Higher Cim Levels 1341 Supervision Level

In complex manufacturing facilities, the coordination of PLCs, to automate operations involving several units (e.g. tanks) at the same time, may become extremely complex. At the limit, each PLC would have to mirror the status of all other involved PLCs for a proper synchronization. This challenge has lead to the development of distributed control systems (DCS), where a supervision tool (often termed supervisory control and data acquisition (SCADA) application) controls a group of separated PLCs, spread over the various equipment units of the plant. Communication is made possible thanks to a field bus system (see Section 13.2.1.3) and local area networks.

The main functions of the supervision tool are to control the process running on the various process units and to display their status, including all measurements and alarms. A key component of the tool is a human-machine interface, which enables the user to enter individual orders, set points or even complete recipes. Supervision tools are also capable of displaying and generating various graphical and text reports. Data are generally stored temporarily, for a few hours or days, before they are archived; this short period is for safety reasons. For data storage, a reduction algorithm is normally applied, which does not select the data at a fixed frequency but on the basis of a significant-change criterion (Sonnleitner 1999). Figure 13.6 shows a simple example of a DCS for a group of three pilot-scale bioreactors. An example of a more complex DCS, for a fermentation pilot plant, can be found in Lam (1992).

With respect to the architecture of DCSs, one can distinguish two main solutions. The first type is a completely homogeneous system, built by a single supplier. The main advantage is the ease of implementation since communication is standardized, and databases and configuration tools are common to both the PLCs and the supervision tool. Development costs and start-up problems are thus reduced. Furthermore, with a single supplier of hardware and software, fewer people are usually required in the project phase. The main drawback is the fact that the user is linked to a single supplier, such that the evolution of the system may be limited. A second type is a hybrid process control system, made of PLCs and associated field devices on one hand, and

Figure 13.6 Simple example of a DCS for a group of three pilot-scale bioreactors. Each bioreactor is controlled by a separate PLC. In this particular example, PLCs have their own human-machine interface (HMI), with a keyboard for entering data (controller parameters, setpoints, etc.); this is rarely the case in large manufacturing plants with complex DCSs, where there is only one HMI, located at the supervision level. For simplification, only two stations at the supervision level are represented: a main one, for instance in the process room, and a second one, for system maintenance and recipe development.

Figure 13.6 Simple example of a DCS for a group of three pilot-scale bioreactors. Each bioreactor is controlled by a separate PLC. In this particular example, PLCs have their own human-machine interface (HMI), with a keyboard for entering data (controller parameters, setpoints, etc.); this is rarely the case in large manufacturing plants with complex DCSs, where there is only one HMI, located at the supervision level. For simplification, only two stations at the supervision level are represented: a main one, for instance in the process room, and a second one, for system maintenance and recipe development.

of a supervision system from another supplier on the other hand. With this type, one can build a customized control system by using the most appropriate devices at each level. Implementation is, however, more complex, takes longer and requires more people with various competencies, since systems and devices from different suppliers must be integrated. The risk of unexpected problems during the project phase is also higher. For the control of complex biotechnology plants, a homogeneous system is thus more common.

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