Pose Calibration Research Articles

Dynamic tread wear measurement method for train wheels against vibrations.

Dynamic tread wear measurement is difficult but significant for railway transportation safety and efficiency. The accuracy of existing methods is inclined to be affected by environmental vibrations since they are highly dependent on the accurate calibration of the relative pose between vision sensors. In this paper, we present a method to obtain full wheel profiles based on automatic registration of vision sensor data instead of traditional global calibrations. We adopt two structured light vision sensors to recover the inner and outer profiles of each wheel, and register them by the iterative closest point algorithm. Computer simulations show that the proposed method is insensitive to noises and relative pose vibrations. Static experiments demonstrate that our method has high accuracy and great repeatability. Dynamic experiments show that the measurement accuracy of our method is about 0.18 mm, which is a twofold improvement over traditional methods.

Enhanced phase measurement profilometry for industrial 3D inspection automation

Industrial metrology and inspection systems commonly rely on phase measurement profilometry (PMP) using sinusoidal fringe patterns projecting, yielding dense, and accurate 3D reconstruction regardless of the presence of texture. However, applying PMP method to industrial 3D inspection is still a big challenging problem due to rigorous industrial measurement conditions including large surface reflectivity variation range and vibration. Aiming to solve these problems, an enhanced phase measurement profilometry (EPMP) is proposed. In EPMP, an optimal exposure time (OET) calibration method is proposed to solve large surface reflectivity variation range problem, and it can avoid saturating the camera sensor in areas of specular reflection while keep the signal-to-noise ratio (SNR) of fringe image in areas of weak reflection at most. To resist the influence of vibration, an improved pose calibration method (IPC) is used to allow fast calibration of pose of cameras by acquiring only one image of planar target. Moreover, an automatic online 3D inspection system for evaluating 3D geometric dimension quality of railway truck adapter (RTA) is developed, and according to the experiments, the EPMP indicates a satisfactory result in accuracy and repeatability, which can meet the requirements of the 3D inspection task in industrial measurement conditions.

Head-pose Invariant Eyelid and Iris Tracking Method

These days, there is more demand for a camera-based gaze estimation method for new interfaces and new marketing measurement tools. Considering these applications, the system should track a new user without any operation such as calibration. It should also admit user's natural head pose changes. Previous methods, however, need a calibration procedure before execution and have less accuracy in a head moving situation. In this paper, we propose a method which tracks the user's eyelids and iris automatically and accurately. Our method is a pretreatment of gaze estimation without any calibration and head pose restraint. First of all, we track the facial feature points from an input face image and estimate its head pose, extracting the eye region image. On the eye region image, we track the eyelid shape based on an eyelid shape model generated beforehand from PCA. Finally we track the iris inside the eyelid based on the eyeball model. The eyelid and iris tracking is processed by Particle Filter. An evaluation against a database including head pose changes confirmed that the accuracy of eyelid and iris tracking was improved compared with previous methods.

Pose Calibration of Inertial Measurement Units on Joint-Constrained Rigid Bodies

Pose Calibration of Inertial Measurement Units on Joint-Constrained Rigid Bodies

Pose transfer geometrical calibration for fringe-reflection optical three-dimensional measurement

System geometrical calibration is the prerequisite condition in fringe-reflection optical three-dimensional (3D) measurement. However, such a calibration procedure is challenging because the fringes displaying on the LCD screen do not lie within the measurement camera's field of view. Commonly, a flat mirror with or without markers can be utilized to complete system geometrical calibration, but the planeness and the refraction of mirror will affect the calibration results. Unlike these methods, the geometrical relationship between the world coordinate and the measurement camera can be calibrated with pose transfer, and the accumulated error of pose transfer is analyzed, the calibration precision just depends on the precision of pose calibration.

Approach of face detection based on pose calibration

Approach of face detection based on pose calibration

Dynamic calibration of the relative pose and error analysis in a structured light system

We investigate the problem of dynamic calibration for our structured light system. First, a method is presented to estimate the rotation matrix and translation vector between the camera and the projector using plane-based homography. Then an approach is introduced to analyze theoretically the error sensitivity in the estimated pose parameters with respect to noise in the projection points. This algorithm is simple and easy to implement. Finally, some numerical simulations and real data experiments are carried out to validate our method.

