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Tablets & Capsules September 2019 21 real-time in-process data generated by the sensors. Some software packages offer graphical plots to evaluate a for- mulation's tabletability, compressibility, compactibility, and scalability profiles. R&D tablet presses Scientists use three main types of tablet presses for developing pharmaceutical and nutraceutical tablets: single-station presses, compaction emulators, and rotary presses. Single-station tablet presses. Scientists typically use single-station tablet presses during the initial stages of R&D because they require the smallest amount of mate- rial. There are many different single-station tablet press designs, including eccentric, hydraulic hand-pump, and automatic simulators driven mechanically or with hydraulics. Single-station presses can be designed for single- or double-ended compression. A single-ended compression system applies the load via the upper punch only; the lower punch receives only the force transmit- ted through the powder bed and die-wall friction. A double-ended compression system applies the load via both the upper and lower punches. Double-ended com- pression is comparable to the compression on a rotary tablet press, which uses compression rollers to cause both the upper and lower punches to travel into the die to compact the material. Single-station tablet presses that use single-ended compression provide valuable information if both the upper and lower punch assemblies are instrumented with force sensors. For example, if the lower punch force is equal to or greater than 90 percent of the upper punch force, then you can assume that there is very little friction in the die during compaction. This information can aid in selecting the appropriate excipients for a material with applied force can affect other variables. An instrumented R&D tablet press can help developers analyze these forces and better understand the material's behavior prior to scaling up for production. Tablet press instrumentation system components A quality instrumentation system is critical to ensuring accurate and actionable data. A software system can dis- play incorrect results just as attractively as it displays cor- rect results, and imprecise data can lead to costly scale-up issues. Collecting accurate and actionable data requires three vital components: a force transducer, a signal con- ditioner, and user interface software. Force transducer. A transducer is a device that con- verts energy from one form to another. On an R&D tablet press, the press applies mechanical force to the trans- ducer during operation, and the transducer generates a proportional electrical output. The transducer's design and placement are critical to ensure that the device responds only to the force that it is intended to measure. For example, a pre- or main compression transducer is designed to measure the force that is being applied by the punch head to the compression roller. The roll pin that supports the roller can be manufactured with strain gauges. The material of the roll pin or spring element (the area where the strain gauge is applied) is also essen- tial and is dependent on the expected force range. The force range of the main compression event is typically much higher than the force range expected for pre-com- pression. The spring element's elastic modulus, or the slope of its stress-strain curve, is important for proper reporting. The transducer's response should be sensitive and linear throughout its range and should show minimal hysteresis, good thermal stability, and an acceptable return to zero when the load is removed. Signal conditioner. The function of the signal condi- tioner is to amplify the transducer's small millivolt signal. This assembly requires a power supply, an amplifier, an analog-to-digital converter, and filters. The power supply is the source of excitation to the transducer. Historically, power supplies were noisy and tended to drift, but today's power supplies are much more stable. It is still prudent to measure the incoming voltage before sampling the voltage from the sensor. Ratiometric measurement is an accurate way to read the sensor's output independent of the voltage supplied by the power supply. The sensor output voltage is normalized by dividing by the output of the applied voltage and is expressed in millivolts per volt applied. The amplifier takes the small millivolt signal and amplifies it by up to 1,000 times. This amplification is needed to convert the analog signal to a digital signal. The analog-to-digital converter converts the incoming analog signal to a series of digital numbers, enabling large quantities of data to be analyzed. Filters remove unwanted analog noise signals that may generate from other machines in the room. User interface software. The user interface software monitors, records, and organizes the large amount of Photo 1: Example of a single-station tablet press (NP-RD10A) that uses single-ended hydraulic hand-pump compression.