m
`©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, Inc
`
`.
`MARCEL DEKKER, INC. - 270 MADISON AVENUE - NEW YORK, NY 10016
`
`K
`
`Tablet Press Instrumentation
`
`Michael Levin
`
`Metropolitan Computing Corporation, East Hanover, New lerse}/, U.$.A.
`
`INTRODUCTION
`
`This article is designed to facilitate the understanding of
`the general principles of tablet press instrumentation and
`the benefits thereof by the fonnulators, process engineers.
`validation specialists, and quality assurance personnel, as
`well as production floor supervisors who would like to
`understand the basic standards and techniques of getting
`information about their tableting process.
`
`HISTORY OF TABLET PRESS
`INSTRUMENTATION
`
`In 1952 1954 Higuchi and his groupm have instrumented
`upper and lower compression, ejection. and punch
`displacement on an eccentric tablet press. and pioneered
`the modern study of compaction process. This work was
`followed by Nelsonlml who was the member of the
`original group.
`In 1966. a U.S. patent was granted to Knoechel and co
`workersm for force measurement on a tablet press. This
`patent was followed by two seminal articles in Journal
`of Pharmaceutical Sciences on the practical applications
`of instrumented rotary tablet machines.[5‘6] A number of
`other patents related to press instrumentation and control
`followed from 1973 on ward.” '5'
`Despite the availability of published designs for
`instrumenting rotary presses. much of the early work on
`compaction properties of materials was done on instru
`merited single punch eccentric presses primarily, due to
`the relative ease of sensor
`installation. as well as
`
`availability of punch displacement measurement.“ '8'
`In mid 1980s, custom made press monitoring systems
`were described.“9‘2°] and the first commercial instrumen
`tation packages became available.
`including both the
`systems for product development and press control. The
`first personal computer based tablet press monitor was
`sold in 1987. and the first Microsoft Windows based press
`instrumentation package in 1995.
`A new era of compaction research has begun with the
`introduction of an instrumented compaction simulatorlz”
`
`E‘Jrc_vcI0pediu afPhurmaceu1icuI Technulrrgy
`DOI: 10.1081/E-EPT l200l2030
`Copyright © 2002 by Marcel Dckker, Inc. All rights reserved.
`
`in 1976. while a compaction simulator patent was issued
`as recently as l996.m]
`A new generation of “compaction simulators” was born
`when a mechanical press replicator was patented in the
`year 200O.[23]
`the early stages of press
`A good exposé of
`instrumentation and resulting research is presented in
`review articles by Schwartzfm Marshall,[25' and Jones.[26]
`A comprehensive review can be found in a relatively
`recent paper by Celik and Rueggenml
`For other historical
`infonnation on the press instru
`mentation. the reader is encouraged to peruse Ridgeway
`Watt’s[28] volume. There have been a number of papers
`published on various disparate instrumentation topics. and
`in a recent volume by Mufios Ruiz and Vromans,[29’ there
`are two good articles on the subject
`but, unfortunately,
`they deal with marginal issues of single station press and
`instrumented punch only.
`
`DATA ACQUISITION PRINCIPLES:
`FROM TRANSDUCERS TO COMPUTERS
`
`To monitor and control a tablet press. certain sensors must
`be installed at specific locations on the machine. These
`sensors are called transducers. In general. a transducer is
`a device that converts energy from one form to another
`(e.g.. force to voltage). Tablet press transducers typically
`measure applied force. turret speed, or punch position.
`Because the signals coming from such transducers are
`normally in millivolts, they need to be amplified and then
`converted to digital form in order to be processed by a data
`acquisition system.
`
`Piezoelectric Gages
`
`Historically, high impedance piezoelectric transducers
`that employ quartz crystals were used in early stages of
`press instrumentation. When subject to stress. the crystal
`accumulates electrostatic charge that is directly propor
`tional to the applied force. Both low and (more modern)
`high impedance piezoelectric gages have high frequency
`response, but may exhibit signal drifting due to charge
`
`1
`
`Page 1 of 26
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`
`Rosellini v. Grunenthal GmbH
`
`IPR2016—00471
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`
`2
`
`Tablet Press Instrumentation
`
`leakage (approximately 0.04% decay per second can be
`seen in modern piezoelectric transducers). Nowadays,
`the preferred way of instrumenting tablet presses is with
`strain gages.
