Direct Measurement Of Acceleration Research Articles

Building an Acceleration Ladder with Tidal Streams and Pulsar Timing

We analyse stellar streams in action-angle coordinates combined with recent local direct acceleration measurements to provide joint constraints on the potential of our galaxy. Our stream analysis uses the Kullback–Leibler divergence with a likelihood analysis based on the two-point correlation function. We provide joint constraints from pulsar accelerations and stellar streams for local and global parameters that describe the potential of the Milky Way (MW). Our goal is to build an “acceleration ladder,” where direct acceleration measurements that are currently limited in dynamic range are combined with indirect techniques that can access a much larger volume of the MW. To constrain the MW potential with stellar streams, we consider the Palomar 5, Orphan, Nyx, Helmi, and GD1 streams. Of the potential models that we have considered here, the preferred potential for the streams is a two-component Staeckel potential. We also compare the vertical accelerations from stellar streams and pulsar timing, defining the function , where Φ is the MW potential determined from stellar streams and α 1 pulsar z is the vertical acceleration determined from pulsar timing observations. Our analysis indicates that the Oort limit determined from streams is consistently (regardless of the choice of potential) lower than that determined from pulsar timing observations. The calibration we have derived here may be used to correct the estimate of the acceleration from stellar streams.

A METHOD FOR THE DETERMINATION OF THE KINETIC FRICTION COEFFICIENT IN FRICTION PAIRS BASED ON DIRECT ACCELERATION MEASUREMENT OF AN OBJECT ON AN INCLINED PLANE

The paper presents and experimentally verifies a method for the determination of the kinetic friction coefficient of friction pairs. The method involves a direct acceleration measurement of an object sliding down an inclined plane and the determination of the momentary friction force based on acceleration. The laboratory test rig was presented and the theoretical foundations of the method were discussed in detail. The laboratory experiments were conducted for the following material pairs: steel-cast iron, steel-bronze, and for various inclined plane angles. The experiments results were presented, acceleration vs. time curves were plotted, and friction coefficients were determined. These values were compared to the values known from literature [L. 1–3]. The uncertainly analysis of kinetic friction coefficient determination was conducted. The method effectiveness and accuracy was verified, noting the sources of possible measurement errors and the ways to eliminate them.

Water Entry of Cylindrical Shells: Theory and Experiments

Modeling hydroelastic response of lightweight structures to water impact plays a pivotal role in the design of lightweight marine vessels and crew capsules. In contrast to the majority of studies that focus on water entry of wedge-shaped bodies, this study addresses the hydroelastic impact of cylindrical shells. A cylindrical shape allows for a systematic study of the interplay between bending and stretching during water entry. This study puts forward a mathematically tractable framework to study the rigid body motion and elastic deformation of cylindrical shells, free falling onto a quiescent water surface. Donnel thin shell theory is used to model the shell, and Wagner theory is adopted to determine the hydrodynamic loading as a function of the rigid body motion and structural deformation. Galerkin method is employed to cast the problem into a manageable system of coupled ordinary differential equations, which are then solved numerically. Model predictions are validated against experiments on shells with two thicknesses, detailing both the flow physics and elastic response through the cogent integration of direct acceleration measurement, high-speed imaging, and particle image velocimetry. The proposed framework and experimental dataset are expected to constitute the foundations on which to formulate new theories for lightweight composites and testing computational schemes.

Vibration Tracking Control of Piezoelectric Bimorph Bender in Novel Reciprocal State Space Form

In this paper, the deflection of a piezoelectric bimorph bender is controlled to track given time varying reference command solely utilizing direct acceleration measurement without integration. Novel estimator that directly applies acceleration measurement and controller that applies estimated state derivative signals have been developed in reciprocal state space (RSS) form. Simulations for the augmented system of both closed loop system and estimator have been carried out to successfully verify the proposed methods. The design approach in this paper is applicable to smart structures with accelerometers as sensors.

