Legend has it that certain divinely inspired engineers in the automation industry are capable of producing bug-free control code on the first pass. Their programs always work as intended the first time the control system is powered up. I am certainly not one of them.

My programs inevitably fail to account for some unforeseen sequence of events or some combination of inputs that only happens once every third Tuesday in June. My programs have even been known to produce erroneous results simply because I mistyped a tag name.

And I'm not alone. That's why so many mere mortal engineers thoroughly validate their control systems with some kind of simulated process before trying to control the real thing. With simulation testing, it's cheaper and safer to find and fix bugs responsible for failures such as tanks overflowing, robot arms colliding, and doors closing too soon or too late.

VPLink from Cape Software (Houston, Tex.) is a PC-based process simulator that can be interfaced to a DCS, PLC, or PC-based control system and configured to behave like the real process. Even without any I/O modules installed in the control system, VP Link can read the output variables generated by a control program, determine how the real process would respond to those signals, then write the simulated response data back into the controller's input variables.

Users can specify the behavior of the simulated process schematically or by coding it directly in VPLink's scripting language. VPLink includes more then 60 preprogrammed algorithms that can be used to represent the behavior of pumps, valves, motors, and many other typical elements of an automated process. This library also includes several purely mathematical operations like random noise, lags, deadtime, and a few I've never encountered before.

VPLink's process simulations can also be used for off-line operator training. Using a spare controller and the same operator interface used with the on-line controller, operators can get a view of what appears to be the process in operation. They can practice starting up the process, running it under normal conditions, and shutting it down without disturbing the real process. Operators and engineers can also work out the bugs in how the operator interface screens display process data even before the process is brought on-line.

VPLink is a far cry from the old fashioned collection of lights, switches, potentiometers, and meters that engineers formerly used to test their control systems off-line. However, there are some things it isn't intended to do. It is not a hardware checker. VPLink doesn't even use the controller's I/O cards. Nor is VPLink a design tool for simulating complex new processes involving one-of-a-kind equipment. Although users can construct their own simulation algorithms if necessary, VPLink works best for simulating the routine devices included in the library of pre-programmed algorithms.

VPLink is typically run on a Pentium PC with an SVGA monitor; 32 Mbytes of RAM are recommended for PCs running Microsoft Windows 95; 64 Mbytes for Microsoft Windows NT. A VPLink license for a PLC application with up to 300 input, output, and auxiliary variables costs $3,000. Auxiliary variables are used to represent values that affect the simulation but are not actually exchanged with the control system (e.g., the ambient temperature, the current state of manual valves, performance parameters, etc.). Additional variables can be added for $1 each.

For larger control systems, a VPLink license costs $9,000 to $14,000. VPLink interfaces are not available for very small control systems like single-loop controllers.

For more information on VPLink, visit www.controleng.com/info