`
`Strain Gages
`
`Strain gages are foil, wire, or semiconductor devices that
`convert pressure or force into electrical voltage. When a
`stress is applied to a thin wire.
`it becomes longer and
`thinner. Both factors contribute to increased electrical
`
`resistance. If an electrical current is sent through this wire.
`it will be affected by the changes in the resistance of the
`conduit. This principle is used in strain gage—based
`transducers. Foil gages. known for robust application
`range, useful nominal resistance, and reliable sensitivity
`control. are most commonly used for instrumentation of
`compression. precompression, and ejection forces. Semi-
`conductor-based strain gages are inherently more sensitive
`but suffer from high electrical noise and temperature
`sensitivity. Such gages are not commonly used in tablet
`press instrumentation except for measurement of die—wall
`pressure and take—off forces.
`
`Wheatstone Bridge
`
`l) is a special arrangement of
`Wheatstone bridge (Fig.
`strain gages that is used to ensure signal balancing. The
`“full" bridge is composed of two pairs of resistors in a
`circle, with two parallel branches used for input and two
`for signal output. By applying the so—called excitation
`voltage (typically, 10V DC) to the bridge input and
`changing the resistance of different “legs” of the bridge by
`adding special resistors. we can make sure that there is a
`zero output voltage when no load is applied to the
`transducer. This is called zero balance. Once the bridge is
`
` Screw _....—-—-V
`
`terminals
`
`T’
`
`1 A typical Wheatstone bridge arrangement of strain
`Fig.
`gages.
`
`Page 2 of 26
`
`balanced. a small perturbation in the resistance of any
`“leg” of the bridge results in a nonzero signal output.
`The output of the Wheatstone bridge is nonnally
`expressed in millivolts per volt of excitation per unit of
`applied force. For example. sensitivity of 0.2 mV/V/kN
`means that applying. say. l0kN force and 10V excitation
`will produce 20mV output. To utilize such output.
`it
`usually needs to be amplified several hundred times to
`reach units of volts.
`
`Another important function of the bridge is balancing
`of temperature effects. Although individual strain gages
`are sensitive to temperature fluctuations. Wheatstone
`bridge arrangement provides for temperature sensitivity
`compensation.
`so that
`the resulting transducer
`is no
`longer changing its output to any significant degree when
`it heats up.
`Because the output of these bridges is in the range of
`millivolts,
`the cables utilized to carry the signal are
`normally shielded with a braided or foil-lined sheath
`around individual wires. The shield. as a rule. is connected
`
`to the amplifier. but never touches the actual instrumented
`equipment (i.e.. tablet press). If this rule is violated. a
`ground loop may generate electrical noise and present a
`dangerous electrical shock hazard.
`
`Gage Factor
`
`The gage factor is a specific characteristic of a strain gage.
`It is calculated as the change in resistance relative to unit
`strain that has caused this change. Strain gages that are
`commonly used for tablet press instrumentation are made
`from copper nickel alloy and have a gage factor of about
`2.
`In a typical bending beam application (such as
`compression roll
`transducer), one side of the beam
`experiences tension while the other side undergoes
`compression. By mounting two gages on each side,
`the
`sensitivity of the transducer can be doubled.
`Strain gages have to be bonded to areas of machine
`parts that are most sensitive to applied force. Such areas
`can be identified with the use of polarized light technique
`that “points out” the stress distribution.
`
`Manufacturing a Force Transducer
`
`instrumentation designs are proprietary and
`Usually,
`specific detail drawings are held in fiducial capacity.
`Each force transducer is custom designed for a particular
`machine part. Overall specifications are taken from the
`actual party and/or manufacturer approved drawings.
`Duplicate steel members. such as pins and cams, are
`normally made from A2 tool steel
`in a fully annealed
`
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`
`Table! Press Instrumentation
`
`3
`
`(softened) state. Original machine parts are first annealed.
`if required. The member is then machined. hardened to
`Rockwell 60 64. tested, and finally. ground to specified
`dimensions.
