Note: Descriptions are shown in the official language in which they were submitted.
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A~ORNEY DOCRI~ No: 050371002001
TRAWL CABLE VIESRATION METER
Backqround o~ the_ ~b~___ on
To catch bottom-dwelling fish, a conically shaped
nat, called a trawl is dragged along the sea bottom. The
trawl is towed by two cables, port and starboard, which are
connected to trawl doors which in turn are tied to opposite
sides of the mouth of the net. The doors ride over the
ocean ~loor about 100 feet apart and open the net over a
wide ~rea.
At the fishing areas, the sea bottom can vary from
flat soft mud, to hard sand and gravel with sculpted wave
patterns, to rocky cobbles, to outcropping ledge. Often the
pattern from soft mud to ledge follows these grades in the
order stated. A fisherman would prefer that the net remain
over the softer bottom. If the net becomes "hung," i.e.
caught upon a rocky hill or ledge, it can cause damage to
the fishing gear and loss o~ time, fuel, and catch. Fish
sometimes congregate near rocky outcrops, luring ~ishermen
to fish close to the hard (rocky) bottom~
In the old days, a crew member would keep a hand or
foot touching the cable at the winch and alert the captain
i~ the cable yanked hard, an indication that the trawl door
had struck hard bottom. The captain would then steer away
from the side which pulled.
In order to assist the fisherman's efforts to avoid
dragging the fishing gear over rocky bottom, today's fishing
vessels may be eguipped with a depth sounder that is
directed downward and/or with a side scan sonar, a device
which scans a conic pattern ~rom side to side ahead of the
boat. These sensors are not always suf~icient to keep the
trawl doors ~rom being hung since the net and doors spread
to about one hundred feet wide and can lag one to three
minutes behind the current vessel position, depending on the
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depth of water, scope of cable, and speed of forward motion.
Wind, waves, and tidal effects keep the net from following
precisely behind the fishing vessel.
A fisherman also prefers that the trawl doors "tend
the bottom," i.e. that the doors are dragged smoothly along
the bottom, holding the net in proper po~ition. Even when
the doors are on smooth, soft bottom, the doors can become
hydrodynamically unstable and periodically lift from the
bottom due to the forces on the door and the cable
attachments. This lifting reduces efficiency since the
shape of the net changes as the doors rise and fall.
Summary of the Invention
The invention features an apparatus which includes
two vibration detection mechanisms, one corresponding to the
port trawl cable, the other to the starboard cable. These
detection mechanisms are mounted on the vessel at a cable
handling mechanism and provide a signal correspondiny to the
vibration in each cable. An operator interface receives the
electrical signals ~rom each detection mechanism and
provides information to the operator regarding the vibration
in each cable.
In the preferred embodiment, a piezo-electric sensor
is mounted at the winch assembly where the stress from the
cable caused a portion to be strained. The interface
includes signal processing circuitry which amplifies and
filters the signal, and compare6 the signal to a threshold
value. The interface includes dual light bars, one
corresponding to each cable, for providing ongoing
information on each cable, and further includes an audible
alarm for alerting when the signal exceeds the threshold
value. When the audible alarm sounds, a light is lit to
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notify the operator whether the port or starboard cable
caused the alarm.
In another embodiment, the apparatus includes a
vibration detection mechanism mounted on the vessel for
providing an output based on the vibration in the cable,
which is indicative of the interaction between the trawl
door and ocean floor, a triggering mechanism for comparing
the output and a threshold value, and an alarm mechanism to
respond to a determination that the threshold was exceeded.
The invention a~lows an operator, such as a vessel
captain, to monitor the vibration in the trawl ~ables as he
is operàting the vessel. The interPace provides ongoing
information on the vibration in each cable, and an alarm ~or
alerting the operator that the vibration exceeds a threshold
and showing which side caused the alarm. Without having to
watch the cables, a captain can discover the potential that
a door will be hung, and steer to avoid the problem. The
captain can also detect when the door is not properly
tending bottom, allowing him to adjust the cable.s or bridles
or towing speed.
The operator interface has several practical
controls for using the meter without requiring great
technical skill. The gain, trigger, and balance are
adjustable by the operator. The visible indicators are
prePerably light bars which are adapted for good visibility,
including in direct sunlight. The preferred embodiment is
thus suitable for practical use by a Pisherman.
Other advantages and features of the invention will
be apparent Prom the Pollowing description of a preferred
embodiment, and from the claims
Brie~ Description of the Drawinqs
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Fig. 1 is a block diagram o~ the trawl cable
vibration meter system.
