PCT
`
`WORLD INTELLECTUAL_ PROPERTY ORGANIZATION
`International Bureau
`
`INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(51) International Patent Classification 7 :
`
`(11) International Publication Number:
`
`WO 00/20895
`
`G01V 1/38, B63B 21/66
`
`A1
`
`(43) International Publication Date:
`
`13 April 2000 (1304.00)
`
`(21) International Application Number:
`
`PCT/IB99/01590!
`
`(22) International Filing Date:
`
`28 September 1999 (28.09.99)
`
`(74) Agent: STOOLE, Brian, David; Geco-Prakla (UK) Limited,
`Schlumberger House, Buckingham Gate, Gatwick, West
`Sussex RH6 ONZ (GB).
`
`(30) Priority Data:
`982l277.2
`
`1 October 1998 (0l.10.98)
`
`GB
`
`(71) Applicant (for all designated States except CA FR US): GECO
`AS
`[NO/NO]; Schlumberger House,
`Solbraveien 23,
`N—l372 Asker (NO).
`
`(71) Applicant (for CA only): SCHLUMBERGER CANADA LIM-
`ITED [CA/CA]; 24th floor, Monenco Place, 801 6th Avenue,
`SW, Calgary, Alberta T2P 3W2 (CA).
`
`PETROLIERS
`SERVICES
`(for FR only):
`(71) Applicant
`SCHLUMBERGER [FR/FR]; 42,
`rue Saint Dominique,
`F—75007 Paris (FR).
`
`(72) Inventors; and
`(75) Inventors/Applicants (for US only): HILLESUND, Oyvind
`[NO/NO]; Juterudaasen 47A, N—13l2 Slependen (NO). BIT-
`TLESTON, Simon, Hastings [GB/GB]; Weathercock Hill
`House, Weathercock Hill, Cheivington, Bury St Edmunds,
`Suffolk IP29 SGR (GB).
`
`(81) Designated States: AE, AL, AM, AT, AU, AZ, BA, BB, BG,
`BR, BY, CA, CH, CN, CR, CU, CZ, DE, DK, DM, EE,
`ES, FI, GB, GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP,
`KE, KG, KP, KR, KZ, LC, LK, LR, LS, LT, LU, LV, MD,
`MG, MK, MN, MW, MX, NO, NZ, PL, PT, RO, RU, SD,
`SE, SG, SI, SK, SL, TJ, TM, TR, TT, TZ, UA, UG, US,
`UZ, VN, YU, ZA, ZW, ARIPO patent (GH, GM, KE, LS,
`MW, SD, SL, SZ, TZ, UG, ZW), Eurasian patent (AM, AZ,
`BY, KG, KZ, MD, RU, TJ, TM), European patent (AT, BE,
`CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC,
`NL, PT, SE), OAPI patent (BF, BJ, CF, CG, CI, CM, GA,
`GN, GW, ML, MR, NE, SN, TD, TG).
`
`Published
`With international search report.
`
`(54) Title: CONTROL SYSTEM FOR POSITIONING OF MARINE SEISMIC STREAMERS
`
`(57) Abstract
`
`A method of controlling a streamer positioning device (18) configured to be attached to
`a marine seismic streamer (12) and towed by a seismic survey vessel (10) and having a wing
`and a wing motor for changing the orientation of the wing. The method includes the steps
`of: obtaining an estimated velocity of the streamer positioning device, calculating a desired
`change in the orientation of the wing using the estimated velocity of the streamer positioning
`device, and actuating the wing motor to produce the desired change in the orientation of the
`wing. The invention also involves an apparatus for controlling a streamer positioning device
`including means for obtaining an estimated velocity of the streamer positioning device, means
`for calculating a desired change in the orientation of the wing using the estimated velocity
`of the streamer positioning device, and means for actuating the wing motor to produce the
`desired change in the orientation of the wing.
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`Codes used to identify States party to the PCT on the front pages of pamphlets publishing international applications under the PCT.
