U.S. patent number 3,855,129 [Application Number 05/232,128] was granted by the patent office on 1974-12-17 for novel pumping apparatus.
This patent grant is currently assigned to Waters Associates, Inc.. Invention is credited to Louis Abrahams, Burleigh M. Hutchins, Jr., James L. Waters.
United States Patent |
3,855,129 |
Abrahams , et al. |
December 17, 1974 |
**Please see images for:
( Certificate of Correction ) ( Reexamination Certificate
) ** |
NOVEL PUMPING APPARATUS
Abstract
A compact pumping system especially useful in liquid
chromatography wherein comprising a liquid path which is fully
flushed on each stroke of a pump, a pump which provides
substantially pulse-free flow, a pressure-sensor integrated into
said flow path and operably connected to the control circuit of a
bifilar stepping motor. This circuit is so designed that the
driving current applied to the motor is only that required to pump
the liquid. The avoiding of heat associated with a greater current
is particularly important in forming a compact package of motor and
pump for use in liquid chromatography.
Inventors: |
Abrahams; Louis (Worcester,
MA), Hutchins, Jr.; Burleigh M. (Framingham, MA), Waters;
James L. (Framingham, MA) |
Assignee: |
Waters Associates, Inc.
(Framingham, MA)
|
Family
ID: |
22871976 |
Appl.
No.: |
05/232,128 |
Filed: |
March 6, 1972 |
Current U.S.
Class: |
210/198.2;
210/137; 417/38; 73/61.56; 222/55; 222/63; 422/70 |
Current CPC
Class: |
G01N
30/36 (20130101); F04B 49/20 (20130101); F04B
17/03 (20130101); F04B 11/0058 (20130101); H02P
8/12 (20130101); G01N 30/32 (20130101); F04B
53/14 (20130101); G01N 2030/324 (20130101); G01N
2030/326 (20130101); G01N 2030/328 (20130101) |
Current International
Class: |
F04B
11/00 (20060101); F04B 11/00 (20060101); F04B
17/03 (20060101); F04B 17/03 (20060101); G01N
30/00 (20060101); G01N 30/00 (20060101); F04B
53/14 (20060101); F04B 53/14 (20060101); F04B
49/20 (20060101); F04B 49/20 (20060101); G01N
30/32 (20060101); G01N 30/32 (20060101); F04B
53/00 (20060101); F04B 53/00 (20060101); G01N
30/36 (20060101); G01N 30/36 (20060101); H02P
8/12 (20060101); H02P 8/12 (20060101); B01d
015/08 (); G01n 031/08 () |
Field of
Search: |
;73/61.1C ;417/2,38,44
;210/31C,198C ;222/55,63 ;23/253R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Quiesser; Richard C.
Assistant Examiner: Roskos; Joseph W.
Attorney, Agent or Firm: Cesari; Robert A. McKenna; John F.
Kehoe; Andrew F.
Claims
What is claimed is:
1. In a chromatography system of the type comprising (A) a liquid
delivery system comprising a pump and discharge port for supplying
liquid under pressure to (B) a compartment containing a stationary
phase forming means to interact with said liquid phase to effect a
separation thereof during its passage through said compartment, and
(C) means to collect the liquid phase leaving the compartment, the
improvement wherein said liquid delivery system comprises
1. interposed between said pump and said compartment, a liquid
pressure-sensing means, and
2. motor control means operably communicating between said
pressure-sensing means and a motor drive means for said pump, said
motor control means forming means to control the flow-output of
said pump by controlling said motor in response to the
pressure-sensed by said pressure-sensing means.
2. A system as defined in claim 1 wherein the output of said motor
is substantially continuously controlled in response to a liquid
pressure between said pump and said compartment.
3. A system as defined by claim 2 wherein said motor in a stepping
motor and said control means controls the current applied to drive
said motor.
4. A system as defined in claim 1 wherein said motor control means
comprises additional control means to modify motor output in
response to pressure, said additional control means forming means
to compensate for compression of said liquid being pumped, and to
maintain a substantially constant input of liquid into said column
despite changes in compression of said liquid with varying
pressure.
5. A system as defined in claim 4 wherein said pump comprises a
two-cylinder piston pump and wherein said pump is operated by a
drive forming means to operate the piston in each said cylinder by
driving a centrally-located member which, in turn, forms means to
simultaneously turn two non-circular members 180.degree. out of
phase with each other, each said non-circular member forming means
to operate a piston in each said cylinder and thereby provide means
for achieving a substantially constant discharge of liquid from
said pump.
6. A system as defined in claim 5 wherein substantially all
liquid-receptive volume between inlet of said pump cylinder and
discharge port from said cylinder is flushed by each cycle of said
piston.
7. Apparatus as defined in claim 5 wherein there are two check
valves mounted in series at both inlet and outlet ports of each
said cylinder.
8. A system as defined by claim 4 wherein said motor is a stepping
motor and said motor control means controls the current applied to
drive said motor.
9. A system as defined in claim 1 wherein said pump comprises a
two-cylinder piston pump and wherein said pump is operated by a
drive forming means to operate the piston in each said cylinder by
driving a centrally-located member which, in turn, forms means to
simultaneously turn two non-circular members 180.degree. out of
phase with each other, each said non-circular member forming means
to operate a piston in each said cylinder and thereby provide means
for achieving a substantially constant discharge of liquid from
said pump.
10. A system as defined by claim 9 wherein said motor is a stepping
motor and said motor control means controls the current applied to
drive said motor.
11. A system as defined in claim 9 wherein substantially all
liquid-receptive volume between inlet of a said pump cylinder and
discharge port from said cylinder is flushed by each cycle of said
piston.