Yig. 2 is a pictorial side view of a fishing vessel
at sea towing a net.
Fig. 2A is a pictorial overhead view of the vessel
towing the net of Fig. 2.
Fig. 3 is a schematic of the cable handling
mechanism.
Fig. 4 is a schematic of the winch assembly with the
sensor mounted on the winch arm.
Fig. 4A is a schematic of the brake band and
stopping mechanism from the plane 4A-4A in Fig. 4.
Fig. 5 is schematic view of a piezo-electric sensor.
Fig. 6 is a schematic showing the connecti~ns from
the sensor to the system circuitry.
Fig. 7 is a schematic of the ~irst stage of
amplification.
Fig. 8 is a graphical representation o~ the
relationship between the current and the strain, the voltage
and current, and the resulting voltage per input strain
relationship.
Fig. 9 is a schemati¢ of the circuitry for
amplification and filtration.
Fig. 10 is a graphical representation of the
frequency response over each stage in Fig. 9.
Fig. 11 is a graph of the system frequency response
resulting Erom the product oE the graphs in Fig. 10.
Fig. 12 is a schematic of the rectifier circuitry.
Fig. 13 is a schematic oE the LED driver and
display.
Fig. 14 is a schematic o~ one LED and the masking
over the LED.
Fig. 15 is a schematic of trigger circuitry.
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Fig. 16 is a schematic of the persistence circuitry
and persistence lights.
Fig. 17 is a schematic of the external interface of
tha meter.
Description o~ the Preferred Embodiment(s)
An overview of the system 15 is shown in block
diagram form in Figure 1. The mechanics of the trawl cable
ar~ identified at blocks 1, 2, and 3. The fishing trawl net
(not shown) is held in the fishing position by two trawl
doors, port and starboard, which are dragged along the sea
bottom as represented by block 1, and are connected by the
trawl cables to the vessel (block 2). The load from each
cable is transferred to the vessel through a cable handling
mechanism. Sensor 4 detects vibration in the cable, which
; 15 is indicative o~ the interaction between the trawl door and
the ocean bottom, and sends a signal through signal wires at
block 5 to circuitry in the system. Signals from the sensor
are amplified, filtered, and rectified, as represented at 6,
7, and 8. Display driver 9 causes LED bar graph 10 to
ligh~, reflecting the vibration signal from the sensor
conditioned by the circuitry. The output from the rectifier
is also passed through trigger circuit block 11 which
detects whether the conditioned signal exceeds an alarm
level. If the slgnal exceeds the alarm level, a buzzer 12
alerts the user, and persistence light timer and driver 13
causes persistence light 14 to stay on, so that the user can
see which side, port or ~tarboard, has triggered the alarm.
Each o~ these components is described in more de.tail below.
The ocean environment of Fig. 2 shows a fishing
vessel 20 dragging port door 22 and starboard door 24,
typically about one hundred feet apart, by port cable 26 and
starboard cable 28. Net 30 is connected by bridles shown
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generally at 32 which are connected to the port and
starboard doors. The bridles are connected to the top of
the net where there are floats, and are also connected to
the bottom of the net where there are weights. These doors,
which are heavy, hydrodynamic "kites", sink the net to the
bottom and then "fly" sideways to spread the net open
horizontally. The trawl cables are mounted on-board by a
cable handling mechanism, here trawl blocks 34 and winch 36.
A sensor (not shown) is connected at a position on-board
where the load is transferred, and results in a graph
represented at 43, which is mounted on the user interface.
The interfaae includes ~wo light bars, representing the
vibration }evels ~rom the port and starboard trawl doors
dragging along the bottom. These light bar displays would
be visible to the vessel captain.~
Two sonar detectors may be used with the fishing
vessel. DPpth sounder 40 detects the depths and "hardness"
of the sea bottom directly below the vessel 20. Side
scanner 42 scans an arc ln front of vessel 20 to detect
upcoming depths and "hardness." The output from these sonar
detectors is represented by the graph 41.
As described above, the sonar detectors can be
inadequate for certain conditions for which this meter is
designed. An example is shown in Fig. 2A which i5 an
25 ~ overhead view of a vessel 20 which is on the surface of the
water and is pulling doors 22 and 24 and net ~0 which are at
the bottom of the sea. Net 30 is in the muddy bottom 20,
but port door 22 has just run onto the hard slope 202 and is
in danger of striking rocky ledge 204. Had vessel 20
followed the relatively straight course shown as dotted line
208, depth sounder 40 (Fig. 2) clearly would not have
alerted the trawler to the danger. Side scanner ~2 (Fig. 2)
might not have detected the risk of getting hung.