`SI
`Slovenia
`Lesotho
`LS
`SK
`Slovakia
`LT
`Lithuania
`SN
`LU
`Senegal
`Luxembourg
`SZ
`Swaziland
`LV
`Latvia
`TD
`Chad
`MC
`Monaco
`TG
`MD
`Togo
`Repulztlic of Moldova
`TJ
`MG
`Tajikistan
`Madagascar
`TM
`Turkmenistan
`MK
`The former Yugoslav
`TR
`Turkey
`Republic of Macedonia
`TT
`Mali
`Trinidad and Tobago
`Ukraine
`UA
`Mongolia
`UG
`Mauritania
`Uganda
`US
`United States of America
`Malawi
`UZ
`Uzbekistan
`Mexico
`VN
`Viet Nam
`Niger
`YU
`Netherlands
`Yugoslavia
`Zimbabwe
`ZW
`Norway
`New Zealand
`Poland
`Portugal
`Romania
`Russian Federation
`Sudan
`Sweden
`Singapore
`
`FOR THE PURPOSES OF INFORMATION ONLY
`
`ML
`MN
`MR
`MW
`MX
`NE
`NL
`NO
`NZ
`PL
`PT
`RO
`RU
`SD
`SE
`SG
`
`AL
`AM
`AT
`AU
`AZ
`BA
`BB
`BE
`BF
`BG
`BJ
`BR
`BY
`CA
`CF
`CG
`CH
`CI
`CM
`CN
`CU
`CZ
`DE
`DK
`EE
`
`Albania
`Armenia
`Austria
`Australia
`Azerbaijan
`Bosnia and Herzegovina
`Barbados
`Belgium
`Burkina Faso
`Bulgaria
`Benin
`Brazil
`Belarus
`Canada
`Central African Republic
`Congo
`Switzerland
`Cote d’Ivoire
`Cameroon
`China
`Cuba
`Czech Republic
`Germany
`Denmark
`Estonia
`
`ES
`FI
`FR
`GA
`GB
`GE
`GH
`GN
`GR
`IIU
`IE
`IL
`IS
`IT
`JP
`KE
`KG
`KP
`
`KR
`KZ
`LC
`LI
`LK
`LR
`
`Spain
`Finland
`France
`Gabon
`United Kingdom
`Georgia
`Ghana
`Guinea
`Greece
`Hungary
`Ireland
`Israel
`Iceland
`Italy
`Japan
`Kenya
`Kyrgyzstan
`Democratic People’s
`Republic of Korea
`Republic of Korea
`Kazakstan
`Saint Lucia
`Liechtenstein
`Sri Lanka
`Liberia
`
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`WO 00/20895
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`PCT/IB99/01590
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`CONTROL SYSTEM FOR POSITIONING OF MARINE SEISMIC STREAMERS
`
`1
`
`BACKGROUND OF THE INVENTION
`
`This invention relates generally to systems for controlling seismic data
`
`acquisition equipment and particularly to a system for controlling a marine
`
`seismic streamer positioning device.
`
`A marine seismic streamer is an elongate cable-like structure, typically
`
`up to several thousand meters long, which contains arrays of seismic
`
`sensors, known as hydrophones, and associated electronic equipment along
`
`its length, and which is used in marine seismic surveying.
`
`In order to perform
`
`a 3D marine seismic survey, a plurality of such streamers are towed at about
`
`5 knots behind a seismic survey vessel, which also tows one or more seismic
`
`sources, typically air guns. Acoustic signals produced by the seismic sources
`
`are directed down through the water into the earth beneath, where they are
`
`reflected from the various strata. The reflected signals are received by the
`
`hydrophones, and then digitized and processed to build up a representation
`
`of the subsurface geology.
`
`The horizontal positions of the streamers are typically controlled by a
`
`deflector, located at the front end or “head" of the streamer, and a tail buoy,
`
`located at the back end or “tail" of the streamer. These devices create
`
`tension forces on the streamer which constrain the movement of the streamer
`
`and cause it to assume a roughly linear shape. Cross currents and transient
`
`forces cause the streamer to bow and undulate, thereby introducing
`
`deviations into this desired linear shape.
`
`The streamers are typically towed at a constant depth of approximately
`
`ten meters, in order to facilitate the removal of undesired “ghost” reflections
`
`from the surface of the water. To keep the streamers at this constant depth,
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`control devices known as “birds”, are typically attached at various points
`
`along each streamer between the deflector and the tail buoy, with the spacing
`
`between the birds generally varying between 200 and 400 meters. The birds
`
`have hydrodynamic deflecting surfaces, referred to as wings, that allow the
`
`position of the streamer to be controlled as it is towed through the water.
`
`When a bird is used for depth control purposes only, it is possible for the bird
`
`to regularly sense its depth using an integrated pressure sensor and for a
`
`local controller within the bird to adjust the wing angles to maintain the
`
`streamer near the desired depth using only a desired depth value received
`
`from a central control system.
