The US and Europe have been taken by storm by the latest PC-based control systems supported by Microsoft Windows. South Africa has only just started exploring this newest trend in process control with the introduction by Xycom SA of Steeplechase SoftPLC software from USA. The debate is now on as to whether or not SoftPLCs are the way of the future and can they be more effective than the traditional PLCs.
Input and output structures I/O is an important factor to consider when selecting the right system. The number of I/O points or devices will dictate the size and cost of a large portion of the control system. Once the number of I/O points have been determined for an application, it is important to remember that PLCs have limitations when it comes to the number of devices that can be connected. Typical limitations include eight or 16-point modules per rack - unless they are analog I/O in which case only two modules per rack are possible.
The important thing to realise with a traditional PLC is that mixing the I/O from different vendors cannot be done. (Three analog modules from PLC vendor 'x' cannot be added to an existing system purchased from vendor 'y' even though they may be much better and more cost-effective.) On the other hand with SoftPLC systems, I/O can be handled from various vendors. Whatever I/O is best for the job may be chosen, and the SoftPLC will talk to that I/O. For reasons of conformity it is unlikely that a user would select digital input cards from vendor 'x' as well as digital input cards from vendor 'y' and then wire them all into the same panel - it would complicate installation unnecessarily. The SoftPLC would have no problem with this. A situation where this can be extremely useful, and often is the case, is where there is a PLC (which has been running a plant for the last 10 years) and the plant requires expanding, but the PLC is obsolete. Perhaps only three more I/O modules are needed - but such modules are no longer available. The traditional PLC vendor expects the user to replace old PLCs with the latest. The SoftPLC system permits replacing the 10-year old PLC CPU with a SoftPLC and allowing it to 'talk' through the existing I/O. Thereafter, what would normally only been achievable by adding more expensive PLCs would be accomplished by adding low-cost I/O expansion cards - from any vendor that suits the client.
PLC CPU
The traditional PLC has a CPU processor, which has the memory, performs the logic and talks to the I/O cards. Most customers are reasonably happy with the reliability of these cards but not with the performance. They have limited memory, have to be programmed with expensive programmer packages which are not very user-friendly and which are dedicated (programming PLC 'x' with a programmer package from PLC vendor 'y' cannot be done).
The SoftPLC runs on a PC - the immediate cry is "unreliable". But one should consider how seldom PC CPUs are known to break down. Xycom says that in their experience of selling PC's, CPU failures are virtually unknown. While PC software may hang up, the hardware does not. In fact, Xycom says that PCs are more reliable than PLC hardware. The reason for this is that PLCs are manufactured in their thousands but PCs are manufactured in their hundreds of millions. A PC manufacturer can afford to spend considerably more money on reliability testing than a PLC manufacture, as a reliability problem on 100 000 000 PCs will quickly bankrupt the manufacturer - so reliability is of paramount importance.
It is not possible to divorce the PC hardware from the PC's software operating system (such as Windows NT) and this can be unreliable. This is the clever thing with the new SteepleChase software, which Xycom SA is now introducing; the SteepleChase SoftPLC software runs on the hardware beneath NT. Xycom says that during an NT crash (the 'blue screen of death') the SteepleChase software will continue to control the plant without a problem.
PCs permit fast sampling rates - a scan time of sub 1 ms is standard (normally the I/O speed is the determining factor). Use can be made of all the standard PC tools, without the need for expensive programmers, or programmer packages. Instead use is made of MS Word, Excel, etc. Communicating with the next level in the control hierarchy (eg a scada) is very simple because the data is already in a PC so the scada can be on the same PC (very fast processing) or by using the PC's Ethernet network (cheap, simple, quick). Redundancy, which is normally a problem with a traditional PLC, is simple to implement with SoftPLCs.
Xycom says that that with today's technology, a SoftPLC can perform better than a PLC in most applications. Speed and device connectivity is not an issue. A personal computer excels when data acquisition in a plant-wide system is required, easily able distribute data over an available network. PCs can store far more information in memory than a PLC due to their extensive RAM and disk space capabilities.
Chrysler Corporation (USA) achieved what is claimed to be the fastest ever launch on a new 70-node line; it reduced its time to commencement of production by 90%, cut its floor space by 40% and reduced design and installation time by 12 weeks. Honda reduced its downtime by 90%; error recovery was reduced from 20 min to less than 2 min. Engineering time devoted to line modifications was reduced by 75% at the same plant.
Xycom believes that there is great potential for similar improvements in South African Industry, and that SoftPLCs are going to control the new millennium.
Xycom
(011) 462 6671
sharonm@xycom.co.za
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