Papers by Keyword: Precision Metrology

Abstract: Laser scanners have become very popular means of point clouds acquisition and 3D reconstruction. However, the accuracy specifications given by laser scanner producers in their publications and pamphlets are not consistent. Experience shows that sometimes these should not be trusted. Therefore, how to reasonably evaluate and quantify main technical parameters of the instruments is particularly important. It is very necessary to investigate in calibration devices and methods that are suitable for laser scanners. The 80m indoor ranging facility of the National Institute of Metrology (NIM) at Beijing, China, is equipped with an interferometer comparator, precise air flotation rail systems and environmental parameter automatical compensation system. The facility is a closed loop feedback control system and can provide a high-precision reference distance value with different intervals. A expanded standard uncertainty (k=2) of U(L_s) is superior to 0.5μm+0.5×10^-6L. This paper describes the calibration principle and procedure, including mounting white standard spheres with surfaces of diffuse reflection at the mobile platform as non-contact targets, scanning and measuring the white spheres , fitting the coordinates of the sphere centers and calculating the relative distance among sphere centers. As an example, this paper also presents analysis and evaluation on the measurement uncertainty of ranging accuracy.

Abstract: A movable pentaprism is a key element in deflectometric profilers, where it directs the beam of an angle measuring device towards the surface under test and enables at the same time the flexible lateral displacement of the beam footprint on the surface. The beam deflection angle of the pentaprism is robust with regard to changes in its angular orientation. Optimal stability is achieved, however, only when the angle measuring device, the pentaprism, and the surface have initially been properly aligned. A newly developed procedure enables the rapid and accurate in-situ adjustment of all angles of the optical components in deflectometric set-ups with an uncertainty of several microrad. In combination with precision mechanical stages, variations in the prism’s deflection angle (caused by changes in its angular orientation) can then be limited to the nanorad level.

Abstract: To overcome the limitations of conventional interferometry, a technique has been developed which allows the absolute topography measurement of near-plane and slightly curved optical surfaces of arbitrary size with low measurement uncertainty. The Extended Shear Angle Difference (ESAD) method combines deflectometric and shearing techniques in a unique way to minimize measurement errors and to optimize measurand traceability. A device for the topography measurement of optical surfaces up to 500 mm in diameter, achieving sub-nanometer repeatability, reproducibility and uncertainty, was built at the Physikalisch-Technische Bundesanstalt (PTB). The ESAD method is optimally suited for creating a primary standard for straightness and flatness with highest accuracy by which the three-flat test or liquid mirrors can be replaced as starting points of the traceability chain in flatness measurement. In the following, the improved ESAD device which uses optimized opto-mechanical components is presented. Central aspects of the proper design and use of deflectometric systems are highlighted, including the optimal use of pentaprisms.

Abstract: The precision positioning device becomes significant requirement in scientific instruments used for the applications of nanotechnology for a few decades. To achieve the high resolution positioning, the sensing methods of displacement sensor become more important in positioning device. In this paper, a novel method is presented for measuring both in-plane/out-plane displacements with the single heterodyne grating interferometry (HGI). We demonstrated the 1D, 2D, and 3D measurement results respectively and the smallest displacement can be detected was better than 6 pm. Furthermore, the nanometer resolution can be ensured within 20 µm displacement. Hence, the in-plane/out-plane measurements with single apparatus can be realized by our method and might be a displacement sensor using in the motorized stage with suitable opto-mechanics structure minimization.

Abstract: A wide-range laser auto-collimation method to measure the surface profiles of targets with steep slopes has been proposed. This method employs a deflecting optical system to enlarge the measurement range of the angle sensor. However, this optical system also changes the sensitivity property of the angle sensor. Therefore, we propose a new in-situ self-calibration method that includes the geometrical designed data of the target and can be applied to wide-range laser auto-collimation method. In the proposed method, the calibration sensitivities at each measurement point are obtained with two sets of measured data, which are acquired before and after performing minute rotation or translation with respect to the measured target, and the simulated shift of the normal angle at each point. The present paper describes the calibration procedure and specific calculation method used in the proposed method.

Abstract: A novel scanning probe measurement system has been developed to achieve precise profile measurements of micro-aspheric surfaces. The system consists of a scanning stage (a spindle and a linear slide) and a sensor unit. The sensor unit consists of a ring artifact, two capacitance sensors and a contact-mode displacement sensor. The two capacitance sensors scan the surface of the ring artifact to measure and compensate the error motions of the scanning stage while the contact-mode displacement sensor scans the surface of a micro-aspheric. In this paper, a new contact-mode displacement sensor that has a small contact force of less than 2.3 mN and a stable output has been developed. After investigating the fundamental performance of the contact-mode displacement sensor, the sensor has been applied to the micro-aspheric surface profile measurement system. The effectiveness of the measurement system has been verified by the measurement results.

Abstract: A simple and practical calibration method is proposed to calibrate the articulated arm flexible CMM. Firstly, a quasi-spherical coordinate system is put forward based on DH method according to the special structure of the articulated arm flexible CMM. This calibration method is presented to determine and separate machine parameters and errors in a creative way based on multiple reversal technology. Experiments were carried out for a Faro Platinum 4ft articulated arm CMM by using a Chameleon 9159 5Q orthogonal CMM. The results prove the feasibility of the proposed method for articulated arm CMM calibration, and all parameters are separated completely. It provides a theoretical base for introducing error compensation and improves the measurement accuracy of the articulated arm CMM.