`
`While the majority of birds used thus far have only controlled the depth
`
`of the streamers, additional benefits can be obtained by using properly
`
`controlled horizontally steerable birds, particularly by using the types of
`
`horizontally and vertically steerable birds disclosed in our published PCT
`
`International Application No. WO 98/28636. The benefits that can be
`
`obtained by using properly controlled horizontally steerable birds can include
`
`reducing horizontal out-of-position conditions that necessitate reacquiring
`
`seismic data in a particular area (i.e. in-fill shooting), reducing the chance of
`
`tangling adjacent streamers, and reducing the time required to turn the
`
`seismic acquisition vessel when ending one pass and beginning another pass
`
`during a 3D seismic survey.
`
`lt is estimated that horizontal out-of-position conditions reduce the
`
`efficiency of current 3D seismic survey operations by between 5 and 10%,
`
`depending on weather and current conditions. While incidents of tangling
`
`adjacent streamers are relatively rare, when they do occur they invariably
`
`result in prolonged vessel downtime. The loss of efficiency associated with
`
`turning the seismic survey vessel will depend in large part on the seismic
`
`survey layout, but typical estimates range from 5 to 10%. Simulations have
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`concluded that properly controlled horizontally steerable birds can be
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`expected to reduce these types of costs by approximately 30%.
`
`One system for controlling a horizontally steerable bird, as disclosed in
`
`UK Patent GB 2093610 B, is to utilize a manually—operated central control
`
`system to transmit the magnitudes and directions of any required wing angle
`changes to the birds. While this method greatly simplifies the circuitry needed
`
`within the bird itself, it is virtually impossible for this type of system to closely
`
`regulate the horizontal positions of the birds because it requires manual input
`
`and supervision. This becomes a particularly significant issue when a
`
`substantial number of streamers are deployed simultaneously and the
`
`number of birds that must be controlled goes up accordingly.
`
`Another system for controlling a horizontally steerable bird is disclosed
`
`in our published PCT International Application No. WO 98/28636. Using this
`
`type of control system, the desired horizontal positions and the actual
`
`horizontal positions are received from a remote control system and are then
`
`used by a local control system within the birds to adjust the wing angles. The
`
`actual horizontal positions of the birds may be determined every 5 to 10
`
`seconds and there may be a 5 second delay between the taking of
`
`measurements and the determination of actual streamer positions. While this
`
`type‘ of system allows for more automatic adjustment of the bird wing angles,
`
`the delay period and the relatively long cycle time between position
`
`measurements prevents this type of control system from rapidly and efficiently
`
`controlling the horizontal position of the bird. A more deterministic system for
`
`controlling this type of streamer positioning device is therefore desired.
`
`It is therefore an object of the present invention to provide for an
`
`improved method and apparatus for controlling a streamer positioning device.
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`An advantage of the present invention is that the position of the
`
`streamer may be better controlled, thereby reducing the need for in—fiIl
`
`shooting, reducing the chance of streamer tangling, and reducing the time
`
`needed to turn the seismic survey vessel.
`
`Another advantage of the present invention is that noise in marine
`
`seismic data associated with streamer position over—correction and streamer
`
`positioning errors can be significantly reduced.
`
`SUMMARY OF THE INVENTION
`
`The present invention involves a method of controlling a streamer
`
`positioning device configured to be attached to a marine seismic streamer
`
`and towed by a seismic survey vessel and having a wing and a wing motor for
`
`changing the orientation of the wing. The method includes the steps of:
`
`obtaining an estimated velocity of the streamer positioning device, calculating
`
`a desired change in the orientation of the wing using the estimated velocity of
`
`the streamer positioning device, and actuating the wing motor to produce the
`
`desired change in the orientation of the wing. The present invention also
`
`involves an apparatus for controlling a streamer positioning device. The
`
`apparatus includes means for obtaining an estimated velocity of the streamer
`
`positioning device, means for calculating a desired change in the orientation
`
`of the wing using the estimated velocity of the streamer positioning device,
`
`and means for actuating the wing motor to effectuate the desired change in
`
`the orientation of the wing. The invention and its benefits will be better
`
`understood with reference to the detailed description below and the
`
`accompanying figures.