12. Apparatus as defined in claim 9 wherein said pump comprises a
plurality of cylinders, a plunger in each cylinder, a liquid inlet
to each cylinder proximate the end of said plunger as positioned at
the end of its forward stroke, a liquid outlet to said cylinder
proximate the end of said plunger as positioned at the backward
stroke, and an annular conduit between said plunger and said
cylinder forming means for said liquid to reach said liquid outlet
in response to the forward movement of said plunger.
13. Apparatus as defined in claim 12 wherein said annular conduit
is from 0.001 to 0.005 inches in width.
14. Apparatus as defined in claim 12 wherein said motor is a
stepping motor and said control means controls the current applied
to drive said motor.
15. Apparatus as defined in claim 9 wherein said pump comprises a
plurality of cylinders, a piston in each cylinder, a liquid inlet
to each cylinder proximate the end of said piston as positioned at
the end of its forward stroke, a liquid outlet to said cylinder
proximate the end of said piston as positioned at the backward
stroke, and an annular conduit between said piston and said
cylinder forming means for said liquid to reach said liquid outlet
in response to the forward movement of said piston.
16. Apparatus as defined in claim 15 wherein the liquid's flow path
is no more than about 0.2 inches from the mean center line of said
flow path.
17. Apparatus as defined in claim 9 wherein the liquid's flow path
is no more than about 0.2 inches from the mean center line of said
flow path.
18. Apparatus as defined in claim 17 wherein there are two check
valves mounted in series at both inlet and outlet ports of each
said cylinder.
19. Apparatus as defined in claim 9 wherein there are two check
valves mounted in series at both inlet and outlet ports of each
said cylinder.
20. In a system of the type defined in claim 1, the improvement
wherein said flow-control means comprises A) Bourdon-type tube, the
interior of which forms a flow path for said liquid B) means for
sensing displacement of said tube in response to pressure of the
liquid therein, and C) motor control means comprising said sensing
means and a motor drive means for said pump.
21. A system of the type defined in claim 20 wherein said
Bourdon-type tube is enclosed within a housing and mounted between
a light source and a light-sensing means, said light source and
sensing means forming said displacement-sensing means, and also
forming means to provide an input signal to said motor control
means.
22. A system as defined by claim 21 wherein said motor is a
stepping motor and said motor control means controls the current
applied to drive said motor.
23. Apparatus as defined in claim 21 wherein the liquid's flow path
is no more than about 0.2 inches from the mean center line of said
flow path.
24. A system of the type defined in claim 20 wherein said Bourdon
tube is bent to form two substantially parallel segments of a
single conduit.
25. A system as defined in claim 20 wherein substantially all
liquid-receptive volume between inlet of a said pump cylinder and
discharge port from said cylinder is flushed by each cycle of said
piston.
26. A system as defined in claim 20 wherein said flow path of said
Bourdon-type tube has an inside diameter of from about 0.005 inches
to 0.10 inches.
27. A system as defined in claim 1 wherein said control means forms
means to controllably drive said motor over a speed ratio of about
100:1 or greater.
28. Apparatus as defined in claim 1 wherein said pump comprises a
plurality of cylinders, a piston in each cylinder, a liquid inlet
to each cylinder proximate the end of said piston as positioned at
the end of its forward stroke, a liquid outlet to said cylinder
proximate the end of said piston as positioned at the backward
stroke, and an annular conduit between said piston and said
cylinder forming means for said liquid to reach said liquid outlet
in response to the forward movement of said piston.
29. Apparatus as defined in claim 28 wherein the liquid's flow path
is no more than about 0.2 inches from the mean center line of said
flow path.
30. In a system as defined in claim 1 suitable for handling heat
sensitive liquids, and said motor and said pump being mounted
adjacent one another, the improvement wherein said motor is a
stepping motor system comprising a control means for controlling
the volume output of said pump by controlling the driving current
applied to the coils of said motor.
31. A liquid delivery system as defined in claim 30 wherein said
motor has an operating range which is substantially free of
resonance effects and is 100:1 or greater.
Description
BACKGROUND OF THE INVENTION
This invention is primarily directed towards improvements in a
liquid feed apparatus for use in liquid chromatography systems.
However, the pumping unit and various aspects thereof are useful
wherever a precise, controlled movement of liquid is desired and,
notably, in the movement of physiological liquids in the medical
field and movement of fluids in analytical processes other than
chromatography. Moreover, the various aspects of the invention
includes specific improvements in the field of controlling a pump
drive means, measuring liquid pressure, and pump design.
Because the invention was made with the immediate aim of solving
problems relating to liquid delivery systems used in liquid
chromatography systems, the background relevant to such problems is
set forth hereinbelow:
Liquid chromatography is a system whereby a mobile liquid phase is
passed through a compartment containing a stationary phase. During
the passage of the liquid phase, it interacts physically or
chemically with the stationary phase. This interaction results in a
separatin of components in the liquid phase. Such a separation is
usually manifested by the fact that different components in the
liquid phase pass through the stationary phase at different rates.
An analysis of samples of the effluent from the compartment taken
over a period of time provides a basis for determing the chemical
composition of the input liquid.
The analysis of the efflux is usually made continuously with a
refractometer or the like.
It is desirable to supply the liquid to the aforesaid
chromatography compartment (usually an elongated column packed with
particles) at very high pressures, usually from 100 to 10,000
psig.
A number of other attributes are desirable for pumping systems used
in chromatography:
They should have a wide operating range in terms of pump throughput
capacity.
They should have a minimal temperature deviation from the
temperature of the fluid being processed. Thus a motor integral
with the pumping system should be either very well insulated from
the liquid being pumped or should run at a temperature close to the
temperature of the liquid.
There should be a minimum flush time in which one liquid being
pumped can be replaced by another liquid being pumped.
