WO2005072520A1

WO2005072520A1 - Position monitoring with regard to bottom contact

Info

Publication number: WO2005072520A1
Application number: PCT/NO2005/000037
Authority: WO
Inventors: Henning Skjold-Larsen
Original assignee: Henning Skjold-Larsen
Priority date: 2004-02-02
Filing date: 2005-02-01
Publication date: 2005-08-11
Also published as: NO20040464L; ES2321897B1; GB0615379D0; GB2425031B; GB2425031A; ES2321897A1

Abstract

The invention relates to a method of monitoring the position of guiding modules for fishing gear with regard to bottom contact, comprising taking measurements of a first angle for the guiding module, calculating average and standard deviation for the angle measurements, based on the average and standard deviation, providing a position indication for the guiding module with regard to bottom contact. The invention also relates to an apparatus for position monitoring.

Description

Position monitoring with regard to bottom contact

The present invention relates to a method and an apparatus for monitoring the position of guiding modules for fishing gear with regard to bottom contact.

The purpose of bottom contact monitoring is to be able to optimally chart the position of the fishing gear in the water.

During trawl operations it is important to know whether the trawl or associated equipment is in contact with the bottom. In bottom trawls, it is, for example, desirable to have bottom contact for most of the time in order to ensure that fish swimming close to the bottom are caught, or to establish a dust cloud on the bottom which scares the fish into the gear.

At other times it is desirable to maintain a certain distance from the bottom (pelagic trawl), for example, to protect coral and other plants and animals.

Not least it is important to keep a certain distance from the bottom or maintain contact with the bottom that is limited to the surface of the bottom in order to prevent the fishing gear from digging into the bottom.

The position of the trawl will of course also depend upon the type of fish that is to be caught.

According to the prior art, bottom contact sensors are used to establish whether the trawl is moving along the bottom or not. A bottom contact sensor is a tubular body fastened to the bottom gear of the trawl at one end, the other end hanging freely. The bottom contact sensor will indicate "bottom contact" / "no bottom contact" depending on the angle between the longitudinal direction of the tubular body and the horizontal direction. When the bottom gear moves freely, the longitudinal direction of the tubular body will form a smaller angle than when there is bottom contact. However, if there are large irregularities in the bottom, i.e., depressions and elevations, the bottom contact sensor will not be capable of providing reliable information.

An example of a tubular body of this kind is described in US 5,347,860, which teaches a device for measurement of actual on-bottom time of a trawl used for benthic sampling. The position of the tubular body activates a counter that records on-bottom time.

Large fishing gear such as trawls comprise guiding modules which ensure that a certain opening of the trawl is maintained during the fishing process. One example of such a module is so-called bottom gear, one of the functions of which is to ensure that the trawl moves along the bottom. Another example are so-called doors, which in their most common form are flat, elongate bodies that are attached to the trawl, for example, by sweeps or wires.

A trawl door that is pulled along the bottom will have a particular angular position, and it will also oscillate or swing about said angular position. The oscillations are due in part to irregularities in the bottom. When the door is about to lose bottom contact, the oscillations will have greater amplitude and frequency than when the door is pulled along the bottom. When the door has lost bottom contact, it will only be subjected to hydrodynamic forces, and this will result in an almost stable angular position. If the door is then lowered, it will experience vibrations again until a new stable angular position is taken up in which frictional forces against the bottom interact with hydrodynamic forces. Hence, it will be possible to monitor the position of the door with regard to bottom contact, that is to say, whether it is pushed along the bottom ("bottom contact"), whether it is moved in the water only ("no bottom contact") or whether there is a transition between these two positions ("transition"), by means of the angular position of the door.

The angular position of the door can be described by means of three angles: a) Angle of attack: The trawl moves in one direction and the door follows this movement, whilst the plane of the door forms an angle with the direction of movement. This first angle, that is the angle between a horizontal axis of the plane of the door and the. direction of movement of the boat, will hereinafter be referred to as the angle of attack. b) Angle of roll: The plane of the door will form an angle with the vertical plane; this will hereinafter be called the angle of roll. c) Angle of pitch: The angle that the horizontal axis of the door forms with a horizontal plane is called the angle of pitch.

