information

Article Design of Hybrid Wired/ Network for Turbine Power Generation System

Sheng Xu 1,2,*, Minrui Fei 1 and Haikuan Wang 1

1 Shanghai Key Laboratory of Power Station Automation Technology, School of Mechatronic Engineering and Automation, Shanghai University, Shanghai 200072, China; [email protected] (M.F.); [email protected] (H.W.) 2 School of Electronic and Information Engineering, Nantong Vocational University, Nantong 226007, China * Correspondence: [email protected]; Tel.: +86-513-81050720

Academic Editor: Willy Susilo Received: 11 April 2016; Accepted: 16 June 2016; Published: 24 June 2016

Abstract: Hybrid fieldbus network integrating wireless networks with existing wired fieldbuses has become new a research direction in industrial automation systems. In comparison to wired fieldbuses, the hybrid wired/wireless fieldbus network has a different system architecture, data transmission mechanism, , etc. This leads to different challenges that need to be addressed. This paper proposes a hybrid wired/wireless fieldbus network which consists of a wireless industrial control network (WICN), a wired -DP (Process Field Bus-Decentralized Periphery) fieldbus network, and a wired /TCP (Mod Bus/Transmission Control Protocol) fieldbus network. They are connected by a new gateway which uses a shared data model to solve data exchange in different network protocols. In this paper, we describe the architecture of the proposed hybrid wired/wireless fieldbus network and data transmission mechanisms in detail, and then evaluate the performance of hybrid fieldbus network via a set of experiments. The experiment results confirm that the proposed hybrid wired/wireless fieldbus network can satisfy the performance requirement of industrial network control systems. Furthermore, in order to further investigate feasibility of the proposed hybrid wired/wireless fieldbus network, it is deployed at a steam turbine power generation system, and the performance figures obtained further verify its feasibility and effectiveness.

Keywords: industrial control; gateway; hybrid wired/wireless fieldbus network

1. Introduction Recently, Industry 4.0 has become a hot topic that is attracting growing interest from governments, manufacturers, researchers, and developers. It is expected to offer a reduction in energy consumption, increase economic benefits, and enable smart production [1]. Therein, wireless communication technology serves as an important approach to realize Industry 4.0. During the last few years, wireless technologies have experienced a great growth in factory automation [2–5]. Applying wireless networks will be a new trend in industrial environments. For example, efforts have been made to provide wireless extension for the Fieldbus and [6–10]. The benefits of wireless networks are fairly obvious: They can provide new advantages such as fully mobile operation, flexible installation, and rapid deployment, while reducing maintenance costs [11,12]. Furthermore, wireless networks can be deployed in harsh environments that are not suitable for cabling setup, such as applications involving mobile subsystems or areas with explosive chemicals. Although wireless networks play increasingly important roles in industrial fields, it is very unlikely that wireless networks will totally replace the wired fieldbus networks in industrial automation for a long time because of well-known problems such as fading, interference, multipath propagation, timeliness and so on [13]. That means that hybrid wireless/wired networks integrating with the wired fieldbus networks is worth focusing

Information 2016, 7, 37; doi:10.3390/info7030037 www.mdpi.com/journal/information Information 2016, 7, 37 2 of 16 on, which may represent truly promising solutions in the context of industrial control systems [14]. The PROFIBUS-DP and MODBUS/TCP fieldbus networks are very mature and powerful solutions for wired communication networks in industrial applications. The PROFIBUS-DP network can deliver real-time services in industrial environments. It is optimized for speed, efficiency, low connection costs, and is especially designed for fast communication between automation systems and distributed I/O peripherals. The MODBUS/TCP network uses TCP/IP protocol to carry the messaging structure and attaches 10,000 or more devices in a single MODBUS/TCP network. These advantages make PROFIBUS-DP and MODBUS/TCP widely used currently in worldwide operations [15–18]. Recently, the implementation of effective Energy Efficient Ethernet (EEE) for Real-Time Ethernet has become a new issue which is attracting the attention of scholars [19,20]. On the other hand, wireless industrial control networks (WICN) (e.g., WirelessHART, ZigBee, and ISA SP100) have also been presented in various process control systems [21–24]. There exist many ways to connect these existing wired fieldbus networks with wireless networks. Some approaches use open database connectivity (ODBC) and object linking and embedding (OLE) for process control (OPC), which are standard database access methods in the of open system interconnection (OSI) reference models. Other methods use a bridge in the of the OSI reference model. Although there are several works to address the combination of wireless communication with wired networks, it can be expected that wireless systems will be an integrated part of fieldbus networks in future standards. A natural approach is to design a new gateway as an information switching node for wired and wireless networks, while leaving the and cable deployment of each network unchanged. This approach is attractive, since the design efforts require only an additional physical device. Furthermore, integrating wireless and wired stations is done in a single local area network. The main work of this paper is to design a new gateway to construct a hybrid wired/wireless fieldbus network. It consists of a wired PROFIBUS-DP fieldbus network, a wired MODBUS/TCP fieldbus network and a wireless industrial control network, which are connected by the gateway as an intermediate station to implement data exchange in different network protocols. The advantage of this hybrid fieldbus network is to reduce the cost of cable deployment and insure the network performance in current complex industrial environments. Meanwhile, the proposed hybrid wired/wireless fieldbus network is successfully deployed at a steam turbine power generation system to verify its feasibility and effectiveness. The rest of the paper is organized as follows. Section2 reviews related works on hybrid wired/wireless fieldbus networks and proposes a new hybrid fieldbus network. Section3 introduces a gateway and corresponding data transmission mechanisms, and then discusses the implementation of the proposed hybrid wired/wireless fieldbus network. Section4 presents the system configuration of the hybrid wired/wireless fieldbus network and evaluates its network performance. Section5 gives a real-world application to verify its feasibility and effectiveness, and conclusions and directions for future research are given in Section6. A preliminary version of this paper was presented in reference [25].

2. Architecture of Hybrid Wired/Wireless Fieldbus Networks This section first gives a brief introduction on several well-known wired and wireless fieldbus networks including PROFIBUS-DP, MODBUS/TCP and WICN (Section 2.1), and then reviews related works on hybrid wired/wireless fieldbus networks (Section 2.2), and finally describes the architecture of the proposed hybrid wired/ (Section 2.3).

2.1. Wired Fieldbus Networks and WICN PROFIBUS-DP is one of variations of PROFIBUS, which is one of the most popular fieldbuses, and consists of , data link layer and application layer in order to satisfy real-time requirements [17]. In industrial automation applications, PROFIBUS-DP is used to operate sensors and actuators via a centralized controller, which uses master/slave communication mechanism between InformationInformation2016 2016, 7, ,7 37, 37 33 of of 16 17

