PLC : Basic Long Explanation

A programmable logic controller, or PLCS, is a software-based equivalent of a relay panel. A programmable logic controller ( PLCS ) is a general-purpose device. One programmable logic controller ( PLCS ) can be programmed to control a variety of machines, and programs can be changed easily for new jobs or changes in production routines.

Programmable logic controllers ( PLCS ) were once primitive devices capable of providing only minimal feedback about machine operation and status. The situation has changed drastically, however, with the advent of more powerful computer chips and new standards that give a programmable logic controller ( PLCS ) access to information throughout a manufacturing plant. Whereas the first programmable logic controllers ( PLCS ) generally provided only limited information about the status of relay contacts, new monitoring capabilities let the user know exactly what is happening on the floor.

Some programmable logic controllers ( PLCS ) are equipped to solve problems involving mathematical functions such as sine, cosine, tangent, xy, y root of x, e sub x , natural logarithms, and common logarithms. Such calculations are often required for energy management, process control, process modeling, real-time error correction, and many other applications.

And while ladder logic is still the standard industry programming language for programmable logic controllers ( PLCS ), the trend is toward state logic, sequential function charts, graphics, and versions that are programmable in Basic, C, or other high-level languages.

The ability to handle analog signals along with arithmetic and other complex calculations has made programmable logic controllers ( PLCS ) suitable for the control of processes as well as for the control of machines. Typical applications for programmable logic controllers ( PLCS ) are mineral and chemical processing, water and waste treatment, and petroleum collection and distribution. In many of these applications a programmable logic controller ( PLCS ) can complement conventional analog control systems by handling sequence problems as well as a portion of the analog calculation and control. In support of those functions, some programmable logic controllers ( PLCS ) now have the ability to store recipes for batch processing, reducing the need for manual inputs.

In further support of their process-control capabilities, some programmable logic controllers ( PLCS ) can be equipped to solve complex equations such as proportional-integral-derivative equations required for the control of many processes. A sophisticated programmable logic controller ( PLCS ) is capable of performing these calculations on many different portions of a process simultaneously.

Programmable logic controllers ( PLCS ) are also capable of producing analog outputs and of providing position control functions. Programmable logic controllers ( PLCS ) can even provide control functions normally performed by numerical controls.

A modern programmable logic controller ( PLCS ) can also pass information back to the operator. It can print out its own ladder diagram for record, review, or change, or it can provide status or progress reports in English on a CRT or printer routinely or on request. A programmable logic controller ( PLCS ) can also display messages in English to summarize data or guide the operator.

Data-analysis programs are becoming increasingly common. A spreadsheet format is often used. Usually each programmable logic controller ( PLCS ) is assigned a tag or number. Parameters such as data type, coil, input, and addresses are tracked. The programmable logic controller ( PLCS ) initiates changes to data within the computer database, which initiate other control tasks. For example, if the programmable logic controller ( PLCS ) closes a valve, a software routine can be started that measures resulting flow through the valve and sounds an alarm if the flow is not within desired limits.

The programmable logic controller ( PLCS ) also can track down external faults. This capability is useful because the machine and externally mounted control elements such as limit switches, solenoids, sensors, transducers, remote pushbuttons and selector switches are usually much less reliable, and more often a cause of machine downtime than the programmable logic controller ( PLCS ).

Other maintenance aids are available to help solve malfunctions. One feature intensifies on a CRT that portion of a circuit that is carrying current. Another feature lets an operator command specific inputs or outputs to be unconditionally turned on or off, thus helping a technician determine whether a problem is being caused by an internal or external failure.

Programmable logic controllers ( PLCS ) robot control: Programmable logic controllers ( PLCS ) are often equipped with special firmware (software programmed by programmable logic controller ( PLCS ) manufacturers) through which the controllers can perform many complex procedures according to simple instructions in user programs. Sequencer firmware for programmable logic controllers ( PLCS ) provides the programming, storing, and accessing of data required for simulating electronic, or electromechanical sequencers and programmers. Sequencer data are stored in a data-table section of programmable logic controller ( PLCS ) memory, a section separate from that available for user programs. Programmable logic controllers ( PLCS ) containing sequencer firmware are especially useful for controlling robots where the final position of each movement is determined by limit switches or other on-off, position-feedback devices.

