Capacitive displacement

A most recent commercial Nano Indenter (Nano Indenter XP (MTS, 2001)) consists of three major components [66] the indenter head, an optical/atomic force microscope, and x-y-z motorized precision table for positioning and transporting the sample between the optical microscopy and indenter (Fig. 28). The load on the indenter is generated using a voice coil in permanent magnet assembly, attached to the top of the indenter column. The displacement of the indenter is measured using a three plate capacitive displacement sensor. At the bottom of the indenter rod, a three-sided... 

When detecting the interface between two liquids, electrical conductivity, thermal conductivity, opacity, or sonic transmittance of the liquids can be used. Interface-level switches are usually of the sonic, optical, capacitance, displacer, conductivity, thermal, microwave, or radiation types. Differential pressure transmitters can continuously detect the interface, but, if their density differential is small relative to the span, the error will be high. On clean services, float- and displacer-type sensors can also be used as interface-level detectors. In specialized cases, such as the continuous detection of the interface between the ash and coal layers in fluidized bed combustion chambers, the best choice is to use the nuclear radiation sensors. 

Figure 2.15 (a) A guarded parallel plate displacement sensor, (b) An instrumentation system with output proportional to capacitance displacement. (Part (a) is from Measurement Systems Application and Design by E O Doebelin, copyright 1990 by McGraw Hill, Inc.)... 

Capacitive displacement transducer, where the capacitance output is a function of position or displacement... 

A more direct definition of a dielectric is a material in which the capacitive (displacement) current is larger than the in-phase current, wC > G or f > gUtze. According to this definition, saline (a = 1 S/m, e = 80 x 8.85 x 10 F/m) is a conductor for f < 250 MHz, and a dielectric for f > 250 MHz. At sufficiently high frequencies, even a metal becomes a dielectric. The definition is frequency dependent, and this is not very practical for a general grouping of materials. In any case, a material is classified as a dielectric if it has an ability to store energy capacitively, not just dissipate it. 

The displacement transducer shown in Fig. 5 is generally one of two types. A linear-variable differential transformer (LVDT) is effectively a transformer where the coupling to the secondary winding(s) is a function of the position of a movable magnetic core. A capacitive displacement transducer (CDT) is a capacitor where the... 

Seismk Accelerometers, Fig. 4 Schematics of variable capacitance displacement transducers, (a) Variable gap. As the central plate (red) moves right, the gap with the left fixed plate (blue) increases (capacitance of left capacitor, yellow, decreases), and the gap with the right plate (blue) decreases (capacitance of the right capacitor, light blue. 

At the start of our d.c. experiment, i.e. immediately after switching on the cmrent, in addition to the pure conduction current Ir there is also a capacitive displacement current flow (Ic), which is responsible for the time dependence. For small stimuli it follows that (Ic = (5/ )Qc Qc=capacitor charge) ... 

The Series 1151 differential pressure transmitter manufactured by Rosemount (MinneapoHs, Minnesota) uses a capacitance sensor in which capacitor plates are located on both sides of a stretched metal-sensing diaphragm. This diaphragm is displaced by an amount proportional to the differential process pressure, and the differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 4—20 m A d-c output. 

Tlte value of / can thus be varied in magnitude and phase displacement to suit a particular location of installation or pi otective scheme by introducing suitable R and /Y into the neutral circuit. When the impedance is inductive, the fault current will also be inductive and will offset the ground capacitive current /". In such a grounding, the main purpose is to offset the fault current as much is possible to immunize the system from the ha/ai ds of an arcing ground. This is achieved by providing an inductor coil, also known as an arc suppression coil, of a suitable value in the neutral circuit. 

In lead zh conate, PbZrOs, the larger lead ions are displaced alternately from the cube corner sites to produce an antifeiToelectric. This can readily be converted to a feiToelectric by dre substitution of Ti" + ions for some of the Zr + ions, the maximum value of permittivity occumirg at about the 50 50 mixture of PbZrOs and PbTiOs. The resulting PZT ceramics are used in a number of capacitance and electro-optic applicahons. The major problem in dre preparation of these solid soluhons is the volatility of PbO. This is overcome by... 

Numerous AFM imaging techniques have been developed and commercialized to monitor topography, friction, mechanical response, capacitance, magnetic properties, etc. However, adhesion measurements require the tip to come into, and out of, contact to measure attractive and adhesion forces. Therefore, other than to select an analysis region, most imaging techniques are not useful for adhesion studies. Instead, measurements are necessarily based on force-displacement curve approaches. 

Asif, S.A.S., Wahl, K.J. and Colton, R.J., Nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer. Rev. Sci. Instrum., 70, 2408-2413 (1999). 

In capacitive circuits the current leads the voltage producing it. Here current is maximum 90 deg earlier than the voltage, as shown hy 4 in sketch D. Again, if the circuit has resistance, the angle of the phase difference (displacement) is less than 90 deg. ... 

This technique can be used to measure displacement where, in effect, the two electrodes are connected to the two bodies. It has also other applications (for example, in moisture meters where the presence of water vapor between the electrodes causes the capacitance change). 

Other, similar methods for measuring surface-surface interactions, which come under the generic heading of surface force apparatus, include the crossed-filament method. This utilizes a beam deflection technique similar to that now being used in some AFMs for the measurement of surface displacement [94]. Another technique for displacement measurement used in a similar SFA is that of a capacitance transducer. Both techniques suffer the criticism that separation is not measured at the point of interest, i.e., the gap between the two surfaces as measured in the FECO technique. 

There is a displacement contribution of SJ arising from the charging of the double layer. This can simply be represented by a capacitance, Cdh that is in parallel with RCT and —W—. 

The experiment is conducted measuring the current corresponding to different voltages (positive and negative) applied to the capacitor. Since the overall voltage applied to the capacitance is V—AT the displacement current is given by I = (V — A< )dC/dt. 

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