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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a pulsationless duplex plunger pump used for a
liquid chromatography, medical inspection apparatus, etc. and, more
particularly, to a pulsationless duplex plunger pump and a control method
thereof in which velocity control is effected so as to minimize pressure
pulsation.
As shown in Japanese Laid-Open No. 57-70975, in a conventional
pulsationless flow pump which is a duplex plunger pump in which two
plungers are reciprocated by one cam to obtain a resultant discharge
through pumping operation of each of the two plungers, there is provided a
mechanism which is connected to a driving motor wherein a revolution
control circuit is connected to the cam and which corrects a detected
output signal of the resultant discharge pressure by reversing the sign of
the signal and adding the signal reversed in sign to a signal outputted
from a revolution setting circuit after passing the detected output signal
through a circuit for removing a component of direct current and through
an amplifier. Further, the revolution control circuit comprises the
revolution setting circuit, a main amplifier, and a tachogenerator which
feeds back an output of the driving motor to the main amplifier.
However, in the above-mentioned prior art, (1) the detected output signal
from a pressure detector is fed back through the amplifier, so that the
control should be effected before a pressure pulsation takes place, while
irrespectively, the revolution control is effected actually with a phase
delay by a time constant which various devices or apparatus have, whereby
a pressure ripple at the starting of pressure fluctuation can not be
removed: (2) although only a pressure fluctuation part is detected because
the detected signal of the pressure detector is passed through the circuit
for removing a component of direct current, factually and accurately, a
value obtained by multiplying a signal resultant from time-differentiation
of the pressure fluctuation part by a constant should be a velocity
correction value of the driving motor, so that the above-mentioned prior
art could not effect sufficient velocity correction.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a duplex plunger pump
which is small in pressure pulsation, and a control method of the pump.
The invention to achieve the above object is characterized in that a
velocity of each of plungers is controlled by detecting the pressure in a
resultant discharge from two reciprocating pumps and a position of the
each plunger and sequentially correcting instructions for velocity control
of the plungers based on the pressure corresponding to each of the
detected plunger positions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view of a control circuit system according to an embodiment of
the present invention;
FIGS. 2 and 3 are sectional views of an example of a pulsationless duplex
plunger pump to which the present invention is applied;
FIGS. 4 and 5 each are diaphragms showing a basic pattern of a plunger
velocity according to the present invention; and
FIG. 6 is a graphical illustration showing the relationship of a plunger
position, a plunger velocity V.sub.p and pressure P.
DESCRIPTION OF THE EMBODIMENTS
An embodiment of the present invention will be described hereunder
referring to the accompanying drawings.
A pump shown here is a pulsationless duplex plunger pump in which one of
two reciprocating pumps is provided with check valves at a suction side
and a discharge side and an outlet of the check valve at the discharge
side of the pump communicates with the other pump at the suction side. One
example of the pump is shown in FIG. 2, and is constructed such that a
pulse motor 1 drives a belt 2 which rotates cams 3 each mounted on the
same shaft. Pistons 5, 6 each have a cam follower 4 contacting a
respective cam 3 by a spring force for reciprocation of the pistons with
rotation of the cams. The pistons, in turn, reciprocate first and second
plungers 8, 9 in respective pump chambers formed in a cylinder head 11 to
effect the pumping operation. The plungers 8, 9 are connected to end
portions of pistons 5, 6 respectively and are made of wear-resistant and
chemical resistant material such as ruby. Seals 10 are provided between
the side walls of the cylinder chambers and the plungers 8, 9. The check
valves 18a, 18b are provided in fluid passages as shown in FIG. 2. In the
pump, the two cams 3 have cam faces formed so that a resultant velocity
from velocities of the two plungers will be constant. The first plunger 8
moves at a velocity twice as fast as the second plunger 9 to discharge
liquid while supplementing the second pump with liquid, and only the
second pump delivers liquid through the operation of the check valves when
the first pump is in a suction stroke.
