Nominal Amplitude Research Articles

Strain Measurement during Quasi-Static and Cyclic Loads in AL-6XN Material Using Digital Image Correlation Technique.

A customized digital image correlation (DIC) system was implemented to monitor the strain produced in a cold-rolled AL-6XN stainless steel plate, 3.0 mm thick, subjected to quasi-static and cyclic loading tests. A comparison of the DIC strain measurements was made against those provided by conventional extensometers. Furthermore, the DIC system was used to monitor the fatigue crack initiation in low-cycle fatigue tests. The true stress-strain behavior for the AL-6XN material was properly captured by the DIC measurements. For low-cycle fatigue tests (strain control), the strain mapping generated by DIC allowed for identifying zones with higher strain than the nominal strain amplitude applied (εa) since the first stages of the fatigue life (FL). These zones become potential fatigue crack initiation sites.

A 3 MHz Low-Error Adaptive Howland Current Source for High-Frequency Bioimpedance Applications.

Bioimpedance is a diagnostic sensing method used in medical applications, ranging from body composition assessment to detecting skin cancer. Commonly, discrete-component (and at times integrated) circuit variants of the Howland Current Source (HCS) topology are employed for injection of an AC current. Ideally, its amplitude should remain within 1% of its nominal value across a frequency range, and that nominal value should be programmable. However, the method's applicability and accuracy are hindered due to the current amplitude diminishing at frequencies above 100 kHz, with very few designs accomplishing 1 MHz, and only at a single nominal amplitude. This paper presents the design and implementation of an adaptive current source for bioimpedance applications employing automatic gain control (AGC). The "Adaptive Howland Current Source" (AHCS) was experimentally tested, and the results indicate that the design can achieve less than 1% amplitude error for both 1 mA and 100 µA currents for bandwidths up to 3 MHz. Simulations also indicate that the system can be designed to achieve up to 19% noise reduction relative to the most common HCS design. AHCS addresses the need for high bandwidth AC current sources in bioimpedance spectroscopy, offering automatic output current compensation without constant recalibration. The novel structure of AHCS proves crucial in applications requiring higher β-dispersion frequencies exceeding 1 MHz, where greater penetration depths and better cell status assessment can be achieved, e.g., in the detection of skin or breast cancer.

An approach to interpreting metastable austenitic material sensors for fatigue analysis

The transformation of metastable austenite to martensite under mechanical loading can be harnessed to create a material sensor which records a measure of the load history without the need for electrical energy and can be read out at arbitrary intervals via eddy current probing, thus leading to an ultra-low-power sensing solution. This paper presents possibilities of processing this load amplitude-dependent evolution of martensite content loading for component fatigue analysis. The general method is based on using a theoretical material model typically used in finite element analyses which includes hardening plasticity and phase transformation to precompute tables of stress amplitude or cumulative damage corresponding to different sensor readings which can be stored on a low power processing system onboard the component for energy-efficient lookup. At nominal single amplitude loading, the sensor can be used as a load cycle counter for known loads or as an overload detection device upon divergent martensite content rise. Interpretation of block program loading is less practical due to resolution issues. Under random loading, sequence effects get averaged out; interpretation is easiest with narrow load spectra, but information can be gained from very wide spectra as well. Multiple sensors at different locations can aid interpretation. Uncertainty due to necessary assumptions and untreated influences of temperature and loading rate is discussed.

