What is PLC ?

A programmable logic controller (PLC) is an industrially hardened computer-based unit that performs discrete or continuous control functions in a variety of processing plant and factory environments. Originally intended as relay replacement equipment for the automotive industry, the PLC is now used in virtually every industry imaginable. Though they were commonly referred to as PCs before 1980, PLC became the accepted abbreviation for programmable logic controllers, as the term “PC” became synonymous with personal computers in recent decades

Difference between PC & PLC

The PC is designed to be flexible and handle thousands of different types of applications, but PLCs are especially designed for control.35 PCs started showing up on the factory floor in the mid-80s in programming and HMI applications. However, now the PC is migrating toward actual control. In response, PLC manufacturers have adapted PC technology, such as Ethernet. Some systems actually provide a “PC on a card” to plug into the PLC back plane.11 PLCs are often thought of as computers. To a certain extent this is true; however, there are important differences between PLCs and computers.

Real-Time Operation/Orientation the PLC is designed to operate in a real-time control environment. The first priority of the CPU is to scan the I/O for status, make sequential control decisions (as defined by the program), implement those decisions, and repeat this procedure all within the allotted scan time. Most PLCs have internal clocks and “watchdog timers” built into their operations to ensure that a software error like “divide by zero” or an endless loop does not send the central processor into an undefined state. When the watchdog time is exceeded, the processor shuts down in a predetermined manner and usually turns off all outputs. In real time systems, reliability is a big concern. PLC manufacturers’ experience shows mean time between failures (MTBF) ranging from 20,000 to 400,000 hours. “This is far in excess of almost any other type of electronic or control equipment.”

Environmental Considerations PLCs are designed to operate near the equipment they control. This means they function in hot, humid, dirty, noisy, and dusty industrial environments. Typical PLCs can operate in temperatures as high as 140F (60C) and as low as 32º F (0C), with tolerable relative humidity ranging from 0 to 95% noncondensing. In addition, they have electrical noise immunities comparable with those required in military specifications.

Programming Languages and Techniques PLC languages are designed to emulate the popular relay ladder diagram format. This format is read and understood worldwide by maintenance technicians as well as engineers. Unlike computer programming, PLC programming does not require extensive special training. Applications know-how is much more important. Although certain special techniques are important for programming efficiency, they are easily learned. The major goal is the control program performance. Another difference between computers and PLCs is the sequential operation of the PLC. Program operations are performed by the PLC in the order they were programmed (Figure 5.4o). This is an extremely useful feature that allows easy programming of shift registers, ring counters, drum timers, and other useful indexing techniques for real-time control applications.

Advantages & Disadvantages of PLC

Advantages:

 Very High Accuracy

 Low Power Consumption (Energy Saving)

 High Level human Safety

 Less Plan Running cost

 Small in Size (Required Lased space)

 Rugged Construction

 Easily programmable

 Easy Maintenance

 Economical & high Flexible

 Shorter project program

 Easy Documentation

Disadvantages:

 High Skilled Engineering Required for that high skilled person required

 Difficulty with changes or replacements

 Some high Initial Cost.

There are many PLC make companies like SIEMENS, ABB, MItshubishi, GE, Omron, Delta,Honeywell. They have their own software for the PLC programming.