Identification and Multi-Objective Controller Tuning of a Plastic Injection Molding Machine

In this paper, a multi-objective approach to controller tuning is described. An algorithm for implementation of this tuning is given. This approach has the following merits: there is no restriction on the type or number of performance criteria; no a 'priori weights need be chosen since performance criteria are not combined into a single objective function. The intention is that the algorithm is implemented to run in real-time so as to provide immediate feedback to a process operator about the current operating point. If the current operating point is not Pareto optimal, the algorithm can automatically adjust the parameters to achieve this. If it is Pareto optimal but not desirable, the algorithm can provide information on how trade-offs can be made so that the desired operating point is achieved. The algorithm is applied to the tuning of a PID controller for the plastication phase of an injection molding process. An analytic model for this process is presented. Experiments are described in which data was collected from this process, and an empirical discrete-time transfer function was identified. This transfer function was then used, within a simulation, to allow the algorithm to tune the controller, with respect to two objective functions. The tradeoff curve for these objectives, and corresponding controller gains is presented.