Note: Descriptions are shown in the official language in which they were submitted.
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THIS IS A DIVISION OF APPLICATION No 508,976
This invention relates to a method for determining
the position of a stirrer, for stirring liquid, in a storage
tank while moving around in the tan]c, thus redissolving
sludge which has separated out from the liquid, and
preventing the accumulation of sludge on the bottom of the
tank. The motion of the stirrer may thus be controlled.
In large tanks used for storage of liquid, (for
instance, crude oil, etc.) sludge may gradually separate out
from the liquid over long periods of time. If this sludge
is allowed to build up on the bottom of the tank, the
quantity of liquid which can be stored in the tank is
effectively reduced, and the tank must therefore be cleaned
regularly to remove accumulated sludge. In order to clean
the tank, it is necessary to drain off all the liquid. A
device was therefore developed to obviate this necessity,
consisting of stirrer rotors inside the tank rotated by a
motor outside the tank, which stirred the liquid and
prevented sludge from separating out. The position of these
stirrer rotors was however fixed, which meant that similar
devices had to be installed at a large no. of points around
the circumference of the tank. This solution is not
practical, and does not permit efficient stirring in the
center area of the tank.
In order to solve this problem, the Applicant, in
Japanese Patent Publication No. 59-46658, proposed a
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device which stirred the liquid while moving around in the
tank. This stirrer has rotors which permit it to move up
and down, and rotors which permit it to move in a horizontal
direction. By controlling these rotors from outside, the
s stirrer can be made to move around freely inside the tank.
The liquid in the tank can therefore ~e stirred uniforml~,
sludge can be redissolved, and sludge can be prevented from
accumulating anywhere on the bottom of the tank.
In order to lighten the weight of the stirrer and
enable it to move smoothly through the liquid, it was
suspended by a rope from a float to give it buoyancy. How-
ever, as the stirrer was suspended from the float, the
stirrer sometimes lost its balance when it was moved hori-
zontally or rotated, and sometimes collided with the tank
inner wall.
Further, in order to move the stirrer to a desired
location somewhere in a large tank by means of an external
operation, it is necessary to determine the actual position
of the stirrer at any time. As li~uid storage tanks are
usually completely e~closed, however, the position of the
stirrer cannot be confirmed from outside.
The Applicant, in Japanese Patent Provisional
Publication No. 58-213210, proposed a device for determin-
ing the position of the stirrer wherein a wire wound round
a drum was suspended from the roof of the storage tank and
connected to the upper part of the stirrer, the direction
and length of wire released, as measured by a potentiometer
or other means, being used to determine the position of the
stirrer.
The structure of this device is however complex,
and if the direction and length of wire released are not
measured with a high degree of precision, it is difficult
to determine the position of the stirrer with accuracy.
Further, as the wire is paid out over a long distance, it
sometimes goes slack or becomes entangled with the electric
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cable of the stirrer.
The object of the present invention, therefore, is
to provide a method for determining the precise position of
the stirrer.
In meeting this and other objects, the present
invention, in its broadest terms provides a method for
determining the position of a stirrer which is movable
through a liquid in a tank, comprising:--
- placing an ultrasonic wave emitter on the stirrer;
- arranging a plurality of receptors at different
locations in the tank;
- emitting ultransonic pulses from the emitter to the
receptors while the stirrer is in a first position in the
tank;
- measuring the time required for the pulses to reach
each of the receptors (the time required for the pulses to
reach each receptor being related to the distance between
the emitter and each of the receptors);
- disposing a computer outside of the tank; and
- calculating the first position of the stirrer in the
tank based on the measured times by feeding the latter to
the computer which computes the position of the stirrer in
the tank.
Preferably the stirrer is motor-driven while
s`tirring the liquid. In this case, Motors operating on the
stirrer move it within the tank and s-topping the motors
allows the stirrer to rest on the bottom of the tank in the
"first position" mentioned above.
For the above purposes, vertical buoyancy tanks
are fitted to the upper part of the frame which constitutes
the skeleton of the stirrer, and a
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perpendicular flow plate is provided on the lower part
of the frame. A pair of motor-driven rotors are
arranged on either side of the flow plate in order to
provide a propulsion in a horizontal direction. In
addition, an elliptical housing is fitted to the under
surface of the flow plate enclosing a pair of rotors,
also motor-driven, which provide a propulsion in a
vertical direction. The rotation of the motors is
controlled by a control unit connected to the stirrer by
a cable. The flow set up in a horizontal direction by
the horizontal rotors causes the stirrer to move
horizontally in the opposite direction to the flow,
while the flow set up in a vertical direction by the
vertical rotors causes the stirrer to rise and fall in
the opposite direction to the flow At the same time, a
flow of liquid is set up in the tank, and this stirring
action causes sludge to redissolve in the liquid. Due
to the vertical buoyancy tanks fitted to the upper part
of the frame in a one-piece construction, there is a
strong vibration damping action and a restoring action
which prevent the stirrer from being upset, and maintain
it in its correct orientation.
