Sensors with Strain Gauges

Diaphragm vacuum gauges where the deflection of the diaphragm is not detected mechanically but by strain gauges are called electromechanical transducers. In a strain gauge a thin element (wire or foil) is stressed by tension or pressure. 

This produces a change of electrical resistance within the element generated by the mechanical strain and not, as often assumed, by the change of length or cross section. 

Strain gauge or piezo-resistive pressure transducers can be bought as versions for absolute and gauge pressure. One disadvantage is the high temperature drift of the zero signal which constrains the measurement range to two pressure decades. 

For applications in corrosive fluids or at elevated temperature CDGs with ceramic diaphragm should preferably be used. AI2O3 is the best material for this application since it shows no creep after overloading and hence its zero stability is very good. Ceramic CDGs are nowadays available even for very low full-scale deflections below 1 mbar. 

The capacitive measurement principle with its very high sensitivity allows a resolution of down to 10 of full scale. To minimize thermal errors, transducers with controlled temperature are available where the sensor cell is kept at a constant temperature (e.g. 45 °C) by electrical heating. 

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A die stamping was produced in just one action. Stock (2) was fed into die (1) and the deformation to obtain a cup was performed by a stamp (3) which moved in a sleeve (4) driven by a piston of a hydraulic cylinder. The strain obtained was measured with strain gauge (5). The temperature of the deformed alloy was maintained by heating device (7) and controlled with sensor (8). After the deformation was completed shedder (9) driven by a piston (10) of hydraulic cylinder (11) ejected the cup. The whole press ram rested upon base (12). 

Fig. 6. Schematic view of the unit for low-temperature impact tests. Legend (1) outer cryostat shell, (2) internal cryostat shell, (3) nitrogen vessel, (4) screen, (5) siphon, (6) circular anvil, (7) crushed sample trap, (8) vacuum transfer, (9) dynamometer with strain gauges, (10) packing, (11) actuating power pneumatic cylinder, (12) hammer, (13) cutter, (14) piston, (15) upper test section, (16) lower test section, (17) rod, (18) cutter movement sensor shutter (19) cutter movement sensor, (20) lock crown, (21) lock, (22) support, and (23) control pneumatic cylinder.

Inductive sensors for displacement measurement are based on the fact that inductance of a cod, L = n Gfi, where n = number of turns of coil, G = form factor, and ju = effective permeability of the medium. Each of these three parameters can be changed by mechanical displacement. Linear variable differential transformer inductive sensors show good linearity over a large range of displacement, high resolution, and better sensitivity compared with strain gauge [11]. 

Further devices use strain-gauge transducers as so-called dead-end instruments or sensors mounted on a pipe with internal flow. The principle of this arrangement is the elastic deformation of a metallic cylinder, measured with the strain gauges. Pressures up to approx. 15 kbar can be measured [11]. 

In order to actively control the thermal bending deformation induced by time-varying temperature Tj, the bending deformation needs to be measured. In model 3, a sensor layer with two strain gauges at position A and B is bounded to the low temperature side... 

Parameters such as impeller speed and shaft power (in a stirred bioreactor) and fluid velocity are indicators of the degree of mixing and thus play an important role in the control of mass transfer. Impeller speed is easily monitored with a tachometer (electronic or mechanical) [39], but the measurement of shaft power input is not as straightforward. The most common method utilizes a torsion dynamometer attached to the impeller drive however, this technique includes losses due to friction in the drive shaft. Better data can be obtained from balanced strain gauges mounted on the impeller [37]. On-line measurement of the liquid velocity in a flowing or stirred system can be obtained by a heat-pulse method in which a resistance thermometer is used to measure a brief temperature increase caused by an upstream pair of electrodes [43]. Use of this sensor system has been limited to laboratory applications. 

Joung et al. [29] have also demonstrated the possibility of combining these sensors with pentacene-based thin film transistors as temperature sensors. The strain sensor consists of a Wheatstone bridge structure where the pentacene film acts as sensing layer of a strain gauge, while the temperature sensors consist of bottom-contact pentacene transistors in which the variations of the drain currents in the subthreshold regime are measured with respect to temperature. 

The 3-D ground reaction force vector, the vertical ground reaction torque and the point of application of the ground reaction force vector (i.e., center of pressure) are measured with force platforms embedded in the walkway. Force plates with typical measurement surface dimensions of 0.5 x 0.5 m are comprised of several strain gauges or piezoelectric sensor arrays rigidly mounted together. 

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