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One side of the cable handling mechanism is shown in
Fig. 3. Starboard trawl cable 28 is connected to a winch 36
through trawl block 34 which i5 mounted on a gallows frame
38. Winch 36 includes brake 40 which trans~ers its load to
the winch foundation. The brake torsion is the product of
the cable tension and the radial arm "r'l, which changes as
the cable is rolled on or of~ the winch. A sensor that
dPtects load vibration indicative of the interaction between
the door and the ocean floor is placed at a place wherP load
is transferred, here at either point A or B~ Point B at the
winch foundation was sel~cted in the preferred embodiment.
of course, the location of the sensor can be in different
places on the cable handling mechanism depending on the
arrangement of the fishing vessel and how the cable is
mounted.
Figure 4 shows winch 36, winch foundation 42, brake
lever arm 44, and a chock 51 which is attached to the winch
drum (Fig. 4A~. Figure 4A shows another view of the chock,
brake, and arm. Chock 51 is rlgidly attached to brake band
47 which is around winch drum 49. As the stress in the
cable changes, the pressure exerted from cable to winch
drum, and from chock to brake arm, varies and is detected by
sensor 50. Dotted line 46 represents the neutral axis of
lever arm 44 at which there is no bending stress. Sensor 50
should be located near the top or near the bottom of lever
arm 44, or i~ possible on the top or bottom surfaces of the
lever arm, ~ar from the neutral axis. The sensors should
also be located as close to the winch foundation 42 as
possible in order to be where the lever arm ~4 experiences
lts maximum bending strain due to braking action.
The winch arrangement shown in Fig. 4 is one of many
possible configurations by which cable load is transferred
to the winch foundation and vessel structure. Other
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arrangements can depend on the manufacture of the winch.
The described em~odiment provides an illustration of the
principle of sensor location: the sensor should be located
to measure the strain of some portion of the mechanism or
structure that resists or holds the entire load from the
cable and experiances significant strain directly in
proportion to load. Furthermore, the geometry of the
mechanism should remain constant over time so that the
proportionality between cable load and measured strain
lo remains constant. For example, if the brake band chock 51
;~ were to land at different places along the top of lever arm
44 from time to time, then the measured strain would be a
function of cable load and chock position, in which case
another location ~or the sensor should be chosen.
The sensor used includes a XYNAR piezo film, a
highly-polar poly-vinylidene fluoride film, available ~rom
Atochem Sensors, Inc., Norristown, PA, as model DTI-028K or
LDTI-028K. (KYNAR is a registered trademark). The sensor
is bonded to the strained element of the winch or trawl
block support with a suitable non-conductive resin matrix,
such as epoxy. A top view o~ sensor 50 is shown at Fig. 5.
The sensor has a variable capacitance, which changes as the
stress-induced strain in the arm 44 changes the geometry of
the piezo film (not shown), which is sandwiched between foil
plates 5 and 53.
Thus, the change in charge is proportional to the
change in strain which is also proportional to the change in
stress. Since current is the change in charge divided by
the change in time, the current is proportional to the
change ln stress divided by the change in time. Thus the
current is proportionate to the time rate of stress change,
i.e. vibration, rather than the actual load. Since the
selected ampli~ier is very sensitive to a change in
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capacitance divided by a change in time, the leads from the
sensor should be held firmly in place and cannot be squeezed
or stepped upon since this would create spurious output.
As an alternative, the vibration detection could be
performed by measuring load with a device, such as a
resistive strain gauge with associated bridge circuitry, and
converting to a signal indicative of vibration.
Sensor 50 is mounted in resin matrix 66, as shown in
Fig. 6. Leads 54 and 56 from the sensor are shielded by
shield 62 leading to circuitry generally represented at 68.
It is preferred that ground connections 60 be made directly
to an "earthl' ground, such as the steel hull of the fishing
vessel. Circuit ground 61 varies from earth ground 60 by
V(noise) which is caused by circuit currents flowing through
the small but finite resistance of the ground leads.
Grounding top lead 54 and shield 62 to earth ground 60
raduces the spurious influences of V(noise) on output.
In the transimpedence amplifier circuitry of Fig. 7,
sensor 50 is modeled as current source 70. A very large
resistancel Rf, in the sensor will cause a small current to
bleed off. For most purposes, this large Rf can be
neglected. Operational ampli~ier 72, a type LF353, i5 used
with an RC circuit in its feedback path. The RC circuit has
impedance Zfb, which provides a pole at 0.72 Hz. The
results of this stage are shown by the graphs in Fig. 8.
