Evaluation of a novel UHMWPE bearing for applications in precision slideways

This paper presents a novel slideway bearing design comprised of a thin-film (0.1 mm-0.2 mm) of ultra-high molecular weight polyethylene (UHMWPE) bound to a rigid hemispherical substrate. Two prototype bearing designs were fabricated and tested to characterize the coefficient of friction (dynamic and static) and wear of the polymer. In addition, similar bearings were incorporated into a kinematically constrained rectilinear carriage to determine the repeatability of motion during multiple traverses. The first bearing had a radius of curvature on the order of 2.38 mm incorporating an UHMWPE film thickness between 0.1 mm and 0.2 mm. The friction coefficient was measured to be between 0.155 and 0.189 for normal loads of 11.5 N and 2.2 N, respectively at a surface speed of 4.2 mm {center_dot} s{sup -1}. This bearing failed after a traverse of approximately 700 m at a load of 11.5 N. A similar evaluation procedure was carried out on a bearing of radius 6.35 mm resulting in a friction coefficient between 0.125 and 0.185 at loads of 27.8 N and 2.2 N, respectively, and the bearing endured a traverse of over 2.2 km at a load of approximately 28 N (in both air and vacuum conditions) with a surface speed of 4.2 mm {center_dot} s{sup -1}. The second bearing prototype was further subjected to a repeatability test. In this setup, a carriage incorporating five bearings was traversed in a nominally linear path while vertical deviations for multiple traverses were measured by a custom built displacement sensor. Deviations from a linear path were observed to repeat to within a few nanometers about nominal variations of less than 10 nm for a traverse distance of 10 mm. This system and other subsystems used to characterize the friction coefficient and noise of the polymer bearing are presented

Early testing of a coarse/fine precision motion control system

This abstract presents a brief overview of key components of a motion control stage for accurate nanometer level positioning for scanning specimens over an area measuring 50 mm x 50 mm. The completed system will utilize a short-range, third generation 6 degree-of-freedom fine motion control platform (4 microns, 160 micro-radians) carried by a long-range, two-axis x-y positioning system (50 mm x 50 mm). Motion of the controlled platform relative to a measurement frame will be measured using a heterodyne laser interferometer and capacitance sensing. The final stage will be mounted onto an isolation table in a vacuum chamber, itself on isolation supports mounted to a granite slab on bed rock and isolated from the main floor of the building. This whole system is housed in a temperature-controlled laboratory. It is envisaged that the current system will provide the ability to ''pick and place'' at nanometer levels and be used for long range scanning of specimens (including biological specimens), micro- /macroassembly, lithography and as a coordinate measuring machine (CMM). Furthermore, the system performance will be compared with other comparable systems at international locations such as, National Physical Laboratory (NPL) in the UK, Technical University of Eindhoven (TUE) in the Netherlands, Physikalisch-Technische Bundesanstalt (PTB) in Germany, and our own sub-atomic measuring machine (SAMM) [1, 2] at UNC-Charlotte. Critical requirements of the system are as follows: (1) Vacuum compatible to better than 20 mPa; (2) Range of 50 mm x 50 mm x 4 microns; (3) Maximum translation velocity of 5 mm {center_dot} s{sup -1}; (4) Sub-nanometer resolution; and (5) System accuracy of better than 10 nm

Controller strategy for a 6 DOF piezoelectric translation stage

A controller for the third generation, 6 degree-of-freedom (DOF) piezoelectric translation stage shown in Figure 1 is presented. This was tested by monitoring all six coordinate motions using an orthogonal array of six, high-resolution capacitance gages. The full 6 DOF matrix transformations and controller block diagrams for this system have been measured and the system operated under closed loop control. Results of early experiments to determine the 21 open loop response functions as well as preliminary results showing the closed loop response for the 3 linear translations are presented in this abstract. The ultimate goal of this project is to incorporate this 6 DOF stage within a long range X-Y scanning system for nanometer pick-and-place capability over an area of 50 x 50 mm. The control strategy and early results from this system will be presented

A comparison of drive mechanisms for precision motion controlled stages

This abstract presents a comparison of two drive mechanisms, a Rohlix{reg_sign} drive and a polymer nut drive, for precision motion controlled stages. A single-axis long-range stage with a 50 mm traverse combined with a short-range stage with a 16 {micro}m traverse at a operational bandwidth of 2.2 kHz were developed to evaluate the performance of the drives. The polymer nut and Rohlix{reg_sign} drives showed 4 nm RMS and 7 nm RMS positioning capabilities respectively, with traverses of 5 mm at a maximum velocity of 0.15 mm{sup -}s{sup -1} with the short range stage operating at a 2.2 kHz bandwidth. Further results will be presented in the subsequent sections

