Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF l'~lE INVENTION
This invention relates to an improvement in a pulsation-
free volumetric pump utiliz~ng a reciprocating plunger-type pump,
and more particularly, to an improvemen~ in a controlling mechanism
for stabilizing the pulsation-free property of the pump.
BACKGROU~ OF TH~ IN~NTION
In. general, the reciprocating pump of single plunger type
generates a greater pulsation of discharged fluid because of its
discharge volume of zero upon suction stroke. In order to avoid
such pulsation o-f the discharged ~luid, a pump for controlling the
total discharge volume to a constant volume has been proposed, in
which two plungers of the same diameter are arranged in such a manner
that the reciprocating phase of each plunger is shifted by 1/2 cycle
from each other. In such type of the pump, one or two cams having
a special curved face are used for providing a constant total volume
discharged by the plungers, as described hereinbefore, whereby the
combined volume discharged by pumping action of the two plungers may
be theoretically free of pulsation.
BRIEF DES~IPTI.ON'OF THE DRAWINGS
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Figure 1 is a schematic illustration showing one embodiment
of the pump structure in the pulsation-free volumetric pump accord-
ing to the invention,
Figures 2(a) to ~c) show pulsation-free characteristic
curves obtained by the pump of Fi.gu.re 1,
Figure 3 is a diagram showing a controlling system for the
pulsation-free volumetric pump according to the invention,
Figures ~(a) to ~c) show working characteristic curves
obtained by the controlling system of Figure 3.
An embodiment of the type of the puIsation-free reciproca-
ting plun:ger pump is illustra:ted in Fi'gure 1, wherein the two plungers
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0, 10 are reciprocated by means of the cam driving for providing
a combined volume discharged through pumping action of these plungers.
The pair of
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plungers 10, 10 are arranged in parallelg with which is
engaged a single rotary cam 12 having a cam sur~ace at its
one end. The cam surface is machined so as to provide a curved
surface for keeplng the constant combined volume discharged by
the pumping action of the two plungers, which move along the
periphery of the cam surface. In Figure 1, each cross-head 14,
14 for transmitting a displacement of the rotary cam 12 to each
plunger 10, 10 is provided with each of cam followers 16, 16 which
is pressed against the cam surface by each spring 18, 18 arranged
around the cross-head 14, lll. Further, the rotary cam 12 is
provided at its center of opposite surface with a rotary shaft 20
to form a cantilever structure. The plungers 10, 10 are inserted
through g~and seals 24, 24 into respective pump chambers 22, 22,
from which are derived discharge pipings 26, 26 which in turn are
~oined together at a point A.
Thus, the pump as illustrated in Fig.l may obtain, from
its constructional view points, the pulsation-free property as
shawn in Fig.2 (a) and provide the pulsation-free combined dis-
charge volume. However, it is difficult to completely avoid the
pulsation of the combined discharge volume due to the following
factors:
(1) A rounded portion must be provided at an inflexion
point of the cam upon machining.
(2) Velocity characteristics of the cam or the plunger
can be varied due to the machining error of the cam.
(3) Accurate and strick machining and arrangement of the
cam must be effected for shifting the phase of two
plungers in 180 from each other.
(~) A slight back-flow occurs through a check valve
connected to a line leading to the pump.
(5) A pumping liquid has its intrlnsic compression.
(6) A pumping liquid contains air bubbles and particles.
As a measure for correcting the ~actors (4) and (5)
listed hereinabove, which vary rather regularly, the following
two methods have been proposed:
[I] A method in which a velocity curve of cam is designed
so as to increase a velocity of plunger only in a
given range of a beginning delivery stroke of the
plunger, thereby to correct the reduction of discharge
volume due to the compression of liquid and the back-
flow through the valve.
~II] A method in which discharging pressures are determined
for several strokes of the plunger to obtain an average
valve, to which is approached a discharging pressure
of subsequent stroke.
However, the former method has such the disadvantages
that the pulsation nevertheless increases in a zone having a
relative low discharging pressure because of less compression
or less back-flow of the liquid~ and that the effect of reducing
the pulsation through correction decreases as the pressure applied
by a corrected curve of the ca~ is deviated from the pressure
actually employed. On the other hand, the latter method has such
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the disadvantage that the average discharging pressure determined
for the several strokes ~an not be followed up by the variation
in the actual discharging pressure. Furthermore, the methods I
and II have a common disadvantage in that an irregular pulsation
incapable of being responded appears so that the variation in
the actual discharge volume depends largely on the cam characte-
ristics.
Now it has been found out that the problems and dis-
advantages described hereinbefore may be solved all at once by
the arrangement in that a cam for driving two plungers is connected
to a driving motor, to which is in turn connected a rotation cont-
rolling circuit, while a pressure detector is provided f'or detecting
a pressure of combined discharge volume and that a variable pressure
signal in the pressure detector is fed to the rotation controlling
circuit as a corrected signal.
SUMMARY OF THE INVENTION
Accordingly, a general ob~ect of the invention is to
provide a pulsation-free volumetric pump which is simple in
construction and may allow the stable and pulsation-free volumetric
pumping.
