Tablet measuring punch position

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. 

Linear variable differential transformer (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. 

The displacement transducers (LVDTs) which measure the absolute punch positions must also be carefully calibrated. This is particularly important when conducting studies which require accurate in-die tablet thickness measurements. Like the punch load cells, the calibration of the LVDTs are also performed in situ using a reference displacement transducer. Statistical analysis has shown that the displacement versus voltage relationship may not be linear over the full range of some transducers, and could be better described by a third-order polynomial. However, over smaller ranges, the relationship is adequately linear (24). These effects should be taken into account for a particular experiment or application as they could propagate error into in-die tablet thickness measure and subsequent. solid fraction calculations. Typically, calibration for both load cells and transducers are conducted once or twice annually. 

The punch pressure required to form a compact for tableting indices measurements is measured at the end of a long dwell time, typically 1.5 minutes, during which the punches remain in fixed positions and stress relaxation within the compact brings about a decay in the applied load. The reported pressure or CS is calculated from the relaxed force and it is dependent on the compact SF. A sample s CS at a standard SF, such as 0.85, can be interpreted to indicate the ease (i.e., the magnitude of the pressure) of forming compacts under standardized conditions. 

Upper precompression and main compression rollers insertion depth adjustments Insertion depth for both precompression and main compression is adjusted in the upper cam section. The insertion depth determines the location of tablet formation in the die cavity relative to the top of the die table as shown in Fig. 5. It is measured as the distance at which the upper punch enters into the die at the tangent between the upper punch head and the compression roller. Insertion depth can be varied between 2 and 6 mm on most machines and is typically maintained between 3 and 4 mm. For precompression and main compression, the insertion depth should be maintained at approximately the same position. On most modern rotary tablet presses, the adjustments for precompression and main compression insertion depth are independent. However, on many older designs, the precompression roller is attached to the main compression roller assembly and its position is measured relative to the main compression roller position. In this way, the ratio of precompression to main compression remains constant as machine adjustments are made. 

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. Instrumentation of the die presents a technological challenge because pressure is distributed non-linearly with respect to tablet position inside the die and depends on tablet thickness. " ... 

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