Patent Publication Number: US-2006007927-A1

Title: Transmission frame structure for control communication network of distributed control system for nuclear power plant

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method for constructing a transmission frame of a control communication network of a distributed control system for use in a nuclear power plant (hereinafter referred to as a nuclear-power-plant distributed control system), which can efficiently use the control communication network of the nuclear-power-plant distributed control system in which real-time characteristics and reliability are the most highly regarded. More particularly, the present invention relates to a transmission frame structure for use in a control communication network, which allows all process control stations contained in the control communication network to share monitoring/control information received from a field communication network or an information communication network, and properly copes with faulty operations of channels (i.e., ring-shaped lines) or process control stations for use in the control communication network.  
      2. Description of the Related Art  
      Typically, a distributed control system can efficiently control a plurality of processors in power plants or the petrochemical industry, and can also allow the processors to share data with each other in the power plants or petroleum-chemical industries. A communication network for use in the nuclear-power-plant distributed control system includes an information communication network, a control communication network, and a field communication network. The information communication network interconnects a plurality of stations distributed in a wide area, such that it can allow the plurality of stations to share a variety of information with each other. The control communication network acts as a transmission medium to allow a plurality of field control devices to share data with each other.  
      The field communication network allows the control communication network to directly communicate with field control devices. In the case of controlling a plurality of systems distributed into a power plant field, the control communication network enables data communication between distributed control devices and other devices, and also allows the distributed control device to share data with the other devices, such that the control communication network is generally considered to be a very important component. The control communication network performs a communication function using a 100M Ethernet. A plurality of process control stations are connected to the control communication network, and are connected to the field communication network via the above-mentioned process control stations.  
      Industrial control communication networks include a common field bus communication network, an IEEE 802.3 (CSMA/CD; Ethernet) communication network, an IEEE 802.4 (Token Bus) communication network, an IEEE 802.5 (Token Ring) communication network, and other communication networks using unique protocols developed by individual production companies, etc. Although the field bus is used to perform a control operation, it should be noted that the field bus is generally used as a low-level communication network rather than a high-level communication network. The Ethernet controls a plurality of stations contained to competitively access the communication network, such that it is unable to satisfy real-time characteristics and reliability needed to control a power plant. The token-bus communication scheme and the token-ring communication scheme allow individual stations to sequentially transmit data to solve unreliability of the Ethernet, such that they can be used for real-time data transmission. However, the present invention exemplarily discloses a specific case in which a conventional 100M Ethernet card is used, such that an Ethernet transmission frame will hereinafter be described.  FIG. 1  shows an Ethernet transmission frame structure in which a CSMA/CD access method from among MAC (Medium Access Control) protocols based on IEEE 802 series indicative of the most popular LAN (Local Area Network) is used.  
      As shown in  FIG. 1 , a preamble  100  is a first area of an 802.3 frame, and includes 7 bytes in which 0 and 1 are repeated, such that it informs a reception system of frame arrival. A Start Frame Delimiter (SFD) area  101  acts as an indicator for designating the beginning of the frame, and includes only 1 byte. A Destination Address (DA) area  102  is assigned the next destination address having 6 bytes. A Source Address (SA) area  103  is indicative of an address of a source indicative of a packet transmission object, and includes 6 bytes. A Length PDU area  104  is indicative of a length of data to be received soon, and includes 2 bytes.  
      A Data and Padding area  105  has a frame length having 46˜1500 bytes, includes an 802.2 (Local Link Control) frame. A field of the Data and Padding area  105  is indicative of a unit which can be configured in the form of a module and can also be removed. Therefore, if a large amount of communication data is generated, a network access method for use in the Ethernet may suffer data collision or transmission failure problems due to the occurrences of unexpected time delay and competition.  
      A field bus from among industrial control communication protocols uses different protocols according to system application objects, and is generally designed to properly control field devices, such that it is improper to be used as a control communication protocol of a nuclear-power-plant distributed control system acting as a large-sized process control system. Also, a plurality of stations contained in the communication network competitively access the communication network over the Ethernet, such that the Ethernet is unable to satisfy real-time characteristics and reliability required to control the power plant. Also, it is difficult to apply an application algorithm, which can permit a fault associated with a communication system of the nuclear-power-plant distributed control system, and can also manage a communication network, to the Ethernet. Particularly, the Ethernet is unable to properly cope with a line failure or station failure of the communication system.  
      In this manner, the conventional Ethernet 802.3 communication scheme controls a plurality of stations contained in a communication network to competitively access the communication network, such that it cannot satisfy real-time characteristics and reliability required to control a large-capacity nuclear-power-plant distributed control system. Although the IEEE 802.4 (Token Bus) communication scheme and the IEEE 802.5 (Token Ring) communication scheme are widely used to implement FA (Factory Automation) and process control, they cannot satisfy real-time characteristics requested by a communication system for use in a nuclear-power-plant distributed control system, cannot perform defect permission and duplexing operations capable of flexibly operating a communication network, and cannot accommodate an application algorithm capable of resolving a malfunction.  
