Underwater Technology, 2000. UT 00. Proceedings of the 2000 International Symposium on Underwater Technology
Autonomous Underwater Vehicle AQUA EXPLORER 2 for Inspection of Underwater Cables Kenichi Asakawa* , Junichi Kojima** ,Yoichi Kato**, Shigetaka Matsumoto** and Naomi Kato*** *, **
KDD R&D Laboratories, 2-1-15, Ohara, Kamifukuoka-shi, Saitama 356, Japan E-mail:
[email protected] *** Tokai University, 3-20-1 Orido, Shimizu, Shizuoka 424, Japan
Abstract Abstract- AQUA EXPLORER 2 (AE2) is a small-sized Autonomous Underwater Vehicle (AUV) which can autonomously track and measure burial depth of underwater cables. Since AE2 first launched in 1997, it has carried out three sea trials and five cable-inspection missions. The total length of inspected cables exceeds 400 kilometers. In this paper, basic design concept, cable tracking performance, acoustic links, and results of sea-trials will be presented. INTRODUCTION In these ten years, the capacity of an underwater fiberoptic telecommunication cable has increased by about 1,000 times. The cost of global communications drastically lowered, which hasten the progress of Internet. Consequently, construction of underwater cable-networks is accelerated still more. If underwater cables are broken by chance, not only large expenditure to repair cables is needed, but also a serious impact on global communications will be imposed. The most of cable-breaks are due to human activities such as fishery and anchoring. In order to prevent such breaks, cables are laid underground in the shallow waters. In such cases, survey of seafloor before cable-laying and inspection of buried depth are important. Usually Remotely Operated Vehicles (ROVs) are used to inspect buried cables. But, when ROV’s tracking underwater cables, mother vessels are required to precisely follow vehicles. Moreover, large vessels are needed to carry big onboard devices. Therefore the operation costs of ROVs are not so low, which limits the extent of their applications. On the other hand, Autonomous Underwater Vehicles *He is now with KDD Submarine Cable Systems Inc./ Visiting Department of Institute of Industrial Science, University of Tokyo
are recently expected in the field of marine inspection. As AUVs have no tether cable between mother vessels and vehicles, they need no large onboard cable-handling system. So, small-sized AUVs are able to be operated with small-sized vessels, and their operation costs are lower. The authors have developed a prototype AUV AQUA EXPLORER 1000(1)-(3) (AE1000) in 1992, the aim of which was to evaluate the possibility of AUVs in the field of underwater telecommunication cables. It was 2.3 meter in length and 500 kg in air. It could track underwater cables up to 500 meters in depth and could measure their buried depth. Since first launched in 1992, five sea trials have been successfully carried out. In the last sea trial, a real under water cable was successfully traced and inspected. Based on the success of AE1000, the authors developed a new practical AUV AQUA EXPLORER 2 (4)-(6)(AE2) in 1997. The hydrodynamic performance was improved and the continuous operation time was enlarged from 4 hours to 24 hours while the weight in air became lighter. Until now four sea trials and five missions have been carried out. The total length of inspected cables exceeds 400 kilometers. Last year, a rechargeable lithium battery was newly developed to replace a non-rechargeable lithium battery, and the maximum velocity was increased from 1.0 meters/sec to 1.5 meters/sec. In this paper, basic design concept, cable tracking performance, acoustic links and results of sea-trials will be discussed. OUTLINE Figure 1 and Figure 2(4) are a photograph of AE2 and an arrangement of equipments respectively. As the vehicle needs to trace the cable laid on a rolling seafloor, high vertical maneuverability is required. To fulfill this requirement, and to
lower the drag coefficient, an airplane-like shape was adopted. To lower operating costs, the vehicle should be handled with a small-sized vessel without special onboard equipment. So, the size and the weight is one of the most important items. The outer cover and the most of the principal strucFigure 1 Photograph of AE2 tures are made of polyethylene. As the specific gravity of polyethylene is smaller than water, extra syntactic form is not necessary. The vehicle has no payload space. But, some small additional sensors can be mounted in place of the buoy releaser and the ballast releaser. The maximum operating depth was designed to be 500 meters. This is because most of human activities such as fishElevator Thruster ing and anchoring are concentrated in the shallower waters Radio than 500 meters, and inspection of cables are more important beacon Radio Magnetometer in these area. modem The vehicle is propelled with two thrusters that are arAcoustic ranged with a little angle to the center axis. The maximum transducer Acoustic Acoustic velocity was 1.0 meters/s at the first stage, but it was imtransponder Buoy transducer releaser Ballast Electronics proved to 1.5 meter/s. After this improvement, it was proved releaser that AE2 could track underwater cables in a strong current such as in Taiwan Straits. Pitching of the vehicle is controlled with the elevators in the forewings. As the size of the elevator is small, the required power to drive the elevator is also small. Sonar Elevator Television Battery Strobe Doppler Computer simulations(7) were carried out to get the opdriver camera sonar timum shape that enable the vehicle to follow the steep of 30 degrees. Figure 3 is an example of computer simulation of Figure 2 Arrangement of AE2(4) longitudinal movement. Figure 4 is another result of computer simulation in which increments of altitude after ten meters in series, and the total rated power was 3.9kVAH. With this advance since the beginning of ascending are estimated. From lithium battery, AE2 could swim more than 24 hours at 0.5 Figure 3 and Figure 4, it is found that when advancing at 1.0 meters/s. If the current is small, this velocity is adequate for meters/s, an obstacle of 5.5 meters in height can be avoided tracking and inspections. with the elevator angle of 15 degrees. Figure 5 is an example At the second stage, a rechargeable lithium battery was of experiment in which the objective of pitching control was developed. Twenty-eight cells make one block, and nine blocks changed from zero to +/- 20 degrees. It is clear that a high are assembled into one battery. To increase the safety, moniperformance of longitudinal movement is obtained while the toring circuit is installed in the battery to electrically remove vehicle is quite stable. wrong blocks. This monitoring circuit also prevents over-disAs a power source, a primary lithium battery was used charging. Although the rated power is reduced to 1.39kVA, at the first stage. The battery consistwdof 38 cells connected the operation cost become lower.