and actuators via a centralized controller, which uses master/slave communication mechanism differentbetween field different devices field and devices a polling and methoda polling as method its medium as its accessmedium control access (MAC) control protocol (MAC) toprotocol achieve to significantlyachieve significantly better real-time better performance.real-time performance MODBUS/TCP. MODBUS/ is a MODBUSTCP is a variant,MODBUS which variant is located, which in is applicationlocated in application layer of the OSIlayer model, of the and OSI is model now one, and of is the now most one commonly of the most available commonly means available of connecting means industrialof connecting electronic industrial devices electronic [16,26]. It devices is designed [16,2 to6] allow. It is industrial designed input/outputto allow industrial equipment input/output such as programmableequipment such logic as controllers programma (PLCs),ble logic computers, controllers operator (PLCs), panels, computers, motors, operator sensors, panels, and so motors, on, to communicatesensors, and viaso on networks., to communicate MODBUS/TCP via networks is based. MODBUS/TCP on the TCP/IP is protocol based on which the isTCP/IP embedded protocol in thewhich frame is ofembed MODBUSded protocol,in the frame and usually of MODBUS uses the protocol, request/reply and usually mechanism uses to the connect request/reply devices onmechanism the Ethernet. to connect WICN isdevices a wireless on the network Ethernet. for low-rateWICN is anda wireless short distance network transmission, for low-rate and which short is baseddistance on thetransmission, chirp spread which spectrum is based (CSS) on the physical chirp layerspread of s thepectrum IEEE 802.15.4a(CSS) physical standard layer [10 of, 27the]. IEEE It is designated802.15.4a standard to the 2450 [10,2 MHz7]. I industrial,t is designated scientific to the and 2450 medical MHz industrial, (ISM) radio scientific bands, andand ismedical expected (ISM) to playradio a keybands, role and in theis expected deployment to play of a WICN key role technologies in the deployment in industrial of WICN fields. technologies It is well knownin industrial that WICNfields. is It veryis well suitable known for that real-time WICN industrial is very suitable control for systems real-time because industrial it synthesizes control systems FSK (Frequency because it Shiftsynthesizes Keying), FSK PSK (Frequency (Phase Shift S Keying),hift Keying) and, ASK PSK (Amplitude(Phase Shift Shift Keying Keying)), and modulation ASK (Amplitude models. Shift It usesKeying) master/slave modulation communication models. It uses between master/ devicesslave communication based on a single between topology devices of a token base ring,d on showna single astopology Figure1 .of a token ring, shown as Figure 1.

Deliver token Terminal Base station Terminal node n node 1

Send Terminal Terminal data Hold token node i node 2

Terminal Terminal node 5 Terminal node 3 node 4 FigureFigure 1. 1.Topology Topology of of token token ring ring in in wireless wireless industrial industrial control control networks networks (WICN). (WICN).

InIn the the WICN WICN depicted depicted in in Figure Figure1, all1, all devices devices are are connected connected as a as ring. a ring. One One of devices of devices is selected is selected to beto a be base a base station station or master or m station,aster station and other, and devicesother device are considereds are considered as terminal as terminal nodes or nodes slave stations.or slave Onlystations the.base Only station the base has station the authority has the toauthority manage to the manage entire wirelessthe entire network wireless and network maintain and the maint tokenain ring.the token All terminal ring. All nodes terminal areassigned nodes are exclusive assigned addresses, exclusive andaddress everyes, nodeand every has its node previous has its node previous and nextnode node and tonext deliver node the to deliver token. Whenthe token. one ofWhen terminal one of nodes terminal gets nodes the token gets in the a specifiedtoken in a period specified of time,period it has of time, the right it has to transmitthe right data to transmit to the base data station. to the Beyond base station. this time, Beyond the terminal this time, node the is terminal able to receivenode is data able from to rece anyive other data nodes from any in the other token nodes ring. in In the this token way, ring. the transmission In this way, isthe collision-free transmission in is WICN,collision and-free the in system’s WICN, reliabilityand the system is enhanced.’s reliability is enhanced.

2.2.2.2. Related Related Works Works on on Hybrid Hybrid Fieldbus Fieldbus Networks Networks ThereThere are are several several ways ways to establish to establish hybrid hybrid wired/wireless wired/wireless fieldbus fie networksldbus networks for various for industrial various applications.industrial applications. In reference [In7], reference authors integrate [7], authors a wireless integrate I/O interfacea wireless with I/O PROFIBUS interface with and PROFINET PROFIBUS forand factory PROFINET automations, for factory and, inautomation references [,6 and], authors, in reference design and[6], implementauthors design a wireless and implement fieldbus for a plasticwireless machineries. fieldbus for A polling-based plastic machine MACries. protocol A polling to improve-based MAC real-time protocol performance to improve in a wireless real-time PROFIBUSperformance is presented in a wireless in reference PROFIBUS [18]. The is presented specifications in reference of these applications [18]. The specifications mainly cover layersof these 2 (dataapplications link layer mainly and MAC) cover and layers 7 (application 2 (data link layer) layer of and the OSI MAC) reference and 7 model. (application Table 1 layer) shows of various the OSI proposedreference methods model. Table and their 1 shows corresponding various proposed advantages methods and disadvantages. and their corresponding It is seen from advantages Table1 that and disadvantages. It is seen from Table 1 that a gateway-based method is a favorable choice to construct

Information 2016, 7, 37 4 of 16 a gateway-based method is a favorable choice to construct a hybrid wired/wireless fieldbus network due to its suitability for all seven OSI layers and its flexibly in converting data stacks between different protocols. In addition, these hybrid wired/wireless fieldbus networks mainly address the extension of the wired PROFIBUS fieldbus, and rarely relate to the wired MODBUS fieldbus. However, there usually exist more than one fieldbus network in practical industrial environments. For example, a steam turbine power generation system is presented in this paper, which includes a wired PROFIBUS-DP fieldbus network and a wired MODBUS/TCP fieldbus network. This motivates us to design a new hybrid wired/wireless fieldbus network for complex industrial environments with various fieldbus networks [28].

Table 1. Various methods proposed in references to establish hybrid fieldbus networks and their advantages and disadvantages. OSI: open system interconnection; ODBC: open database connectivity.

OSI Reference Methods Advantages Disadvantages Examples Models ODBC data Easy implementation and Do not assure real-time mapping-based Application layer configuration; no need to References [25,26] performance method change system architectures Short response time and Need highly similar Bridge-based Data link layer timeout; simultaneously handle protocols between different References [27,28] method the traffic in different networks networks Multiple segments and Do not assure the Repeater-based multiple wireless cells are Physical Layer synchronization, and References [13,28] method interconnected; create a queuing delays may appear broadcast network Time delay of information Gateway-based Easily convert data stack All seven OSI layers exchange is uncertainty in a Reference [29] method between different protocols gateway

2.3. Hybrid Wired/Wireless Fieldbus Network Architecture This paper proposes a hybrid wired/wireless fieldbus network architecture, shown as Figure2. It consists of a wired MODBUS/TCP network, a wired PROFIBUS-DP network, WICN, various network devices, and a gateway which connects WICN with PROFIBUS-DP and MODBUS/TCP [29]. In this architecture, the gateway is taken as a slave station of a PROFIBUS-DP network, and a client station of a MODBUS/TCP network, but as a base station in WICN. This architecture is attractive because of three main reasons. Firstly, it takes into account the increasing need for mobile devices in industrial environments, where different wired fieldbus networks simultaneously exist. Because most of the design options on previous hybrid fieldbus networks are used in one wired fieldbus network [5,15,16,18], it cannot satisfy the need for the reconstruction of more than one wired fieldbus network. One wise solution is to extend the traditional wired fieldbus networks with wireless communication techniques. Secondly, the current wired fieldbus networks have been mature and developed very well, and it is not realistic for them to be completely replaced by wireless networks, at least for a long time. Therefore, hybrid wired/wireless fieldbus networks are the best choices. Finally, this proposed architecture requires only converting one protocol into another among different sub-networks by a gateway, but the protocol stack and deployment remains unchanged within each sub-network. Namely, it does not need the high similarity for the protocols in different sub-networks, and can better connect WICN with a wired PROFIBUS-DP network and a wired MODBUS/TCP network. Information 2016, 7, 37 5 of 16 Figure 2

Figure 2. Hybrid wired/wireless fieldbus network architecture based on MODBUS/TCP, PROFIBUS-DP and WICN networks. Figure 3 3. Implementation of a Hybrid Wired/Wireless Fieldbus Network This section first introduces a shared data model in a gateway in the hybrid wired/wireless fieldbus network (Section 3.1), and then describes the data transmission mechanisms between various network protocols (Section 3.2).