Technical parameters analyses of different types of impact-vibration soil compacting machines

Introduction. Impact-vibration machines, which perform soil compaction with periodic removal of the working body from the ground, are the most common for mechanization of soil compaction in road construction: self-propelled non-reversible and reversible vibratory plates, vibratory rammers, mounted excavator vibratory plates, trench vibratory rollers and soil vibratory rollers. It is necessary to analyze the main technical characteristics of impactvibration machines of different types in order to assess the possibility of developing a mathematical model of soil compaction, combining several types of impact-vibration machines at once.Materials and methods. The technical characteristics of different types of impact-vibration machines for the analysis: 342 models of non-reversible self-propelled vibratory plates; 312 models of reversible self-propelled vibratory plates; 311 models of vibratory ground rollers; 63 models of mounted vibratory plates for excavators; 24 models of vibratory rammers and 21 models of trench vibratory rollers are used. The machine mass (for vibratory plates of different types) or the mass per compaction module (for vibratory rollers of different types) was taken as a main parameter.Results. The graphical dependencies have been obtained. They illustrate that for the given types of shock-vibration machines the excitation force, relative excitation force and oscillation frequency are located on mutually contiguous and continuing parts of curves. This is a common pattern: as the mass of the machine increases, the excitation force increases, but the relative excitation force and frequency of oscillation decrease. Vibratory rammers are out of this general pattern, because at comparable weight they have significantly lower frequency of oscillation (9...12 Hz) and high amplitude (50...75 mm). Also, the mounted excavator vibratory plates stand out from the general pattern, having a smaller range of vibration frequencies for machines of similar weight, moreover, they perform soil compaction positionally, and not in the process of movement at a certain speed, as other impact-vibration machines.Discussion and conclusions. The common pattern of changes in the main technical characteristics of selfpropelled non-reversible and reversible vibratory plates, trench rollers and soil vibratory rollers at close nominal amplitude of their vibrations gives grounds for the development of a unified theoretical description of the interaction of the working bodies of these types of machines with the compacted soil and a common method of calculating the influence of technical characteristics and operating modes of these machines on the results of soil compaction.

Using Dynamic Phasors To Model and Analyze Selective Harmonic Compensated Single-Phase Grid-Forming Inverter Connected to Nonlinear and Resistive Loads

In an islanded microgrid, a grid-forming inverter (GFMI) is required to maintain a stable sinusoidal voltage at nominal frequency and amplitude with minimal distortion regardless of loading conditions. As a result, the accurate design of GFMI control schemes in an islanded microgrid requires accurate understanding of the converter dynamics and system harmonics. Electromagnetic transient (EMT) simulations based on detailed switching models give an accurate picture of the GFMI's behaviour. However, EMT simulations take a relatively long time to produce accurate results in large-scale systems. On the other hand, conventional phasor models are unsuitable for harmonic studies. To address this limitation, this article leverages the dynamic phasor (DP) method to develop a new model of a single-phase GFMI connected to single-phase nonlinear (diode-bridge rectifier) and resistive loads. The benefit of the DP method lies in its inherent feature of transforming time-domain signals into slow-varying or DC signals, thereby enabling faster simulations. In this article, the relationship between the control gains of the proposed DP model and the conventional detailed model is established for the first time through small-signal and frequency-domain-based comparative analyses. User-written codes of the proposed DP model are scripted in MATLAB, and simulations on MATLAB/Simulink show that the proposed DP model results have a high degree of correlation with the detailed switching model results. A significant savings in computation time is exhibited by the proposed DP models. The simulation results are validated against experimental results to confirm the accuracy of the proposed DP model.

Compaction method development and materials performance evaluation of large size cement-treated macadam materials

Existing research mainly focuses on cement-treated macadam materials with a maximum nominal particle size (MNPS) of ≤ 37.5 mm, and there is little research on large size cement-treated macadam (LSCM) with an MNPS of ≥ 53 mm. In this paper, a self-developed vertical vibration testing equipment (VVTE) was used to compact an LSCM with an MNPS of 53 mm (hereafter, LSCM-50). Then, an appropriate specimen size for compacting LSCM-50 was determined. Moreover, a self-developed vertical vibration compaction method (VVCM) for laboratory-based compaction of LSCM-50 was proposed and its reliability was verified. Subsequently, the indoor mechanical properties and durability of the VVCM-fabricated LSCM-50 were studied. The results show that when the working frequency of VVTE = 32 Hz, upper system weight of VVTE = 122 kg, lower system weight of VVTE = 180 kg, nominal amplitude of VVTE = 1.2 mm, and static eccentric moment of VVTE = 0.215 kg·m, the compaction effect of LSCM-50 is the best. The correlation between the laboratory-VVCM-fabricated LSCM-50 samples and the on-site-drilled LSCM-50 cores is more than 90%. The mechanical properties of the LSCM-50 samples increase nearly linearly with the increase in the cement content but increase nonlinearly with the increase in the curing period. Additionally, the strength deterioration coefficient of the LSCM-50 samples does not decrease when the freeze–thaw cycles exceed seven, and the mass loss percentage of the LSCM-50 samples does not increase after being soured for 25 min. This paper provides a new method for compacting the LSCM and an evaluation of its mechanical properties and durability.

