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Observational evidence of two extratropical pathways to forcing tropical convective disturbances is documented through a statistical analysis of satellite-derived OLR and ERA5 reanalysis. The forcing mechanism and the resulting disturbances are found to strongly depend on the structure of the background zonal wind. Although Rossby wave propagation is prohibited in easterlies, modeling studies have shown that extratropical forcing can still excite equatorial waves through resonance between the tropics and extratropics. Here this “remote” forcing pathway is investigated for the first time in the context of convectively coupled Kelvin waves over the tropical Pacific during northern summer. The extratropical forcing is manifested by eddy momentum flux convergence that arises when extratropical eddies propagate into the subtropics and encounter their critical line. This nonlinear forcing has similar wavenumbers and frequencies with Kelvin waves and excites them by projecting onto their meridional eigenstructure in zonal wind, as a form of resonance. This resonance is also evidenced by a momentum budget analysis, which reveals the nonlinear forcing term is essential for maintenance of the waves, while the remaining linear terms are essential for propagation. In contrast, the “local” pathway of extratropical forcing entails the presence of a westerly duct during northern winter that permits Rossby waves to propagate into the equatorial east Pacific, while precluding any sort of resonance with Kelvin waves due to Doppler shifting effects. The intruding disturbances primarily excite tropical “cloud plumes” through quasigeostrophic forcing, while maintaining their extratropical nature. This study demonstrates the multiple roles of the extratropics in forcing in tropical circulations and illuminates how tropical–extratropical interactions and extratropical basic states can provide be a source of predictability at the S2S time scale. Significance Statement This study seeks to understand how circulations in the midlatitudes excite the weather systems in the tropics. Results show that the mechanisms, as well as the types of tropical weather systems excited, are strongly dependent on the mean large-scale wind structure. In particular, when the large-scale wind blows from east to west, a special type of eastward-moving tropical weather system, the Kelvin wave, is excited owing to its resonance with remote eastward-moving weather systems in the extratropics. On the contrary, when the average wind blows from west to east, midlatitude systems are observed to intrude into the lower latitudes and directly force tropical convection, the cloud plumes, while maintaining their extratropical nature. These results speak to how the midlatitudes can excite distinct types of tropical weather systems under different climatological wind regimes. Understanding these tropical weather systems and their interactions with the midlatitudes may ultimately help to improve predictions of weather beyond 2 weeks.

The dominant structural variability of African easterly waves (AEWs) is explored using an empirical orthogonal function (EOF) approach. The structure of AEWs is obtained by projecting the wind fields from reanalysis data and satellite-derived brightness temperature Tb onto the principal components associated with EOF patterns of filtered Tb (Tb EOF) and 700-hPa meridional wind (v700 EOF). The wave structure depicted by the Tb EOF has confined convection and circulation mostly south of the African easterly jet. It shares many characteristics with AEWs analyzed and discussed in the literature. In contrast, the v700 EOF exhibits less familiar characteristics and includes interactions with the equatorial and subtropical regions. The convective patterns are characterized by a “checkerboard” pattern of convection that has not been emphasized before. The most striking feature is the broad meridional extent, which depicts interactions with a mixed Rossby–gravity wave (MRG) in the equatorial region and interactions with the basic-state potential vorticity in the subtropics. The southern portion of the wave has a modified MRG structure, and this AEW–MRG hybrid cannot be separated using the EOF technique, indicating the prevalence of such structures. The subtropical interaction at mid- to lower levels establishes a vortex off the coast of Morocco that results in dry-air advection into the tropics in tandem with the northern vortex. At upper levels, a subtropical wave train is induced by the AEW-associated convective inflow and outflow. The contrasting AEW circulations are associated with differences in the precipitation rates and patterns over Africa. These results highlight the variability of AEW structures and their interactions with equatorial and subtropical waves.

Image matching is one of the key technologies in the image processing. In order to increase its efficiency and precision, a new method for image matching which based on the improved SURF and Delaunay-TIN is proposed in this paper. Based on the original SURF algorithm, three constraint conditions, color invariant model, Delaunay-TIN, triangle similarity function and photography invariant are added into the original SURF model. With the proposed algorithm, the image color information is effectively retained and the erroneous matching rate of features is largely reduced. The experimental results shows that this proposed method has the characteristics of higher matching speed, uniform distribution of feature points to be matched, and higher correct matching rate than the original algorithm does.

This study proposes a 3-axis AC servo motor parallel platform with intersecting rails. The 3-actuators are designed to be horizontally on the base platform with their extensions intersect in the base platform center. The forward kinematics, inverse kinematics and work space of this mechanism are established and the singularity distribution is investigated. Optimal maneuvering regions are recommended from the viewpoints of singularity and stiffness.

Parallel mechanisms could be hardly used in contour tracking because of their mechanism features. This study proposed a link-space real time contour tracking for a 3 DOF (Z、α and β) hydraulic parallel mechanism. The essence of this approach is to convert control points of command trajectory to link space by inverse kinematics. A real-time interpolator was created and the multi-axis cross-coupled pre-compensation control (MCCPM) was constructed for link-space contour tracking. It was shown that a contour-accurate trajectory tracking could be performed which was impossible in the original Z-α-β space. Other advantages of this link-space approach were time efficiency and the uniform tracking velocity.

Machine tools, robots and parallel manipulators are useful platforms in manufacturing. For parallel manipulators, developing contour tracking ability and clarifying the role of force computation are of significance. This work presents a novel contour tracking control with force computation for hydraulic parallel manipulators. Different trajectories are used to evaluate the tracking errors on an empirical hydraulic parallel manipulator. The proposed control law and tracking systems are effective and the force computation is proven highly effective in the frame of the contour tracking system.