Due to the floats, the weight of the stirrer
in the liquid is practically zero. The driviny force
required to lift the stirrer is thus reduced and, by
decreasing the speed of the stirrer when it is falling,
a collision with the bottom of the tank is prevented.
The flow plate is parallel to both the
horizontal and vertical directions of motion, which
stabilizes the orientation of the stirrer when it is
moving.
In this invention, in order to determine the
position of said stirrer in the liquid storage tank, an
ultrasonic wave emitter is fitted to the stirrer, and
several receptors are installed at different points
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inside the tank to receive the ultrasonic wave so
generated A computing device is provided to compute
the position of the stirrer, which functions by
determining the time taken by each receptor to receive
this ~ltrasonic pulse The time taken to receive the
pulse from the ultrasonic wave emitter is directly
proportional to the distance to each receptor, and so
the position of the stirrer can be found from these
times. To reduce errors due to noise when the
measurements are being carried out, all motors are
stopped, and the stirrer allowed to settle on the tank
bottom before generating an ultrasonic pulse from the
emitter. This operation is repeated several times. The
period between generation of pulses is chosen to be
greater than a time interval equivalent to the time
taken to traverse the greatest distance inside the tank
plus the time taken for attenuation of echos due to
reflections from the tank inner wall. As measuring
errors arise if the echos of the ultrasonic pulses are
used, distances are measured using the time taken for
the first pulse to be received in each emission cycle.
The position of the stirrer is computed for pulses in
several cycles, and the average value obtained is taken
to be the actual position.
The above object, other objects and advantages
of this invention will be made clearer by the following
description and reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a front view of the stirrer of this
invention.
Fig. 2 is a side view of the stirrer of this
invention.
Fig. 3 is a plan view of the stirrer taken on
the line A-A in Fig. 2.
Fig. 4 is a vertical sectional view through a
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liquid storage tank containing the stirrer of this
invention
Fig. 5 is a diagrammatic view of the parts
involved in determining the position of the stirrer.
Fig 6 is a waveform diagram showing thP
relation between the ultrasonic pulse emitted and the
pulse received.
Fig. 7 is a waveform diagram showing the
relation between the ultrasonic pulse and the echos
o inside the tank.
Fig. 8 is a vertical sectional view through a
tank showing how the stirrer cable is cor~ected.
Fig. 9 is a waveform diagram showing control
of the stirrer rotors, and control of ultrasonic pulse
emission.
DETAILED DESCRIPTION OF TE~ PREFERRED EMBODIMENT
As can be seen from Figs. 1-3, this device
consists of a vertical frame 1, and an elliptical
housing 2 fixe~ to the lower part of the frame provided
with several windows 2a. These elements together form
frame 4, which is the skeletal structure of the stirrer
3.
On the upper part of frame 1, a motor 5 is
provided which drives the vertical rotors 6a, 6b
arranged symmetrically on either side of elliptical
housing 2. A rotation is transmitted to rotors 6a, 6b
by means of gearboxes 5a, 5b connected to the shaft of
motor 5. Rotors 6a, 6b consist of impellers with
several blades, and their rotation sets up a practically
perpendicular fluid flow. On either side of said motor
5, there are 2 auxiliary motors 7a, 7b, and gearboxes
8a, 8b installed in vertical frame 1 below. On either
side of frame 1, there are also horizontal rotors 9a, 9b
oriented so as to set up a practically horizontal fluid
flow. These rotors 9a, 9b are driven by said motors 7a,
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7b.
Rotating brushes 11a, 11b are arranged via
bearing boxes 1Oa, 1Ob on the ~ase of elliptical housing
2, and are connected to the shafts of said rotors 6a, 6b
s respectively such that they rotate together with them
On the upper part of frame 4, a palr of
primary buoyancy tanks 12a, 12b, which have a
semicylindrical form and are hollow, are arranged so as
to enclose between them the upper part of frame 1 and
said motors 5, 7a, 7b. The lower parts of the tan~s are
fixed to frame 1, and the upper parts are fixed in a one
piece construction to the joint box 13 provided at the
upper end.
