Nonlinear Spatial Filtering of Multichannel Surface Electromyogram Signals During Low Force Contractions

This study introduces the application of nonlinear spatial filters to help identify single motor unit discharge from multiple channel surface electromyogram (EMG) signals during low force contractions. The nonlinear spatial filters simultaneously take into account the instantaneous amplitude and frequency information of a signal. This property was used to enhance motor unit action potentials (MUAPs) in the surface EMG record. The advantages of nonlinear spatial filtering for surface MUAP enhancement were investigated using both simulation and experimental approaches. The simulation results indicate that when compared with various linear spatial filters, nonlinear spatial filtering achieved higher SNR and higher kurtosis of the surface EMG distribution. Over a broad range of SNR and kurtosis levels for the input signal, nonlinear spatial filters achieved at least 32 times greater SNR and 11% higher kurtosis for correlated noise, and at least 15 times greater SNR and 1.7 times higher kurtosis for independent noise, across electrode array channels. The improvements offered by nonlinear spatial filters were further documented by applying them to experimental surface EMG array recordings. Compared with linear spatial filters, nonlinear spatial filters achieved at least nine times greater SNR and 25% higher kurtosis. It follows that nonlinear spatial filters represent a potentially useful supplement to linear spatial filters for detection of motor unit activity in surface EMG at low force contractions.     

Subisokinetic sampling characteristics of high speed aircraft inlets: A new CFD-based correlation considering inlet geometries

Large spatial-scale atmospheric aerosol characterization is commonly accomplished from high-speed aircraft platforms using subisokinetic sampling. The aspiration efficiencies in these inlets are commonly determined from the empirical correlation of Belyaev and Levin [1974. Techniques for collection of representative aerosol samples. Journal of Aerosol Science, 5, 325–338] (B&L), though the correlation was obtained from experiments under conditions very different from those encountered during high-speed aircraft sampling. Aircraft inlets are generally thick-walled, often have a blunt-body downstream, operate at a large freestream to sampling velocity ratio, and under conditions of low ambient pressures. The dependence of sampling characteristics on these parameters is studied using computational fluid dynamics (CFD) modeling. The CFD calculations show that the anisokinetic inlet sampling characteristics are strongly influenced by the operating conditions and sampler geometry, especially the presence of an aft blunt body. It is observed that these dependencies are not accurately captured by the existing correlations. The presence of even a moderately sized blunt-body aft of a sampling inlet can greatly influence the sampling characteristics of a straight tube inlet. A new correlation is derived based on the CFD simulation results, and this correlation is seen to greatly improve prediction of inlet sampling characteristics considering their geometries.     

Adaptive output feedback actuator failure compensation for a class of non‐linear systems

An adaptive compensation control scheme using output feedback is designed and analysed for a class of non‐linear systems with state‐dependent non‐linearities in the presence of unknown actuator failures. For a linearly parameterized model of actuator failures with unknown failure values, time instants and pattern, a robust backstepping‐based adaptive non‐linear controller is employed to handle the system failure, parameter and dynamics uncertainties. Robust adaptive parameter update laws are derived to ensure closed‐loop signal boundedness and small tracking errors, in general, and asymptotic regulation, in particular. An application to controlling the angle of attack of a non‐linear hypersonic aircraft dynamic model in the presence of elevator segment failures is studied and simulation results show that the developed adaptive control scheme has desired actuator failure compensation performance. Copyright © 2004 John Wiley & Sons, Ltd.     

Stress relaxation and the structure size-dependence of plastic deformation in nanotwinned copper

Stress-relaxation experiments were performed on nanotwinned Cu to characterize the twin size-dependence of the activation volume and mobile dislocation density. We find that the variation of activation volume as a function of twin lamellae thickness can be captured well by a Hall–Petch-type relation. This structure size-dependence is interpreted to arise from a transition of the rate-controlling mechanism from intra-twin to twin boundary-mediated processes with decreasing twin thickness. Furthermore, we find that the exhaustion rate of mobile dislocations reduces with decreasing twin thickness. Such a twin size-dependence is attributed to the increased strain-hardening rate associated with a high density of coherent twin boundaries. Our results demonstrate that twin boundary-mediated dislocation processes can effectively promote the strain hardening and preserve mobile dislocations, leading to ultrahigh strength while retaining ductility in nanotwinned Cu.