By Richard A. Baker Jr.
Engineering Manager
Schneider Electric Automation Business
North Andover, Mass.
EDITED BY MILES BUDIMIR
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The Momentum M1E line of PLCs come with embedded Web pages. The line includes a full range of I/O bases that accept a snap-on communications adapter, making it usable with Profibus, Interbus-S, DeviceNet, Modbus Plus, and Ethernet networks. |
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A ladder logic diagram from a PLC can be called up on a browser screen indicating the status of system components such as pilot lights, relay contacts, and motors. Applets can be written to represent the start and stop boxes allowing manual control of stopping and starting the motor with the click of a mouse. |
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The Web server provides a direct connection from the PLC to the Internet. A real-time operating system in the Web server controls the interaction of the various system components in addition to allocating tasks to the CPU. An Ethernet driver provides the connection to the TCP/IP network. Security in the form of a firewall is generally a part of the user's network infrastructure. |
Imagine being in Paris, drinking coffee at an outdoor cafe, taking out a laptop computer, connecting to the Internet, and checking the status of a machine in Cleveland. The Momentum M1E PLC from Schneider Electric, North Andover, Mass., makes this a reality. Using a standard Ethernet inter-face, the PLC links to the World Wide Web via a built-in Web server, allowing anyone with a PC and an Internet connection to access information on the PLC from anywhere around the world.
In the past, remote monitoring and control of industrial systems and processes took many forms. Dedicated lines were at one time the most common form of communication between a control system and a remote location. This had limited application because the control system was not accessible from multiple locations. Modems made it possible to access the control system from different locations, but they are generally restricted to down-loading and uploading data files and require a customized interface to access the control system. Also, providing any type of control function between locations is rather limited.
The Internet and the World Wide Web offer a common and easily accessible way to deliver data through hypertext links. A client server system gives each end user the same interface with universal access to services on the Web. Combining a Web browser and a PLC gives users a simple, graphical means to access data and reset parameters from any PC tied to the World Wide Web. This makes it easier for users dependent on the graphic interfaces available for their desktop PC applications to simplify the frustrating task of extracting useable information from windowless PLCs.
The Web interface on the M1E runs Web pages on the PLC CPU and includes an HTTP (Hyper Text Transfer Protocol) interpreter, a TCP/IP stack, and the normal PLC kernel and language support. The Web interface provides access to the PLC by a user at a remote location through the Internet. The interface translates the industry standard Ethernet, TCP/IP, and HTTP protocols used on the Internet into data recognizable to the PLC. Using this interface, the user can retrieve data such as PLC configuration, I/O and register status, operating statistics, diagnostics, and distributed I/O configurations. Updates to the operating software can also be down-loaded through the Internet.
There is a distinct advantage in retrieving the information directly from the PLC instead of through a PC network. PC-based Web servers require information travel between the PLC and the PC through the multiple networks in factories and plants. For instance, a typical SCADA package polls numerous points in a PLC to retrieve live factory data. The polling involves executing the protocol stacks on both the PC and the PLC network board. Data is then retrieved from the PLC memory across the backplane and sent back through the same protocol levels. This makes it unsuitable for time-sensitive information.
Embedding the Web server in the PLC ensures the timely flow of information required on the factory floor. A Web server in the PLC has direct access to this information. At the same time, the built-in Ethernet interface allows this information to be easily shared across the enterprise for faster decision making.
PULLING IT ALL TOGETHER
At the heart of the M1E is an 80186ER microprocessor running at 50 MHz with a Crystal CS8900 Ethernet MAC and PHY chip. The board includes fast memory timing so that the CPU is not delayed waiting for memory access. Directly accessible memory is limited to 512k SRAM and 256k flash RAM due to the addressing limitations of the 186. A page mechanism that allows banks of flash RAM to be addressed removes the 1-Mbyte memory-addressing limitations of the 80186. Other processors in the x86 family avoid this limitation but use too much electrical power.
Because the Ethernet stack and the HTTP server are embedded into the PLC, determinism is maintained both in the TCP/IP communications and in the PLC executive allowing it to be used to control I/O networks and synchronization between PLCs. As a result, the M1E executes 4,000 TCP/IP round-trip messages per second.
One of the biggest challenges was developing a file system for the PLC's Web server. Web browsers and servers communicate with HTTP, which is file-based and assumes a file system. However, the M1E PLC did not have a file system. To create a file system, the Web pages were assem-bled in a linked list and downloaded with the PLC executive to the M1E. The M1E follows the linked list chain until the requested file name is found in memory.
These pages provide information on the status of the PLC, its configuration, the I/O map, and the Ethernet statistics. The user interface is designed to combine the look and feel of a web browser, the PLC programming software, and input from a user-interface expert.
NOW AND IN THE FUTURE
Web-enabled PLCs can significantly change the way plants are run, reducing downtime, and increasing productivity. A device equipped with an embedded Web server can communicate proactively. If it breaks, or reaches certain preset parameters that indicate it is about to break, it can send a message to the OEM or maintenance department, describing the specific problem, and requesting service.
When breakdowns do occur, Web-enabled equipment allows the OEMs who built it to remotely monitor and even re-pair their machines, reducing response time and expenses that ultimately are passed to the user, including staff time and travel expenses.
Plant engineers can now fix problems within a factory over the Web from their office or even from home, eliminating those late-night trips to the plant. This same technology allows plant engineers to simultaneously monitor multiple plants, making it possible to back up colleagues when they are out or on vacation, or even to manage multiple plants from a single location.
Collecting plant data centrally also permits business managers to assign new orders based on which plant can deliver the quickest response time or the lowest cost of production at a particular point in time due to variations in utility rates, labor costs or other factors.