      Therefore, the present invention provides a method for constructing a transmission frame when operating a large-capacity nuclear-power-plant distributed control system to which a maximum of 64 stations are accessible, such that the transmission frame structure controls the distributed control system to receive update information from a field control device, and allows all stations contained in a control communication network to share data in real time, and controls an erroneous control communication system to be normally operated by conducting fault permission of the control communication system.  
     SUMMARY OF THE INVENTION  
      Therefore, the present invention has been made in view of the above problems, and it is an object of the invention to provide a transmission frame structure for use in a large-capacity nuclear-power-plant distributed control system using a 100M Ethernet-based communication network acting as a control communication network, such that the transmission frame structure receives monitoring/control information from a field communication network or information communication network over the 100M Ethernet communication network, controls all process control stations contained in the control communication network to share data in real time, and properly copes with faulty operations of a control communication system node or the communication network.  
      It is another object of the present invention to provide a transmission frame structure for additionally adding a network management field acting as a transmission frame core field to a frame field, in which the network management field can manage token scheduling information for pre-designating a station to which data transmission authority will be assigned, and can isolate a faulty communication line from a normal communication line.  
      In accordance with the present invention, the above and other objects can be accomplished by the provision of a transmission frame structure of a control communication network for use in a nuclear-power-plant distributed control system which broadcasts data received from a node having transmission authority to all nodes via a bypass line, and allows a ring accelerator to pass the data by a roundabout way and to isolate an erroneous station from normal stations, comprising: a transmission frame. The transmission frame includes: a destination address for performing the broadcasting operation; a source address for recording a source node address (ID) therein; a type/length field for classifying frames into a control data frame and a network management event frame; a network management (NM_TYPE) field which is valid only when it is designated by type/length field, and performs different roles according to network management event frame types; a Seq&amp;Ver field for including the number of transmissions of a data frame and frame upgrade version information; a NS_ID field for recording number information of a node equal to the next token reception node, and being used when one station transmits a token to the next station; a data field having a predetermined maximum size of 1 kbyte, for including not only general control information according to a value of the type/length field, but also 7 event frames (i.e., NM_TOKEN_FRAME, NM_DUAL_CH_CHECK_FRAME, NM_DUAL_CH_READY_FRAME, NM_NM_LINK_CHECK_FRAME, NM_NM_NODE_FAIL_FRAME, NM_NODE_CHECK_FRAME, and NM_TX_MODE_FRAME frames) such as a token frame; and a CRC (Cyclic Redundancy Code) field for inspecting the presence or absence of a CRC error, whereby the transmission frame operates the communication network, solves a malfunction or error of the communication network, and recovers the communication network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above objects, and other features and advantages of the present invention will become more apparent after reading the following detailed description when taken in conjunction with the drawings, in which:  
       FIG. 1  shows an Ethernet transmission frame structure;  
       FIG. 2  shows a control communication network structure and a data transmission structure;  
       FIG. 3  shows a structural diagram illustrating an apparatus for improving topology of a control communication network;  
       FIG. 4  shows a control communication network management algorithm and a state transition diagram according to the present invention; and  
       FIG. 5  shows a transmission frame structure for controlling a nuclear-power-plant distributed control system according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.  
      The present invention relates to a method for constructing a transmission frame of a control communication network of a nuclear-power-plant distributed control system, which can efficiently use the control communication network of the nuclear-power-plant distributed control system in which real-time characteristics and reliability are the most highly regarded. The transmission frame structure allows all process control stations contained in the control communication network to share monitoring/control information received from a field communication network or an information communication network, and properly copes with faulty operations of channels (i.e., ring-shaped lines) or process control stations for use in the control communication network.  
       FIG. 2  shows an exemplary control communication network for use in a nuclear-power-plant distributed control system. Although the control communication network is indicative of a ring-shaped communication network as shown in  FIG. 2 , it is substantially considered to be a bus-shaped communication network. Therefore, if a node  204  having transmission authority transmits data, the data is broadcast to all nodes  201 ,  202 ,  203 , and  204  over a bypass line. Each of the nodes  201 ˜ 204  includes a transmitter T, a receiver R, and a buffer. A ring acceleration unit  200  selectively performs data transmission or data broadcasting. The data successively moves counterclockwise from a node (i) having transmission authority to other nodes i+1 and i+2.  
