Yongjie Ren

A Vision-Based Self-Calibration Method for Robotic Visual Inspection Systems

A vision-based robot self-calibration method is proposed in this paper to evaluate the kinematic parameter errors of a robot using a visual sensor mounted on its end-effector. This approach could be performed in the industrial field without external, expensive apparatus or an elaborate setup. A robot Tool Center Point (TCP) is defined in the structural model of a line-structured laser sensor, and aligned to a reference point fixed in the robot workspace. A mathematical model is established to formulate the misalignment errors with kinematic parameter errors and TCP position errors. Based on the fixed point constraints, the kinematic parameter errors and TCP position errors are identified with an iterative algorithm. Compared to the conventional methods, this proposed method eliminates the need for a robot-based-frame and hand-to-eye calibrations, shortens the error propagation chain, and makes the calibration process more accurate and convenient. A validation experiment is performed on an ABB IRB2400 robot. An optimal configuration on the number and distribution of fixed points in the robot workspace is obtained based on the experimental results. Comparative experiments reveal that there is a significant improvement of the measuring accuracy of the robotic visual inspection system.

Calibration technology in application of robot-laser scanning system

Abstract. A system composed of laser sensor and 6-DOF industrial robot is proposed to obtain complete three-dimensional (3-D) information of the object surface. Suitable for the different combining ways of laser sensor and robot, a new method to calibrate the position and pose between sensor and robot is presented. By using a standard sphere with known radius as a reference tool, the rotation and translation matrices between the laser sensor and robot are computed, respectively in two steps, so that many unstable factors introduced in conventional optimization methods can be avoided. The experimental results show that the accuracy of the proposed calibration method can be achieved up to 0.062 mm. The calibration method is also implemented into the automated robot scanning system to reconstruct a car door panel.

Study on reducing the precision loss of a portable probe in large-scale measurements

Abstract. The traditional measurement method for a hand-held probe with two sensors in large-scale measurements magnifies the uncertainty significantly when the distance between the tip and the nearest sensor is increased in order to touch the measurement points in the shadowed zone. To reduce the precision loss in that case, we propose a new method that only utilizes the colinear characteristic of the sensors on our newly designed probe. Hence, the principle of intersection can be used to determine the coordinate of the measurement point by holding the probe to touch the point in two or more different directions. This method is helpful to keep a low uncertainty in the case where increasing the distance between the tip and the nearest sensor is necessary. Correspondingly, the algorithms used to make the probe work its best, as well as the detailed derivations, are given. An experiment is then shown to verify the efficacy of the new design and the proposed method. The results indicate that the new design and the new method can reduce the precision loss remarkably in the case where the distance between the tip and the nearest sensor has to be increased.

Design of an omnidirectional single-point photodetector for large-scale spatial coordinate measurement

Abstract. In high precision and large-scale coordinate measurement, one commonly used approach to determine the coordinate of a target point is utilizing the spatial trigonometric relationships between multiple laser transmitter stations and the target point. A light receiving device at the target point is the key element in large-scale coordinate measurement systems. To ensure high-resolution and highly sensitive spatial coordinate measurement, a high-performance and miniaturized omnidirectional single-point photodetector (OSPD) is greatly desired. We report one design of OSPD using an aspheric lens, which achieves an enhanced reception angle of −5  deg to 45 deg in vertical and 360 deg in horizontal. As the heart of our OSPD, the aspheric lens is designed in a geometric model and optimized by LightTools Software, which enables the reflection of a wide-angle incident light beam into the single-point photodiode. The performance of home-made OSPD is characterized with working distances from 1 to 13 m and further analyzed utilizing developed a geometric model. The experimental and analytic results verify that our device is highly suitable for large-scale coordinate metrology. The developed device also holds great potential in various applications such as omnidirectional vision sensor, indoor global positioning system, and optical wireless communication systems.

Fast Recovery Technology of Tool Center Point in Robotic Visual Measurement System

To recover measurement quickly after robot collision in robotic online visual measurement system, two simple and feasible methods to recalibrate the tool center point(TCP) of robot quickly are proposed. When there is a robot collision in the industry field, only several measurements are performed by moving the robot, then the tool coordinate is recalibrated and the robot TCP is recovered accurately, which can effectively avert the complex work of re-teaching the robots measurement trajectory. The tool coordinate and TCP of the robot are defined in this paper first, based on the structure parameters of the light plane in visual sensor. Then the reference sphere based tool coordinate calibration method and common point deviation based calibration method are presented respectively. Simulation experiment testifies that the two methods can recover the robot tool coordinate and TCP quickly, and meet the demand of recovery measurement after robot collision in robotic visual measurement system.