In addition a secondary buoyancy tank 14,
which has an elliptic cylindriacal form, is vertical and
is of large volume, is positioned on the upper part of
joint box 13 to which it is fixed as one piece.
The under surfaces of primary buoyancy tanks,
12a, 12b slant away from frame 1 towards the exterior so
as not to offer any resistance to the vertical flow of
fluid set up by said rotors 6a, 6b.
Between primary buoyancy tanks 12a, 12b, frame
1 and elliptical housing 2, a flow plate 15 is arranged
at right angles to frame 1, that is, parallel to the
fluid flow set up by both said rotors 6a, 6b and said
rotors 9a, 9b. This flow plate 15 is fixed to tanks
12a, 12b, frame 1 and elliptical housing 2, and also
serves to provide greater strength.
Several obstacle detectors 16 are installed on
the outer wall of elliptical housing 2. These obstacle
detectors 16 use tape switches, limit switches or
pressure sensitive switches, or any other common device
which emits a signal when it comes into contact with an
obstacle.
A depth finder 17 is fitted to joint box 13,
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and may for example consist of a strain gauge,
potentiometric gauge or Bourden tube pressure gauge. It
is so designed as to output a depth signal due to the
pressure detected when the stirrer 3 is immersed in the
liquid in the tank.
A de~ice is provided, as shown in Figs. 4 and
5, for determining the position of the stirrer in the
liquid in the tank
This position finding device involves the use
of an ultrasonic wave system. An ultrasonic emitter 19,
for example of the cylindrical piezo-electromagnetic
type, is fitted to the center of the upper part of
secondary buoyancy tank 14 Three receptors 20a, 20b,
20c, which are intended to receive the ultrasonic wave
pulse and are suspended from the roof 27 of tank 22, are
then placed in their respective positions.
As can be seen from Figs. 5 and 6, when an
ultrasonic pulse is emitted at fixed time intervals from
the emitter 19, the time (t) reguired for this pulse to
reach the receptors 20a, 20b and 20c (tl, t2, t3), is
directly proportional to the distance from the emitter
to the receptor. The distance from emitter 19 to each
of the receptors 20a, 20b and 20c, that is Ql, ~2 ~ Q3,
may then be measured by multiplying the respective times
tl, t2, t3 recorded by measuring instrument 32, by the
velocity of sound. A computer 33 is provided to compute
the position of stirrer 3 in the liquid storage tank
from the distance5 Ql, Q2, Q3-
As shown by Fig. 5, if the position of the
ultrasonic emitter 19 in three-dimensional coordinates
is (x, y, z), the positions of the ultrasonic receptors
20a, 20b, 20c in the same coordinate system are (x,, y,,
zl), (x2, Y2, Z2), (X3 ~ y3 ~ Z3 ), and the distances Ql,
02, Q 3 are determined the point (x, y, z) may be found
from the following equations (1) - (3):
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(X-xl ) ~ (y-y, ) ~ ~Z-ZI ) = ~
(X-X2 ) 2 ~ (Y-Y2) 2 + (Z-~2) 2 = ~ ~ 2 ... (2)
(X-X ~ 2 + (y-y3) 2 + (Z_Z3 ) 2 = 1~3 ''' (3)
If the receptors 20a, 20b, 20c are arranged at
5 the same height (z, = Z2 = Z3 ), the computation is
simplified
The ultrasonic emitter 19 is made to emit
several pulses separated by a given time internval, and
the computer 33 performs the computation described above
for each pulse.
By repeating this computation several times
and taking the average, it is possible to obtain an
accurate measurement for the position of the stirrer
with very little error. If, however, motors 5, 7a and
7b are running when the ultrasonic pulse is emitted,
errors of measurement can easily arise due to noise.