The pumping system should be capable of providing means to moderate
the pump output capacity in response to pump output pressure, and
this moderating means should be sensitive enough to allow
compensation for compressibility on the pump output side of the
system, whether this compressibility be a characteristic of fluid
being pumped or of the pumping system itself.
The pumping system should include protective means to shut down the
pump in an emergency, i.e., when the outlet pressure reaches a
certain level.
The pump should provide as little mixing of the liquid being pumped
as is possible and preferably so little mixing that recycling of
the liquid through the pump is feasible.
The pumping action should be steady, i.e., pulseless.
In general the above-listed attributes tended to be
self-conflicting when a chromatography system was operated
according to the methods of the prior art. For example, the use of
pulse dampeners tended to result in undesirable mixing of the
fluid. Yet pulse-imparting pumps were those most acceptable for
reaching the desired high free pressures. And motors used to drive
said pumps generated considerable heat -- enough to affect the
characteristics of the fluids being pumped when the motor was
packaged proximate the pump as is convenient and customary.
Moreover, conventional pressure sensing means required dead space
in which liquid would become relatively stagnant and from which it
could not be quickly flushed when one wished to change the liquid
being pumped.
Applicants therefore set as their objectives to provide liquid
delivery apparatus and processes having a number of the
above-listed attributes and overcoming many of those problems
relating to conflicting design requirements.
SUMMARY OF THE INVENTION
In the pumping system of the invention, the primary advances in the
art have been made under the general categories of 1) pump motor
and control means, 2) construction of the pump itself, 3)
pressure-sensing means, and the various combinations of these
advances. It will be manifest, to those skilled in the art on
reading the instant application, that some of the advantages
obtainable by using all the features of the invention will be
attainable by constructing apparatus using only some of the
features of the invention.
Pump Construction.
The pump of the invention is provided with a plurality of plungers
operated with drive linkage between the motor and the plungers
which cause the output of the pump to be substantially constant
with time. Although any operable drive linkage may be used to
achieve this purpose, a particularly advantageous linkage comprises
the use of a drive gear between a) and pump motor and b) a
plurality of eccentric gears connected to the plunger of the pump
so that the plungers will be dependently operated in proper cycle
relationship to obtain the desired constant output of the pump.
Other advantageous features of the pump include a) novel check
valve means, which assure the timely and leak-proof closure of the
inlet and outlets to the pump chambers, b), novel drive linkage
between each plunger and its drive shaft whereby the plunger is
allowed to seek its own center line, and wearing of the pump
chambers seal means by the plunger is fully avoided, c) a feature
whereby the plunger is machined to have a diameter which is
sufficiently smaller than the chamber in which it reciprocates so
that, on the plunger's movement through the chamber, fluid being
displaced from the chamber follows a flow path generally defined by
the annular space between the advancing plunger and chamber wall.
This last feature allows the pump chamber to have what has been
found to be highly important feature: a chamber has its inlet at
one end thereof, its outlet at the other end thereof and,
consequently, a full-chamber flushing action at each stroke of the
pump.
Pressure sensing means.
If the pump motor and control means is to be responsive to the
pressure of the liquid being pumped, it is necessary to provide a
sensor for this pressure. It has been found most advantageous, in
order to conserve the advantages inherent in the flow
characteristics imparted to the liquid by the pump, to avoid use of
conventional sensing means which require some dead liquid volume in
which to place a sensor or which require some dead liquid volume to
form a hydraulic contact with the sensor. A "flow-through" meter
has been developed which comprises a conduit as its
pressure-sensitive element. This conduit forms an integral,
serially aligned, flow path of the liquid between the pump and the
outlet of the liquid delivery system. In general, this device
comprises, as its fluid-conducting element, a Bourdon tube and a
displacement sensor which senses both the degree of movement of the
tube and is operably attached to the motor control system.
After construction of the apparatus generally described above, it
was found that a surprisingly large number of advantages were
derived from its use in chromatographic applications. For example,
solvent changes which took up to an hour in apparatus of the prior
art could be activated within 5 to 10 minutes with the apparatus of
the invention.
So little peak-spreading, or mixing, was achieved with the system
that it becomes entirely practical to recycle material to be
analyzed through the pump again and still achieve so little mixing
during recycle that analysis was not unduly hindered. Moreover,
thermally sensitive liquids, i.e., liquids which have any
undesirablle physical or chemical response to heat can be processed
most favorably with the system. For example, troublesome gas
bubbles in the liquid can be avoided even on the suction stroke,
because they are not promoted by excessive heat.
The chemical analysis, e.g., those made by a refractometer which is
fed from a column supplied by the liquid feed system of the
invention, show a better base line, and more accuracy in general
because of the lack of pulses and thermally induced drift.
In summary, therefore, applicants have constructed a novel pump
which can provide a constant flow over a period of hours, a
generally pulseless flow, and an even flow. They have coupled with
this pump, in a novel liquid delivery system, a novel
pressure-sensing device which requires no liquid dead space and
which serves to form part of a motor control circuit wherein it
functions as both a safety, i.e., shut-down device and as a
motor-speed governing means. Finally, applicants have incorporated
a motor having a novel control circuit into their liquid delivery
system, thereby providing not only a means to control the speed of
the motor but a means to limit the current to said motor to that
current actually required to drive the pump.
By so limiting the current, the amount of heat dissipated in the
drive means or its control circuit is markedly reduced. This is
extremely important when the motor and pump are both mounted on a
common base or in a common housing or so close to one another that
there would be a significant flow of heat from the motor to the
pump where the pump maintained close to ambient and the motor
reached an elevated temperature, e.g., 140.degree. F or higher. In
liquid chromatography, if motor-generated heat reaches the liquid
being pumped, there is a problem of interfering bubbles being
formed by gasses forced out of solution. The problem can be
especially severe on the suction stroke of a piston pump. The
formation of such bubbles can unduly affect the compressibility of
the liquid.
