ic controllers The workhorse of factory automation keeps things on track Programmable Logic Controllers are at the forefront of manufacturing automation. Many factories use Pro- grammable Logic Controllers to cut pro- duction costs andor increase quality. Since its predecessor was hard-wired relay panels, the Programmable Logic Controller uses a unique language called ladder logic. Although other lan- guages are used, ladder logic presently remains the dominant language of automation. The Programmable Logic Controller (PLC) is sometimes called a Programmable Controller (PC), but the abbreviation PLC is preferred to distin- guish it from the Personal Computer. PLCs developed out of the need to replace the hard-wired relay panels. In the 1960s, a typical automated assem- bly or other manufacturing line had a cabinet of relays wired to control the operation. As one might expect, debug- ging relay failures could be time-con- suming, and changing functionality by modifying the sequence of operations was time-consuming and costly because of the required rewiring. In 1968, the Hydramatic Division of General Motors Corporation (GM) spec- ified design criteria for the first PLC. (They had to rewire many relay panels annually for car model year changes.) Some major specifications were: 1. Easily programmed and repro- grammed, preferably in-plant to alter its sequence of operations. 2. Easily maintained and repaired- preferably with plug-in modules. 3. Capable of operation in a plant environment. 4. Smaller than relay equivalent. 5. Capable of communicating with central data collection system. 6. Cost-competitive with solid-state and relay logic systems then in use. A handful of companies responded to develop the device we now call a PLC in late 1969 and early 1970. The fKst PLCs just basically replaced hard- wired relay logic. Today, PLCs are available in a wide range of capabilities and cost. There are five general categories of PLCs avail- able. The general capabilities of each category are: Micro PLCs: Generally have the basic relay instructions, counters, and timers with up to 32 digital input/output (UO) points (fined number of each) and 2K words of program memory built into a compact unit. Small PLCs: Added capabil- ities of analog YO, expandable I/O of up to 128 points, 4K words program memory, shift register and sequencer instruc- tions, and primitive communi- cations with other PLCs. Medium PLCs: Expand- able U0 of up to about 1024 points and 32K words pro- gram memory, remote I/O, basic math and data handling instructions, subroutines, interrupts, functional block or high-level language, local area network connection. Large PLCs: Expandable and 256K words program memory, enhanced math and data handling instructions, PID control. Very Large PLCs: Expandable I/O of up to about 8192 points and 4M words program memory. PLC architecture The architecture of a general PLC is shown in Fig. 1. The main parts of a PLC are its processor, power supply, and input/output (I/O) modules. In a micro PLC, all three main parts are YO of up to about 2648 points Fig. 1 Architecture of typical PLC 14 0278-6648/96/$5.00 0 1996 IEEE IEEE POTENTIALS Authorized licensed use limited to: Zehaie Hailu. Downloaded on January 14, 2010 at 20:48 from IEEE Xplore. Restrictions apply.enclosed in a single unit. For larger PLCs, these three parts are separately purchased (depending on desired func- tionality), and combined to form a PLC. The programming device, often a per- sonal computer, connects directly to the processor through a serial port or remotely through a local area network. Depending on the manufacturer, the local area network interface may be built into the processor, or may be a separate module. Many of the PLC local area networks are proprietary to one manufacturer. However, interfaces to standard networks, such as Ethernet, have recently been introduced. The architecture of the PLC is basi- cally the same as a general purpose computer. In fact, some of the early PLCs were computers with special I/O. However, some important characteris- tics distinguish PLCs from general pur- pose computers. They can be placed in an industrial environment that has extreme temperatures (typically up to 160”F), high humidity (up to 95%), electrical noise, electromagnetic inter- ference, and mechanical vibration. They are easy to use by plant technicians. Hardware interfaces are easily connect- ed. Modular and self-diagnosing inter- face circuits pinpoint malfunctions and are easily replaced. They are pro- grammed using ladder logic, which is easy to learn. The PLC executes a sin- gle program in an orderly and sequen- tial fashion. However, most medium to large PLCs have instructions that allow subroutine calling, interrupt routines, and the bypass of certain instructions. Also, many PLCs have modules that implement higher-level languages, such as C and BASIC. Ladder logic The IEC 113 1 international standard defines four PLC languages: ladder logic, sequential function charts, func- tion blocks, and a text language. By far, ladder logic is the most prevalent lan- guage. The ladder logic symbology was developed from the relay ladder logic wiring diagram. In order to explain the symbology, simple switch circuits will be converted to relay logic and then to PLC ladder logic. Consider the simple problem of turn- ing on a lamp when both switches A and B are closed, Fig. 2(a). Fig. 2(b) is a truth table of all possible combinations of the two switches and the consequent lamp action. To implement this function using relays, the switches A and B do not con- circuit, d) equivalent ladder logic nect to the light directly. Instead, control relay coils, whose contacts are normally open, control the light, Fig. 2(c). The switches appear as inputs to the circuit. The output (lamp in this case) is not driven directly, but driven by another relay to provide voltage isolation from the relays implementing the logic. The switches control relay coils so that the inputs are isolated from the logic. Also, this way one input can be used multiple times by using the multiple poles (con- left to right. One would interpret the rung symbology as: “When input (switch) A is ON and input (switch) B is ON then the lamp is ON.” If the example is changed to turn on a lamp when either switch A or B is closed, then the two contacts are placed in parallel. Now consider the implementation of a logical NOT
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