Using PCs for machine condition monitoring: Part 5

Networking considerations

11/09/2004



Robert Jackson, National Instruments, Austin, TX

Ethernet-based technologies are migrating to the plant floor. One example of the impact of Ethernet on the plant floor is networked machine condition monitoring (MCM) systems. Networked MCM systems use nodes or computers connected together through a common network to monitor rotating machinery. Each system includes signal conditioning, analog-to-digital conversion (ADC), and embedded processors that perform complex analyses. Ideally, a networked MCM system would use a single development environment that can scale the entire plant enterprise, standard Ethernet components to facilitate networking among devices at the embedded hardware and software layer, and systems across the entire plant.

Traditional machine monitoring systems have used proprietary software and hardware. These closed architectures have resulted in different proprietary communication protocols at the embedded layer and enterprise layer. These proprietary protocols do not support hardware at the embedded layer β€” not to mention support for standard networking across the enterprise.

Industrial Ethernet protocols deliver Ethernet across plant floor
Based on the adoption of Ethernet technologies, industrial protocols and factory automation networks are beginning to embrace standard networking protocols. New industrial Ethernet protocols such as EtherNetI/P and ETHERNET Powerlink (EPL) are emerging, and existing communication busses are also moving towards Ethernet-based implementation.

Emerging Industrial Ethernet Protocols
Protocols such as EtherNet/IP and ETHERNET Powerlink (EPL) use layers 1%%MDASSML%%4 of the 7-layer Open System Interconnection (OSI) model. The first and second layers of the OSI model define the physical connection. Ethernet is the most common implementation of the physical layer. The third and fourth layers define the network and transport protocols. TCP/IP (Transmission Control Protocol/Internet Protocol) has emerged as the most common network and transport protocols.

EtherNetI/P is based on IEEE 802.3 physical and data link layer, and the Ethernet TCP/IP protocols. EtherNetI/P creates a Common Industrial Protocol (CIP) that supports interoperability with DeviceNet, ControlNet, and CAN devices. EPL is based on IEEE 802.3u physical and data link layer and the Ethernet TCP/IP protocols. Because EPL is compatible with the IEEE 802.3u Fast Ethernet, standard Ethernet wiring, such as 100BaseFx, can be used to accommodate distances between nodes of up to 2000 m. Cycle times as low as 200 msec with jitter guaranteed to be below 1 msec are possible.

Existing protocols are migrating to Ethernet
Many popular industrial communication protocols are migrating to support standard Ethernet networks. Modbus was the first protocol to make the transition to Ethernet 5 years ago. EtherNetI/P and recently PROFInet followed Modbus TCP/IP. Modbus TCP/IP uses layers 1%%MDASSML%%4 of the 7 Layer OSI Model and adds Modbus messaging at the Application Layer (Layer 7). PROFInet was developed by the Profibus organization, and offers three levels. PROFInet Open Channel is based on the standard TCP/IP networking protocol and offers 100 msec cycle times. PROFInet real time channel adds PROFIBUS DP to deliver 5%%MDASSML%%10 msec cycle times. Then PROFInet Isochronous Real Time channel uses a custom ASIC to increase speeds to 250 msec.

The clear trend is that industrial communication protocols are migrating to Ethernet. This is a positive trend for plant engineers because they now can choose between powerful easy-to-use PC-based technologies and proprietary systems programmed using closed software.

New types of embedded devices offer standard networking and reliability
A new type of hardware β€” the programmable automation controller (PAC) β€” is emerging. PACs offer the benefits of standard Ethernet technologies and adds support for real-time operating systems. These new PACs are embracing the trend toward standard networking technologies and addressing security and reliability concerns.

An example of this trend is the industry standard PXI (PCI eXtensions for Instrumentation). A PXI based system offers an embedded controller running a real-time operating system. For example, an autonomous MCM system that monitors machine health can communicate over a standard Ethernet network to a PC-based system at the supervisory level. This system can acquire vibration data, perform signal conditioning, and analog-to-digital (A/D) conversion. The embedded processor performs octal analysis, and then streams data such as machine health or alarms and events to the supervisory level.

Software to span the enterprise
Plant engineers need a simple easy-to-use application development environment (ADE) to achieve the full productivity that standard networks are prepared to deliver. With industrial communication protocols embracing standard networking technologies from the device to the enterprise level, a single ADE must also deliver communication across the enterprise. A single ADE is needed to program PLCs, PACs, and embedded devices such as FPGAs. The same ADE should serve as the HMI and SCADA system at the supervisory level, and use standard networking technology to pass data to servers across the enterprise.

Benefits of a networked MCM system
A networked MCM system is a reality today with existing PC-based hardware and software. Some systems based on the PXI industry standard are capable of handling up to 112 channels per system, and multiple systems support expandability to 5000 channels with sampling rates up to 200 kS/sec (kilo-samples per sec).

The same technology also offers multisystem synchronization. Compared to stand-alone systems, a PC-based system for machine monitoring offers expandability, increased sampling rates, a single ADE that spans from the embedded level to enterprise level, and commercial networking technology.

Increasing Ethernet performance
Open architectures based on PC technology have continually improved in performance while reducing costs. So will industrial communication protocols based on Ethernet also benefit from the same increase in speed and performance as PCs? To address this question just look at the history of IEEE protocols, beginning with IEEE 802.3, speeds up to 10 Mbits/sec were possible. With the introduction of IEEE 802.3u, Fast Ethernet speeds improved by 10 times to 100 Mbits/sec.

The latest Ethernet technology is Gigabit Ethernet operating at 1000 Mbits/sec. The IEEE 802.3z standard describes the specifications for the 1000BASE-X fiber optic Gigabit Ethernet, and IEEE 802.3ab describes the specifications for the 1000BASE-T twisted-pair Gigabit Ethernet system. Currently IEEE 802.3ae is in development and is expected to define speeds up to 10,000 Mbit/sec or 10 Gbit/sec Ethernet.

As Ethernet speeds continue to increase from Mbit/sec to 10 Gbit/sec, systems from the embedded level to the enterprise level can benefit from advances in commercial networking technology. Advances in commercial technologies and software ADEs will allow plant engineers to reduce control system costs and development time while improving system performance and flexibility. This is what PC-based technology can bring to the industrial market.

Robert Jackson is a PXI product manager at National Instruments, Austin, TX, His current projects include PXI product management, coordinating web development and product launches for PX,I and developing opportunities for PXI in industrial applications. Robert joined National Instruments in 2002, and holds a Chemical Engineering degree from Oklahoma State University. He can be reached at 512-683-5440 or robert.jackson@ni.com .





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