The quick-flushing characteristic of the pump and liquid delivery
system of the invention is particularly efficient when no part of
the liquid flow path through the pump or the system is more than
0.2 inches from the mean center line of the flow path of the
liquid. By "flow path" in this connotation is meant the volume
which is filled with liquid from the time it enters any given pump
chamber to the time it leaves a downstream pressure sensor if one
be used.
When "flushing" is referred to in this application it is defined as
an action whereby fresh liquid displaces existing liquid in a
segment of a flow path. Thus, if a conduit is 4 inches long, the
entire conduit need not be flushed by each stroke as long as the
liquid advances through the conduit, segment by segment, without
bypassing any substantial dead space. By substantial dead space is
meant that such as would be formed by a dampening devices, pressure
sensors known to the prior art and other such devices which
comprise compartments for receiving the liquid being pumped which
compartments are not in any sense flushed on each stroke of the
pump.
In the most favorable embodiment of the invention for use in liquid
chromatography, the total liquid volume within the liquid delivery
system from pump inlet to pressure--sensor outlet will be less than
3 milliliters. It is advantageously lower, ie, less than 1.5
milliliters as is the case in the specific system described
below.
ILLUSTRATIVE EXAMPLE OF THE INVENTION
In this application and accompanying drawings there is shown and
described a preferred embodiment of the invention and suggest
various alternatives and modifications thereof, but it is to be
understood that these are not intended to be exhaustive and that
other changes and modifications can be made within the scope of the
invention. These suggestions herein are selected and included for
purposes of illustration in order that others skilled in the art
will more fully understand the invention and the principles thereof
and will be able to modify it and embody it in a variety of forms,
each as may be best suited in the condition of a particular
case.
FIG. 1 is a diagrammatic perspective view indicating an arrangement
of various parts of a pump constructed according to the
invention.
FIG. 2 is a perspective view of a novel pressure sensor elements
useful in constructing the liquid delivery system of the invention.
The housing is omitted to better illustrate the relative position
of the various parts.
FIG. 3 is an outlet manifold used in conjunction with the pump such
as shown in FIG. 1.
FIG. 4 is a fragmentary view of a portion of a pump constructed
according to the invention showing the elements between the drive
linkage and reciprocating plunger and numerous other parts in
section.
FIG. 5 is a detailed view of a pump chamber and inlet and outlet
ports therefrom which have been constructed according to the
invention.
FIG. 6 shows a portion of a mechanical linkage between the motor
drive of the pump and the pump plunger.
FIG. 7 is a schematic diagram of a chromatography system
incorporating the invention.
FIG. 8 is a highly schematized block and line diagram of motor
control used in connection with the present invention.
FIG. 9 is a schematic diagram of a preferred form of control
circuit useful in the present invention.
FIG. 10 is a sketch of driving wave forms for the circuit of FIG.
9.
FIG. 11 is a sketch of a pressure gage useful in the present
invention.
FIG. 12 is a schematic diagram of a power supply useful in the
present invention.
Referring to FIG. 1, it is seen that a pumping unit 41 comprises a
frame 42, a stepping motor 43 mounted thereon, a gear train 44
mounted to drive two crank arms 45 in two elongate cylindrical
housings 46 each of which house plunger driving means, better seen
in FIG. 4. Mounted at one end of housing 46 are pump heads 48, the
details of which are also shown in FIGS. 4 and 5.
In general, stepping motor 43 drives worm gear 50, thence rotary
gear 52 turns shaft 53 and a master elliptical gear 54. Gear 54
causes two eccentric gears 56, also elliptical in shape to operate
crank arms 45 180.degree. out of phase with one another so that the
sum of the instantaneous displacement per unit time of all of the
pistons in the pressurizing direction is a constant. This gear
system is shown in more detail in FIG. 6.
FIG. 3 shows a manifold 57 which has inlet conduits from pump heads
48. It is within this manifold 57 that streams from the various
pumpheads 48 are integrated into a single effluent stream. This
effluent will normally exit from the manifold through conduit 62,
but a valve 64 forms means to divert the flow through conduit 66 to
flush the reference liquid of a differential liquid chromatographic
detector.
FIG. 2 illustrates a particularly advantageous pressure-sensing
device utilized in the invention. It comprises a housing 70, a
light source 72, a light sensor 74, and a Bourdon-tube 73 which
forms an extension of conduit 62. In operation, the position of the
straight portion 75 of Bourdon tube 73 is caused to change
dependent on the pressure of liquid within the tube. This change in
position of portion 75 of tube 73 results in a different amount of
light from source 72 reaching sensor 74. The sensor, normally a
photoresistor, provides a direct means for sensing and signalling
the pressure within tube 73. The signal so generated is applied to
controlling the circuit of motor 43 and this mode of control will
be described elsewhere in the specification.
Tube 73 is mounted to housing 70 by means of a stainless steel
block 76 of about one-half inch in thickness. The tube is
one-sixteenth inch in diameter and is comprised of two generally
parallel segments 77 meeting at the midpoint 79 of straight portion
75 which extends beyond the helical segment 80 of the tube. Tube 73
is conveniently 0.005 to 0.10 inches in diameter.
Referring now to FIG. 4, it is seen that each cylindrical housing
46 comprises mechanical plunger driving means 82. This driving
means is actuated by the movement of a crank arm 45 already
described. Crank arm 45 is connected through a spherical bearing 84
positioned in a recess 89 of a piston 86. Bearing 84 is connected
to crank arm 45 by fastening means 87.