As mentioned above, the angular position of the door will vary depending on the position of the door. In the transitions to and from the bottom, the angular position is unstable, and this will be reflected in relatively marked changes in some of the said angles. Thus, it will be possible to use the angular position of the door and variations thereof to monitor the position, i.e., check whether there is bottom contact or not, and optionally to derive characteristics relating to the bottom contact situation. By the latter is meant that the analysis of the angular position will allow it to be known whether the door lies deep in the bottom, whether the lower edge of the whole door or just a part thereof has bottom contact, whether there are any irregularities in the bottom etc.

The method according to the invention is thus characterised in that it comprises: - taking measurements of a first angle for the guiding module; - calculating the average and standard deviation for the angle measurements;

- based on the average and the standard deviation, providing a position indication for the guiding module with regard to bottom contact.

The first angle may be the angle of roll, the angle of pitch or the angle of attack, and in a preferred embodiment of the invention the angle of roll is used to provide the position indication.

It is also possible to measure a second angle in addition to the first, and to use the mean and standard deviation for the second angle in combination with the values for the first angle in order to provide the position indication. If the first angle is the angle of roll, it would be natural to choose the angle of pitch as the second angle.

The position indication could assume various forms. In some cases it will simply result in a display of the type "bottom contact / no bottom contact" on the module, or it will indicate the situation directly by showing the value of the average and standard value, whilst in other cases a position indication signal will be provided that could show elapsed time in the position relative to bottom contact.

The position indication signal or signals could also be transmitted to a receiver at the surface by means of a cable connection, acoustically, by radio signals or wirelessly in some other way. This would allow to have an indication of the type "bottom contact/transition/no bottom contact" on board the boat. This would be very useful, especially where transition is concerned, since it would be possible to initiate rapid measures to prevent bottom contact/free floating.

The position indication signal or signals may also be stored in a memory in the module. The invention also comprises transmission of the position indication signal or signals to a display device.

Lastly, the method according to one embodiment of the invention will comprise control of the module's position on the basis of the indication signal or signals and desired values. In this connection, the desired value may be values for the average, standard deviation or other variables that indicate the position of the module. Both an alternative where desired value is position = bottom contact (for bottom trawls) and an alternative where the desired value is position = no bottom contact (for pelagic trawls) are included by the control function. The main object of the control function is to keep the module out of the transitional positions.

In one embodiment of the invention, the method is carried out in connection with fishing gear in the form of at least one trawl. In this case, the guiding module consists of a trawl door, lead, sweep or bottom gear. According to one embodiment of the invention, the angle measurement will be made using an inclinometer. Depending on how many angles are employed, one, two or three-axis inclinometers will be used. The inclinometer can be incorporated in the module or secured thereto by wires, welding etc. The invention also comprises an apparatus for monitoring the position of guiding modules for fishing gear with regard to bottom contact, comprising:

- a measuring unit for measuring a first angle for the guiding module;

- a processing unit adapted to calculate the average and standard deviation for the angle measurements, and to provide, based on the calculated standard deviation, a position indication for the guiding module as regards bottom contact;

- a memory unit.

In one embodiment of the invention the measuring unit is adapted for measuring the angle of roll, the angle of pitch or the angle of attack. *

In another embodiment, the measuring unit is adapted for measuring a second angle selected from the angle of roll, the angle of pitch or the angle of attack. In connection with this, the processing unit is adapted to calculate the average and standard deviation for the measurements of the second angle, and to provide a position indication based on the average/standard deviation for both angles.

In one embodiment the apparatus comprises an output unit connected to the processing unit for providing a position indication signal based on the position indication, and/or a transmitter unit for transmitting the position indication signal to a receiver.

