Yongchun Fang

Motion-Estimation-Based Visual Servoing of Nonholonomic Mobile Robots

A 2-1/2-D visual servoing strategy, which is based on a novel motion-estimation technique, is presented for the stabilization of a nonholonomic mobile robot (which is also called the “parking problem”). By taking into account the planar motion constraint of mobile robots, the proposed motion-estimation technique can be applied in both planar and nonplanar scenes. In addition, this approach requires no matrix estimation or decomposition, and it avoids ambiguity and degeneracy problems for the homography or fundamental matrix-based algorithms. Moreover, the field-of-view (FOV) constraint of the onboard camera is largely alleviated because the presented algorithm works well with few feature points. In order to incorporate the advantages of position-based visual servoing and image-based visual servoing, a composite error vector is defined that includes both image signals and the estimated rotational angle. Subsequently, a smooth time-varying feedback controller is adopted to cope with the nonholonomic constraints, which yields global exponential convergent rate for the closed-loop system. On the basis of the perturbed linear system theory, we show that practical exponential stability can be achieved, despite the lack of depth information, which is inherent for monocular camera systems. Both simulation and experiment results are collected to investigate the feasibility of the proposed approach.

Phase plane analysis based motion planning for underactuated overhead cranes

Inspired by the desire to achieve fast payload transportation as well as sufficient swing suppression, a novel phase plane based motion planning method is proposed for underactuated overhead cranes. Specifically, the variation law of the underactuated system states in the phase plane is firstly derived via mathematical analysis for the phase portraits. Based on this, an analytical three-segment acceleration trajectory (namely, a trapezoid velocity trajectory) with the coupling behavior being taken into consideration is obtained under actual crane control constraints. To deal with the jerk (discontinuity) problem, we then present two modified acceleration trajectories by introducing some transition stages and performing some rigorous analysis. Moreover, the trajectories generated by the proposed method can evaluate the maximum payload swing and the arrival time for a given transportation task in advance, which provides essential control indexes for crane operation. Simulation results are provided to illustrate the superior performance of the proposed trajectory planning method.

Note: A novel atomic force microscope fast imaging approach: Variable-speed scanning

Imaging speed is one of the key factors limiting atomic force microscopes (AFM) wide applications. To improve its performance, a variable-speed scanning (VSS) method is designed in this note for an AFM. Specifically, in the VSS mode, the scanning speed is tuned online according to the feedback information to properly distribute imaging time along sample surface. Furthermore, some practical mechanism is proposed to determine the best time of moving the AFM tip to the next scanned point. The contrast experiment results show that the VSS method speeds up the imaging rate while ensuring image quality.

System modeling of an AFM System in Z-axis

Motivated by increasing the scanning performance of the atomic force microscope (AFM), many efforts have been made to analyze the system behavior of an AFM system, mainly in Z-axis, and then to develop more advanced controllers. However, most of the previously derived models involve complex physical or mathematical analysis, and many parameters need to be identified for actual application. In this paper, an empirical model is obtained for the Z-axis dynamics of an AFM system by utilizing experimental data. Specifically, the model consists of a dynamical component and multiple static gains. As introduced in the paper, the N4SID algorithm is first employed to derive the dynamical part based on input-output data. Then the static gains of the piezo-actuator are calibrated experimentally. It can be seen from the experimental data that the main source of time delay in Z-axis is the finite retraction/protraction velocity of the piezo-actuator.

AM-AFM System Analysis and Output Feedback Control Design With Sensor Saturation

This paper analyzes the dynamics of an amplitude-modulation atomic force microscopy (AM-AFM) system, and designs a novel output feedback robust adaptive control (OFRAC) law to improve the scanning performance of the AM-AFM system. That is, a control-oriented reduced model is proposed to approximate the mapping from tip-sample separation to oscillation amplitude, whose accuracy is verified by experimental results. Considering the facts that the parameters of an AM-AFM system vary with different combinations of piezo-scanner and cantilever as well as detected samples, and measurement saturation occurs frequently in dynamic AFM systems, an OFRAC strategy for the piezo-scanner is designed to keep the oscillation amplitude of the cantilever staying at the desired setpoint under various complex situations. It is shown theoretically that the proposed control strategy pushes the system away from the saturation state in finite time, and it ensures uniform ultimate boundedness result for the control error. The OFRAC strategy is applied to a virtual AM-AFM system, and the collected results clearly demonstrate that it presents superior imaging performance for high-speed scanning tasks.