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`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Figure 1 is a schematic diagram of a seismic survey vessel and
`
`associated seismic data acquisition equipment;
`
`Figure 2 is a schematic horizontal cross—sectional view through a
`
`marine seismic streamer and an attached streamer positioning device;
`
`Figure 3 is a schematic vertical cross—sectional view through the
`
`streamer positioning device from Figure 2; and
`
`Figure 4 is a schematic diagram of the local control system architecture
`
`of the streamer positioning device from Figure 2.
`
`DETAILED DESCRIPTION OF THE INVENTION
`
`In Figure 1, a seismic survey vessel 10 is shown towing eight marine
`
`seismic streamers 12 that may, for instance, each be 3000 meters in length.
`
`The outermost streamers 12 in the array could be 700 meters apart, resulting
`
`in a horizontal separation between the streamers of 100 meters in the regular
`
`horizontal spacing configuration shown. A seismic source 14, typically an
`
`airgun or an array of airguns, is also shown being towed by the seismic
`
`survey vessel 10. At the front of each streamer 12 is shown a deflector 16
`and at the rear of every streamer is shown a tail buoy 20. The deflector 16 is
`
`used to horizontally position the end of the streamer nearest the seismic
`
`survey vessel 10 and the tail buoy 20 creates drag at the end of the streamer
`
`farthest from the seismic survey vessel 10. The tension created on the
`
`seismic streamer by the deflector 16 and the tail buoy 20 results in the
`
`roughly linear shape of the seismic streamer 12 shown in Figure 1.
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`Located between the deflector 16 and the tail buoy 20 are a plurality of
`
`streamer positioning devices known as birds 18. Preferably the birds 18 are
`
`both vertically and horizontally steerable. These birds 18 may, for instance,
`
`be located at regular intervals along the streamer, such as every 200 to 400
`
`meters. The vertically and horizontally steerable birds 18 can be used to
`
`constrain the shape of the seismic streamer 12 between the deflector 16 and
`
`the tail buoy 20 in both the vertical (depth) and horizontal directions.
`
`In the preferred embodiment of the present invention, the control
`
`system for the birds 18 is distributed between a global control system 22
`
`located on or near the seismic survey vessel 10 and a local control system
`
`located within or near the birds 18. The global control system 22 is typically
`
`connected to the seismic survey vessel’s navigation system and obtains
`
`estimates of system wide parameters, such as the vessel's towing direction
`
`and velocity and current direction and velocity, from the vessel’s navigation
`
`system.
`
`The most important requirement for the control system is to prevent the
`
`streamers 12 from tangling. This requirement becomes more and more
`
`important as the complexity and the total value of the towed equipment
`
`increases. The trend in the industry is to put more streamers 12 on each
`
`seismic survey vessel 10 and to decrease the horizontal separation between
`
`them. To get better control of the streamers 12, horizontal steering becomes
`
`necessary.
`
`If the birds 18 are not properly controlled, horizontal steering can
`
`increase, rather than decrease, the likelihood of tangling adjacent streamers.
`
`Localized current fluctuations can dramatically influence the magnitude of the
`
`side control required to properly position the streamers. To compensate for
`
`these localized current fluctuations, the inventive control system utilizes a
`
`distributed processing control architecture and behavior-predictive model-
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`based control logic to properly control the streamer positioning devices.
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`In the preferred embodiment of the present invention, the global control
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`system 22 monitors the actual positions of each of the birds 18 and is
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`programmed with the desired positions of or the desired minimum separations
`
`between the seismic streamers 12. The horizontal positions of the birds 18
`
`can be derived, for instance, using the types of acoustic positioning systems
`
`described in our U.S. Patent No. 4,992,990 or in our PCT International Patent
`
`Application No. WO 98/21163. Alternatively, or additionally, sate|lite—based
`
`global positioning system equipment can be used to determine the positions
`
`of the equipment. The vertical positions of the birds 18 are typically
`
`monitored using pressure sensors attached to the birds, as discussed below.
`
`The global control system 22 preferably maintains a dynamic model of
`
`each of the seismic streamers 12 and utilizes the desired and actual positions
`
`of the birds 18 to regularly calculate updated desired vertical and horizontal
`
`forces the birds should impart on the seismic streamers 12 to move them from
`
`their actual positions to their desired positions. Because the movement of the
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`seismic streamer 12 causes acoustic noise (both from seawater flow past the
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`bird wing structures as well as cross current flow across the streamer skin
`
`itself), it is important that the streamer movements be restrained and kept to
`
`the minimum correction required to properly position the streamers. Any
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`streamer positioning device control system that consistently overestimates
`
`the type of correction required and causes the bird to overshoot its intended
`
`position introduces undesirable noise into the seismic data being acquired by
`
`the streamer.
