Design, Setup, and Evaluation of a Compensation System for the Light Deflection Effect Occurring When Measuring Wrought-Hot Objects Using Optical Triangulation Methods

Abstract

<jats:p>In this paper, we present a system to compensate for the light deflection effect during the optical geometry measurement of a wrought-hot object. The acquired 3D data can be used to analyze the formed geometry of a component directly after a hot forging process without waiting for the needed cooling time to room temperature. This may be used to parameterize the process and to detect defect components early in the production process, among others. The light deflection as the deviation from the linear path of the light is caused by an inhomogeneous refractive index field surrounding the hot object. We present the design and setup for a nozzle-based forced air flow actuator, which suppresses the light deflection effect. The design process includes a simulation of the developing field, as well as of the interaction of the field with an external forced air flow. The cooling effect of the air flow is evaluated, and conclusions are drawn from the conflicting interests of good measurement conditions against the forced cooling of the hot object. The findings are then implemented in the physical setup of the suppression system. The system is evaluated using a previously established method based on optical triangulation and fringe projection. Other occurring effects and their influence on the evaluation are considered and discussed.</jats:p>

Adapted Fringe Projection Sequences for Changing Illumination Conditions on the Example of Measuring a Wrought-Hot Object Influenced by Forced Cooling

“…In reference to the reconstruction quality metric, we presented a setup for compensating the light deflection effect when measuring the geometry of hot specimen [ 19 ]. The setup consists of an additional forced air flow actuator based on nozzles and pressurized air to suppress the inhomogeneous refractive index field developing around a hot object.…”

“…An extensive description of the physical setup is beyond the scope of the paper. The interested reader is referenced to [ 17 , 19 ].…”

“…The presented methods are also evaluated in the design application, i.e., when measuring hot components. By this means, the compensation method for the light deflection effect [ 19 ] is used and the presented methods are applied in order to shorten the gap w.r.t. the reconstruction quality metric between a cold reference measurement and a hot measurement under the influence of a forced air flow.…”

“…the delay time between activation of the flow actuator and the start of the measurement. This delay time was previously shown to be an influence on the established reconstruction quality metric [ 19 ]. In the present paper, the following delay times are investigated of which are assumed to be the time between the activation of the flow actuator and the full development of the flow field.…”

“…In previous papers, we have presented a method to estimate the influence of refractive index gradients on the results of optical 3D geometry measurements of hot specimen by evaluation of the reconstruction quality [ 17 ]. Additionally, we have discussed the use of a forced air flow similar to the air knife of [ 18 ] to compensate the light deflection effect when measuring the 3D geometry of red-hot specimens [ 19 ]. The validity of the compensation was evaluated using the developed reconstruction quality.…”