`
`is highly advantageous to determine stress dis-
`It
`tribution using polarized light beams identifying the areas
`of maximum strain yet avoiding areas of uneven strain.
`A typical procedure of transducer manufacturing in a
`professional gaging lab might be as follows. Foil
`type
`strain gages are bonded to the steel member utilizing a
`high-performance 100% solid epoxy system adhesive
`under controlled heating rate conditions. After intrabridge
`wiring is completed using solid conductor wiring (to
`prevent electrical noise). multistrand wire cabling with a
`combination of foil and braided insulation is connected to
`
`the bridge. Wire anchoring is achieved utilizing epoxy
`adhesive and protected with a combination of latex-based
`adhesives and/or epoxy resins. Lead wire cabling is
`protected by Teflon outer shield. as well as inner braided
`wire shield. The Teflon to steel joint is sealed with epoxy
`but should not be subject to stresses that would cause the
`cable to kink or yield in such a way as to expose the inner
`braided wire shield. Protective coatings are then applied
`and final postcure heating is performed for at least 2hr at
`a temperature approximately 50°C above the transducer
`normal operating temperature (approximately 175 or
`I05 °C). A silicon-based adhesive. such as RTV. is used to
`
`lzuge cavities while maintaining a low modulus of
`fill
`elasticity. preventing undue infiuence upon the actual
`strain measurement. The coating is resilient to moderate
`mechanical abrasion. as well as most solvents. oils.
`
`intended for protection against
`is not
`It
`cleaners. etc.
`penetrating sharp objects.
`A high—quality connector is then attached to the cable.
`The next step is to perform offset zeroing with fixed.
`l‘7c
`precision. low-temperature coefficient resistors. followed
`by NIST (National Institute for Standards and Technol-
`ogy) traceable calibration.
`It is worth noting that, in general. the duplicate members
`are not made of corrosion-resistant steel. because high
`tensile strength and ability to be easily machined are
`required. Prior to storage.
`the surface should be treated
`like any high—grade tablet press punch steel will be treated:
`a thin coating of oil should be used after wiping with
`alcohol.
`
`Load Cells
`
`In some cases, where appropriate. a load cell can be used
`in place of bonded strain gages.
`A load cell is a strain gage bridge in an enclosure
`forming a complete transducer device.
`
`Page 3 of 26
`
`it produces an
`Like any properly made transducer.
`output signal proportional
`to the applied load. Unlike
`custom-made transducers that require application of strain
`gages directly to press parts, load cells are self-contained
`and can be placed on a press in specially machined cavities
`to be easily replaced or serviced. As a drawback. load cells
`generally are less sensitive or less suited to measure the
`absolute force than custom-made strain gage transducers.
`Load cells can be used on punch holders in a single station
`press. Another example of load cell use is a die assembly
`for calibration of existing traducers on a press.
`
`Linear Variable Differential Transformer
`
`Linear variable differential transfonner (LVDT. Fig. 2) is
`a device that produces voltage proportional to the position
`of a core rod inside a cylinder body.
`It measures
`displacement or a position of an object relative to some
`predefined zero location. On tablet presses. LVDTs are
`used to measure punch displacement and in—die thickness.
`They generally have very high precision and accuracy. but
`there are numerous practical concerns regarding improper
`mounting or maintenance of such transducers on tablet
`presses.
`
`Proximity Switch
`
`Proximity switch (Fig. 3) is a noncontact electromagnetic
`pickup device that senses the presence of metal. On tablet
`presses. it is widely used to detect the beginning of a turret
`
`
`
`Fig. 2 Linear variable differential transfonner (LVDT).
`
`

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`
`4
`
`Tablet Press Instrumentation
`
`
`
`Fig. 3 Proximity switch.
`
`to identify stations and to facilitate peak
`revolution.
`detection in tablet force control applications.
`
`Instrumentation Amplifier and Signal
`Conditioning
`
`Signal conditioning involves primarily an amplifier that
`provides the excitation voltage. as well as gain (a factor
`used in convening millivolt output of the strain gage
`bridge to the volt-based range of the input of the data
`acquisition device).
