MOMA: Visual Mobile Marker Odometry

In this paper, we present a cooperative odometry scheme based on the detection of mobile markers in line with the idea of cooperative positioning for multiple robots [1]. To this end, we introduce a simple optimization scheme that realizes visual mobile marker odometry via accurate fixed marker-based camera positioning and analyse the characteristics of errors inherent to the method compared to classical fixed marker-based navigation and visual odometry. In addition, we provide a specific UAV-UGV configuration that allows for continuous movements of the UAV without doing stops and a minimal caterpillar-like configuration that works with one UGV alone. Finally, we present a real-world implementation and evaluation for the proposed UAV-UGV configuration

Subtleties of extrinsic calibration of cameras with non-overlapping fields of view

The calibration of the relative pose between rigidly connected cameras with non-overlapping fields of view (FOV) is a prerequisite for many applications. In this paper, we focus on the subtleties of experimental realization of such a calibration optimization method presented in [1]. We evaluate two strategies to adapt a given optimization process to find better local minima. The first strategy is the introduction of a quality measure for the image data used for calibration, which is based on the projection size of known planar calibration patterns on the image. We show, that introducing an additional weighting to the optimization objective chosen as a function of that quality measure improves calibration accuracy and increases robustness against noise. The second strategy to further improve accuracy is a careful data acquisition of pose pairs used for the calibration. We integrate the above strategies into different setups and demonstrate the improvement both in simulation and real-world experiment

New Methods of Eye-to-Eye Calibration and its Application to Cooperative Localization

During the past few decades, an explosive development of multiple camera systems has occurred. For example, a multiple camera system can be used by an advanced driver-assistance system. For cooperative tasks among robots, a multi-camera rig can be used to increase the localization accuracy and robustness. In the logistics industry, a cargo drone mounted with a multi-camera system obtains a panorama view. In these or other high-demanding tasks that heavily depend on multi-camera systems, accurate extrinsic calibration of cameras is an absolute prerequisite for precise visual localization. In this dissertation, a weighted optimization method and a data selection strategy for extrinsic calibration are proposed that relieve the inherent imbalance between pose estimates existing in Liu’s setup. Besides, two new extrinsic calibration methods are proposed to improve the extrinsic calibration accuracy further. Other contributions of the thesis are two cooperative localization methods MOMA and S-MOMA, which can be applied to a robot group. These methods aim at overcoming the localization challenges in indoor environments where repetitive or lack of features are usually the case. The weighted optimization method introduces a quality measure for all the entries of camera-to-marker pose estimates based on the projection size of the known planar calibration patterns on the image. The data selection strategy provides valuable suggestions on the selection of measurements leading to a better coverage in pose space used for the calibration procedure. By introducing a highly accurate tracking system, the first proposed calibration method disconnects the calibration objects, which are rigidly linked in Liu’s setup. With the aid of the tracking system, the method improves calibration accuracy further. The second calibration method uses active calibration patterns realized with two electronic displays. By regulating the fiducial patterns displayed on the monitors, the approach can actively perceive the best possible measurements for the calibration estimation. The configuration of the dynamic virtual pattern aims at maximizing the underlying sensitivity of the objective function, which is based on the sum of reprojection errors, with regard to the relative pose between the camera and the fiducial pattern. State-of-the-art calibration methods, together with different configurations, are conducted and compared in simulation as well as in real experiments validating that both the optimization method and the twonew calibration methods improve the calibration results in terms of accuracy and robustness. In the second part of the dissertation, two novel, purely vision-based cooperative localization approaches MOMA and S-MOMA for a multi-robot system are introduced. MOMA realizes visual odometry via accurate MObile MArker-based positioning. The movement pattern of the robots mimics the movement of a caterpillar. The introduced fiducial marker board, which is mounted on one of the robots, serves as a mobile landmark, based on which the relative pose between the robots is recovered. The absolute positioning of each robot is deduced from the concatenation of the relative poses of previous phases. The second localization algorithm S-MOMA (MOMA with a stereo camera) extends the original MOMA approach. By fusing absolute pose estimates from static environment features with relative pose estimates from known mobile fiducial features, S-MOMA is formulated as an optimization problem combining two different objectives for these two different feature sources based on the same error measure, namely the reprojection error. A comparison between the proposed cooperative localization approaches MOMA, S-MOMA, as well as state-of-the-art localiza- tion algorithms for different configurations, is given validating the improvement in accuracy and robustness against various challenging testing environments

