R
R. Suresh
(Originally posted Mon. 1/26/98)
> Don Lavery wrote:
> Was the questioning due to the fact that PLC's were prone to software
> crashes and hardware failures, or was it pure reluctance to use > > something new and different?
> PC's are NOT noted for working every time,
> even right out of the box ?
> it seems to me that those who are currently
> reluctant to implement PC's in a control environment have a pretty > > solid foundation on which to base their opinions.
> >Carl Lemp <[email protected]> wrote:
> > It's funny how history repeats itself. I seem to
> > remember control engineers questioning the reliablility and
> appropriateness of PLCs
I can confirm this. I come with a background of 20 years with Siemens, in Germany and in India , right from the days before PLCs were born (or were essentially LCs - ie wired-"programmable" modules). The first system I implemented for automation of a complete cement plant had 14
logic centers, but no PLCS! The clients had not much confidence in the PLCs. (We had even less! - No not because we suspected the electronics
robustness, but the functioning in our Indian ambiance!)
The logic centers were constructed modularly , entirely with contactor logic, but suitable for direct replacement with PLCs at a later date
(remove the contactor baseplate, reconnect the terminal wires to the PLC mounted base plate. We did implement the PLCs 3 years later at the same plant.
The first PLC locations (Why, even Variable frequency drives) required back up systems to be parallel wired !
These are not entirely due to experience of the new electronics failing, but more a mind set.
I now run several companies engaged in designing bus-linked modules, PLCs (CAN Bus) and applications concentrating on drives, industrial
controls and BMS. We have implemented several systems with bus-linked modules (upto 120 nodes in some cases) entirely orchestrated for signal exchange and logging by the central PC. The programs were originally under DOS platform. The PCs work 24 hours, 365 days a year. NULL PROBLEMO! We have at worst, 1 breakdown call per year, and this is normally due to a bus disconnection.
Today we offer Windows based systems. We have also developed buffer intelligent (controller) interfaces to the buses.
Our personal experience is -
1. PLCs are indeed far more robust than commercial PCs. I would not include industrial grade PCs in this comparison.
2. Intelligent buffers developed to link the nodes to the PCs were a result of power considerations (UPS for PC costs more than a 24/12 V battery backup system)
3. PC failures at hard disk levels have almost been negligible, even though we would have normally placed this as the most failure prone
area (moving mechanism)
4. Windows OS (and beyond) add a large amount of code and hardware superiority to make the OS function efficiently, but reduce the MTBF, for the very same reason. We have had more crashes in Windows based systems than in DOS based. Clearly, the systems are better looking, more powerful, more salable - but, also more failure prone! I am sure this would change too, given time. Given that more efficient programmers are needed for the newer OS, less lazy too (!) the time needed to master each level of the new hardware and software is getting worse than keeping up with the Jones's !
5. The observation 4 above is NOT a mind set, but very real.
6. PC programming does permit use of a variety of Tricks (undocumented or otherwise), but the PLCs are not beyond these (different methods to
achieve a more efficient end). The CAN open poses us enough challenges to carry out multi CPU dialog as would probably a PC level program to
build simultaneously tabular compilations of field data and graphical display of same.
7. Given the tasks of controlling and monitoring, every solution, be it using a PC or a PLC is as good, unless these systems are so expensive and need a longevity without upgradation of several decades. The sole criteria would be that the solution clearly meets the need.
I am sure that the above views are debatable, and look forward to more views on the subject.
Best regards to all
Suresh
========================================================
From: ICON microcircuits & Software Technologies pvt ltd
12, First Street, Nandanam Extension, Madras-600035, INDIA
Ph: +91-44-4321857
Fax :+91-44-4335578
EMail : [email protected]
> Don Lavery wrote:
> Was the questioning due to the fact that PLC's were prone to software
> crashes and hardware failures, or was it pure reluctance to use > > something new and different?
> PC's are NOT noted for working every time,
> even right out of the box ?
> it seems to me that those who are currently
> reluctant to implement PC's in a control environment have a pretty > > solid foundation on which to base their opinions.
> >Carl Lemp <[email protected]> wrote:
> > It's funny how history repeats itself. I seem to
> > remember control engineers questioning the reliablility and
> appropriateness of PLCs
I can confirm this. I come with a background of 20 years with Siemens, in Germany and in India , right from the days before PLCs were born (or were essentially LCs - ie wired-"programmable" modules). The first system I implemented for automation of a complete cement plant had 14
logic centers, but no PLCS! The clients had not much confidence in the PLCs. (We had even less! - No not because we suspected the electronics
robustness, but the functioning in our Indian ambiance!)
The logic centers were constructed modularly , entirely with contactor logic, but suitable for direct replacement with PLCs at a later date
(remove the contactor baseplate, reconnect the terminal wires to the PLC mounted base plate. We did implement the PLCs 3 years later at the same plant.
The first PLC locations (Why, even Variable frequency drives) required back up systems to be parallel wired !
These are not entirely due to experience of the new electronics failing, but more a mind set.
I now run several companies engaged in designing bus-linked modules, PLCs (CAN Bus) and applications concentrating on drives, industrial
controls and BMS. We have implemented several systems with bus-linked modules (upto 120 nodes in some cases) entirely orchestrated for signal exchange and logging by the central PC. The programs were originally under DOS platform. The PCs work 24 hours, 365 days a year. NULL PROBLEMO! We have at worst, 1 breakdown call per year, and this is normally due to a bus disconnection.
Today we offer Windows based systems. We have also developed buffer intelligent (controller) interfaces to the buses.
Our personal experience is -
1. PLCs are indeed far more robust than commercial PCs. I would not include industrial grade PCs in this comparison.
2. Intelligent buffers developed to link the nodes to the PCs were a result of power considerations (UPS for PC costs more than a 24/12 V battery backup system)
3. PC failures at hard disk levels have almost been negligible, even though we would have normally placed this as the most failure prone
area (moving mechanism)
4. Windows OS (and beyond) add a large amount of code and hardware superiority to make the OS function efficiently, but reduce the MTBF, for the very same reason. We have had more crashes in Windows based systems than in DOS based. Clearly, the systems are better looking, more powerful, more salable - but, also more failure prone! I am sure this would change too, given time. Given that more efficient programmers are needed for the newer OS, less lazy too (!) the time needed to master each level of the new hardware and software is getting worse than keeping up with the Jones's !
5. The observation 4 above is NOT a mind set, but very real.
6. PC programming does permit use of a variety of Tricks (undocumented or otherwise), but the PLCs are not beyond these (different methods to
achieve a more efficient end). The CAN open poses us enough challenges to carry out multi CPU dialog as would probably a PC level program to
build simultaneously tabular compilations of field data and graphical display of same.
7. Given the tasks of controlling and monitoring, every solution, be it using a PC or a PLC is as good, unless these systems are so expensive and need a longevity without upgradation of several decades. The sole criteria would be that the solution clearly meets the need.
I am sure that the above views are debatable, and look forward to more views on the subject.
Best regards to all
Suresh
========================================================
From: ICON microcircuits & Software Technologies pvt ltd
12, First Street, Nandanam Extension, Madras-600035, INDIA
Ph: +91-44-4321857
Fax :+91-44-4335578
EMail : [email protected]