FIG. 3 shows another example of a pulsationless duplex plunger pump to
which the present invention is applied.
Rotational movement of each of two individual pulse motors 1 is converted
to a linear reciprocating movement of a piston 5, 6 through a drive and
transmission system comprising an epicyclic reduction gear 14, a thrust
bearing 15, and a ball nut screw 16, whereby first and second plungers 8,
9 connected to end portions of the pistons 5, 6 are driven by the
respective individual pulse motors 1. A numeral 10 denotes a sealing, and
numerals 18a, 18b check valves.
An embodiment of the present invention using one of the above-mentioned
pumps is described hereunder, referring to FIG. 1.
A preset flow signal which is set by a flow setting device 26 (or an outer
flow controller) is converted into a binary code by a binary-coded decimal
to binary code conversion circuit 27 to be inputted into a micro-computer
24. When in addition to this signal, a starting signal from a start-stop
button 28 is inputted into the micro-computer 24 through a pulse generator
29, the micro-computer 24 calculates pulse-motor driving frequencies,
etc., and a pulse train for generating a plunger velocity driving pattern
as shown in FIG. 4, signals for determining a rotating direction and
electric current control signals are inputted into a pulse-motor driver 25
from an output port of the micro-computer 24. In this embodiment, these
signals are distributed at the pulse-motor driver 25 to drive individually
two pulse motors 1. Upon rotation of the pulse motors 1, which are
reversible electric motors, the rotation of each is decelerated by the
respective epicyclic reduction gears 14, and converted into a linear
reciprocating motion through the drive and transmission system comprising
the thrust bearing 15, the ball nut screw 16, so that the plungers 8, 9
are reciprocated to pressurize a liquid to make it high in pressure and
discharge the pressurized liquid. In this case, timing deviates so that
pulsation takes place because of a difference in bulk modulus between used
liquids when liquid of a low pressure is pressurized to be high in
pressure, as well as a response delay in the check valves. Further, in
time other than the switching period of time also, there is caused
fluctuation in the plunger velocity because of errors in measurement in
manufacturing parts constituting the driving apparatus, so that a small
pulsation occurs. A pressure sensor 20 is provided to detect a line
pressure and pressure pulsation. The detected signal is inputted into a
differentiator 21 through an amplifier and a filter for removing noises,
both of which are not illustrated. Output of the differentiator 21 is
converted into a digital signal by a A/D converter 22 and inputted into
the micro-computer 24. On the other hand, there are provided rotary
encoders 19 mounted on the pulse motors 1 and a rotation angle detecting
circuit 23 as means for detecting a position at which pressure pulsation
takes place. By this means, a rotation angle of the pulse motors at which
pressure pulsation takes place, or the number of pulses inputted into the
pulse motor 1, from a reference point is detected. Further, as means for
detecting a plunger position and a position where pressure pulsation takes
place, a linear scale which detects a plunger position is preferable.
Here, a resultant discharge pressure at a position of the plunger is
obtained as follows. Namely, in the starting of the pump, both of the
first and second pistons are driven and reach initial setting positions,
respectively. Plunger positions are nearly equivalent to the piston
positions. The piston position, which is described later, can be obtained
by multiplying a time-integral value of driving frequency, having + or -
sign according to a rotation direction of the motor, by a constant, and by
adding the initial setting position thereto. The value thus obtained and
the pressure signal (value) are stored in a memory means, whereby the
pressure at a position of the plunger can be detected. Further, when the
piston position is detected by the linear scale, an output of the linear
scale is acknowledged as a plunger position. The relationship between the
plunger position and the pressure, detected in this manner is shown in
FIG. 6.
