design of communication and video system for a multi-legged subsea robot

8/12/2019 Design of Communication and Video System for a Multi-legged Subsea Robot

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Design of Communication and Video System

for a Multi-legged Subsea RobotBanghyun Kim, Sung-Woo Park, Pan-Mook Lee and Bong-Huan Jun

Ocean Engineering Research Department, MOERI-KORDI

1312gil 32, Yuseongdaero, Yuseong-guDaejeon, 305-343, Rep. of Korea

This work was supported by Ministry of Land, Transportation and Maritime Affairs (MLTM) of Korea for the Development of a Multi-legged Walking FlyingSubsea Robot.

Abstract-The MOERI-KORDI has launched a new project todevelop multi-legged subsea robot technologies. The objective of1st stage is to develop a multi-legged robot walking in high tidalcurrent and high turbidity environment while the objective of2nd stage is to develop a walking and flying robot with 6 legs indeep sea where is calm and clear environment. This paperpresents the design of communication and video system for themulti-legged subsea robot and its test bed. The basic designconcept of communication system is simple connectivity based on

Gigabit Ethernet network. Several serial networks for sensorsand motors in the robot are grouped into one Ethernet line usinga small I/O computer. The focus of video system design is on

compressed digital transmission using network cameras andvideo encoder for analog cameras. This approach can minimizeelectronic and magnetic interference in analog data transmission

and decrease the amount of transmission data. We constructedthe test bed to verify the designed system. The results ofperformance evaluation using the test bed show that the networkthroughput is enough to connect all designed devices and theoptimal method of video encoding is the H.264/MPEG-4 AVCformat with 30% compression rate.

I.

INTRODUCTION

Most of underwater robots use screws for their propulsion.

Screws have been used as a means of underwater propulsion

for long time, and the theoretical mechanism is well

established. Moreover, the propulsion efficiency is high incertain region. However, in the western coast of Korea, the

underwater robots with screws have a lot of difficulties to

conduct precision work because of the disturbances from high

tidal current. The direction of tidal current changes four times

a day, the maximum flow rate amounts to 3 or 7 knots in the

western coast of Korea. In addition, the propeller flow makes

problems when vehicle investigates in deep-sea sediment

environment.

To overcome these problems, the Maritime and Ocean

Engineering Research Institute (MOERI), a branch of the

Korea Ocean Research and Development Institute (KORDI),

is developing a multi-legged subsea robot. Newly developing

robot will move on the seabed by walking and flying with 6legs which is different moving mechanism from the existing

underwater thrust system such as screw or caterpillar. The

robot was named 'Crabster' as the concept of the robot is

similar to the crab and lobster. In other words, the Crabster

can walk on sea floor like as lobster and can swim like as

flying-crabs. The project of the Crabster development consists

of two stages each of which has three years. The objective of

first stage is to develop a 200m-class multi-legged seabed

robot the missions of which are survey of shipwreck and

scientific research in coastal area. The focus of development is

on the working technologies in high tidal current and high

turbidity environment. In the second stage, we plan to develop

a 6,000m-class walking and flying robot in deep sea where is

calm and clear environment [1].

This paper presents the design of communication and video

system for the Crabster and its test bed. The Crabster systemconsists of the surface control unit, the depressor, and the

Crabster. The basic design concept of communication system

is simple connectivity based on the Gigabit Ethernet network.

Several serial networks, such as RS-232/422/485 and CAN

(Controller Area Network), for sensors and motors in the

Crabster or the depressor are grouped into one Ethernet line

using a small I/O computer. The Crabster and the depressor

are connected to the surface control unit through the optical

fiber and the fiber optic Gigabit media converter supports

connecting Ethernet networks each other. This approach can

minimize the complexity of network system and make the

communication software easy.

The design concept of video system is on full digitaltransmission using network cameras and video encoder for

analog cameras. They are directly connected to Ethernet

network and unique IP address. The digital video data can be

compressed by the H.264/MPEG-4 or the motion JPEG. This

method can minimize electronic and magnetic interference in

analog data transmission and decrease the amount of

transmission data.

We constructed the test bed to verify our design of

communication and video system for the Crabster. The test

bed includes two test computers with Gigabit Ethernet

interface, two Soltek SFC2000-TWL fiber optic converters,

two 3COM 3CGSU08 Gigabit Ethernet hubs, an AXIS M1114

network camera, an AXIS Q7404 video encoder, and a

Kongsberg oe15-108 analog camera. By the experimental

results of the test bed, we confirmed the real network

throughput of the designed system and found the optimal

compression ratio of video data.

978-1-61284-4577-0088-0/11/$26.00 2011 IEEE


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The remainder of this paper is organized as follows. Section

II introduces the overview of the Crabster, and Section III

describes the design of communication and video system. The

experimental results of the test bed are presented in Section IV,

and the last section provides concluding remarks and the

future of our work.

