Feedback control of oxygen uptake during robotics-assisted end-effector-based stair climbing

A heart rate (HR) feedback control system for end-effector gait rehabilitation robots was previously developed and successfully tested, but oxygen uptake ( $\dot {{\rm V}}{\rm O}_2$ ) is thought to better characterize physiological exercise intensity. The aim of the present study was to identify and compare $\dot {{\rm V}}{\rm O}_2$ and HR dynamics, and to develop and test a $\dot {{\rm V}}{\rm O}_2$ controller for an end-effector robot operated in stair climbing mode. Six able-bodied subjects were recruited for controller testing. Command response, disturbance rejection and robustness were assessed by means of three quantitative outcome measures: root-mean-square (RMS) error of $\dot {{\rm V}}{\rm O}_2$ ( ${\rm RMSE}_{\dot{\rm V}O_2}$ ), average control signal power ( $P_{\Delta P}$ ) and RMS error of volitionally controlled power ( ${\rm RMSE}_P$ ). The nominal first-order linear model for $\dot {{\rm V}}{\rm O}_2$ had time constant $\tau =52.4$  s and steady-state gain k=0.0174 (l/min)/W. The mean time constant $\tau = 67.3$  s for HR was significantly higher than for $\dot {{\rm V}}{\rm O}_2$ , where $\tau = 53.4$ (p=0.048). Command responses for a target $\dot {{\rm V}}{\rm O}_2$ profile gave consistent and accurate tracking with ${\rm RMSE}_{\dot{\rm V}O_2} = 0.198 \pm 0.070$  l/min, $P_{\Delta P} = 2.15 \pm 0.70$  W2 and ${\rm RMSE}_P = 39.2 \pm 15.4$  W ( ${\rm mean} \pm {\rm SD}$ ). Disturbance rejection performance was also found to be satisfactory. The results of the controller tests confirm the feasibility of the proposed $\dot {{\rm V}}{\rm O}_2$ feedback control strategy. Robustness was verified as the single LTI controller was specific to only one of the subjects and no difference in outcome values was apparent across all subjects. Subject-specific variability in breath-by-breath respiratory noise is the main challenge in feedback control of $\dot {{\rm V}}{\rm O}_2$

Atomic force microscopy phase imaging of epitaxial graphene films

International audienceDynamic mode atomic force microscopy phase imaging is known to produce distinct contrast between graphene areas of different atomic thickness. But the intrinsic complexity of the processes controlling the tip motion and the phase angle shift excludes its use as an independent technique for a quantitative type of analysis. By investigating the relationship between the phase shift, the tip-surface interaction, and the thickness of the epitaxial graphene areas grown on silicon carbide, we shed light on the origin of such phase contrast, and on the complex energy dissipation processes underlying phase imaging. In particular, we study the behavior of phase shift and energy dissipation when imaging the interfacial buffer layer, single-layer, and bilayer graphene regions as a function of the tip-surface separation and the interaction forces. Finally, we compare these results with those obtained on differently-grown quasi free standing single- and bilayer graphene samples

Giant Increase of Hardness in Silicon Carbide by Metastable Single Layer Diamond-Like Coating

: Silicon carbide (SiC) is one of the hardest known materials. Its exceptional mechanical properties combined with its high thermal conductivity make it a very attractive material for a variety of technological applications. Recently, it is discovered that two-layer epitaxial graphene films on SiC can undergo a pressure activated phase transition into a sp3 diamene structure at room temperature. Here, it is shown that epitaxial graphene films grown on SiC can increase the hardness of SiC up to 100% at low loads (up to 900 µN), and up to 30% at high loads (10 mN). By using a Berkovich diamond indenter and nanoindentation experiments, it is demonstrated that the 30% increase in hardness is present even for indentations depths of 175 nm, almost three hundred times larger than the graphene film thickness. The experiments also show that the yield point of SiC increases up to 77% when the SiC surface is coated with epitaxial graphene. These improved mechanical properties are explained with the formation of diamene under the indenter's pressure

Fabricating Nanoscale Chemical Gradients with ThermoChemical NanoLithography

Production of chemical concentration gradients on the submicrometer scale remains a formidable challenge, despite the broad range of potential applications and their ubiquity throughout nature. We present a strategy to quantitatively prescribe spatial variations in functional group concentration using ThermoChemical NanoLithography (TCNL). The approach uses a heated cantilever to drive a localized nanoscale chemical reaction at an interface, where a reactant is transformed into a product. We show using friction force microscopy that localized gradients in the product concentration have a spatial resolution of ∼20 nm where the entire concentration profile is confined to sub-180 nm. To gain quantitative control over the concentration, we introduce a chemical kinetics model of the thermally driven nanoreaction that shows excellent agreement with experiments. The comparison provides a calibration of the nonlinear dependence of product concentration versus temperature, which we use to design two-dimensional temperature maps encoding the prescription for linear and nonlinear gradients. The resultant chemical nanopatterns show high fidelity to the user-defined patterns, including the ability to realize complex chemical patterns with arbitrary variations in peak concentration with a spatial resolution of 180 nm or better. While this work focuses on producing chemical gradients of amine groups, other functionalities are a straightforward modification. We envision that using the basic scheme introduced here, quantitative TCNL will be capable of patterning gradients of other exploitable physical or chemical properties such as fluorescence in conjugated polymers and conductivity in graphene. The access to submicrometer chemical concentration and gradient patterning provides a new dimension of control for nanolithography