抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

A heuristic method has been developed for registering two sets of 3-D curves obtained by using an edge-based stereo system, or two dense 3-D maps obtained by using a correlation-based stereo system. Geometric matching in general is a difficult unsolved problem in computer vision. Fortunately, in many practical applications, some a priori knowledge exists which considerably simplifies the problem. In visual navigation, for example, the motion between successive positions is usually approximately known. From this initial estimate, our algorithm computes observer motion with very good precision, which is required for environment modeling (e.g., building a Digital Elevation Map). Objects are represented by a set of 3-D points, which are considered as the samples of a surface. No constraint is imposed on the form of the objects. The proposed algorithm is based on iteratively matching points in one set to the closest points in the other. A statistical method based on the distance distribution is used to deal with outliers, occlusion, appearance and disappearance, which allows us to do subset-subset matching. A least-squares technique is used to estimate 3-D motion from the point correspondences, which reduces the average distance between points in the two sets. Both synthetic and real data have been used to test the algorithm, and the results show that it is efficient and robust, and yields an accurate motion estimate.

/pdf/iterative-point-matching-for-registration-of-free-form-5fl1ftde76.pdf

Iterative point matching for registration of free-form curves and surfaces

A scheme is developed for classifying the types of motion perceived by a humanlike robot. It is assumed that the robot receives visual images of the scene using a perspective system model. Equations, theorems, concepts, clues, etc., relating the objects, their positions, and their motion to their images on the focal plane are presented. >

http://ieeexplore.ieee.org/iel2/815/3148/00100651.pdf

Robot vision

Geometric matching in general is a difficult unsolved problem in computer vision. Fortunately, in many pratical applications, some a priori knowledge exists which considerably simplifies the problem. In visual navigation, for example, the motion between successive positions is usually either small or approximately known, but a more precise registration is required for environment modeling. The algorithm described in this report meets this need. Objects are represented by free-form curves, i.e., arbitrary spaces curves of the type found in practice. A curve is available in the form of a set of chained points. The proposed algorithm is based on iteratively matching points on one curve to the closest points on the other. A least-squares technique is used to estimate 3-D motion from the point correspondences, which reduces the average distance between curves in two sets. Both synthetic and real data have been used to test the algorithm, and the results show that it is efficient and robust, and yields an accurate motion estimate. The algorithm can be easily extended to solve similar problems such as 2-D curve matching and 3-D surface matching.

/pdf/iterative-point-matching-for-registration-of-free-form-b0pre910b7.pdf

Iterative point matching for registration of free-form curves

In many areas of industry, it is desirable to create geometric models of existing objects for which no such model is available. This paper reviews the process of reverse engineering of shapes. After identifying the purpose of reverse engineering and the main application areas, the most important algorithmic steps are outlined and various reconstruction strategies are presented. Pros and cons of various data acquisition techniques are described with related problems of boundary representation model construction. Specific issues addressed include characterization of geometric models and related surface representations, segmentation and surface fitting for simple and free-form shapes, multiple view combination and creating consistent and accurate B-rep models. The limitations of currently known solutions are also described, and we point out areas in which further work is required before reverse engineering of shape becomes a practical, widely-available engineering tool.

Reverse engineering of geometric models—an introduction

In this paper, a precision tracking control scheme for linear discrete time nonminimum-phase systems is proposed. This control scheme consists of a preview filter, a tracking-performance filter, a command feedforward controller, and a feedback controller. A command feedforward controller, whose design is based on the minimal order inverse model of the plant being controlled, will result in a completely decoupled system. The preview filter is introduced to compensate the phase and gain errors induced by the nonminimum phase zeros or lightly damped zeros of the system. Using the command feedforward controller along with the proposed preview filter, the tracking performance of the proposed control scheme can be characterized by the frequency response of the tracking-performance filter. For the design of the preview filter, a generalized Nth order preview filter and its associated penalty function that quantifies the tracking error of a design are defined. It is shown that, given the desired bandwidth and the order of the preview filter, the optimal solution for the design of the preview filter can be obtained explicitly. The proposed control scheme together with the optimal preview filter is shown to be very effective in achieving precision tracking control of discrete time MIMO nonminimum phase systems. It is also shown that the tracking performance is improved as the order N of the preview filter is increased.

