Force transducers strain gauges

In general, one should be aware that tablet press instrumentation involves the use of strain gauges or piezoelectric transducers to provide a voltage signal proportional to the force applied for the compaction operation. Let us say we can measure forces, such as those applied to the granulation by the punches, that applied to the die wall, that required for tablet ejection, etc. With tlie use of other transducers we can also measure distance. With the measurement of force and distance, we can calculate work, energy, etc. 

The tensile force and birefringence were simultaneously measured as functions of the strain. In brief, the samples were suspended vertically between two clamps the lower clamp was fixed, and the upper clamp was connected to a force transducer (Statham strain gauge). The output of the transducer was monitored by a Hewlett-Packard chart recorder (7, 8). Values of the birefringence An were determined by using a single-frequency He-Ne laser according to well-established procedures (2, 7, 8). Values were calculated directly from the sample thickness and the relative retardation R, which was measured with a Babinet-type compensator. The measurements on the 660-21.3 X 10 samples were carried out at 0-90 °C, and those on the 880-21.3 X 10 samples were carried out at 25 °C. 

The most common method for reducing the effects of environmental inputs is isolation. Here, each of the measuring elements is effectively isolated from environmental changes. Examples are the placement of reference junction of a thermocouple in a temperature-controlled enclosure rmd the use of active vibration-isolation tables to isolate a measuring system (e.g., atomic force microscope) from external mechanical vibrations. Of course, it is possible to reduce environmental influences by selecting a transducer material that is completely insensitive to a specific environmental parameter. An example is the use of a metal alloy in strain gauges that has a zero coefficient of thermal expansion. But such an ideal material is often difficult to find and quite expensive. 

Force/pressure transducers The simplest force measures use load cells based on an LVDT, which displaces by a small amount as a force is applied. Strain gauges can measure the (small) deformation of thin film attached to a surface by measuring the resistance of an attached conducting element. 

Static force measurements (in which there is no apparent motion) usually rely on measurement of strain due to the applied force. Figure 18.6(c) illustrates the typical construction of a common force transducer called a load cell. The applied force produces a proportional strain in the S-shaped structural member, which is measured with a sensitive displacement transducer, usually a strain gauge resistor or a piezoelectric crystal. 

In the case of actively controlled structural dynamics, sensors may measure a variety of signals, such as accelerations (accelerometers), displacements (Hall sensors, capacitive sensors, laser interferometers, etc.), forces (force transducers), or - typically for adaptronic structures - strain or strain velocities (strain gauges, piezoelectric sensors, etc.). Most of these cases can be represented by the following sensor equation ... 

S37.8 Specifications and Tests for Strain Gauge Force Transducers. 

Force measurement spring balances, strain gauges, piezoelectric transducers. 

The forced oscillation technique can be used over a wider frequency range than the torsion pendulum ranging from 10 to 10 Hz. Uniaxial extension (Fig. 10.8(b)), bending, torsion and shear are used in different commercial instruments. A sinusoidal strain is applied to the specimen and the stress (force) is accurately measured using a strain gauge transducer as a function of time. Knowing both the strain and stress as functions of time enables the complex modulus ( or G ) to be determined. 

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