In another embodiment the memory unit is adapted for storage of the position indication signal. In yet another embodiment, the apparatus comprises a control unit connected to the output unit for controlling the position of the module based on the bottom contact indication signal and desired values.

The measuring unit is, in a preferred embodiment of the invention, a one, two or three-axis inclinometer. Although the different embodiments of the method and the apparatus according to the invention are referred to individually, it will be clear to the skilled person that they can be combined in different ways, so that, for example, there could be a method in which two angles are measured, the position indication signal is stored in the memory and used as the actual signal for controlling the position of the module. Or an apparatus with which one angle is measured and the position indication signal is transmitted acoustically to the boat for display. Measurement and provision of position indication using three angles is also possible.

For the invention to achieve its object, it is important to use the correct sampling frequency. The sampling frequency is the frequency of the angle measurements, which must be. determined with regard to the frequencies of the oscillations that are to be detected. The sampling frequency will also have to allow for the transmission of the angle measurements in the case where the processing unit is at a distance from the measuring unit.

If the processing unit is located in the boat moving the fishing gear and only a single measurement is taken each time data is to be transmitted to the boat, in most cases any oscillatory movement will be under-sampled. When in addition there is filtration of the measured value in the bridge instrument, a good indication of the actual behaviour of the module will certainly not be obtained. The oscillations will not be picked up by the apparatus. With small oscillatory movements, a fairly correct picture of the average angle of pitch and angle of roll may just about be obtained. Of course, it is possible to envisage that data is measured and sent up very quickly so that any oscillatory movement is not under-sampled, but because of limitations in the transmission methods and duration requirements, this solution is not always optimal. The apparatus according to the invention will therefore perform measurements

(sample) so quickly that a possible oscillatory movement is not under-sampled, and will calculate complete relevant user data that can be sent up to the boat at a much slower rate than the actual angle measurement down on the door. In a preferred embodiment of the invention, the processing unit will therefore be in the immediate vicinity of the measuring unit.

In a simplified embodiment of the invention, the position indication signal is identical to the position indication, and simply comprises the average and standard deviation for the angle measurements. In this embodiment, at least two position indication signals are transmitted to the boat. These are the average angle in addition to standard deviation for one of the angles which define the position of the module. The average value will represent the nominal angle at which the module may be positioned and oscillate around. The standard deviation is the "normal" deviation in relation to the mean, and will indicate the extent of the oscillations the module makes in relation to the mean. This standard deviation will be a "chatter factor" which will make it very easy for the user to know whether the module is moving in a stable manner or not. If there is a small standard deviation (chatter factor), the module is moving in a stable manner, and if there is a large standard deviation, the module is unstable and oscillates a great deal. In the following the invention will be explained in more detail by means of an exemplary embodiment illustrated in the drawings. The example relates to the measurement of the angle of roll and angle of pitch for trawl doors and the provision of a position indication in relation to bottom contact based on the average and standard deviation for these values. As mentioned above, it is quite possible to use the invention in connection with other guiding modules and for other fishing gear, and also to establish a position indication based on the angle measurements of a single angle or of three angles.

List of figures Fig. 1 is a view of a door, and the definition of the angles of roll, pitch and attack.

Fig. 2 shows the position of a bi-directional inclinometer in relation to the door.

Fig. 3 is a block diagram for the measured value calculation.

Fig. 4 shows standard deviation and measured value for the angle of pitch and the angle of roll. Figs. 5, 6, 7 and 8 show details of the diagrams in Fig. 4.

Fig. 9 shows a screen image comprising a position indication provided by the invention.

Detailed description

Fig. 1 shows an outline of a door and the definitions of the angles of roll and pitch. The door 1 is, as mentioned, a relatively flat body that is used to control the opening of a trawl. The door will move during the trawl operation, and this movement can be described as turning about axes coincident with or parallel to the "main" axes of the door 1. The main axes of the door 1 are axis x', coplanar with the door and extending in the longitudinal direction of the door, y¹ coplanar with the door and at right angles to x', and z', at right angles to the plane of the door.