3.1. Shared Data Model in the Gateway The gateway uses a shared data model to solve communication problems in different fieldbus networks. Figure3 shows a shared data model, where the shared data area is used as a public data buffer for information exchange. The size of the data blocks in the shared data area is 256, which are divided into two parts: the read (R) data block and the write (W) data block. The read data block saves input data, which comes from various network devices, and the write data block sends output data in the reverse direction. In addition, every data block in a MODBUS/TCP client station and a WICN base station exchanges information with the shared data area using the data mapping method because they have the highly similar network protocols, but every data block in a PROFIBUS-DP slave station exchanges information with the shared data area using the data converting method because they have different styles of data stack between two protocols. In fact, a PROFIBUS-DP slave station uses input and output stacks to switch data with the shared data area. Figure4 presents an example of the data mapping method and the data converting method to illustrate their difference, where the data stacks in MODBUS/TCP and WICN are the same to the shared data area, but are different from those in PROFIBUS-DP.

1 Figure 2

InformationFigure2016, 73, 37 6 of 16

Figure 3. Shared data model of the gateway in the hybrid wired/wireless fieldbus network. R: read dataFigure block; 4 W: write data block.

1

FigureFigure 8 4. An example of the data mapping method and the data converting method.

3.2. Data Transmission Mechanisms In this subsection, data transmission mechanisms in different sub-networks are described in detail. The message sequence charts, from Figure 5 to Figure 8, are used to show the data transmission process [25]. The data transmission mechanisms consist of four parts: data transmission from WICN terminal nodes to the gateway’s WICN base station; data transmission from the gateway’s WICN base station to WICN terminal nodes; data transmission between MODBUS/TCP server stations and the gateway’s MODBUS/TCP client station, and; data transmission between the PROFIBUS-DP master station and the gateway’s PROFIBUS-DP slave station.

3.2.1. Data Transmission from WICN Terminal Nodes to the Gateway’s WICN Base Station In a WICN sub-network, the gateway acts as a base station for all wireless terminal nodes. Figure5 shows the data transmission from WICN terminal nodes to the gateway’s base station. Terminal nodes first wait to get a token from the WICN sub-network. Once one of terminal nodes obtains the token from the previous node, it records the token holding time and sends a data packet to the gateway’s base station. After the base station receives the data packet successfully, it sends a success message Information 2016, 7, 37 7 of 17 Information 2016, 7, 37 7 of 17 nodes first wait to get a token from the WICN sub-network. Once one of terminal nodes obtains the nodesInformation first2016 wait, 7, 37to get a token from the WICN sub-network. Once one of terminal nodes obtains7 ofthe 16 tokentoken from from the the previous previous node, node, it it records records the the token token holding holding time time and and sends sends a a data data packet packet toto the the gatewaygateway’’ss basebase station.station. AfterAfter thethe basebase stationstation receivesreceives thethe datadata packetpacket successfully,successfully, itit sendssends aa successsuccess messagemessageto this terminal toto thisthis tete noderminalrminal holding nodenode holdingholding the token. thethe token. Thetoken. process TThehe processprocess is executed isis executedexecuted circularly circularlycircularly until until theuntil running thethe runningrunning time timetimeterminates, terminates,terminates, and thenandand then thethen terminal thethe terminalterminal node nodenode releases releasesreleases the token thethe tokentoken to the toto next. thethe next.next.

Ternimal Node: i Ternimal Node: k WICN-Base Station Ternimal Node: i Ternimal Node: k WICN-Base Station get token get token

send data send data receive success receive success release token to next release token to next

Figure 5. Data transmission from the WICN terminal nodes to the gateway’s WICN base station. Figure 5 5.. DataData transmission from the WICN terminal nodes to the gateway’sgateway’s WICN base station.station.

3.2.3.2.3.2.2.22.. DD Dataataata TransmissionTransmission from from the the Gateway’s Gateway’s WICN WICN B BaseBasease Station Station to to WICN WICN TerminalTerminal NodesNodesNodes AfterAfter the the gateway’sgatewaygateway’’ss base basebase station stationstation gets getsgets the thethe token tokentoken from fromfrom the thethe WICN WICNWICN sub-network, subsub--network,network, it first itit firstfirst maps mapsmaps the data thethe datadatablocks blocksblocks from fromfrom the WICN thethe WICNWICN terminal terminalterminal nodes nodesnodes to the toto sharedthethe ssharedhared data datadata area. area.area. Meanwhile, MMeanwhile,eanwhile, the the datathe datadata blocks blocksblocks from fromfrom the thetheshared sharedshared data datadata area areaarea are alsoareare alsoalso mapped mappedmapped to the toto gateway’sthethe gatewaygateway base’’ss basebase station stationstation upon uponupon request. request.request. Then, ThenThen the,,WICN thethe WICNWICN base basebasestation stationstation transmits transmitstransmits data to datadata each toto terminal eacheach terminalterminal node. Whennode.node. theWWhenhen time thethe that timetime the that gateway’sthat thethe gatewaygateway base station’’ss basebase holds stationstation the holdsholdstoken thethe runs tokentoken out, run itrun releasesss outout,, itit the releasesreleases token thethe to the tokentoken next toto terminal thethe nextnext node. terminalterminal Such node.node. a round SSuchuch of aa informationroundround ofof informationinformation exchange exchangeexchangein a WICN inin sub-network aa WICNWICN subsub- is-networknetwork shown asisis shown Figureshown6 asas. FigureFigure 66..

Terminal Node: i WICN Base Station Shared Data Area Terminal Node: i WICN Base Station Shared Data Area

get token get token map data block map data block

Data Mapping method Data Mapping method request request map data block send data map data block send data

release token to next release token to next

Figure 6. Data transmission from the gateway’s WICN base station to WICN terminal nodes. FigFigureure 6 6.. DDataata transmission from the gateway gateway’s’s WICN base station to WICN terminal nodes.nodes. 3.2.3. Data Transmission between a MODBUS/TCP Server Station and the Gateway’s MODBUS/TCP 3.2.3.2.3.3. D Dataata Transmission betweenbetween aa MODBUS/TCPMODBUS/TCP Server Station and thethe Gateway’sGateway’s MODBUS/TCPMODBUS/TCP Client Station Client Station In a MODBUS/TCP sub-network, the server station obeys a standard MODBUS/TCP protocol to IInn a MODBUS/TCP sub sub-network,-network, the the server server station station obeys obeys a astandard standard MODBUS/TCP MODBUS/TCP protocol protocol to poll each MODBUS/TCP client station. When the server station needs to communicate with the pollto poll each each MODBUS/TCP MODBUS/TCP client client station. station. When When the the server server station station needs needs to to communicate communicate with with the gateway’s client station, it emits a read/write request. If the gateway’s client station receives the gatewaygateway’s’s client client station, station, it it emits emits a read/write a read/write request. request. If the If gateway’sthe gateway client’s client station station receives receives the server the server station’s read request, it responses the request and uploads data to the gateway’s client serverstation’s station read’ request,s read request, it responses it responses the request the and request uploads and data uploads to the datagateway’s to the client gateway station’s client from station from the shared data area using the data mapping method. Then, the gateway’s client station stationthe shared from data the areashared using data the area data using mapping the data method. mapping Then, method. the gateway’s Then, the client gateway station’s sendsclient datastation to sends data to the server station. If the gateway’s client station receives the server station’s write sendsthe server data station. to the If server the gateway’s station. If client the stationgateway receives’s client the station server receives station’s the write server request, station it responds’s write request, it responds to the request and receives data from the server station, then updates data in the requestto the request, it respon andds receives to the request data from and recei the serverves data station, from the then server updates station, data then in the updates shared data data in area the

using the same data mapping method. Such a round of information transmission is shown as Figure7. Information 2016, 7, 37 8 of 17 sharedInformation data2016 area, 7, 37 using the same data mapping method. Such a round of information transmission8 of is 16 shown as Figure 7.

