Batch processes convert raw materials to products during a finite period of time by following prescribed processing recipes. A high degree of reproducibility is necessary to obtain successful batches. Monitoring and control of batch processes are crucial for detecting deviations from reference trajectories and interfering with undesirable trends to bring the operation to conditions that assure acceptable product quality. The goal of statistical process monitoring (SPM) is to detect the existence, magnitude, and time of occurrence of changes that cause a process to deviate from its desired operation.
Traditional statistical process control (SPC) has focused on monitoring quality variables at the end of a batch and if the quality variables are outside the range of their specifications making adjustments (hence control the process) in subsequent batches. An improvement of this approach is to monitor quality variables during the batch run and make adjustments in the same run if they deviate from their expected ranges. Monitoring of quality variables usually involves measurement and reporting delays. Information about quality variations is encoded in process variables, and measurement of process variables is often frequent and highly automated. Hence, monitoring of process variables is useful not only for assessing the status of the process, but also for controlling product quality.
In traditional quality control of multivariable processes, several quality variables are monitored using univariate SPC techniques such as Shewhart charts. This approach considers each variable in isolation. In contrast, multivariate techniques focus on the whole picture and generate an alert when many process variables make small moves from their mean values in a way that indicates a specific trend. They leverage the interaction between variables and monitor changes in the correlation structure of the variables.
The book presents many process monitoring and quality control tools in Chapter 6, starting with simple univariate SPC charts. Several multivariate SPM techniques for end of batch and real-time on-line monitoring are introduced and integrated with quality control. Furthermore, these tools are linked with fault diagnosis to offer an automated process monitoring and diagnosis environment in Chapter 8.
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