Heui Jae Pahk

Ultra precision positioning system for servo motor–piezo actuator using the dual servo loop and digital filter implementation

The ultra precision positioning technique has become one of the important parts in the development of precision machines. For positioning systems having a long stroke with ultra precision, a combined system including a global stage (coarse stage) and a micro stage (fine stage) is designed in this paper. A ball screw based servo motor is used as the global stage and a piezo actuator as the micro stage, in which some design parameters are analysed. To improve positional accuracy and remove the noise components of motion, the digital Chebyshev filter has been designed and implemented. The implemented digital filter has been found to improve accuracy by as much as three times. Positional accuracy has been readily achieved within 10 nm over the typical 200 mm stroke, and therefore this technique can be applied to develop high precision semiconductor equipment such as lithography steppers and probers.

A new technique for volumetric error assessment of CNC machine tools incorporating ball bar measurement and 3D volumetric error model

This paper presents a useful technique for assessing the volumetric errors in multiaxis machine tools using a kinematic double ball bar. This system has been developed based on a volumetric error model which describes the three-dimensional errors of machine tools. The developed system requires input of the measured radial data performed on the three orthogonal planes, and analyzes parametric errors such as positional, straightness, angular, squareness, and backlash errors. The developed system can also assess the dynamic performance of machine tools such as errors due to the servo gain mismatch. The developed system has been tested using an actual machine tool, showing high potential for error assessment of multiaxis machine tools.

Development of straightness measurement technique using the profile matching method

The property of straightness is one of fundamental geometric tolerances to be strictly controlled for guideways of machine tools and measuring machines. Straightness measurement for long guideways is usually difficult to perform, and it requires additional equipment or special treatment with limited applications. In this paper, a new approach is proposed using the profile matching technique for the long guideways, which can be applied to most straightness measurements. An edge of relatively short length is located along a divided section of a long guideway, and the local straightness measurement is performed. The edge is then moved to the next section with several overlap points. After the local straightness profile is measured for every section along the long guideway with overlap, the global straightness profile is constructed using the profile matching technique based on the least-squares method. The proposed technique is numerically tested for two cases of known global straightness profile: arc profile and irregular profile, with and without random error intervention, respectively. The developed technique has been practically applied to a vertical milling machine of the knee type, and demonstrated a good performance. Thus the accuracy and efficiency of the proposed method are demonstrated, and show great potential for a variety of applications for most straightness measurement cases, including straight edges, laser optics and angular measurement equipments.

Precision inspection system for aircraft parts having very thin features based on CAD/CAI integration

In this paper, a precision inspection technique using CAD/CAI integration is proposed for parts having very thin and sharply curved features. The technique begins with feature reconstruction of turbine blades which have combined geometry, such as splines, and thin small radius circles. The alignment procedures consists of two phases — rough and fine phases: the rough phase alignment is based on the conventional 6 points probing on the clear cut surfaces, and the fine phase alignment is based on the initial measurement of the curved parts using the least-squares technique based on iterative measurement feedback. For the analysis of profile tolerance of parts, the actual measured points are obtained by finding the closest points on the CAD geometry by the subdivision technique developed. The Tschebyscheff norm is applied iteratively, giving an accurate profile tolerance. The inspection technique developed is applied to practical blade manufacturing, and has demonstrated good performance.

A new method of noncontact measurement for 3D micro pattern in semiconductor wafer: implementing a new optical probe

In this paper, a new type of noncontact/nondestructive measuring method has been developed, comprising a new optical probe, precision stages, and controllers. The new optical probe has been designed using the reflected slit beam through the optical window. The optical measurement system has been implemented around light illumination, optical lenses, optical window, and beam splitter. Efficient analysis algorithm has been developed in order to analyse the 3D micro pattern and some parameters of the surface. The developed system has been successfully applied to measure the wafer surface, giving nanometer vertical resolution.

Thermal Error Measurement and Real Time Compensation System for the CNC Machine Tools

Thermally induced errors have been significant factors affecting the machine tool accuracy. In this paper, the spindle thermal error and thermal feed axis error have been focused, the measurement/compensation of thermal error is introduced. In order to compensate the thermal error in real time bases, the coordinates of the CNC machine tool are modified in PC-based environment. After the real time compensation, the machine tool accuracy has been improved about 4–5 times typically.

Precision alignment technique for parts having thin features using measurement feedback iterative method in CAD/CAI environment

Abstract In this paper, a new technique of CAD/CAI integration is proposed for the three-dimensional (3D) curved parts having very thin and sharp geometry. The technique begins with feature reconstruction of turbine blades which have 3D combined geometry, such as splines, and thin circles. The alignment procedures consist of two phases: rough phase and fine phase alignments. The rough phase alignment is based on the conventional six points probing on the clear cut surfaces, and the fine phase alignment is based on the initial measurement for the 3D curved parts, using an iterative least squares technique based on the measurement feedback. As the results of the alignment, the CNC code of measurement operation is generated in DMIS file format, thus dimensional inspection of complicated parts having thin sections is completed. The developed inspection technique has been applied to practical procedures of blade manufacturing, and demonstrated high performance.

Residual vibration reduction of white-light scanning interferometry by input shaping.

White-light scanning interferometry is widely used for precision metrology of engineering surfaces. It needs a mechanical scanning for capturing an interferogram that determines where the surface of a measured sample is located. The residual vibration during the scanning procedure distorts the interferogram and it reduces the accuracy and the precision of the system. The residual vibration becomes bigger as the proportional gain gets higher for the fast response. So it is hard to achieve the fast and precise measurement simultaneously. In this study, input shaping which convolves a reference signal with the input shaper is investigated to reduce the residual vibration of the scanning system. The step response data is analyzed using Continuous Wavelet Transform (CWT) to design the input shaper. Using proposed method, the residual vibration of the white light scanning interferometry is reduced and it achieved both faster measurement speed and more accurate measurement.

Computer aided measurement and compensation system for focal length of lenses in camera manufacture based on the MTF performance using the line CCD sensor

Abstract A computer aided system has been developed for the focal length measurement/compensation in camera manufacture. Signal data proportional to light intensity are obtained and sampled very rapidly from the line CCD. Based on the measured signal, the MTF performance is calculated, where the MTF is the ratio of magnitude of the output image to the input image. In order to find the optimum MTF performance, an efficient algorithm has been implemented using the least squares technique. The developed system has been applied to a practical camera manufacturing process, and demonstrated high productivity with high precision

Critical dimension measurement of transparent film layers by multispectral imaging

An optical microscopy system as a non-destructive method for measuring critical dimension (CD) is widely used for its stability and fastness. In case of transparent thin film measurement, it is hard to recognize the pattern under white light illumination due to its transparency and reflectance characteristics. In this paper, the optical measurement system using multispectral imaging for CD measurement of transparent thin film is introduced. The measurement system utilizes an Acousto-Optic Tunable Filter (AOTF) to illuminate the specimen with various monochromatic lights. The relationship between spectral reflectance and CD measurement are deduced from series of measurement experiments with two kinds of Indium Tin Oxide (ITO) patterned samples. When the difference of spectral reflectance between substrate and thin film layers is large enough to yield a large image intensity difference, the thin film layer can be distinguished from substrate, and it is possible to measure the CD of transparent thin films. This paper analyzes CD measurement of transparent thin film with reflectance theory and shows that the CD measurement of transparent thin film can be performed successfully with the proposed system within a certain wavelength range filtered by AOTF.