`
`In current systems, this type of over—correction noise is often
`
`balanced against the “noise" or “smearing" caused when the seismic sensors
`
`in the streamers 12 are displaced from their desired positions.
`
`The global control system 22 preferably calculates the desired vertical
`
`and horizontal forces based on the behavior of each streamer and also takes
`
`into account the behavior of the complete streamer array. Due to the
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`relatively low sample rate and time delay associated with the horizontal
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`position determination system, the global control system 22 runs position
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`predictor software to estimate the actual locations of each of the birds 18.
`
`The global control system 22 also checks the data received from the vessel’s
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`navigation system and the data will be filled in if it is missing. The interface
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`between the global control system 22 and the local control system will
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`typically operate with a sampling frequency of at least 0.1 Hz. The global
`
`control system 22 will typically acquire the following parameters from the
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`vessel's navigation system: vessel speed (mls), vessel heading (degrees),
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`current speed (mls). current heading (degrees), and the location of each of
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`the birds in the horizontal plane in a vessel fixed coordinate system. Current
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`speed and heading can also be estimated based on the average forces acting
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`on the streamers 12 by the birds 18. The global control system 22 will
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`preferably send the following values to the local bird controller: demanded
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`vertical force, demanded horizontal force, towing velocity, and crosscurrent
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`velocity.
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`The towing velocity and crosscurrent velocity are preferably “water-
`
`referenced” values that are calculated from the vessel speed and heading
`
`values and the current speed and heading values, as well as any relative
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`movement between the seismic survey vessel 10 and the bird 18 (such as
`
`while the vessel is turning), to produce relative velocities of the bird 18 with
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`respect to the water in both the “in-line” and the “cross-|ine” directions.
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`Alternatively, the global control system 22 could provide the local control
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`system with the horizontal velocity and water in—flow angle. The force and
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`velocity values are delivered by the global control system 22 as separate
`
`values for each bird 18 on each streamer 12 continuously during operation of
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`the control system.
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`The “water—referenced" towing velocity and crosscurrent velocity could
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`alternatively be determined using flowmeters or other types of water velocity
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`sensors attached directly to the birds 18. Although these types of sensors are
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`typically quite expensive, one advantage of this type of velocity determination
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`system is that the sensed in—|ine and cross—line velocities will be inherently
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`compensated for the speed and heading of marine currents acting on said
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`streamer positioning device and for relative movements between the vessel
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`10 and the bird 18.
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`Figure 2 shows a type of bird 18 that is capable of controlling the
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`position of seismic streamers 12 in both the vertical and horizontal directions.
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`A bird 18 of this type is also disclosed in our PCT International Application
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`No. WO 98/28636. While a number of alternative designs for the vertically
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`and horizontally steerable birds 18 are possible, including those utilizing one
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`full-moving wing with ailerons, three full-moving wings, and four full-moving
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`wings, the independent two—wing principal is, conceptually, the simplest and
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`most robust design.
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`In Figure 2, a portion of the seismic streamer 12 is shown with an
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`attached bird 18. A communication line 24, which may consist of a bundle of
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`fiber optic data transmission cables and power transmission wires, passes
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`along the length of the seismic streamer 12 and is connected to the seismic
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`sensors, hydrophones 26, that are distributed along the length of the
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`streamer, and to the bird 18. The bird 18 preferably has a pair of
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`independently moveable wings 28 that are connected to rotatable shafts 32
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`that are rotated by wing motors 34 and that allow the orientation of the wings
`28 with respect to the bird body 3t) to be changed. When the shafts 32 of the
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`bird 18 are not horizontal, this rotation causes the horizontal orientation of the
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`wings 28 to change and thereby changes the horizontal forces that are
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`applied to the streamer 12 by the bird.
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`The motors 34 can consist of any type of device that is capable of
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`changing the orientation of the wings 28, and they are preferably either
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`electric motors or hydraulic actuators. The local control system 36 controls
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`the movement of the wings 28 by calculating a desired change in the angle of
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`the wings and then selectively driving the motors 34 to effectuate this change.
`
`While the preferred embodiment depicted utilizes a separate motor 34 for
`
`each wing 28, it would be also be possible to independently move the wings
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`28 using a single motor 34 and a selectively actuatable transmission
`
`mechanism.