Experimental studies on the effects of draft condition, current heading and mooring stiffness on vortex-induced motions of a Tension-Leg Platform

The paper presents experimental studies on the effects of draft conditions, current headings and mooring stiffnesses on the Vortex-Induced Motion (VIM) responses of a Tension-Leg Platform (TLP) with four circular columns. The nominal transverse and yaw amplitudes, and the corresponding spectra obtained by fast Fourier Transform (FFT) and continuous wavelet transform (CWT) are analyzed to examine the characteristics of the motion responses. Four current headings in the range of 0° to 45° are adopted in the model test. The results show that the maximum response amplitude in both transverse and yaw directions are achieved at 0° current incidence. For the transverse response, the maximum nominal amplitude decreases and the lock-in range narrows as the current heading increases. The spectral analyses for the transverse and yaw motions at 0° current heading show that the dominant peak frequencies increase as the reduced velocity increases. It is observed that the low-frequency spectral peak of the in-line motion amplitude spectra increases as reduced velocity increases. A color band or a ribbon concentrates stably around the frequency of f<0.05 in the wavelet analysis. It is primarily caused by the initial acceleration of the model in the towing tank. The absolute values of correlations between the spring tensions and the transverse motion of the platform are much larger than those between the spring tensions and the in-line or yaw motions. Moreover, it is found that the VIM trajectories of the TLP are primarily always along the direction perpendicular to the current heading.

Transition induced by an egg-crate roughness on a flat plate in supersonic flow

Hot-wire measurements in a Mach 3.5 quiet tunnel were made in the wake of a roughness patch on a flat plate. These measurements were used to determine mode shapes and frequencies of the dominant instabilities leading to boundary-layer transition. The egg-crate roughness pattern is an analytic function described by a sinusoidal equation, similar to an array of discrete elements that are positioned in a spanwise and streamwise grid, but containing both protuberances and dimples. This is an intermediate configuration towards understanding the underlying physics of a pseudorandom distributed roughness, and ultimately, the underlying physics of roughness-induced boundary-layer transition. The roughness pattern had a wavelength of 6.25 mm, with a nominal amplitude of 272 ${\rm \mu}{\rm m}$ (0.49 times the boundary layer thickness at the first row of protuberances). The roughness was positioned near the leading edge of the flat plate and contained 3.5 wavelengths in the streamwise direction and 7.5 wavelengths in the spanwise direction. The dominant instability was centred near 74 kHz at a free stream unit Reynolds number of $12.9\times 10^{6}\,{\rm m}^{-1}$ and resembled an antisymmetric mode downstream of each of the protuberances in the roughness patch. Computations using linear stability analysis based on the plane-marching parabolized stability equations (PSE) showed limited agreement with measurements when comparing the growth of the wake instability. Better agreement with the measurements was observed when considering the modification of first mode waves by the egg-crate roughness patch and the solution of the three-dimensional harmonic linearized Navier–Stokes equations was used as the in-flow to the PSE. The agreement confirms the significance of disturbance growth both upstream of and above a finite length roughness patch and the effect on the growth of instabilities in the wake.