`from other
`is different
`Instrumentation amplifier
`types of amplifiers in that the signal from each side of
`the transducer bridge is amplified separately and then
`the difference between the two amplified signals is,
`in
`tum. amplified. As a result. noise from both sides of the
`sensors is reduced.
`
`filter
`also offer
`instrumentation amplifiers
`Some
`functionality. A filter circuit combines resistors and
`capacitors that act to block the undesirable frequencies.
`It
`is a fact. however.
`that a good transducer should
`provide a clean signal. Use of analog or digital filters
`may cause the loss of some important portions of the
`signal
`that one is attempting to measure. Frequency
`filters may cause the measured compression force peak.
`for example.
`to be skewed toward the trailing edge of
`the peak and can yield a lower than actual peak force.
`Because
`filters distort
`the signal.
`they must be
`avoided unless absolutely necessary.
`In many cases,
`better electronics may make filter use obsolete. For
`example.
`the so—called antialiasing filter that is used to
`condition a high—frequency signal for a slow sampling
`rate
`is generally not
`required for
`tablet press
`applications
`if modem fast
`speed data acquisition
`devices are used for signal processing.
`In addition to amplification, excitation. and filtering.
`signal—conditioning devices may provide isolation. voltage
`division.
`surge protection.
`and current—to—voltage
`conversion.
`
`Instrumentation Terms: Definitions
`
`Several important terms may be now defined with respect
`to transducers and signal conditioning:
`
`a
`
`interval over which a
`Full Scale (F5): The total
`transducer is intended to operate. Also. it can define
`the output from transducer when the maximum load is
`applied.
`a Excitation: The voltage applied to the input terminals
`of a strain gage bridge.
`a Accuracy: The closeness with which a measurement
`approaches
`the true value of a variable being
`measured. lt defines the error of reading. Good tablet
`press transducers have at least 1% accuracy ( with this
`level of accuracy. for example. a compression force
`transducer with 50 RN FS will produce at most an error
`of SOON).
`
`o
`
`Precision: Reproducibility of a measurement, i.e., how
`much successive readings of the same fixed value of a
`variable differ from one another. If a person is shooting
`darts. for example. the accuracy is determined by how
`close to the bull's eye the darts have landed. while the
`precision will be indicated by how close the darts are
`to each other.
`
`o Resolution: The smallest change in measured value
`that the instrument can detect.
`
`o Dynamic range of a transducer: The difference
`between the maximal FS level and the lowest
`
`it
`(dB),
`detectable signal. Measured in decibels
`indicates the ratio of signal maximum to minimum
`levels:
`
`Some press sensors may have a rather narrow dynamic
`range not necessarily correlated with accuracy. For
`example, a very accurate compression roll pin designed
`to measure S0kN force may not detect 5 kN signal.
`o Calibration: Comparison of transducer outputs at
`standard test loads to output of a known standzud at the
`same load levels. A line representing the best fit to data
`is called a calibration graph.
`o Calibration factor: A load value in engineering units
`that a transducer will indicate for each volt of output.
`after amplifier gain and balance. Calibration factor is
`usually expressed in relation to FS.
`Shunt calibration: A procedure of transducer testing
`when a resistor with a known value is connected to one
`
`0
`
`leg of the bridge. The output should correspond to the
`voltage specified in the calibration certificate. If it does
`not. something is wrong and the transducer needs to be
`inspected for possible damage or recalibrated.
`
`Page 4 of 26
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`
`Tablet Press Instrumentation
`
`5
`
`0
`
`o
`
`Sensitivity: The ratio of a change in measurement
`value to a change in measured variable. For example, if
`a person ate a I lb steak and the bathroom scale shows
`2 lb increase in the body weight, then the scale can be
`called as insensitive (ratio is far from unity).
`Traceability: The step-by-step transfer process by
`which the transducer calibration can be related to
`
`primary standards. During any calibration process,
`transducer is compared to a known standard. National
`or
`international
`institutions usually prescribe the
`standards. In the United States, such governing body
`is the NIST.
