Compliance transducer

Transducers that use torsion bars or springs of known compliance may also oscillate at the beginning of the test as the step is imposed. This ringing is symptomatic of the detector and not the sample response, and thus needs to be removed by signal filtering. In a creep test this kind of behavior only occurs if the sample is... 

Because the cartilage, even when swollen. Is very thin (approx. 1-1.5 mm thick), the compliance of the load cell had to be taken into account in determining the axial displacement of the tissue during compression. This was done by subtracting the displacement of the platen-load cell at each load (as determined from the displacement-load curve of the load cell) from that measured by the displacement transducer. Likewise It was necessary to add an appropriate displacement (as calculated) during the relaxation phase of the experiment. With the particular load cell used In our apparatus, these corrections ranged from 3 to 5% of the displacement at peak loads, to negligible amounts at low loads. 

Acceptors, e.g, replacing Zr"" with Fe with the concomitant formation of oxygen vacancies. Domain reorientation is limited, and hence acceptor additives lead to poorly developed hysteresis loops, lower k, low tan 5, low compliance, and high aging rates. Typical applications are in high-power devices such as sonar and ultrasonic transducers. 

The horizontal and vertical deformation necessary to determine, primarily, the creep compliance and Poisson s ratio are measured by LVDTs, or other types of transducers/gauges, on both specimen faces (see Figure 7.32). 

Moving coil transducers are the most commonly encountered design class but, as with loudspeakers, membrane resonances must be carefully minimized through damping strategies. Cushion compliance and cup resonances must also be factored into high-fidelity designs. 

Introduction Motion and Force Transducers Process Transducers Transducer Performance Loading and Transducer Compliance... 

Because of these complicating effects it can be advantageous to determine 544 and 566 froni experiments in simple shear. Lewis, Richardson and Ward [13] have used a Hall effect transducer to measure this type of deformation and obtain shear compliances that agree with those measured in torsion. 

There is considerable practical interest in developing low-density, compliant, flexible piezoelectric transducers. A low-density piezoelectric would have better coupling to water and have a more easily adjusted buoyancy than the higher-density ceramics used for hydrophones. A complaint material would have a better resistance to mechanical shock than a conventional ceramic transducer, and a large compliance would also mean high damping, which is desirable in a passive device. A flexible material could also be formed to any desired profile. 

Meissner (1972) shows how this small transient deflection of the torque transducer can affect short time start-up data (see Figure 8.3.7). Gottlieb and Macosko give relations to correct G and G" for compliance. These corrections are made in the software in some rheometers. Corrections are reliable only if the spring constant K is known accurately, and then typically only when the measured strain is 10% or more of the commanded value. 

One solution to transducer deflection is to eliminate it with a feedback control servo see Figure 8.2.5 (Franck, 1985a). The transducer is essentially a dc motor in which torque is measured by the current needed to prevent any deflection. No servo system is instantaneous, and a combination of high frequency and torque can lead to transducer compliance (Mackay and Halley, 1991). The upper frequency is about 100 rad/s, whereas stiff torsion bar systems have resonant frequencies above 10 rad/s and piezoelectric sys-... 

With its low acoustic impedance, extreme bad width, high piezoelectric coefficient, and low density (only one-quarter the density of ceramic materials), PVDF is ideally suited as a transducer for broad band underwater receivers in lightweight hydrophones. The softness and flexibility of PVDF give it a compliance 30 times greater than ceramic. PVDF can thus be utilized in a hydrophone structure using various device configurations, such as compliant tubes, rolled cylinders, discs, and planar stacks of laminated material. 

Instrument compliance is another cause of concern in making mechanical measurements, since the forces generated by the fluid in response to a deformation will tend to twist, bend or compress the rheometer components that also experience these forces. These include shear and normal force transducers as well as the frame of the instrument. A sophisticated approach to dealing with this problem is the use of a Force Rebalance Transducer (product of Rheometrics/TA Instruments described, for example, by Vermant et al. [102]) in which a feedback loop provides torsional and axial motions to compensate for the corresponding compliances and thus minimize their effects on data. In the case of torsional motion, there remains some compliance due to the twisting of the shafts supporting the fixtures [103]. 

The measurement of normal stress differences in transient deformations is extremely sensitive to small variations in gap spacing, which can arise from instrument compliance or minute temperature variations. Venerus and Kahvand [43] have shown how to evaluate the effect of instrument compliance by measuring the response using several sets of cone-plate fixtures. If a Force Rebalance Transducer is used for a transient normal stress measurement to compensate continuously for compliance in order to keep the gap constant, the response time of the transducer may affect the data. Also, the thermal expansion that results from the power dissipated in the transducer can affect the gap spacing and is of particular concern when normal stresses are being measured [104]. 

Veneras [106] carried out simulations to explore the effects of several instrument effects, as well as slip, on step strain results, and he concluded that both transducer compliance and slip can result in stress relaxation data showing type C behavior in step strain. However, Archer et al. [28,105,107] showed that slip is in fact the primary cause of this phenomenon. 

In Section 10.8.1 it was noted that molecular orientation results in flow birefringence in a polarizable polymer, and if the melt is transparent, optical techniques can be used to determine the three components of the stress tensor in uniform, shear flows [91-93]. To determine the transient normal stress differences, the phase-modulated polarization technique was developed by Frattini and Fuller [ 126]. Kalogrianitis and van Egmond [ 127] used this technique to determine the shear stress and both the normal stress differences as functions of time in start-up of steady simple shear. Optical techniques are particularly attractive for measurements of normal stress differences, since such methods do not require the use of a mechanical transducer, whose compliance plagues measurements of normal stress differences by mechanical rheometry. 

Mackay, M. E., Halley, P. J. Angular compliance error in force rebalance torque transducers./. Rheol. 

Dynamic Modulus. The complex shear compliance is being obtained in a Periy-Fitzgerald transducer apparatus and preliminary results are illustrated here.f9) Figure 10 shows the real part of the compliance, reduced to 25 0 Figure 11, the imaginary part. 

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