A principal ob~ect of the invention is to provide a pul-
sat,ion-free reciprocating volumetric pump in which two plungers
are reciprocated by means o~ cam driving so as to provide a com-
blned discharge volume through pumping action of the plungers,
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which is characteri~ed in that the volumetrie pump comprises
t~10 plungers, a cam for driving said plungers, a driving motor
connected to said cam, a eircuit connected to said driving
motor for eontrolling a rotational rate, and another circuit
for detecting a pressure of said combined discharge volume and
for correcting a control signal of said rotation eontrolling
cireuit through the detected signal.
PREFERRED ~MBODIMENT OF THE INVENTION
In a preferred embodiment of the invention, the eircuit
for controlling the rotational rate may prefera~ly eomprises a
circuit for setting the rotational rate, a main amplifler and a
tachogenerator for feeding baek an output of the driving motor
to the main amplifier.
In a further preferred embodiment of the invention, the
eireuit for eorreeting the eontrol signal of the rotation eont-
rolling eireuit may eomprises a pressure deteetor eonneeted to a
piping whieh provides the eombined diseharge volume through pumping
aetion of the two plungers, a eireuit for eliminating a direet
eurrent (DC) portion from an output signal of the pressure deteetor,
and an amplifying eireuit wherein a signal from the amplifying
eireuit being eonverted to a reverse phase and added to a signal
from the rotation setting eireuit for eorreetion.
Most preferably~ the driving motor eomprises a DC motor
having a meehanieal time eonstant below 12 msee.
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Now the invention will be described in more detail herein--
after with reference to the drawings which illustra:te the preferable
embodiments.
DETAILED DESCRIPTION OF THE EMBODIMENT
Figure 3 illustrates a diagram of a controlling system for
the pump according to the inventiong wherein the reference 30 shows
a reciprocating plunger-type pump having two plungers as shown~ for
example, in Figure 1. From two pumping chambers of the plunger-type
pump 30 are derived respective discharge pipints which are joined
together to form a piping 32, to which is connected a pressure
detector 34. A signal generated from the pressure detector 34 is
fed back through ~ circuit 36 for eliminating a DC
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component and an ~mplifying circuit 38 in order to control
variation in a discharging pressure. On the other hand, there
is provided a driving motor 40 for driving a rotary cam of the
plunger-type pump 30, an output of which motor 40 is transmitted
to the rotary cam shaft optionally through a convenient gear 4 2 .
The driving motor 40 is controlled for its rotational rate through
a main amplifier 4~, while the rotational rate provided by the
rotation setting circuit 44 is corrected by the signal from the
pressure detector 34. Further, to the driving motor 40 is
connected a tachogenerator 48, an output of which is fed back to
the main amplifier 46.
The working of the pump according to the invention will
be described hereinbelow with reference to the working characte-
ristics, as shown in Figure 4.
When a somewhat pulsation appears in the ~orking characte-
ristics of the plunger-type pump 30, an output signal as shown
in Fig. 4(a) is obtained in the pressure detector 34. In this
case, the resulting output signal is freed from a DC component
by a circuit 36 for eliminating the latter, because only a variable
or ripple portion is to be treated, but not an absolute value.
Then the signal after elimination of the DC component is amplified
by the amplifying circuit 38 to give a characteristic curve, as
shown in Fig. 4(b). The signal thus obtained is converted to a
reverse phase, as shown in Fig. 4(c) and added to a signal from the
rotation setting circuit 44, thereafter fed to the main amplifier
46 for obtaining an output, as shown in Fig. 4(d). Thus, lf the
pressure detected by the detector 34 increases, then the rotational
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rate of the driving motor 40 may be decreased. On the contrary,
if the pressure detected by the detector 34 decreases~ then the
rotational rate of the driving motor 40 may be increased. In this
way, in accordance with the invention, the variation in the dis~
charging pressure of the pump may be kept extremely low through
detection of the discharging pressure and thereby negative feed-
back control of the rotational rate of the driving motor.
In order to control the pump according to the invention
most effectively, the inertia of each component should be ~ept
as low as possible, and especially a response characteristic of
the driving motor 40 is most important. For this purpose, various
experiments have been carried out to find out that the pulsation
may be readily reduced to much lower level, for example one fifth
or lower, in comparison with the conventional system only based on
the cam property, by use of a DC motor having a mechanical time
constant below 12 msec. When the mechanical time constant is
higher than 15 msec., it has been found out that the variation of
the discharging pressure is difficult to be sufficiently followed
up and controlled, so that the reduction of pulsat~on could not
be expected.
It will be appreciated from the embodiment described
hereinbefore that the pump according to the invention has such
the advantages that the pulsation of the discharged fluid from
the pump may be corrected and controlled by means of the electrical
means, thereby allowing the electrical correction of the ripple
in the discharging pressure without replacing the cam, and that
the magnitude of pulsation may be suitably controlled within the
desired range, as shown in Fig. 2(c).
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Although the invention has been described hereinabove
with the reciprocating plunger-type pump as shown in Fig. 1, it
will be appreciated that the invention may be applied widely and
effectively to many other types of pulsation free volumetric
pumps.
The foregoing is descriptive of an embodiment of the
pulsation-free volumetric pump, and rnany changes and modifications
may be made without departing from the scope and spiri.t of the
invention.