6.0
AE2 is equipped with a video camera and a strobe light that takes pictures of the seafloor. The video-image is captured synchronously with the strobe light, and is recorded on a built-in hard disc (Figure 6). When a picture per second is recorded at velocity of 0.5 meters/s, continuous image of the seafloor can be recorded. A video cassette recorder also can be used, but continuous recording time is restricted. The captured video-signal is compressed and simultaneously sent to the mother vessel through a newly developed acoustic videosignal link. With this link, the operator can know the quality of recorded pictures in real-time. AE2 has another low-bit-rate acoustic link for control and monitor. These acoustic links will be explained later. The other sensors and devices include an acoustic Doppler sonar, an obstacle-avoidance sonar, a depthometer, and an acoustic transponder for acoustic locator.
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Figure 4 Result of computer simulation Increments of altitude after ten meters advance since beginning of ascending.
When tracking cables, low frequency (16~25Hz) currents are fed on conductors in optical submarine cables. Coaxial alternating magnetic filed produced by the currents is utilized to locate cables. The relative location and the direction of cables can be measured with two tri-axial magnetometers, which are placed in the forewings. To increase the accuracy of positioning of cables, the distance between the two magnetometers should be large. So the forewings are suitable place
604A11: +++ Pitch step, Fwd=1m/s +++ 0 pitch (deg)
CABLE TRACKING
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Figure 3 Example of computer simulation of longitudinal movement Velocity = 1.0meter/s, angle of elevator = 15 deg.
Figure 5 Example of pithing control The objective of pithing control was changed from zero to +/- 20 degrees.
for the cable-tracking sensor. Moreover, as the sensor is placed in the fore part of the vehicle, the sensor can detect and locate the cable earlier when approaching the cable. The airplanelike shape of AE2 is the most suitable shape for cable-tracking vehicles. Figure 7 is an example of cable tracking. As the cable being laid on the seabed, the altitude measured with the magnetometers and the vehicles altitude measured with the acoustic altimeter coincide with each other.
alternating current decays about 1/10 when propagating 1,000 kilometers from landing stations, AE2 can find and track cables if 45mA current is supplied from landing stations.
There is some offset in lateral distance from the cable. This is caused by water currents. As the cable is not laid straight, there are some fluctuations in the lateral distance from the cable. But this offset was eliminated by recent studies, and the vehicle could move right on cables. As the magnetic field around the cable is quite small, it is very important to reduce the noise generated by the vehicle itself. As the noise level of the magnetometer is very low (0.01nT/(Hz) 1/2), the main source of noise is the magnetic field produced by the electric motors that drive the thrusters and elevators. In order to reduce this noise, magnetic shield is adopted to all electric motors. Besides, the thrusters are placed apart from the magnetometers, the typical noise level of AE2 could be reduced to less than 0.05nT. This noise level means that assuming alternating current in cables being 50mA, and needed Signal to Noise ratio (S/N) being 10dB, AE2 can find cables from sixty meters away. If the required S/N for cable tracking being 20dB, and if the distance from the cable being three meters, the required alternating current for cable tracking is 4.5mA. Although the
multiple reflection, wind, waves, vessels and living things. To cope with these noises and to lower power consumptions, the bit-rate of the link is intentionally lowered to 125bits/s. As the bit-rate is low and the transmission delay is large, only high-level commands can be transmitted from the vessel. A built-in computer does low-level controls such as cable tracking and altitude/depth keeping to which quick response is needed.