`
`When the bird 18 uses two wings 28 to produce the horizontal and
`
`vertical forces on the streamer 12, the required outputs of the local control
`
`system 36 are relatively simple, the directions and magnitudes of the wing
`
`movements required for each of the wings 28, or equivalently the magnitude
`
`and direction the motors 34 need to be driven to produce this wing
`
`movement. While the required outputs of the local control system 36 for such
`
`a two full moving wing design is quite simple, the structure and operation of
`
`the overall system required to coordinate control of the device is relatively
`
`complicated.
`
`Figure 3 shows a schematic vertical cross-sectional view through the
`
`streamer positioning device shown in Figure 2 that will allow the operation of
`
`the inventive control system to be described in more detail. The components
`
`of the bird 18 shown in Figure 3 include the wings 28 and the body 30. Also
`
`shown in Figure 3 are a horizontal coordinate axis 38 and a vertical
`
`coordinate axis 40. During operation of the streamer positioning control
`
`system, the global control system 22 preferably transmits, at regular intervals
`
`(such as every five seconds) a desired horizontal force 42 and a desired
`
`vertical force 44 to the local control system 36.
`
`The desired horizontal force 42 and the desired vertical force 44 are
`
`combined within the local control system 36 to calculate the magnitude and
`
`direction of the desired total force 46 that the global control system 22 has
`
`instructed the local control system to apply to the streamer 12. The global
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`WO 00/20895
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`PCT/IB99/01590
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`11
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`control system 22 could alternatively provide the magnitude and direction of
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`the desired total force 46 to the local control system 36 instead of the desired
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`horizontal force 42 and the desired vertical force 44.
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`While the desired horizontal force 42 and the desired vertical force 44
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`are preferably calculated by the global control system 22, it is also possible
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`for the local control system 36 in the inventive control system to calculate one
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`or both of these forces using a localized displacement/force conversion
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`program. This type of localized conversion program may, for instance, use a
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`look-up table or conversion routine that associates certain magnitudes and
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`directions of vertical or horizontal displacements with certain magnitudes and
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`directions of changes in the vertical or horizontal forces required. Using this
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`type of embodiment, the global control system 22 can transmit location
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`information to the local control system 36 instead of force information.
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`Instead of the desired vertical force 44, the global control system 22 can
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`transmit a desired vertical depth and the local control system 36 can calculate
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`the magnitude and direction of the deviation between the desired depth and
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`the actual depth. Similarly, instead of transmitting a desired horizontal force
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`42, the global control system 22 can transmit the magnitude and direction of
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`the displacement between the actual horizontal position and the desired
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`horizontal position of the bird 18. One advantage to this alternative type of
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`system is that the required vertical force can be rapidly updated as the local
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`control system receives updated depth information from the integrated
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`pressure sensor. Other advantages of this type of alternative system include
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`reducing communication traffic on the communication line 24 and simplifying
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`the programming needed to convert the measured vertical and/or horizontal
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`displacements into corresponding forces to be applied by the birds 18.
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`When the local control system 36 has a new desired horizontal force
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`42 and desired vertical force 44 to be applied, the wings 28 will typically not
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`be in the proper orientation to provide the direction of the desired total force
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`EX. PGS 1046
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`Ex. PGS 1046
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`

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`WO 00/20895
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`PCT/IB99/01590
`
`12
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`46 required. As can be seen in Figure 3, the wings 28 introduce a force into
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`the streamer 12 along an axis perpendicular to the rotational axis of the wings
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`28 and perpendicular to the streamer. This force axis 48 is typically not
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`properly aligned with the desired total force 46 when new desired horizontal
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`and vertical force values are received from the global control system 22 or
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`determined by the local control system 36 and some rotation of the bird 18 is
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`required before the bird can produce this desired total force 46. As can be
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`seen, the force axis 48 is directly related to the bird roll angle, designated in
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`Figure 3 as go.
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`The local control system 36 optimizes the control process by projecting
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`the desired total force 46 onto the force axis 48 (i.e. multiplying the magnitude
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`of the desired total force by the cosine of the deviation angle 50) to produce
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`an intermediate desired force 52 and then adjusting the wing common angle
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`a (the angle of the wings with respect to the bird body 30, or the average
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`angle if there is a non-zero splay angle) to produce this magnitude of force
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`along the force axis. The calculated desired common wing angle is compared
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`to the current common wing angle to calculate a desired change in the
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`common wing angle and the wing motors 34 are actuated to produce this
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`desired change in the orientation of the wings.