This work investigates the spatial contour tracking on a new kind of motion platform that is constructed by extending the parallel platform with a proven planar motion table. Multi-axis cross-coupled tracking control schemes were developed on the basis of this hybrid structured Stewart platform, which formed a 5-axis working machine. A framework for the functions of this system was established. Roadmap for further development that leads to a highly versatile and efficient precision motion platform was made.

An improved dynamic cutting force model for end-milling operation is developed based on a developed algorithm for calculation of instantaneous shear plane area and taking into account into the cutter geometry, influence of vibration, cutting speed variation, and friction between cutter and chips. To determine the instantaneous cutting force, the cutting edges of end-milling cutter are discretized into limited number of elements, and the forces exerted by each element regarded as oblique cutting are calculated. By summing up the cutting forces generated by elemental edges, the instantaneous cutting forces of end milling can be determined. A theoretical–empirical method to determine the parameters needed for cutting force calculation was proposed. Cutting experiments on aluminum were performed to compare with the proposed model.

The objective of this paper is to propose two different modified sinusoidal pulse width modulation (SPWM) control schemes, MPWM-S and MPWM-T, for the dc/ac inverter with a non-constant dc voltage source. The amplitude distortion of the pulse width modulated waveforms is caused by the input dc voltage source variation and has the most significant impact on the dc/ac inverter’s ac output voltage spectral errors. The proposed MPWM-S and MPWM-T can achieve the desired fundamental amplitude and eliminate the low order harmonic components caused by the dc voltage source variation. The mathematical analysis of the amplitude distortion and the systematic development of the MPWM-S and MPWM-T are presented in this paper. The proposed MPWM-S and MPWM-T are feed-forward compensation methodologies and can be integrated with any practical feedback control schemes. Hardware implementations of the proposed control schemes can be easily accomplished by adding one or two analog multipliers to the traditional SPWM control c...

Contour accuracy is obtained by efforts in algorithms. Most motion control algorithms reduce either position error or contour error to gain accuracy. In this paper, we investigate the idea of constructing a Fuzzy-logic controller to a proven algorithm, the cross-coupled precompensation method (CCPM), and using both position and contour error to generate compensation term. Simulation comparison with known methods and experimental comparisons among the uncoupled system, the cross-coupled system, the original CCPM and the proposed method were given. It was shown that the performance of the original CCPM was further enhanced. The proposed method also showed a better performance when subjected to external load.

CNC machining has been studied from the perspective of either cutting or feeding. However, machining quality is the outcome of both of these processes. This work investigates the contour errors of a complete CNC machine system. A system model is developed to cover all groups of functions, including trajectory planning, trajectory tracking, cutting process and machine structure. Analysis results reveal the limitations of traditional studies. The dependence of contour errors on trajectory curvature, feed-rate, cutting depth and tracking control is investigated as well. A new model of CNC machining is developed.

To evaluate the effect of placement of short iliac screw using Galveston technique in lumbosacral fusion.From October 2003 to August 2007, 18 consecutive patients (mean age 46 years ranging from 25 to 62 years) received placement of short iliac screw in lumbosacral fusion. The patients were followed up for a mean of 18 months (12-23 months), and the effect of lumbosacral fusion was evaluated according to standing anterior-posterior and lateral plain films taken before and after the operation and at the follow-up and also on the basis of symptom relief.The mean time of surgery was 210 min (180-290 min). No complications occurred during and after the operation. According to the evaluation criteria of surgical treatment of low back pain formulated by the spine group of Chinese Orthopedic Association, excellent clinical outcome was achieved in 12 cases, good outcome in 3 cases, and tolerable outcome in 2 cases, with the excellent and good outcome rate of 83%.The Galveston technique for short iliac screw placement can obtain satisfactory outcome in the lumbosacral fusion.

An AC-DC-AC converter using the valley-fill filter for power factor correction is proposed in this paper. An AC-DC bridge rectifier with high power factor can be easily achieved by using the valley-fill filter. The operation modes of the valley-fill filter are introduced. It has the advantages of high power factor and simplicity compared with other passive filters and active filters, respectively. The DC ripple voltage is the drawback of the valley-fill filter. However, the DC-AC converter can tolerate the DC ripple voltage by using the direct pulse width modulation control scheme. Computer simulation results of the proposed AC-DC-AC converter are presented in this paper.

Two different modified SPWM control schemes, MPWM-S and MPWM-T, for three-phase inverters with nonconstant DC voltage sources are proposed in this paper. The proposed modified SPWM control schemes can achieve the desired fundamental amplitude and eliminate the low order harmonic components caused by the DC voltage source variation. The analysis of the inverter amplitude distortion and the developments of the proposed MPWM-S and MPWM-T control schemes are presented. Hardware implementation of the proposed control schemes can be easily accomplished by adding one or two analog components to the traditional SPWM control circuit. The performance of the two proposed modified SPWM control schemes is verified by experimental results.

The proposed analog-based modified PWM control scheme for the DC-AC inverter can eliminate the low order harmonic components caused by the nonconstant DC voltage source. A simple high-power factor full-bridge rectifier with the valley-fill filter is selected to realize the nonconstant DC voltage source. The development of a modified triangular signal for the PWM control is introduced in this paper. The performance of the proposed modified PWM control scheme is verified by using computer simulation and experimental results.

The proposed analog-based modified PWM control scheme for the DC-AC inverter can eliminate the low order harmonic components caused by the nonconstant DC voltage source. A simple high power factor full-bridge rectifier with the valley-fill filter is selected to realize the nonconstant DC voltage source. Operation modes of the valley fill filter are included in the paper. The performance of the proposed modified PWM control scheme is verified by computer simulation and experimental results.