Avalanches in a Stochastic Model of Spiking Neurons

Neuronal avalanches are a form of spontaneous activity widely observed in cortical slices and other types of nervous tissue, both in vivo and in vitro. They are characterized by irregular, isolated population bursts when many neurons fire together, where the number of spikes per burst obeys a power law distribution. We simulate, using the Gillespie algorithm, a model of neuronal avalanches based on stochastic single neurons. The network consists of excitatory and inhibitory neurons, first with all-to-all connectivity and later with random sparse connectivity. Analyzing our model using the system size expansion, we show that the model obeys the standard Wilson-Cowan equations for large network sizes ( neurons). When excitation and inhibition are closely balanced, networks of thousands of neurons exhibit irregular synchronous activity, including the characteristic power law distribution of avalanche size. We show that these avalanches are due to the balanced network having weakly stable functionally feedforward dynamics, which amplifies some small fluctuations into the large population bursts. Balanced networks are thought to underlie a variety of observed network behaviours and have useful computational properties, such as responding quickly to changes in input. Thus, the appearance of avalanches in such functionally feedforward networks indicates that avalanches may be a simple consequence of a widely present network structure, when neuron dynamics are noisy. An important implication is that a network need not be “critical” for the production of avalanches, so experimentally observed power laws in burst size may be a signature of noisy functionally feedforward structure rather than of, for example, self-organized criticality

Emergent Oscillations in Networks of Stochastic Spiking Neurons

Networks of neurons produce diverse patterns of oscillations, arising from the network's global properties, the propensity of individual neurons to oscillate, or a mixture of the two. Here we describe noisy limit cycles and quasi-cycles, two related mechanisms underlying emergent oscillations in neuronal networks whose individual components, stochastic spiking neurons, do not themselves oscillate. Both mechanisms are shown to produce gamma band oscillations at the population level while individual neurons fire at a rate much lower than the population frequency. Spike trains in a network undergoing noisy limit cycles display a preferred period which is not found in the case of quasi-cycles, due to the even faster decay of phase information in quasi-cycles. These oscillations persist in sparsely connected networks, and variation of the network's connectivity results in variation of the oscillation frequency. A network of such neurons behaves as a stochastic perturbation of the deterministic Wilson-Cowan equations, and the network undergoes noisy limit cycles or quasi-cycles depending on whether these have limit cycles or a weakly stable focus. These mechanisms provide a new perspective on the emergence of rhythmic firing in neural networks, showing the coexistence of population-level oscillations with very irregular individual spike trains in a simple and general framework

Top Quark Physics

We review the prospects for studies of the top quark at the LHC.We review the prospects for studies of the top quark at the LHC. Members of the working group who have contributed to this document are: A.Ahmadov, G.Azuelos, U.Baur, A.Belyaev, E.L.Berger, W.Bernreuther, E.E.Boos, M.Bosman, A.Brandenburg, R.Brock, M.Buice, N.Cartiglia, F.Cerutti, A.Cheplakov, L.Chikovani, M.Cobal-Grassmann, G.Corcella, F.del Aguila, T.Djobava, J.Dodd, V.Drollinger, A.Dubak, S.Frixione, D.Froidevaux, B.Gonzalez Pineiro, Y.P.Gouz, D.Green, P.Grenier, S.Heinemeyer, W.Hollik, V.Ilyin, C.Kao, A.Kharchilava, R. Kinnunen, V.V.Kukhtin, S.Kunori, L.La Rotonda, A.Lagatta, M.Lefebvre, K.Maeshima, G.Mahlon, S.Mc Grath, G.Medin, R.Mehdiyev, B.Mele, Z.Metreveli, D.O'Neil, L.H.Orr, D.Pallin, S.Parke, J.Parsons, D.Popovic, L.Reina, E.Richter-Was, T.G.Rizzo, D.Salihagic, M.Sapinski, M.H.Seymour, V.Simak, L.Simic, G.Skoro, S.R.Slabospitsky, J.Smolik, L.Sonnenschein, T.Stelzer, N.Stepanov, Z.Sullivan, T.Tait, I.Vichou, R.Vidal, D.Wackeroth, G.Weiglein, S.Willenbrock, W.W