Jong-Ahn Kim

Six-degree-of-freedom displacement measurement system using a diffraction grating

Six-degree-of-freedom displacement measurement systems are applicable in many fields: precision machine control, precision assembly, vibration analysis, and so on. This article presents a new six-degree-of-freedom displacement measurement system utilizing typical features of a diffraction grating. It is composed of a laser source, three position sensitive detectors, a diffraction grating target, and several optical components. Six-degree-of-freedom displacement is calculated from the coordinates of diffracted rays on the detectors. A forward and an inverse problem were solved to compute the full pose of an object through kinematic analysis. The experimental results show that the measurement system had a maximum error of ±10 μm for translation and ±0.012° for rotation. The repeatability is about 10 μm for translation and 0.01° for rotation.

Design methods for six-degree-of-freedom displacement measurement systems using cooperative targets

Six-degree-of-freedom displacement measurement systems generally employ various kinds of cooperative targets, and calculate displacement from the coordinate values of reflective rays. To improve the performance of measurement systems, a new design methodology is required, which considers the relationship between each axial output and the coordinate values, Jacobian matrix. Several performance indices are derived from Jacobian matrix and the design parameter values are determined through the optimization process. In a design example, the improvement of performance is verified by evaluating resolution, accuracy and crosstalk. The newly designed measurement system shows a maximum error of ± 0.5 μm for translation and ± 2 arcsec for rotation.

A passive method to compensate nonlinearity in a homodyne interferometer

This study presents an analysis of the nonlinearity resulting from polarization crosstalk at a polarizing beam splitter (PBS) and a wave plate (WP) in a homodyne interferometer. From a theoretical approach, a new compensation method involving a realignment of the axes of WPs to some specific angles according to the characteristics of the PBS is introduced. This method suppresses the nonlinearity in a homodyne interferometer to 0.36 nm, which would be 3.75 nm with conventional alignment methods of WPs.

An optical absolute position measurement method using a phase-encoded single track binary code

We present a new absolute position measurement method using a single track binary code where an absolute position code is encoded by changing the phase of one binary state representation. It can be decoded efficiently using structural property of the binary code, and its sub-division is possible by detecting the relative positions of the binary state representation used for the absolute position encoding. Therefore, the absolute position encoding does not interfere with the sub-division process and so any pseudo-random sequence can be used as the absolute position code. Because the proposed method does not require additional sensing part for the sub-division, it can be realized with a simple configuration and efficient data processing. To verify and evaluate the proposed method, an absolute position measurement system was setup using a binary code scale, a microscopic imaging system, and a CCD camera. In the comparison results with a laser interferometer, the measurement system shows the resolution of less than 50 nm and the nonlinearity error of less than ±60 nm after compensation.

High resolution interferometer with multiple-pass optical configuration

An interferometer having fourteen times higher resolution than a conventional single-pass interferometer has been developed by making multiple-pass optical path. To embody the multiple-pass optical configuration, a two-dimensional corner cube array block was designed, and its symmetric structure minimized the measurement error. The effect from the alignment error and the imperfection of corner cube is calculated as picometer level. An experiment proves that the suggested interferometer has about 45 nm of optical resolution and its nonlinearity is about 0.5 nm in peak-to-valley.

Multidimensional motion measurement of a bimorph-type piezoelectric actuator using a diffraction grating target

Precision actuators, such as pick-up actuators for HDDs or CD-ROMs, mostly show multidimensional motion. So, to evaluate them more completely, multidimensional measurement is required. Through structural variation and optimization of the design index, the performance of a measurement system can be improved to satisfy the requirement of this application, and so the resolution of each axis is higher than 0.1 μm for translation and 0.5 arcsec for rotation. Using this measurement system, the multidimensional motion and frequency transfer functions of a bimorph-type piezoelectric actuator are obtained.

Application of sensitivity analysis for the design of six-degree-of-freedom measurement system

We present a sensitivity analysis and performance evaluation of a six-degree-of-freedom measurement system that uses a diffraction grating as a cooperative target. To design the measurement system, we require a theoretical analysis of performance, such as sensitivity of each sensing direction. The intensity distributions of the diffracted beams are calculated with the scalar diffraction theory and sensitivity of the mea- surement system is analyzed against the variations of design parameter values. Using the results of sensitivity analysis, we design the measure- ment system and evaluate its performance with resolution, measurement error, and crosstalk. The resolution is less than 0.2 mm in translation and 0.5 arcsec in rotation. In experiments, measurement error and crosstalk between sensing channels are within 60.5 mm in translation and 62 arcsec in rotation.

Note: Nonlinearity error compensated absolute planar position measurement using a two-dimensional phase-encoded binary grating

This Note presents a new absolute planar position measurement method using a two-dimensional phase-encoded binary grating and a sub-division process where nonlinearity error is compensated inherently. Two orthogonally accumulated intensity profiles of the image of the binary grating are analyzed separately to obtain the absolute position values in each axis. The nonlinearity error caused by the non-ideal sinusoidal signals in the intensity profile is compensated by modifying the configuration of the absolute position binary code and shift-averaging the intensity profile. Using an experimental setup, we measured a circular trajectory of 100 nm radius, and compared the measurement result with that of a laser interferometer. Applying the proposed compensation method, the nonlinearity error was reduced to less than 15 nm.

Compensation of surface inclination for detecting in optical triangulation sensors

Optical Triangulation Displacement Sensors (OTDSs) are widely used for their simple structure, high resolution, and long operating range. However, errors originating from speckle, inclination of the object, source power fluctuation, ambient light, and noise of the detector limit their usability. In this paper, we propose new design criteria for an error-reduced OTDS. The light source module for the system consists of an incoherent light source and a multimode optical fiber for eliminating speckle and shaping a Gaussian beam intensity profile. A diffuse-reflective white copy paper, which is attached to the object, makes the light intensity distribution on the charge-coupled device (CCD). Since the peak position of the intensity distribution is not related to the various error sources, a sub-pixel resolution signal processing algorithm that can detect the peak position makes it possible to construct an error-reduced OTDS system.

A modified ladar system with a Geiger mode APD to remove a dead time problem

A laser radar (LADAR) system with a Geiger mode avalanche photodiode (GAPD) is used extensively due to its high detection sensitivity. However, this system requires a certain amount of time to receive subsequent signals after detecting the previous one. This dead time, usually 10 ns to 10 μs, is determined by the material composition of the detector and the design of the quenching circuits. Therefore, when we measure objects in close proximity to other objects along the optical axis using the LADAR system with GAPD, it is difficult to separate them clearly owing to the dead time problem. One example for that is a case of hidden objects behind partially transparent blinds. In this paper, we suggested a modified LADAR system with GAPD to remove the dead time problem by adopting an additional linear mode avalanche photodiode (LAPD) as a complementary detector. Because the LAPD does not have dead time while still maintaining relatively low detection sensitivity, the proposed system can measure an object placed within the dead time with high detection sensitivity. Light is emitted from the pulsed laser of a light source and is delivered into a fast photodiode to generate a start signal. Most of laser pulses are directed onto the target and scattered from the surfaces of targets. The scattered light in the field-of-view of the system is divided by a polarizing beam splitter, after which it becomes incident to two different types of APDs, the GAPD and the LAPD. The GAPD receives the signals from the target with high sensitivity, and the signals scattered in the dead time zone are then detected by the LAPD. The obtained signals are analyzed at the same time. In this way, the signals scattered from objects placed within the dead time can be distinguished clearly.