Using PCs for machine condition monitoring: Part 6
Chris DeFilippo and Robert Jackson, National Instruments, Austin, TX
Search the internet for the term “wireless” and you’ll get more than 131 million hits. The term “wireless” has become a buzzword for everything from consumer cell phones and PDAs to commercial internet routers and industrial networks. Today in industrial applications, wireless technology is used in applications where normal Ethernet wiring is difficult or impossible to install. How can this same technology be used to make asset management and predictive maintenance (PdM) simpler? This article outlines new wireless technologies, discusses potential applications for wireless machine condition monitoring (MCM), and highlights trends that show commercial-off-the-shelf (COTS) wireless technologies are reducing costs.
New wireless technologies
There are a number of COTS wireless industry standards suitable for use in the industrial environment. This development of wireless technology is similar to trends for increased performance and reduced costs that are common in the personal computer (PC) market. Products that use PC-based technology, such as programmable automation controllers (PACs), can benefit from these same trends in wireless technology.
PACs combine the flexibility and performance of PCs with the reliability and ruggedness of programmable logic controllers (PLCs). With PCs and PACs, maintenance engineers have more control over their PdM equipment and the ability to do advanced diagnostics. By combining wireless technology with PACs it is easier to monitor machine health for critical machinery and then wirelessly notify operators of alarm conditions.
Wireless technology offers an opportunity to reduce wiring costs in certain industrial applications. Physical wiring is often expensive. Wiring in a typical chemical plant may cost up to $40/ft, while in a nuclear power plant it may be up to $2000/ft. Certain assets such as rotating equipment, vehicles, and cranes are difficult if not impossible to add infrastructure. Machine monitoring systems that implement wireless technologies are easier to relocate than machine monitoring systems with wired infrastructure.
Wireless LAN, IEEE 802.11, and Wi-Fi
Arguably, the mostly widely accepted and available wireless standard so far is IEEE 802.11a/b/g, which is also known as wireless local area network (LAN), Wi-Fi, and wireless Ethernet. Introduced in June 1997, IEEE 802.11 specifies one medium access control (MAC) protocol layer, and three physical layers: frequency hopping (FHSS), direct sequence (DSSS), and infrared. The FHSS and DSSS specifications operate in the 2.4 GHz band and deliver data rates up to 54 Mbps. IEEE 802.11 delivers interoperability among devices, in much the same way that Ethernet devices are interoperable at the physical and MAC layers. For more information of the different physical layers, see Using PCs for machine condition monitoring: Part 5.
One application of 802.11 technologies is wireless MCM applications for large rotating machinery. Typically, distributed PAC-based MCM systems are deployed throughout the factory floor. Each PAC collects vibration data and performs analysis. Alarms and reporting data are transferred wirelessly to a control system using a standard 802.11 networks. Benefits of this type of wireless MCM system include:
Reduced wiring costs
Ability to easily retrofit existing machinery without adding additional infrastructure
Flexibility to relocate machinery and the MCM system without relocating infrastructure
Makes MCM possible for rotating or mobile machinery where wires are not feasible
You can also take advantage of 802.11 and wireless LANs by combining this technology with the industrial Ethernet network already present in your facility. Modern, PC-based programming tools for MCM have built-in web server capabilities. Through the 802.11 standard, you can wirelessly monitor or control your system from any PC in your facility or any PC in the world simply by enabling these built-in web servers.
Second generation cellular modems had limited bandwidth and connectivity. The next generation of cellular systems, known as 3G for third generation, offers up to 500 Kbps, although not as fast as 802.11. The tradeoff for this bandwidth limitation is increased base range. Cellular modems offer extended distances because they can transmit at many times the power of an 802.11 devices.
One advantage of cellular technology is long-distance distributed systems. With cellular modems, remote PACs can connect to central monitoring systems and notify operators on cell phones many miles away. Wireless cellular modems typically include built-in antennas to simplify setup. One example application is an alarm system for oil field equipment such as pumps and compressors that are spread over large distances. Cellular modems can quickly and easily provide alarms to operators and engineers.
