(714d) Loss-in-Weight Control of a Low Dose Powder Feeding System

Accurate and robust powder feeding is the key for successful continuous pharmaceutical manufacturing. Feeding systems available on the market are typically suitable for feed rates above 200 g/h. We show the development of loss-in-weight (LIW) control for a novel feeding system capable of continuously feeding powder at feed rates in the range of few g/h. Our in-house developed prototype is equipped with a LIW scale, a powder pump and a scraping device. Additionally, a high precision gain-in-weight (GIW) scale is used as reference. First, a feed-forward (FF) control strategy was established to compensate for the material dependent displacement density along the powder container. This approach is similar to the feed factor correction based on the hopper fill level of commercially available large scale feeders. In a second step, an iterative learning (IL) control strategy was developed, which updates the model for FF control after regularly spaced time intervals based on the measured feed rate deviation from the set-point.

In the presentation, we compare operation with constant powder pump speed to feed-forward controlled experiments. It can be shown that the feed rate deviations, caused by the reproducible displacement density, can be mitigated. Furthermore, we show the principle of IL control in order to compensate model inaccuracies as a result of changing material behavior due to batch-to-batch variability or inconsistent powder conditioning by the operator.

A LIW control strategy for a novel low dose feeding system was developed, successfully transferred to the prototype and tested. Accurate and robust feeding for feed rates as low as 1.5 g/h could be achieved and by means of IL control, the effect of changing material behavior could be significantly reduced.