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Pumps & Systems, January 2008

Even though control technology has become an integral part of many pumping systems, over half of our readers have not expressed much interest in pump controls. I think I know why - today's controls are simply not as intuitively understandable as they once were. Who wants to read about the advantages of PID control without a basic understanding of P, I, and D itself?

This two-part series takes a brief look at the hierarchy of process control technology and explains the function of those three (often confusing) letters. Remember, this column is entitled Pump Ed 101, not 301, so our purpose here is to present the basics and hopefully make some of those more advanced control articles a little more understandable.

Before we start, let's define a term and the three words it encompasses that may lead to some of the confusion surrounding controls. The term is "process control loop." Not too long ago, we referred to pump control by application - constant pressure, pump up, pump down, circulation, etc. But today we tend to lump all of these into the generic term "process control."

A process is a systematic series of actions that result in a desired end product. A simple process example is the removal of water from a sump. A more complex one could result in the manufacture of a 1,000-ft reel of 20 conductor cable. But the key words, systematic and desired, apply equally to both. A process should be repeatable and provide the same result each time it is performed.

Control is the application of direction or restraint on how that process proceeds. Basically, control supervises the actions of the process. For example, a sump pump might use simple float switch activation as a control mechanism. Or, it might go a step further and employ a level sensor and operate at different speeds in an attempt to keep the sump at some constant level.

A loop consists of the set of instructions (digital or analog), from start to finish, that controls the process. It is called a loop because once the instructions are completed they will be repeated when the process begins again. There are many different types of loops, and we will discuss several in detail a little later.

A Little History

Is there any rhyme or reason to those schematics often found on the inside of a control panel door? Are they the result of trial and error, or is there some form of logic involved?

Well, back in the early 1800s an English mathematician named George Boole developed a system of logic known as Boolean algebra. It uses simple operators such as "if, and, or, & not" that can be combined to form precise and logical statements that test the "truth" of a series of events.

This is the basis for the design of all control systems, regardless of whether they are composed of simple switches and relays or complex microprocessors. It is also the basis for several modern computer programming languages. The schematics that define the logic of those controllers are simply a graphic representation of Boolean algebra. Let's look at a simple example.

Suppose you want your porch light to turn on for five minutes each time the doorbell rings. If "A" is the input from the doorbell and "C" is a timer that controls the light, the Boolean logic would be: "If A Then C." Translated into English, this statement says that if A is "true" (on), then C is also "true" (on). So every time the doorbell rings, the light will turn on and remain on until the timer shuts it off.

Now if you are a tightwad, you probably don't want that light to turn on during daylight hours. So you add a sensor (B) that provides an input when it is dark outside. If we add the sensor to our original logic, it becomes: "If A And B Then C." This statement says that both A and B must be true if the light is to turn on. In other words, it must be dark outside and the doorbell must ring before the light will turn on.

If you change "And" to "Or" the logic would be quite different. During the day the light will turn on each time the bell rings and, at night, it will stay on continuously. Figure 1 is a schematic representation of the different versions of this "ABC" logic. Boolean algebra is an elegantly simple method of defining the steps necessary to control some process.

 Until the 1980s, much of the hardware used to implement this logic consisted of electromagnetic relays. Relays are still popular today in simpler controllers because failures are easy to diagnose and they can be replaced as an individual component.

But the transistor has taken over much of this market because of price, function, and footprint. Now a couple of chips or integrated circuits (IC), measuring just a few square inches, contain hundreds of transistors and can replace dozens of relays.

The reason the transistor fits in so well is that, like the relay, it is also a switch. It is part of a family known as semiconductors. Picture it as a switch with three leads. Under normal circumstances the semiconductor material will not allow current to flow between leads 1 & 2, but if a separate current (an input from a switch or another transistor) is applied to lead 3, the semiconductor material becomes a conductor and leads 1 & 2 are connected.

When these little switches are integrated into a programmable logic controller (PLC) we end up with a computerized switching system that can provide a wide range of control functions. And, if the process requirements change, you can reprogram it to meet those changing conditions without rewiring the logic section.