W. K. Zhu

Reflection high-energy electron diffraction intensity oscillations during growth of GaN(0001)A by plasma-assisted molecular beam epitaxy

Abstract Reflection high-energy electron diffraction (RHEED) intensity oscillations have been observed during radio-frequency plasma-assisted molecular beam epitaxy of GaN on its (0001) A face. The starting A (Ga) face was prepared by growing a micrometer-thick GaN layer directly on a low-index 6H–SiC(0001) substrate at 650°C. RHEED intensity oscillations are measured with substrate temperatures less than 550°C in both Ga-limited and N-limited growth conditions. In the N-limited condition, an initial transient high-frequency oscillation is observed before it reaches a steady-state frequency. If the Ga flux is subsequently stopped while keeping the N flux unchanged, a few extra oscillations are recorded. Scanning tunneling microscopy images of surfaces quenched during growth show triangular-shaped islands, verifying a two-dimensional growth mode. At substrate temperatures greater than 550°C, neither island nucleation nor intensity oscillation is observed, suggesting a step-flow growth mode.

A dynamic priority-based approach to concurrent toolpath planning for multi-material layered manufacturing

This paper presents an approach to concurrent toolpath planning for multi-material layered manufacturing (MMLM) to improve the fabrication efficiency of relatively complex prototypes. The approach is based on decoupled motion planning for multiple moving objects, in which the toolpaths of a set of tools are independently planned and then coordinated to deposit materials concurrently. Relative tool positions are monitored and potential tool collisions detected at a predefined rate. When a potential collision between a pair of tools is detected, a dynamic priority scheme is applied to assign motion priorities of tools. The traverse speeds of tools along the x-axis are compared, and a higher priority is assigned to the tool at a higher traverse speed. A tool with a higher priority continues to deposit material along its original path, while the one with a lower priority gives way by pausing at a suitable point until the potential collision is eliminated. Moreover, the deposition speeds of tools can be adjusted to suit different material properties and fabrication requirements. The proposed approach has been incorporated in a multi-material virtual prototyping (MMVP) system. Digital fabrication of prototypes shows that it can substantially shorten the fabrication time of relatively complex multi-material objects. The approach can be adapted for process control of MMLM when appropriate hardware becomes available. It is expected to benefit various applications, such as advanced product manufacturing and biomedical fabrication.

A multi-material virtual prototyping system for biomedical applications

This paper describes a multi-material virtual prototyping (MMVP) system for modelling and digital fabrication of discrete and functionally graded multi-material objects for biomedical applications. The MMVP system consists of a DMMVP module, an FGMVP module, and a virtual reality (VR) simulation module. The DMMVP module is used for design and process planning of discrete multi-material (DMM) objects, while the FGMVP module is for functionally graded multi-material (FGM) objects. The VR simulation module integrates these two modules to perform digital fabrication of multi-material objects, which can be subsequently visualized and analyzed in a virtual environment to optimize MMLM processes for fabrication of product prototypes. Using the MMVP system, two biomedical objects, including a human dextrocardic heart made of discrete multi-materials and a hip joint assembly of FGM are modelled and digitally fabricated for visualization and analysis in a VR environment. These studies show the MMVP system is a practical tool for modelling, visualization, process planning, and subsequent fabrication of biomedical objects of discrete and functionally graded multi-materials for biomedical applications.

A spline-based flexible method of virtual force design for dynamic motion planning of robots

ABSTRACTVirtual force approach is preferable for motion planning of mobile robots in dynamic environments. It composes virtual attractive forces to drive robots to destinations and virtual repulsive forces to steer robots away from neighbouring robots. However, most traditional methods use functions with limited controllable parameters, compromising design flexibility essential for smooth yet responsive robot motions.This paper proposes a spline-based method to enhance design flexibility of virtual forces that streamline robot motions. We take advantage of local controllability of interpolating cubic splines to generate desirable smooth virtual forces for robots. This merit facilitates responsive robot motions in dynamic situations. A case study of autonomous military robots is presented to validate the approach, in terms of enhanced motion safety and shortened operational time.

Performance optimisation of mobile robots in dynamic environments

This paper presents a robotic simulation system, that combines task allocation and motion planning of multiple mobile robots, for performance optimisation in dynamic environments. While task allocation assigns jobs to robots, motion planning generates routes for robots to execute the assigned jobs. Task allocation and motion planning together play a pivotal role in optimisation of robot team performance. These two issues become more challenging when there are often operational uncertainties in dynamic environments. We address these issues by proposing an auction-based closed-loop module for task allocation and a bio-inspired intelligent module for motion planning to optimise robot team performance in dynamic environments. The task allocation module is characterised by a closed-loop bid adjustment mechanism to improve the bid accuracy even in light of stochastic disturbances. The motion planning module is bio-inspired intelligent in that it features detection of imminent neighbours and responsiveness of virtual force navigation in dynamic traffic conditions. Simulations show that the proposed system is a practical tool to optimise the operations by a team of robots in dynamic environments.

Performance optimisation of mobile robots for search-and-rescue

This paper presents a team performance optimisation system for multiple mobile robots in search-and-rescue operations, in which refugees are first discovered and subsequently robots are dispatched to transport themto shelters. Coordination of mobile robots involves two fundamental issues, namely task allocation and motion planning. While task allocation assigns jobs to robots, motion planning generates routes for robots to execute the assigned jobs. Task allocation and motion planning together play a pivotal role in optimisation of the robot team performance. These two issues become more challenging in dynamic search-and-rescue environments, where the refugees are unpredictably discovered at different locations and the traffic conditions of rescue zones keep changing. Weaddress these two issues by proposing an auction-based closed-loop module for task allocation and a bio-inspired intelligent module for motion planning. The task allocation module is characterised with a closed-loop bid adjustment mechanism to improve the bid accuracy even in light of stochastic rescue requests. The motion planning module is bio-inspired intelligent in that it features detection of imminent neighbours and responsiveness of virtual force navigation in dynamic traffic conditions. Simulations show that the proposed system is a practical tool to optimise the dynamic operations of search-and-rescue by a team of mobilerobots.

Concurrent toolpath planning for multi-material layered manufacturing

Reviewed, accepted September 10, 2008 This paper proposes an algorithm for planning efficient concurrent toolpaths to reduce the build-time of fabricating multi-material prototypes by layered manufacturing. The algorithm first sorts and partitions slice contours into hierarchical families of specific materials to enhance concurrent tool movements. It then detects overlapping of parametric tool envelopes with a “Two-phase Overlap Query Algorithm” to avoid potential tool collisions. Finally, it plans concurrent movements with an “Immediate Fabrication Algorithm” (IFA) to enhance fabrication efficiency by reducing idle time of tools. The algorithm is being implemented in a multi-material virtual prototyping system. It can be adapted for control of physical fabrication of multi-material prototypes when appropriate hardware becomes available.