Background oriented schlieren measurement of the refractive index field of air induced by a hot, cylindrical measurement object

In the manufacturing process of Tailored Forming components, the inline inspection of the joining zone directly after each single process step can yield advantages -such as early error detection and real-time process control. Since measuring times need to be synchronized with the production chain, there is no time to cool down the components in between two hot forming processes. On the one hand, the chosen measurement technique needs to be non-tactile due to the heat of the measurement object. On the other hand, the object's areal surface texture needs to be captured rapidly to realize a fast inline inspection. These requirements are only matched by optical 3d measurement systems. Additional challenges arise due to the high temperature of the Tailored Forming components: the ambient air is heated up and the air's temperature increase results in an inhomogeneous refractive index field surrounding the hot workpiece, effecting the light's path emitted by the illumination unit of the optical sensor. We present a simple measurement setup based on the laser light section method to visualize the measurement accuracy loss induced by the convectional heat flow from a hot cylindrical measurement object. To attain a direct validation of the measurement results, the measurements are performed with and with reduced influence of the inhomogeneous refractive index field induced by the hot object.

show abstract

“…‡ The figure is taken from Beermann et al. 8 The journal paper is accepted and will be published in 2017.…”

Section: Discussionmentioning

confidence: 99%

Light section measurement to quantify the accuracy loss induced by laser light deflection in an inhomogeneous refractive index field

et al. 2017

Self Cite

View full text Add to dashboard Cite

show abstract

“…22 Equation 5allows the derivation of six coupled first-order ordinary differential Eqs. (6) and (7) given in vector notation:…”

Section: Theoretical Backgroundmentioning

confidence: 99%

“…6 The resulting 3-D refractive index field's (RIF) shape, extension, and magnitude is time-variant and highly depends on the object's temperature, geometry, and the present air flow conditions. 7 The light propagation is affected, as its path is bent toward more dense air layers. Most articles in this field neglect this deflection effect, [8][9][10][11] which is legitimate if the light path deflection is too small to be reproduced by the applied measurement system.…”

Section: Introductionmentioning

confidence: 99%

Full simulation model for laser triangulation measurement in an inhomogeneous refractive index field

et al. 2018

Self Cite

View full text Add to dashboard Cite

The optical inspection of wrought hot workpieces between subsequent forming steps of a multistage process chain can yield diverse advantages. Deficient components can be detected in an early forming stage. Moreover, the eliminated cooling economizes heating energy. The present workpiece temperature can be exploited in the following chain steps. Challenges arise due to the heat input into the air surrounding the workpiece, as triangulation techniques rely on homogeneous optical conditions. The effect of an inhomogeneous refractive index field (RIF) in air on a 3-D geometry measurement by optical triangulation is modeled exemplary by a virtual measurement of a hot cylinder. To our knowledge, this is the first simulation approach that fully considers both light deflection from the illumination unit to object and from object to camera. Simulated measurement results in a homogeneous and an inhomogeneous RIF are compared. The presented approach predicts measurement deviations in inhomogeneous optical media and can help to design actuated or computer-assisted compensation routines in order to reduce deflection effects when measuring hot objects.

show abstract

“…The principle outline of the developed BOS setup in combination with a vacuum chamber is depicted in figure 2. The 2D BOS setup design is based on a former implementation, 6 but is now adjusted to the specific low pressure application. It comprises a monochromatic camera and a heating rod that is used to generate an inhomogeneous refractive index field by convective density flow due to heat input into the surrounding air.…”

Section: Refractive Index Measurement: Theory and Principle Outline Omentioning

confidence: 99%

2D refractive index field measurements in air in different pressure scenarios

Beermann

Quentin

Reithmeier

et al. 2018

Optical Metrology and Inspection for Industrial Applications V

Self Cite

View full text Add to dashboard Cite

The optical geometry characterization of wrought hot components can help to quantify material distortion effects during air-cooling. The component's shrinkage behavior is affected by inhomogeneous heat dissipation due to the object's complex geometry and-in case of hybrid materials-differing thermal expansion coefficients. As optical triangulation techniques rely on the rectilinear expansion of light, the hot component's heat input into the surrounding medium air influences the reachable accuracy of optical geometry measurements due to an inhomogeneous refractive index field around the hot component. In previous work, the authors identified low pressure measurements in air as a possible approach to reduce the magnitude and expansion of the inhomogeneous refractive index field above cylindrical high-temperature objects and thereby allow precise geometry acquisition. We now present experimental data of the 2D refractive index field above a hot cylinder in different pressure scenarios using the well-known background oriented schlieren (BOS) method in order to illustrate the decrease in refractive index variations dependent on the pressure state. For this purpose, a ceramic rod is placed in a vacuum chamber and heated up to temperatures of about 1000 • C. Using a monochromatic camera, a wavelet background and an optical flow algorithm, the developing 2D refractive index field for a low pressure scenario is compared to ambient pressure conditions. The experimental data illustrates a reduction in the convective heat flow above the hot heating rod at lower pressure values and therefore a homogenization of the density-coupled refractive index in air, validating former simulation results.

show abstract

Background oriented schlieren measurement of the refractive index field of air induced by a hot, cylindrical measurement object

Cited by 29 publications

References 14 publications

Light section measurement to quantify the accuracy loss induced by laser light deflection in an inhomogeneous refractive index field

Light section measurement to quantify the accuracy loss induced by laser light deflection in an inhomogeneous refractive index field

Full simulation model for laser triangulation measurement in an inhomogeneous refractive index field

2D refractive index field measurements in air in different pressure scenarios

Contact Info

Product

Resources

About