Control Devices and PLC

Control Devices and PLC

Joseph La Fauci

From Wikipedia, the free encyclopedia

Several types of control devices are used in industry to satisfy the following control needs.

Mechanical Control、Pneumatic Control、Electromechanical Control、Electronic Control、Computer Control、Programmable Logic Control (PLC) Mechanical control includes cams and governors. Although they have been used for the control of very complex machines, to be cost effectively, today they are used for simple and fixed-cycle task control. Some automated machines, such as screw machines, still use cam-based control. Mechanical control is difficult to manufacture and is subject to wear.

Pneumatic control is still very popular for certain applications. It uses compressed air, valves, and switches to construct simple control logic, but is relatively slow. Because standard components are used to construct the logic, it is easier to build than a mechanical control. Pneumatic control parts are subject to wear.

As does a mechanical control, an electromechanical control use switches, relays, timers, counters, and so on, to construct logic. Because electric current is used, it is faster and more flexible. The controllers using electromechanical control are called relay devices.

Electric control is similar to electromechanical control, except that the moving mechanical components in an electromechanical control device are replaced by electronic switches, which works faster and is more reliable.

Computer control is the most versatile control system. The logic of the control is programmed into the computer memory using software. It not only can be for machine and manufacturing system control, but also for data communication. Very complex control strategies with extensive computations can be programmed. The first is the interface with the outside world. Internally, the computer uses a low voltage (5 to 12 volts) and a low current (several milliamps). Machine requires much higher voltages (24, 110, or 220 voltages) and currents (measured in amps). The interface not only has to convert the voltage difference, but also must filter out the electric noise usually found in the shop. The interface thus must be custom-built for each application.

In order to use the advantages of all those controllers and eliminate the difficulties, the programmable logic controllers were invented. A PLC was a replacement for relay devices. They are programmed using a ladder diagram, which is standard electric wiring diagram. As PLCs become more flexibility, high-level as well as low-level languages are available to PLC programmers. PLCs have the flexibility of computers as well as a standard and easy interface with processes and other devices. They are widely accepted in industry for controlling from a single device to a complex manufacturing facility.

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Automatic of many different processes, such as controlling machines or factory assembly lines, is done through the use of small the computers called a programmable logic controller (PLC), PLCs were first created to serve the automobile industry, and the first PLC project was developed in 1968 for General Motors to replace hard-wired relay systems with an electronic controller. Since the advent of PLCs, the ability to centralize factory processes, especially in the automotive industry, has improved greatly.

Automatic control has become an important consideration in most industrial processes where certain repetitive operations are performed. This applies to situations such as the automatic assembly of modules and products where a cycle of events is conducted in a consistent and uniform manner. Applications generally include a combination of feeding, handing, drilling, cutting, assembling, discharging, inspecting, packaging and transporting by conveyor. Prior to the introduction of computer-based control systems the automation of such events was achieved by using either electrical relay logic circuits or pneumatic logic circuits. Although these are conceptionally simple and easy to maintain, they are somewhat bulky and can be expensive. More important is the fact that the resulting control circuits are inflexible and do not lend themselves to easy system control alterations.

The late 1960s saw the introduction of the programmable logic controller (PLC) as a direct replacement for the relay sequence controllers. In essence the PLC replaces the hardwired relay or pneumatic logic with a more flexible programmable logic. It offers a simple, flexible and low-cost means of implementing a sequence control strategy where outputs for switching devices on and off are set according to input conditions as read from digital sensor states. It should be noted that, particularly in the USA, the PLC is often referred to as a „programmable controller‟ with the abbreviation of PC. It should not be confused with the personal computer „PC‟ or IBM-PC.

The PLC is composed of the same ingredients as a microcomputer such as a microprocessor, memory and input/output facilities. The processor executes the instructions held in memory by operating on inputs derived from the controlled process and providing outputs in accordance with the logic sequence defined in the control program. Its basic principle of operation during the execution of the program is that the program is scanned very fast, typically 1 to 20 us per step, to record all input states. The outputs are then set according to the logic specified in the program. The sequence is continually repeated for each scan period of the controller.

Small PLCs dedicated to sequential control have typically 12 inputs and 8 outputs with the possibility of expansion up to 128 I/O lines. They come complete with an input interface to accommodate a range of input signals from the controlled process which are then converted to an appropriate from for the processor. Similarly, provision is made at the output of the PLC to interface with a variety of process hardware such as lamps, motors, relays and solenoids. The

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typical handing voltages are 24V DC and 110V AC.

Program instructions can be input into the battery backup RAM of a PLC by means of either a hand-held programming keypad or a connected PC with an appropriate software development package. Some LCD programming consoles incorporate a limited graphical display which illustrates the program in ladder logic format as the programmer builds it up using symbolic keys. This is also the principle of the PC-based development system where additionally the programmer has access to a lager visual display and the PC‟s disk operating system for data storage and retrieval. Once the program has been debugged and the control strategy verified by simulation, the codes can be loaded into an erasable and programmable read only memory chip (EPROM) which can then be installed in the PLC.

