Communication Disconnection Prevention System by Bandwidth Depression-Type Traffic Measurement in a Multi-Robot Environment using an LCX Network

Kei Sawai, Satoshi Aoyama, Takumi Tamamoto, Tatsuo Motoyoshi, Hiroyuki Masuta, Ken'ichi Koyanagi, Toru Oshima
2019 International Journal of Advanced Computer Science and Applications  
In this paper, we propose and develop a method for determining the transmission amount of each mobile robot connected to a network constructed with a leaky coaxial cable (LCX) by using broadcast packets. Tele-operation of mobile robots using an LCX network is more effective as an information collection method in closed spaces compared with existing methods in terms of the maintenance of the mobile robots' running performance and the stability of the communication quality for disaster reduction
more » ... ctivity. However, when the transmission and reception of information exceeds the maximum transmission amount, communication disconnection and transmission amount reduction occur because of band division in the communication path, and there is a risk that mobile robots will be separated from the LCX network. Therefore, to prevent the network division and the decrease of transmission amount during multi-robot operation on an LCX network, we propose a method for determining the transmission amount of each mobile robot using broadcast packets. The proposed method is evaluated on an LCX network, and its effectiveness is confirmed by evaluating the transmittability of broadcast packets and operability of mobile robot. Keywords-Multi-robot; tele-operation; leaky coaxial cable; LCX networks; operability; broadcast packets; transmittability of broadcast packets; network disconnection prevention; disaster reduction activity I. INTRODUCTION After a disaster occurs, disaster reduction activities are conducted to minimize damages, injuries, and fatalities. The investigation of the Hanshin-Awaji Earthquake indicated that survivors buried in the rubble had a survival rate of less than 5% after 72 hours. Therefore, the prompt implementation of disaster reduction activities is required to save lives [1-2]. To carry out effective disaster reduction activities, it is necessary to quickly gather information about the affected area for disaster deduction activities [3] [4] . Drones and existing sensor systems are often used to gather disaster information. However, in closed spaces such as underground facilities and factories, it is impossible to gather information using drones, and there is a high risk that the existing infrastructure is disconnected from the outside [5] [6] . In these situations, information must be collected by rescue teams, but there may be risks of secondary disasters such as fire and collapse. Therefore, a rapid information-gathering system using multiple robots is considered. In a multi-robot system, it is necessary to choose communication methods to use between robots and between robots and operators according to the operation environment. Wired communication and wireless communication can be selected for the communication of mobile robots operating in the disaster area according to the environmental conditions. Wired communication is excellent for maintaining the communication quality between the remote operator and the mobile robot, but in a multi-robot environment, there is a high risk that cable tangle and disconnection may cause communication failure. Although wireless communication improves the running performance of a mobile robot, it is difficult to maintain the communication quality between the operator and the mobile robot because of the influence of structures and obstacles. Therefore, the wireless tele-operation range of the mobile robot is smaller than that of wired communication. When tele-operation of a mobile robot is required, the communication method that matches the disaster area must be determined, but it is difficult to estimate the situation in the disaster area in advance. Hence, it is important to carry out various studies on tele-operation methods that can be used in closed spaces for effective disaster mitigation activities. It is important to be able to operate all the robots in a multi-robot system without entanglement or damage of cable to collect information quickly in a closed space affected by a disaster. A. Tele-Operation of a Mobile Robot using an Ad-hoc Network Many studies on the wireless tele-operation of mobile robots in closed spaces have used the Robot Wireless Sensor Network (RWSN) system, which uses an ad-hoc network as a communication method [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] . RWSN is a system in which the mobile robot extends the tele-operation area by deploying a wireless communication repeater on a moving route (Fig. 1) . However, when the number of relays on the communication path increases, the throughput value between the operator and the mobile robot decreases. The decrease of the throughput value not only reduces the operability but also increases the communication disconnection risk. Therefore, the operator must constantly monitor the communication quality with the mobile robot. In addition, in a multi-robot environment constructed using RWSN, the routing becomes complicated, so the number of relays is likely to change, thus causing the throughput to change. The fluctuation of the throughput leads to the disconnection of communication; thus, multi-robot operation using RWSN is difficult. In this study, to overcome the shortcomings of RWSN, we develop a tele-operation system for mobile robots using an LCX network as the communication infrastructure for the purpose of effective information-gathering activities in disaster areas. B. A Multi-Robot System using an LCX Network as Wireless Communication Infrastructure An LCX network has a large number of holes called slits in the outer conductor of the coaxial cable that are designed to leak the electrical signal transmitted in the cable to the outside (Fig. 2) . Information transmitted by the remote operator to the