A Linux-Based Steam Turbine Test Bench
All computers are connected to the local network, split into three segments (see Figure 3). The first segment includes all the DAS-Flow computers; the second, the DASOP computers and office computers; and the third looks to the outer world via the leased line. The third segment includes only one computer, which acts as our gateway to the Internet, with a firewall based on ipchains and mail server.
In the “middle” of the network is our main server. It acts as file- and print-server for all the computers, but this is not its main task. During the tests, we collect a large amount of data. All this data, as raw, measured values and later as evaluated parameters, is automatically stored in a MySQL database. An Apache web server provides a powerful interface to the database for all users—our local researchers and our customers abroad—through HTTPS.
Any registered user needs to have only a browser to access the database, search the data and get results in a text or graphical form. PNG, CGM and PDF formats are available. We use mostly PHP, as an Apache mod_php module, for generating data-driven pages. Almost all graphs are generated on the fly using the gnuplot program via Perl CGI scripts, which select the parameters from the database, pipe them to gnuplot and then pass the generated image to Apache. We wrote more than 50 different CGI scripts to provide users with all possible kinds of plots, where the user can select everything—what parameters to plot, search conditions, kind of characteristics to plot, auto or manual axes scaling, kind of smoothing and approximation and other choices.
I have to mention specifically the important role of gnuplot in our project. In my point of view, it's one of the greatest scientific plotting utilities with a wide range of capabilities and output formats. It's still under active development, and I'm always trying to use the latest development versions, which are quite stable even for my production environment. I also use the well-designed JpGraph PHP classes for generating certain plots, especially some kinds of fast search results.
Another important part of the software we developed is the technological data acquisition system DASOP (see Figure 4). It has a modular structure and includes the data acquisition module, data evaluation module, socket communication module and application modules.
The data acquisition module works with a Programmable Data Controller (PDC) connected by an RS232 interface to a PC. It gets about 150 values from the PDC every second and performs some manipulations with PDC digital I/O if needed. All the measured data is placed in a shared-memory segment as a two-dimensional array, where each column is a full set of all parameters' raw values. The number of columns is fixed, so we always have a fixed number of last-measured datasets in memory.
The evaluation module, which is synchronized with data acquisition modules through a mechanism of semaphores, gets the last measurement set from the shared memory, makes some on-line evaluations and places evaluated data in the same column, extending its length.
The socket communication module provides access to the shared-memory segment for the remote application modules. There are several application modules. Some of them can be run locally with direct access to the shared-memory segment with measured and evaluated data; another can do it remotely via the communication module. Application modules include data logging modules, a safety control module and data presentation modules.
Data presentation modules provide different kinds of graphical presentation of the data in real time. Some examples are parameters over time plots, bar plots (where the color of the bar shows the state of the parameter—normal, warning or emergency) and panels looking like real external devices.
Because of our scheduling requirements we do not need hard real-time operation. Soft real time is enough for us, so we use the normal Linux kernel for our hardware. Data acquisition, evaluation and communication modules are written in plain C and work on the same PC. Safety, logging and some of the presentation modules work on that PC also. Part of the presentation modules work on another PC, which works as an X terminal for the first one. Both PCs, with their monitors, are located in the control room of the test bench to provide all the information to operational personnel. Some presentation modules work on the researchers' PCs, getting information via the communication module.
The development of presentation modules has changed over time. At first, they were ncurses-based programs for the Linux text console. Later, I switched to X, using only standard X11 and Xt libraries. The next step was trying Motif, which we bought from SuSE. The open-source GTK appeared one or two years later, and I switched to it. Over the last two years, almost all presentation and other modules have been written in Tcl/Tk, with an extensive use of the BLT extension. I found Tcl/Tk useful for fast GUI development, serial and socket communications and data presentation.