
154.
[0293] Once the pressure PV2 is reached, the pressure PV2 and the
updated reference volume V2 is communicated to the control block or
system element 160. The control block or system element 160 then
computes the elastic capacitance by the expression
C_Est=.DELTA.V/.DELTA.P=(V2-V1)/(PV- 2-PV1). An iterative technique
can also be utilized for determining C_Est based on using updated
and continuous or periodic pressure readings and volumes. This
completes the Calculate C_Est dispense submode.
[0294] The system pressure is now reset to the initial pressure Po
to initiate the Calculate Fluid Parameters dispense submode. TABLE
6 below is a mode matrix illustrating the Calculate Fluid
Parameters dispense submode (represented by the control block or
system elements 160, 162 in FIG. 12) and summarizes and clarifies
some of the various operations as illustrated in FIG. 12 via the
positions of the switches SW1, SW2, SW3, SW4, SW5, SW6, SW7, SW8,
SW9 and SW10. Note that all inputs in TABLE 6 are sample and hold,
and the values are read when the switches open. (The "init" switch
position in TABLE 6 refers to corresponding "init density" and
"init viscosity" switch positions in FIG. 12 which respectively
represent the initial or user input density and viscosity.)
6TABLE 6 Mode Matrix of the Calculate Fluid Parameters Dispense
Submode of FIG. 12 MODE SW1 SW2 SW3 SW4 SW5 SW6 SW7 SW8 SW9 SW10
Measure open closed closed init init open open open open X Po
Measure open open closed init init open open open open X P1 Measure
open open open init init open open open open X Pss
[0295] The transient pressure measurement scheme, as described
above, is used to estimate or determine the density and/or
viscosity. The pressure Po as measured by the pressure sensor(s) 50
is communicated to the 162. The nozzle 38 is positioned over the
waste position 31 and the stepper motor 26 (and syringe pump 22)
are incremented or operated in the forward direction to dispense
fluid. The control block or system element 140 communicates stepper
frequency (and stepper step size) commands to the stepper motor 26.
The valve 20 is also provided with commands from the control block
or system element 140 to operate the valve 20 at a predetermined
valve on or open time, valve open frequency and/or duty cycle.
[0296] After a time t1, the pressure Pl as measured by the pressure
sensor(s) is communicated to the control block or system element
162. This pressure Pl can be an average value to reduce signal
noise. Dispensing is continued for a long time until the pressure
reaches or decays to a steady state value Pss at time tss. This
steady state pressure is communicated to the control block element
162. The valve open time Tv, the valve open frequency Fvalve and
the stepper flow rate Qstep are provided to the control block or
system element 162. The control block or system element 162
computes the capillary flow resistance Rc and orifice flow
resistance Ro using equations (47) to (49) and communicates these
values to the control block or system element 164. The control
block or system element 164 is also provided with the nominal
nozzle diameter D_nom, the nominal nozzle length L_nom and the
nozzle discharge coefficient Cd. The control block or system
element 164 computes the estimated fluid density and viscosity
using equations (51) and (52).
[0297] Once the values of the fluid density and viscosity are
estimated, these values are communicated to the control block or
system element 140. This is illustrated below by the Hold submode
TABLE 7. Note that all inputs in TABLE 7 are sample and hold, and
the values are read when the switches open. (The "calc" switch
position in TABLE 7 refers to corresponding "init density" and
"init viscosity" switch positions in FIG. 12 which respectively
represent the initial or user input density and viscosity.)
7TABLE 7 Hold Mode to Transfer Calculated Fluid Parameters MODE SW1
SW2 SW3 SW4 SW5 SW6 SW7 SW8 SW9 SW10 Hold open open open calc calc
open open open open X
[0298] The operations of FIG. 12 to determine the elastic
capacitance and the fluid density and viscosity can also be
represented by the state diagrams of FIGS. 13 and 14. FIG. 13 is a
simplified state diagram 170 schematically illustrating the
operation of the fluid parameter calculator 142. If the fluid
density, viscosity and system compliance are not available, the
system compliance is determined in step 172. Then in step 174, the
fluid density and fluid viscosity are calculated. In step 176, the
calculated or estimated values of the density and viscosity are
communicated to the control block or system elements 140 and/or
146.
[0299] FIG. 14 is a more detailed schematic of the state diagram
170 of FIG. 13. Step 172 comprises a step 180 of starting the
calculations, a step 182 of measuring the initial pressure PV1 and
a step 184 of measuring the second predetermined pressure PV2
achieved by the actuations of the syringe pump 22. This allows the
elastic capacitance to be calculated as discussed above.
[0300] The system pressure is then reset to the initial pressure
Po. Step 174 comprises a step 186 of measuring the initial pressure
Po, a step 188 of measuring the pressure Pl at a later time t1 and
a step 190 of measuring the steady state pressure Pss at a time
tss. The fluid density and viscosity are then calculated based on
these transient pressure measurements and other parameters.
[0301] Surface Tension
[0302] If the surface tension is not known, it can be estimated
using an off-line calibration or operation. Drops of gradually
decreasing size are dispensed and the detachment from the nozzle 38
is observed by the user. At the onset or close to the onset of
unreliable and/or improper drop detachment, it can be assumed that
the Weber number is close to 1, that is, We.congruent.1. Equation
(58) and/or other suitable equation, model or correlation can then
be used to estimate the fluid surface tension .sigma..
[0303] While the components and techniques of the present invention
have been described with a certain degree of particularity, it is
manifest that many changes may be made in the specific designs,
constructions and methodology hereinabove described without
departing from the spirit and scope of this disclosure. It should
be understood that the invention is not limited to the embodiments
set forth herein for purposes of exemplification, but is to be
defined only by a fair reading of the appended claims, including
the full range of equivalency to which each element thereof is
entitled.
* * * * *