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SCADA goes long

2010-03-28 13:42:40views: 823Stuart Boyer (ISA)

SCADA goes long

Supervisory control and data acquisition (SCADA) is the technology we use to control processes that extend over long distances.  Occasionally, in a plant, there are parts of the process that are so far away from the control room that a SCADA system can be included in the plant distributed control system (DCS) to reach them. Usually controlling these processes is simple, involving measuring flows or temperatures, monitoring alarms, opening or closing valves, turning motors on or off, opening or closing switches, and adjusting set points on controllers located near the process.  While this control is simple, it is usually very important.  Also, the speed with which we respond to information about these processes is not critical—the response time is not urgent.  If these statements are true, why has SCADA become such a popular technology?

A long time ago, all processes were controlled manually.  Most processes were “batch.”  That is, the person that was in charge would mix the correct amount of each ingredient, would add heat to bring the mixture to a selected temperature, would stir for an experimentally determined time, and would cool the product.  Often, the method of telling if the amount of each ingredient was correct was a personal preference of the mixer, and the temperature was estimated by knowing how long the heat had been applied.  The speed of cooling was controlled by the ambient temperature.

Eventually, someone noticed the quality of the product depended on who was doing the mixing.  Shortly after this, experiments were conducted to determine what made one mixer’s product better than that of another mixer.  Better measurements of the ingredients were tried.  Thermometers were used to control the temperature.  Mechanical stirring was applied to eliminate the human tendency to stir less rather than more.  Different rates of change of cooling were evaluated, and the best rates were selected.  In short, process instrumentation was introduced to improve the quality of the product.

At about this time, the person who controlled these factors was reading all of the instruments and manually adjusting fuel and stirring rates, adding ingredients at the right time, and removing the heat and providing cooling water of the right temperature and at the right rate.  That person was probably also cleaning the pots after every batch of product was made.

As process instrumentation improved, it became practical to convert some classes of process from “batch” to “continuous”.  Process fluids could now be added to and removed from the process container without stopping the flow.  Generator voltages and currents could be controlled without turning off the generator to adjust settings of the machine.  Materials could be shaped to the proper dimension without stopping the shaping process to measure if they were now the correct size.  When done properly, this continuous processing resulted in massive increases in throughput.  But it also required more dedicated attention of the operator because failure to notice a change in process measurement would surely result in product that was out of specification.

The next developments integrated the measuring and control functions. A controller that pinched down on the fuel when the mixture got too hot was easy to build and could be relied upon to do a better job than an operator whose attention may wander.  It was not much of a stretch to develop devices that could control the rate of change of temperature, other devices that could change from adding heat to removing heat after a preselected time, and even more devices that could add and stir the proper amounts of ingredients.  The operator’s function became one of monitoring all of these devices to ensure they did the right things.  When the operator was no longer continuously occupied running one process, management decided to assign two or three processes to each operator.  Control shifted from one corner of the process to a room dedicated to control.  The increased distance from the process to the control room meant pneumatic and hydraulic lines became prohibitively expensive and pairs of copper electric lines, each pair dedicated to one signal, replaced them.

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