mobile robot is leaked to the outside as radio waves from the slit of the LCX. Slits can also receive radio waves, enabling mutual communication between the mobile robot and the operator. Therefore, the LCX can be used as a single antenna by connecting it to a TCP/IP-compliant access point (Fig. 3) . By laying LCX, a network can be constructed in a wide area along the cable; thus, LCX can be used as wireless communication infrastructure in a multi-robot environment. C. Tele-Operation of Multiple Robots using an LCX Network An information-gathering system for multiple robots on an LCX network in a closed space affected by a disaster is illustrated in Fig. 4 [24] . In the flow of information-gathering activities, a cable-laying robot first lays LCX as the communication infrastructure. Multiple robots that gather information are operated by wireless tele-operation without degrading the running performance by using radio waves leaked from the laid LCX. The communication protocol of this system uses TCP / IP in consideration of mobile robot installation and tele-operation. At present, it is common to use a general-purpose PC for the control system of the mobile robot, and TCP/IP is often used as a communication protocol for tele-operation. Therefore, in the proposed system, we adopted IEEE802.11g as the communication method of the mobile robot, and we performed tele-operation by packet communication. The purpose of this experiment is to confirm that broadcast packets can be received in an environment where communication load is applied to the communication band. The communication load that can be applied to the communication band is set by assuming the load when information is transmitted and received. In this experiment, we constructed an IEEE 802.11g network using LCX in a real environment and evaluated the availability of the proposed method by transmitting and receiving broadcast packets. The equipment used for the experiment adopted Raspberry Pi2 (Model B) for the control system of the tele-operator and the mobile robot, and LCXF-8D-LCX (HC Networks, Ltd.) for LCX. In addition, AP-214 (Aruba Networks, Ltd.) was used as the access point compliant with TCP/IP. Broadcast packets were sent and received by connecting four tele-operator computers and four computers simulating mobile robots to the LCX network. To put a sufficient communication load on the LCX network, we used four pairs of tele-operators and mobile robots. Tele-operation computers (SN1-4) and the mobile robots (MSN1-4) were placed at 0 m in measurement environment, and an arbitrary communication load was applied on the communication path between SN1-MSN1 and SN3-MSN3. Next, computers other than SN2 were set to the broadcast packet reception standby mode. In an environment where the communication load is applied to the network, SN2 transmits a broadcast packet that contains information that assumes the amount of data actually transmitted and received 10 times. After that, we checked whether the computers in the reception mode could receive the information stored in the packet. For packet reception confirmation, the number of packet losses was counted as an evaluation of packets that could not be received. The communication loads on the communication path were 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0 Mbps. When the communication load applied to the network was 5.0 to 15.0 Mbps, SN1 generated the communication load. The communication load up to 20.0-30.0 Mbps was generated by SN1 and SN3. In addition, MSN1-4 was moved from 0 m to 100 m at intervals of 10 m, and evaluation experiments were conducted at each 10 m interval. 96 | P a g e www.ijacsa.thesai.org of packet losses is not constant, it is conceivable that the communication load changes somewhat due to the change of the communication environment. Furthermore, in this experiment, it was confirmed that broadcast packets were delivered to other computers, although packet loss occurred when the communication load was within 30.0 Mbps. Hence, it is considered that the proposed method can prevent the decrease of the transmission amount and communication division due to band division by determining the maximum transmission amount of the communication route to be used. In this experiment, IEEE 802.11g was adopted as the communication method, but even when other communication methods are adopted, this method can be used to determine the maximum transmission amount of the communication path. VI. CONCLUSION In this paper, as a method to prevent disconnection in remote operator-mobile robot communication due to bandwidth compression assuming a multi-robot environment constructed using LCX, a bandwidth non-compression type communication status determining system using broadcast packets was proposed. In LCX cables, the transmission loss of electrical signals increases from the beginning to the end of the cable. The loss of the electrical signal affects the radio waves leaking from the LCX, and the throughput value also decreases. Moreover, when multiple robots connected to LCX are at the same distance from the access point along LCX, the maximum amount of transmission that can be communicated between each remote operator and a mobile robot is divided into bands. The decrease of the throughput value and the bandwidth division reduce the amount of transmission available per mobile robot. As a result, the amount of transmission necessary for tele-operation cannot be secured, and the risk of communication disconnection is increased. In this paper, we proposed a system of preventing communication disconnection and band division by using broadcast packets to obtain information on the transmission amount that each mobile robot connected to LCX transmits and receives for tele-operation. Remote operators can pay attention to communication disconnection and band division because other remote operators can share the amount of transmission that other mobile robots are transmitting and receiving.
doi:10.14569/ijacsa.2019.0100714 fatcat:eduwuel3knhbxiwfgaggbn4uxu