Water entry of compliant slender bodies: Theory and experiments

Hydroelastic phenomena play a critical role on the dynamics of lightweight marine vessels, but a comprehensive understanding of their potential benefits and drawbacks in water impact remains elusive. In contrast with the literature on hydroelastic impact that focuses on plane strain deformations and two-dimensional fluid flows, we address the water entry of compliant slender bodies. We propose a mathematically-tractable framework to predict the rigid body motion and elastic deformation of a beam impacting the water surface in free fall. At each location on the beam, the hydrodynamic loading is determined from the level of submersion of the cross-section and its local velocity and acceleration. Slamming, added mass, and drag are all included in the model and cogently weighted based on the level of submersion of the cross-section. Edge effects are incorporated in the model through a shape function that modulates the hydrodynamic loading as a function of the distance from the edges. Model predictions are validated through experiments on compliant beams of circular cross-section, which detail both the structural response and the resulting flow physics, through the integration of high-speed imaging, direct acceleration measurement, and particle image velocimetry. Results of this study are expected to aid in the design of lightweight ship hulls, experiencing dynamic bending deformations during sailing and maneuvering.

Direct measurement of the positive acceleration of the universe and testing inhomogeneous models under gravitational wave cosmology

One possibility for explaining the apparent accelerating expansion of the universe is that we live in the center of a spherically inhomogeneous universe. Although current observations cannot fully distinguish ΛCDM and these inhomogeneous models, direct measurement of the acceleration of the universe can be a powerful tool in probing them. We have shown that, if ΛCDM is the correct model, DECIGO/BBO would be able to detect the positive redshift drift (which is the time evolution of the source redshift z) in 3–5 year gravitational wave (GW) observations from neutron-star binaries, which enables us to rule out any Lemaître-Tolman-Bondi (LTB) void model with monotonically increasing density profile. We may even be able to rule out any LTB model unless we allow unrealistically steep density profile at z ∼ 0. This test can be performed with GW observations alone, without any reference to electromagnetic observations, and is more powerful than the redshift drift measurement using Lyman α forest.

Some observations on Statnamic pile testing

Limited guidance on usage and methodology is currently available to engineers for the most common rapid load pile test, known as Statnamic. In order to improve the analysis of Statnamic testing in clay soils a full-scale instrumented auger bored pile was installed and tested in glacial lodgement till. As a result, improvements have been suggested in the test methodology and analysis. The inclusion of an accelerometer at the pile head would allow direct measurement of acceleration and verification of displacement measurements and velocity calculations. High-precision optical levelling of the pile before and after test cycles would allow multiple loading cycles to be considered cumulatively with greater confidence. The separation of the laser reference source from the test pile should be sufficient to avoid surface wave disturbance.

Aerodynamic force measurement on a large-scale model in a short duration test facility

A force measurement technique has been developed for large-scale aerodynamic models with a short test time. The technique is based on direct acceleration measurements, with miniature accelerometers mounted on a test model suspended by wires. Measuring acceleration at two different locations, the technique can eliminate oscillations from natural vibration of the model. The technique was used for drag force measurements on a 3m long supersonic combustor model in the HIEST free-piston driven shock tunnel. A time resolution of 350μs is guaranteed during measurements, whose resolution is enough for ms order test time in HIEST. To evaluate measurement reliability and accuracy, measured values were compared with results from a three-dimensional Navier–Stokes numerical simulation. The difference between measured values and numerical simulation values was less than 5%. We conclude that this measurement technique is sufficiently reliable for measuring aerodynamic force within test durations of 1ms.

