Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design - Part 77 potx

744 5 Safety and Risk i n Engineering Design Fig. 5.97 Petri net-based optimisation algorithms in system simulation dynamic modelling, time series prediction, adaptive control, etc., of various engi- neering design problems. A typical design problem that is ideal for ANN mod- elling is the formulation and evaluation of stream surge p ressures in continuous flow processes, given in the simulation option of the AIB blackboard as illustrated in Fig. 5.97. The NeuralExpert c  (NeuroDimension 2001) program, imbedded in the AIB blackboard, asks specific questions and intelligently builds an ANN. The first step in building an ANN is the specification of the problem type, as illustrated in Fig. 5.102. The four currently available problem types in the NeuralExpert are classification, function approximation,prediction, and clustering. Once a problem type is selected, the program configures the parameters based on a description of the problem. These settings can be modified in the AIB blackboard, or in the NeuralExpert. Input data selection is the next step in constructing an ANN model. The input file selection panel specifies where the input data file is located by choosing the ‘browse’ button and searching through the standard Windows tree structure to find the relevant file referenced in the AIB blackboard database, or by clicking on the triangle at the right edge of the text box to indicate a list of recently used text files in th e NeuralExpert. 5.4 Application Modelling of Safety and Risk in Engineering Design 745 Fig. 5.98 AIB blackboard model with CAD data browser option Figure 5.103 illustrates typical input data attributes for the example used to de- termine the stream surge pressure given in the simulation model option of the AIB blackboard illustrated in Fig. 5 .97. In this case, the sample data would represent x 1 , x 2 , x 3 , x 4 values of the input attributes. The pressure surge Petri net given in Fig. 5.97 includes conditions of flow surge criteria that now become the ANN input attributes, such as the pipe outlet diameter, pipe wall thickness, the fluid bulk mod- ulus, and Young’s modulus. The goal is to train the ANN to determine the stream surge pressure (desired output) based on these attributes. Typical computational problems associated with artificial neural network pro- grams, with regard to specific as well as general engineering d esign requirements, include the following: Classification problems are those where the goal is to label each input with a specified classification. A simple example of a classification problem is to la- bel process flows as ‘fluids’ and/or ‘solids’ for balancing (the two classes, also the desired output) using their volume, mass and viscosity (the input). The input can be either numeric or symbolic but the output is symbolic in nature. For example, the desired output in the process balancing problem is the ratio of fluids and solids, and not necessarily a numeric value of each. 746 5 Safety and Risk in Engineering Design Fig. 5.99 Three-dimensional C AD integrated model for process information Function approximation problems are those where the goal is to determine a nu- meric value, given a set of inputs. This is similar to classification problems, except that the output is numeric. An example is to determine the stream surge pressure (desired output) in numeric values, given the pipe outlet diameter, the pipe wall thickness, the fluid bulk modulus and Young’s modulus. These problems are called function approximation because the ANN will try to approximate the functional re- lationship between the input and desired output. Prediction problems are also func- tion approximation problems, except that they use temporal information (e.g. the past history of the input data) to make predictions of the available data. Prediction problems are those where the goal is to determine an output, given a set of inputs and the past history of the inputs. The main difference between pre- diction problems and the others is that prediction problems use the current input and previous inputs (the temporal history of the input) to determine either the cur- rent value of the output or a future value of a signal. A typical example is to predict process pump operating performance (desired output) from motor current and de- livery pressure performance values. Clustering problems nformation is to be extracted from input data without any desired output. For example, in the analysis of process faults in designing for safety, the faults can be clustered according to the severity of hazard consequences risk. 5.4 Application Modelling of Safety and Risk in Engineering Design 747 Fig. 5.100 CAD integrated models for process information The fundamental difference between the clustering problem and the others is that there is no desired output (therefore, there is no error and the ANN model cannot be trained using back propagation). For classification problems to label each input with a specified classification, the option is given to randomise the order of the data before presenting these to the network. Neural networks train better if the presentation of the data is not ordered. For example, if the design problemrequires classifying between two classes, ‘fluids’ and/or ‘solids’, for balancing these two classes (as well as the desired output) using their volume, mass and viscosity (the input), the network will train much better if the fluids and solids data are intermixed. If the data are highly ordered, they should be randomised before training the artificial neural network. One o f the primary goals in training neural networks in the process of ‘iterative prediction’ is to ensure that the network performs well on data that it has not been trained on (called ‘generalisation’). The standard method of ensuring good general- isation is to divide the training data into multiple datasets or samples, as indicated in Fig. 5.104. The most common d atasets are the training, cross validation, and testing datasets. The cross validation dataset is used by the network during training. Periodi- cally while training on the training dataset, the network is tested for per formance 748 5 Safety and Risk in Engineering Design Fig. 5.101 ANN computation option in the AIB blackboard on the cross validation set. During this testing, the weights are not trained but the performance of the network on the cross validation set is saved and compared to past values. The network shows signs of becoming over-trained on the training data when the cross validation performance begins to degrade. Thus, the cross validation dataset is used to d etermine when the network has been trained as best as possible, without over-training (i.e. maximum generalisation). Although the network is not trained with the cross validation set, it u ses the cross validation set to choose the best set of weights. Therefore, it is not truly an out- of-sample test of the network. For a true