25-08-2017, 09:32 PM
Basic Experiments in PID Control for Non-electrical Engineers
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Introduction
As you read this little book and follow its directions you will be guided to an
understanding of proportional-integral-derivative (PID) control. However, the purpose of this
manual, and the value of learning PID control goes beyond PID control itself.
Assuming that you are not an electrical engineering student, it is possible that you lack a
basic understanding of electrical engineering. You may ask yourself, what is it that electrical
engineers do? Of course, a lack of understanding of basic principles in electrical engineering
limits your ability to understand most mechanical devices--since most are really
"electromechanical." This lack of understanding limits your imagination and prevents you from
dreaming up and designing new electromechanical devices to solve whatever problem you face--
whether the device is to be sold on the open market, integrated into a manufacturing process, or
simply used in a laboratory.
So, what are these basics that electrical engineers follow? The answer is really quite
simple. Also, you might be surprised to find out that these basics have been followed for over a
century and that they haven’t changed. Certainly, the devices themselves have changed--they’ve
become smaller, more reliable and more efficient - but not the basic principles.
Getting Ready
The readers of this little book are mechanical and aerospace engineering students, both
undergraduates and graduates, and others who wish to learn about basic analog design and PID
control. The belief followed here is that the best way to learn this material is not by watching a
video or through classroom instruction, but by a hands-on experience that progress at your own
pace.
So, to get ready, find a couple of hours of solitary time, and read over this little book
before you do anything else. In fact, read it over several times! When you’re ready, familiarize
yourself with the components that you’ll use. You will then be ready to build your PID control
system.
Analog Components
The Resistor
Perhaps the most basic component in an electric circuit is the resistor. Resistors dissipate
energy (which is converted into heat). A drawing of a typical resistor and the symbol that we use
to represent a resistor in circuit diagrams are shown in Fig. 1.
The Capacitor
The second component in a simple circuit is the capacitor. Capacitors are used to store
electrical charge q. If you apply a voltage v across the two leads of a capacitor, a specific
amount of charge will accumulate in the capacitor. The amount of charge is proportional to the
voltage. The proportionality constant is called the capacitance C. In other words, q = Cv, where
q is measured in Coulombs, C is measured in Farads, and again v is measured in volts (V).
The Operational Amplifier
As the word amplifier suggests, the function of an operational amplifier (op amp) is to
amplify a voltage. However, the operational amplifier does much more than that. It also
functions as a buffer and as a cascader—which are two functions that enable simple circuits to be
assembled into complex circuits to create higher level functions which are called operations¾
hence the name operational amplifier.
Op amps have five terminals that are important. The voltage that is amplified is the
difference between the voltage at the ‘+’ terminal vp and the voltage at the ‘-’ terminal vn, as
shown in Fig. 4. The amplified voltage is the output voltage vo.
Unlike the resistor and capacitor, which are both “passive” (unpowered) devices, the op
amp is an “active” device. Indeed, the op amp needs a voltage supply for the amplification. The
vs
+ and the vs
- terminals are the positive and negative supply voltages, respectively. The op amp
schematic and the chip that we’ll use are shown in Fig. 4.
Operational Amplifiers
In order to appreciate how to use op amps, it’s important to appreciate how they are used
to create analog circuits. Specifically, as mentioned earlier in this little book, op amps fulfill
three needs. First, they enable voltages to be amplified. Secondly, they can act as buffers, which
means that they can isolate the input voltage from the output voltage. Third, the output voltages
are independent of the output load, which means that the output voltages remain the same
regardless of the resistance (load) that the output is connected to. This enables op amp circuits to
be cascaded with other components. This property of buffering and cascading enables op amps
to be used with other analog components to create more complicated circuits which can perform
various operations—this is why they are called operational amplifiers.