-RA TE ACOUSTIC LINK LO W-BIT LOW -BIT-RA -RATE Certainty and reliability is required to acoustic links for control and monitor of vehicles, but especially in the shallow waters, there are so many sources of acoustic noises such as
Table 1 Specifications of low-bit-rate acoustic link
Magnetic Field (nT)
Items Modulation Bit rate Direction Directivity Carrier Frequency Error correction
(4)
Specifications Frequency Shift Keying 125 bits/s Full Duplex Omnidirectional 40kHz/48kHz Reed-Solomon code
2.0 1.5 1.0 0.5
Altitude(m) Distance from Cable(m)
0.0
Figure 6 Example of image of seafloor taken by AE2
1.0 0.5 0.0 -0.5 -1.0 4
Altimeter
3 2 1
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Distance (m) Figure7 Example of cable tracking
The specifications of the low-bit-rate acoustic link are (4) listed in Table 1 . Almost all the signal processing, such as filtering, modulation, demodulation and synchronization are done with a DSP. The data is formed into packets of fixed length. When these packets not being correctly received, they are resent at predetermined intervals. ACOUSTIC VIDEO-SIGNAL LINK The specifications of the acoustic video-signal link are (4) listed in Table 2 . Video images are compressed with JPEG (Joint Photographic Experts Group) algorithm. In case of visual inspection of the sea floor, transmission rate of images is more important than quality and resolution. For example, if a full color image of 160x120 pixels is compressed to 1/30, the amount of data will be reduced from 460.8 kbits to 15.36 kbits. Therefore, one image can be transmitted within one second when the data transmission rate is 16 kbits/s. An error correcting technique is essential because even one error is not allowed for decompression. So, Reed-Solomon coding is adopted for error correction. Transmitting data are broken into 128-bytes blocks and encoded. One block consists of information of 108 bytes and redundant data of 20 bytes. Although, the real transmission rate is reduced from 16kbits/s to 13.500 kbits/s, errors of up to 10 bytes in a block can be corrected. Modulation is done by a DSP. The output of the DSP is QPSK (Quadrate Phase Shift Keying) signals with a center frequency of 8 kHz. Before sending image data, training signals are transmitted. These training signals consist of two parts; 0-p
(4)
sequential signals for signal detection and synchronization, and random pattern signals to adjust the adaptive equalizer. Transmitted signals are demodulated by two DSPs. Timing signals are extracted from the received signals. The extracted timing signal changes the sampling timing of the AD converter. This feedback loop compensates the change of timing caused by Doppler shift. The adaptive filter is a double sampling equalizer with 40 taps. Least mean square method is used to update the tap coefficients. CONCLUSION Design concept, cable tracking performance, acoustic links and results of sea-trails were presented. Until now four sea-trails and five missions were carried out. Total length of inspected cables exceeds 400 kilometers. Last year, AE2 succeeded in inspection of the cable in strong currents of Taiwan Strait. It is a hard task to protect underwater cables laid on the deep ocean floor against natural forces and human activities. Although the topography of the seafloor and character of bottom soils is complicated, it is difficult even to inspect them. AUVs are one of solutions to these problems, and are proved to have the ability by AE2. The authors wish to extend the application of AE2 to other field, such as environmental assessment, survey of biological resources and mineral resources. REFERENCES (1) K.Asakawa, J. Kojima, Y. Ito, Y. Shirasaki and N. Kato, ‘Development of Autonomous Underwater Vehicle for Inspection of Underwater Cables’, Pro. of UNDERWATER INTERVENTION ‘93, pp.208-2162, 1993.
Table 2 Specifications of acoustic video-signal link Items Resolution Signal Compression Bit rate Error correction Modulation Carrier frequency Transmission level Directivity Power consumption Doppler immunity
Specifications 80 x 60 to 640 x 480 pixels JPEG 16, 24, 32kbits/s Reed-Solomon QPSK 90 to 110kHz 183 dB re 1µPa 45 degrees 20 Watt max. 0.3% (4.5meter/s)
(2) J. Kojima, Y. Ito, K. Asakawa, Y. Shirasaki and N. Kato, ‘Cable Tracking of Autonomous Underwater Vehicle AQUA EXPLORER 1000', Proc. of UNDERWATER INVENTION ‘94, pp.349-356, 1994 (3) K. Asakawa, J. Kojima, Y. Ito, S. Takagi, Y. Shirasaki and N. Kato,‘Autonomous Underwater Vehicle AQUA EXPLORER 1000 for Inspection of Underwater Cables’, Proc. of AUV’96, 1996 (4) J. Kojima, Y. Kato, K. Asakawa, S. Matsumoto, S. Takagi,
N. Kato, ‘Development of Autonomous Underwater Vehicle AQUA EXPLORER 2 for Inspection of Underwater Cables’, Proc. of OCEANS’97, 1997 (5) J. Kojima, Y. Kato, K. Asakawa and N. Kato, ‘Experimental Results of Autonomous Underwater Vehicle ‘AQUA EXPLORER 2’ for Inspection of Underwater Cables’, Proc. of OCEANS’98, 1998 (6) J. Kojima, Y. Kato and K. Asakawa, ‘Autonomous Underwater Vehicle for Inspection of Submarine Cables’, Proc. of the Ninth Inter. Offshore and Polar Engineering Conf., 1999 (7) N. Kato, J. Kojima, Y. Kato, S. Matsumoto, K. Asakawa, ‘Optimization of Configuration of Autonomous Underwater Vehicle for Inspection of Underwater Cables’, Pro. of IEEE International Conference on Robotics and Automation, 1998