Piston 86 which has been seen to comprise a recess 89 at one end
thereof comprises at the other end thereof a recess 91 for
receiving a plunger holder 92. Plunger holder 92 comprises a
rounded spheroid surface 94 which normally abuts against the center
of a flat bearing surface 96 on piston 86. Holder 92 is further
positioned by a spring 98 retained by a circlip 100 snapped into an
annular recess on the interior wall 102 of recess 91. Spring 98 is
a biasing means which pulls the plunger assembly back during its
return stroke while biased against piston 86. It will be noticed
however, that piston holder 92 is neither rotationally restrained
nor is it restrained against axial movement relative to piston 96
when stresses on the piston require such movement. There is a
sufficient difference in the outer diameter of positioning head 104
carrying surface 94 and the interior wall 102 of recess 91 to
permit some positional adjustment of piston holder 92.
Attached to plunger holder 92 by a single pin 106 is plunger 108.
Plunger 108 extends through a plunger support 109 and a seal means
110, both best shown in FIG. 5, into pummp head 48. There is a
cylindrical seal, 111, formed of a halocarbon resin which supports
the plunger within support 109. The pump head is fitted with an
inlet port 112, an outlet port 113 and check valve means 114 and
115 mounted in said port. Self-lubricating seal means 110 is
mounted within head 48 and is held therein by plunger support 109.
Ports 116 are provided in plunger holder 109 in case drainage is
required because of seal 110 failure.
FIG. 5 illustrates in more detail, the pumping heads 48 and parts
of the system directly associated with the heads. Seal 110 is
constructed of a fiber-reinforced poly(tetrafluoroethylene) as such
it has strength and a self-lubricating characteristic which helps
to further minimize binding and wear between seal and plunger.
Plunger 108 is noted to have a forward path just passes inlet port
112 and a backward motion, the rearwardmost position of which is
defined by dotted line 116 just in back of outlet port 113. In the
illustrative example, the plunger is about one-eighth of an inch in
diameter and the pump chamber 118 in which it reciprocates is about
0.006 inches larger in diameter, thus forming an annulus 120
through which liquid sucked into chamber 118 during its backward
stroke of the plunger can flow backwardly toward outlet 113 during
the forward stroke of the plunger.
A proper register of the mechanical driving means 82 and the
chamber 118 is assured by the use of plunger support means. 109 as
a precision positioning coupling between the driving means 82 and
the pump head 48.
FIG. 5 also discloses a novel check valve construction which can be
illustated with reference to outlet valve 115. Valve 115 comprises
two, serially-arranged, flow checking assemblies 117 each
comprising a ball element 119, a ball-seat 121, a gasket support
tube 125. The support tube forms an integral part of the flow path
and prevents the inward radial distortion of gasket 123. In case of
the outwardmost support tube 125a, it is integral with the housing
128 of the check valve. A sleeve 127 formed out of a
self-lubricating resin is placed between the valve seat element and
the housing element. Valve 114 is similar to, and shown in somewhat
more detail than, Valve 113.
Note an annular slot 142 in seal 110 holds a spring forming means
to assist in biasing the seal means in sealing relationship against
the plunger.
FIG. 6 shows an elliptical gear system typical of non-circular gear
systems useful with the invention which system comprises 3
identical gears. A shaft 130 drives centrally-located gear 132.
Gear 132, inturn, causes gears 134 and 136 to turn. The crank arms
are affixed to the gears to operate at 180.degree. out of phase
with one another. FIG. 7 shows a complete liquid flow schematic of
a liquid chromatography system of the type described herein. The
liquid to be analyzed is maintained in a reservoir 150. Material to
be analyzed is pushed into the manifold 152 and into pump chambers
118 and a manifold 57. From manifold 57, the flow may be all caused
to go through a pressure-sensor such as Bourdon tube 154 or,
alternatively, it may be partly diverted to a reference means
156.
From the pressure sensor 154, the material to be analyzed is pumped
through a chromatography column 158, thence to an analytical device
160.
The device described will allow fluid to be pumped at a
substantially constant rate, i.e., at an output of plus or minus 1
percent from stroke to stroke; therefore flow will be essentially
pulseless.
In FIG. 8, a motor 210 drives a pump 212 which supplies fluid under
pressure to a utilization device 214 by way of a fluid line 216
having a pressure transducer 218 in it. The transducer 218 supplies
electrical signals indicative of the pressure in line 216 to a
control unit 220 connected to motor 210. The unit 220 controls the
driving current applied to the motor 210. The motor 210 is
preferably a stepping motor, that is, it has a number of distinct
driving coils which can be separately energized to drive a rotor
through steps of discrete angular increment. It is desired that the
driving current applied to this motor be only such as to drive the
load to which the motor is connected and that excessive driving
current, which results in increased heat dissipation, not be
applied to the motor. The control unit 20 performs this function,
among others.
Turning now to FIG. 9, the control circuit on the present invention
is shown in detail. In FIG. 9, a transformer 230 has a primary
winding connected to an AC power source and a secondary winding
connected across a full-wave diode bridge rectifier 232. The bridge
232 has a resistor 234 connected from its output terminal to ground
for reasons to be described hereinafter. The output of the bridge
232 is applied to a filter 236 through a controllable switching
element preferably in the form of a silicon controlled rectifier
238. The filler 236 is formed from an inductor 240, capacitor 242,
and diode 244. A capacitor 246, resistor 248, and diode 250 are
interposed between the filter 236 and rectifier 238 for reasons
described hereinafter.
