Batch processes are widely used in several industrial sectors, such as food and pharmaceutical manufacturing. In a typical batch, raw materials are loaded in the processing unit and submitted to a series of transformations, yielding the final product. Process performance is described by variables which are monitored as the batch progresses. Data arising from such processes are likely to display a strong correlation-autocorrelation structure, and are usually monitored using control charts based on multiway principal components analysis (MPCA charts). In this paper we investigate the special (and rather frequent) case of batches with varying duration, which cannot be directly monitored using MPCA charts. We propose a new quality control strategy for monitoring such batches. In our proposition, batches are not aligned or time warped with respect to their trajectories, but are rather completed using a straightforward scheme. Thus all information on the variability in batch profiles along the time axis is preserved. The data set completed is reduced using the Statis method and monitoring of batch performance is accomplished directly on principal plane graphs, from which non-parametric control charts are derived. A simulated example illustrates the proposed method.
Unsynchronized batches; Multivariate quality control; Statis method