Analysis on the dynamic error for optoelectronic scanning coordinate measurement network

Large-scale dynamic three-dimension coordinate measurement technique is eagerly demanded in equipment manufacturing. Noted for advantages of high accuracy, scale expandability and multitask parallel measurement, optoelectronic scanning measurement network has got close attention. It is widely used in large components jointing, spacecraft rendezvous and docking simulation, digital shipbuilding and automated guided vehicle navigation. At present, most research about optoelectronic scanning measurement network is focused on static measurement capacity and research about dynamic accuracy is insufficient. Limited by the measurement principle, the dynamic error is non-negligible and restricts the application. The workshop measurement and positioning system is a representative which can realize dynamic measurement function in theory. In this paper we conduct deep research on dynamic error resources and divide them two parts: phase error and synchronization error. Dynamic error model is constructed. Based on the theory above, simulation about dynamic error is carried out. Dynamic error is quantized and the rule of volatility and periodicity has been found. Dynamic error characteristics are shown in detail. The research result lays foundation for further accuracy improvement.

Compensation of sampling error in frequency scanning interferometry

Absolute distance measurement techniques are of significant interest in the field of large volume metrology. Ones which could offer an ability of ADM and high accuracy will improve the efficiency and the quality of large assemblies. Frequency scanning interferometry (FSI) is a kind of ADM technique which use a variable synthetic-wavelength achieved by tuning the optical frequency continuously. FSI could offer a relative accuracy of several ppm in a range of tens of meters. In a FSI ranging system, it is necessary to get knowledge of the tuning range of optical frequency, which could be done by using of gas absorption cell, femtosecond laser comb, F-P etalon and the most used: a predicted auxiliary interferometer. As the result of the measurement is calculated by the tuning range of optical frequency, a length drift of the auxiliary interferometer will make a contribution in error of the result. Analysis of sampling error caused by the drift of the auxiliary interferometer has been done and a real-time compensation system has been proposed to minimize the drift of the auxiliary interferometer. The simulation has proved the analysis and the error has been decreased.

Rotating machinery vibration analysis of the rotary-laser scanning measurement system

The workshop Measurement and Positioning System (wMPS) based on the rotary-laser scanning technology has been widely applied in the manufacturing industry to provide an accurate and robust coordinate measurement. However, some vibration-related problems in the rotating machinery inevitably exist. These problems influence the measurement accuracy of wMPS and even reduce its service life. In this paper, the rotating machinery vibration analysis of wMPS is introduced. Some significant factors causing vibrations, such as the mass imbalance of the rotor, are discussed. The vibration signals of rotating machinery are captured experimentally by the three-axis accelerometer. These raw vibration signals are processed by the data pretreatment, the time-domain analysis and the frequency-domain analysis. Based on these analyses, some evaluation criteria of rotating machinery vibration are introduced. These criteria provide guidance to the fault detection and ensure the ongoing operational condition of wMPS.

Study of network topology effect on measurement accuracy for a distributed rotary-laser measurement system

Abstract. The workshop Measurement Positioning System (wMPS) is a large-scale measurement system that better copes with the current challenges of dimensional metrology. However, as a distributed measuring system with multiple transmitters forming a spatial measurement network, the network topology of transmitters relative to the receiver exerts a significant influence on the measurement accuracy albeit one that is difficult to quantify. An evaluation metric, termed the geometric dilution of precision (GDOP), is introduced to quantify the quality of the network topology of the wMPS. The GDOP is derived from the measurement error model of wMPS and its mathematical derivation is expounded. Two significant factors (density and layout of the transmitter) affecting the network topology are analyzed by simulations and experiments. The experimental results show that GDOP is approximately proportional to the measurement error. More transmitters, and a relatively good layout thereof, can decrease the value of GDOP and the measurement error.

Photoelectric scanning-based method for positioning omnidirectional automatic guided vehicle

Abstract. Automatic guided vehicle (AGV) as a kind of mobile robot has been widely used in many applications. For better adapting to the complex working environment, more and more AGVs are designed to be omnidirectional by being equipped with Mecanum wheels for increasing their flexibility and maneuverability. However, as the AGV with this kind of wheels suffers from the position errors mainly because of the frequent slipping property, how to measure its position accurately in real time is an extremely important issue. Among the ways of achieving it, the photoelectric scanning methodology based on angle measurement is efficient. Hence, we propose a feasible method to ameliorate the positioning process, which mainly integrates four photoelectric receivers and one laser transmitter. To verify the practicality and accuracy, actual experiments and computer simulations have been conducted. In the simulation, the theoretical positioning error is less than 0.28 mm in a 10  m×10  m space. In the actual experiment, the performances about the stability, accuracy, and dynamic capability of this method were inspected. It demonstrates that the system works well and the performance of the position measurement is high enough to fulfill the mainstream tasks.