The output of filter 236 is applied to control windings 260a-260d
of motor 210. The motor 210 is a stepping motor having a permanent
magnet rotor and pairs of bifilar-windings, that is coils 260a and
260b are bifilar wound as are coils 260c and 260d. Motors of this
type are sold under the mark Slo-Syn by the Superior Electric
Company. The motor windings are connected in series with diodes
262a-262d as well as with the collector-emitter of transistor pairs
264a-264d connected in a Darlington configuration. Diodes 264a-264d
are connected from one of the inductors 260a-260d, respectively, to
ground through a Zener diode 266. The conduction state of the
transistor pairs 264a-264d is controlled by the outputs of a
flip-flop 268. The flip-flop 268 is a 4-phase flip-flop commonly
known as a "Johnson" flip-flop or a "Johnson" counter which
provides output pulses on four discrete leads, each pulse having a
duration of approximately 180.degree.. The pulses are of positive
polarity such that, when applied to the base of the respective
transistor pairs 262a-262d, they cause these transistors to conduct
so that current is drawn down through the corresponding motor
winding to step the motor to a new angular position. The control
outputs of flip-flop 268, and thus the driving currents in windings
260a-260d, are shown in FIG. 10. Flip-flop 268 is driven from an
oscillator 270 through an amplifier 272 and a capacitor 274.
Oscillator 270 will be described in detail hereafter.
The current being drawn through the motor 210 at any given time is
sensed by means of a resistor 276 and a voltage proportional to
this current is applied through a resistor 278 to the summing
junction of an integrator 280. A second input is applied to the
integrator from a pressure transducer 218 through a resistor 284. A
negative bias current is also supplied to integrator 280 through a
resistor 286. The common node to which resistors 278, 284 and 286
are connected comprises a summing junction and the output of
integrator 280 is the time sum of the net current injected into
this node. This output is applied through a resistor 288 to a
capacitor 290. The charge on the capacitor 290 is discharged
through a unijunction transistor 292 into a resistor 294. The
firing point of the unijunction transistor 292 is determined by
means of a voltage applied to its gate through a voltage divider
network 296.
When sufficient charge has been accumulated on capacitor 290 to
cause transistor 292 to conduct, a voltage is generated across
resistor 294 which is coupled through a capacitor 298 to the gate
of the silicon controlled rectifier 238 to turn the rectifier "on"
and thus connect the output of bridge 232 to filter 236. A resistor
300 connected between gate and cathode of rectifier 238 helps to
insure turn-off of the rectifier.
The integrator 280 compares the current called for by both the
pressure sensor 218 and the fixed bias source connected to resistor
286 with the current actually supplied to the motor 210. As long as
the motor current is less than that called for, the integrator 280
supplies an output which charges capacitor 290 with its upper
electrode positive with respect to ground. The rate at which
capacitor 290 is charged depends on the net current applied to the
integrator input; the larger this current, the greater is the rate
at wich capacitor 290 is charged, the earlier the point at which
the unijunction transistor 296 is triggered, and thus the earlier
the rectifier 238 is fired. As the firing point of rectifier 238
advances in each half wave, it delivers more power to the filter
236 and thus to the motor 210.
In order to insure fine control of power supplied to the motor 210,
the charge on capacitor 290 is dumped during each half cycle of the
full wave rectified output from the bridge rectifier 232. This is
accomplished by means of a diode 300 which is connected to the
bridge output. Each time the bridge output drops below the voltage
on capacitor 290, the capacitor discharges to ground through diode
300 and resistor 234.
As the power demands of the load increase, the point at which the
rectifier 238 fires shifts to a point earlier and earlier in each
half-wave cycle. When the firing point lies between 90.degree. and
180.degree. in the half-wave cycle, the voltage at the filter
output will generally be less than that applied to the anode of the
controlled rectifier and the rectifier will fire normally to
maintain the required filter output voltage. If more power is
demanded, the firing point is advanced toward 90.degree. and the
anode voltage is made even larger than the output voltage at firing
time. When, however, the firing point is advanced beyond 90.degree.
toward 0.degree., the anode voltage at firing time drops and may
become less than the filter output voltage. Normally, this would
cause the rectifier to misfire, the output voltage would drop, and
the control would advance the firing point to an even earlier
position so that the rectifier would quickly block. This is avoided
in the present circuit by the provision of capacitor 246 and
resistor 248. Capacitor 246 provides a low-impedance path to ground
for initial rectifier turn-on, hold resistor 248 provides a DC
current path through which a "trickle" current (current of small
magnitude) can flow should the rectifier be fired prematurely (that
is, when the filter output voltage is greater than the rectifier
anode voltage). This current is maintained until the anode voltage
rises above the output voltage during the 0.degree.-90.degree.
portion of the power cycle.
The inductor 236, in connection with capacitor 242, filters the
wave-form passed by rectifier 238 and provides a nearly smooth DC
voltage for motor 210. It does this by limiting the current surges
associated with turn-on and turn-off storing energy during the
transient changes and then delivering it over subsequent and longer
time intervals to thereby smooth any peak surges and limit ohmic
losses in the motor and elsewhere. When the current flow from
rectifier 238 is cut off at the end of each half cycle, the
inductor 240 generates a reverse voltage which tends to maintain
current flow in the inductor for a brief interval. This current
would naturally be drawn through rectifier 238 and thus this
rectifier would tend to remain "on". To prevent this, a diode 250
is connected from ground to the inductor and provides a path
through which the transient current for inductor 240 may be drawn,
thus allowing rectifier 238 to turn off.
Limiting the current applied to the motor in the manner described
limits the ohmic losses in the motor and its associated circuitry
and thus significantly lowers the power dissipation and heat level
of the motor and its controller. This prolongs motor lift, lowers
ambient temperature, and simplifies circuit design problems by
minimizing the temperature range over which the control circuit is
required to operate. Additionally, however, it extends the speed
range over which the motor achieves a given torque level. The
reason for this is that the motor is inductive at high speeds and
must be fed from a higher potential source (e.g. 15 volts), but is
resistive at low speeds being then fed from a lower potential
source (e.g. 2 volts). A series resistor is usually inserted in the
current supply leads to limit the current at low speeds;
unfortunately, this also limits the current that can be drawn at
high speeds and thus the output torque diminishes at higher speeds.
