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4.3. MONITORINGEarlier, we defined monitoring as the activity of tracking and comparing observations to expectations. Many examples of monitoring expert systems, using the MBR approach, can be found in the manufacturing and process industries. These industries already use monitoring systems based on conventional techniques. For example, this may mean tracking the value of a particular measurement over time, and warn the operator if the value exceeds a particular fixed alarm threshold. The resulting monitoring performance is often very crude, and may lead to excessive alarming (too narrow alarm limits), or under-reporting (too permissive limits). The trouble is that the monitoring limits are fixed and not related to the actual running conditions of the plant (device). This is in contrast to the MBR approach, where monitoring is done by running a model of the physical system in parallel the system itself, producing expected values for different observations (measurements), comparing the actual observations of the system with those that the model predicts (dynamically), and reporting significant deviation. Dvorak and Kuipers (1991) presented an MBR monitoring application, called MIMIC. It exploits three different techniques:
The MIMIC architecture was applied to a water heating system on an experimental basis. Among the advantages quoted by the authors of their approach are:
Several other MBR monitoring systems have been reported in the literature, often (as for MIMIC) as part of an overall monitoring-diagnosis application. Some examples are:
4.4. CONTROLControl means governing the behavior of the target system to meet stated goals. In the AI/expert system area in general, and in MBR-based systems in particular, there are not many real-world applications of closed-loop control systems, i.e., systems that directly control the target system without human intervention. On the other hand, there are numerous examples of open-loop systems, where the human is an essential link between observations and control actions. In manufacturing and process industries, one often refers to such systems as operator support systems. Their role is to provide the operator with insight into the status of the target system, and to suggest corrective or optimizing control action that the operator may implement. Such systems generally exist as an add-on to an underlying closed-loop control system that carries out low-level control in real time. More systematically, we may distinguish between the following different ways in which expert system technology (and MBR) are being applied to control problems:
In this list, the last type most directly applies MBR in practice. As explained above, most existing systems of this type are open-loop operator support systems.
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