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`A splay angle is then introduced into the wings 28 to produce a
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`rotational movement in the bird body 30 (i.e. to rotate the force axis 48 to be
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`aligned with the desired total force 46). The splay angle is the difference
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`between the angles of the wings 28 with respect to the bird body 30. As the
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`bird body 30 rotates and the force axis 48 becomes more closely aligned with
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`the desired total force 46, the bird roll angle and the bird roll angular velocity
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`are monitored, the splay angle is incrementally reduced, and the common
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`angle is incrementally increased until the intermediate desired force 52 is in
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`the same direction and of the same magnitude as the desired total force. The
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`local control system 36 carefully regulates the splay angle to ensure that the
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`EX. PGS 1046
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`Ex. PGS 1046
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`

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`WO 00/20895
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`PCT/[B99/01590
`
`13
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`streamer is stable in roll degree of freedom. The calculated common wing
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`angle and the splay angle are also regulated by the local control system 36 to
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`prevent the wings 28 from stalling and to ensure that the splay angle is
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`pnonflzed.
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`When using the type of birds described in our published PCT
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`international Application No. WO 98/28636, where the bird 18 is rigidly
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`attached, and cannot rotate with respect, to the streamer 12, it is important for
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`the control system to take the streamer twist into account.
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`If this is not taken
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`into account, the bird 18 can use all of its available splay angle to counter the
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`twist in the streamer 12. The bird 18 will then be unable to reach the
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`demanded roll angle and the generated force will decrease. The inventive
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`control system incorporates two functions for addressing this situation; the
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`anti-twist function and the untwist function.
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`in the anti—twist function, the streamer twist is estimated by
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`weightfunction filtering the splay angle measurements instead of simply
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`averaging the splay angle measurements to improve the bandwidth of the
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`estimation. The anti—twist function engages when the estimated twist has
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`reached a critical value and it then overrides the normal shortest path control
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`of the calculated roll angle. The anti-twist function forces the bird 18 to rotate
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`in the opposite direction of the twist by adding +/- 180 degrees to the
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`demanded roll angle. Once the twist has been reduced to an acceptable
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`value, the anti—twist function disengages and the normal shortest path
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`calculation is continued.
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`The untwist function is implemented by the global control system 22
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`which monitors the splay angle for all of the birds 18 in each streamer 12. At
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`regular intervals or when the splay angle has reached a critical value, the
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`global control system 22 instructs each local control system 36 to rotate each
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`bird 18 in the opposite direction of the twist. The number of revolutions done
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`EX. PGS 1046
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`Ex. PGS 1046
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`WO 00/20895
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`PCT/[B99/01590
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`14
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`by each bird 18 is monitored and the untwist function is disengaged once the
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`twist has reached an acceptable level.
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`Figure 4 is a schematic diagram of the architecture of the local control
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`system 36 for the bird 18. The local control system 36 consists of a central
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`processor unit 54, having EEPROM 56 and RAM 58 memory, an input/output
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`subsystem 60 that is connected to a pair of motor drivers 62, and an analog
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`to digital conversion unit 66. The motor drivers 62 are connected to and
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`actuate the wing motors 34 to produce the desired change the orientation of
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`the wings 28 with respect to the bird body 30.
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`The wing motor 34/wing 28 units are also connected to wing position
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`indicators 64 that sense the relative positions of the wings and provide
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`measurements to the analog to digital conversion unit 66 which converts the
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`analog wing position indicator 64 measurements into digital format and
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`conveys these digital values to the central processor unit 54. Various types of
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`wing position indicators 64 can be used, including resistive angle or
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`displacement sensors, inductive sensors, capacitive sensors, hall sensors, or
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`magneto-restrictive sensors.
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`A horizontal accelerometer 68 and a vertical accelerometer 70, placed
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`at right angles with respect to one another, are also connected to the analog
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`to digital conversion unit 66 and these accelerometers convey measurements
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`that allow the central processor unit 54 to determine the roll angle and roll
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`rate of the bird 18. An angular velocity vibrating rate gyro (rategyro) can also
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`be used to measure the roll rate of the bird 18. A temperature sensor 72 is
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`connected to the analog to digital conversion unit 66 to provide temperature
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`measurements that allow the horizontal accelerometer 68 and the vertical
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`accelerometer 70 to be calibr