Inline three-dimensional measurements are a growing part of optical inspection. Considering increasing production capacities and economic aspects, dynamic measurements under motion are inescapable. Using a sequence of different pattern, like it is generally done in fringe projection systems, relative movements of the measurement object with respect to the 3d sensor between the images of one pattern sequence have to be compensated.
Based on the application of fully automated optical inspection of circuit boards at an assembly line, the knowledge of the relative speed of movement between the measurement object and the 3d sensor system should be used inside the algorithms of motion compensation. Optimally, this relative speed is constant over the whole measurement process and consists of only one motion direction to avoid sensor vibrations. The quantified evaluation of this two assumptions and the error impact on the 3d accuracy are content of the research project described by this paper.
For our experiments we use a glass etalon with non-transparent circles and transmitted light. Focused on the circle borders, this is one of the most reliable methods to determine subpixel positions using a couple of searching rays. The intersection point of all rays characterize the center of each circle. Based on these circle centers determined with a precision of approximately 1=50 pixel, the motion vector between two images could be calculated and compared with the input motion vector. Overall, the results are used to optimize the weight distribution of the 3d sensor head and reduce non-uniformly vibrations. Finally, there exists a dynamic 3d measurement system with an error of motion vectors about 4 micrometer. Based on this outcome, simulations result in a 3d standard deviation at planar object regions of 6 micrometers. The same system yields a 3d standard deviation of 9 µm without the optimization of weight distribution.
The requirement for a non-transparent Lambertian like surface in optical 3D measurements with fringe pattern projection cannot be satisfied at translucent objects. The translucency causes artifacts and systematic errors in the pattern decoding, which could lead to measurement errors and a decrease of measurement stability. In this work, the influence of light wavelength on 3D measurements was investigated at a stereoscopic system consisting of two filter wheel cameras with narrowband bandpass filters and a projector with a wide-band light source. The experimental results show a significant wavelength dependency of the systematic measurement deviation and the measurement stability.
Fringe projection is an established method for contactless measurement of 3D object structure. Adversely, the coding of fringe projection is ambiguous. To determine object points with absolute position in 3D space, this coding has to be unique.
We propose a novel approach of phase unwrapping without using additional pattern projection. Based on a stereo camera setup, an image segmentation of each view in areas without height jumps larger than a fringe period is necessary. Within these segments, phase unwrapping is potentially without error. Alignment of phase maps between the two views is realized by an identification process of one correspondence point.
An optical three-dimensional (3-D) sensor based on a fringe projection technique that realizes the acquisition of the surface geometry of small objects was developed for highly resolved and ultrafast measurements. It realizes a data acquisition rate up to 60 high-resolution 3-D datasets per second. The high measurement velocity was achieved by consequent fringe code reduction and parallel data processing. The reduction of the length of the fringe image sequence was obtained by omission of the Gray code sequence using the geometric restrictions of the measurement objects and the geometric constraints of the sensor arrangement. The sensor covers three different measurement fields between 20 mm×20 mm and 40 mm×40 mm with a spatial resolution between 10 and 20 μm, respectively. In order to obtain a robust and fast recalibration of the sensor after change of the measurement field, a calibration procedure based on single shot analysis of a special test object was applied which works with low effort and time. The sensor may be used, e.g., for quality inspection of conductor boards or plugs in real-time industrial applications.
Fringe projection is an established method to measure the 3D structure of macroscopic objects. To achieve both a high accuracy and robustness a certain number of images with pairwise different projection pattern is required. Over this sequence it is necessary that each 3D object point corresponds to the same image point at every time. This situation is no longer given for measurements under motion. One possibility to solve this problem is to restore the static situation. Therefore, the acquired camera images have to be realigned and secondly, the degree of fringe shift has to be estimated. Furthermore, there exists another variable: change in lighting. The compensation of these variances is a difficult task and could only be realized with several assumptions, but it has to be approximately determined and integrated into the 3D reconstruction process. We propose a method to estimate these lighting changes for each camera pixel with respect to their neighbors at each point in time. The algorithms were validated on simulation data, in particular with rotating measurement objects. For translational motion, lighting changes have no severe effect in our applications. Taken together, without using high-speed hardware our method results in a motion compensated dense 3D point cloud which is eligible for three-dimensional measurement of moving objects or setups with sensor systems in motion.
Measuring the three-dimensional (3D) surface shape of objects in real time has become an important task e.g. in
industrial quality management or medical sciences. Stereo vision-based arrangements in connection with pattern
projection offer high data acquisition speed and low computation time. However, these coded-light techniques
are limited by the projection speed which is conventionally in the range of 200. . .250Hz.
In this contribution, we present the concepts and a realized setup of a so-called 3D array projector. It is
ultra-slim, but nonetheless able to project fixed patterns with high brightness and depth of focus. Furthermore,
frame rates up to the 100 kHz range are achievable without any need of mechanically moving parts since the
projection speed is limited mainly by the switching frequency of the used LEDs. According to the measurement
requirements, type and structure of the patterns can be chosen almost freely: linear or sinusoidal fringes, binary
codes such as the Gray code, square, hexagonal or random patterns and many more.
First investigations on the functionality of such a 3D array projector were conducted using a prototype with
a combination of Gray codes and phase-shifted sinusoidal fringes. Our contribution proves the high brightness
of the proposed projector, its sharpness and the good Michelson contrast of the fringe patterns. We deal with
the patterns’ homogeneity and the accuracy of the phase shift between the sinusoidal patterns. Furthermore, we
present first measurement results and outline future research which is, inter alia, addressed to the use of other
structured light techniques with the help of new purpose-built 3D array projector prototypes.
A sensor based on fringe projection technique was developed which allows ultrafast measurements of the surface of flat
measuring objects which realizes a data acquisition rate up to 8.9 million 3D points per second. The high measuring
velocity was achieved by consequent fringe code reduction and parallel data processing. Fringe sequence length was
reduced using geometric constraints of the sensor arrangement including epipolar geometry. Further reduction of the
image sequence length was obtained by omission of the Gray code sequence by using the geometric constraints of the
measuring objects. The sensor may be used e.g. for inspection of conductor boards.