Preliminary Study of the Intel RealSense D415 Camera for Monitoring Respiratory Like Motion of an Irregular Surface

Motion of the tumour during a radiotherapy treatment can compromise the clinical outcome for the cancer patient. To address this challenge the suitability and performance of the Intel RealSense™ D415 depth camera has been investigated as a tool for measuring the respiratory motion of the body surface. The precision of the camera depth data has been characterised as a function of the measurement distance up to 1.2 meters from a stationary and moving surface. The latency of the camera system was also measured. The average standard deviation of depth data in stationary measurements ranged from less than 0.2 mm for the shortest camera-surface distance of 400 mm to 3 mm for a distance of 1200 mm. Similar accuracy is reported for the acquisition of breathing motion with a discrepancy from nominal amplitudes of around 0.5 mm and 5 mm for the two extreme measurement distances. No dependence on the measurement precision has been observed as a function of the motion pattern. Finally, the camera was shown to be able to measure the more patient-like motion of a deformable respiratory motion phantom with an amplitude of surface motion varying from 1.5 - 2.5 mm. Although noisier, the camera data showed good agreement when compared with a NDI Polaris Spectra system. The latency of the Realsense™ system was measured to be 68.6 ms ± 9.6 ms. The results of this work indicate that the D415 RealSense™ depth camera is capable of measurement of external respiratory type motion of an irregular surface.

High-Efficiency Can Be Achieved for Non-Uniformly Flexible Pitching Hydrofoils via Tailored Collective Interactions

New experiments examine the interactions between a pair of three-dimensional (AR = 2) non-uniformly flexible pitching hydrofoils through force and efficiency measurements. It is discovered that the collective efficiency is improved when the follower foil has a nearly out-of-phase synchronization with the leader and is located directly downstream with an optimal streamwise spacing of X*=0.5. The collective efficiency is further improved when the follower operates with a nominal amplitude of motion that is 36% larger than the leader’s amplitude. A slight degradation in the collective efficiency was measured when the follower was slightly-staggered from the in-line arrangement where direct vortex impingement is expected. Operating at the optimal conditions, the measured collective efficiency and thrust are ηC=62% and CT,C=0.44, which are substantial improvements over the efficiency and thrust of ηC=29% and CT,C=0.16 of two fully-rigid foils in isolation. This demonstrates the promise of achieving high-efficiency with simple purely pitching mechanical systems and paves the way for the design of high-efficiency bio-inspired underwater vehicles.

Nonlinear Dynamics of the Rigid Drum for Vibratory Roller on Elastic Subgrades

In this paper, a coupling nonlinear dynamic model of the drum and subgrade is established for the vibratory roller. The dynamic characteristics of the rigid drum of the vibratory roller in the process of vibratory compaction are comprehensively investigated by time history, phase diagram, frequency spectrum, Poincare map, and bifurcation diagram. During the compaction process, the stiffness of the subgrade increases and the motion of the rigid drum of the vibratory roller changes from a single period to multiple periods and finally enters chaos by the way of period doubling. Moreover, the roller parameters also significantly affect the dynamic characteristics of the rigid drum and the compaction effect of the subgrade. Based on detailed numerical results, a parameter adjustment strategy about the roller frequency and nominal amplitude is proposed, which can avoid the “bouncing” of the drum during compaction and improve the compaction efficiency.

Numerical modeling of the low cycle fatigue: effect of manufacturing imperfections caused by machining process

The majority of mechanical components in nuclear power plants must be designed to withstand extreme cyclic loading conditions. In fact, when these components are subjected to low cycle fatigue, machining imperfections are considered one of the most significant factors limiting their service life. In the present work, using finite element analysis, a methodology has been suggested to predict the fatigue life of cylindrical parts made of 316 SS, at ambient temperature, under nominal strain amplitude ranging from ± 0.5 to ±1.2% with various surface roughness conditions. Two different multiaxial strain-life criteria have been considered to estimate the fatigue life, namely Brown-Miller and maximum shear strain. The comparison between the predicted and the experimental fatigue lifetimes has revealed that the adopted multiaxial strain life criteria can successfully estimate the fatigue life of 316 SS grade under uniaxial loading conditions. Furthermore, it has been found that the fatigue life decreases as the surface roughness average value increases, which indicates that surface regularities have a significant impact on low cycle fatigue life. Therefore, the proposed methodology is found to be capable of assessing the impact of surface roughness on the fatigue life of this specific steel in the low cycle fatigue regime.

