Inertial Member Research Articles

A torsion balance as a weak-force testbed for novel optical inertial sensors

Torsion balances (TBs) are versatile instruments known for their ability to measure tiny forces and accelerations with high precision. We are currently commissioning a new TB facility to support the development and testing of novel optical inertial sensor units for future gravity-related space missions. Here, we report on the status of our apparatus and present first sensitivity curves that demonstrate acceleration and torque sensitivities of 5⋅10−11ms−2 and 1⋅10−12NmHz−1 at frequencies around 4mHz , respectively. Capacitive sensors and optical levers measure the dynamics of the system with a displacement sensitivity of down to 9⋅10−10mHz−1 for the former and 2⋅10−11mHz−1 for the latter. Combining the readout of the suspended inertial member (IM) with environmental sensor signals, the system is characterized, and limiting noise sources are identified. We find that, in particular, the coupling of ambient seismic motion is limiting over a broad frequency range and show that due to its high susceptibility to ground motion, our TB is also a promising platform for exploring ground motion sensing in multiple degrees of freedom. Future upgrades will focus on mitigating seismic noise by controlling the torsion fiber suspension point using piezoelectric actuators and the integration of precision interferometric readout of the IM. These improvements will further increase the sensitivity towards the thermal noise limit which constrains the performance to 1⋅10−13ms−2Hz−1 at 4mHz .

Stroboscopic torsion pendulum

We report on a simple, inexpensive readout for torsion pendulums, suitable for robust applications like teaching-lab equipment or monitoring of large amplitude oscillations. A short light pulse is recorded every time a reflective band on the pendulum inertial member passes in front of the sensor, an infrared LED pair (emitter–receiver). Simple algebraic manipulations on the time series of these pulses arrival times allow one to extract the resonant frequency as well as the decay time of the pendulum. This readout is insensitive to the amplitude of oscillation and is therefore suitable for monitoring torsional oscillations of large amplitude, where traditonal readouts like optical levers or autocollimators encounter dynamic range limitations.

A new torsion pendulum for gravitational reference sensor technology development.

We report on the design and sensitivity of a new torsion pendulum for measuring the performance of ultra-precise inertial sensors and for the development of associated technologies for space-based gravitational wave observatories and geodesy missions. The apparatus comprises a 1 m-long, 50 μm-diameter tungsten fiber that supports an inertial member inside a vacuum system. The inertial member is an aluminum crossbar with four hollow cubic test masses at each end. This structure converts the rotation of the torsion pendulum into translation of the test masses. Two test masses are enclosed in capacitive sensors which provide readout and actuation. These test masses are electrically insulated from the rest of the crossbar and their electrical charge is controlled by photoemission using fiber-coupled ultraviolet light emitting diodes. The capacitive readout measures the test mass displacement with a broadband sensitivity of 30 nm∕Hz and is complemented by a laser interferometer with a sensitivity of about 0.5 nm∕Hz. The performance of the pendulum, as determined by the measured residual torque noise and expressed in terms of equivalent force acting on a single test mass, is roughly 200 fN∕Hz around 2 mHz, which is about a factor of 20 above the thermal noise limit of the fiber.

Microscope System Having Vibration Dampening Mechanism

A movable part to which a specimen or optical component is fixed is effectively damped in a short period of time at low cost, while saving space and energy and avoiding heat generation, thus allowing high-precision observation of a specimen to be carried out quickly after stopping at a certain position. The invention provides a microscope system including a driving mechanism for driving a movable part to which a specimen or optical component is fixed and stopping the movable part at a certain position; and a vibration damping mechanism for damping vibrations generated when stopping the movable part with the driving mechanism. The vibration damping mechanism includes an inertial member having a prescribed mass and a viscoelastic member sandwiched between the inertial member and the movable part. Dimensions in a plurality of directions of the viscoelastic member are set on the basis of resonance frequencies of vibrations generated in multiple directions when stopping the movable part.