There are a large number of manufacturers of PLCs. Although some use their own particular software language the majority are based on the ladder logic diagram. Historically this was introduced in order to gain the acceptance of customers who were interested in moving from hardwired relay control systems to the PLC. In addition to the basic input/output facilities the PLC also contains timers, counters and other special functions.

As PLC technology has advanced, so have programming languages and communications capabilities, along with many other important features. Today‟s PLCs offer faster scan times, space efficient high-density input/output systems, and special interfaces to allow non-traditional devices to be attached directly to the PLC. Not only can they communicate with other control systems, they can also perform reporting functions and diagnose their own failures, as well as the failure of a machine or process.

Size is typically used to categorize today‟s PLC, and is often an indication of the features and types of applications it will accommodate. Small, non-modular PLCs (also known as fixed I/O PLCs) generally have less memory and accommodate a small number of inputs and outputs in fixed configurations. Modular PLCs have bases or racks that allow installation of multiple I/O modules, and will accommodate more complex application.

When you consider all of the advances PLCs have made and all the benefits they offer, it‟s easy to see how they‟ve become a standard in the industry, and why they will most likely continue their success in the future.

Communicating with a variety of other control devices has not been strength of traditional PLC networks. Many industrial controllers are quipped with an RS232 serial port for the transfer of data to and from other digital control devices in a system.

PLCs face ever more complex challenges these days. Where once they quietly replaced relays and gave an occasional report to a corporate mainframe, they are now grouped into cells, given new jobs and new languages, and are force to compete against a growing array of control products. For this year‟s annual PLC technology update, we queried PLC makers on these topics and more.

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Higher level PLC programming languages have been around for some time, but lately their popularity has been mushrooming. As Raymond Leveille, vice president & general manager, Siemens Energy & Automation, Inc. Programmable Controls Division, points out: “ As programmable controls are being used for more and more sophisticated operations, languages other than ladder logic become more practical, efficient, and powerful. For example, it‟s very difficult to write a trigonometric function using ladder logic.” Language gaining acceptance includes Boolean, control system flowcharting, and such function chart languages as Graphcet and its variations. And there‟s increasing interest in languages like C and BASIC.

Thus far, PLCs have not been used extensively for continuous process control. Will this continue? “The feeling that I‟ve gotten,” says Ken Jannotta, manager, product planning, Series one and Series Six product, at GE Fanuc North America, “is that PLCs will be used in the process industry but not necessarily for process control.”

Several vendors-obviously betting that the opposite will happen-have introduced PLCs optimized for process applications. Rich Ryan, manager, commercial marketing, Allen-bradley Programmable Controls Div. cites PLCs‟ increasing use in such industries as food, chemicals, and petroleum.

While there are concerns about the lack of compatible communications between PLCs from different vendors, the connection at the other end-the I/O-is even more fragmented. With rare exceptions, I/O is still proprietary. Yet there are those who feel that I/O will eventually become more universal. GE Fanuc is hoping to do that with its Genius smart I/O line. The independent I/O makers are pulling in the same direction.

Many say that I/O is such a high-value item that PLC makers will always want to keep it proprietary. As Ken Jannotta, says: “The I/O is going to be a disproportionate amount of the hardware sale. Certainly each PLC vendor is going to try to protect that.” For that reason, he says, PLC makers won‟t begin selling universal I/O systems from other vendors. “If we start selling that kind of product,” says Jannotta, “what do we manufacture?”

With more intelligent I/O appearing, Sal Provanzano feels this will lead to more differentiation among I/O from different makers. “Where the I/O becomes extremely intelligent and becomes part of the system,” he says, “it really is hard to define which is the I/O and which is the CPU. It really starts to become distributed processing. Now, in order for that distributed processing to work, the CPU, if you will, is equally integrated into the system as the I/O.”

While different PLCs probably will continue to use proprietary I/O, several vendors make it possible to connect their I/O to IBM PC (personal computer)-compatible equipment. Allen-bradley, Gould, and Cincinnati Milacron already have, and rumor has it that GE is planning something along these same lines.

“There are inherent architectural differences between a general purpose

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computer,” says Rich Ryan, “and a programmable controller. There are hardware constructs built into almost every manufacturer‟s programmable controller today that customize the hardware to run ladder logic and to solve machine code.” One fundamental difference he cites is called state of the machine. Ryan: “When you shut the machine off, or interrupt the cycle, or you jump to another spot in the cycle, programmable controllers inherently remember the state of the machine: what the timers were, what the counters were, and what the states of all the latches were. Computers don‟t inherently do that.”

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