Phase 3: Grouping and segmentation of I/Os and Racks (functional distribution)
In this article we discuss the importance of the functional distribution of I/Os, especially in medium size and large control systems (index of design phases).
3-Grouping and segmentation of signals and racks – Functional distribution
The way we distribute I/O signals, modules and racks is an essential part of the design of the control system, often we don’t pay much attention to this. The quality of the final design can become bad or very bad if we have not done a good grouping and segmentation of functional signals with the focus placed on the field instruments and equipment. It is true that it seems obvious, but it is not if we see the amount of bad designs that can be found in this aspect.
This functional distribution of signals must always be a priority of the design regardless of the size of the system and whether it is redundant or not. Logically if the system is small then the functional distribution will be simple or even there will be no such distribution if we have very few signals. In medium and large systems it is very important.
On the other hand, it is very different to distribute the signals in the case of high availability systems (redundant signals, logic 2oo3) than in the case of systems without redundancy.
The signal functional distribution must be made in such a way that the failure of an I/O module does not cause a total or partial shutdown of the plant or equipment and it will depend on the criterion used in each case.
The most critical signals are those that produce the total shutdown and one of the usual practices is to triple the instruments and to make a logic 2 out of 3 (2oo3) in the controller. In process plants, it is a common practice to use 4-20 mA transmitters for these cases. The design must be done so that each channel of the triple signal is located in a different module and, if possible, in a different I/O rack. If all three modules are in the same rack, it must have a redundant power supply.
At the next level we have redundant signals with logic 2 out of 2 (2oo2). They are also critical signals related to main equipment whose malfunction causes total or partial shutdown, e.g. limit switches and open / close commands to the main valves. Each channel of the redundant signal must be wired to a different module and, if possible, to a different rack. If both modules are in the same rack, it must have a redundant power supply.
At the last level we have simple signals that we can divide into several categories. On the one hand we have the signals that do not cause an equipment shutdown directly, such as starting and stopping permissives, lamp outputs, equipment signals that are only used during start-up processes, etc. An example may be the igniter signals from the burners of a boiler.
Another category of non-redundant signals are those that affect non-critical equipment or duplicate equipment (primary and secondary equipment). The classic example is the pumping group consisting of two pumps of 100% capacity. In normal operation only one of the pumps works and in case of failure it starts the other automatically. Another example is when we have a boiler with several burners in which it is not critical to lose one of the burners.
Functional distribution of I/Os in different modules is also important in all these cases. For example, in the case of the two pumps we have to avoid using the same module for both, and in the case of the burners it is advisable to make a distribution that affects the smallest number of burners in case of a module failure.
In short, we must avoid so-called common mode failures.
Another aspect to consider is the number of signals per module, i.e. density, because using high density modules (32 or 64 I/O) does not imply that the cost of the PLC will be lower. This is easily understood if we take into account the above on the functional distribution of the I/O (based on the field equipment). What is clear is that each case is different and should be analyzed well.