Subtleties of extrinsic calibration of cameras with non-overlapping fields of view

The calibration of the relative pose between rigidly connected cameras with non-overlapping fields of view (FOV) is a prerequisite for many applications. In this paper, the subtleties of the experimental realization of such calibration optimization methods like in (Z. Liu, et al., Measurement Science and Technology, 2011, Z. Li, V. Willert, Intelligent Transportation Systems (ITSC), 2018) are presented. Two strategies that could be adapted to certain optimization processes to find better local minima are evaluated. The first strategy is a careful measurement acquisition of pose pairs for solving the calibration problem, which improves the accuracy of the initial value for the following non-linear refinement. The second strategy is the introduction of a quality measure for the image data used for the calibration, which is based on the projection size of the known planar calibration patterns on the image. We show that introducing an additional weighting to the optimization objective chosen as a function of that quality measure improves calibration accuracy and increases robustness against noise. The above strategies are integrated into different setups and their improvement is demonstrated both in simulation and real-world experiment

Optimal design on the high-temperature mechanical properties of porous alumina ceramics based on fractal dimension analysis

Abstract Fractal theory and regression analysis were employed for the first time to investigate the effect of pore size and pore distribution on high-temperature mechanical properties of porous alumina ceramics (PAC). In the present work, PAC with the comparable porosity, different pore sizes and pore distributions were prepared using carbon black as the pore-forming agent. Particular emphasis in this study was placed on the establishment of correlation between the thermal shock resistance and pore properties. The relationship between fractal dimension (D f) andthermal shock resistance parameter (R st) in specimens presented the negative power function, indicating that low D f could benefit the improvement of thermal shock resistance in specimens. The results showed that the increase of pore size and pore sphericity leads to a reduced D f, the enhanced hot modulus of rupture (HMOR) and. The decrease of proportion of micro-pores below 2 μm, the increase of mean pore size and pore sphericity could result in the decrease of D f, and then improve R st and HMOR of specimens. Based on the correlation between R st and pore characteristics, PAC with improved thermal shock resistance could be achieved when their pore structure meets the above features

Cadmium biomonitoring and renal dysfunction among a population environmentally exposed to cadmium from smelting in China (ChinaCad).

Cadmium, an environmental pollutant, can have adverse effects on the human body. The kidney is the critical organ. In order to improve the understanding of the dose-response relationship between cadmium exposure and health effects, and especially renal dysfunction, a study on a general population group in China was performed. This study was therefore concerned with cadmium exposure biomarkers, such as the concentrations in blood (BCd) and urine (UCd), and effect biomarkers of renal dysfunction, such as beta2-microglobulin (beta2m), retinol binding protein (RBP) and albumin (ALB). To improve the evaluation of exposure levels in relation to the adverse health effects of cadmium exposure in the general population, a quality control program was conducted to determine analytical quality in the determination of cadmium in blood and urine and for beta2m, creatinine, ALB and RBP. The measurements showed that analytical quality was adequate. The exposure and effect biomarkers were studied in the population groups living in three areas, namely a control area and two Cd polluted areas. In the highly exposed area, most of the BCd values were higher than 5 microg/l and most of the UCd values were higher than 5 microg/g creatinine. Beta2-microglobulin, retinol binding protein, and albumin in urine were all significantly higher in the population living in the heavily polluted area than in that in the control area. Based on data from all three areas, a marked dose-response relationship between UCd or BCd and the prevalence of renal dysfunction was demonstrated. The number of abnormalities in kidney was related to the level of cadmium exposure. Only one index of renal tubular dysfunction was affected in subjects exposed to low levels of cadmium, but more than two indices of renal function were affected in those exposed to high levels