M
Michael Griffin
(Originally posted Thurs, 3 Sep 1998)
I’ve used both PLCs and PCs (for test equipment), and I think we should be carefull about jumping to conclusions without considering the application in which they are being applied. I think we also need to define just what we mean by a “PC”.
I define a PLC as an industrial controller which is ready to apply right out of the box. There is little or no integration effort to get different components to work with each other.
A PC is the beige box you see sitting on your desk. It requires loading an operating sytem, and the application software before you can do anything. If you want a working system, you have to do all the system testing and integration yourself.
Industrial computers seem to fall somewhere in between the above two catagories. They have been around for decades, and actually predate the PLC.
I work in an industry with many small machines, most of which are custom built for our application by a variety of companies (both large and small). Typically each machine has its own small PLC. For these types of our applications, a PC (however defined) offers no advantages to us at all, and has the disadvantages of higher cost, larger size, etc. The total number of PLCs we have is about two hundred or more.
Other applications we have involve test equipment where we need a full keyboard, graphic screen, and a hard drive. We also need to be able to plug in specialised data aquisition boards, GPIB boards, vision processors, etc. In other words we need a PC. These systems are generally desk top type PCs in EEMAC 12 rated enclosures. These include both systems which we have built ourselves, and systems which we have purchased from other companies.
The software is written in Quick Basic, C, and Visual Basic. The operating systems used include DOS, Windows 95, and Windows NT. I have heard on this list many complaints about the “blue screen of death”, but this has not been a problem for us, not even with Windows 95 (although Windows NT has caused lots of problems when used in office applications).
However, we do have problems with hardware. Monitors, keyboards, power supplies, and hard drives all fail. Monitors and keyboards are easy enough to change, but power supplies and expecially hard drives are more of a maintenance problem. These last two items are beyond what we expect our electricians to be able to replace, and so have to be serviced by qualified computer repair technicians (we get the same company which services our office PCs to do this - they have at least one person permanently stationed on site at our plant). If a PC fails at night, it has to wait until morning to get repaired.
The PCs seem to run OK for about 3 or 4 years, and then start to give trouble. When they do go wrong, they are much more of a headache, and take much longer to fix than a PLC would. Fortunately, the hardware which has Windows 95 and NT on it is all relatively new, so those ones haven’t started to fail on us yet. Some of the DOS systems are AT&T 6300s which were probably built in the late 1980s. This would not be considered very old for normal industrial equipment, but these PCs are difficult to keep going.
I had one 4 year old PC fail late last week (right about when this thread started), and another somewhat newer one failed a couple of weeks ago. Both were a big headache.
Another problem with PCs is their lack of standardisation. There have been so many bus systems, graphics card standards, motherboard layouts, and operating systems changes that each new PC (even ones that are supposed to be the same model) can be an adventure. Data aquisition and other similar board drivers, even those from large reputable companies, can be of unpredicable quality and may not work simply for reasons such as the clock rate of the CPU is higher than on the PC they were written for.
We have other machines which use STD Bus industrial computers. These are controlling standard machines (e.g. lathes) which are produced in some volume by a large company. I would not consider these to be ‘PC control’ though. STD Bus has been around for quite a while, and was designed for exactly this sort of application. These machines do not have hard drives, monitors, or keyboards (they do have special operator interface panels though). The boards do occasionally fail, but unlike PCs though, these boards are comparitively simple to change.
STD or other similar industrial computers do take more engineering effort to apply than a PLC, but may offer advantages for large OEM equipment builders making standard products. I would like to note that the company I referred to in the above paragraph still prefers to use PLCs for any one-off machines they may build.
Michael Griffin
London, Ont. Canada
[email protected]
I’ve used both PLCs and PCs (for test equipment), and I think we should be carefull about jumping to conclusions without considering the application in which they are being applied. I think we also need to define just what we mean by a “PC”.
I define a PLC as an industrial controller which is ready to apply right out of the box. There is little or no integration effort to get different components to work with each other.
A PC is the beige box you see sitting on your desk. It requires loading an operating sytem, and the application software before you can do anything. If you want a working system, you have to do all the system testing and integration yourself.
Industrial computers seem to fall somewhere in between the above two catagories. They have been around for decades, and actually predate the PLC.
I work in an industry with many small machines, most of which are custom built for our application by a variety of companies (both large and small). Typically each machine has its own small PLC. For these types of our applications, a PC (however defined) offers no advantages to us at all, and has the disadvantages of higher cost, larger size, etc. The total number of PLCs we have is about two hundred or more.
Other applications we have involve test equipment where we need a full keyboard, graphic screen, and a hard drive. We also need to be able to plug in specialised data aquisition boards, GPIB boards, vision processors, etc. In other words we need a PC. These systems are generally desk top type PCs in EEMAC 12 rated enclosures. These include both systems which we have built ourselves, and systems which we have purchased from other companies.
The software is written in Quick Basic, C, and Visual Basic. The operating systems used include DOS, Windows 95, and Windows NT. I have heard on this list many complaints about the “blue screen of death”, but this has not been a problem for us, not even with Windows 95 (although Windows NT has caused lots of problems when used in office applications).
However, we do have problems with hardware. Monitors, keyboards, power supplies, and hard drives all fail. Monitors and keyboards are easy enough to change, but power supplies and expecially hard drives are more of a maintenance problem. These last two items are beyond what we expect our electricians to be able to replace, and so have to be serviced by qualified computer repair technicians (we get the same company which services our office PCs to do this - they have at least one person permanently stationed on site at our plant). If a PC fails at night, it has to wait until morning to get repaired.
The PCs seem to run OK for about 3 or 4 years, and then start to give trouble. When they do go wrong, they are much more of a headache, and take much longer to fix than a PLC would. Fortunately, the hardware which has Windows 95 and NT on it is all relatively new, so those ones haven’t started to fail on us yet. Some of the DOS systems are AT&T 6300s which were probably built in the late 1980s. This would not be considered very old for normal industrial equipment, but these PCs are difficult to keep going.
I had one 4 year old PC fail late last week (right about when this thread started), and another somewhat newer one failed a couple of weeks ago. Both were a big headache.
Another problem with PCs is their lack of standardisation. There have been so many bus systems, graphics card standards, motherboard layouts, and operating systems changes that each new PC (even ones that are supposed to be the same model) can be an adventure. Data aquisition and other similar board drivers, even those from large reputable companies, can be of unpredicable quality and may not work simply for reasons such as the clock rate of the CPU is higher than on the PC they were written for.
We have other machines which use STD Bus industrial computers. These are controlling standard machines (e.g. lathes) which are produced in some volume by a large company. I would not consider these to be ‘PC control’ though. STD Bus has been around for quite a while, and was designed for exactly this sort of application. These machines do not have hard drives, monitors, or keyboards (they do have special operator interface panels though). The boards do occasionally fail, but unlike PCs though, these boards are comparitively simple to change.
STD or other similar industrial computers do take more engineering effort to apply than a PLC, but may offer advantages for large OEM equipment builders making standard products. I would like to note that the company I referred to in the above paragraph still prefers to use PLCs for any one-off machines they may build.
Michael Griffin
London, Ont. Canada
[email protected]