Torsion resonators loom large in the history of precision measurement; however their role in modern nanomechanics experiments is limited. In this presentation I will describe a new class of ultra-high-Q torsion nanoresonators fashioned from strained nanoribbons, and how they might be used for imaging-based quantum optomechanics experiments and chip-scale intertial sensing. Specifically, using an optical lever, we have resolved the rotation of one such nanoribbon with an imprecision 100 times smaller than the zero-point motion of its fundamental torsion mode, paving the way towards observation of radiation pressure shot noise in torque. We have also found that a strained nanoribbon can be mass-loaded without changing its torsional Q. We have used this strategy to engineer a chip-scale torsion pendulum with an ultralow damping rate of 7 micro-hertz, sufficient to resolve micro-g fluctuations of the local gravitational field.
We present a new class of ultra-high-Q nanomechanical resonators based
on torsion modes of high-stress nanoribbons, and explore their
application for quantum optomechanics experiments and precision
optomechanical sensing. Specifically, we show that nanoribbons made of
high stress silicon nitride support torsion modes which are naturally
soft-clamped, yielding dissipation dilution factors as high as 10^4
and Q factors as high as 10^8 for the fundamental mode. We show that
these modes can be read out with optical lever measurements with an
imprecision below that at the standard quantum limit, paving the way
for a new branch of torsional quantum optomechanics. We also show
that nanoribbons can be mass-loaded without changing their torsional Q
factor. We use this strategy to engineer a chip-scale torsion balance
with an damping rate of 10 micro-hertz. We use this torsion balance
as a clock gravimeter to sence micro-g fluctuation in the local
gravitational field strength.
If nanomechanical testing is to evolve into a tool for process and quality control in semiconductor fabrication, great advances in throughput, repeatability, and accuracy of the associated instruments and measurements will be required. A recent grant awarded by the NIST Advanced Technology Program seeks to address the throughput issue by developing a high-speed AFM-based platform for quantitative nanomechanical measurements. The following paper speaks to the issue of quantitative accuracy by presenting an overview of various standards and techniques under development at NIST and other national metrology institutes (NMIs) that can provide a metrological basis for nanomechanical testing. The infrastructure we describe places firm emphasis on traceability to the International System of Units, paving the way for truly quantitative, rather than qualitative, physical property testing.
We discuss a vibration measurement and control system developed to investigate the complex dynamics of boring-bar chatter. A fundamental question is how best to integrate the sensors and actuators for effective control. Is it sufficient to control only motions normal to the machined surface? We consider a smart structure that consists of two actuator/sensor pairs oriented orthogonally to control the motions of a boring tool in directions normal and tangential to the machined surface. Actuation is achieved with Terfenol-D struts that sting the tool near its base. We develop a control strategy by considering a single-degree- of-freedom chatter model for tool motions normal to the machined surface, showing that enhanced structural damping is an effective chatter control. We adapt a second-order feedback compensation scheme from the literature and point out the special design considerations engendered by the use of Terfenol-D actuation. We consider chatter signatures obtained using the system with and without feedback control and show that the system is very effective at chatter suppression. Because we may control each of the dominant structural modes independently, we examine the validity of a single-mode approximation by considering chatter signatures obtained with only tangential control active. We find that so-called mode coupling effects persist; hence, we expand our modeling efforts to include a coupling due to the cutting forces. We see that a variety of chatter modes exist, depending on the operating parameter, and that, to achieve the most robust performance from the controlled system, it is advisable to have control of both directions.
An active non-linear vibration absorber for flexible structures is developed. The absorber exploits the inherent quadratic nonlinearity of the actuator material Terfenol-D to produce a two-to-one autoparametric resonance between the forced vibrations of a structure and a second-order analog controller circuit. Nonlinear resonance of this type exhibits the well-known saturation phenomenon. When the structure is forced near resonance, its response saturates to a small value. This type of control has been demonstrated by previous researchers using linear actuators where nonlinearities were introduced via the analog circuit. In contrast, we use the natural nonlinearity of the Terfenol-D material to achieve the same results. We develop the theory and present experimental results for the control of the first and second modes of a cantilever beam. We also consider the application of the strategy experimentally when the forcing is due to a rotating imbalance. In this case, the excitation source is nonideal. Our results indicate that the saturation based control technique implemented with a Terfenol-D actuator constitutes an effective nonlinear vibration absorber.
A self-sensing magnetostrictive actuator design based on a linear systems model of magnetostrictive transduction for Terfenol-D is developed and analyzed. Self-sensing, or the ability of a transducer to sense its own motion as it is being driven, has been demonstrated for electromechanical transducers such as moving voice coil loudspeakers and, most recently, piezoelectric distributed moment actuators. In these devices, self-sensing was achieved by constructing a bridge circuit to extract a signal proportional to transducer motion even as the transducer was being driven. This approach is analyzed for a magnetostrictive device. Working from coupled electromechanical magnetostrictive transduction equations found in the literature, the concept of the transducer's `blocked' electrical impedance and motional impedance are developed, and a bridge design suggested. However, results presented in this paper show that magnetostrictive transduction is inherently non-linear, and does not, therefore, lend itself well to the traditional bridge circuit approach to self-sensing.
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