These signals are inputted into the micro-computer 24. The above-mentioned
digitalized time-differential signal of pressure pulsation is converted
into a correction signal of the plunger velocity, that is to say, a
correction signal of the pulse motor driving frequency by multiplying the
signal by a constant, and added to the pulse-motor driving frequency
before one revolution at the detected position of pressure pulsation
occurrence. In case of the pump shown in FIG. 2 being used, a resultant
velocity from two plunger velocities is corrected taking an effect of the
check valves into consideration. In case the pump shown in FIG. 3 is used,
the correction of the plunger velocity is effected on one of the plungers
which is in a discharge stroke.
Next, a plunger velocity control method will be explained.
Velocity patterns of the two plungers are as shown in FIGS. 4, 5. The
plungers are operated so that the sum of the two plunger velocities will
be constant including an effect of the check valves, whereby on general
principles, a pulsationless pump is made. However, in order to
continuously discharge liquid of a high pressure, a switching must be
effected in a moving direction and a velocity of the plunger. On this
switching, pressure is lowered because of the response delay of the check
valve and leakage of the liquid, and as a result, a pressure ripple is
generated. Therefore, in this switching period of time, it is necessary to
minimize pressure pulsation through control of the plunger velocity.
However, in case of the plunger velocity control, unless a magnitude of
velocity correction corresponding to pressure pulsation, and a phase of
the velocity correction to the pressure pulsation are proper, the pressure
ripple is left. For example, in case of the pulse motor 1 being used as a
motor, a plunger velocity
##EQU1##
is given as follows:
##EQU2##
wherein a is a constant, and f is driving frequency of the pulse motor 1.
In case of the second plunger 9, the following equation is established:
##EQU3##
wherein A is a sectional area of the plunger,
Q.sub.1, an amount of liquid leakage,
Q.sub.2, flow rate,
V.sub.2, volume of a cylinder and a line passage,
K, apparent bulk modulus of liquid, and
P.sub.2, pressure.
Therefore, a time differential value of the pressure pulsation corresponds
to an amount of velocity correction of the plunger, that is to say, an
amount of driving frequency correction.
Accordingly, in the present invention, when the two plungers are driven by
the two cams mounted on a shaft common thereto, an amount of correction of
a resultant velocity of the two plungers is set so as to be proportional
to a time differential value of the pressure pulsation. Where pressure
starts to decrease, it is necessary to increase the velocity of the
plunger. On the contrary, where the pressure starts to rise, it is
necessary to lower the plunger velocity. Therefore, in case of a resultant
velocity of the plungers being set, a sign of time differential value of
the pressure pulsation is made reverse. Further, signals of the detected
pressure pulsation are passed through the amplifier and the
differentiator, so that a delay by an amount of a time constant which the
device or apparatus have is caused. Therefore, as for the plunger
velocity, at least an amount of a phase differential value is corrected by
the operation of the micro-computer. Further, in case two plungers are
driven individually using two motors, a leakage takes place because of a
response delay of the check valves when a discharge stroke is shifted to a
suction stroke, so that the above-mentioned amount of piston velocity
correction is added to one of the plungers which is in discharge stroke.
In this manner, feed back control is effected so that the phase is set
proper by shifting the phase while detecting pressure pulsation of the
discharged liquid.
By controlling as mentioned above, velocity correction of the plunger
corresponding to pressure pulsation can be carried out, and effected at a
suitable time to the pressure pulsation.
Next, a method of setting a proper phase of the velocity correction of the
plunger in order to make a small pressure pulsation will be described
hereunder.
As mentioned above, when the pressure is detected, the pressure pulsation
obtained through the amplifier, the filter and the differentiator is
delayed by time constants at such devices or apparatus, compared with the
pressure pulsation appearing in the pressure sensor mounted right near to
the delivery port of the pump, namely, the pressure pulsation which is
delayed in phase compared to one in the pressure sensor is taken in, so
that the timing of velocity correction which is obtained through the
detection of rotation angle of the pulse motor 1 is not necessarily
proper. Therefore, for first correction, a velocity correction time is
shifted by a delayed time which can be prospected, and after that, an
amount of shift of the phase is determined judging whether or not there is
a change in a position of occurrence of the pressure pulsation and whether
or not there is a change in the sign of the positional signal. When the
variation is within a preset range, locking is effected. Concretely, in
case the position of occurrence of the pressure pulsation does not change
and in case the sign also does not change, the preset timing remains the
same. If the sign changed, the control is effected so as to be delayed by
one half of the before value because the phase was excessively advanced.
