PC-Based Control Software Runs on Schedule

Vance J. VanDoren, Ph.D., P.E. -- Control Engineering, 1/1/1998

One of the advantages that programmable logic controllers (PLCs) have traditionally offered for the purposes of real-time control has been deterministic operations. That is, they have the ability to apply their control efforts at precisely timed intervals. This feature allows a PLC to work with sampled data, knowing that each sample represents a uniform period of process activity.

Personal computers with conventional operating systems, on the other hand, were not originally intended to operate in real time. Most PCs respond to keyboard inputs whenever they occur and take as much time as necessary to update the screen, access the disk drives, and run programs. A PC has no means of scheduling these operations and no means of timing a series of control efforts accurately.

Several PC software vendors, including Intellution (Norwood, Mass.), have addressed this shortcoming by modifying the PC's operating system to provide deterministic operations. Intellution's PC-based control software, Paradym-31, (from the acquisition of Wizdom Controls) adapts Microsoft Windows NT to give priority to operations that must occur on schedule. Lower priority support functions are relegated to the time remaining between scans (which can be as little as 100 msec). If the user-specified scan interval turns out to be too short to allow the entire program to run, Paradym-31 automatically increases it at start-up.

Paradym-31 is based on the IEC 1131 standard. It allows users to program real-time control operations using relay ladder logic, sequential function charts, and function block diagrams. User-defined function blocks are also available for creating custom control programs in C.

Programming is accomplished graphically with all three languages. Users place graphical elements on screen to represent the operations to be performed, then connect the graphics with lines indicating the required flow of data among operations. Function-block diagrams are built from function-block graphics that represent mathematical operations, PID loops, and other continuous calculations. These are generally used to specify continuous operations where data flow from block to block non-stop.

Sequential-function charts, on the other hand, are generally used for applications where the functions represented by each block are executed periodically in a prescribed sequence. The only data that flows between the blocks is a conditional START signal that indicates when the next block in the sequence is to be executed. For relay ladder logic diagrams, the graphics depict the traditional contacts, coils, and embedded function blocks familiar to PLC users. Data flowing through the rungs of a relay ladder are the results of Boolean calculations performed by the relays.

Of the three options, I find the relay ladder logic to be the least useful, particularly for programming continuous or batch operations. Function blocks and sequential function charts are much easier to construct, debug, and maintain. In general, I find that a relay ladder program with more than a hundred rungs or so becomes virtually unintelligible to anyone but the original programmer.

In addition to real-time control, Paradym-31 offers off-line simulation. After completing a control program, users can test their logic without applying it to a real process. Simulated I/O is used instead. The simulation can be run in real time or in an accelerated time base.

Paradym-31 version 4.0 is available in three editions that offer varying degrees of functionality. All three are available with development, runtime, and execute-only licenses. A basic development package starts at $1,995. System requirements include Windows NT, a 133 MHz Pentium processor (166 MHz is recommended), 32 MByte of RAM, about 1 GByte of hard-disk space, and a hardware key. Drivers for a variety of industrial I/O products are included with the software.

For more information on Paradym-31 visit www.controleng.com/info.