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Comparison of Incremental and Inductive Displacement Transducers on an Eccentric Tablet Press

Jochen A. Dressler, Karl G. Wagner, Martin A. Wahl, and Peter C. Schmidt

Department of Pharmaceutical Technology, Eberhard-Karls-University Tübingen, Tübingen (Germany)

Instrumented eccentric tablet presses are widely used in tablet development. Commonly, they are equipped with strain gauges or piezo-electric load washers to measure the compression force and with inductive displacement transducers to record the displacement of the punches. In the present investigation, an inductive and an incremental displacement transducer were compared simultaneously on a Korsch EK II single-punch eccentric tablet press. An automatic computer-controlled data acquisition system was developed to compare both transducer techniques. The Heckel equation was used as a model to describe the quality of the displacement measurement, and the parameters were calculated automatically by a self written script application using Catman software. The Heckel parameters are known to be sensitive to errors in the displacement measurement which was confirmed by a model calculation.
Pregelatinized starch, a-lactose monohydrate and tribasic calcium phosphate were used as model substances. The results indicate the high precision of the incremental displacement transducer and reveal the characteristic differences between the plastic-elastic and brittle nature of the excipients used in this study.

Key words Eccentric tablet press · Displacement transducer · Heckel plot · a-Lactose monohydrate · Pregelatinized starch · Tribasic calcium phosphate