The principal of safety and basic process control controllers is very well known in the process automation or turbomachinery control system environment. However with the latest controllers, often these two separate controller could be integrated on one hardware and software platform. How does this work? iAUTOMATION will explain the ICSS concept in this article.
The Process safety certified controller is usually a PLC with special controller functionality to check the memory and data consistency while executing data manipulations. This will make sure that data and memory are not changed or corrupted during execution or storage in the controller memory. The communication for a safety controller is usually a protocol like profisafe, these protocols have a validation on communication data frames that make sure that data is sent and receive in the correct order, unchanged, from the correct station and in time.
In most control systems the modules and controller have a logo or colour coding to highlight the fact that the controller or IO is safety. This can help onsite engineer to find the correct controllers, and not change a failsafe controller while they are not allowed to for instance.
In a lot of cases a safety controller is made redundant, this is a bad thing for the actual safety number on the loop, but increases the availability of the functionality. Because it is not good for a safety system to be designed as a redundant system, an engineer should really keep the system as a single controller, unless the controller has multiple tasks. The IO system might need to have a 1oo2 voting system when a safety loop is concerned, so the considerations for IO and PLC are very different. This is obviously dependent on the make and SIL rating of the PLC, IO, field device and SIL requirement for the safety loop.
Because the safety controller can be the same make and model as the basic process controller, and the IO systems are often the same and can be mixed, it is also possible for a controller to run both a safety and non safety control program in the same PLC. Obviously the same principles apply to the way of communication and data processing in the PLC. So the basic process data will go through the controller in a normal way, and the safety data will be processed in a failsafe way.
The controller will obviously be loaded more, then when it has just basic process control(BPC) running on it. But for most PLC firmware that allow this integration of systems, a firmware system is put in place where is will delay the BPC execution for 1 or more cycles to allow the safety processing to continue in case of processor overloading, or in case processing groups are running at the same time interval.
Single units or smaller processes therefore get a lot cheaper when integrating the basic process control and safety system, and there is nothing that will get impacted in the safety system by doing so. Added benefits come in to play when connecting HMI software to the controller, as now a single HMI solution can be selected, and the safety and non safety equipment can be show in the same process overview. Also the engineering will become cheaper, as there is one engineering environment, and the same engineers can do the programming. This way less computers and less spares are needed on site.
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