It's Not Just a Loop Tuner, It's a Plant Manager

Reprinted with permission from CONTROL Magazine, May 1996.

Manual tuning of a PID loop's proportional band or gain, integral action or reset and derivative rate is a rewarding but tedious task. That's where time-saving software tuning and loop analysis tools like ExperTune come in. This package, from ExperTune (formerly Gerry Engineering), Hartland, WI, is used to import process data, simulate the effects of PID parameters on the process, and save hours on tough loops - that is, if you can get them into the plant and run the gauntlet of cost justification.

According to John Gerry, president of the software company, ExperTune's benefits only begin with loop-tuning time savings. Because when loop behavior is optimized, the process, the plant and arguably the business can run more efficiently. "We had one plant that was looking at installing some process equipment," says Gerry, "but when they used our package to optimize their loops, it turned out they increased their throughput by 50%. And they found they didn't need the new equipment in, after all."

A new version of the package, v.6.05, ships this month in 16-bit form, with a 32-bit version due out within the next few months. With each license comes a year's worth of free technical support and upgrades. A free demo disk is available.

Prices vary, but in cost analyzes the company has performed for customers, loop-tuning time savings alone are estimated at $125 per loop. Factor in lower labor costs, multiple loops and tunings per year, dollars lost to product waste and rework, inefficient utility fuel usage costs, and downtime attributable to loops not optimized ... and you have a recipe for success. And while it's difficult to measure the value of process consistency and product quality, these factors certainly come into play.

Once ExperTune has been justified, it can be purchased in several configurations: with direct drivers for controllers from Allen-Bradley, GE Fanuc, Modicon, Moore Products, and Siemens; with DDE services for Windows or ASCII for raw data import; in a vendor- specific version such as Tune-A-Fish for the Fisher Provox control system; or in conjunction with a small data acquisition system (as needed). In its full direct driver or integrated control system version, the full Windows user interface is most complete.

There are added features in v.6.05, such as pop-up notices of controller connection errors, and lots of new highlighted hypertext terms to help new or uninitiated users with loop tuning terminology. But most of the improvements are below the surface of tuning, modeling and simulation analyzes (which are based on imported process data).

Under-the-skin upgrades have been made in the program's modeling algorithms. These, says Gerry, result in better and more accurate analysis features, which measure robustness, frequency response, and tuning performance. There also are valve sizing checks, analysis hysteresis, filtering, and power spectral density, which shows power at various frequencies.

"Whenever you model a process or tune a loop, you want the model to match a real-world linear differential equation," says Gerry. "In some cases, there will be a difference." One convenient visual cue is an index that provides a "good/fair/poor" indication of how well the imported data matches the classic, linear PID model.

Another helpful visual tool is a robustness plot that can be viewed simultaneously with changes of dead-time or gain in a simulation plot. Again, these tools display only the surface results of underlying enhancements to the modeling algorithms.

According to Gerry, specific enhancements have been made to "second-order" loop simulations characterized by two large lags or time constants that might be 10 times larger than the dead time in a loop-for example, lags in the controller's effort to ramp-up or achieve steady-state temperature regulation in a process. Incidentally, unlike the earlier release of ExperTune, users no longer need to differentiate between first-order and second-order loops. The software automatically selects which type of model to use.

"Because of greater confidence in tuning loops like these," says Gerry, "we are able to tune those loops aggressively without sacrificing safety margin." After all, the goal is to tune as tightly as possible, at the brink of performance, without falling off the edge of process instability.

If higher productivity is mandated by plant managers, then a tool like ExperTune just might be a necessity for process control engineers.