Bluetooth is a wireless standard for short distance communication between computers and peripheral devices. In the industrial environment, Bluetooth enables a PDA or laptop to retrieve information wirelessly from a distributed monitoring system. In this application, your PDA acquires data and alarms from a PAC system without direct connectivity to the system. The biggest benefit of Bluetooth is that it supports automatic connection between devices.
Bluetooth uses a 2.4 GHz radio frequency band and has a range of 30—40 ft. It was initially developed by electronics manufacturers to connect a keyboard or mouse to a PC without wiring. Creating low-level Bluetooth communications is similar to TCP communication in that there are server and client applications. PAC development environments also support low-level Bluetooth functions to create the server and client applications.
For more detailed information on Bluetooth, click here ( www.bluetooth.com )
Infrared Data Association
Infrared Data Association (IrDA) is the protocol used for most TV remotes. IrDA is relatively inexpensive and reliable; however it suffers from the same limitations as your remote control, it requires line-of-sight between devices.
IrDA networks are similar to isolated TCP/IP networks, where you can assign unique IP addresses randomly. An IrDA network is dynamic and devices can enter and leave the network frequently. There are no fixed IrDA addresses that a client device can use to establish communication with a server. The network detects a device by its name (usually specified by the user) and dynamically generates a unique ID to establish communication. Like Bluetooth, IrDA is most often found in short range applications.
Zigbee or IEEE 802.15.4
Zigbee is the first wireless standard to be designed for sensors, monitoring, and control devices. The IEEE 802.15.4 standard was approved in mid-2003 and products are beginning to show up in the marketplace. Zigbee operates in the 2.4 GHz band with a range of up to 300 ft. The advantage of Zigbee is lower power consumption. This allows the technology to be used in sensors and devices with limited power. The tradeoff for decreased power is limited bandwidth. Zigbee devices are limited to 250 kbps. For more information on Zigbee, click here ( www.zigbee.org )
Adoption of wireless technologies drives down costs
Wireless technology is not a replacement for industrial Ethernet architectures. Instead it is an extension to Ethernet for unique applications. Widespread adoption of wireless technologies makes IEEE 802.11 devices relatively inexpensive. It is used in business networks, home networks, automobiles, and built into laptops and PDAs.
The trend toward increased performance and reduced costs is shown by the price of network cards. In December of 1999, Dell released the Dell 4800LT wireless network card for Inspiron Notebooks. Initially selling for $139 for an 802.11b networking card, just 5 years later in November of 2004, a Dell TrueMobile 1300 802.11b/g wireless network card sells for $44.10. This is the benefit of using COTS technology.
As wireless technologies develop, the best MCM software packages will support multiple targets, integrate a broad range of commercial technology, and deliver distributed software control. Some of the challenges that application development environments (ADEs) must overcome include:
Multiple Targets — support for a wide range of hardware targets with single ADE
Agnosticism — ability to transfer execution between processors and targets without changing source code
Interoperability — support of 3rd party hardware so engineers benefit from wide range of industrial devices.
Multiple targets supported from a single development environment currently include wide range of processors, specific FPGAs, and DSPs. These same environments can re-compile code for different environments, and support a wide range of 3rd party device support.
PACs that benefit from a single (ADE) will scale from the device to enterprise level and offer a wide range of hardware targets. With this single development environment, various networking technologies such as 802.11 and Zigbee will extend the ability to easily share data and control execution between different applications.
Personal computers (PCs), commercial technology, and PACs are affecting how plant maintenance is done. New predictive MCM maintenance systems are making PCs an essential part of the monitoring system. PCs have proven their reliability, flexibility, and performance for use on the factory floor. With a combination of PCs and COTS technology, predictive maintenance systems deliver higher performance at a lower cost compared to traditional turnkey systems. The results of these technologies are lower-cost, open-architecture hardware platforms. The reduced cost makes machine monitoring more cost effective and more widespread. This means that today’s plant engineers are preventing catastrophic failures and expensive downtime; predicting maintenance requirements; and protecting valuable assets and machinery; meeting the goals of engineers everywhere.
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