Using Ethernet technology to optimise PLC networks

Traditional fieldbus networks were - and some still are - isolated automation systems that required extensive local monitoring and routine on-site maintenance. To increase productivity and reduce operating costs, many manufacturers and plant operators have deployed Industrial Ethernet to converge remotely isolated fieldbus systems for centralised control and monitoring.

The term automation was introduced in 1946¹, later published in a magazine in 1948 and defined as “the art of applying mechanical devices to…perform these tasks in timed sequence…under pushbutton control at strategic stations”. What engineers had in mind in the 1940s remains fundamentally the same as what control engineers intend to achieve today with automation: minimising human involvement while maximising output capacity, increasing production speed and ensuring consistent repeatability. Until recently, automation systems were isolated and generally consisted of I/O devices connected directly to PLCs, which in turn connected to SCADA systems in local control rooms. Today, a major difference exists in the architecture of modern industrial automation networks - in the form of a technology that is increasingly being deployed to connect large-scale distributed systems with remote monitoring and control centres. That technology is Industrial Ethernet.

Industrial Ethernet technology consists of various Ethernet-based protocols, developed with deterministic capabilities, as robust alternatives to costly proprietary automation systems. More importantly, they enable a highly scalable and unified infrastructure to converge all network nodes at the administrative, control and field levels. Many advanced industrial automation systems utilise gateways to efficiently bridge fieldbus I/O devices to Ethernet-based PLCs, and Ethernet switches to connect PLCs and SCADA terminals at remote control centres, reducing operating costs and improving production efficiency by expanding automation networks.

According to recent studies conducted by IMS Research, three of the most pervasive Ethernet-based protocols, EtherNet/IP, Profinet and Modbus TCP, account for more than 65% of the world’s existing industrial automation deployments, and their collective growth from 2010 to 2015 is expected to exceed 18 million new nodes.

Figure 1: Worldwide market share by protocol. (Source: IMC Research 2011)

However, the worldwide number of existing fieldbus devices (all protocols) will increase from 183 million in 2010 to over 326 million by 2015. This steady growth, despite the increasing popularity of Industrial Ethernet, is primarily due to the fact that fieldbuses are still considered to be the easier, and more cost-effective, protocols to deploy at field-level networks than Industrial Ethernet. Another contributing factor is that there are exponentially more nodes in field-level networks than nodes in control- or supervisory-level networks. As growing industrial automation networks expand to increase productivity, and factory systems converge for centralised control, the integration of various Ethernet-based protocols and bridging of fieldbus systems will be inevitable.

An Ethernet-based single-network infrastructure offers operators the efficiency of centralised network management with greater network scalability and flexibility, higher bandwidth availability and faster failure recovery. To optimise system performance and improve network manageability, switches and gateways should integrate seamlessly with industrial automation networks for centralised control and monitoring. However, integrating industrial fieldbus with Industrial Ethernet presents integrators and engineers with two considerable challenges:

• Centralised monitoring of all network nodes, including switches, should be available on control and SCADA systems. However, standard Industrial Ethernet switches do not process industrial automation protocols and therefore are unable to be monitored on the same control system as the PLC and other I/O devices.
• Existing fieldbus devices can account for a substantial portion of a company’s assets, and their integration with Industrial Ethernet networks can be done via PLC modules. However, for large-scale control systems, deploying Industrial Ethernet gateways would be a more cost-effective solution to integrate fieldbus devices, but a substantial amount of time can be required to manually configure the switches and gateways.

Many manufacturers offer switches and gateways to integrate fieldbus-to-Ethernet communication for automation systems consolidation, with emphasis on device features and system interoperability between components and PLCs. But these are just the basic requirements of systems convergence. Most industrial switches and gateways available today are designed only with automation-centric perspectives. The optimisation of the PLC network, such as overall performance, configuration/management efficiency and application flexibility, seems to have been overlooked.

Components for PLC network optimisation

As device networks expand into larger control networks through fieldbus-to-Ethernet integration, network deployment and management become more complex, and network performance becomes highly critical. To optimise PLC network performance and management, Industrial Ethernet switches and gateways should be designed with a network-centric approach to enhance overall performance, improve configuration efficiency, simplify network management and provide application flexibility.

High performance ensures high network capability

Performance optimisation of industrial automation networks is critical for maximum productivity and reliability. High bandwidth, high data transmission rates, switch ASICs and hardware features are indispensable components of high network performance. However, these factors are useless if the network is unavailable. Maintaining high network availability not only requires reliable network devices, but also speedy recovery of the network and its components during maintenance or in the event of a connection or device failure.

Industrial Ethernet switches

High-speed redundancy

Redundancy is a must-have component to ensure industrial automation reliability. Prolonged periods of unplanned system downtime can jeopardise on-site personnel safety, severely impact plant productivity and possibly damage expensive factory equipment and machinery. Recent refinements in redundancy technologies not only provide millisecond-level network recovery, they can also substantially reduce the costs of deployment.

Faster-than-PLC power cycling

Power cycling of switches and PLCs is generally required for plant-wide maintenance updates. Most managed switches can take approximately three minutes, if not more, to reboot because they have relatively low processing power. PLCs, on the other hand, operate by using simple ladder logic, and power cycling normally takes less than one minute. Administrative-level nodes (such as SCADA and HMI) will have no access to control-level and device-level nodes until switches have completed POST (power-on self-test) diagnostics and the bootstrap loading process. The difference between the duration of power cycling times of PLCs and switches will not only delay maintenance efforts, it can even cause port lockouts in some older time-sensitive automation systems. Advanced managed switches are now capable of completing a power cycle in the 10-second range.

