237 research outputs found
Dynamic Control of Local Field Emission Current from Carbon Nanowalls
We report on a systematic study of modulation of the field emission current
from carbon nanowalls using a sharp probe as the anode in an ultrahigh vacuum
system. Modulation of the local emission current was achieved by either varying
the anode-cathode distance (d) with the aid of an AC magnetic field or
superimposing a small AC bias on a DC bias during the field emission
measurement. Current modulation ratio of over two orders of magnitude was
achieved with the modulation becoming more efficient at a smaller d. The
experimental results are discussed using the Fowler-Nordheim theory in
combination with a simple cantilever model to account for the modulation
effect. The experimental results demonstrated good static stability and dynamic
controllability of local field emission current from the carbon nanowalls
Command shaping with constrained peak input acceleration to minimize residual vibration in a flexible-joint robot
Rapid point-to-point motion is limited when flexibilities exist in the system. In order to minimize the vibrations related to joint flexibilities, much work has been done, including modifying the system so that vibrations can be damped out more quickly, calculating the inverse dynamics of the system and constructing shaped input profiles that avoid system natural frequencies. In this work, the earlier fixed-time command shaping method has been extended to a peak-acceleration-constrained approach with two basis functions, the ramped sinusoid function and the versine function, such that the maximum acceleration is guaranteed without overconstraining the input profiles. The approach is developed and then validated with a two-link flexible-joint robotic arm. The effect of peak input acceleration and weighting factor on residual vibrations has been studied. A performance metric has been developed to assess residual vibrations. Input profiles with two basis functions are compared with each other, as well as the results of a bang-bang profile. All simulations and experiments have shown the effectiveness of the command shaping method with constrained peak input acceleration on residual vibration reduction. In addition, the ability to weigh the trade-off between actuation time and settling time warrants the optimization of total move time. Lastly, there exists an optimal weighting factor for each peak input acceleration to minimize the total move time and the total move time decreases with higher peak input acceleration
Self-current induced spin-orbit torque in FeMn/Pt multilayers
Extensive efforts have been devoted to the study of spin-orbit torque in
ferromagnetic metal/heavy metal bilayers and exploitation of it for
magnetization switching using an in-plane current. As the spin-orbit torque is
inversely proportional to the thickness of the ferromagnetic layer, sizable
effect has only been realized in bilayers with an ultrathin ferromagnetic
layer. Here we demonstrate that, by stacking ultrathin Pt and FeMn alternately,
both ferromagnetic properties and current induced spin-orbit torque can be
achieved in FeMn/Pt multilayers without any constraint on its total thickness.
The critical behavior of these multilayers follows closely three-dimensional
Heisenberg model with a finite Curie temperature distribution. The spin torque
effective field is about 4 times larger than that of NiFe/Pt bilayer with a
same equivalent NiFe thickness. The self-current generated spin torque is able
to switch the magnetization reversibly without the need for an external field
or a thick heavy metal layer. The removal of both thickness constraint and
necessity of using an adjacent heavy metal layer opens new possibilities for
exploiting spin-orbit torque for practical applications.Comment: 28 pages, 5 figure
Anomalous Hall magnetoresistance in a ferromagnet
The anomalous Hall effect, observed in conducting ferromagnets with broken
time-reversal symmetry, offers the possibility to couple spin and orbital
degrees of freedom of electrons in ferromagnets. In addition to charge, the
anomalous Hall effect also leads to spin accumulation at the surfaces
perpendicular to both the current and magnetization direction. Here we
experimentally demonstrate that the spin accumulation, subsequent spin
backflow, and spin-charge conversion can give rise to a different type of spin
current related magnetoresistance, dubbed here as the anomalous Hall
magnetoresistance, which has the same angular dependence as the recently
discovered spin Hall magnetoresistance. The anomalous Hall magnetoresistance is
observed in four types of samples: co-sputtered (Fe1-xMnx)0.6Pt0.4, Fe1-xMnx
and Pt multilayer, Fe1-xMnx with x = 0.17 to 0.65 and Fe, and analyzed using
the drift-diffusion model. Our results provide an alternative route to study
charge-spin conversion in ferromagnets and to exploit it for potential
spintronic applications
Motion Control of Robotic Arm with Command Shaping Method
In the manufacturing industry, lots of rapid point-to-point motion is required while the residual vibration is unfavorable. Residual vibrations caused by flexible elements are limiting the performance of mechanical system, especially when the system needs to make rapid point-to-point motion. As proved in earlier studies, avoiding natural frequencies of the mechanical system reduces the residual vibrations. This work is based on a non-linear, two-link flexible jointed robot with configuration dependent resonance. Command shaping method consisting different combinations of base functions and weighting factors are compared in this work. The compatibility of command shaping with classical feedback control structure allows a computationally effective method for real time implementation. By measuring the acceleration after the input stops, the residual vibration is analyzed. The best case would be the one with the least peak-to-peak residual acceleration and reasonable peak acceleration during the motion. Experimental results show that the residual vibrations can be reduced considerably after the implementation of command shaping method. As verified in this work, command shaping method is a practical way to control motion with flexible elements without exciting the system’s natural frequencies and demanding significantly more computation capability
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