Precision Tracking Control of Discrete Time Nonminimum-Phase Systems

This article presents the development of a faster control loop for oscillation amplitude regulation in tapping mode operation of atomic force microscopy. Two techniques in relation to actuation and measurement are developed, that together significantly increase the bandwidth of the control loop. Firstly, magnetic actuation is employed to directly control the tip position of the cantilever to improve both the speed and the dynamics of the positioning system. Secondly, the signal path for oscillation amplitude regulation is separated from that for topography estimation in order to eliminate measurement delay that degrades the performance of the feedback loop. As a result, the phase-crossover frequency and gain margin of the control system are both increased, leading to a faster and more stable system. Two experiments are performed, one in air and the other in aqueous solution, to compare the developed control system with a commercial one and demonstrate the improvement. The results verify that the combination of the two techniques along with other existing methods eliminates all limitations associated with the instrument for the purpose of oscillation amplitude regulation, which is therewith dictated by the bandwidth of the cantilever.

http://biophys.w3.kanazawa-u.ac.jp/References/High-speed_AFM/Menq-RSI-2006.pdf

Direct tip-position control using magnetic actuation for achieving fast scanning in tapping mode atomic force microscopy

The control of tip-to-sample distance in atomic force microscopy (AFM) is achieved through controlling the vertical tip position of the AFM cantilever. In the vertical tip-position control, the required z motion is commanded by laser reading of the vertical tip position in real time and might contain high frequency components depending on the lateral scanning rate and topographical variations of the sample. This paper presents a dual-actuator tip-motion control scheme that enables the AFM tip to track abrupt topographical variations. In the dual-actuator scheme, an additional magnetic mode actuator is employed to achieve high bandwidth tip-motion control while the regular z scanner provides the necessary motion range. This added actuator serves to make the entire cantilever bandwidth available for tip positioning, and thus controls the tip-to-sample distance. A fast programmable electronics board was employed to realize the proposed dual-actuator control scheme, in which model cancellation algorithms were implemented to enlarge the bandwidth of the magnetic actuation and to compensate the lightly damped dynamics of the cantilever. Experiments were conducted to illustrate the capabilities of the proposed dual-actuator tip-motion control in terms of response speed and travel range. It was shown that while the bandwidth of the regular z scanner was merely a small fraction of the cantilever's bandwidth, the dual-actuator control scheme led to a tip-motion control system, the bandwidth of which was comparable to that of the cantilever, where the dynamics overdamped, and the motion range comparable to that of the z scanner.

Control of tip-to-sample distance in atomic force microscopy: a dual-actuator tip-motion control scheme.

This paper presents the development of a new compact six-axis compliant stage employing piezoelectric actuators to achieve six-axis actuation with nanometer resolution. The integration of direct metrology in the object space, based on real-time visual feedback, enables high-precision motion control. In order to achieve greater motion range, the simple and compact decoupled mechanical structure utilizes two-tap displacement amplifiers for in-plane motion and semibridge amplifiers for out-of-plane motion. The kinematic analysis of the stage is presented. Laterally sampled white light interferometry was implemented to measure the out-of-plane motion of the stage, and a measurement model associated with the designed target patterns is developed to estimate the in-plane motion in real time. Together, they form a visual tracking system and are integrated with the six-axis compliant stage to realize precision six-axis real-time visual servo-control. Experimental results demonstrate that the six-axis compliant stage has the motion range of 77.42 microm, 67.45 microm, 24.56 microm, 0.93 mrad, 0.95 mrad, and 3.10 mrad, and the resolution of +/-5 nm, +/-8 nm, +/-10 nm, +/-10 murad, +/-10 murad, and +/-20 murad for x-axis, y-axis, and z-axis translation and rotation, respectively.

Design, implementation, and control of a six-axis compliant stage.

3-D active vision systems that project artificial structured light for coordinate measurements have been adopted in many industrial applications. With advances of electronic projection display technology, the digital projector is becoming an important component of various 3-D active vision systems. However, current projector models or structured light calibration techniques for 3-D active vision systems are limited to stripe-type structured light and the majority of them do not consider projector lens distortion. In order to overcome these limitations, a digital projector calibration method is developed to calibrate light beams projected from all pixel elements of a digital projector. Since the digital projector is fully programmable, various structured light patterns can be projected for coordinate acquisition, whose models can be obtained by interpolating parameters of light beams that synthesize the structured light patterns. With proper interpolation functions, experimental results indicate that the projector lens distortion can be successfully compensated and measurement errors are significantly reduced. When the digital projector is moved, a simple rigid body transformation calibration method is developed to rapidly obtain the transformation without re-calibrating the projector The precision of the 3-D active vision system using the proposed digital projector calibration method and rigid body transformation calibration technique is experimentally evaluated.

Chia-Hsiang Menq

Papers

Precision Tracking Control of Discrete Time Nonminimum-Phase Systems

Direct tip-position control using magnetic actuation for achieving fast scanning in tapping mode atomic force microscopy

Control of tip-to-sample distance in atomic force microscopy: a dual-actuator tip-motion control scheme.

Design, implementation, and control of a six-axis compliant stage.

Digital Projector Calibration for 3-D Active Vision Systems