Yuki Terada

In situ passivation of InP surface using H2S during metal organic vapor phase epitaxy

An in situ surface passivation of InP(100) using H2S during metal organic vapor phase epitaxy has been characterized by x-ray photoemission spectroscopy and photoluminescence. X-ray photoelectron spectra indicate that the H2S-treated InP at 300 °C is free of P and In oxides even after exposure to air. The enhancement of photoluminescence intensity confirms that H2S passivation of an InP epilayer can reduce the surface defects. It is shown that H2S treatment results in In–S bonds, which dominate the sulfur-passivated InP surface, effectively suppressing interface oxidation during the subsequent ultrathin Al2O3 dielectric film growth.

High-frequency-variation speed control of spindle motor for chatter vibration suppression in NC machine tools

This paper proposes a spindle motor control method for self-excited chatter vibration suppression in numerical control (NC) machine tools. In conventional NC machine tools, spindle speed is set to constant value during machining, and the spindle speed is determined according to analysis or operators experience. The proposed method reduces self-excited chatter vibration by varying spindle speed with high frequency during machining. This method can also suppress the disturbance derived from cutting resistance by repetitive control, and achieve spindle speed variation with high frequency by perfect tracking control. Finally, we show the advantages of the proposed method by simulations and experiments.

Cantilevered carbon nanotube hygrometer

We investigate the effects of humidity on the vibrations of carbon nanotubes (CNTs) using two types of CNT cantilevers: open-ended and close-ended CNT cantilevers. As the humidity increases, the resonant frequency of the open-ended CNT cantilever decreases due to the adsorption of water molecules onto the CNT tip, whereas that of the close-ended CNT cantilever increases probably due to the change in the viscosity of the air surrounding the CNT cantilever, which is negatively correlated with the humidity of air. Our findings suggest that a close-ended CNT cantilever is more suitable for a quick-response and ultrasensitive hygrometer because it continuously reads the viscosity change of moist air in the vicinity of the CNT.

External sensorless adaptive chatter avoidance in NC machining by applying disturbance observer using high resolution linear encoder

The productivity of numerical control (NC) machining is highly limited by the occurrence of chatter vibration. Chatter stability analyses are widely studied to predict chatter vibration. However, these analyses often fail to provide reliable prediction results because the parameters in the vibration model are sensitive to identification errors, mechanical deterioration and thermal changes. On the other hand, to implement robust avoidance of chatter vibration to process changes, additional external sensors have been used, e.g. accelerometers and force sensors. However, this results in increase in cost and decrease in rigidity of the installation site. Meanwhile in recent years, there has been a growing trend toward including a high resolution linear encoder as a standard equipment for a NC machine. In this paper, by applying a disturbance observer using a high resolution linear encoder, an external sensorless adaptive spindle speed selection method is proposed for the robust chatter avoidance. The proposed method is verified by experimental studies on a NC end milling machine.

Adaptive vibration suppression perfect tracking control for linear time-varying systems with application to ball-screw feed drives

In high-precision positioning of feed drive systems, mechanical resonances are one of the main limiting factor for fast and precise motion. Vibration suppression tracking controllers are employed to improve positioning performance, although precise plant models are generally required. Hence, in systems with time-varying modes, the increasing modeling errors often result in the loss of the needed performance. This paper presents an adaptive tracking controller for vibration suppression of linear systems with time-varying dynamics. The technique is based on multirate feedforward, alleviating the unstable-zeros problem of discrete time plant models, for which the nominal plant model is adjusted using recursive estimation. A novel estimation scheme is presented to estimate the modal parameters of the plant based on the relative motion between the mechanical components. The proposed control scheme is applied to ball-screw feed drives with varying dynamics during the cutting process and the advantages demonstrated through simulations and experiments.

Joint torque control for backlash compensation in two-inertia system

In controlling ball-screw driven stages of machine tools, industrial robots, and welfare robots, they are modelled as two-inertia systems to consider their transmission characteristics such as low rigidity and nonlinearity. To obtain precise position at the load side, the number of devices with load-side encoders is increasing. The precise joint torque control method and the load-side torque control method for a two-inertia system with backlash are proposed. The proposed methods utilize load-side encoder information effectively. Simulation and experimental results demonstrate the advantages of proposed methods quantitatively.

Experimental verification of chatter suppression in end milling process using cooperative control of spindle and stage motors

Chatter vibration is well known as an undesirable phenomenon, so various methods for chatter suppression have been proposed. Spindle speed variation as a triangular trajectory in the cutting process is one of the methods for chatter suppression. However, this method cannot suppress chatter vibration completely. Moreover, the triangular spindle speed variation requires large torque and a good response of the spindle motor. To solve these problems, this paper proposes a cooperative control of spindle and stage motors. The proposed methods are evaluated by simulations and experiments.

Metalorganic Vapor Phase Epitaxy of GaAs with AlP Surface Passivation Layer for Improved Metal Oxide Semiconductor Characteristics

Growth of an AlP epitaxial layer on the top of GaAs was proposed as a novel in situ passivation method. The AlP layer was almost converted to AlOx upon air exposure, forming a part of a gate dielectric. Removal of AlAs at the GaAs/AlP interface was mandatory for avoiding As-oxide formation upon air exposure, which necessitated complete As removal from the surface when switching growth from GaAs to AlP. H2S treatment allowed us to obtain low-As-content c(8×2) surface reconstruction for the first time with metalorganic vapor phase epitaxy (MOVPE). AlP on the c(8×2) GaAs surface has made it possible to obtain a smooth morphology and complete suppression of arsenic oxide. PL intensity increased by a factor of three with the AlP growth for 10 s at 500 °C, corresponding to a thickness of 0.5 nm. Accumulation capacitance obtained from capacitance–voltage (C–V) curves was the largest with that growth a condition of AlP, suggesting reduction in interface states.

Metalorganic Vapor Phase Epitaxy of GaAs with AlP Surface Passivation Layer for Improved MOS Characteristics

Univ. of Tokyo, Dept. of Electrical Engineering and Information systems, School of Engineering 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan Phone: +81-3-5841-6753 E-mail: yuki@hotaka.t.u-tokyo.ac.jp Univ. of Tokyo, Dept. of Materials Eng., School of Engineering 3 Univ. of Tokyo, Research Center for Advanced Science and Technology Univ. of Tokyo, Institute of Engineering Innovation, School of Engineering