      The ring acceleration unit  120  controls transmission data to pass by a roundabout way, and is also used to isolate a faulty station from a normal station as can be seen from  FIG. 3 . A medium control scheme for use in the inventive communication network is established by tokens. A predetermined manager node  201  generates an initial token or manages the flow of tokens.  
      The manager node  201  inspects the flow of tokens and data of an overall communication network, such that it discriminates a specific node  204  ( i ) having current transmission authority from among a plurality of nodes, and monitors whether individual nodes preferably perform data transmission/reception. In order to resolve a malfunction of the manager node  201 , the next node  202  acting as a manager backup node periodically monitors soundness of the manager node  201 . If a malfunction or error occurs in the manager node  201 , the manager backup node  202  detects the malfunction or error, takes manager authority from the manager node  201 , and isolates the erroneous manager node  211  from a communication network, such that it can normally manage the overall communication network.  
       FIG. 3  is a structural diagram illustrating a ring acceleration unit  200  shown in  FIG. 2 . As shown in  FIG. 3 , a transmission line is denoted by TxN and TxP, and a reception line is denoted by RxN and RxP. If a controller  300  determines a reception mode  320  or a transmission mode  330 , a switch is operated according to the determined mode, such that the reception mode  320  receives data and at the same time transmits reception data to the next node via a bypass line connected to an analog switch  312 , and the transmission mode  330  switches off the analog switch  312  and at the same time transmits data to be transmitted to a neighboring node. The controller  300  is implemented with software, and can transmit a control signal to the control signal line  301  to perform a switch operation between the reception mode  320  and the transmission mode  330 .  
      Two analog switches  311  and  312  contained in an analog switching unit  310  are properly operated according to state information of the controller  300 . In the case of the reception mode  320 , the analog switch  312  is switched on and the other analog switch  311  is switched off when the controller  300  selects the reception mode  320 , input data is applied to the reception mode  320 , and at the same time is transmitted to a neighboring node via the analog switch  312 . In the case of the transmission mode  330 , the analog switch  312  is switched off and the other analog switch  311  is switched on when the controller  300  selects the transmission mode  330 , such that data to be transmitted can be transmitted to a neighboring node.  
       FIG. 4  shows a data transmission state transition diagram  400  of a control communication network manager node and a malfunction solution algorithm 410 performed by the manager node when faulty operations of lines or stations occur in a communication system. If all stations in a reception mode  401  receive tokens, they perform data transmission  402 . If data transmission  402  is completed, all stations perform token transmission, and return to the reception mode  401 .  
      If timer interruption occurs due to a malfunction of the communication system, a channel soundness inspection frame is transmitted as denoted by  411 , and determines the presence or absence of a channel (i.e., a line) error. If the channel error occurs, a channel switching frame is transmitted as denoted by  412 , such that it performs channel switching. If a station (i.e., a node) error occurs, an erroneous station is inspected as denoted by  413 , a frame of the inspected erroneous station is transmitted to all stations as denoted by  414 , token transmission is performed, and the process returns to the reception mode  401 . Also, if all stations receive information of an erroneous station from a manager station during the reception mode  401 , they update an address of the erroneous station as denoted by  415 , and do not transmit data to the erroneous station.  
       FIG. 5  shows a transmission frame structure for use in a data link layer capable of performing data communication over a control communication network. A token passing method for controlling a token of a ring-shaped transmission line to circulate in all nodes according to a predetermined order is generally used as a communication scheme of the control communication network. Provided individual stations of the control communication network occupy a token, this indicates that each station has transmission authority. If a node having a token finishes data transmission, it gives the next node the token, such that the next node can perform data transmission.  
      An algorithm for operating a communication network, solving a malfunction or error of the communication network, and recovering the communication network is performed by the transmission frame shown in  FIG. 5 . A destination address  501  (i.e., a destination node address) having 6 bytes is used to perform a broadcasting operation. A source address  502  (i.e., a source node address) having 6 bytes is used to record an address ID of a transmission node. A type/length field having 2 bytes is used to discriminate a frame type. There are two kinds of frames, i.e., a data frame for a control operation and an event frame for network management. Different values are assigned to the type/length field  503  for use in the data frame and the other type/length field  503  for use in each of seven event frames (See the following Table 1) having token transmission.  
      If the type/length field  503  designates the event frame, the network management field (NM_TYPE)  506  must designate one event frame from among seven event frames. In more detail, the network management field  506  having 1 byte is valid only when the type/length field  503  designates the network management field  506  itself, and its role is determined according to NM_TYPE information shown in Table 1.  
