A control loop is built from mechanical and electrical devices. These usually include
The controller will usually be located in a control room; typically, it exists as software within a Distributed Control System (DCS) computer. The other parts are physical equipment built into or adjacent to the process equipment.
Information is passed around the control loop in the form of signals. These may be analog or digital, electrical or pneumatic. Converters or transducers transform signals from one type to another.
A control system needs to be able to pass information around the plant. Valves need to be told whether to be wide open, partially open, or closed. Controllers need to know whether the measured variable is where it is supposed to be. Standardized signals are used to convey this information.
Signals can be digital or analog. Digital signals are encoded as binary numbers. Analog signals vary continuously from small to large. Many plants use a mixture of digital and analog; the age of the plant and control system usually determines what signals are used where.
Signal converters can be used to change from analog to digital (A/D converter) or vice versa (D/A converter). These converters often exist as software or a chip built into other hardware.
Digital signal handling is a little more complicated than analog; so it is usually best to learn to solve problems with analog signals and then later learn how to convert the methods to digital.
Signals can be represented as percentages, as electrical current, or as pressures in a pneumatic system.
Conceptually, many problems are best approached by thinking of the signal as a continuously varying percentage; numbers from 0-100 or 0-1.0. It is usually easier to solve problems with percentages first, and then convert to the appropriate physical signal.
Most analog signals today take the form of electrical current of continuously varying amperage. Standard signals range from 4 to 20 milliamps. Older pneumatic systems use compressed air of continuously varying pressure. Pneumatic signals are often commonly used to operate control valves and elements.
Electrical signals are routinely converted to pneumatic (I/P transducer); pneumatic signals can be converted to electrical as well (P/I transducer) although these are needed primarily in older plants with legacy pneumatic hardware.
Notice that the physical signals don't start at zero. This is to provide an easy way to distinguish between a minimum signal and a broken signal. If you're reading 4 mA, the signal corresponds to 0%; if you see 0 mA, you know there is a problem in the system.
You should be able to quickly convert from one type of signal to another.
A pneumatic signal ranges from 3 to 15 psig, or a total of 12 units. 25% of 12 is 3, so a 5% signal in a pneumatic system is 3+3 = 6 psig. Similarly, in an electronic signal it is 4 + 0.25(16) = 8 mA.
A 15 mA signal is 11 units above minimum, or 11/(20-4) = 68.75%. The pneumatic equivalent is 3 + 0.6875(12) = 11.25 psig.
A 12 psig signal is (12 - 3)/(15-3) = 75%, so it is the same as 4 + 0.75(20-4) = 16 mA.
References:
R.M. Price
Original: 10/15/93
Modified: 3/1/96, 2/24/97, 3/23/98; 4/25/2003
Copyright 1996, 2003 by R.M. Price -- All Rights Reserved