Transducer, force balance

Analyzers are available for both strain (displacement) and stress (force) control. On the one hand, under strain control, the probe is displaced and the resulting stress of the sample is measured by implementing a force balance transducer, which utilizes different shafts. The advantages of strain control include a better short-term response for materials of low viscosity, which allows experiments of stress relaxation to be performed with relative ease. On the other hand, under stress control a set force is applied while several other experimental conditions (temperature, frequency, or time) are varied. Although stress control analyses are... 

K. Chau, S. Lewis, Y. Zhao, R. Howe, S. Bart, R. Marcheselli, An integrated force-balanced capacitive accelerometer for low-g applications, in Tech. Dig. 8th Ini. Conf. Solid-State Sensor and Actuators (Transducers 95), Stockholm, Sweden, June 1995, 593-596. 

It is convenient to express this energy in eV-mL-i h-i according to Eq. 13, with V as the volume of the irradiated liquid. The sonochemical yield can be expressed in the G unit commonly used in radiolysis, i.e., representing the number of molecules or equivalents transformed per 100 eV of energy absorbed. Other force balances are described. A method of calibrating high-frequency transducers by measuring the radiation force on spherical objects set in an axisymmetric wave field is reported. ... 

Force Restoration Transducer A device that uses electronic feedback to adjust the current in an electromagnet until it balances the force put against it by a mass, permitting a highly accurate determination of weight. 

FIGURE 18.5 Seismic accelerometers (a) rectilinear acceleration transducers, (b) rotary accelerometer, (c) force-balance accelerometer, (d) piezoelectric accelerometer. 

Pressure measurement force-balance and elastic element systems, and solid state pressure transducers. 

Variable gap transducers are often used in feedback systems, like force-balance accelerometers, since the mass displacement is held very smaU - within the linear zone - by the servo control. 

Seismic accelerometers sense the ground or stmcture seismic vibrations and, together with a suitable recorder, are called accelerographs. Most modem seismic accelerometers are of force-balance type (EBA), a servo system in which a feedback force is applied to the suspended inertial mass to keep its motion as small as possible. This improves the instmment linearity and dynamic range. Usually the mass motion is measured by a sensitive capacitive transducer. 

Active Sensors The heart of the active sensor is a device measuring the ground acceleration, the so-called force balanced accelerometer (FBA) (Fig. 3). The FBA has a feedback coil, which can exert a force equal and opposite to the inertia force due to the acceleration. The displacement transducer sends a current to this force coil... 

Sensors, Calibration of. Fig. 3 Simplified principle behind the force balanced accelerometer. The displacement transducer normally uses a capacitor C, whose capacitance varies with the displacement of the mass. A current, proportional to the displacement transducer output, will force the mass to remain stationary relative to the frame (Figure from Havskov and Alguacil 2010)... 

Sabolich Research and Development (Oklahoma City, Okla.) recently has developed commercial systems for the upper and lower extremities that attempt to supplement sensation lost by amputation. Their Sense of Feel leg provides sensory feedback to the residu limb. Output from force transducers in the sole of the foot is transmitted to the residual limb via electrodes in the prosthetic socket. The amplitude of the tingling sensation is proportional to the force measured at the foot, thereby providing direct feedback to the amputee regarding the relative loading of the forefoot versus the heel. Such information is hypothesized to improve lower extremity balance. Clinical trials are currently underway. 

At the start of a test the cylinder is completely filled by the solid/liquid mixture which can exhibit the characteristics of a slurry, where both filtration and consolidation processes take place, or by a networked semi-solid where consolidation commences from the start of the test. In general the mixture will be concentrated and/or composed of fine particulates that do not settle appreciably under gravity. The piston is introduced to the open end of the cylinder and the constant applied force causes pressure to be generated within the cylinder. Liquid is expelled through the filter medium and collected in either a measuring cylinder or, in more sophisticated units, a vessel mounted on a load cell or electronic balance. The volume or mass of liquid is measured at various time intervals from the start of expression. The thickness of suspension (or cake towards the end of the test) in the cylinder can be calculated from a knowledge of the volume of liquid removed from the cylinder, or from measurements of the piston displacement from the beginning of the test should an appropriate transducer be fitted. 

Figure 4.19 shows a schematic representation of state-of-the-art expression apparatus. The more conventional piston press apparatus described in Section 4.5.1 is augmented with the addition of a computer driven pressure regulator which allows the pressure applied to the piston to be carefully controlled and varied according to a preset pattern (e.g. ramp, sinusoidal etc.). The interfaced rotary or linear encoder, force transducer and electronic balance allow semi-continuous measurements of piston displacement, transmitted pressure and... 

Lateral capillary forces between vertical cylinders or between spherical particles have been measured by means of sensitive electromechanical balance [317], piezo-transducer balance [318], and torsion microbalance [319]. Good agreement between theory and experiment has been established [318,319]. 

Force measurement spring balances, strain gauges, piezoelectric transducers. 

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