Papers by Author: Wei Gao

Abstract: This paper presents a design study of an optical three-axis angle sensor for measurement of angular error motions of long-stroke precision linear stages. The three-axis angle sensor, which is based on a laser autocollimation method, can detect angular motions about the XYZ-axes simultaneously by using zeroth-and first-order diffraction beams from a diffraction grating mounted on a measuring target as a reflector. In this paper, an optical sensor head is designed in such a way that the three-axis angle sensor shares a diffraction grating with an optical sensor head of a linear encoder, which is already installed in a stage system. The optical sensor head of the three-axis angle sensor is designed in a size of smaller than 50 mm × 30 mm × 30 mm so that the sensor head can be compatible with the linear encoder. Details of the optical configuration designed for the sensor head, and some results of experiments are reported.

Abstract: In this study, a long stroke atomic force microscope (LS-AFM) has been developed for the measurement of the micro-optics which have micrometric amplitude surface structures. An electrochemically polished tungsten wire is used as the AFM probe tip. Since the effective length of the tungsten probe is more than 100 μm, the probe tip can access valley part without the interference between the steep slope and the sidewall of the probe tip. By using the long stroke PZT Z scanner whose long stroke of 70 μm and a resolution of 1 nm, micrometric amplitude of the optical elements can be measured. A linear encoder is employed to measure the displacement of Z scanner in Z direction. A well-detected result of a prism sheet film profile has been obtained by the LS-AFM and its tapping mode.

Abstract: A strain-gauge-type precision displacement sensor, which is developed for a usage of micro-XY stage, is described in this paper. A thin-film strain-gauge element, which is made by Cr-N alloy, is directly fabricated on the base of the strain gauge. The direct fabrication and using the Cr-N element are expected to achieve higher sensitivity for displacement detection and better stability against the change of ambient temperature. In this study, several designs of the thin-film strain gauge, including both of two-gauge-type and four-gauge-type, are prepared to compare sensor performances such as sensitivity, stability and so on. The designed patterns of the strain-gauge element are directly fabricated on zirconia plates by using photolithography processes. The fabricated strain gauges are then evaluated as precision displacement sensors. At first, stability of the fabricated Cr-N strain-gauge-type displacement sensor was confirmed by comparing with the one made by a conventional strain gauge. Resolution of the fabricated Cr-N strain-gauge-type displacement sensors was then evaluated by comparing with a commercially-available laser displacement sensor, while giving sub-micrometer-order deformation to the strain-gauge-type displacement sensor. Details of the design, fabrication and evaluation results of the Cr-N strain-gauge-type displacement sensor are described.

Abstract: This paper presents an optical probe employed for edge contour measurement of single point diamond cutting tools. The laser beam from a laser diode (LD) is focused by an objective lens to form a small light spot with a radius of approximately 20 μm, and is then received by a photodiode (PD) after passed through the focal point of the objective lens. The cutting edge of the tool, which is placed on a line with the LD and the PD, is inserted into the laser beam section. At an each X-position, the output of the PD is recorded while the light spot is moved along the Z-direction, scanning across the cutting edge of the tool with its rake face aligned in the XZ plane. The Z-position of the cutting edge is evaluated by determining a specific value from the obtained outputs of the PD. The edge contour of the diamond cutting tool can thus be measured by repeating the Z-scanning at different X-positions. Computer simulation was carried out to investigate the influences of error factors on the evaluation of the cutting edge. Experiments were also carried out to measure a round nose of a tool with nominal nose radius of 2 mm.

Abstract: This paper presents the dynamic response of an air-bearing displacement sensor for on-machine surface form measurement of micro structures. The on-machine measurement system has some merits in terms of measurement efficiency and re-machining for error compensation after measurement. On the other hand, the performance of the contact type measurement system based on stylus methods is dominated by the dynamic characteristics of the system because the system is subject to random vibrations of the machine tool including the spindle during measurement. Therefore, the dynamic response of the system is useful for analyzing the mechanical vibrations of the practical on-machine measurement system. In this paper, the equation of motion for the air-bearing displacement sensor to the excited input displacement is derived and the displacement behavior of the system by using a PZT actuator is investigated within certain frequency ranges experimentally for identifying the dynamics of the system.

Abstract: A contact stylus-type displacement sensor is proposed for a surface form measurement of micro-structures. The stylus is supported by an air-bearing, thus it can be moved smoothly without friction between the stylus and the air-bearing. In order to reduce the measurement force, a novel controlling technique of a contact force has been developed. During the measurement one edge of the stylus is maintained contact with the measured surface by its own weight. Another end of the stylus is connected with a counter weight by a string and the stylus is hanging on the string in order to reduce the contact force between the edge of the stylus and the measured surface. Measuring force is estimated to confirm the effectiveness of the counter weight. Surface measurement of a steel ball is carried out using the developed air-bearing displacement sensor.

Abstract: A deign study of a thermal-type contact sensor for the detection of small defects, the heights of which are less than 16 nm on the wafer surface, is described in this paper. The feasibility of the contact sensor, which would detect frictional heat generated at the contact with defects, was theoretically investigated focusing on the temperature rise of the sensor element. To investigate the temperature rise of the contact sensor due to the generated frictional heat, both the theoretical calculation with simple model of heat transfer and a simulation with a finite element model (FEM) was carried out. Relationship between the sensor size and the response of the temperature rise of the contact sensor was also investigated by using FEM simulation.

Abstract: A form error characterization of a reflective-type scale grating, which is used in three-degree-of-freedom (3-DOF) encoders for position measurement of a planar motion stage, is presented. The scale grating has a micro-structured surface, the pitch of which is 1 µm in both X- and Y- direction. The periodic pattern on the scale grating generates diffracted beams when a laser beam incidents to the grating surface. The ±1st order diffracted beams from the grating contain information about the stage motions of not only X- or Y- directional in-plane displacement but also Z-directional out-of-plane displacement, and are therefore able to be utilized for multi-axis position detection. Accuracies of the position detection are mainly determined by a period deviation and a Z-directional out-of-flatness of the scale grating. The form error characterization of the grating is possible by using Fizeau interferometer, although the form error of a reference mirror in the Fizeau interferometer still remains as a measurement error in the form of the measured scale grating. In this paper, a new method was proposed to evaluate the form error characterization of the scale grating for the 3-DOF encoder, while eliminating the form error of the reference mirror in the Fizeau interferometer.

Abstract: In this paper, a novel Z-θZ micro-stage is designed and constructed by utilizing an impact friction drive. The stage is configured by two driving units, a moving element and a stage base. Each driving unit consists of two PZTs and a friction element. One end of each PZT is fixed on the stage base, and the other end is attached on a permanent magnet as the friction element. The moving element, which is a steel cylinder, is supported and moved by the friction element. Magnetic force generated by permanent magnet is utilized to stabilize the driving condition between the moving element and the friction element for the friction drive. The size of the stage is less than 1 cm3. The stroke and the maximum speed of the stage in the Z-direction are 3.8 mm, 5.7 mm/s, respectively. The stroke in the θZ-direction is unlimited and the maximum rotational velocity is 26 rpm.

Abstract: An eccentric error compensation method is presented for the pitch deviation measurement of gears. The eccentric error compensation method mainly consists of the evaluation of the assembly accuracy, the reconstruction of the reference curve and the coordinate compensation of intersection points. The geometrical model of the measurement system was analyzed and the corresponding mathematical model was proposed. A gear with small modulus was adopted as the specimen for the pitch deviation measurement. Experimental results reveal that the values of eccentricity can be extracted from the measured profile successfully and the eccentric error compensation is effective for the pitch deviation measurement of gears.