The first graph de~onstrates that current is proportional to
strain divided by time, as explained above. The second
graph i5 the transfer function of output voltage and input
current. The combination, as shown in the third graph, is a
high pass filter~ Low frequency motions, 1 Hz and lower,
due to pitch and roll Oe the sea can cause substantial load
changes in the cable and should be filtered.
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One of two identical channels of the filter and
ampli~ication circuitry is shown in Fig. 9. ~edundancy is
shown in certain places where controls are cross-connected.
Output Vl from the first stage (Fig. 7) is the input to
balance control 78, a cross-connected stereo potentiometer
which is adjusted such that one channel goes up while the
othar goes down. Sea filter 80 is switch selectabla so that
a high pass filter of either 0.072 Hz or 0.72 Hz may be
employed. This filter also blocks out any DC output from
the first stage due to the bias current and voltage through
R~. Sea filter 80 has different levels for calmer and
windier weather. For windy weather fishing, the 2.2~F
capacitor 82 can be selected, and for calm weather fishing
the 22~F capacitor 84 can be selected. Additional filtering
of wave motion induced "noise" is desired when there is
increased wave motion in the sea. Other switches, values,
or factors can be considered for the sea filter which is
selectad by a switch located on the meter housing (Fig. 17).
In calmer weather, there is less vessel motion so
less filtering of low frequency signals is required,
allowing increased sensitivity of these signals. The
fisherman can watch for low frequency vibrations that can be
caused by the trawl door becoming hydrodynamically unstable.
This condition occurs when the door periodically leaves the
soft bottom of the ocean and lands again, changing the net
shape and reducing trawling e~ficiency. If the Pisherman
determines that a door is not tending bottom well, the
cables, bridles, or towing speed can be adjusted to try to
minimiæe the effect. Gain control stage 86 is also cross-
connected to the other channel and is adjustable at themeter by an operator. Unlike balance control 78, the gain
controls are arranged so that the sensitivity of both the
port and starboard channels is increased or decreased
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together. Second stage 90, like thP first stage, includes
an operational amplifier and an RC feedbacX circuit 9~. RC
circuit 92 yields another pole at 18 Hz. Higher frequencias
are filtered out so that the flashes in the light bar will
not appear as constant steady light. The ground connections
represented at 91 are to "earth" ground as discussed with
reference to ~ig. 6.
The graphs in Figs. 10 and 11 show the results of
the filtering in Figs. 7 and 9. Graph 94 corresponds to the
resulting third graph of Fig. 8. The sea filter stage 80
has a transfer function shown by graph 96, with double lines
~t lower frequencies representing the summer and winter
selection. The transfer function for second stage 90 is
shown in graph 98. The overall system frequency response is
represented by graph 100 of Fig. 11, and reflects the
product of graphs 94, 96, and 98 in Fig. 10.
Rectifier circuit 104 in Fig. 12 also includes a
gain of 22. An alternative is to use truncation, but
rectification was selected because the vibrations are
bipolar, but not necessarily symmetric; the vibration
polarity is arbitrary; and the vibration intensity is based
on absolute value. Rectification, but not trunc~tion,
doubles the frequency of the output. In order to prevent
the frequency doubling, diode Dl or D2 could be removed and
left open circuited, thus causing truncation of signals of
one polarity, and amplification of the signals of the other
polarity.
The display driver and LED bar graph, previously
shown as blocks 9 and 10 in Fig. 1, are shown ln Fig. 13.
Display driver 110 and display 112 are well-known, employing
an LED light bar and a driver chip, which may be an LM3914.
Resistance Rs sets range for the meter. The illuminated
height of each LED light bar indicates the amplitude of the
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vibration of a door. The frequency of the light bar flash
is driven by the frequency of the vibration, possibly
doubled as explained above. Impacts caused by the trawl
door striking rock are rich in all vibrational frequencies
and cause a distictive and intense flash of the display.
The display light ~lashes caused by dragging the door over
soft bottom or by the doors lifting from the bottom each
have their own qualitative frequency characteristics that
can be read and understood, in addition to the information
represented by the height or amplitude of the light bar.
Several changes have been ~ade to the LED elements
for practical benefits. For better visibility, two driver
; chips and displays are cascaded ~or each display to drive
twenty LEDIs per channel to produce an expanded reading.
Figure 13 shows the driver and display for ten LED's in on~
channel, so a second display is~added for each channel, ~or
a total of forty LED elements. The port channel LED I 5 are
red, and the starboard LED's are green, corresponding to
traditional marine colors for navigation lights.
