7.1.1 Distributed Control System
The vessel management system will manage the functions of control, monitoring and alarm management of vital machinery required to control the vessel including engine and propulsion auxiliary systems, fluid and cargo systems and other ancillary systems. A modern vessel management system will utilise a distributed control system to perform these functions.
As the name implies a distributed control system does not concentrate control of all processes at a central location, rather it distributes the control of tasks to different nodes (PLCs or RCUs). These nodes will normally be housed close to the plant they are controlling, thereby cutting down on wiring. Suitable communication networks will link all aspects of the system together.
DCS architecture generally consists of one or more HMI, PLCs or RCUs, I/O modules and a data communication highway linking the PLCs to the HMI and vice versa. The I/O modules within the controller nodes will use appropriate communication for the plant they are controlling. This could be analogue or digital I/O as required. Figure 109 shows a basic DCS with A60 compartment separation to allow some continued operation following the loss of any compartment to fire or flood etc.
RCU or PLC
Figure 109 – Basic distributed control system
In reality it can be argued that current systems are a SCADA/DCS hybrid, in as much as the software within the HMIs will monitor aspects of the plant, alarm when an event is detected and carry out some supervisory action such as starting a standby pump.
This is the basis of a SCADA system where data gathering is the prime objective and actions are event driven. A DCS is usually accepted as being process driven, meaning that all tasks allocated to a node are normally run sequentially, usually within a ladder programme of a PLC, and mismatch alarms etc. are not generated until the process looking at that particular I/O is run.
7.1.2 Industrial Networks
Industrial networks is a general term given to any type of communication connecting devices for process control applications. Industrial networks can be divided into several types:
control networks;
fieldbus networks;
sensor networks.
Control network: Sometimes called the process network, this is the high level network used to transmit instructions and data between the HMI and the PLCs which control the plant. Different technologies and protocols have been used over the years but the preferred solution used by most companies at this time is Ethernet/IP, where the IP stands for industrial protocol.
It has been argued that Ethernet/IP is a non-deterministic protocol and when the system is using network hubs, half duplex, CSMA/CD architecture this is true to some extent, depending on overall bandwidth usage. However with newer installations, where data switches are used in place of hubs, and full duplex 100M networks are in use, the network can be considered deterministic for all practical purposes. The use of switches also allows additional quality of standard (QoS) rules to be implemented, i.e. the switches can assign higher priorities to specific traffic.
Fieldbus network: The origin of the fieldbus was to replace hard wired connections between complicated field devices and their controllers by a single digital link where all data could be transmitted in a serial string. To accommodate information from several sources the data is time multiplexed.
PSU CPU Anlogue Input Digital Input
Anlogue Output Digital Input START / STOP / RESET
ALARM DATA RUNNING
VOLTAGE / CURRENT /FREQUENCY BAUD RATE
PLC / RTU MOTOR CONTROL CENTER
PROFIBUS SERIAL LINK
Figure 110 – Fieldbus communications using Profibus DP protocol
With all devices or equipment that support the Profibus DP protocol, a general system data (GSD) file (text file) should be supplied by the manufacturer. This GSD file contains general information about the device, type of device, supported baud rates, a summary of functions supported within the protocol and all cyclic data (data that is exchanged every bus scan with the master).
Figure 110 shows how this operates in practice. The PLC will have the MCC as one of the devices it must interrogate and control during its scan. Data recovered from the MCC will be repackaged and sent to the HMI on the control network.
Sensor network: This is the most basic network and works by detecting the status of a sensor and transmitting this information as a discrete 1 or a 0 to the PLC. This might be the position of a valve (open or closed) or the state of switch.
7.1.3 Network Topologies
With different types of networks as described above there are different network topologies.
Historically, these were designated as star, bus and ring, however as technology has progressed these have changed somewhat from their original design and a more correct name for each one is now:
physical star logical bus;
physical bus logical ring;
physical ring logical ring.
Physical star logical bus topology: In its simplest form a star network consists of a single device connected to each node by a separate cable, as shown in Figure 111. In earlier fieldbus iterations this would have been all field devices possibly connecting to a single main
frame computer in the control room. All control functions would be sent to the field devices using point-to-point hardwired connections. This was a true star topology as all data would flow back through the mainframe. The system was expensive due to the amount of wiring involved, complex to configure and difficult to maintain. In addition the lack of effective standards led to expensive upgrades when new technology was introduced.
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Figure 111 – Star topology
With the introduction of distributed control and agreements on standards for industrial data communications new models were required. Figure 112 shows a small network with three switches and twelve nodes. Although each switch may physically look as though it is the centre of a star, internally each switch has a linear bus as a backbone, making this a physical star logical bus topology.