II.

OVERVIEW OF THE CRABSTER

Though the screw has been used as propulsion means for

most of the underwater vehicles, there are difficulties in fine

attitude control due to the highly nonlinear dynamic

characteristics caused by dead zone, delay and saturation. In

particular, it is hard to guarantee the maneuverability and

stability in high current environment [2]. As a result,

underwater robots propelled by screws have difficulties to get

precise positioning, accurate manipulation and clear acoustic

image particularly in high current environment. The direction

of tidal current changes four times a day, the maximum flow

rate amounts to 3 or 7 knots in the western coast of Korea. In

such harsh environment, we needed a new concept of

underwater robot to substitute conventional screw-propelled

robots which suffer from instability and high energy

consumption.

The Crabster robot will be implemented to two types of

underwater robots. The one is a 200m-class Crabster which

works in shallow and strong tidal current environment by

crawling sea floor. The other is a 6000m-class Crabster which

works on deep-sea sediment without disturbing the

environment by flying on seabed. The Crabster has 6 legs for

underwater walking and working. The forward two legs have

respectively 6 degrees of freedom which can function as both

of manipulators and legs while middle and rear legs have 4degrees of freedom for only leg function. All of the joints are

actively controlled by electric motors [1].

The walking and endurance in tidal current is very

important issue in the study on locomotion of subsea legged

robot [3]. The Crabster endures the tidal current by controlling

the posture of its streamlined body and legs based on the

detected environmental information. The Crabster controls the

joint angles of legs so that the hydrodynamic forces on the

body and legs work to improve the stability margin. In order

to estimate the environmental status, the Crabster senses speed

and direction of sea current, contact force of each foot, and

force/torque of each leg.

Generally most of the high current environment is also high

turbidity environment. The Crabster has real time vision

system consists of optical and acoustic cameras for high

turbidity environment. Figure 1 shows the equipment of

Crabster for stable locomotion control while Figure 2 shows

the equipment for high turbidity mission implementation.

Cable drag may be another strong disturbance to the Crabster

in high current. In order to minimize the effect of cable drag, a

depressor works as a damper between the robot and support

vessel.

The main strategy for developing the Crabster is utilization

of existing technologies. Since joint mechanism and walking

technologies are well established in the field of on-land multi-

legged or humanoid robots [4], they can be shared with the

Crabster. On the other hand, watertight, hydrodynamics andfield operation technologies of underwater vehicles are well

studied in the field of autonomous underwater vehicle (AUV)

and remotely operated vehicle (ROV) [5-8]. The Crabster can

share those technologies with existing underwater robot fields.

Another strategy is selection of core technologies and

concentration of efforts to develop the selected technologies.

The four technologies necessary to develop the Crabster are

the underwater joint mechanism, the modeling and analysis of

hydrodynamic forces on legs, the drag-optimized path

planning, and the posture control against external disturbances.

The specifications of Crabster are derived from the

requirement analysis for the required ability to conduct a

given missions in western coast of Korea. The 200m-classCrabster has 2.2m length, 1.0m width, 1.1m height, and

maximum 300kg weight. The walking speed is 0.5m/s and the

enduring speed of current is 2knots. The Crabster can be

operated in condition of sea state 3 and survived in condition

of sea state 4.

Figure 2. Crabster equipment for high turbidity mission implementation

Figure 1. Crabster sensing equipment for stable locomotion


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III. COMMUNICATION AND VIDEO SYSTEM

Figure 3 shows the designed overall communication and

video system of the Crabster. The structure of the Crabster

system is similar to the ROV system which consists of a

surface control unit, a depressor and a ROV. The main

network in each unit is Gigabit Ethernet (1000BASE-TX) with

1,000Mbps bandwidth. The surface system communicates tothe Crabster and the depressor through two single mode

optical fiber lines (1000BASE-SX). The network bandwidth is

enough for all designed devices to communicate each other

simultaneously. The I/O computer groups several serial lines,

which are connected to sensors and actuators, into one

Ethernet line. This communication design can minimize the

number of communication lines and decrease the complexity

of network system.

The surface system has 10 general computers connected

Gigabit Ethernet. Two spare computers are prepared for

special scientific purpose. The surface system can provide

comfortable operation space because it uses wireless keyboard,

wireless mouse, wireless joystick, and LED monitor. Twofiber optic converters connect the surface system to the

depressor and the Crabster. Solteck SFC2000-TWL was

chosen as fiber optic converter, and it has one Gigabit

Ethernet connector and one is one single mode fiber cable

connector. The convertor supports one fiber wavelength

division multiplexing (WDM) conversion, maximum 10km

transmission distance, and 1000Mbps bandwidth.