               TABLE 1                          Control communication network event frames                                 Network management event frames               Numbers   (NM_TYPE)   Usages   Remarks               601   NM_TOKEN_FRAME   Token   Transmission authority                   possession       602   NM_DUAL_CH_CHECK_FRAME   Inspect backup channel   Channel duplexing       603   NM_DUAL_CH_READY_FRAME   Prepare backup channel   Channel duplexing               Switching       604   NM_NM_LINK_CHECK_FRAME   Inspect Communication   Periodic inspection               network link       605   NM_NM_NODE_FAIL_FRAME   Notify node (station)   Erroneous node               Malfunction   Information       606   NM_NODE_CHECK_FRAME   Inspect node (station)   Confirm Erroneous                   node recovery       607   NM_TX_MODE_FRAME   Notify switching to   Isolate erroneous node               transmission mode   and return                  
 
      A Seq&amp;Ver field  504  having 1 byte includes the number of transmissions of a data frame, and also includes frame upgrade version information. A NS_ID field  505  having 1 byte includes number information of a node indicative of the next token reception node, such that it is used when one station transmits a token to the next station.  
      A data field  507  having 43˜1024 bytes has a predetermined maximum size of 1 kbyte, and may include general control information according to a value of the type/length field  503 . Also, the data field  507  may include seven event frames such as a token frame. A CRC (Cyclic Redundancy Code) field  508  having 4 bytes is used to inspect the presence or absence of a CRC error.  
      The Seq&amp;Ver field  504 , the NS_ID field  505 , and the NM_TYPE field  506  contained in the inventive transmission frame are newly proposed to operate a control communication network of a nuclear-power-plant distributed control system according to the present invention. The destination address node  501 , the source address node  502 , the type/length field  503 , the data field  507 , and the CDC field  508  perform functions similar to those in construction fields of the conventional Ethernet transmission frame. However, the data field  507  can perform a data transmission function as in the conventional Ethernet transmission frame, and can include event frame information therein, differently from the conventional Ethernet transmission frame. The inventive transmission frame can accommodate data capacity having a maximum of 1024 (1K) bytes, whereas the Ethernet transmission frame can accommodate data capacity having a maximum of 1500 bytes.  
      As shown in Table 1, the NM_TOKEN_FRAME  601  unit is indicative of a token frame needed to operate a network. A general station receives the NM_TOKEN_FRAME  601 , gains transmission authority, and broadcasts its own data. The NM_DUAL_CH_CHECK_FRAME  602  unit determines whether a link of a channel to be switched is alive when a current channel is in an abnormal state, such that it determines the presence or absence of the channel link soundness. The NM_DUAL_CH_READY_FRAME  603  unit informs general nodes of a preparation state of communication switching to a backup channel, when a main channel is in an abnormal state.  
      The NM_LINK_CHECK FRAME  604  unit periodically determines whether a backup line (i.e., a link) indicative of a communication network backup channel is alive whenever it receives a token predetermined reception times. The NM_NODE_FAIL_FRAME  605  unit transmits new token scheduling information to all nodes when a node is isolated or encounters a malfunction or error, such that it indicates an address of the abnormal node. The NM_NODE_CHECK_FRAME  606  unit periodically determines whether the node isolated by the malfunction or error returns to a normal state. If the node recovers from the malfunction or error, the NM_NODE CHECK_FRAME  606  unit is re-included in the communication network, such that it can be assigned transmission authority.  
      The NM_TX_MODE_FRAME  607  unit automatically operates (i.e., switches on) a bypass line of a topology improvement device in association with the erroneous node, such that it can always maintain a reception (Rx) mode. If the erroneous node returns to a normal state, the NM_TX_MODE_FRAME  607  unit does not maintain the reception (Rx) mode any more, such that it can enable the erroneous node to perform data transmission (Tx).  
      The present invention provides a unique transmission frame without using transmission frames for use in conventional communication networks (i.e., an Ethernet communication network, a token bus communication network, and a token ring communication network), and applies the transmission frame to a communication system. The transmission frame according to the present invention is characterized in that a network management field, which manages token scheduling information capable of pre-designating a station to which data transmission authority will be assigned and isolates a faulty communication line from a normal communication line, is additionally provided, and is then added to a frame field.  
      In other words, the transmission frame according to the present invention can perform data and token transmission using only one frame structure, whereas a token ring or an Fiber Distributed Data Interface (FDDI) includes a data frame and a token frame separated from each other. The network management field includes a total of seven event frames. A token frame is one of the event frames, and is included in the network management field.  
      As apparent from the above description, a method for constructing a transmission frame according to the present invention can properly cope with faulty operations of lines or stations of a communication system incapable of being included in IEEE 802-series communication networks, receives update information from a field control device when operating a nuclear-power-plant distributed control system to which a maximum of 64 stations are accessible, quickly transmits the received update information to all stations contained in a control communication network, and controls the system in real time, resulting in increased stability and reliability of the system.  
      Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.