MODBUS/TCP server MODBUS/TCP client Shared Data Area

read request read response upload data block

Data mapping method send data Figure 4 update data block

write request write response

FigFigureure 7.7.Data Data transmission transmission between between a MODBUS/TCP a MODBUS/TCP server station server and station the gateway’s and MODBUS/TCPthe gateway’s MODBUSclient station./TCP client station.

3.2.3.2.4.4. D Dataata Transmission Transmission between a PROFIBUS PROFIBUS-DP-DP Master Station and the Gateway’s PROFIBUS PROFIBUS-DP-DP SlaveSlave Station TheThe PROFIBUS-DPPROFIBUS-DP sub-network sub-network plays plays the the role role of backbone of backbone in the hybridin the wired/wireless hybrid wired/wireless fieldbus fieldbusnetwork network because itbecause is widely it is used widely in industrial used in industrial process control process systems. control Thesystems gateway. The is gateway taken as is a takenPROFIBUS-DP as a PROFIBUS slave station-DP slave in this station sub-network. in this sub The-network. master station The master obeys a station standard obeys PROFIBUS-DP a standard PROFIBUSprotocol to- pollDP protocol each PROFIBUS-DP to poll each slavePROFIBUS station-DP distributed slave station in the distributed sub-network. in the The sub gateway’s-network. slave The gatewaystation acknowledges’s slave station received acknowledges messages received from the messages master station, from andthe thenmaster sends station, response and messagesthen sends to responsethe master messages station upon to the request. master Ifstation the master upon stationrequest. requests If the mas outputter station data, the request gateway’ss output slave data, station the gatewayreceives’ datas slave from station the masterreceives station data from and thenthe master updates station data inand the then shared updates data areadata usingin the the shared data dataconverting area using method. the data If the converting master station method. requests If the input master data, station the gateway’s requests input slave stationdata, the uploads gateway data’s slavefrom station the shared uploads data data area from using the the shared same datadata convertingarea using methodthe same and data then converting sends them method to the and master then sendsstation. them Such to athe round masterFigure of informationstation. 8 Such transmission a round of information is shown as transmission Figure8. is shown as Figure 8.

PROFIBUS-DP master PROFIBUS-DP slave Shared Data Area

request message response message output data update data

Data converting method

input data upload data

Figure 8. Data transmission between a PROFIBUS-DP master station and the gateway’s PROFIBUS-DP Figure 8. Data transmission between a PROFIBUS-DP master station and the gateway’s PROFIBUS-DP slave station. slave station. 4. Experimental Performance Evaluation 4. Experimental Performance Evaluation This section first introduces the implementation of the gateway and system configuration of the This section first introduces the implementation of the gateway and system configuration of the proposed hybrid wired/wireless fieldbus network (Section 4.1), and then evaluates and discusses proposed hybrid wired/wireless fieldbus network (Section 4.1), and then evaluates and discusses their their experimental performance (Section 4.2). experimental performance (Section 4.2).

Information 2016, 7, 37 9 of 16 Information 2016, 7, 37 9 of 17

4.1.4.1. Gateway I Implementationmplementation and SystemSystem ConfigurationConfiguration The AT91RM9200AT91RM9200 processor processor (Atmel (Atmel Corporation, Corporation San, San Jose, Jose, CA, CA USA), USA) is used is used as the as hardwarethe hardware core ofcore the of gateway, the gateway, which iswhich an industrial-grade is an industrial embedded-grade embedded microprocessor microprocessor including including abundant abundant hardware resourceshardware and resources interfaces. and The interfaces. T runninghe software in the running gateway in employs the gateway VxWorks employs (version VxWorks 6, Wind River(version Systems 6, Wind Inc., River Alameda, Systems CA, Inc., USA) Alameda, which is CA, a real-time USA) which embedded is a real operation-time embedded system. The operation internal wirelesssystem. nodeThe internal uses a nanoPAN5360 wireless node RF uses module a nanoPAN5360 produced by Nanotron RF module Technologies produced GmBH by Nanotron (Berlin, Germany),Technologies which GmBH is based (Berlin, on IEEE Germany 802.15.4a-CSS), which (chirp is based spread on spectrum). IEEE 802.15.4a It has extremely-CSS (chirp low s powerpread consumptionspectrum). It has over extremely a wide range low power of operating consumption temperatures. over a wide Figure range9 shows of operating the physical temperature figure ofs. theFigure gateway. 9 shows the physical figure of the gateway.

FigFigureure 9 9.. Physical figure figure of the gateway gateway..

The system configuration of the hybrid wired/wireless fieldbus network consists of WICN, a The system configuration of the hybrid wired/wireless fieldbus network consists of WICN, a MODBUS/TCP sub-network and a PROFIBUS-DP sub-network which is the backbone network. The MODBUS/TCP sub-network and a PROFIBUS-DP sub-network which is the backbone network. The gateway is the key device to interconnect different sub-networks in the hybrid fieldbus network. gateway is the key device to interconnect different sub-networks in the hybrid fieldbus network. Namely, the gateway served as a slave station of a PROFIBUS-DP sub-network, and a client station Namely, the gateway served as a slave station of a PROFIBUS-DP sub-network, and a client station of of a MODBUS/TCP sub-network, but it is taken as the base station in WICN. a MODBUS/TCP sub-network, but it is taken as the base station in WICN. Real-time performance and the packet loss rate are the most important measurement Real-time performance and the packet loss rate are the most important measurement parameters parameters in industrial network control systems. They provide methods for simultaneously in industrial network control systems. They provide methods for simultaneously capturing the capturing the timeliness and reliability capabilities of hybrid wired/wireless fieldbus networks. This timeliness and reliability capabilities of hybrid wired/wireless fieldbus networks. This paper defines paper defines a polling period (Tpp) and a control cycle time (Tctrl) to denote real-time performance. a polling period (Tpp) and a control cycle time (Tctrl) to denote real-time performance. The polling The polling period Tpp indicates the time that the gateway’s base station polls each terminal nodes in period Tpp indicates the time that the gateway’s base station polls each terminal nodes in WICN, and WICN, and the control cycle time Tctrl covers the whole time consumption in different sub-networks. T theNamely, control a cyclesensor time in WICNctrl covers sends the a datawhole packet, time consumption and the gateway in different receives sub-networks. this data packet Namely, and auploads sensor to in the WICN controller sends ain data a PROFIBUS packet, and-DP the sub gateway-network receives. Then the this controller data packet transmits and uploads a control to thesignal controller to the gateway, in a PROFIBUS-DP which continues sub-network. to send Thenit to an the actuator controller in transmitsa MODBUS/TCP a control sub signal-network to the. gateway, which continues to send it to an actuator in a MODBUS/TCP sub-network. Figure 10 shows Figure 10 shows the whole control cycle, and the control cycle time Tctrl is calculated as the whole control cycle, and the control cycle time Tctrl is calculated as

