We present a novel optical system for distance measurement based on the combination of optical time-of-flight metrology and digital holography. In addition absolute calibration of the measurement results is performed by a sideband modulation technique. For the time-of-flight technique a diode laser (1470 nm) is modulated sinusoidally (128 MHz). The light reflected and scattered by an object is detected by an avalanche-photo-diode. The phase difference between the sent and detected modulation is a measure for the distance between the sensor and the object. This allows for distance measurements up to 1.17 m with resolutions of ~2 mm. The interferometric setup uses 4 whispering-gallery-mode lasers to perform multiwavelengths-holographic distance measurements. The four wavelengths span the range from 1547 nm to 1554 nm. The unambiguous measurement measurement-range of the interferometric setup is approx. 7 mm while resolutions of 0.6 μm are observed. Both setups are integrated into one setup and perform measurements synchronously. Exact knowledge of the frequency differences of hundreds of GHz between the four lasers is crucial for the interferometric fine scale measurement. For this aim the light of the lasers is phase-modulated with frequencies of 36 GHz and 40 GHz to produce optical sidebands of higher order, thus generating beat signals in the hundreds-of-MHz regime, which can be measured electronically. The setup shows a way to measure distances in the meter range with sub-micron resolution.
A continuous increase in production speed and manufacturing precision raises a demand for the automated detection of small image features on rapidly moving surfaces. An example are wire drawing processes where kilometers of cylindrical metal surfaces moving with 10 m/s have to be inspected for defects such as scratches, dents, grooves, or chatter marks with a lateral size of 100 μm in real time. Up to now, complex eddy current systems are used for quality control instead of line cameras, because the ratio between lateral feature size and surface speed is limited by the data transport between camera and computer. This bottleneck is avoided by “cellular neural network” (CNN) cameras which enable image processing directly on the camera chip. This article reports results achieved with a demonstrator based on this novel analogue camera – computer system. The results show that computational speed and accuracy of the analogue computer system are sufficient to detect and discriminate the different types of defects. Area images with 176 x 144 pixels are acquired and evaluated in real time with frame rates of 4 to 10 kHz – depending on the number of defects to be detected. These frame rates correspond to equivalent line rates on line cameras between 360 and 880 kHz, a number far beyond the available features. Using the relation between lateral feature size and surface speed as a figure of merit, the CNN based system outperforms conventional image processing systems by an order of magnitude.
This paper reports measurement results and some design details of railway measurement systems based on optical principles.
The quality of railway lines is crucial for reliability and safety and therefore to be controlled regularly. Special measuring vehicles operate permanently on all railway lines, even during regular traffic situations. Therefore the measurement systems used to record the data have to be fast enough even at high speeds and robust enough to provide reliable data under almost any environmental conditions. The application of optical methods is advantageous concerning accuracy and speed but of course limited by external influences. We report here measures enabling even a sensitive optical measurement principle, the phase measurement technique, to be applied under these harsh environmental conditions. Exemplarily the optical and mechanical design of a clearance profile scanner is described. It is shown how to make the sensor insensitive against environmental conditions like contamination by dust or water or temperature changes. Measurement results of this scanner and of another system to measure the position of the contact wire are presented.
This paper reports a theoretical description of a recently proposed length sensor using the laser-feedback technique. It is found that the optical feedback causes a sinusoidal fluctuation of the emission frequency with respect to time. The amplitude of the fluctuation strongly depends on the power reflectivity of the target and therefore the target distance. For target power reflectivities below 10-4 stable single-mode laser oscillation is maintained and accurate measurements of target displacement are possible. Good agreement between theoretically calculated device output and measured device output has been obtained. Furthermore typical application examples for dielectric and metallic target materials are presented and some performance characteristics are discussed.
The acquirement of 3D-information is currently achieved by stereophotography, line and grid projection techniques or by laser scanning in combination with a fast distance measuring device. This paper describes a new principle using a single CCD-camera with an optical demodulator in front of it. The scene is illuminated by a high frequency intensity modulated light source. Demodulating the backscattered light by a gateable image intensifier yields a grey level image which directly corresponds to the object's form. Intensity variations within the image due to inhomogeneous object reflectivity or illumination intensity are overcome by a phase shift technology. Possible applications for such a 3D- camera industrial automation, medical and industrial endoscopic analyses, robotics or 3D-digitalization.
We present a novel velocity and length measuring instrument based on laser-feedback interferometry. Uncertainties down to 10-4 in length measurement of moving materials can be achieved by combining the measurement signals of two separate sensors. Furthermore the dependence on parameters like the angle of incidence can be eliminated mathematically and no accurate alignment is required. The signal processing unit utilizes a real-time fast Fourier transformation. Thus good signal to noise ratios and very accurate measurements of most scattering surfaces used in industrial production are possible.
The Optical Radar System (ORAS) provides a complete set of data to derive 3D surface contours of bodies or of body-like scenes primarily in industrial environment. In addition, the intensity as back-scattered from the object is recorded in order to create combined (3D and conventional) images for special tasks in object characterization. ORAS as described in this paper is based on the time of flight principle. Its specifications, measurement results and applications are presented.
The Optical Radar System (ORAS) provides a complete set of data to derive three-dimensional
surface contours of bodies or of body-like scenes primarily in industrial environment. In addition,
the intensity as back-scattered from the object is recorded in order to create combined (3-D and
conventional) images for special tasks in object characterization.
ORAS as described in this paper is based on the time of flight principle. Its specifications,
measurement results and applications are presented.
In the present paper an attempt has been made to immerse the micromechanical resonant structure in the gas medium and investigate the pressure and temperature sensitivities.
By means of a closed loop between the exciting laser diode source,the mechanical resonator and the interrogating detector diode it is possible to design a self-oscillating system,which automatically provides the correct mechanical resonance frequency.This simple and economic data reduction system seems to operate quite efficiently.
Dimensional measurement techniques are requested to determine distance and length measurement, for two and three dimensional profiling or for volume determination. To meet the requirements of numerous tasks in process monitoring or quality control different measurement principles are available. Out of the 3-D methods, the reported optical radar shows special advantages: * short data acquisition period, * co-axial optics, * high accuracy for the determination of the distance, and * quick and straightforward data evaluation. In this context, a system based on the method of optical radar is described in more detail. The system provides data for three-dimensional scene analysis with the spatial coordinates of a surface to be determined within a pre-defmed volume. A complete 3-D picture with 500 x 500 pixels is recorded and evaluated within less than 2 seconds. The role of speckles limiting the accuracy of the measurement is described in some detail, too.
For quality control, volume determination, process monitoring and other tasks in industry, the application of 3-D measurement techniques is required. Out of the available 3-D methods, the optical radar shows special advantages. In this context, a system based on the method of optical radar as well as first results is described in more detail. The new system provides data for 3-D scene analysis with the spatial coordinates of a surface to be determined in a pre- defined volume. A complete 3-D picture with 500 X 500 pixels is recorded and evaluated within less than 2 seconds.
A new type of moire reference grid on the surface of a CCD-chip is presented. In surface contouring an improvement of the resolution by a factor of 4 is achieved. The modification steps to generate the new reference grid are simple. Any further modification of the internal image size or the electronic design of the CCD-camera is not necessary.
Fluctuations of the refractive index of air causes nowadays the largest errors in high precise interferometric displacement measuring systems. We present in this paper a new method for compensating these influences by tuning the optical frequency of the laser. Variing laser temperature and current regulates the frequency (wavelength) of a GaA1As-semiconductor laser in such a manner that the wavelength in air is constant.