Web-enabling Your PLC
Enough has been written during the past couple years to convince most plant managers of the benefits of connecting plant floor equipment and processes to the Internet or Intranet. So, now that we're all convinced of the benefits of web-enabled automation, what pieces and parts are required to put this technology to work? The basic parts required for web-based data acquisition and control are:
· an interface to the machine or process to be monitored or controlled via the web (network) connection;
· a web server to make the desired display and/or control pages available to the remote browser or client; and
· a data service or interface to handle exchanging data between the local machine/process (server) and the remote system (client).
For remote viewing of the data and/or web pages, the only requirement is a standard browser interface. For applications requiring SPC, optimization, or enterprise level software to exchange real time data with the machine/process, a remote server PC and a compatible data exchange service are required.
In a simplified block diagram form, the parts might look like the figure below.
In this system, the web server/data service device ("thin server") provides the connection between the machine or process and the Internet. In a typical installation, a connection to an existing PLC or proprietary controller will be required to extract data or enable control over the equipment. Most PLCs, including products from AB, Schneider/Modicon, and Siemens, support at least a serial connection using communication protocols available from the PLC vendor or third-party suppliers. In many cases, the communication driver will be available from the thin server vendor as part of the embedded software application.
If the equipment uses a standard PLC protocol or supports another open standard protocol such as Modbus RTU or TCP, then the job of connecting the thin server is usually greatly simplified. However, this is not always the case.
If the equipment in question uses a non-standard PLC or a proprietary controller, a connection to the thin server device is still possible as long as the controller supports some kind of serial protocol and the vendor can provide the protocol documentation. In this situation, either the thin server vendor or a third-party developer will be involved to implement the driver required to connect the thin server to the equipment controller.
In the extreme case, the equipment to be monitored may have a controller with no external communication port, or the protocol may be unavailable for some reason. In this situation, it may be necessary to externally instrument the equipment or process using additional sensors and I/O. These I/O devices can be installed to monitor strategic data or control points on the equipment. If this is required, the I/O devices should be chosen so that they support a standard communication protocol (e.g. Modbus or an open ASCII protocol) and can be connected to the thin server. Alternatively, an I/O device may be selected that includes an integral thin server function. If the I/O device includes an integrated server, then the separate thin server may not be required.
With the thin server installed in the equipment enclosure and the proper driver installed or selected, the next step is to configure the thin server to link the equipment communication data and control (inputs and outputs) to the network link. The details of this step vary depending on the features and configuration software provided with the thin server. For example, a gateway server may simply map PLC registers to network variables or a remote PC connection. More sophisticated thin servers may allow configuration of a web page to view equipment data, enable alarm monitoring and data trending, add standard connectivity interfaces such as OPC, and may even support paging on programmable equipment fault conditions. Make sure that the thin server selected for the project supports an adequate feature set to allow for future expansion.
Chiller Box (courtesy of Dumont Associates)
Here is a close-up of the chiller control cabinet, showing the chiller control PLC and the embedded web server (gray box). The serial cable connecting the web server to the PLC for data exchange can be seen, as well as the blue Ethernet LAN connection for Intranet
The Internet or Intranet (LAN) connection to the thin server may include a standard hardwired Ethernet line, a modem/phone line (dialup), or a wireless connection (802.11b Ethernet, for example). The type of connection is typically determined by the type of thin server and the existing plant network infrastructure – or lack of network infrastructure.
If a hardwired LAN already exists near the equipment to be connected, then a network connection is relatively simple with the addition of another cable drop to the equipment. If a hardwired connection is difficult or expensive due to complex cable runs or distances, a wireless connection may be the best alternative. However, when selecting wireless connections, be sure to carefully evaluate the proposed installation site and consider factors such as proper range and signal integrity. If using a wireless network, look for a thin server that directly supports a wireless LAN interface to simplify the installation at the equipment site.
With a LAN connection established, the remote system with its application software can now access the machine equipment at the thin server's assigned IP address via the factory network. Applications such as an SPC package can now take advantage of two-way data exchange with the equipment, made possible by the thin server. If the thin server and remote application software support a common interface, such as OPC, setting up the data exchange can usually be accomplished in a matter of minutes.