Most of the ROV system uses analog video transmission

because commercial underwater cameras are mainly analog

camera. Analog transmission is prone to electronic and

magnetic interference, so video noise can be generated. Digital

video transmission method can prevent this noise and decrease

the size of transmission data, and the video encoder is used for

converting analog video data to digital video data. We

designed the video system using both analog camera with the

video encoder and network camera as digital camera, so full

digital video transmission of all cameras is possible. Network

camera, or internet protocol (IP) camera, is a type of digital

video camera commonly employed for surveillance, and

which unlike analog cameras can send and receive data via a

computer network and the Internet.

Network cameras are directly connected to Ethernet while

analog cameras are connected to Ethernet through video

encoder. The video computer can display and save all videos

in the real time and any computer can access any video

camera. AXIS Q7404 video encoder is chosen as digital video

converter, and it supports 4 video channels and 1 audio

Figure 3. Overall communication and video system of the Crabster


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channel. The encoder can convert analog video of NTSC

(National Television System Committee) and PAL (Phase

Alternating Line) system. The maximum conversion

resolution is 720x576, and the maximum frame rate is 30fps in

all resolutions. The encoder provides H.264/MPEG-4 AVC or

M-JPEG as video compression format.

The H.264/MPEG-4 AVC (advanced video coding) is a

standard for video compression, and is currently one of the

most commonly used formats for the recoding, compression,

and distribution of high definition video. The video format is a

block-oriented motion-compensation-based codec standard

developed by the ITU-T Video Coding Experts Group

(VCEG) together with the ISO/IEC Moving Picture Experts

Group (MPEG). The H.264 is perhaps best known as being

one of the codec standards for Bly-ray Discs. The M-JPEG

(Motion JPEG) is an informal name for a class of video

formats where each video frame or interlaced filed of a digital

video sequence is separately compressed as a JPEG image.

Originally developed for multimedia PC applications, where

more advanced formats have displaced it, M-JPEG is now

used by many portable devices with video-capture capability,

such as digital cameras.

Generally, the H.264 is more efficient than the M-JPEG for

video monitoring and saving in underwater environment since

the change of scene is little and the H.264 is motion-

compensation-based codec. The M-JPEG is useful for still

image, but there is slice difference with the H.264.

IV.

EXPERIMENTS USING TEST BED

We constructed the test bed which is subset of Figure 3 to

verify the designed video and communication system as

shown in Figure 4. The test bed includes two test computers

with Gigabit Ethernet interface, two Soltek SFC2000-TWL

fiber optic converters, two 3COM 3CGSU08 Gigabit Ethernet

hubs, an AXIS M1114 HDTV (high-definition television)network camera, an AXIS Q7404 video encoder, an AXIS

M7001 video ender, and a Kongsberg oe15-108 analog

camera.

The AXIS M7001 is one video channel encoder, and it has

same encoding ability of the AXIS Q7404. The AXIS M1114

provides maximum 1280x800 resolution and maximum 30fps

in all resolutions. The network camera supports H.264 or M-

JPEG video compression format and 100Mbps power-over-

Ethernet (PoE) connector. The PoE technology describes a

system to pass electrical power safely along with data, on

Ethernet cabling. Because of the network camera is not

underwater camera, the case for waterproof is needed. The

Kongsberg oe15-108 is black and white CCD (charge-coupleddevice) camera for general purpose underwater viewing. Its

horizontal resolution is 400 TV lines and the vertical scanning

ability is 625 line / 50Hz.

We benchmarked the network performance using NTttcp

program and Ping program on MS Windows XP

environment. The Microsofts NTttcp is a multithreaded,

asynchronous application that sends and receives data between

Figure 4. Test bed for the Crabster communication and video system


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two or more endpoints and reports the network performance

for the duration of the transfer. It is essentially a Winsock-

based port of the ttcp tool that measures networking

performance in terms of bytes transferred per second and CPU

cycles per byte. Ping is a computer network administration

utility used to test the reachability of a host on an IP network

and to measure the round-trip time for messages sent from the

originating host to a destination computer.

By the test result of NTttcp, the network throughput was

771.545Mbps when 6553.6MB data was transferred using

TCP/IP protocol. The throughput was less than 1Gbps because

of the overhead of TCP/IP protocol such as user-system copy,

TCP checksum, network memory copy, Ethernet driver,

TCP/IP/ARP processing, and operating system overhead. The

network delay among two computers is less than 1ms by the

result of Ping test.

The test of video display and storing using the AXIS

Camera Station software were successful and there wasnt anynoise. The AXIS Camera Station is a complete monitoring and

recording system for up to 50 cameras when the AXIS camera

system is used. The next test was conducted to find optimal

compression method and compression rate. The compression

rate was increased from 0% to 100% by 10% interval, and the

M-JPEG and H.264 compression method were used. The tests

were performed three times on each condition. The cameras

were pointed at rotating fan, and Figure 5 is the still scene of

AXIS M1114.