`
`o Measurement errors: Any discrepancies between the
`measured values and the reported results over the
`entire FS. Such errors include, but are not limited to:
`
`o Nonlinearity: The maximum deviation of the
`calibration points from a regression line (best fit
`to the data), expressed as a percentage of the rated
`FS output and measured on increasing load only.
`o Repeatability: The maximum difference between
`transducer output readings for repeated applied
`loads under identical loading and environmental
`conditions. It indicates the ability of an instru-
`ment
`to give identical
`results
`in successive
`readings.
`o Hysteresis: The maximum difference between
`transducer output readings for the same applied
`load. One reading is obtained by increasing the
`load from zero and the other reading is obtained
`by decreasing the load from the rated FS load.
`Measurements should be taken as rapidly as
`possible to minimize creep.
`0 Return to zero: The difference in two readings:
`one, at no load, and the second one, after the FS
`
`load was applied and removed.
`
`A good transducer is one with the combined (or
`maximum) error of less than I% of the FS.
`
`Analog-to-Digital Converters
`
`from a
`(in volts)
`In order to convey analog output
`transducer to a computer, it has to be converted into a
`sequence of binary digital numbers. Modern analog-to-
`digital converter (ADC) boards are sophisticated high-
`speed electronic devices that are classified by the input
`resolution, as well as the range of input voltages and
`sampling rates.
`Resolution of an ADC board is measured in bits.
`
`is 55
`Bit (abbreviation for binary unit) is a unit of information
`equal to one binary decision (such as “yes or no,
`on or
`
`off’). A I2-bit system provides a resolution of one part in
`2'2
`4096, or approximately 0.025% of FS. Likewise, I6
`bits correspond to one part
`in 216
`65,536, or
`approximately 0.00l5% of FS (for tablet press appli-
`cations, such resolution is usually excessive).
`Thus, resolution of ADC board limits not only the
`dynamic range but also the overall system accuracy.
`Altematively, a higher resolution may be required to retain
`a certain level of accuracy within a given dynamic range.
`For example, a 0.5% accurate transducer with 80dB
`dynamic range requires at least I2-bit ADC resolution.
`Amplifying a low-level signal by 10 or 100 times
`increases the effective resolution by more than 3 and 6 bits,
`respectively. On the other hand,
`increasing an ADC
`resolution cannot benefit the overall system accuracy if
`other components, such as amplifier or transducer, have a
`lower resolution.
`
`When an input signal change is smaller than the
`system’s minimum resolution,
`then that event will go
`undetected. For example, for an FS of IO V (corresponding
`to, say, a compression transducer output of 50 kN), using a
`12-bit ADC, any signal that does not exceed 2.44 mV
`(12.2 N) will not be seen by the system.
`The ADC boards also differ by the effective sampling
`rate range. Sampling rate speed is measured in Hertz
`(times per second). The signals coming from a tablet press
`have a frequency of not more that l00Hz (compression
`events per second). To avoid aliasing (losing resolution of
`the incoming signal due to slow sampling rate),
`the
`sampling rate should be at
`least
`twice the highest
`frequency of the signal. Most ADC boards used for data
`collection in tableting applications have a sampling rate of
`5 20 times larger than signal frequency. That
`is why
`antialiasing filters are not required.
`
`Computers and Data Acquisition Software
`
`Overall accuracy of a data acquisition system is
`determined by the worst—case error of all its components.
`One should be aware of the fact that most system errors
`come neither from transducers (0.5% 1% accuracy) nor
`from A/D converters (0.025% accuracy) but from the
`software analyzing the data (round—off errors, improper
`sampling rate, or algorithms).
`The speed and capacity of a data acquisition system
`depend on the computer’s processor and hand drive
`specifications. The real—time data from transducers is
`streamlined to both the screen (for monitoring) and the
`disk (for replay and analysis). Generally speaking, “real-
`time” processing means reporting any change in the
`phenomena under study as it happens. Interestingly, but
`
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`
`6
`
`Tablet Fl'L“39 Instrumentation
`
`a high-speed data collection from a tablet press and a
`bookkeeping home finance program used on a monthly
`basis to balance the checkbook can both be related to as
`“real-time" software. The difference is in the time frames.