Numerical studies on flow-induced motions of a semi-submersible with three circular columns

Abstract The semi-submersible with three circular columns is an original concept of efficient multifunctional platform, which can be used for marginal oil, gas field, and floater of wind turbines (FOWT). However, under certain flow conditions, especially in uniform current with specific velocities, the eddies will alternatively form and drop behind columns, resulting in the fluctuating lift force and drag force. Consequently, the semi-submersible will subject to the flow-induced motions (FIM). Based on the detached eddy simulation (DES) method, the numerical studies were carried out to understand the FIM characteristics of the three-column semi-submersible at two different parameters, i.e., current incidences (0°, 30°, and 60°-incidences) and reduced velocities ( 4 ≤ U r ≤ 14 ). The results indicate that the lock-in range of 6 ≤ U r ≤ 10 for the transverse motions is presented, and the largest transverse non-dimensional nominal amplitude is observed at 60°-incidence, with a value of A y / D = 0.481 . The largest yaw amplitude A y a w is around 3.0° at 0°-incidence in the range of 8 ≤ U r ≤ 12 . The motion magnitude is basically the same as that of a four-column semi-submersible. However, smaller responses are presented compared to those of the three-column systems revealing the mitigation effect of the pontoon on FIM.

Proposal of novel chatter stability indices of spindle speed variation based on its chatter growth characteristics

Chatter is one of the most critical problems that causes poor surface quality and restriction of machining efficiency. Spindle speed variation (SSV) is a well-known technique for suppression of regenerative chatter. However, in the authors’ understanding, the chatter suppression effect diminishes when the spindle speed difference between the present and previous cutting moments is small. Furthermore, the stability changes largely according to the spindle speed variation profile which changes with the set condition of SSV parameters, e.g., nominal spindle speed, variation period and variation amplitude. Therefore, SSV parameters should be adequately set to avoid this limitation and to exert its effect throughout the entire duration of cutting. However, there is no clear methodology to determine the optimal condition. This paper presents the characteristics of chatter growth during SSV focusing on the change of chatter frequency, which lead to novel indices to evaluate the chatter stability when cutting with SSV. To verify the validity of the indices, time-domain simulations and the cutting experiments with triangular spindle speed variation (TSSV) are carried out. The influence of SSV parameters on the chatter stability is investigated from the simulation and experimental results. The limitations of widely utilized SSV profiles are discussed.

Prediction of beam losses during crab cavity quenches at the high luminosity LHC

Studies of the crab cavities at KEKB revealed that the RF phase could shift by up to 50o within ~50 us during a quench; while the cavity voltage is still at approximately 75% of its nominal amplitude. If such a failure were to occur on the HL-LHC crab cavities, it is likely that the machine would sustain substantial damage to the beam line and surrounding infrastructure due to uncontrolled beam loss before the machine protection system could dump the beam. We have developed a low-level RF system model, including detuning mechanisms and beam loading, and use this to simulate the behaviour of a crab cavity during a quench, modeling the low-level RF system, detuning mechanisms and beam loading. We supplement this with measurement data of the actual RF response of the proof of principle Double-Quarter Wave Crab Cravity during a quench. Extrapolating these measurements to the HL-LHC, we show that Lorentz Force detuning is the dominant effect leading to phase shifts in the crab cavity during quenches; rather than pressure detuning which is expected to be dominant for the KEKB crab cavities. The total frequency shift for the HL-LHC crab cavities during quenches is expected to be about 460 Hz, leading to a phase shift of no more than 3o. The results of the quench model are read into a particle tracking simulation, SixTrack, and used to determine the effect of quenches on the HL-LHC beam. The quench model has been benchmarked against the KEKB experimental measurements. In this paper we present the results of the simulations on a crab cavity failure for HL-LHC as well as for the SPS and show that beam loss is negligible when using a realistic low-level RF response.

