This paper discusses the measurement precision loss problem introduced by the inconsistent between measurement space and calibration space and presents a hybrid calibration method to solve this problem. The proposed method establishes the hybrid adjustment model based on the ray intersection constraint, ruler length constraint, and sparse control points' rescan constraint. The Levenberg-Marquardt algorithm is employed to solve the objective function. The initial value is calculated in two steps, in which the ruler length constraint is used to calculate the relationship between each transmitter, and the global coordinate system's transformation parameter is calculated by space intersection. In addition, the part of the adjustment objective function introduced by the ray intersection constraint is extended by considering the ray intersection constraint of the measured point. About 78% improvement in measurement accuracy is achieved compared with the traditional way.
Limited by the principle of multi-angle intersection measurement, photoelectric scanning measurement systems such as workshop Measuring and Positioning System (wMPS) and indoor Global Positioning System (iGPS) have systematical positioning errors when measuring moving targets. Fusing target movement information can effectively compensate the positioning error introduced by the relative motion between the target and the transmitter. Inertial measurement systems are capable of high-frequency, all-weather and autonomous rotation and acceleration measurement. They have good measurement complementarity with photoelectric scanning measurement systems. Integrating movement information from inertial measurement unit (IMU) is an effective method to improve the dynamic measurement performance. Inertial information fusion needs two preconditions: the first one is clock synchronization and the second one is spatial alignment. In response to the two problems above, we research on wMPS and propose a wMPS-IMU integrated online calibration method. The time offset of the two systems is precisely calibrated by compensating the delay between the wMPS reference clock and the IMU clock. Due to the fact that wMPS receivers and the IMU are connected as a rigid body, the rotation R and the translation T between the wMPS and the IMU can be easily calibrated through a linear estimation method. Our novel method benefits to achieve real-time online temporal and spatial calibration without the help of any external measurement equipment and prior information and significantly improves the applicability of wMPS. Verification experiments were carried out. A three-axis turntable and a linear guide were utilized as the roundness and straightness references. The calibration experiment shows that the calibration accuracy of our method is at the same level as that achieved from the high accuracy coordinate measuring machine (CMM). The precision evaluation experiment shows that the hybrid wMPS-IMU system calibrated through the proposed method has slightly better dynamic performance than that calibrated on a CMM. This method is fully verified to be effective.
Dynamic three-dimensional surface measurement based on line-scan cameras has been developing gradually. Line-scan cameras have the advantage of high resolution and a high line rate. In this article, a measurement system based on triple line-scan cameras is presented. The measurement model is established considering the asynchronous acquisition of the same point. The point cloud optimization algorithm helps to minimize the error caused by six-degree-of-freedom vibration. Theoretical analysis reveals that the resolution of the method is approximately 0.07 mm, and the measurement uncertainty is about 0.025 mm. Experiments demonstrated the feasibility of the method.
This paper introduces an improved 6DOF measurement method for the non-overlapping multi-camera systems. In contrast to the existing equal-weighted bundle adjustment methods, the proposed method assigns an uncertainty based weight to each of the non-overlapping cameras. The measurement uncertainty evaluation framework is established based on the implicit function theorem. The weight assignment criterion enables the camera in a better observation condition to provide stronger constraints. The 6DOF parameters are obtained by optimizing the uncertainty-weighted space resection model with the nonlinear squares solver. The experiment results demonstrate the robustness of the proposed method to cope with poor observation conditions.
This paper presents a multi-dimensional evaluation method of the angle measurement performance for the transmitter of the photoelectric scanning measurement network. As a distributed measurement system based on the multi-angle intersection, the accuracy of the angle measurement of the transmitter directly determines the measurement accuracy of the entire system. At present, there are few methods of evaluating the angle measurement performance for the transmitter. Multiple receivers are placed around the transmitter in a certain order and kept at the same level as much as possible. Based on circumferential closure and statistical knowledge, this method can evaluate the angle repeatability of different angle positions and the rotational speed volatility of the angular separations between adjacent receivers. The rotational speed volatility of the transmitter is important for the optimization of the angle measurement performance of the transmitter. Through experiments, we find that the method can reflect more information about the shafting, which can provide a good reference for the subsequent system upgrade.
With the increase in demand for advanced military and civilian aircraft , aircraft assembly cycle time is getting shorter and shorter while the quality requirement is becoming higher and higher, meanwhile the costs of aircraft assembly are getting lower and lower. It is supposed that intelligent assembly is one of important approaches to solve the issue above. Aircraft assembly state perception and process control are the most essential factors for achieving assembly intelligence. This essay illustrates indoor GPS to build large-scale measurement field respectively on whole plane testing station, docking stations of full aircraft and materials transportation routes, which achieved the goal that whole plane level testing is characterized with high efficiency and accuracy. Dynamic posture-adjustment guidance of intelligent logistic AGV is realized. While quantitative monitoring the quality and process of aircraft assembly, the number of employees in the assembly logistics are reduced, the efficiency and quality of aircraft assembly are greatly improved.
This paper describes a method of measuring distance and angle by dispersive interferometer using a femtosecond pulse laser. The measurement system is set up based on the modified Michelson interferometer, which combines the advantages of using retroreflector and plane-mirror reflector. The two parallel measured beams and one reference beam can undergo a mutual interference in frequency domain. The three shutters are set to two measure arms and reference arm, respectively. the shutters can be used to change the work mode of interference system. The optical path difference of distance and angle can be acquired by the modulated frequency of interference signal. Angle measurement has been compared to a calibrated rotation stage in the range of 1800 arcsec, the obtained mutual agreement is better than 3 arcsec in ordinary laboratory conditions. The results show the method has the potential for further research of absolute multiple degree-of-freedom measurements.