`
`For a tablet press. we are collecting data that need to be
`sampled and processed in milliseconds
`(a typical
`compression event may take 25 msec). while for home
`bookkeeping once a month will do.
`Most vendors supplying transducers, signal condition-
`ing. and computer hardware adhere to practical standards.
`e.g.. there are some accepted norms for strain gage factors,
`combined errors, sensitivity. ADC resolution. and
`sampling rate. The difference between vendors is apparent
`when we compare software because there are no
`universally established standards of user interface. Yet
`the hardware is “transparent" (i.e..
`invisible) to the end
`user
`day in tuid day out the user is facing the screen.
`keyboard. and mouse. The ease of software use. bug-free
`analysis of signals coming from transducers, reliability of
`statistical computations. quality of graphs and reports. and
`validatability of the system all of the above contrfloute to
`the quality of the data acquisition software.
`Proper validation tests of a data acquisition system
`should include calculation of an overall system error when
`the input is known and controlled (eg. an NIST traceable
`signal generator providing a sinusoidal signal with a
`known amplitude and frequency, to simulate compression
`events on a press). Comparing the output (for example.
`peak heights as reported by the software) to the known
`input. the overall system error can be reliably established.
`
`TABLET PRESS INSTRUMENTATION FOR
`PRODUCT AND PROCESS DEVELOPMENT
`
`R&D Grade Instrumentation
`
`In a production environment, typical tablet weight control
`mechanism is driven by signals that are coming from a
`compression force transducer. Strain gages may be
`installed on a column connecting the upper and lower
`compression wheel assemblies. This transducer measures
`what is generally known as “main compression." i.e.. a
`diluted average of the forces acting on the upper and lower
`punches. Altematively. a load cell may be positioned in
`some convenient
`location to register a transmitted
`compression force.
`It
`is measured away from the force
`application axis. and some force is lost in the transmission.
`The resulting measurement may be adequate for tablet
`weight control. but. even if properly calibrated. it does not
`represent the absolute value of the compression force.
`
`The R&D grade instrumentation, in contrast. requires
`placing the strain gages as close to the punch tips as
`possible in a vertical alignment with the direction of force
`application.l3°l in practice. it means placing the gages in a
`compression roll pin. so that the resulting measurement
`would reflect the absolute force. Thus. we can differentiate
`
`between the force on the upper and lower punch, and
`moreover. we can compare readings of the compression
`force from different tablet presses.
`
`Compression Force Measurement
`
`On a typical R&D grade compression transducer. a
`compression roll pin is machined with incisions made for
`placing strain gages (Fig. 4). The actual form of these
`cavities constitutes the very art of the transducer design
`that is usually proprietary and is based on the know-how of
`instrumentation vendor.
`
`It hast to be noted that there could be an upper or lower
`instrumented compression roll pin.
`The resulting measurement of a compression force is
`highly correlated with a variety of tablet properties. As
`compression increases. so does tablet hardness and weight
`(at constant thickness and true density). along with a force
`required to eject a tablet. Many variables affect the force
`of compression: press settings. press speed. punch length
`variation. punch wear. and damage.
`formulation and
`excipient pmperties.
`
`Precompression Force Measurement
`
`Similar to instrumented compression pin, there is a way
`to instrument an upper or lower instrumented preCom—
`pression mll pin. Precompression, if it exists on a press.
`is used for de-aerating and initial tamping of the powder
`mass in the die and usually helps to achieve the desired
`hardness without capping or lamination.l3°‘3” The force
`
`
`
`Fig. 4 Compression force transducer.
`
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`
`Tablet Press Instrumentation
`
`7
`
`of precompression is normally a
`compression force.
`
`fraction of
`
`the
`
`Ejection Force Measurement
`
`There are many ways to instrument an ejection cam.
`A preferred zurangement of strain gages (the so-called
`“shear force" design) does not require any discontinuities
`in the cam surface, and, most importantly, it provides for a
`very good linearity of the resulting signal (Fig. 5).
`Larger ejection forces may lead to an increased wear on
`tooling and eject cam surface. Ejection force may also be
`used to evaluate the effectiveness of lubrication (of both
`
`the press and the product) and punch sticking. Sensitivity
`and linearity of an ejection transducer are design
`dependent: shear force designs are always preferable
`over split cam or cantilevered beam designs.
