The PLC has come a long way from its development in the seventies, whilst their speed will never match that of the PC but who wants a 3GHz PLC, its overall reliability, customization and ability to work under virtually any conditions around the world make the PLC the only choice when process control is required.
While the PLC is without doubt the number one choice in process control, this doesn't mean it is without problems there are two areas that have caused some problems for engineers. The first problem was that the original concept back in the seventies was to develop a device that could be easily and quickly programmed, so Ladder logic was developed to make it familiar to the electrical engineer, however over the years PLC's were being supplied by several major names such as: Allen Bradley - Omron - Siemens - Mitsubishi - Square D, and here the problem was created, each of them developed their own style of Ladder logic.
For developers this meant learning several different styles of programming and also how to use the differing development tools, while the developer could cope with this, a large number of maintenance engineers found it very difficult. I personally know one international company that has 3 different PLC's and the maintenance engineers only know one and they struggle with that one, the lack of control within the company means the company downtime and efficiency is far below what it could be.
The second problem is new, PLC's have to be robust to survive the industrial environment, the designs up till 2001 were very well suited and also lent themselves to rapid replacement of faulty modules, however some PLC designers have now developed PLC's where the surface area of plastic is only 50%, this is easily damaged, and during assembly into racks small metal objects such as screws can fall into the sensitive electronics.
Also gone is the backplane that allowed rapid replacement, now modules have to be slotted together, so if the middle one fails you have to strip the PLC down. Finally the miniature size causes wiring problems, the engineers I know tend to have fingers the size of bananas, so you can imagine the fun they have in rack assembly.
Despite these little drawbacks, the PLC is still far better than the PC, which has more the two failings one of which tends to be the Operating System.
Rockwell - a US product. Their Logix Platforms supply the end user with everything necessary to build a reliable application in control integration from Control to Drive to HMI Logix, operating over both Open and Proprietary networks.
Allen Bradley PLC
Omron - extensively used throughout Europe and Far East. They have extended their range of PLC's with the new CJ1 series for total machine control. The CJ series is set to replace the very popular and reliable C200H alpha series. Program development is via Omron's Cx Programmer, the latest version is v6.0, replacing the dated Syswin ver3.4.
Siemens, European product. For many years Siemens PLC's have been amongst those at the top, with safety, reliability and flexibility as their selling points.The Simatic range is very extensive and can provide an overall solution to process and machine control. Cost is expensive but then you are paying for a good product, however licensing and their custom programming laptop I consider too much, paying several thousand pounds against Omron and Mitsi few hundred pounds. Support is vital and if you are unfamiliar with the product then support is a priority.
Mitsubishi now have the FX3u series PLC, a more powerful and higher performance PLC.
It is very flexible and modular, allowing it to be expanded up to 384 IO, with instructions being processed in 0.065usec. Communicating via Profibus and Ethernet. Further details on this can be found via LC Automation and their INFORMATION ZONE.
Mitsubishi provide an extensive range of products from PLC's to Drives, Robots and HMI's and most importantly their overall customer support is good. The PLC development tool is GxDeveloper, this superceded Medoc, GX Developer is a bit dated, however they now have available GXiec, which is a very good development package, allowing you to create programs based upon IEC61131.
GXiec in our opinion is very good.
Keyence The world's smallest PLC with AC power supply built-in. Easy-to-use Access Window, Compact operator interface panel available. Fast processing with 10-µs interrupt and 30-kHz high-speed counters. Complete with User-friendly Windows® ladder logic software.
Typical PLC Control System