Tablet Press Instrumentation - mcc-online.com

1 Tablet Press InstrumentationMichael LevinMetropolitan Computing Corporation, East Hanover, New Jersey, article is designed to facilitate the understanding ofthe general principles of Tablet Press Instrumentation andthe benefits thereof by the formulators, process engineers,validation specialists, and quality assurance personnel, aswell as production floor supervisors who would like tounderstand the basic standards and techniques of gettinginformation about their tableting OF Tablet PRESSINSTRUMENTATIONIn 1952 1954 Higuchi and his group[1]have instrumentedupper and lower compression, ejection, and punchdisplacement on an eccentric Tablet Press , and pioneeredthe modern study of compaction process.

2 This work wasfollowed by Nelson[2,3]who was the member of theoriginal 1966, a patent was granted to Knoechel and co-workers[4]for force measurement on a Tablet Press . Thispatent was followed by two seminal articles in Journalof Pharmaceutical Sciences on the practical applicationsof instrumented rotary Tablet machines.[5,6]A number ofother patents related to Press Instrumentation and controlfollowed from 1973 on ward.[7 15]Despite the availability of published designs forinstrumenting rotary presses, much of the early work oncompaction properties of materials was done on instru-mented single punch eccentric presses primarily, due tothe relative ease of sensor installation, as well asavailability of punch displacement measurement.

3 [16 18]In mid-1980s, custom-made Press monitoring systemswere described,[19,20]and the first commercial instrumen-tation packages became available, including both thesystems for product development and Press control. Thefirst personal computer-based Tablet Press monitor wassold in 1987, and the first Microsoft Windows-based pressinstrumentation package in new era of compaction research has begun with theintroduction of an instrumented compaction simulator[21]in 1976, while a compaction simulator patent was issuedas recently as 1996.[22]A new generation of compaction simulators was bornwhen a mechanical Press replicator was patented in theyear 2000.

4 [23]A good expose of the early stages of pressinstrumentation and resulting research is presented inreview articles by Schwartz,[24]Marshall,[25]and Jones.[26]A comprehensive review can be found in a relativelyrecent paper by C elik and Ruegger.[27]For other historical information on the Press instru-mentation, the reader is encouraged to peruse RidgewayWatt s[28]volume. There have been a number of paperspublished on various disparate Instrumentation topics, andin a recent volume by Mun os-Ruiz and Vromans,[29]thereare two good articles on the subject but, unfortunately,they deal with marginal issues of single station Press andinstrumented punch ACQUISITION PRINCIPLES:FROM TRANSDUCERS TO COMPUTERSTo monitor and control a Tablet Press , certain sensors mustbe installed at specific locations on the machine.

5 Thesesensors are called transducers. In general, a transducer isa device that converts energy from one form to another( , force to voltage). Tablet Press transducers typicallymeasure applied force, turret speed, or punch the signals coming from such transducers arenormally in millivolts, they need to be amplified and thenconverted to digital form in order to be processed by a dataacquisition GagesHistorically, high impedance piezoelectric transducersthat employ quartz crystals were used in early stages ofpress Instrumentation .

6 When subject to stress, the crystalaccumulates electrostatic charge that is directly propor-tional to the applied force. Both low and (more modern)high impedance piezoelectric gages have high-frequencyresponse, but may exhibit signal drifting due to chargeEncyclopedia of Pharmaceutical TechnologyDOI: 120012030 Copyrightq2002 by Marcel Dekker, Inc. All rights 2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, DEKKER, INC. 270 MADISON AVENUE NEW YORK, NY 10016leakage (approximately decay per second can beseen in modern piezoelectric transducers).

7 Nowadays,the preferred way of instrumenting Tablet presses is withstrain GagesStrain gages are foil, wire, or semiconductor devices thatconvert pressure or force into electrical voltage. When astress is applied to a thin wire, it becomes longer andthinner. Both factors contribute to increased electricalresistance. If an electrical current is sent through this wire,it will be affected by the changes in the resistance of theconduit. This principle is used in strain gage-basedtransducers. Foil gages, known for robust applicationrange, useful nominal resistance, and reliable sensitivitycontrol, are most commonly used for Instrumentation ofcompression, precompression, and ejection forces.

8 Semi-conductor-based strain gages are inherently more sensitivebut suffer from high electrical noise and temperaturesensitivity. Such gages are not commonly used in tabletpress Instrumentation except for measurement of die-wallpressure and take-off BridgeWheatstone bridge (Fig. 1) is a special arrangement ofstrain gages that is used to ensure signal balancing. The full bridge is composed of two pairs of resistors in acircle, with two parallel branches used for input and twofor signal output. By applying the so-called excitationvoltage (typically, 10 V DC) to the bridge input andchanging the resistance of different legs of the bridge byadding special resistors, we can make sure that there is azero output voltage when no load is applied to thetransducer.

9 This is called zero balance. Once the bridge isbalanced, a small perturbation in the resistance of any leg of the bridge results in a nonzero signal output of the Wheatstone bridge is normallyexpressed in millivolts per volt of excitation per unit ofapplied force. For example, sensitivity of mV/V/kNmeans that applying, say, 10 kN force and 10 V excitationwill produce 20 mV output. To utilize such output, itusually needs to be amplified several hundred times toreach units of important function of the bridge is balancingof temperature effects.

10 Although individual strain gagesare sensitive to temperature fluctuations, Wheatstonebridge arrangement provides for temperature sensitivitycompensation, so that the resulting transducer is nolonger changing its output to any significant degree whenit heats the output of these bridges is in the range ofmillivolts, the cables utilized to carry the signal arenormally shielded with a braided or foil-lined sheatharound individual wires. The shield, as a rule, is connectedto the amplifier, but never touches the actual instrumentedequipment ( , Tablet Press ).