Figure 1 a shows an angle of pitch α, which is the angle between the axis x' and the horizontal direction (marked with the letter x). Figure lb shows the angle of roll β, which is the angle between the axis y' and the vertical direction (marked with the letter y). Figure lc is a top view of the door. In this figure the direction of travel of the vessel is indicated by the letter R and the angle of attack indicated by γ is the angle between x' (direction of the door) and the direction R.

Figure 2 shows the location of a measuring unit in the form of an inclinometer 2 in relation to a door 1. In some cases (Fig. 2a), the inclinometer 2 will be fastened to the door 1 by wires, welding etc. In other cases, it will be embedded in the door 2 or in another guiding or measuring module. Depending on the type of inclinometer (with 1 , 2 or 3 axes) and the correspondence between the axes of the inclinometer and the axes of the door, it will be possible to measure the angles of roll, pitch and attack. In the illustrated example, the measuring unit is a two-axis inclinometer 2. All necessary filtration and measured value calculation in this embodiment takes place in the immediate vicinity of the measuring unit.

The invention is based on the calculation of the standard deviation and mean value for a series of angle measurements, and the use of these values to establish the position of the module. The theoretical basis for the calculations follows. The variance of a variable x is defined as:

var(x) = j ( -3c) • p x)dx = E {x-x) >

wherein p(x) is the probability function and 3c = E(x) is the mean value of x:

3c = E( ) = J x- p(x)dx

The variance can in turn be written as

var( ) = E{(x-3c)²} = E( ²)-[E(x)]²

This means that if the variance of a measurement is to be calculated, it can be done by first finding the mean value of the squared measured values and then subtracting the squared mean of the measurement. The mean value can be estimated using a low-pass filter, and this gives a method for calculating the continuous variance of a measurement. As described above, we wish to indicate a "chatter factor" in the form of the standard deviation, as the standard deviation is easier to give a physical interpretation than the variance. The standard deviation is given as the square root of the variance:

std(x) = var(x)

Fig. 3 is a block diagram for measured value calculation and illustrates the present embodiment of the invention. The apparatus illustrated in the figure comprises a measuring unit, here in the form of an accelerometer 10, a filter device 11, an analogue/digital converter 12, a first processing unit 13 for calculating average Av and standard deviation Std, a second processing unit 14 for providing a position indication Pos for the module with regard to bottom contact, and a memory unit 15 for processing instructions, variables etc. As shown in the block diagram, before the signal is digitalised (sampled), it is sent through the filter 11 which in one embodiment of the invention is an analogue low- pass filter. The low-pass filter ensures that all frequencies higher than half the sampling frequency are removed. Thus, with a sampling frequency of 2 Hz (1/2 second sampling rate), frequencies above 1 Hz will be filtered away. The characteristics of the filter device and the sampling could however be altered to adapt this first filtration to the type of seabed or bottom. Although this example shows the processing unit that consists of two units 13 and 14, it is quite possible to carry out all the calculation steps in a single unit. It is also possible to establish a wireless connection between the units 13 and 14, and to place the unit 14, for example, on the vessel moving the trawl. The block diagram also shows additional components of the apparatus according to the invention: - an output unit 16 for providing a position indication signal, i.e., a signal which comprises a number of position values ("bottom contact", "transition", "no bottom contact"); - a display unit 17 for showing the position indication in the form of letters/ numbers/sound; - a control unit 18 connected to a desired value input unit and an actuating unit, so that it is possible to control the position of the module on the basis of desired values and measured values (actual values); - a transmitter unit 20 for transmitting the position indication signal to a receiver 21 which, for example, is on board the vessel. Figure 3b illustrates a part of the processing that is carried out in the processing unit 13. The figure shows that the measurement signals from the measuring unit 10 are first converted/linearised (block 101) and then filtered to remove wild points (block 102). The purpose of the wild-point filter is to remove erroneous measured values. One possibility is a three-point median filter which will remove an erroneous measurement point that appears alone. However, if two erroneous measurements come in succession, they will pass through. Subsequently, the signals are filtered (blocks 103 and 106) by RC filters, which, for example, are a digital first order RC filter:

^' ^ - j / - i) + [tt(i) -j<ι - l)] - _Jg: wherein y is the filtered measured value and u is the unfiltered raw value. K is a filter constant in the range of 0 to 1. As regards the choice of time constant in the filter, this must be chosen on the basis of the slowest oscillation the door may have. A typical period for a possible oscillation in the angle of roll is assumed to be between 5 and 10 seconds, but in the case of large pelagic doors, the period may be as much as 20 seconds. We therefore assume a maximum period of 20 seconds when choosing the filter constant K. Simulations show that with a sampling rate of 0.5 seconds K = 0.050 gives a good response. This filter constant is used in both filters that are included in the standard deviation calculation.

After the filtration, the values are raised to the second degree (blocks 104 and 105), subtracted and the square root of the result is calculated (block 107). The output signal from block 103 represents the mean of the angle measurements Av (the process is carried out separately for different angles), whilst the output signal from block 107 gives standard deviation for the angle measurements.

Figure 4 shows standard deviation and measured value for the angles of roll and pitch of a door, where the door at certain times is in contact with the bottom. In the diagrams Meas 1 is mean Av for the angle of roll β, Meas 2 is standard deviation Std for the angle of roll β, Meas 3 is mean Av for the angle of pitch α and Meas 4 is standard deviation for the angle of pitch α. The horizontal axis represents time in seconds.

It can be seen in this figure (Meas 1 and Meas 2) that the door has a first angle of roll in the first 600 seconds, that subsequently there is a transition (between 600 seconds and 800 seconds) and that the door then stabilises at a second angle. The first angle of roll has a typical value for bottom contact whilst the second is typical for no bottom contact. The transition shows how the door oscillates as it leaves the bottom. In Meas 2 the transition is shown as a marked peak in the standard value around 750 seconds. By establishing limits for the mean, the limits being between the means for bottom contact and for no bottom contact, a position indication can be provided. In the same way, by establishing limits for the standard deviation and not least for the increases therein (derived, peak value), it will be possible to provide a position indication with regard to transition.

It is also possible to provide a position indication simply by showing the values for average and standard deviation of the angles. Figure 9 shows a screen image comprising a position indication provided by means of the invention. The screen image shows the parameters associated with a bottom trawl that is controlled by two doors.

The upper fields on the screen image show, from left to right:

- depth of the port door: 111 m; variation: 0 m/min - depth of the trawl headline: 108 m; variation: 0 m/min

- depth of the starboard door: 1 10 m; variation 1 m/min

- temperature: 10.3°C, variation: 3°C/min - speed: 1.6 knots; variation: 0 knots/min, direction of water flow coincident with direction of trawl

- distance between doors: 38.3 m; variation: Om/min

- angle of roll for the first door (mean Av): 19.4°; standard deviation for angle of roll ("chatter factor"): 4, angle of pitch for the first door: 6.9°; standard deviation for angle of pitch: 2

- angle of roll for the second door (mean Av): -14.8°; standard deviation for angle of roll: 6; angle of pitch for the second door: 7.6°; standard deviation for angle of . pitch: 3 - height of trawl (between headline and bottom gear): 2.7 m; trawl opening: 2.7 m, distance from bottom: 0.0 m.

The lower left field shows, from the top of the picture down and from left to right:

- the distance between the doors: 38.3

- tension in the port wire (that pulls the trawl): 0.6 tonnes, variation: 0 tonnes/min - length of the port wire (that pulls the trawl): 250.7 m; variation: 0 m/min

- depth of the trawl headline: 108 m; variation: 0

- height of the trawl: 2.7 m

- tension in the starboard wire: 0.6; variation: 0

- length of the starboard wire: 251.1 m; variation: 0 - trawl opening: 2.7 m

- angle of roll for the first door: 19°

- depth of the first door: 111 m; variation 0 m/min

- distance from bottom: 0 m, relative depth of the doors: OS

- depth of the second door: 110 m; variation lm/min - angle of the second door: -15°.