Force Measurement of Large-scale Scramjet Model within Short Test Duration

Aerodynamic force measurement technique was developed for large scramjet models within short test time. The technique was based on direct acceleration measurement with miniature accelerometers mounted on a test model, which was suspended with stainless steel wires to make the restorative force negligible. In order to cancel the natural mode of vibrations of the test model, sum averaging signal processing technique was proposed. The technique was applied on drag force measurement of a 3 m length rectangular duct scramjet in the high enthalpy shock tunnel HIEST. Measurement result was evaluated with a three-dimensional Navier-Stokes numerical simulation. The evaluation with numerical simulation showed that the difference between measurement and numerical simulation is less than 5%. It is demonstrated that the technique has sufficient reliability to measure aerodynamic force with time resolution of 350μs, which is equivalent to frequency response of 2.88 kHz.

Unsteady force measurement technique in shock tubes

An aerodynamic force measurement technique with an extremely short test time was developed. A signal recovery method based on a frequency domain deconvolution technique was applied on a direct acceleration measurement with a weakly restrained test model. With the technique, an unsteady drag force of an 80 mm diameter sphere model was measured when a planar shock wave of Ms=1.22 passed the sphere in a vertical shock tube of 300×300 mm square cross section. The measured drag force was evaluated with a numerical simulation, the reliability of which was verified with pressure and optical measurements. The evaluation revealed that the present force measurement technique has enough accuracy and time resolution for phenomenon with a duration of a few hundred μs. Additionally, the unsteady force measurement of a sphere revealed that drag force had been negative for approximately 200 μs after the shock wave passed over the sphere. Numerical analysis showed that this negative drag was caused by the high pressure produced by shock waves converging at the lower part of the sphere.

First Direct Measurement of Acceleration in the Outflow of a Nova: U Scorpii (1999)

We report the first direct measurement of acceleration in the outflow of a nova, U Sco, observed 5 days after the start of an outburst. Spectroscopic time series of the intense H? emission line, obtained with the Space Telescope Imaging Spectrograph camera on board the Hubble Space Telescope, shows a bright emission component moving across the spectrum at a rate of +0.16 ? 0.02 ? in 30 minutes. This is consistent with a clump of emitting material having a line-of-sight acceleration ar = +4.1 ? 0.5 m s-2. The data suggest that this clump resides within a moderately collimated bipolar outflow. Due to projection effects, this implies an actual acceleration of the clump relative to the star of a +25 m s-2.

An output feedback sliding mode speed regulator for DC drives

Synthesis of the switching surface equation in a variable structure speed controller is accomplished by using a phase lead type compensator. In addition, the switching control is designed using only the sign of the switching surface function and an estimate of the maximum value of the equivalent sliding control. As a result, the proposed variable structure controller relies only upon output feedback. The problem of control chattering is addressed by substituting a saturation function for the switching control. The practical significance of the work is demonstrated by designing a robust speed regulator for a DC drive, without direct acceleration measurement or the use of a state estimator. Results from a control experiment complement the discussion.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On adaptive inverse dynamics control of rigid robots

The authors derive an adaptive inverse dynamics control law for rigid robots that overcomes the most restrictive difficulty of such schemes to date, namely, the requirement that the inverse of the estimated inertia matrix remain bounded. The control scheme still requires the joint accelerations for its implementation. In practice, one would implement this scheme by estimating the acceleration from the measured velocity. Good performance could still be expected without direct acceleration measurement, assuming sufficient encoder resolution and sufficiently fast sampling. Another practical advantage of the authors' scheme is that since the 'nominal' inverse dynamics or inner loop portion of their control law is fixed, it is more amenable to the development of a dedicated interface and can be 'burned' onto a chip to reduce online computation requirements. >

Measurement of Average Mechanical-Power Input to a Structure Excited by Frequency Bands of Noise

An impedance head is used to measure the average power fed into a complex structure excited by frequency bands of random noise. The same technique also yields the average driving-point impedance and the correlation coefficient between the force and the velocity. The technique is described, and measurements are presented for a plate with and without damping. The results are compared with those obtained by direct measurement of acceleration and reverberation time of the plate under the assumption that a reverberant vibration field exists. Satisfactory agreement is obtained. [This work was supported under U. S. Air Force Contract No. AF 33(657)-9118.]