By controlling thus, plunger velocity correction of a proper timing and a
proper value can be effected.
Another control method according to the present invention is explained
hereunder.
In case of the pulse motor being used as a motor, for example, a plunger
velocity
##EQU4##
is given as follows:
##EQU5##
wherein a is a constant and
f, a driving frequency of the pulse motor. In case of attention being paid
on the first and second plungers 8, 9, the following equations are
established:
##EQU6##
wherein A is a sectional area of each of the plungers;
Q, a flow rate to column;
Q.sub.12, a flow rate from the first plunger 8 to the second plunger 9 or a
leakage amount,
Q.sub.10, a flow rate from a container to the first plunger 8 or a leakage
amount;
V.sub.2, a volume of the second cylinder and a pipe passage;
V.sub.1, a volume of the first cylinder;
K, an apparent bulk modulus of the liquid, and
P.sub.1, P.sub.2, pressure in the first and second cylinder, respectively.
In a liquid chromatography apparatus, a column is used for separating
components, so that, in general, a flow rate Q is proportional to the
pressure. Therefore, taking .alpha. as a proportional constant, the
following equations are given by adding the above two equations one to
another;
##EQU7##
When the second piston 6 is in a discharge stroke, the following is
considered to be established;
##EQU8##
Assuming that .epsilon.(t) is a term of disturbance from the outside,
pressure pulsation and a pressure level in the resultant discharge liquid
are determined according to a change in the disturbance term and the
velocity. If the disturbance term is included in a term of velocity
fluctuation, by determining a base line of pressure to be Pm through
observation of the pressure P.sub.2 and obtaining a time differential
value
##EQU9##
a velocity correction value can be obtained by the following equation:
##EQU10##
Namely, a time differential value of pressure pulsation and a differential
between the base line pressure and a measured pressure correspond to an
amount of plunger velocity correction, that is, an amount of correction of
the driving frequency.
Accordingly, in the present invention, in case the two plungers are driven
by the two cams mounted on the same shaft, an amount of correction of a
resultant velocity of the two plungers is set so as to be proportional to
the sum of the time differential value (reversed sign) of pressure
pulsation and the differential between the base line pressure and the
measured pressure. Further, signals of the detected pressure pulsation
pass through the apparatus or devices, so that a time delay by a time
constant of the apparatus or the device takes place. Therefore, as for the
plunger velocity, at least an amount of this phase differential is
corrected by the operation of the micro-computer.
Further, in case of the two plungers being independently driven by the two
motors, a leakage takes place due to a response delay of the check valve
where a discharge stroke is shifted to a suction stroke, so that at such a
switching time, the above-mentioned piston velocity correction amount is
added to one of the two pistons which is in a discharge stroke. In a time
other than the switching time, the above-mentioned piston velocity
correction amount is added to the second plunger so that the pressure in
the second cylinder can be directly controlled. In this manner, the
plunger velocity can be corrected corresponding to the pressure pulsation,
and at a proper time to the pressure pulsation. Further, the control
encloses a pressure clause, so that the control can be effected even in
case there is a pressure difference between the suction stroke and the
discharge stroke of the second plunger because of a liquid leakage in the
check valves.
Further, constants A.sub.0, B.sub.0 have different values according to the
kinds of liquid, however, the constants can be determined by detecting the
time differential value of pressure, pressure and the sum of plunger
velocities.
As mentioned above, according to the invention, the plunger velocity
correction of a magnitude suitable to remove fluctuation of the pressure
pulsation can be effected and the plunger velocity correction can be
effected with a suitable phase differential, so that the pressure
pulsation can be minimized.
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Description |
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