Tctrl1   up   2     3   down   4 (1) Tctrl “ τ1 ` τup ` τ2 ` τc ` τ3 ` τdown ` τ4 (1) where τ1 denotes the wireless transmission time that a data packet comes from the sensor in WICN whereto the τ1 gateway.denotes theτup wirelessdenotes transmission the part of time protocol that a data converting packet comes time from which the is sensor from in WICN WICN to thePROFIBUS gateway.-DPτup indenotes the gateway. the part τ of2 denotes protocol the converting time that time the which controller is from in a WICN PROFIBUS to PROFIBUS-DP-DP master instation the gateway. gets a dataτ2 denotes packet the from time the that gateway. the controller τc denotes in a the PROFIBUS-DP process time master of a data station packet gets ain data the packetcontroller. from τ3the denotes gateway. the timeτc denotes that the the gateway process receives time of aa datacontrol packet signal in thefrom controller. the controller.τ3 denotes τdown thedenotes time another that the gatewaypart of the receives protocol a control converting signal time from from the controller.PROFIBUSτ-DPdown todenotes MODBUS/TCP anotherpart in the of thegateway. protocol τ4 denotes converting the transmission time from PROFIBUS-DP time for the gateway to MODBUS/TCP to send the in control the gateway. signal toτ4 thedenotes actuator the transmissionin the MODBUS/TCP time for thenetwork gateway. Note to that send τ theup + controlτdown denotes signal the to theentire actuator protocol in the converting MODBUS/TCP time in network.the gateway. Note that τup + τdown denotes the entire protocol converting time in the gateway.

Information 2016, 7, 37 10 of 16

Information 2016, 7, 37 10 of 17

Actuator τ4 τdown τc MODBUS/TCP network τ3 Protocol converter Controller Information 2016, 7, x τ2 10 of 17 T No.1 pp No.k τup τ PROFIBUS-DP No.n 1 Sensor network WICN

Figure 10. Control cycle time in the proposed hybrid wired/wireless fieldbus network. Figure 10. Control cycle time in the proposed hybrid wired/wireless fieldbus network. Packet loss rate is another indicator of network measurement performance in this paper. It is defined as the ratio of the number of lost packets Slost to the total number of packets Ssum in the whole Packet lossnetwork rate control is another cycle. This indicator indicator reflects of network data loss in measurement the whose network, performance which is calculated in this as paper. It is defined as the ratio of the number of lost packets Slost to the total number of packets Ssum in the whole Loss S S 100% (2) network control cycle. This indicator reflects datalost loss sum in the whose network, which is calculated as Figure 10. Control cycle time in the proposed hybrid wired/wireless fieldbus network. 4.2. Experimental Results Packet loss rate is another indicatorLoss “ ofS networklost{Ssum measurementˆ 100% performance in this paper. It is (2) defined4.2.1. Polling as the Period ratio of T ppthe with number Different of lost Number packets of S lostNodes to the and total Length number of Dataof packets Packets Ssum in in WICN the whole 4.2. ExperimentalnetworkF Resultsigure control 11 shows cycle. This the experimentalindicator reflects results data ofloss 50 in runs the whose with differentnetwork, numberwhich iss calculatedof nodes andas lengths of data packets in WICN, where the=× number of nodes includes five nodes, 10 nodes and 15 Loss Slost S sum 100% (2) 4.2.1. Pollingnodes, Period andT thepp withlength Differentof data packet Numbers involves of4 bytes, Nodes 12 bytes and and Length 20 bytes. of F Datarom the Packets figure, it in is WICN seen that polling periods Tpp are almost the same in the cases of 15 nodes and different bytes, but Figure 114.2.they shows Experimental are different the experimentalResults in the cases of results 4 bytes and of 50 different runs with nodes. differentThe results numbers illustrate that of nodespolling and lengths of data packetsperiod ins WICN,Tpp are more where influenced the by number the number of of nodes WICN nodes includes than the five length nodes, of data 10 packet nodess in the and 15 nodes, 4.2.1. Polling Period Tpp with Different Number of Nodes and Length of Data Packets in WICN and the lengthproposed of data hybrid packets fieldbus involves network. 4Tobytes, further 12 justify bytes the and experimental 20 bytes. results, From the confidencethe figure, it is seen intervalFigure with 11 a levelshows of the95% experimental is used based resultson these of observed 50 runs samplewith different data. A confidencenumbers of interval nodes gives and that polling periodslengthsan estimated of Tdatapp range arepackets almostof valuesin WICN, thewhich where same is likely the in number theto include cases of nodes an of unknown 15 includes nodes population five and nodes, different parameter 10 nodesbytes, and and 15 is but they are different in thenodes,calculated cases and from ofthe 4length a bytes given of setanddata of packets differentsample involvesdata. nodes. Table 4 by 2 tes, Theshows 12 results bytesthe mean and illustrate 20and bytes. corresponding From that the polling figure,confidence it periods is Tpp are more influencedseeninterval that by of thepolling polling number periods periodof Ts ppT WICN ppare in almostdifferent nodes the cases. same than T inhethe the results lengthcases verify of 15 of thatnodes data the and packets length different of indata bytes, the packets proposed but hybrid theydoes arenot differentinfluence inthe the polling cases period, of 4 bytes but theand number different of WICNnodes. nodesThe results is the importantillustrate thatfactor polling to the fieldbus network.periodspolling periodT Topp are further more[30]. influenced justify theby the experimental number of WICN results, nodes than the the confidence length of data intervalpackets in the with a level of proposed hybrid fieldbus network. To further justify the experimental results, the confidence 95% is used based on these observed sample data. A confidence interval gives an estimated range of interval with a level of 95% is used based on these observed sample data. A confidence interval gives values which is likely to include100 an unknown population parameter and is calculated from a given an estimated range of values which is likely to include an unknown population parameter and is set of samplecalculated data. Table from 2a showsgiven set theof sample mean data. and Table corresponding 2 shows the mean confidence and corresponding interval confidence of polling periods 80 Tpp in differentinterval cases. of polling The results periods verifyTpp in different that thecases. length The results of data verify packets that the doeslength notof data influence packets the polling period, but thedoes number not influence of WICNthe polling nodes period, is but the the important number of WICN factor nodes to the is the polling important period factor to [30 the]. polling period [30]. 60

time (ms) time 10040 15 nodes and 20 bytes 15 nodes and 12 bytes 8020 15 nodes and 4 bytes 10 nodes and 4 bytes 5 nodes and 4 bytes 600 0 10 20 30 40 50 running num. time (ms)time 40 Figure 11. Polling period Tpp with different number of nodes15 nodesand length and 20 of bytes data packets in WICN. 15 nodes and 12 bytes 20 15 nodes and 4 bytes 10 nodes and 4 bytes 5 nodes and 4 bytes 0 0 10 20 30 40 50 running num.

Figure 11. Polling period Tpp with different number of nodes and length of data packets in WICN. Figure 11. Polling period Tpp with different number of nodes and length of data packets in WICN.

