Consider two units, for example, a mixing tank and a three-bed dryer. The dryer's throughput sets the tank's output. Normally the dryer uses two beds while regenerating the third — this allows the mixer to operate at full capacity. What happens when the dryer suddenly requires one of the operating beds to be regenerated even though the bed in regeneration isn't finished or needs maintenance?
In a conventional automation design the operator must reduce output from the mixing tank. The operator must devote attention to the mixer until all necessary changes are made there, rather than address the root cause of the problem on the dryer. This extends the mean time to repair (MTTR) for the dryer and impacts first prime production and OEE. When a second dryer bed finally is available the operator needs to be aware and return the mixer to full production rates — perhaps dealing with many faceplates and many changes. This delays the time to return to normal and once again affects performance, first prime and overall unit availability, the three cornerstone OEE factors. At the same time, it's likely the mixer's reduced production conditions made control loops not perform optimally, raised energy usage per unit of product and triggered alarms. The dryer probably also had unnecessary alarms because it was operating with only one bed. Failure to provide adequate alarm management in abnormal situations and resulting nuisance alarms or alarm floods have been shown to significantly reduce overall operator performance and contribute to errors and safety risks. (For more on alarm management, see "Adroitly Manage Alarms.")
SBC provides situational optimization directly through state engine functionality and coordination control. First, it automatically would make the mixer turn down from full production without any operator intervention. The operator's attention would remain on correcting the root cause problem at the dryer, improving MTTR. When the dryer returns to normal the mixer, again automatically, would return to full production without operator intervention.
The state engine design also can automatically make many environment changes during the different state conditions -- e.g., implementing optimal loop tuning factors and appropriate active alarm and interlock conditions for each operating state. It can set the right limits to avoid nuisance alarms as well as loop setpoints to make the unit as energy efficient as possible.
Taking advantage of SBC can measurably enhance OEE.
SBC also may improve human asset utilization. In most facilities today the individual loop designs of systems lead to operators being overwhelmed with the amount of information they need to manage. SBC's lower number of interaction points can significantly reduce the effort operators need to keep track of everything. This can free time to work on improvement projects and other value-added activities. Fewer interface points also mean there're fewer things to learn, decreasing necessary training and generally boosting its effectiveness and retention. Additionally, SBC eases moving operators between units because the control environment is nearly the same in each — only actual states and conditions of the states differ.
Engineering staff also may benefit. The same tool used to create the specification and verify final implementation during system test can remain as a live tool to check system performance over time or to amend configuration as the plant evolves. Many of the engineering hours that now go into creating, tracking and maintaining specification and configuration information on the automation system now become available to focus in other improvement areas that will positively impact OEE and RONA.
State-based control is a design philosophy whose time has come. New automation platforms and their software features and tools provide a real opportunity to use SBC to improve OEE and operational excellence.
David Huffman is a systems marketing manager, Control Systems and Products, for ABB Inc., Wickliffe, OH. E-mail him at email@example.com.