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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 3989-3992 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A synchronous detector electronics package with self-contained oscillator is described. It uses only three integrated circuits, all of which contain operational amplifiers. The performance of the detector was evaluated with a capacitance sensor. Linearity and stability were found to compare favorably with general purpose commercial systems costing hundreds to thousands of dollars. Although the noise performance was less impressive, the dynamic range was still in excess of 90 db, permitting better than 100 μrad resolution over a range of nearly 3 rad. As presently configured, the detector is compatible only with high impedance sensors such as the capacitance type; however, coupling the oscillator output through a power amplifier would permit its use with LVDTs.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 2789-2793 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A linear sensor of the differential capacitance type, possessing more than 120 dB dynamic range, has been developed. The electrical equivalent circuit is that of a symmetric bridge, whose four capacitor elements derive from two semicircular segments each of a rotor and a stator. When driven at a constant excitation voltage, and configured with a differential amplifier and synchronous detector; the sensor output is proportional to the angular displacement, from the nominal position, of the rotor compared to the stator.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 57 (1990), S. 1825-1827 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A preliminary study has been made of the free decay of a physical pendulum whose mass is dominated by two metal bobs, and whose period can be adjusted to exceed 30 s. The decay mechanism is predominantly anelasticity from internal friction of creep. The relaxation spectrum of this creep suggests a mechanical Barkhausen effect. In surprising contrast to exponential decay at large energy, the motion at low energy and long period is characterized by repeatable metastable states. Phase is preserved in the transition between most states even though the transitions may be chaotic. There is a lowest harmonic state which is different for Pb and Cu bobs, and below which the system transitions from free decay to stochastic fluctuation-dissipation. Noise of this stochastic state is larger than electronic noise, and power spectral density plots show that it contains mixtures of subharmonics at integral multiples of 1/4 the pendulum frequency.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 68 (1990), S. 1616-1621 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Number densities and energies for desorption of gas molecules on solid surfaces have been determined using Langmuir recoil force measurements and numerical modeling for the first time. Activation energies are computed from the time-dependent relative recoil force and associated temperature history of the sample. Subsequently, the model uses the activation energy and the absolute recoil force to estimate absolute desorption rates and number densities of adsorbed molecules. Desorption is by laser-induced thermal means and is detectable for sample temperature increases smaller than 100 K above room temperature. The resulting recoil forces are measured with a torsion balance, which is mechanically similar to the Cavendish type. Measurements of deflection are different, however, by using a capacitance transducer instead of an optical sensor.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 2256-2261 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Differential capacitive sensors are common to pressure gauges. Previous sensors of the bridge type have utilized only two variable (active) components, resulting in a half-bridge electrical equivalent circuit. There are numerous advantages of a symmetric full bridge, using four active components. The symmetry improves linearity and increases signal to noise ratio, especially when supported by electronics based on synchronous detection. Another advantage of symmetry is invariance to scaling, through immunity to stray capacitance. This feature is especially important in relationship to the current trend toward micro-electro-mechanical systems. A crude, inexpensive, macroscopic prototype of the present design was built with attention to high sensitivity. Constructed with a thin aluminized Mylar diaphragm, this device can readily resolve pressure changes smaller than 10−2 Pa.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 2250-2255 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Capacitive transducers have proven to be very effective sensors of small displacements, because of inherent stability and noninvasive high resolution. The most versatile ones have been those of a differential type, in which two elements are altered in opposite directions in response to change of the system parameter being monitored. Oftentimes, this differential pair has been incorporated into a bridge circuit, which is a useful means for employing synchronous detection to improve signal to noise ratios. Unlike previous differential capacitive dilatometers which used only two active capacitors, the present sensor is a full-bridge type, which is well suited to measuring low-level thermal expansions. This analog sensor is capable of 0.1 μm resolution anywhere within a range of several centimeters, with a linearity of 0.1%. Its user friendly output can be put on a strip chart recorder or directed to a computer for sophisticated data analysis.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 810-813 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A variant of the linear rotary differential capacitance transducer has been used to build an angle sensor which is essentially linear over an infinite range, while maintaining microradian resolution. This capability is realized by ganging two of the transducers on a common axle, in quadrature. By sequentially monitoring the output from the pair of transducers with a personal computer, estimates of axle position are obtained through a switching algorithm.
    Type of Medium: Electronic Resource
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