`
`Take-Off Force Measurement
`
`Take—off force is monitored by mounting a strain gage to a
`cantilever beam on a press feed frame (in front of the take-
`off blade, Fig. 6). It is done to measure adhesion of tablets
`to lower punch face. Such adhesion is indicative of
`underlubricated granulation, poor tooling face design, die-
`wall binding, and tablet capping.[32'33l
`
`Speed Measurement and Station ID
`Determination
`
`Station identification and press speed are usually obtained
`by means of a revolution counter (proximity switch). It is
`installed on the press in order to mark the beginning of
`a turret revolution and thus enable station identification
`
`and speed calculations by system software.
`In addition, a linear arrangement of proximity switches
`can mark the beginning and end of a compression event
`
`
`
`Fig. 5
`
`Instrumented ejection cam.
`
`
`
`Fig. 6
`
`lnstrumented take off bar.
`
`(information that is vital for tablet weight controllers) and
`also indicate missing punches.
`Other points of instrumentation on a tablet press rue as
`follows:
`
`a
`
`a A rotary press pull-up cam can be instrumented to
`measure the upper punch pull-up force (the force
`required to pull up the upper punch from the die).
`Likewise,
`the
`lower punch pull-down force is
`measured on a bolt holding the pull-down camm] It
`is useful
`in determining the smoothness of press
`operation (extent of lubrication. cleanliness of the
`machine, and long batch fatigue buildup).
`Punch displacement measurements are easily done on
`a single station press by attaching LVDT to the punch.
`On a rotary press. such measurements can be done by
`means of slip ring. telemetry. or instrumented punch.
`Punch displacement profiles may be used in conj-
`unction with compression force to estimate work of
`compression and work of expansion (measure of
`elasticity). Because capping tendency increases with
`the punch penetration depth,
`it may be desirable to
`monitor actual punch movement
`into the die. The
`shape of a force displacement curve is an indication
`of the relative elasticity or plasticity of the material;
`whereas plastic deformation is desirable for stronger
`tablets, excess plasticity usually results in tablets that
`tend to cap and larninate.”5'37'
`o Radial and axial die—wall force measurements also
`
`provide an insight into the compaction mechanism of
`the material and may indicate a die—wall binding
`(sticking) that is,
`in effect, a negative pull on lower
`punch. The radial die—wall pressure due to friction is
`material—specific and is more evenly distributed inside
`the die with an addition of a lubricant.'3""""
`
`Instrumentation of the die presents a technological
`challenge because pressure is distributed nonlinearly
`
`Page 7 of 26
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`

`
`©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form unthout the express written permission of Marcel Dekker, Inc.
`
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`
`8
`
`o
`
`to tablet position inside the die and
`with respect
`depends on tablet thickness."‘5'46'
`In-die temperature can be monitored for heat-sensitive
`fomrulation, such as ibuprofen.[47’48'
`
`Instrumented Punch
`
`Several vendors offer instrumented punch, i.e., a punch
`that has strain gages and other instrumentation built-in.
`The data are then accumulated or transmitted via telemetry
`to a stationary computer. Such devices are versatile
`enough to report compression forces and either punch
`displacement or acceleration, and, at least in theory, they
`can be easily moved from press to press.[49’5°] However,
`one should keep in mind that each instrumented punch is
`limited to one size and shape of the tooling, and is limited
`to one station, compared to roll pin instrumentation that
`reports data for all stations and any tooling. In addition,
`instrumented punches are either rather cumbersome to
`install, or else they report a useless measurement of punch
`acceleration instead of punch displacement. Attempts to
`calculate displacement from acceleration so far could not
`be validated.
`
`Single Punch Press for Ft&D
`
`Single punch eccentric presses are often used in early
`stages of formulation development because they do not
`require a large amount of powder. Another benefit is that
`they allow relatively inexpensive measurements of die
`forces and punch displacement (there is no rotation of die
`table and therefore no need to use expensive telemetry
`methods). This is the primary reason why so much basic
`research and product deve