Industrial Ethernet gateways

Processing power

Typical gateways have access to 16-bit processing power, which limits the gateway to eight simultaneous connections and delays response times. Newly developed 32-bit Industrial Ethernet gateways can handle up to 16 concurrent connections (from SCADA/HMI/PLC) without compromising response time or transmission reliability.

Data prioritisation

Most switches already offer packet prioritisation between administrative-level (SCADA/HMI) and control-level (PLC) devices. Some Ethernet gateways are now also capable of providing device-level (fieldbus I/O) data prioritisation to enhance QoS (quality of service) and improve determinism.

The core of industrial automation consists of SCADA, PLC and I/O devices. Switches and gateways enable fieldbus-to-Ethernet convergence by providing network communication between these core devices at field, control and supervisory-level networks. For performance optimisation of large-scale industrial automation networks, it is imperative that each network component operates with timely efficiency.

Figure 2: Ethernet at the control level.

Advanced efficiency simplifies configuration and monitoring

Efficiency of switch and gateway configuration during fieldbus-to-Ethernet integration has previously been reliant on the expertise of system integrators. Gateway configuration can be extremely time-consuming as it requires manual input and is verified through trial and error. In addition, the isochronous nature of hard real-time industrial automation systems does not tolerate delays and requires precise calibration of critical parameters, such as gateway response time-out settings, to prevent system errors. Furthermore, optimising PLC network management requires tools for network configuration and monitoring, not only to ensure reliable operation and immediate event notification, but also to provide effortless maintenance updates to minimise system downtime.

Industrial Ethernet switches

Plug-and-play simplicity

Switches deployed on the factory network should also be enabled with Industrial Ethernet protocols such as Profinet, Modbus TCP, and EtherNet/IP. This allows switches to be integrated into the SCADA/HMI of industrial automation systems for optimised consolidation of network configuration, management and control without the need for additional configuration or modifications.

Device parameters and monitoring

In addition to relay output warnings for on-site network performance monitoring, switch parameters, such as power, port and redundancy statuses, should also be accessible by SCADA systems. Switch visibility in SCADA systems is vital to maintain healthy PLC networks, and access to device parameter allows remote monitoring of switch components, providing operators complete control-level network information.

Industrial Ethernet gateways

Intelligent auto-detection

The latest innovations in Ethernet gateways can eliminate the tedious tasks of fieldbus parameter input and device configurations, which, even for experienced systems integrators, can be extremely complex and time-consuming. Recent gateway developments enable fieldbus-to-Ethernet integration in a fraction of the time by automatically detecting Profibus I/O modules and Modbus device settings to eliminate the possibility of manual input errors.

Monitoring and diagnostic tools

When network communication errors halt production lines on the factory floor, control engineers must quickly locate and resolve the root cause. When problems occur, gateways should be equipped with relay output warnings to immediately trigger an alarm at the control room or a shutoff mechanism on the factory segment to minimise the impact of a system failure. Many intelligent gateways also offer software diagnostic tools to assist control engineers in troubleshooting. When communication errors are determined to be the root cause of a system failure, engineers can quickly access gateway data transmission logs and filter out erroneous fieldbus frames for review and analysis.

Manual input of I/O modules and device time-out settings can take many hours, if not days, to complete. More intelligent switches and gateways can assist system integrators and control engineers to effortlessly deploy, configure and update industrial automation segments within the network to quickly enable production on the factory floor.

Maximum flexibility to converge various applications

Industrial automation network infrastructures can span across multiple plants in multiple remote locations, where various applications are performed in various industrial environments. Switch and gateway flexibility allows operators to optimise PLC network deployments with devices that are best suited for applications in industries such as chemical plants, wastewater treatment and oil refineries.

Industrial automation networks will require different types of gateways and switches for different applications. These applications can require different port densities and various combinations of port types, such as fibre, PoE, SFP ports with Fast Ethernet, Gigabit or 10GbE connectivity. Furthermore, industrial automation applications, whether indoors or outdoors, can present some of the most challenging conditions, pushing devices to their limits. Environmental factors such as extreme temperatures, vibration, shock, corrosion, dust, moisture, humidity, power surges and EMI will require the deployment of industrial-grade devices with industry-specific certifications and compliance. Listed below are a few types of application-specific requirements.

• Wide-temperature operation is necessary for many outdoor environments, where temperatures can dip well below -30°C at night and rise to well over 70°C in the daytime.
• Gigabit PoE can be deployed to combine control-level communication with video surveillance of the factory floor, eliminating the need for external power cabling.
• Fibre is a media commonly used to provide long-distance communication between remote networks. Also, EMI immunity makes fibre the choice for many power industry applications.
• 10GbE and layer 3 switching are required in control rooms with large-scale surveillance systems to provide the data rates required for HD-quality video transmission.
• M12 connectors are required for severe outdoor applications, such as mining and transportation industries, where intense vibration and dust can compromise network communication.
• Ingress protection (IP) is important for application environments where contaminants and high humidity can cause device malfunction and even failures.

Many vendors offer a selection of switches and gateways specifically designed for a targeted application, without much consideration of how these devices can impact the automation network as a whole. Consequently, operators can be left with a multivendor network that does not work as a cohesive whole and can severely impact overall network performance and manageability. Converging various systems and applications within the PLC network requires a wide spectrum of switches and gateways, not only designed and calibrated specifically for each application to provide maximum deployment flexibility, but also to optimise the configuration, performance and reliability of PLC networks as a whole.

References

1. Pursell C, 2008, A Companion to American Technology, Blackwell Publishing