This is obviated by the control circuit of the present invention
which sets the current drawn at all times to that required to drive
the load.
As was previously noted, the windings 260a and 260b, as well as the
windings 260c and 260d, are bifilar wound and thus magnetically
coupled to each other. I have found that it is this coupling which
in fact largely causes the oscillations or "resonance" effects
noted in stepping motors of this type. Considering the current in
each winding to be in the positive or forward direction when it
flows downward in the windings in FIG. 9 and in the negative or
reverse direction when it flows upward in these windings, the motor
210 is designed to operate properly when positive current flows in
the windings in accordance with the schedule shown in FIG. 10.
However, due to inductive coupling between the windings, a large
negative voltage is coupled to winding 260b when current in winding
260a turns off, and vice versa. The same is true of windings 260c
and 260d. This voltage greatly exceeds the forward driving voltage
from the power supply and its forward-biases the base-collector
path of the transistor switch with which it is associated. Thus, it
causes a current in the reverse direction through the transistor
and through the winding. It is this current which causes the
undesired motor oscillations, since it momentarily drives the motor
in the reverse direction. If the switch used to control the current
in each winding were in fact a perfect switch having infinite
impedence, no reverse current could flow, despite the reverse
voltage and the motor would not oscillate. However, practical
switches are always less than ideal and thus may conduct
substantial reverse current.
Having thus recognized the problem, its harmful effects are
mitigated simply by placing a diode in series with each motor
winding and oriented such as to pass current through the winding
when he associated transistor switch is turned "on" in response to
a control input and to block current from passage through the
winding when the transistor is "off." This is the function of
diodes 262a-262d in series with the windings 260a-260d
respectively. In the forward direction these diodes present a very
low impedance to current flow; in the reverse direction, however,
they present an extremely high impedance to current flow and thus
effectively protect the motor against reverse current flow caused
by voltages coupled in from other windings.
Returning now to oscillator 270, the oscillator 13 formed from an
amplifier 310, a capacitor 312, and a unijunction transistor 314.
The transistor 314 has a control potential applied to it from a
voltage divider 316 as well as from a potentiometer 318 which is
connected between the output of pressure transducers 218 and
ground. The oscillator 270 receives an input from a switching
network 320 through first and second independently actuable
switches 322 and 324, respectively. The network 320 comprises
transistors 326 and 328 and resistors 330, 322 and 334. A source of
negative potential is applied to the emitter of transistor 328 and
is coupled to a number of series-connected resistors 336, 338, 340,
etc. in the collector circuit of transistor 328 when this
transistor is turned "on." The resistor string 336-340 has
intermediate terminals or "taps" at which selected fractions of the
voltage applied at the emitter of 328 may be obtained. The voltages
at the taps selected by switches 320 and 324 are coupled to the
oscillator 270 through a resistor 342 in the case of switch 322 and
through resistors 344 and 346 and capacitor 348 in the case of
switch 324.
The magnitude of resistor 342 is such that as switch 322 moves from
one tap to another on switching network 320, the current supplied
through resistor 342 to oscillator 270 changes by one unit.
Similarly, the magnitude of the resistors 344 and 346 is such that
as switch 324 moves from tap to tap along the switching network
320, the magnitude of the current supplied through these resistors
to oscillator 270 changes by 10 units. The capacitor 348 slows the
rate at which the current to oscillator 270 is allowed to change
when the switch 324 moves from tap to tap. A biasing current of
selectable polarity is also applied to oscillator 270 from a
network 350.
Oscillator 270 comprises a very simple yet effective sawtooth wave
generator. The amplifier 310 and capacitor 312 form an integrator
which provides an output voltage proportional to the magnitude and
polarity of the current supplied to its input. The time constant of
the integrator, which is determined by the capacitor 312 and by the
magnitudes of the impedences connected to its input, is such that
the output voltage rises essentially linearly during the time over
which the integrator is to integrate. When the output voltage
reaches a magnitude equal to that applied to the gate of
unijunction transistor 314, this transistor "fires," thus
discharging the capacitor 312 through it. After firing, the
transistor 314 turns "off" and capacitor 312 again starts charging.
Thus a repetitive ramp wave form is generated. The duration of the
ramp is determined by the magnitude of the signal applied to the
input of oscillator 270, as well as by the magnitude of the gate
control signal on transistor 314. By decreasing the latter, or
increasing the former, the repetition frequency of the oscillator
for 270 is increased. Conversely, it is decreased by increasing the
magnitude of the gating signal applied to the transistor 314 or by
decreasing the driving input applied through the switches 342 and
344.
The operation of switching network 320 is controlled from an
amplifier 360 which has an input connected through a resistor 362
to the output of pressure sensor 218 and through a resistor 364 to
the wiper arm on a potentiometer 366 to which a positive biasing
potential is applied. A resistor 368, a diode 370 and a pushbutton
switch 372 are connected between one input of the amplifier 360 and
its output, and a resistor 374 is connected between this input and
ground.
The amplifier 360 compares the output of pressure sensor 218, which
is proportional to the pressure in the fluid line driven by motor
210, with a preestablished "set point" determined by the setting of
potentiometer 366. As long as the pressure corresponding to the
output of the sensor 218 is less than that corresponding to the set
point, the output of amplifier 360 is negative. This holds the
transistors 326 and 328 "on," and a portion of the negative
potential applied to the emitter of transistor 328 is therefore
coupled through the thumb wheel switches 322 and 324 to oscillator
270. When, however, the pressure rises to such a point that the
output of pressure sensor 218 exceeds that corresponding to the set
point of potentiometer 366, the output of amplifier 360 switches to
a positive polarity, transistors 322 and 324 are turned "off," and
the input to oscillator 270 from switches 322 and 324 is cut off.