Lastly, the screen image shows schematically the signal from a trawleye, i.e., an echo sounder positioned on the headline that points towards the bottom. The left side of the diagram shows that the bottom "is approaching the sensor" or in other words, that the trawl is on its way down to the bottom. Subsequently, it shows the relative position of the bottom gear and the bottom. This sensor is not capable of charting the position of the trawl with regard to bottom contact in a satisfactory manner. A position determination of this kind is possible using the invention.

As can be seen, the invention allows the provision of a distinct and reliable position indication for guiding modules. The position indication is provided quickly and it ensures the possibility of hourly correction of the control parameters of the trawl.

Claims

PATENT CLAIMS

1. A method for monitoring the position of guiding modules for fishing gear with regard to bottom contact, comprising:

- taking measurements of a first angle for the guiding module; - calculating the average and standard deviation for the angle measurements;

- based on the average and standard deviation, providing a position indication for the guiding module with regard to bottom contact.

2. A method according to claim 1, characterised in that the first angle is the angle of roll, the angle of pitch or the angle of attack.

3. A method according to one of the preceding claims, characterised in that it comprises taking measurements of a second angle selected from among the angle of roll, the angle of pitch and the angle of attack.

4. A method according to claim 3, characterised in that it comprises calculating the average and standard deviation for the measurements of the second angle, and providing a position indication for the guiding module with regard to bottom contact based on the average/standard deviation for both angles.

5. A method according to one of the preceding claims, characterised by providing a position indication signal based on the position indication.

6. A method according to claim 5, characterised by transmitting the position indication signal or signals to a receiver at the surface by means of a cable connection, acoustically, by radio signals or wirelessly in some other manner.

7. A method according to claim 5, characterised in that it comprises storing the position indication signal or signals in a memory unit on the module.

8. A method according to claim 5, characterised by transmitting the position indication signal or signals to a display device.

9. A method according to claim 5, characteri sed in that it comprises controlling the position of the module on the basis of the indication signal or signals and desired values.

10. A method according to claim 1, characterised in that the module is a door, a lead, a sweep or bottom gear for fishing gear, for example, a trawl.

11. A method according to one of the preceding claims, characterised in that the angle measurements are performed using an inclinometer.

12. An apparatus for monitoring the position of guiding modules for fishing gear with regard to bottom contact, comprising:

- a measuring unit for measuring a first angle for the guiding module;

- a processing unit adapted to calculate the average and standard deviation for the angle measurements, and, based on the average and standard deviation, to provide a position indication for the guiding module with regard to bottom contact;

- a memory unit.

13. An apparatus according to claim 12, characteri s ed in that the measuring unit is adapted for measuring the angle of roll, the angle of pitch or the angle of attack.

14. An apparatus according to one of preceding claims 12 or 13, characteris ed in that the measuring unit is adapted for measuring a second angle selected from among the angle of roll, the angle of pitch and the angle of attack.

15. An apparatus according to one of preceding claims 12-14, characterised in that the processing unit is adapted to calculate the average and standard deviation for the measurements of the second angle, and to provide a position indication based on the average/standard deviation for both angles.

16. An apparatus according to one of preceding claims 12-15, characteri sed in that it comprises an output unit connected to the processing unit for providing a position indication signal based on the position indication.

17. An apparatus according to claim 16, characteris ed in that it comprises a transmitter unit for transmitting the position indication signal to a receiver.

18. An apparatus according to claim 16, characteris ed in that the memory unit is adapted for storing the position indication signal.

19. An apparatus according to claim 16, characterised in that it comprises a control unit connected to the output unit for controlling the position of the module based on the bottom contact indication signal and desired values.

20. An apparatus according to claim 13, characterised in that the measuring unit is a one, two or three-axis inclinometer.