Since the meter may be used in an open area, the LED
elements should be clearly visible in sunlight, but not
appear to be "on" when they are not. Preferably, LED's with
clear lenses should be used, since these LED's appear clear
when ofP, even when viewed in bright sunlight. Many LED's
have tinted lenses which may appear to be ~'on" when viewed
in bright sunlight.
An LED which is e~fective in practical use is shown
in Fig. 14. LED 114 may be the CMD5760 (red) and CMD5460
(green), Chicago miniature brand, a size T-1 with source
intensity o~ 12 mcd, and a view angle o~ 60 (30 each side
o~ vertical). Also possible are the HLMP 1340 (red) and
~LMP 1540 (green), with 60 mcd and 40 view angle. Above
LED 114 is a layer of clear tape 120, di~user tape 118
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~which can be Scotch brand "Magic" tape), and a paper mask
116. The appearance o~ the rather ~'tight" beam from the
tiny LED source varies noticeably as the position of the
viewsr's eye moves relative to the optical axis of the LED.
The diffuser tape interrupts the tight beam and makes it
appear as a large bright spot which has uniform appearance
as the viewer's eye moves through a wide range o~ positions.
The tape also gives the display a uniform appearance despite
many variations in the alignment of the optical axis each
LED in the light bar array. Clear tape 120 hides the sticky
side of the diffuser tape and holds the diffuser tape to
paper mask 116. Wax paper may also be used as a substitute
~or the diffusar tape.
The trigger circuit, shown in Fig. 15 at 124
generally, includes a manually adjustable variable resistor
126 which allows an operator to set and vary the trigger
~evel at which an alarm occurs. When Vs exceeds trigger
reference voltage Vt, the normally "high" output of the
operational amplifier goes "low", causing current to flow
; 20 from the external voltage through the buzzer with driver
130. Switch 132 enables an operator to turn the alarm on or
o~f. Diode 134 is inserted to prevent cross-talk betwe~en
the two channels.
Triggering could be performed with techniquPs other
than analog circuitry, e.g. the output from the sensor could
be digitized and methods employing software could compare
the digitized signal with a threshold.
Persistence light timer and driver and the
pQrsistencQ light LED's are shown generally at 140 in Fig.
16. Persistence circuitry 140 and trigger circuitry 124 are
connected at common node 136 of Figs. 15 and 16. R and C
are chosen to have a discharge time constant of about 3.3
seconds. When the output of the op-amp in Fig. 15 goes low
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and node 136 goes low, current is pulled through the
capacitor and the branch with diode D, resistor R being
large and negligible at this point, lowering the voltage of
the negative terminal of the op-amp. This causes the output
of the op-amp 144 to go high causing persistence LED's 142
to light. Persistence LED's 142 are two additional LED
elements placed next to or above the 20 LED' 5 of each
channel, as described in Fig. 13. These two LED's are
referred to as persistence lights because they stay on for
about 2 seconds, a period controlled by the value of R and
C, to enable the operator to look at the meter interface
after the buzzer sounds so that the operator can determine
which side, port or starboard, triggered the alarm, and
steer the vessel accordingly.
The external portion of the user interface 150,
shown in Fig. 17, includes switches and controls which are
adjustable by an operator. The red port side LED bar graph
152 and port side persistence lights 156 are shown, along
with corresponding green starboard LED's 154 and 158. The
trigger is set at knob 160, corresponding to pot 126 in Fig.
15, and the balance control is set at knob 162,
corresponding to balance 78 in Fig. 9. Power switch 166 has
three positions: off, dim, or bright. When the power is
turned on, a light or LED 174 is lit. Filter switch 168 is
the switch used to control sea filter 80 (Fig. 9) for calm
or windy use. Alarm switch 170 allows the buzzer to be
turned of~ or on, as shown at 132 in Fig. 15. Gain control
172 allows the sensitivity to be adjusted for sea bottom
roughnes~ and ~or variations in the radius o~ the cable on
the winch drum (Fig. 3). Gain switch 172 corresponds to
gain control 86 (Fig. 9). The interface could also provide
information on the vibrat:Lon in each cable with in~ormation
in ways other than necessarily having one bar corresponding
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to each oable. An additive signal could indicate total
vibration, and a differential signal could indicate a high
difference between each side. The user should have
instantaneous and/or alarm information regarding each side
in some manner to assist in steering, or to alert i~ a door,
and which door, is not tending bottom well.
The operation o~ the trawl cable vibration meter is
fairly simple, and an operator need not have special
technical skill. Only a few switches and knobs are actually
needed to be adjusted in order to operate the meter, thus
fulfilling an objective as a practical instrument for
isherman.
What is claimed is;
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