Visual servoing of nonholonomic mobile robots based on a new motion estimation technique

This paper presents a novel visual servoing strategy for a nonholonomic mobile robot, which is based on a new motion estimation technique. By taking into account the planar motion constraint of mobile robots, the proposed motion estimation technique does not require the estimation and decomposition of the homography or fundamental matrix, and it dose not cause any ambiguity problems. Moreover, the camera field-of-view (FOV) constraint and the partial occlusion problem are largely alleviated because the presented algorithm works well with few feature points. In order to incorporate the advantages of position-based visual servoing (PBVS) and image-based visual servoing (IBVS), a novel hybrid error vector is defined including both image signals and the estimated rotational angle. Furthermore, a smooth time-varying feedback controller is adopted to cope with the nonholonomic constraints, which yields global exponential stability for the closed-loop system despite the lack of depth information. Simulation results demonstrate the performance of the proposed approach.

Automatic tuning of PI controller for atomic force microscope based on relay with hysteresis

An atomic force microscope (AFM) usually employs proportional-integral (PI) control strategy to sustain a constant cantilever deflection. However, it is well known that the tuning of PI parameters is a tedious and complicated procedure, especially for those unfamiliar with control theory. In this paper, we employ and implement relay controller to automate the tuning procedure for contact-mode AFM PI controller during different scanning speed operations based on relay with hysteresis. Experimental results show that this approach offers system with satisfactory step response with typical settling time of about 2 ms. Moreover, better sample topography image can be obtained after auto-tuning the control gains during different scanning speed.

Adaptive learning control of complex uncertain systems with nonlinear parameterization

In this paper, an adaptive learning control law is proposed to address complex uncertain systems with nonlinear parameterization. Specifically, the controller consists of: (i) a feedback type term, (ii) an adaptive mechanism for the unknown system parameters, and (iii) a learning-based technique to estimate the unknown periodic functions. As proven by a Lyapunov-based stability analysis, the designed adaptive learning control achieves global asymptotic tracking result for the system state while compensates for the uncertainty associated with the system parameters and the unknown periodic functions simultaneously

Sterilization of polydimethylsiloxane surface with Chinese herb extract: a new antibiotic mechanism of chlorogenic acid

Coating of polydimethylsiloxane (PDMS) surface with a traditional Chinese herb extract chlorogenic acid (CA) solves the contemporary problem of sterilization of PDMS surface. The E. coli grows slower and has a higher death rate on the CA-coated PDMS surfaces. A smoother morphology of these E. coli cell wall is observed by atomic force microscopy (AFM). Unlike the reported mechanism, where CA inhibits bacterial growth by damaging the cell membrane in the bulk solution, we find the CA-coated PDMS surface also decreases the stiffness of the cell wall. A decrease in the Young’s modulus of the cell wall from 3 to 0.8 MPa is reported. Unexpectedly, the CA effect on the swarming ability and the biofilm stability of the bacteria can be still observed, even after they have been removed from the CA environment, indicating a decrease in their resistance to antibiotics for a prolonged time. The CA-coated PDMS surface shows better antibiotic effect against three types of both Gram-positive and Gran-negative bacteria than the gentamicin-coated PDMS surface. Coating of CA on PDMS surface not only solves the problem of sterilization of PDMS surface, but also shines light on the application of Chinese traditional herbs in scientific research.

Development of functional biointerfaces by surface modification of polydimethylsiloxane with bioactive chlorogenic acid

The effect of physicochemical surface properties and chemical structure on the attachment and viability of bacteria and mammalian cells has been extensively studied for the development of biologically relevant applications. In this study, we report a new approach that uses chlorogenic acid (CA) to modify the surface wettability, anti-bacterial activity and cell adhesion properties of polydimethylsiloxane (PDMS). The chemical structure of the surface was obtained by X-ray photoelectron spectroscopy (XPS), the roughness was measured by atomic force microscopy (AFM), and the water contact angle was evaluated for PDMS substrates both before and after CA modification. Molecular modelling showed that the modification was predominately driven by van der Waals and electrostatic interactions. The exposed quinic-acid moiety improved the hydrophilicity of CA-modified PDMS substrates. The adhesion and viability of E. coli and HeLa cells were investigated using fluorescence and phase contrast microscopy. Few viable bacterial cells were found on CA-coated PDMS surfaces compared with unmodified PDMS surfaces. Moreover, HeLa cells exhibited enhanced adhesion and increased spreading on the modified PDMS surface. Thus, CA-coated PDMS surfaces reduced the ratio of viable bacterial cells and increased the adhesion of HeLa cells. These results contribute to the purposeful design of anti-bacterial surfaces for medical device use.