Robot movements can be altered by using different masks for different robot tasks. One method of loading or programming sequencers is with a "teach" mode. This is a technique for loading or setting up the on-off contacts in a sequencer to correspond with the on-off status of I/O points. For this method, a robot is jogged into a desired position. Pressing a "teach" pushbutton energizes a sequencer load instruction that causes the on-off status of all pertinent inputs to be copied into a sequencer step. By jogging the robot through steps sequentially, in each case pressing the "teach" pushbutton, a programmable logic controller ( PLCS ) is "taught" a sequence of movements.

Robots having closed-loop systems are controlled by programmable logic controllers ( PLCS ) through digital-to-analog (d/a) I/O modules. These modules convert digital PC signals to analog outputs having ±10-V range. The outputs serve as speed references for the hydraulic or electrical servosystems that typically power each axis. Each axis is mechanically coupled to a potentiometer or encoder that feeds position and velocity data back to a programmable logic controller ( PLCS ), closing a feedback control loop. Digital commands from a programmable logic controller ( PLCS ) to a converter initiates motion, sets acceleration rates, determines speeds, and initiates deceleration. Motion is halted when the feedback indicates the robot has reached the proper position.

Suitably equipped programmable logic controllers ( PLCS ) can generally control point-to-point or vectored motion. Point-to-point movement along each axis is initiated and halted independently of other axes. This type of motion is easily programmed, requires little memory, and is suitable for many robot applications.

Vectored motion, however, requires that the movements along two or more axes be interdependent. The programmable logic controller ( PLCS ) adjusts acceleration rates and speed for each axis so that all movement terminates simultaneously. By this means a robot arm moves from point to point along the shortest path. Vectored motion control usually reduces the time required for each move. To perform vectored control, programmable logic controllers ( PLCS ) must be equipped with arithmetic firmware that calculates speeds required for each axis. Although standardized algorithms are used for these calculations, vectored motion programs are somewhat more complex and require more memory than those for point-to-point motion.

Computers have expanded programmable logic controller ( PLCS ) power through greater speed and programming flexibility. Today's programmable logic controllers ( PLCS ) almost always have a port that permits a user to tie into a computer. Three developments have helped bring about this integration of Programmable logic controllers ( PLCS ) and computers: "smart" programmable logic controllers ( PLCS ) with their own microprocessors and memory, multitasking software, and local-area networks (LANs).

New software has enhanced the capability of computers, particularly personal computers to operate with programmable logic controllers ( PLCS ). Until recently, small computers were limited to performing one task at a time. If a computer was being used to check the status of a programmable logic controller ( PLCS ), it could not perform data analysis or generate a report at the same time. However, with the development of concurrent DOS (disk operating system), the simultaneous juggling of two different tasks can be done.

More powerful microprocessors have resulted in programmable logic controllers ( PLCS ) able to perform multiaxis control and able to link with sophisticated vision systems. One reason new programmable logic controllers ( PLCS ) are taking on such a wide range of duties is that they can be installed in modules, thus simplifying any needed customization and future expansion. Because each programmable logic controller ( PLCS ) contains its own internal communication highway or bus, a programmable logic controller ( PLCS ) can be upgraded with additional memory or processing capability and can be added by snapping in additional modules.

Various programmable logic controller ( PLCS ) modules add RS-232 communication ports, multiaxis control and fault annunciation.

Programmable logic controller ( PLCS ) software can be developed either on or off line; data-management and analysis programs are available. Many of the programmable logic controllers ( PLCS ) can be programmed from an IBM PC or compatible machine, and special industrially hardened programmers are also available when extreme temperature, dust, and vibration are problems. I/O stations can be located up to 2,000 ft from the CPU.

A programmable logic controller ( PLCS ) is no more powerful than the software available for it. Two relatively recent developments for programmable logic controllers ( PLCS ) are menu-driven software and concurrent operating systems, which have simplified programming and made it more useful. Programs using menus allow an operator with only minimal training to monitor, analyze, and manage processes. Concurrent operating systems switch back and forth between two different programs so fast that both appear to be running at the same time.