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As aging kilns get older and the original instrumentation system can't be repaired, many plants are making the next logical move and upgrading their control technology. The costs for most of these systems are very similar, but the difference in usability can be extreme. If you are contemplating a conversion to a computer interface system with PLC control, it's important to keep a number of factors in mind.
Functional DisplaysMy biggest concern when looking at any instrumentation panel is the ability to see-at a glance-if all control loops are within reasonable deviation values. I never want to have to scroll through endless screens in order to find key information. If you have a human machine interface(HMI) system that requires repetitively scrolling through screens to find the data you need, you know how frustrating that can be. And yet, the norm in screen design seems to be to spend time developing "wow" graphics instead of putting time into crystal-clear organization.
You must think about what is most important and insure that the main screen shows this data at first glance. The use of color to indicate deviation bands and simple (but clear) graphics mean that this will be the screen that will be used 90% of the time.
When operating a tunnel kiln, the factors that affect the control of the kiln include various internal pressures; set-point temperature; process variables and output of each loop; burner operation/burner failure; pushing speed; and, in some cases, O2 levels. The best screen design will include all of these variables clearly, along with a kiln curve that shows all of the thermocouple temperatures in correct graphical scale. All that should be necessary to drill down into any of these data points is to click on that piece of data to see what's behind it.
For example, if you are concerned about the performance of a control loop, clicking on the loop icon should immediately open a window to all tuning parameters, output limits and graphical loop performance, including standard deviation from set-point and output. You should end up with fewer menu buttons but easier access to data. Being able to simply click on the data icon means you spend less time finding the data and have more time to correct any problems.
The key parameters are similar for periodic kilns. It's nice to see a curve with a cursor to tell you the state of the program, along with modes of firing and maximum deviations. The ability to zoom an area is often very helpful. Individual zone performance screens, and an overall view of the kiln, burner positions, deviation from set-point, and output, give a good top-down view of the system. Remember, unlike the analog systems of old, where the entire system (plus strip chart) was right in front of you, you only have a 15-in. monitor to view all of the data.
Data StorageWhen logging data, store everything! Any PV that has an input to the PLC should be recorded, preferably in Excel format, and files should be auto-labeled by date. Graphical storage is also possible and desirable. All of this should be fully automatic.
AccessibilityFinally, always make sure that the HMI system is accessible over the Internet, which enables kiln operators to view the system operation from both inside and outside the plant. Internet access makes middle-of-the night solutions easier, and remote access reduces the time of correction-especially without the midnight drive to the plant to see what's going on. In addition, specialists can view the system and assist remotely to solve problems.
Find the Best FitIf you are working with a developer, sketch out the kind of screens that you think are necessary. Don't accept the canned, one-size-fits-all design, because you will be the one operating the system for years to come. As we all know, sometimes it's tough to find problems in kiln control, and time is always limited. A good HMI system reduces the amount of time required to find the problem, and that strikes me as what HMI should be all about.
Any views or opinions expressed in this column are those of the author and do not represent those of Ceramic Industry, its staff, Editorial Advisory Board or BNP Media.