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.
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.
With outstanding features of high resolution and high acquisition rate, line-scan imaging holds great potentials for high-speed applications. This paper presents a stereo line-scan system for 3D shape measurement of fast moving objects. The principle and key technologies are addressed. The system setup and 3D imaging model are introduced first and the stereo matching scheme and calibration approach are described subsequently. The system is verified by experiments. The results demonstrate the validity and accuracy of the proposed system.
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.
Precise distance measurement is of interest for large-scale manufacturing, future space satellite missions, and other industrial applications. The ranging system with femtosecond optical frequency comb (FOFC) could offer high accuracy, stability and direct traceability to SI definition of the meter. Here, we propose a scheme for length measurement based on the frequency-modulated continuous-wave (FMCW) ladar with a FOFC. In this scheme, the reference interferometer in the FMCW ladar is calibrated by the intensity detection using the FOFC in the time domain within an optical wavelength resolution. With analysis of the theoretical model, this system has the potential to a high-speed, high-accuracy absolute distance measurement. Then, based on the experimental results, the evaluation of the performance of the calibration of the reference arm is discussed. In addition, the performance of this system is evaluated by a single position measurement with different tuning velocities of wavelength. The experimental results show that the reproducibility of the distance measurement is 10-5 level.
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.
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.
Multi-task and real-time measurement of relative displacement is widely needed in the present industrial field. Existing measuring methods require complex preparation and data processing, or are unable meet the requirement of automation, multi-task and real-time. The instruments used to measure absolute coordinates are inefficiency because of the measured target is relative displacement. A new single-station wMPS (Workspace Measuring Position System) measurement method combined distance measurement is presented in this paper. It learns from measuring principle of total station, measures angle based on rotating scanning laser plane measuring method, and measures distance based on optoelectronic scanning multi-angle intersection location principle, uses the angle-length resection measuring method, builds a new mathematical model to measure the relative displacement of the target. The result of experiment proves that it increases measuring efficiency and achieves multi-task and real-time measurement of relative displacement.
The present scanning system consists of an industrial robot and a line-structured laser sensor which uses the industrial robot as a position instrument to guarantee the accuracy. However, the absolute accuracy of an industrial robot is relatively poor compared with the good repeatability in the manufacturing industry. This paper proposes a novel method using the workspace measurement and positioning system (wMPS) to remedy the lack of accuracy of the industrial robot. In order to guarantee the positioning accuracy of the system, the wMPS which is a laser-based measurement technology designed for large-volume metrology applications is brought in. Benefitting from the wMPS, this system can measure different cell-areas by the line-structured laser sensor and fuse the measurement data of different cell-areas by using the wMPS accurately. The system calibration which is the procedure to acquire and optimize the structure parameters of the scanning system is also stated in detail in this paper. In order to verify the feasibility of the system for scanning the large free-form surface, an experiment is designed to scan the internal surface of the door of a car-body in white. The final results show that the measurement data of the whole measuring areas have been jointed perfectly and there is no mismatch in the figure especially in the hole measuring areas. This experiment has verified the rationality of the system scheme, the correctness and effectiveness of the relevant methods.
This paper presents a new 3D coordinate measurement method based on linear camera equipped with cylindrical lenses (LCEWCL). The LCEWCL has prominent advantages in high-precision coordinate measurement and dynamic position-tracking owning to the high resolution and outstanding frame rate of linear CCD. Every LCEWCL is a one-dimensional angle-measuring unit which can confirm a plane passing through the light spot. With three or more LCEWCLs arrangement in front of the measurement field, the 3D spatial coordinate of the light spot is reconstructed by multi-angle intersection. Experiments are conducted to validate the accuracy and effectiveness of the proposed method and good results have been obtained.
KEYWORDS: Transmitters, Calibration, Receivers, Control systems, 3D metrology, Optimization (mathematics), Head, Optical engineering, Time metrology, Computing systems
The workspace measurement and positioning system is a three-dimensional (3-D) coordinate system based on laser scanning, which is widely applied in large-scale metrology. As a core part ensuring measurement accuracy, the transmitter parameters’ calibration is the critical technique of the system. The present transmitter parameter calibration method relies on auxiliary measurement equipment, which is more error prone and less efficient. This paper will focus on the improvement of the transmitter parameters’ calibration by using a highly precise 3-D coordinate control network. Several calibration points with known coordinates are set in the workspace to establish the precise 3-D coordinate control network. After the new model of transmitter parameter calibration has been explained, both the calculation method for optimization and the production of the initial iteration value are given. As indicated by the results of the verifying experiment, the accuracy and efficiency of the transmitter calibration can be distinctly improved by using the proposed method. The experimental data show that the 3-D coordinate measurement error has obviously decreased from 0.3 to 0.15 mm as a merit of the proposed method.
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.
Occlusion is a major problem for real-time position and orientation measurement with distributed optical large-scale metrology systems. This paper presents two novel methods with occlusion handling to address this issue, which should be used in combination for practical applications. These two approaches are based on the constraints established by three control points and six control points, respectively, and then the position and orientation can be calculated through iterative optimization algorithms. In this paper, all the work is carried out by using the workspace measuring and positioning system as a verification platform. The experimental results show that the orientation accuracies of the three-point method and the six-point method are kept within 0.1 and 0.04 deg, respectively, and the position accuracy exceeds 0.15 and 0.08 mm.
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