It is often refreshing to rip out old technology and replace it with a state of the art system when the bean counters (unwittingly, perhaps) sign off on a new project. As we know, it is often a frustratingly slow business interfacing new equipment to an existing system; so starting fresh is often an exhilarating prospect. However there is a significant amount of older automation equipment still around from the 1990’s (and indeed, even earlier when you look at some oil and gas and mining installations). Proprietary industrial networks started disappearing in the late nineties and now most are open and based around Ethernet.
A Strong Drive
However, there is an enormous drive today to tie data all the way from the instrument on the shopfloor to the company’s boardroom and business systems (maintained by the IT department). However, existing PLC systems, I/O, instruments, valves and drives from over two decades ago are often still chugging along happily and to simply replace these in the name of interconnectivity with a modern Ethernet network is not viable and would boost the costs of a project into the stratosphere.
IT versus Control Systems
There is still the age-old challenge of the IT guys working reluctantly with the control system group. The skills and know-how required to maintain an IT system (with ERP/email/Microsoft Office and database management) is often considerably different to that for working with SCADA systems, PLCs, drives and instruments. The plant electrician of today often has a Notebook computer in his toolkit and is becoming exceptionally skilled in IT issues. Much to the bemusement of our IT brethren; who often come from more rarified surrounds.
Legacy Systems Galore
These include older PLCs which interfaced to serial RS-232/Profibus/Modbus type networks plus a slew of proprietary networks such as Modbus Plus / Allen Bradley DH485 or DH+; but sadly didn’t have any direct interface to Ethernet at the time. Plus there are an enormous of proprietary control systems written in C and running on proprietary hardware with vendors that have long since disappeared.
Gateways Galore to the rescue
If you are looking to upgrade, a good strategy is to first approach your existing vendor for an upgrade path. Many of the blue chip vendors know that everyone needs to be able to upgrade and often have a variety of solutions to connect to older equipment. If the vendor doesn’t exist any longer or they don’t have a credible solution; there are numerous third-party proven solutions out there able to integrate between Ethernet, DeviceNet, EtherNet/IP, Profibus, ProfiNet, Modbus and Foundation Fieldbus et al - the list is seemingly endless. However, ensure that your proposed gateway solution actually does work consistently and has proven performance with data for the entire range of operation (hardware as well as protocol types).
Other approaches that can work
Some strategies that work are to carefully upgrade the existing PLC (e.g. a Rockwell SLC 5/03 with an SLC 5/05) which can then interface to Ethernet and thus to the SCADA system and to avoid disturbing your existing I/O and simply running your old program on a new processor. OPC can then be used to exchange data.
Another strategy is to capitalise on the low cost industrial wireless systems appearing on the market and to install wireless data acquisition nodes on the machine control and sensor panels (minimal wiring and effort). The robustness of industrial wireless means it can easily and dynamically handle the chaotic plant environment with tanks, conveyers, piping and other metal structures scattered at every conceivable place.
However, as noted in the previous section, before doing any massive re-engineering of your existing systems, check for whether there are no easily available gateways or interfaces currently available from the device to Ethernet and TCP/IP and whatever protocol you are using on top of this. Remember of course, that simply providing Ethernet and TCP/IP is only part of the story. You still need to define what protocol runs on top of TCP/IP (at the application layer of the good old OSI model).
Finally, ensure when you open up your previously proprietary network, that you are aware that you are now releasing data into a more public field (and exposing your system to the outside world) and need to be aware of network safety.
As painful as it often is, we need to constantly re-invent ourselves by creating new approaches; as Cecil DeMille remarked: Creativity is a drug I cannot live without.
Thanks to Don Hebert of ControlDesign and Automation.com for a series of interesting discussions.
Yours in engineering learning