test of the performance of the network, an independent (i.e. out of sample) testing set is used. This provides a true indication of how the network will perform on new data. The ‘out of sample testing’ panel shown in Fig. 5.105 is used to specify the amount of data to set aside for the testing set. It is important to find a minimal network with a minimum number of free weights that can still learn the prob lem. The minimal network is more likely to generalise well with new data. Therefore,once a successful training session has been achieved, the process of decreasing the size of the network should commence, and the training repeated until it no longer learns the problem effectively. The genetic optimisation component shown in Fig. 5.106 implements a genetic algorithm to optimise one or more parameters within the neural network. The most common network parameters to optimise are the input columns, the number of 5.4 Application Modelling of Safety and Risk in Engineering Design 749 Fig. 5.102 ANN NeuralExpert problem selection hidden processing elements (PEs), the number of memory taps, and the learning rates. Genetic algorithms combine selection, crossover a nd mutation operators with the goal of finding the best solution to a problem. Genetic algorithms are general- purpose search algorithms that search for an optimal solution until a specified ter- mination cr iterion is met. Network complexity is determined by the size of the neural network in terms of hidden layers and processing elements (neurons). In general, smaller neural net- works are preferable over large ones. If a small one can solve the problem suffi- ciently, then a large one will not only require more training and testing time but also may perform worse on new data. This is the generalisation problem—the larger the neural network, the more free parameters it has to solve the problem. Excessive free parameters may cause the network to over-specialise or to memorise the training data. When this happens, the performance of the training data will be much better than the performance of the cross validation or testing datasets. The network complexity panel shown in Fig. 5.107 is used to specify the size of the neural network. It is essential to start ANN analysis with a ‘low-complexity’ net- work, after which analysis can progress to a medium-or high-complexitynetwork to determine if the performance results are significantly better. A disadvantage is that medium- or high-complexity networks generally require a large amount of data. 750 5 Safety and Risk in Engineering Design Fig. 5.103 ANN NeuralExpert example input data attributes In the NeuralExpert c  (NeuroDimension 2001) program, imbedded in the AIB blackboard, several criteria can be used to evaluate the fitness of each potential so- lution. The solution to a problem is called a chromosome. A chromosome is made up of a collection of genes, which are simply the neural network parameters to be optimised. A genetic algorithm creates an initial population (a collection of chro- mosomes) and then evaluates this population by training a neural network for each chromosome. It then evolves the population through multiple generations in the search for the best network parameters. Performance measures of the error crite- rion component provide several values that can be used to measure the performance results of the network f or a par ticular dataset. These are: • the mean squared error (MSE), • the normalised mean squared error (NMSE), • the percent error (% error). The mean squared error (MSE) is defined by the following formula MSE = ∑ P j= 0 ∑ N i=0 (d ij −y ij ) 2 NP (5.115) 5.4 Application Modelling of Safety and Risk in Engineering Design 751 Fig. 5.104 ANN NeuralExpert sampling and prediction where: P = number o f output processing elements N = number of exemplars in the dataset y ij = network output for exemplar i at processing elements j d ij = desired output for exemplar i at processing elements j. The normalised mean squared error (NMSE) is defined by NMSE = P·N ·MSE P ∑ j= 0  N ∑ N i=0 d 2 ij −  ∑ N i=0 d ij  2 N  (5.116) where: P = number o f output processing elements N = number of exemplars in the dataset MSE = mean squared error d ij = desired output for exemplar i at processing elements j. 752 5 Safety and Risk in Engineering Design Fig. 5.105 ANN NeuralExpert sampling and testing The percent error (%E) is defined by the following formula %E = 100 NP P ∑ j= 0 N ∑ i=0 |d ij −dd ij | dd ij (5.117) where: P = number o f output processing elements N = number of exemplars in the dataset d ij = denormalised network output for exemplar i at elements j dd ij = denormalised desired output for exemplar i at elements j. Knowledge-based expert systems Expert knowledgeof how to solve complex en- gineering design problems is not often available. Knowledge-based expert systems are programs that capture that knowledge and allow its dissemination in the form of structured questions, to be able to determine the reasoning behind a p articular de- sign problem’s solution. The knowledge-based expert systems incorporated in the AIB blackboardare based on the classical approach to expert systems methodology, which incorporates the following: 5.4 Application Modelling of Safety and Risk in Engineering Design 753 Fig. 5.106 ANN NeuralExpert genetic optimisation • User interface, • Working memory, • Inference engine, • Facts list, • Agenda, • Knowledge base, • Knowledge acquisition facility. The user interface is a mechanism by which the user and the expert system com- municate. The working memory consists of a global database of facts used by rules. The inference engine controls the overall execution of queries or selections related to problems and their solutions based around the rules. The facts list contains the data o n which in ferences are derived. An agenda is a list of rules with priorities created by the inference engine, the patterns of which are satisfied by facts in the working memory. The knowledge base contains all the knowledge and rules. The knowledge acquisition facility is an automatic way for the user to enter or modify knowledge in the system, rather than by having all the knowledge explicitly coded at the onset of the expert systems design. The user interface of the AIB blackboard is an object-oriented application in which the designer can point-and-click at digitised graphic process flow diagr ams . methodology, which incorporates the following: 5.4 Application Modelling of Safety and Risk in Engineering Design 753 Fig. 5.106 ANN NeuralExpert genetic optimisation • User interface, • Working memory, • Inference. Application Modelling of Safety and Risk in Engineering Design 749 Fig. 5.102 ANN NeuralExpert problem selection hidden processing elements (PEs), the number of memory taps, and the learning rates Application Modelling of Safety and Risk in Engineering Design 747 Fig. 5.100 CAD integrated models for process information The fundamental difference between the clustering problem and the others