The oscillator is thus effectively disabled, exceptt for a residual
driving current supplied to it from potentiometer 350.
When the output of pressure sensor 218 exceeds that obtained from
potentiometer 366, the output of amplifier 360 goes positive, diode
370 conducts and feeds a portion of the output back to the input.
This rapidly drives the amplifier to saturation and holds it in the
saturated state such that it is thereafter insensitive to any
changes in the input. The positive output of amplifier 360 turns
off transistor 326 and thus transistor 328. Further, it turns on a
transistor 380 and lights a warning light 382 to indicate that
preset pressure limits have been exceeded. The amplifier 360 is
reset by means of pushbutton switch 372. Momentarily depressing
this switch disconnects the positive feedback around the amplifier
and allows it to return to its usual monitoring state.
As noted earlier, the period of oscillator 270 can be changed by
changing its input or by changing the control voltage applied to
the gate of unijunction transistor 314. As the pressure in the line
to which motor 210 is connected increases, the output of pressure
sensor 218 becomes increasingly negative. This output is coupled
through potentiometer 318 to the gate of transistor 314 and thus
the control potential on this gate is lowered as the pressure
increases. This increases the repetition frequency of the
oscillator 270 and thus speeds up the motor 210. Accordingly, as
the compression in the line increases due to pressure of the fluid
increasing in the line, the motor driving rate is speeded up in
order to maintain a constant mass flow rate. This is desirable in
applications such as chromatography.
The pressure sensor 218 of the present invention is especially
simple in design and operation. Referring briefly to FIG. 11, the
sensor is formed from a mechanical pressure sensor such as Bourdon
tube 390 having a mechanically movable indicator element 392 which
moves between a light source 394 and a light responsive transducer
such as a photocell 396. The tube 390 is connected into the fluid
line whose pressure is being measured, as the tube moves the
indicator 392 to block a greater or lesser amount of light from the
photocell 396 and the resistance of this cell thus varies in
accordance with line pressure.
Referring back to FIG. 9, the photocell 396 is connected in a
bridge circuit with resistors 396, 398, 400 and 402. The bridge
output, which is proportional to the resistance of photocell 396
and thus to the position of indicator 392 in response to the fluid
line pressure, is applied to an amplifier 404 which provides an
output proportional to the deviation of this pressure from a
convenient "zero point" determined by the magnitude of resistors
398-402. For the configuration shown, this output preferably ranges
between 0 volts and some negative voltage level. This voltage
determines the motor driving rate and motor drive current as
previously described.
Turning now to FIG. 12, an especially advantageous power supply for
supplying the necessary voltages for the active elements in the
control circuit is shown. In FIG. 12, a transformer 410 has a
primary winding 412 and a pair of secondary windings 414 and 416.
The winding 414 has a diode 418 in series with it and a capacitor
420 connected across it. A series pass regulator in the form of a
transistor 422 controls the voltage drop between the cathode of
diode 418 and a first output terminal 424. A resistor 426 is
connected from collector to base of transistor 422 and a zener
diode 428 is connected between base and a second output terminal
430. Voltage divider resistors 432, 434 and 436 are connected
across terminals 424 and 430.
An amplifier 440 has one input terminal connected between the
junction of resistor 434 and 436 and a second input terminal
connected to its output terminal and thence to ground. The
amplifier 440 is chosen to have sufficiently high gain (of the
order of tens or thousands of more) such that the junction of
resistors 434 and 436 is effectively at ground potential. In this
case, terminal 424 is thus at a potential above ground, while
terminal 430 is at a potential below ground. The power inputs for
amplifier 226 are taken from the terminals 424 and 426.
The amplifier 440 effectively provides a low impedence return path
to ground for currents drawn from terminals 424 or 430. If the
junction of resistors 434 and 436 were directly grounded without
the use of such an amplifier, the return path to ground would
include the resistors 432 and 434 in case of current drawn from
terminal 424, and would include resistor 436 in the case of current
drawn from terminal 430. The currents through these resistors would
vary with the load demands and this the output voltage would vary
accordingly. Though the use of amplifier 440, however, this effect
is mitigated and the voltage regulation of the power supply is this
greatly improved.
An auxilliary power supply is formed from transformer secondary
winding 416, which is center-tapped, in connection with diodes 442
and 444 which provide full wave rectifivation for the voltage
applied across transformer 416. A filter and regulator if formed
from capacitor 446, series pass regulator transistor 448, and
amplifier 450 and resistor 452. The auxilliary supply receives a
reference voltage from the common connection point of resistors 432
and 434 in the primary power supply. The auxilliary power supply is
otherwise conventional and will not be described in further
detail.
From the foregoing, it will be seen that there is provided an
improved motor control circuit. The circuit limits power
dissipation in a stepping motor by limiting the motor drive current
to that demanded by the load, so that heat dissipation in the motor
is kept to a minimum. In connection with the control circuit, there
is provided a useful driving circuit for operating a
silicon-controlled rectifier in a phase-controlled power supply
network and additionally have provided a very simple yet effective
variable frequency oscillator for use in conjunction with a motor
drive circuit.
Further, there is provided an inexpensive and useful power supply
for supplying the active elements in a control circuit.
Moreover, undesired oscillations or resonances formerly encountered
in connection with driving a stepping motor have been
inexpensively, yet effectively, obviated.
* * * * *