Process Control (Part One): Smart and Not So Smart Control

Written by Joe Evans, Ph.D

Page 2 of 2

Smart and Not So Smart Control

The typical residential sprinkler system is controlled by a simple timer that turns the system on at some preset time, then turns it off at some other preset time. It doesn't matter if it is raining cats and dogs - it will turn on and off based on the timer settings.

In the process environment, this control scheme is known as "open loop" control. The key characteristic of open loop control is that the controller has no clue what is going on within the system. It simply follows its instructions, to the letter, regardless of its surroundings. Open loop control works well as long as the event it controls is repetitive and no damage could result from its action. If our sprinkler activates during a rainstorm, the water is wasted but no damage occurs. The figure below is a graphic representation of this control loop.

proc con fig 2 0108.jpg

It is a bit different when it comes to your home heating system. Although open loop control could be used, the results would be less than satisfactory. You would experience periods when it is too warm and others when it is too cold, as the heater would start and stop based on a simple timing cycle.

A better control method would provide some "feedback" to the heater based on the desired temperature and the actual measured temperature at any point in time. The system could then make its own decision as to when it should start and how long it should operate.

In the typical home heating system this is accomplished with a thermostat. When the temperature drops below a certain predetermined level, the thermostat starts the heating system and runs it at its full capacity until the temperature rises to some preset maximum. The thermostat then stops the heating system and waits to begin another cycle.

This is a simple example of "closed loop" control. More specifically, it is known as "on/off, closed loop control" as the heater is either fully on or fully off and there are no intermediate settings. The key characteristic of the closed loop controller is that it receives some form of feedback as to what is going on within the system and can therefore make smarter decisions. The next figure is a graphic representation of this closed loop example.

proc con fig 3 0108.jpg

Smarter Control: The P in PID

Now suppose for a moment that our home heating example does not use an on/off thermostat, but instead uses one that can transmit the actual measured temperature in the room back to the heating system controller. Let's also suppose that the heating system can vary its output based upon the temperature reading it receives from the thermostat.

As the temperature in the room approaches its "set point," the heater would not necessarily turn off, but would instead reduce its output and attempt to keep the room at the desired temperature. If the temperature drops, it would increase its output. If the temperature increases, it would either reduce its output further or shut off completely.

Furthermore, these changes in output would be in "proportion" to the change in temperature. A small change in temperature results in a small change in output, while larger changes in temperature would lead to proportionally larger changes in heat output.

The example above is one of "proportional, closed loop" control and is the "P" in "PID." In the pumping environment, proportional control is seen daily. Constant speed, multi-pump booster systems use pressure switches to start or stop additional pumps based upon changes in system pressure. Sewage lift stations use level switches to accomplish the same mission based on changes in the liquid level.

I like to refer to these applications as "fixed, proportional control" as there is a limited number of "proportions" (pump combinations) available to the control loop. Proportional control works best in systems where the feedback measurement changes slowly. In the booster system example above, feedback (pressure) can change quickly, so we will often employ a fairly large pressure differential and maybe even delay timers to keep lag pumps from cycling on or off too quickly.

But unlike the constant speed booster, today's variable speed (dynamic) systems rely on a microprocessor or PLC to execute the Boolean logic necessary to control a pump's pressure. A computer program, or algorithm, monitors pressure and makes its own decisions about changes in pump speed. But when conditions change quickly in a dynamic system, proportional control alone doesn't always do a good job.

Next month we will take a look at how the "I & D" of "PID" can help proportional control do a better job in applications where feedback changes quickly.

Joe Evans is the western regional manager for Hydromatic Engineered Waste Water Systems, a division of Pentair Water, 740 East 9th Street Ashland, OH 44805. He can be reached via his website at www.pumped101.com. If there are topics that you would like to see discussed in future columns, drop him an email.

Comments (0) Add Comment