Switch 1 Switch 2 Switch 3 12
Figure 112 – Physical star logical bus
Some of the main advantages of this network are scalability with no disruptions to the network when adding or removing devices and easy fault detection. In addition, the failure of a node will have no effect on the rest of the network. With the use of switches and 100M full duplex communication a node communicates with any other node using point-to-point communication. Each node believes it has total use of the bandwidth, so no collisions occur and retransmitting of data is not required.
In addition to contributing to the advantages of this topology, the switches can be seen as the main disadvantage. Failure of a switch will mean any nodes connected to it will be unreachable. As discussed above each node believes it has total use of the bandwidth, therefore although the bandwidth used between nodes in point-to-point communication may be low, if several nodes are communicating simultaneously the cabling between the switches may become a bottleneck. This is usually overcome by using fibre optic cabling between switches connected to Gigabit uplink modules within the switches. As modern switches normally have a Gigabit backbone this effectively means the network can accommodate 10 x 100M individual networks.
The disadvantages discussed above are overcome by utilising a dual network, nominally called net A and net B, Figure 113 illustrates this configuration. The two networks are completely independent with separate switches, separate cabling and dual isolated network adapters within the network nodes. Separately sourced power supplies are also provided for switches on the different networks. All data is transmitted on both networks simultaneously with the
same time stamp. Only one message is processed and the other one is dropped with no further action being carried out.
This dual redundant topology is ideal for Ethernet connections and is used extensively for control networks, i.e. between the HMIs and the RCUs etc.
Switch 1
Figure 113 – Dual redundant star topology
Physical bus logical ring topology: In this topology all nodes are connected to a shared backbone using multi-drop lines connected to medium access units (MAU) as shown in Figure 114. The MAU devices ensure the connected node is ready for network traffic before it is allowed on to the bus. Data placed on the bus is propagated to all operating nodes and is therefore sometimes described as a broadcast system. A terminating load (resistor) is fitted at each end. This is required to optimise signal quality and prevent signal reflection.
Incorrect termination can make a bus unusable especially at high transmission speeds.
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Figure 114 – Bus topology
This topology greatly reduces cabling costs when compared to a star network. It is easy to implement and fault finding is relatively easy. Failure of a node should have minimal effect on the rest of the network.
There are several disadvantages of this basic system, the main one being that as a single backbone is used only one data packet can be transmitted at any time. All transmissions should therefore be carefully controlled by some arbitration system. This explains why most industrial bus systems use some type of token passing system.
A basic token passing system would consist of a data object that is passed around the nodes in a ring. When a node receives the token it has a finite time (token hold time) to take control of the bus, to either send data or receive data. It will send any high priority packets first and if any time is left will send any low priority packets. Once the token hold time has expired the next lowest addressed node receives the token and the cycle continues.
The token passing protocol is easy to implement in software, in addition when adding new nodes to the network the ring is reinitialised and the target token rotation time (time for a complete cycle around the bus) is recomputed.
The token passing bus is not a physical bus but a logical ring, in other words once the token has been passed to node 12 on our diagram it is then handed to node 1, thereby completing the ring. Figure 115 illustrates this setup.
Physical Bus Logical Ring
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3 2
1 4
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12 10 9 8 7
Figure 115 – Physical bus logical ring
Physical ring topology: Figure 116 shows a basic ring network where each node connects to the nodes directly adjacent to it on both sides forming a single continuous route around the ring. Data travels in one direction with each node regenerating the network packets and forwarding them to the next node. A token passing system is used as discussed above, except that the token is passed to the next physical node as opposed to logical node, as seen in the physical bus logical ring topology. If any node is not switched on, electronics within the MAUs ensure data is still forwarded to the next node.
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3 2
1 4
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12 10 9 8 7
MAU
Figure 116 – Physical ring topology
Advantages of this arrangement are that data transmission is structured with every node having access to the token for a specific time. Large networks can be configured without complex mapping being required.
Historically, logical and physical ring token passing networks performed better than star networks at heavy network loads due to the complexities and time delays introduced by the retransmissions of data caused by the CSMA/CD aspect of a star network. However this advantage has been negated by the use of full duplex point-to-point operations as discussed above. Typical ring transmission rates are 4MHz or 16MHz which means they are considerably slower than a star/bus topology using fast Ethernet which will operate at 100MHz.
Because the basic ring topology is uni-directional a failure of a single cable, multi-drop link or node could cause severe disruption to network traffic. To overcome this problem it is now common practice to install a counter-rotating ring to provide a redundant topology as shown in Figure 117. Until recently this was uneconomic due to the cost (and perhaps weight) of the additional cabling. With the introduction of cheap fibre cabling more vendors are offering this solution as an alternative to the star/bus system, arguing that it is a more deterministic solution.
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2
1 4
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RING 2
RING 1
Figure 117 – Dual ring topology