Table 2. Mean and corresponding confidence interval with a level of 95% of polling periods Tpp.

15 Nodes and 15 Nodes and 15 Nodes and 10 Nodes and 5 Nodes and 20 Bytes (ms) 12 Bytes (ms) 4 Bytes (ms) 4 Bytes (ms) 4 Bytes (ms) Mean 78.4 79.3 78.9 63.3 47.9 Confidence interval (75.3, 81.5) (75.9, 82.7) (75.7, 82.1) (61.3, 65.4) (44.5, 49.3) Information 2016, 7, x 11 of 17

Table 2. Mean and corresponding confidence interval with a level of 95% of polling periods Tpp.

15 Nodes and 15 Nodes and 15 Nodes and 10 Nodes and 5 Nodes and

20 Bytes (ms) 12 Bytes (ms) 4 Bytes (ms) 4 Bytes (ms) 4 Bytes (ms) Information 2016, 7, 37 11 of 16 Mean 78.4 79.3 78.9 63.3 47.9 Confidence interval (75.3, 81.5) (75.9, 82.7) (75.7, 82.1) (61.3, 65.4) (44.5, 49.3)

4.2.2. Control Cycle Time Tctrl with Different Wireless Nodes in the Hybrid Wired/Wireless Fieldbus4.2.2. Network Control Cycle Time Tctrl with Different Wireless Nodes in the Hybrid Wired/Wireless Fieldbus Network Figure 12 shows control cycle times Tctrl with the same length of data packets and different number Figure 12 shows control cycle times Tctrl with the same length of data packets and different of wireless nodes in the hybrid wired/wireless fieldbus network. From the figure, it is apparently seen number of wireless nodes in the hybrid wired/wireless fieldbus network. From the figure, it is that theapparently control seen cycle that time the is control the largest cycle in time the is case the oflargest 20 wireless in the case nodes of and20 wireless the smallest nodes isand in the case of fivesmallest wireless is in nodes, the case which of five indicates wireless that nodes, the wh numberich indicates of wireless that the nodes number can of affect wireless control nodes cycle can times Tctrl.affect Table control3 shows cycle the times mean Tctrl and. Table corresponding 3 shows the mean confidence and corresponding interval with confidence a level interval of 95% with of T actrl in differentlevel wireless of 95% of nodes Tctrl in baseddifferent on wireless these observed nodes based sample on these data. observed sample data. Figure 4 100

80

60

time (ms)time 40 20 nodes 15 nodes 20 10 nodes 5 nodes 0 0 10 20 30 40 50 running num.

Figure 12. Control cycle time Tctrl with different wireless nodes in the hybrid wired/wireless fieldbus Figure 12. Control cycle time Tctrl with different wireless nodes in the hybrid wired/wireless network. fieldbus network.

Table 3. Mean and corresponding confidence interval with a level of 95% of control cycle times Tctrl. Table 3. Mean and correspondingFigure 8 confidence interval with a level of 95% of control cycle times Tctrl. 20 Nodes (ms) 15 Nodes (ms) 10 Nodes (ms) 5 Nodes (ms)

Mean 20 Nodes (ms)78.2 15 Nodes56.3 (ms) 10 Nodes46.1 (ms) 543.4 Nodes (ms) MeanConfidence interval 78.2 (75.9, 80.4) (55.1, 56.3 57.5) (45.3, 46.1 46.9) (42.8, 44.0) 43.4 Confidence interval (75.9, 80.4) (55.1, 57.5) (45.3, 46.9) (42.8, 44.0) 4.2.3. Converting Time in the Gateway

4.2.3. ConvertingThe protocol Time converting in the Gateway time τup + τdown represents the real-time performance of the gateway. Figure 13 shows converting times of 50 runs in the gateway in the cases of 15 wireless nodes and a Thedata protocolpacket length converting of 12 bytes. time Weτ upcan+ seeτdown clearepresentsrly that the the minimum, real-time the performance maximum, and of the the mean gateway. Figurevalues 13 shows of converting converting times times are 15 ofms, 50 30 runs ms and in the 22.4 gateway ms, respectively in the cases. The results of 15 wireless illustrate nodesthat the and a data packetconverting length time ofis the 12 bytes.smaller Weand canthe real-time see clearly performance that the minimum, is the better thefor the maximum, proposed andgateway. the mean values of converting times are 15 ms, 30 ms and 22.4 ms, respectively. The results illustrate that the converting time is the smaller and the real-time performance is the better for the proposed gateway. Figure 13 50

40

30

(ms)time 20

10

0 0 10 20 30 40 50 running num.

Figure 13. Converting times τup + τdown in the gateway in the cases of 15 wireless nodes and a data packet length of 12 bytes. 2 Information 2016, 7, 37 12 of 16

4.2.4. Packet Loss Rate for the Whole Hybrid Network Figure 14 depicts packet loss rates of 50 runs in the cases of 15 wireless nodes and a data packet length of 12 bytes in the hybrid wired/wireless fieldbus network. It is shown that the maximum value of packet loss rate is less than 0.1%, which is a standard reference value to measure reliability performance of network control systems [10]. It is further obtained that the mean and corresponding standard deviation values ofFigure packet 14 loss rates are 0.044% and 0.021%, respectively. The results indicate that the packet loss rates of the proposed hybrid fieldbus network satisfy the network reliability.

0.1

0.08

0.06 % 0.04

0.02

0 0 10 20 30 40 50 running num. Figure 14. Packet loss rates in the hybrid wired/wireless fieldbus network in the cases of 15 wireless nodes and a data packet length of 12 bytes.

Based on the real-time performance and packet loss rates obtained by the above experiments, it is confirmed that the results satisfy performance requirement of networked control systems. Namely, the proposed hybrid wired/wireless fieldbus network can get better network performances, and qualify for the environment of industrial automation.

5. Application In order to further investigate feasibility of the proposed hybrid wired/wireless fieldbus network, it is applied to an experimental measurement and control platform of a steam turbine power generation system, which is composed of a gateway, a wired PROFIBUS-DP sub-network, a wired MODBUS/TCP sub-network, WICN and some power generation devices. In this system, there exist two wired fieldbuses because it is a complex distributed control plant; some devices are connected by PROFIBUS-DP fieldbuses, and other devices are connected by MODBUS/TCP fieldbuses to satisfy the cabling requirement. In fact, some interesting classes of industrial applications, such as the health monitoring of machine colonies and the system applications, involve more than one wired fieldbus and various kinds of networked embedded devices. An important characteristic in these application areas is that technology improvement based on a wireless industry network is required. Figure 15 shows a schematic representation of the system, where electrical energy generation using a steam turbine involves three energy conversions: extracting thermal energy from the fuel in a petroleum gas tank and using it to raise steam by a steam boiler; converting the thermal energy of the steam into kinetic energy in a steam turbine, and; using a rotary generator to convert the turbine’s mechanical energy into electrical energy [31]. From Figure 15, it is known that the measured values of this system are transmitted by different wired/wireless sub-networks to the gateway, which changes the styles of data stacks and continues to transmit to the controller in a backbone PROFIBUS-DP network. Thus, the gateway becomes a bridge between wired sub-networks and wireless sub-networks. In the process of electrical energy generation, some important variables including generator voltage, generator current, generator speed, turbine inlet pressure and temperature, and turbine outlet pressure and temperature are controlled by the flow control valve. In this experiment, we use the conventional

