Communications in and around a production environment is the key to modern automation. Over the last two decades we have seen a series of developments in this field, each allowing productivity to increase by a significant amount. To the outsider, the developments can seem slow and unimportant, but we have now reached a point where all aspects of manufacturing can be integrated into one control system. This includes the actual manufacturing processes, and also stock control, sales order fulfilment, production management information and enterprise management reports.
A critical aspect of achieving this fully vertically and horizontally integrated control system is the speed of data transfer. Industrial Ethernet has stepped up to this plate with a ten-fold increase in communications rates to 1Gbps(gigabit per second) with the release of a new open communication standard by the CC-Link Partners Association (CLPA). Called CC-Link IE (Control and Communication Link Industrial Ethernet), it is the first completely integrated gigabit Ethernet based network for industrial automation. Launched by the CLPA, it defines the new threshold for open standards for Industrial Ethernet.
CC-Link IE combines the best of many existing technologies and applies them to an optical industrial network system with a redundant architecture that enables extremely high-speed and reliable data transfer between field devices and other controllers via Ethernet links. The signalling rate of 1Gbps will redefine users’ expectations and systems capabilities; it being more than enough to cater for the real-time communications requirement of today’s manufacturing industries.
In addition to enabling control data transmission to equipment such as PCs, PLCs, HMIs and motion controllers, CC-Link realises seamless transmission of data between the various communications layers from shop floor to top floor.
It uses cyclic communication as a function that refreshes the control data among all the stations connected to the same network at a constant transmission rate. This makes it independent of load and therefore constant for any given network, so it is predictable and deterministic. Data updates, therefore, do not slow down when data traffic is heavy, such as during major plant operations or emergency actions. In fact data can be sent from any station to any other station (and read by the receiving station), even across interlinked networks. This means that any station can be monitored, programmed, reset and reprogrammed from any other station within the network.
Redundancy is based on a double loop architecture which ensures that the communication will continue even if a cable is cut, because loop-back is done at both sides' node of cut cable.
The duplex ring topology also allows very large networks to be developed. In fact a single network can include up to 66km (42 miles) of fibre optic cabling with no loss in communication speed. As many as 120 stations can be integrated into each network, and 239 of these networks can be directly linked together creating total systems with over 14,000km of cabling, integrating 25,000 or more nodes – enough to solve most users’ problems!
Implementation
Now let us look at some practical aspects of high-speed industrial networking.
The first premise of CC-Link IE is that it provides stable and easy-to-use real time communications for efficient manufacturing. Critically, it allows vertical integration through the various layers of management, collecting data from the shop floor and processing it into information suitable for every aspect of enterprise management. It does this seamlessly so that everyone involved is working on the same systems, with the same data, nothing is lost in translation and there are no time delays.
The CC-Link IE controller network adopts the token passing method for data transmission control. Token passing results in increased communication throughput by eliminating the possibility of data collisions. This provides highly deterministic data exchange and constant link scan time. CC-Link IE controller network can be configured as a single network or as a multi-network system, allowing for or very flexible system configuration.
Each station accesses its own, shared memory, which contains data from all network stations. Shared network memory is refreshed in real-time with data from all stations by cyclic communication.
The control station of CC-Link IE controller network maintains the cyclic transmission between all stations on the network. In the event of a failure of the control station on the CC-Link IE controller network for any reason, another network station can assume the role of the network control station. This ‘floating master’ function enables the cyclic transmission to continue between all stations.
In effect, the system is made up of a number of specialist networks, each dedicated to a given task and each communicating with other networks as required. This configuration means communications times can be minimised and efficiency maintained. Every part of the system is kept informed with the data it requires, but is not bombarded with irrelevant data. Data handling is fast - a network of 64 stations can be updated in about 4ms.
The local network architecture is also a major contributor to the ease of initial system installation, allowing engineers to concentrate on one issue at a time.
Similarly, it also allows easy reconfiguration of installed systems. For instance, if a new machine or line is to be added, its ‘internal’ network can be set up so that it works in standalone mode. Then the information that needs to be imported to, or exported from, the line can be identified and appropriate communication nodes built into the network. Integration with the wider system would simply be a matter of connecting the nodes.
If a part of the network fails or needs to be taken out of service for maintenance, the network configuration will automatically reroute transmissions so that data is still delivered as required.
Determinism
In a complex communications system, data transmissions can be divided into those that are time critical and those that are not. Much of the shop floor data is time critical - machines must work in the correct sequence, often to an accuracy of fractions of a second. However, enterprise management information – the sort of reports scrutinised by directors - is often not so time dependent, a delay of minutes or even hours may be perfectly acceptable.
CC-Link IE is able to distinguish between these two types of information and treat them accordingly. The time critical applications are handled by the cyclic communications, which perform station-to-station exchange of data through the use of shared network memory in the network modules. Non time-critical information is handled by transient communication, which allows peer-to-peer communication between network stations.
Each station accesses its own shared memory which contains data from all network stations. Shared network memory is refreshed in real-time with data from all stations by cyclic communication. This high speed communication enables reduced cycle times and improved manufacturing yields.
This is called determinism and allows time critical information to be delivered virtually instantaneously and in the right order because cyclic communication always takes precedence over transient communication.
Industrial Ethernet, such as CC-Link IE, may seem to offer an answer for every possible problem a production engineer or manufacturing company is ever likely to encounter. To a cynic (and it behoves all engineers to question what they are told, rather than passively accepting it), this may seem too good to be true.
However, it should be realised that CC-Link IE was born out of Japanese manufacturing, and their philosophy is constant improvement towards optimised production. CC-Link IE is not something that was developed in a sterile laboratory or that sprang from the mind of a theory-based academic. It has evolved over two decades of progression in industrial automation, addressing one issue at a time and working – patiently – toward an ultimate goal.
In simple term, two generations worth of development have gone into perfecting CC-Link IE, a continuum that is very difficult to sustain within western business practises.
Steve Jones is general manager, CLPA