Measuring the De Sitter precession with a new Earth\u2019s satellite to the simeq 10^{-5} level: a proposal

The inclination I of an Earth’s satellite in polar orbit undergoes a secular De Sitter precession of -,7.6 milliarcseconds per year for a suitable choice of the initial value of its non-circulating node Omega . The competing long-periodic harmonic rates of change of I due to the even and odd zonal harmonics of the geopotential vanish for either a circular or polar orbit, while no secular rates occur at all. This may open up, in principle, the possibility of measuring the geodesic precession in the weak-field limit with an accurately tracked satellite by improving the current bound of 9times 10^{-4} from Lunar Laser Ranging, which, on the other hand, may be even rather optimistic, by one order of magnitude, or, perhaps, even better. The most insidious competing effects are due to the solid and ocean components of the K_1 tide since their perturbations have nominal huge amplitudes and the same temporal pattern of the De Sitter signature. They vanish for polar orbits. Departures of simeq {10^{-5}}^circ mathrm{to} {10^{-3}}^circ from the ideal polar geometry allow to keep the K_1 tidal perturbations to a sufficiently small level. Most of the other gravitational and non-gravitational perturbations vanish for the proposed orbital configuration, while the non-vanishing ones either have different temporal signatures with respect to the De Sitter effect or can be modeled with sufficient accuracy. In order to meet the proposed goal, the measurement accuracy of I should be better than simeq 35~text {microarcseconds}=0.034~text {milliarcseconds} over, say, 5 year.

Array Synthesis for Optimal Microwave Power Transmission in the Presence of Excitation Errors

Beam capture efficiency (BCE) is one key factor of the overall efficiency in a microwave power transmission (MPT) system. However, excitation errors of transmitting array are inevitable in practice, which always cause deviation of BCE from the designed one. In this paper, we propose a synthesis method of transmitting array for optimal MPT by using cooperatively coevolving differential evolution (CCDE) algorithm while considering excitation errors including amplitude errors and phase errors. Toward this purpose, a statistical analysis (SA) method is also presented to evaluate the achievable worst BCE in the presence of excitation errors. The tolerance of BCE against the random excitation errors is simulated by computer program, and the positions and nominal excitation amplitudes are both simultaneously optimized to improve the worst BCE. Simulated results show that the worst BCE is improved about 4% from 81% to 85% based on the SA-CCDE synthesis method for the excitation errors $(\sigma _{\delta }, \sigma _{\Phi }) =(0.1,10^{\circ})$ , where $\delta _{n}$ denotes the amplitude error in percent and $\Phi _{n}$ represents the phase error.

Fatigue resistance of branching phase-transformation fronts in pseudoelastic NiTi polycrystalline strips

Inhomogeneous martensitic phase transformation induced by tensile stress can trigger Lüders-like band formation in pseudoelastic NiTi polycrystalline which has influence on its fatigue behaviors. It is often intuitively conjectured that the phase-transformation front (band front) has lower fatigue resistance than the regions inside or outside the band (i.e., the Martensite or Austenite domains), but no experiment in literature can confirm or disprove this conjecture. In this paper, a systematic fatigue study is performed on NiTi strips with nucleated bands under cyclic “elastic” loading, where the nominal strain amplitude is set as a small value (0.25%) to keep the band front immobile. Meanwhile, a close relation between the front morphology and the strip's fatigue failure is revealed with the help of a “global camera” to achieve the full-field strain distribution (by Digital Image Correlation) and a “local camera” to observe the local front morphology. It is found that the band front branches into many small-scale narrow bands in thin strips while the front has no branching and just becomes diffusive in thick strips, and the fatigue life of the thin strip is shorter than the thick strip. Moreover, for the strips cut along the rolling direction, the fatigue failures of the strips with different thicknesses always occur at the phase-transformation front. The reason is attributed to the stress concentration in the front due to the branching front morphology and geometry necking, which depend on the structure geometry (strip thickness here), and the dissipative microscopic interface motion of Austenite-Martensite phase mixtures.