All motors are therefore first stopped, and the pulse is
emitted after a certain time has elapsed, that is, after
the stirrer has settled on the bottom of tank 22. In
carrying out the measurement, the period of ultrasonic
emission is determined by the maximum value of the
distance to be measured (equivalent to the diameter of
the tank). If however there are considerable echos due
to collision of the ultrasonic wave with and reflection
from the tank inner wall, the period is determined as
shown in Fig 7 by an interval consisting of the time sl
corresponding to the greatest distance of traverse, plus
the time s2 required for the echos to die away
completely
Even in this case, however, the ultrasonic
receptors 20a, 20b, 20c may receive echos after they
have received the first pulse until the next pulse cycle
begins The measuring instrument 32 therefore treats
the first pulse received as correct information, and
35 ignores the subsequent echos This is because the
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shortest distance between the emitter 19 and the
receptors 20a, 20b, 20c is the straight line distance
connecting them, and echos which do not take this path
but are reflected must necessarily be received after the
regular pulse has arrived
The motor drive and signal cable 21 attached
to the stirrer 3 is wired along the secondary buoyancy
tank 14. This calbe 21, as shown in Fig. 4, is
supported by floats 24, 25 positioned in the liquid 23
contained in tank 22, and is connected to the control
device 26 outside tank 2~.
If the roof 27 of the liquid tank 22 is of the
floating type, the ends of the 1Oats 25, as shown in
Fig. 8, come into contact with roof 27. The length ~ of
the floats 25 is greater than the length ~ t of the
columns 2~, so that cable 21 does not become tangled
around columns 28 which support floating roof 27 when it
reaches the bottom of the tank. If a cable duct 29 is
provided for cable 21 in the floating roof 27, the
length Q of the floats 25 is of course chosen to be
greater than the length Q2 of the duct~
The control unit 26 determines the position of
stirrer 3 when the above position finding device is
operated. It also drives motors 5, 7a, 7b and controls
the motion of the stirrer based on the position found,
the contact signals received from said obstacle
detectors 16, and the depth signal from depth finder 17.
Alarm indicators are provided on the control
unit to show the status of stirrer position, depth and
contact with obstacles A manual operation switch and
auto operation switch are provided so that the said
control operations can be performed either manually or
automatically as desired.
As shown in ~ig~ 4, the stirrer is introduced
into tank 22 through a manhole 30 in the roof. When the
stirrer is introduced, it is adjusted by means of the
primary and secondary buoyancy tanks 12a, 12b and 14,
such that its weight in the fluid is practically zero.
The motors S, 7a, 7b are started from control
unit 26 and, by rotating the vertical rotors 6a, 6b and
the horizontal rotors 9a, 9b in a given direction or in
the reverse direction, the stirrer is made to rise,
fall, move forwards or backwards horizontally, or change
its direction.
If the vertical rotors 6a, 6b are rotated
clockwise, liquid is pushed vertically downwards by the
impeller blades, and stirrer 3 therefore rises. If
rotors 6a, 6b are rotated counter-clockwise, liquid is
pushed vertically upwards and stirrer 3 falls.
lS If the horizontal rotors 9a, 9b are rotated
clockwise, liquid 23 is pushed back horizontally, and
the stirrer moves forward. If rotors 9a, 9b are rotated
counter-clockwise, liquid 23 is pushed forwards, and
stirrer 3 moves backward. Further, if rotors 9a, 9b are
rotated in mutually opposite directions, the direction
of the stirrer changes.
Thus, as the stirrer moves almost uniformly
throush liquid storage tank 22, the rotors 6a, 6b or
rotors 9a, 9b stir the liquid at the same time. This
causes the sludge which tends to separate out from the
liquid and accumulate on the bottom of storage tank 22
to be stirred, lifted up by the flow, dispersed, and
prevented from accumulating on the bottom. Further,
when the stirrer is on the bottom of tank 22, the
rotatiny brushes 11a, 11b are driven to mechanically
pulverize the sludge and shear through it. The sludge
thus becomes finer, floats up and is carried on the
upward flow produced by rotors 6a, 6b, tending to
promote its redissolution in the liquid.
As has already been mentioned, vertical
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buoyancy tanks 12a, 12b and 14 are fitted to stirrer
frame 4 in a one piece construction As a result, when
the stirrer 3 is moving forwards or backwards and loses
its balance or oscillates, there is a strong vibration
damping effect and restoring force tending to maintain
its orientation stable.
Due to buoyancy tanks 12a, 12b and 14, the
weight of stirrer 3 in the liquid is practically zero.
It does not therefore require such large driving force
to make the stirrer rise. Similarly, there are no
sudden falls when the stirrer is falling, and damage due
to collision of the stirrer with the bottom of storage
tank 22 is avoided
The stirrer 3 is also provided with a flow
plate 15. When the stirrer is moving forwards or
backwards, therefore, its direction is constrained by
this flow plate 15, so that motion is executed in a
perfectly straight line.
This sequence of operations can be performed
manually while watching various indicators on control
unit 26, and automatically by means of a control program
set up in control unit 26.