Staircase pattern in an oscillating torus He II analogous of the superconducting rf SQUID

We are developing a superfluid 4He analog of the superconducting rf SQUID to be used as a gyroscope. A 1∼μ m radius orifice supported by a thin partition, that works as a weak-link, is placed inside a toroidal container filled with liquid helium and the superflow through the orifice is induced by rotating the whole torus, which is the inertial member of a torsional oscillator. Connected with the occurrence of the critical velocity for the flow, the torus oscillation amplitude shows a small but reproducible staircase pattern, that is also observed in the resonant frequency vs. drive curve. This pattern can be seen from 1.3 K up to 2.05 K, where the system resolution is suppressed by the reduction of the superfluid fraction, while at lower temperatures a single plateau is observed. A computer simulation ascribes this behaviour to multiple phase slip occurrence in the orifice.

Staircase pattern in a superfluid He-II torsional-oscillator analog of the radio-frequency SQUID.

We are developing a superfluid [sup 4]He analog of the rf SQUID. A thin partition with a micrometric orifice is placed inside a toroidal container filled with liquid helium; ac and dc superflows through the orifice are induced by rotating the whole torus, which is the inertial member of a torsional oscillator. The torus oscillation amplitude vs drive curve shows a small but reproducible staircase pattern, which is detectable between 1.4 and 2.05 K, but is not modulated by a superimposed steady rotation. We discuss this behavior in terms of the occurrence of multiple phase slips.

Vibration and torsional damping coupling for a power transmission

Several embodiments of combined torsional and elastic couplings for transmitting power from an engine output shaft that experiences torque fluctuations and a driveshaft. In each embodiment, the flexible coupling includes an inertial member that is coupled either elastically or by means of a viscous body to the engine output shaft for absorbing the torsional vibrations. In addition, an elastic coupling couples the engine output shaft to the driveshaft for permitting limited relative rotation to absorb torque flux variations in the engine output shaft.

Rotationally induced dissipation in superfluid helium.

In an attempt to make a superfluid $^{4}\mathrm{He}$ analog of the radio-frequency superconducting quantum interference device (SQUID) that could be used as a gyroscope, we measured the critical velocity of the ac superflow through an orifice 5 \ensuremath{\mu}m in radius. The orifice was supported by a septum placed inside a hollow torus filled with liquid helium. The superflow through the orifice was induced by rotating the whole torus. The torus was the inertial member of a high-quality-factor torsion pendulum forced to oscillate at its resonance frequency. The occurrence of dissipation was detected by analyzing the oscillator motion. The principle on which this device works is analyzed, and results are reported of experiments in which quite reproducible critical oscillation amplitudes\char22{}well accounted for by intrinsic vortex formation\char22{}were obtained. Depending on the value of the forcing torque, the critical behavior manifested itself either as the occurrence of sudden collapses of the oscillation amplitude or as a steady excess dissipation. The energy involved in the process was estimated. Temperature dependencies of the critical amplitude up to 2.115\ifmmode\pm\else\textpm\fi{}0.003 K were measured. The results were found to be in agreement both with the ac data obtained with the Helmholtz-resonator technique and with the data obtained with dc flow measurements.

Progress towards a superfluid 4He analog of the superconducting rf squid

Abstract We are working to realize a superfluid 'He analog of the superconducting rf SQUID to be used as a gyroscope. As a first step we developed an experimental method to induce an ac superfluw across a 5 μm radius orifice by using a rotation. The orifice was supported by a septum placed inside a hollow torus filled with liquid helium and the flow was induced by rotating the whole torus, that was the inertial member of a torsional oscillator. We report here measurements of the critical velocity for the orifice, obtained by detecting the occurrence of a dissipation in the oscillator's motion. The energy and the number of circulation quanta involved in the dissipative process are evaluated.

The dynamic shear modulus and internal friction of a fiber vibrating in the torsional mode

A torsional pendulum consisting of an 8-mil boron–tungsten fiber and inertial member is described. The torsional oscillations are excited electrostatically and detected by a frequency modulation technique. Both the dynamic shear modulus and the internal friction can be measured. In addition, this system exhibits the resonance behavior characteristic of a high Q oscillator.