Figure 6 shows the test result using the AXIS M114 HDTV

camera when the resolution is 1280x800 and the frame rate is

30fps. The image quality on humans eye is almost same when

the compression rate is less than 30%, and becomes bad from

40%. On the side of data size, the H.264 format is better than

the M-JPEG format. Figure 7 shows the test result using

Kongsberg oe15-108 camera when the resolution is 720x480

and the frame rate is 30fps. The image quality is almost same

when the compression rate is less than 50%, and becomes bad

from 40%. The data size of the Kongsberg oe15-108 camera is

small than the AXIS M1114 camera because the Kongsberg

oe15-108 camera is mono camera and its resolution is small

than the AXIS M1114 camera.

By the test result, we chose the H.264 format with 30%

compression rate as video encoding format because and the

data size is smallest while the image quality is kept. When the

compression rate is 30%, the data size of the AXIS M1114

camera is 544.1KB/s. If 10 cameras are used, the maximum

required space is 5.5441MB/s. When the Crabster is operated

for an hour, the saving data size is less than 20GB. This size is

acceptable since the capacity of recent commercial hard disk

is more than 1TB.

The data size is same to required network throughput. In the

case of the AXIS M1114 camera, the required network

throughput is 544.1KB/s. When 10 cameras are used, the

camera system consumes less than 1% in the network

bandwidth. This means that the Crabster system has manycameras as possible and the digital video transmission doesnt

affect the control data transmission such as sensor data and

actuator order.

Figure 5. Scene of AXIS M1114 camera for video test

Figure 6. Video test result of AXIS M1114 camera

Figure 7. Video test result of Kongsberg oe15-108 camera


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V.

CONCLUSION

This paper introduced the design of communication and

video system for a multi-legged subsea robot named

Crabster. We verified the designed system by the test bed,

and found that the optimal method of video encoding is the

H.264 format with 30% compression rate. We also confirmedthat the camera system consumes less than 1% in the network

bandwidth when 10 HDTV cameras are used. Full digital

network system based on Gigabit Ethernet is efficient for

hardware and software implementation since the approach

decrease the complexity of the system. For example, we can

use the simple commercial fiber optical converter with low

cost, and dont need any device for analog camera except

video encoder. Especially, the main advantage is that the noise

by electronic and magnetic interference can be minimized.

The presented method in this paper is applicable to other ROV

system.

We will make entire communication and video system for

the Crabster after more tests, which are the performance test

of data transmission using designed message, concurrent

saving test using 5 analog cameras, network delay test on

heavy traffic, and stress test for long time.

REFERENCES

[1] B.H Jun, H. Shim, J.Y. Park, B. Kim and P.M. Lee, A New Concept

and Technologies of Multi-legged Underwater Robot for High Tidal

Current Environment, International Symposium on Underwater

Technology 2011 & International Workshop on Scientific Use of

Submarine Cables and Related Technologies 2011, pp. 1-6, April 2011.

[2] S. M. Savaresi, F. Previdi, A. Dester, S. Bittanti and A. Ruggeri,Modeling, Identification, and Aalysis of Limit-Cycling Pitch and

Heave Dynamics in an ROV, IEEE Journal of Oceanic Engineering,

Vol. 29, No. 2, pp. 407-417, April 2004.

[3] A. Joseph, Underwater Walking, Arthropod Structure & Development,

Vol. 33, pp. 347-360, June 2004.

[4] I.W. Park, J.Y. Lee and J.H. Oh, Mechanical Design of the HumanoidRobot Platform, HUBO, Advanced Robotics, 2007.

[5] B.H. Jun, J.Y. Park, F.Y. Lee, P.M. Lee, C.M. Lee, K. Kim, Y.K. Limand J.H. Oh, Development of the AUV ISiMI and a free running testin an Ocean Engineering Basin, Ocean Engineering, Vol. 36, No. 1, pp.2-14, 2009.

[6] F.Y. Lee, B.H. Jun, P.M. Lee and K. Kim, Implementation and test ofISiMI100 AUV for a member of AUVs Fleet, OCEANS 2008 Quebec,

pp. 1-6, 2008.[7] J.Y. Park, B.H. Jun, P.M. Lee and J.H. Oh, Experiments on vision

guided docking of an autonomous underwater vehicle using onecamera, Ocean Engineering, Vol 36, No 1 , pp. 48-61, 2009.

[8] P.M, Lee, B.H, Jun and Y.K. Lim, System design and development ofthe 6000m class deep-sea ROV Hemire, 2nd International Conferenceof USYS, pp. 1-6, 2008.

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