In the case of automatic operation, control
unit 26 first lifts the stirrer from a resting position
on the bottom of the tank, moves it horizontally by a
given distance, allows it to settle on the bottom again,
and stirs the fluid. This normal control sequence is
effected as follows:
Motors 5, 7a and 7b are driven so as to rotate
vertical rotors 6a, 6b clockwise, and horizontal rotors
9a, 9b both clockwise. These rotations cause the
stirrer to move forwards while rising. After rotors 6a,
6b have been rotated clockwise for a time T,, they are
rotated counter-clockwise for a time T2 (Tl < T2), and
rotors 9a, 9b are stopped after a time T3
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These operations cause the stirrer 3 to move
forward a given distance, and to fall to the bottom of
the tank 22 The rotating brushes 11a, 11b are then
rotated so as to remove sludge 31 adhering to the bottom
of the tank, and clean the bottom After the rotors 6a,
6b have rotated counter-clockwise for time T2, control
unit 26 again rotates rotors 9a, 9b for a time T3, and
again rotates rotors 6a, 6b clockwise for a time Tl. By
repeating these operations over and over again, the
stirrer 3 is made to rise, fall and move forward,
stirring and cleaning liquid storage tank 22 as it does
so .
If, during the above operations~ a signal is
received from the obstacle detectors 16 indicating that
the stirrer has come into contact with an obstacle or
with the inner wall of tank 22, evasive action is taken.
This evasive action consists of lifting the
stirrer from its position and rotating it, the direction
of this rotation and the magnitude of the angle of
rotation being completely random. If this action were
given some regularity, the stirrer's motion would fall
into a steady pattern (such as, for example, executing a
back and forth motion between opposite walls), which it
is intended to avoid. This control sequence is effected
as follows.
If rotors 6a, 6b are rotating counter-
clockwise, control unit 26 drives motor 5 so as to
rotate them clockwise; while if rotors 6a, 6b are
rotating clockwise, motor 5 is driven so as to continue
this clockwise rotation. At the same time, motors 7a,
7b are driven so as to rotate rotors 9a, 9b in the
opposite direction to one another Due to the clockwise
rotation of rotors 6a, 6b, and the rotation of rotors
9a, 9b in mutually opposite directions, stirrer 3 rises
3s while rotating about its own axis.
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The stirrer is made to rise for a time T~,
following which rotors 6a, 6b are rotated counter-
clockwise for a time Ts (T, < Ts)~ and rotors 9a, 9b are
stopped after a time T~ ~T6 < 2T,~. Due to tnese
operations, the stirrer body rotates, and then falls and
settles on the bottom of the tank The direction of
this rotation is determined by the mutually opposed
directions of rotation of rotors 9a, 9b, and the angle
of rotation is determined by T6. The choice of
direction and the magnitude of T6 depend on the signals
from the random signal generator incorporated in control
unit 26.
If control unit 26 continues to receive
signals from the obstacle detectors 16 even after the
end of said period T6, the above control operations are
repeated until no further signals are received. When no
further signals are received, the stirrer is operated
normally as described previously.
Control unit 26, apart from carrying out
normal control and obstacle evasion control, also
determines the depth and position of the stirrer by
receiving signals from depth finder 17 and the position
finding device. The range of movement of the stirrer
can be set depending on its depth and position.
The position of the stirrer is determined as
described previously. In carrying out the
determination, control unit 26 operates the position
finding device when the stirrer is at rest, as shown by
Fig. 9 As shown in Fig. 9, the stirrer body 3 rises
when rotors 6a, 6b are rotating clockwise. When
rotation is stopped, however, the stirrer body 3 falls
under its own weight, and settles on the bottom of the
tank after a time To~ After a further time T~o~ an
ultrasonic wave pulse is emitted from ultrasonic emitter
19 in the position finding system, and the position of
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the stirrer is determined This determination is
carried out with the stirrer stationary on the bottom of
the tank in order to eliminate the effect of noise due
to motors 5, 7a and 7b, and to giv~ a stable reading for
the position
When the stirrer reaches the limit of the set
range of ~ovement, depending on the position found,
control unit 26 rotates rotors 6a, 6b and rotors 9a, 9b,
and changes the direction of stirrer 3 as in the
obstacle evasion sequence. If stirrer 3 was rising, it
is made to fall. By setting the range of movement
beforehand, therefore, the stirrer moves only over the
set range, and stirs the liquid in the tank efficiently.