Calibration of focal length and 3D pose based on the reflectance and depth image of a planar object

The estimation of position and orientation of a Photonic Mixer Device (PMD) camera in a global reference frame is required by many measurement applications based on such systems. PMD cameras produce a depth as well as a reflectance image of low resolution compared to standard optical cameras, so that calibration of the cameras based on the reflectance image alone is difficult. We will present a novel approach for calibrating the focal length and 3D pose of a PMD camera based on the depth and reflectance image of a planar checkerboard pattern. By integrating both sources of information higher accuracies can be achieved. Furthermore, one single image is sufficient for calibrating the focal length as well as the 3D pose from a planar reference object. This is because the depth measurements are orthogonal to the lateral intensity measurements and provide direct metric information.

Calibrating and optimizing poses of visual sensors in distributed platforms

Many novel multimedia, home entertainment, visual surveillance and health applications use multiple audio-visual sensors. We present a novel approach for position and pose calibration of visual sensors, i.e., cameras, in a distributed network of general purpose computing devices (GPCs). It complements our work on position calibration of audio sensors and actuators in a distributed computing platform (Raykar et al. in proceedings of ACM Multimedia `03, pp. 572---581, 2003). The approach is suitable for a wide range of possible --- even mobile --- setups since (a) synchronization is not required, (b) it works automatically, (c) only weak restrictions are imposed on the positions of the cameras, and (d) no upper limit on the number of cameras under calibration is imposed. Corresponding points across different camera images are established automatically. Cameras do not have to share one common view. Only a reasonable overlap between camera subgroups is necessary. The method has been sucessfully tested in numerous multi-camera environments with a varying number of cameras and has proven itself to work extremely accurate. Once all distributed visual sensors are calibrated, we focus on post-optimizing their poses to increase coverage of the space observed. A linear programming approach is derived that determines jointly for each camera the pan and tilt angle that maximizes the coverage of the space at a given sampling frequency. Experimental results clearly demonstrate the gain in visual coverage.

An experimental study on the vibration-free high-speed operation of a three-dimensional coordinate measuring machine

This paper presents a novel method that can accurately calibrate structured light system requiring only one directional structured patterns. This method leverages the existing flexible camera calibration to extract each calibration pose for three-dimensional coordinate determination for each pixel, and then establish pixel-wise relationship between each coordinate and phase. The phase artifacts caused by high-contrast calibration target was addressed to eliminate its impact on the calibration. Comparing with the traditional structured light system calibration method that requires two directional fringe patterns, our method requires only one single directional fringe patterns yet achieves 6.6 times higher accuracy for flat surface measurements.

Calibration-based absolute localization of parts for multi-robot assembly

In multi-robot assembly of parts, for successful mating, the grasped parts must be located with sufficiently small position and orientation errors so that assembly can be achieved. This paper describes a new approach for determining the absolute three-dimensional spatial location of parts grasped by robots during assembly. Through a combination of robot pose calibration and part-sensor calibration, the robot, used to grasp the part, is calibrated to accurately position and orient parts to a designated mating location. First, by employing a robot pose measurement system, the 6 DOF robot pose errors relative to a reference coordinate frame are compensated. Second, with the implementation of a part pose measurement, the 6 DOF part pose errors, relative to the robot tool frame, are estimated in real time. An experimental verification of the proposed methodology using a single FANUC S–110 robot manipulating an automotive sheet metal part is described.

Calibration of a hexapod machine tool using a redundant leg

Parallel configurations are recently being applied to the machine tool with the hopes of greater rigidity, stability, and accuracy than conventional multi-axis structures allow. However, the many calibration methods presently available for serial machine tools are not applicable to hexapod type structures. A calibration method is presented that uses a ball–bar or other simple length measuring device to act as an ‘extra leg,’ allowing calibration of the hexapod's true kinematic parameters. This method utilizes a total least squares minimization, does not require any special hexapod configuration or difficult six degree of freedom pose measurements, and is effective with as few as one additional length sensor. Selection of calibration pose sets is briefly discussed, as well as the influence of measurement noise on calibration accuracy. Simulations show the potential for this algorithm to significantly reduce errors to the point where machining errors are within 5–10 times the measurement errors.