3 Information 2016, 7, 37 13 of 17 connected by PROFIBUS-DP , and other devices are connected by MODBUS/TCP fieldbuses to satisfy the cabling requirement. In fact, some interesting classes of industrial applications, such as the health monitoring of machine colonies and the Internet of things system applications, involve more than one wired fieldbus and various kinds of networked embedded devices. An important characteristic in these application areas is that technology improvement based on a wireless industry network is required. Figure 15 shows a schematic representation of the system, where electrical energy generation using a steam turbine involves three energy conversions: extracting thermal energy from the fuel in a petroleum gas tank and using it to raise steam by a steam boiler; converting the thermal energy of the steam into kinetic energy in a steam turbine, and; using a rotary generator to convert the turbine’s mechanical energy into electrical energy [31]. From Figure 15, it is known that the measured values of this system are transmitted by different wired/wireless sub-networks to the gateway, which changes the styles of data stacks and continues to transmit to the controller in a backbone PROFIBUS-DP network. Thus, the gateway becomes a bridge between wired sub-networks and wireless sub-networks. In the process of electrical energy generation, some important variables including generator voltage, generator current, generator Information 2016, 7, 37 13 of 16 speed, turbine inlet pressure and temperature, and turbine outlet pressure and temperature are controlled by the flow control valve. In this experiment, we use the conventional PID (proportionalPID (proportional-integral-derivative)-integral-derivative) control control to regulate to regulate the theflow flow control control valve, valve, and and the the model model of of the the plantplant is is obtained obtained by by system system identificat identificationion based based on on data data acquired acquired during during a a series series of of experiments. experiments.

MODBUS/TCP Networks

Gateway

Condense Turbine Outlet Valve Position Tower Temperature Fuel Flow Turbine Inlet Pressure Boiler Pressure Generator Petroleum Resistive Load Gas Tank

Turbine Inlet Control Valve Generator Voltage Steam Turbine Temperature Generator Current Turbine Outlet Boiler Temperature Pressure Valve Output Gennerator Speed Steam Boiler Temperature

Wireless Industrial Control PROFIBUS-DP Networks Networks(WICN)

FigureFigure 15 15.. ScSchematichematic representation representation of of a asteam steam turbine turbine power power generation generation system system based based on on the the hybrid hybrid wired/wirelesswired/wireless fieldbus fieldbus network network..

TableTable 44 showsshows the the performance of the proposed hybrid wired/wireless fieldbus fieldbus network network in in the the steamsteam turbine turbine power power generation generation system, system, which which includes includes the the network network performance performance denoted denoted by by the the meanmean transmission transmission times times from from the the field field devices devices to to the the control control panel, panel, and and the the control control performance performance dendenotedoted by by the the mean mean measurement measurement values values from from different different sub sub-networks.-networks. N Noteote that that some some parts parts of of variablevariable values, values, such such as indoor temperature,temperature, are are only only obtained obtained by by WICN, WICN, other other parts, parts such, such as fuel as fuel flow, floware, obtained are obtained by WICN by WICN and and one ofone the of wiredthe wired fieldbus fieldbus sub-networks sub-networks and and others, others such, such as turbine as turbine inlet inletpressure, pressure are, simultaneouslyare simultaneously obtained obtained by allby the all threethe three sub-networks. sub-networks. From From this this table, table, it is it found is found that thatmean mean transmission transmission times times from the from field the devices field devices to the control to the panel control are very panel small are for very all small sub-networks for all suband-networks completely and satisfy completely the real-time satisfy therequirements real-time requirementsfor industrial for automation. industrial This automation. indicates This that the proposed hybrid wired/wireless fieldbus network is very feasible in the steam turbine power

generation system. Furthermore, we can know that the mean transmission times of WICN are lower than those of the corresponding wired sub-networks in all cases. This illustrates that WICN can reduce transmission times and obtain better network performance. On the other hand, it is obvious that the measured values are relatively stable through different wired/wireless sub-networks. This denotes that control strategy is feasible in the proposed hybrid wired/wireless fieldbus network, and such network architecture can obtain the stable control performance. Information 2016, 7, 37 14 of 16

Table 4. Network performance (mean transmission times) and control performance (mean measurement values) are obtained by a PROFIBUS-DP sub-network, a MODBUS/TCP sub-network and WICN in the steam turbine power generation system. Note that “–” denotes that there is no measurement value in the corresponding sub-network.

PROFIBUS-DP Network MODBUS/TCP Network WICN Performance Network performance Control performance Network performance Control performance Network performance Control performance (Mean transmission time) (Mean measurement value) (Mean transmission time) (Mean measurement value) (Mean transmission time) (Mean measurement value) Indoor Temperature – – – – 22.8 ms 20.35 ˝C Fuel Flow – – 60.1 ms 8.42 ˆ 10´5 m3¨ s´1 56.2 ms 8.39 ˆ 10´5 m3¨ s´1 Boiler Pressure – – 42.7 ms 380,629.5 Pa 33.4 ms 38,0524.1 Pa Boiler Temperature – – – – 25.3 ms 142.17 ˝C Valve Position – – 59.4 ms 32.71% 49.7 ms 32.06% Turbine Inlet Pressure 32.8 ms 32110.7 Pa 43.5 ms 32,291.3 Pa 31.6 ms 32,219.5 Pa Turbine Inlet Temperature – – – – 22.8 ms 102.71 ˝C Turbine Outlet Pressure 37.2 ms 7800.3 Pa 44.1 ms 7784.3 Pa 34.5 ms 7805.6 Pa Turbine Outlet Temperature – – – – 23.2 ms 98.33 ˝C Generator Speed 62.8 ms 2614 r¨ min´1 – – 51.7 ms 2586 r¨ min´1 Generator Voltage 67.1 ms 3.25 V – – 50.2 ms 3.15 V Generator Current 64.0 ms 0.12 A – – 52.1 ms 0.11 A Information 2016, 7, 37 15 of 16

6. Conclusions In this paper, a hybrid wired/wireless fieldbus network is proposed to solve complex communication path problems in industrial networked control systems. The proposed hybrid fieldbus network uses a gateway to connect a wired PROFIBUS-DP fieldbus network and a wired MODBUS/TCP fieldbus network with a wireless industrial control network. Since the network protocols are different in various sub-networks, they employ a shared data model to address data exchange in different network protocols. In order to evaluate the feasibility and effectiveness of the proposed hybrid fieldbus network, real-time performance and packet loss rates are adopted as performance indicators. The experiment results indicate that the hybrid wired/wireless fieldbus network can satisfy the performance requirement of industrial automation systems. Meanwhile, it is deployed at a steam turbine power generation system to infer this and to further verify that it has better network performance and control performance. There are several important directions for future work. This paper mainly focuses on designing the gateway and implementing the proposed hybrid wired/wireless fieldbus network, but there is room for improvement. For example, it is of interest to optimize the architecture of the proposed hybrid fieldbus network. The second important direction for future work is to investigate the feasibility and effectiveness of this hybrid fieldbus network in other industrial real-world applications. The third important direction for future work is to develop new control strategies to improve the network control performance based on this hybrid fieldbus network.