Numerical studies on vortex-induced motions of a multi-column deep-draft oil and gas exploration platform

This paper presents numerical studies on vortex-induced motions (VIM) of a multi-column floating oil and gas exploration platform. Numerical computations are performed using an improved delayed detached eddy simulation (IDDES) together with a moving grid approach. The transverse (sway) and yaw motion responses, motion trajectories, motion frequencies and power spectral density of motions are computed and analyzed systematically. After extensive comparisons with experiments, it is confirmed that the present numerical solutions using IDDES agree well with the experimental results and are better than those via delayed detached eddy simulation (DDES). The differences of transverse motion responses between computational results and experiments are less than 10% in the lock-in region. The numerical simulations reveal that the transverse VIM responses occur in a range of reduced velocities from 7.0 to 14.0 at H/D = 1.44 (H and D are the column height and width, respectively). The largest nominal transverse amplitude, around 35% of the column width, occurs for 22.5∘ current incidence. It is found that the VIM responses mainly perform along the platform diagonals for 15∘, 22.5∘ and 45∘ current incidences. The transverse and yaw motion frequencies for 15∘, 22.5∘ and 45∘ current incidences are higher than those for 0∘ current incidence. The energy levels of the yaw motion responses for 15∘, 22.5∘ and 45∘ current incidences are about 10% of that for 0∘ current incidence. Moreover, parametric studies have been performed to examine the effect of submerged column height on VIM. It demonstrates that VIM in transverse direction grows significantly when submerged column height H/D is greater than 1.0. To be more specific, compared to the case with H/D = 1.44, VIM in transverse direction at H/D = 3.0 increase by around 120%.

Unintentional drifts during quiet stance and voluntary body sway.

We explored unintentional drifts in voluntary whole-body sway tasks following the removal of visual feedback. The main hypothesis was that the unintentional drifts were produced by drifts of referent coordinates for salient performance variables. Young healthy subjects stood quietly on a force platform and also performed voluntary body sway at 0.5Hz both in the anterio-posterior and medio-lateral directions. Visual feedback on the center of pressure (COP) coordinate was provided and then turned off. During quiet stance trials, the subjects matched the initial COP coordinate with a target shifted by 3cm anterior, posterior, left, or right from the coordinate during natural standing and activated the right tibialis anterior to 30% of its maximal voluntary contraction. During cyclical voluntary sway task, the nominal sway amplitude was always 4cm while the midpoint was at one of the four mentioned locations. Removing visual feedback caused COP drifts during quiet stance trials that were consistent across trials performed by a subject but could be in opposite directions across subjects; there was a consistent drop in the activation level of tibialis anterior. During voluntary body sway, removing visual feedback caused a consistent increase in the voluntary sway amplitude and a drift of the midpoint that was consistent within but not across subjects. Motor equivalent and non-motor equivalent inter-cycle motion components were quantified within the space of muscle groups (muscle modes) under visual feedback and at the end of the period without visual feedback. Throughout the trial, there were large motor equivalent motion components, and they increased over the period without visual feedback. The results corroborate the idea that referent coordinate drifts at different levels of the control hierarchy can lead to unintentional drifts in performance. It suggests that directions of COP drifts are defined by two main factors, drift of the body referent coordinate toward the actual coordinate (that can lead to fall) and an opposite drift to ensure body motion to a safer location. Analysis of motor equivalence suggests that postural stability is not compromised during unintentional drifts in performance in contrast to earlier studies of multi-finger tasks. This may be due to the vital importance of postural stability for everyday actions.