Using this same Internet connection, a browser PC (desktop, laptop, PDA, or a "thin client") can access a web page resident in the thin server. This web page can be used to monitor data from the equipment as well as send data to the equipment via the thin server. And, since the web page is resident in the thin server, no software is required on the browser other than Internet Explorer or Netscape.
Chiller Screen (courtesy of Dumont Associates)
This is the main chiller system overview screen, as viewed from a remote PC via a browser. This web page with live data from the embedded web server gives a quick overview of the chiller status and allows adjustment of setpoints and other parameters remotely with proper login credentials
As a side benefit to this implementation, if the only requirement of the browser PC is to view web pages from the thin server, the PC can be solely dedicated to this task. In this case, the browser "PC" can be a very simple device. It requires only an LCD, CPU, and a small amount of memory; a Thin Client device. This can result in a lower installed cost and lower maintenance cost than a standard PC. Another advantage is that the device is not limited to viewing web pages from only the thin server. It can also view web pages from any source accessible via the network. So, a browser PC or Thin Client located on the shop floor could be used by an assembler to access HTML help files, with graphics and instructions, located on a remote server. The ruggedness and low cost of this type of thin client browser make it possible to provide data and control access in places previously not possible with standard or even industrial PCs.
For data exchange with the application software running on a remote system (as shown in the diagram) a common "language" is required. Several possibilities exist, including OPC, mentioned previously, as well as XML. XML, or Extensible Markup Language, is a standard defined by the World Wide Web Consortium (W3C) that provides for a method of exchanging data between systems via the Internet and is an important element of Microsoft's .NET architecture. The data is exchanged along with a "tag" that defines the data in a manner that is independent of the sender's and receiver's hardware platform, OS, and application. This makes XML very powerful when implementing open systems within a company or between different companies, as in business-to-business applications.
And, since web enabled automation is all about enabling the use of data within and between companies to improve processes and reduce costs, XML is a very good fit in this distributed web architecture. Choosing a thin server that supports XML can be a real benefit to future plant floor connectivity.
Now that we've seen the power of web enabled automation and reviewed how it can be implemented, where do we go from here? Let's take the sample architecture from the previous section and see how it could be implemented using eAutomation products from Advantech Automation.
Thin server: Advantech's new WebLink was designed for this task. WebLink is a complete "intelligent embedded server" solution, including all hardware and runtime software required to web enable a system. It can connect to a device (machine/process controller, I/O, sensor, etc.) using a standard RS-232/485 serial port or an optional fieldbus adapter. A network connection is then made through WebLink's standard Ethernet 10/100BaseT port or via optional modem or wireless network/Internet connections. Development software enables web pages and data connections to remote application software to be easily created and maintained from anywhere via the network connection. Security is provided by WebLink through password protected user login and optional restricted access by user IP.
For applications where a local HMI is required to be web enabled, Advantech offers the WebOIT operator interface terminal. This product series combines the features of WebLink with an integrated LCD and HMI software functionality.
Browser/Thin Client: There are several choices here, depending on the need. Any standard laptop or desktop with browser software can access the web pages resident in WebLink, providing instant access to information about your equipment, plant, process, building, or business worldwide. For access from the plant floor or other locations requiring a ruggedized viewer, a standard industrial HMI from Advantech's Industrial Panel PC line can be used to provide browser capability along with full PC expandability and functionality. If full PC functionality is not required, Advantech's WebView can be a very cost-effective solution. WebView is a thin client terminal that supports viewing of remote web pages via its integrated Ethernet connection and standard Internet Explorer software. WebView also supports third-party or custom HMI applications based on Windows CE or Linux, including support for Java.
Web enabled automation does have real benefits and provides real competitive advantages for your enterprise. And, with solutions like WebLink, WebOIT, and WebView it can be easy and cost effective to make web enabled automation a reality.
This article was written and provided by Mike Rothwell, director of Advantech Automation Corp.'s eAutomation Engineering Center. He founded the R&D and product group in Cincinnati's Industrial Automation Group headquarters and is responsible for Advantech's web-enabled eAutomation products worldwide. Rothwell holds a BS in Electrical Engineering and a Masters in Business Administration, both from the University of Cincinnati (Cincinnati, Ohio, USA). He has almost 20 years experience in the industrial automation and controls industry, primarily focused on development of real-time control and operator interface systems for robotics, CNC, and plastics processing machinery.
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