Sara Rosendahl

Shape verification using dual-wavelength holographic interferometry

In automotive industry there is an interest of controlling the shape of a large number of identical components on-line in the manufacturing process. We propose a method to do this by capturing a digital hologram of the object and then using information from its computer aided design (CAD) model to calculate the shape and determine the agreement between the manufactured object and the CAD-model. The holographic recording of the object is done using dual wavelengths with a synthetic wavelength of approximately 400 μm. The optical measurement results in a wrapped phase map with the phase values in the interval [ − π, π]. Each phase interval represents a depth distance on the object of about 0.2 mm. The phase unwrapping is done iteratively using information from the CAD-model. This implies that it is possible to measure large discontinuities on the surface of the measured object. The method also gives a point-to-point correspondence between the measurement and the CAD-model which is vital for tolerance control.

Phase errors due to speckles in laser fringe projection

When measuring a three-dimensional shape with triangulation and projected interference fringes it is of interest to reduce speckle contrast without destroying the coherence of the projected light. A moving aperture is used to suppress the speckles and thereby reduce the phase error in the fringe image. It is shown that the phase error depends linearly on the ratio between the speckle contrast and the modulation of the fringes. In this investigation the spatial carrier method was used to extract the phase, where the phase error also depends on filtering the Fourier spectrum. An analytical expression for the phase error is derived. Both the speckle reduction and the theoretical expressions for the phase error are verified by simulations and experiments. It was concluded that a movement of the aperture by three aperture diameters during exposure of the image reduces the speckle contrast and hence the phase error by 60%. In the experiments, a phase error of 0.2 rad was obtained.

Shape measurement with one fringe pattern recording including a digital master

We present a method in which the 3D shape of an object can be measured and compared to the shape of the digital master of the object, e.g., the computer-aided design model. The measurement is done using a stereo camera system and a single projected fringe pattern. Because the digital master is available, i.e., the expected shape is known, only one projection and image recording is necessary; thus, the method becomes fast. The idea in this work is to find homologous points in the cameras, i.e., points corresponding to the same object point, using the object information. An algorithm to find the homologous points is presented and a method to calculate shape is described. Given the ambiguity due to the fact that the phase in the images is wrapped, there is a maximum deviation from the master that can be correctly detected. An analytical expression for this deviation is derived. Results from the shape measurement of an object both with and without deviations from the digital master are also presented. In these measurements, where the measurement volume is approximately 1 dm3 and the fringe period on the object plane is about 1 mm, the accuracy is ≈±40 μm, and a deviation of max ≈±1.6 mm can be correctly detected.

Investigation of ice surface change during vehicle testing

Today, there are a lot of vehicles and tyre testing carried out on lake ice surfaces. Thus, it is important to have knowledge about parameters that affect roadgrip. The thesis within this paper is ...

Dual-wavelength holographic shape measurement with iterative phase unwrapping

In order to measure the shape of a large number of identical components in a manufacturing industry we propose a method where digital holography is used to capture an image of the object and then the shape of the object is achieved by using information from the CAD-model. The holographic recording of the object is done using dual wavelengths giving a synthetic wavelength of about 400 μm. This gives a phase map where the phase intervals represent a depth distance on the object of about 0.2 mm. To find the shape of the object the phase map has to be unwrapped. Since the surface contains discontinuities we use information from the CAD-model of the measured object and unwrap the phase iteratively. The result becomes a digital point representation of the measured surface that can either be used just as a description of the object shape or as a way to describe how well the object has been manufactured compared to the CAD-model. The measurement process that is proposed is adapted for on-line purposes; hence it is fast and reliable.

Evaluation of velocity and curvature dependence for roadgrip measured by low lateral slip

Roadgrip is an important parameter for vehicle testing and road maintenance. Therefore, an evaluation of the velocity and curvature effects on roadgrip measurement was performed on asphalt roads and on two ice tracks using the continuous roadgrip apparatus RT3 Curve. The aim was to find suitable driving patterns for measurements on public roads and test tracks to ensure the repeatability of roadgrip measurements. During the evaluation, it was concluded that in order to achieve a reliable roadgrip value, regardless of road conditions, the radius of curvature should not be less than 20 m. The velocity dependency of the RT3 Curve is different for the two road conditions, with the measurements on ice being much more sensitive to velocity changes than the measurements on the dry asphalt.

Phase errors in speckle reduced laser fringe projection

When measuring 3D-shape with triangulation and projected interference fringes it is of interest to reduce the phase error in the fringe pattern. A study has been carried out concerning parameters that will affect the phase error and an analytical expression has been derived. It is concluded that the phase error depends on the speckle contrast, C, and the modulation, M, of the fringes and since the phase in this investigation is determined using the spatial carrier method the phase error also depends on the filtering of the Fourier spectrum. To reduce the phase error this work has been focusing on suppressing the speckle contrast. For this the method with a moving aperture is used; a disk with several apertures is rotated in the aperture plane of the camera lens. To verify the derived expression for the phase error and the method to suppress speckles both numerical simulations and experiments have been performed. In the measurements made it was concluded that after an aperture movement of three aperture diameters the speckle contrast and hence the phase error was reduced by 60 %. A phase error of 0.15 radians was obtained in the experiments, thus approximately 1/40 of a fringe period.