Acknowledgments: This work is supported in part by the Funds of National Natural Science Foundation of China (grant No. 61473182), in part by Program for the Key Project of Science and Technology Commission of Shanghai Municipality (14JC1402200, 15JC1401900), in part by Program for “Blue project” of colleges and universities in Jiangsu province, in part by Program for “Overseas Study Plan” of colleges and universities in Jiangsu province, and in part by the Funds of Natural Science Special Program of Nantong Vocational University (grant No. 1407001). Author Contributions: Sheng Xu conceived the concept, performed the research and wrote the manuscript. Minrui Fei conceived and reviewed the manuscript. Haikuan Wang evaluated the experimental performance of the proposed hybrid wired/wireless fieldbus network. All authors have read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Li, X.; Li, D.; Wan, J.; Vasilakos, A.V.; Lai, C.F.; Wang, S. A Review of Industrial Wireless Networks in the Context of Industry 4.0. Wirel. Netw. 2015.[CrossRef] 2. Chang, C.; Lee, C. A secure single sign-on mechanism for distributed computer networks. IEEE Trans. Ind. Electron. 2012, 59, 629–637. [CrossRef] 3. Chen, J.; Cao, X.; Cheng, P.; Xiao, Y.; Sun, Y. Distributed collaborative control for industrial automation with wireless sensor and actuator networks. IEEE Trans. Ind. Electron. 2010, 57, 4219–4230. [CrossRef] 4. Flammini, A.; Ferrari, P.; Marioli, D.; Sisinni, E.; Taroni, A. Wired and wireless sensor networks for industrial applications. Microelectron. J. 2009, 40, 1322–1336. [CrossRef] 5. Sisinni, E.; Depari, A.; Flammini, A. Design and implementation of a for temperature sensing in hostile environments. Sens. Actuators A. 2016, 237, 47–55. [CrossRef] 6. Flammini, A.; Marioli, D.; Sisinni, E.; Taroni, A. Design and implementation of a wireless fieldbus for plastic machineries. IEEE Trans. Ind. Electron. 2009, 56, 747–756. [CrossRef] 7. Kjellsson, J.; Vallestad, A.E.; Steigmann, R.; Dzung, D. Integration of a Wireless I/O interface for PROFIBUS and PROFINET for factory automation. IEEE Trans. Ind. Electron. 2009, 56, 4279–4288. [CrossRef] 8. Lehr, W.H.; Chapin, J.M. On the convergence of wired and wireless access network architectures. Inf. Econ. Policy. 2010, 22, 33–41. [CrossRef] 9. Maestro, J.A.; Reviriego, P. Energy efficiency in : The case of powerlink. IEEE Trans. Ind. Electron. 2010, 57, 2896–2903. [CrossRef] 10. Gaj, P.; Jasperneite, J.; Felser, M. Computer Communication within Industrial Distributed Environment—A Survey. IEEE Trans. Ind. Inform. 2013, 9, 182–189. [CrossRef] Information 2016, 7, 37 16 of 16

11. Gungor, V.C.; Lu, B.; Hancke, G.P. Opportunities and challenges of wireless sensor networks in . IEEE Trans. Ind. Electron. 2010, 57, 3557–3564. [CrossRef] 12. Luo, R.C.; Chen, O. Mobile sensor node deployment and asynchronous power management for wireless sensor networks. IEEE Trans. Ind. Electron. 2012, 59, 2377–2385. [CrossRef] 13. Vitturi, S.; Tramarin, F.; Seno, L. Industrial Wireless Networks: The Significance of Timeliness in Communication Systems. IEEE Ind. Electron. Mag. 2013, 7, 40–51. [CrossRef] 14. Seno, L.; Tramarin, F.; Vitturi, S. Performance of Industrial Communication Systems: Real Application Contexts. IEEE Ind. Electron. Mag. 2012, 6, 27–37. [CrossRef] 15. Alves, M.; Tovar, E. Real-time communications over wired/wireless PROFIBUS networks supporting inter-cell mobility. Comput. Netw. 2007, 51, 2994–3012. [CrossRef] 16. Elarag, H.; Moedinger, A.; Hogg, C. TCP friendly protocols for media streams over heterogeneous wired-wireless networks. Comput. Commun. 2008, 31, 2242–2256. [CrossRef] 17. Miorandi, D.; Vitturi, S. Hybrid wired/wireless implementations of PROFIBUS DP: A feasibility study based on Ethernet and . Comput. Commun. 2004, 27, 946–960. [CrossRef] 18. Willig, A. Polling-based MAC protocols for improving real-time performance in a wireless PROFIBUS. IEEE Trans. Ind. Electron. 2003, 50, 806–818. [CrossRef] 19. Meike, D.; Pellicciari, M.; Berselli, G. Energy efficient use of multirobot production lines in the automotive industry: Detailed system modeling and optimization. IEEE Trans. Autom. Sci. Eng. 2014, 11, 798–809. [CrossRef] 20. Chang, Q.; Xiao, G.; Biller, S.; Li, L. Energy saving opportunity analysis of automotive serial production systems (March 2012). IEEE Trans. Autom. Sci. Eng. 2013, 10, 334–342. [CrossRef] 21. Baker, N. ZigBee and Bluetooth strengths and weaknesses for industrial applications. J. Comput. Control Eng. 2005, 16, 20–25. [CrossRef] 22. Ferrari, P.; Flammini, A.; Marioli, D.; Rinaldi, S.; Sisinni, E. On the implementation and performance assessment of a wirelessHART distributed packet analyzer. IEEE Trans. Instrum. Meas. 2010, 59, 1342–1352. [CrossRef] 23. Petersen, S.; Carlsen, S. WirelessHART versus ISA100. 11a: The format war hits the factory floor. IEEE Ind. Electron. Mag. 2011, 5, 23–34. [CrossRef] 24. Song, H.; Mok, A.K.; Chen, D.; Lucas, M.; Nixon, M. WirelessHART: Applying wireless technology in real-time industrial process control. In Proceeding of IEEE Real-Time and Embedded Technology and Applications Symposium, St. Louis, MO, USA, 22–24 April 2008; pp. 377–386. 25. Wang, H.; Hou, W.; Fei, M. Polling-based protocol gateway for the Integration of hybrid wired/wireless industrial control networks. In Proceedings of the 29th Chinese Control Conference, Beijing, China, 29–31 July 2010; pp. 5772–5777. 26. Mondal, S.A.; Luqman, F.B. Improving TCP performance over wired–wireless networks. Comput. Netw. 2007, 51, 3799–3811. [CrossRef] 27. Jiang, D.; Fei, M.; Wang, H.; Li, T. Wireless network performance test in hybrid wired/wireless network system. In Proceedings of the 8th World Congress on Intelligent Control and Automation, Taipei, Taiwan, 21–25 June 2011; pp. 1029–1034. 28. Rutagangibwa, V.; Krishnamurthy, B. A Survey on the Implementation of Real Time Systems for Industrial Automation Applications. Int. J. Innov. Res. Sci. Technol. 2015, 1, 174–177. 29. Wu, L.; Xu, S.; Jiang, D. MFAHP: A novel method on the performance evaluation of the industrial wireless networked control system. In Proceedings of the Sixth International Conference on Electronics and Information Engineering, Dalian, China, 26–28 September 2015. 30. Bertocco, M.; Narduzzi, C.; Tramarin, F. Estimation of the delay of network devices in hybrid wired/wireless real-time industrial communication systems. In Proceedings of the 2012 IEEE International Instrumentation and Measurement Technology Conference (I2MTC), Graz, Austria, 13–16 May 2012. 31. Ma, H.; Fei, M.; Yang, Z.; Wang, H. Wireless networked learning control system based on Kalman filter and biogeography-based optimization method. Trans. Inst. Meas. Control 2013.[CrossRef]

© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).