Scale Grating Research Articles

Design and testing of a two-axis surface encoder with a single Littrow configuration of a first-order diffraction beam

A simple but effective optical design is proposed to expand the measurement range of a surface encoder in the out-of-plane Z-direction, which had been much shorter than that in the in-plane X-direction. A zeroth-order and a first-order diffraction beams generated at a transparent grating are projected onto a parallelly aligned scale grating. The reflected zeroth-order beam from the scale grating interferes with a beam from a reference plane mirror for the Z-directional measurement over an expanded range of 13 mm. A single Littrow configuration is established for the first-order diffraction beam to travel to and from the scale grating on the same path so that it can interfere with the reflected zeroth-order beam for the X-directional measurement regardless of the Z-position of the scale grating. A prototype sensor is constructed for demonstrating the effectiveness of the proposed optical design for expansion of Z-range. Uncertainty analysis on the measurement results is also conducted.

Concept of Error Compensation for Nonorthogonality in Two-Axis Displacement Measurement System Utilizing Single Grating Scale and Littrow Configuration

Concept of Error Compensation for Nonorthogonality in Two-Axis Displacement Measurement System Utilizing Single Grating Scale and Littrow Configuration

Design of an Optical Head with Two Phase-Shifted Interference Signals for Direction Detection of Small Displacement in an Absolute Surface Encoder

This paper presents the design and construction of a new optical head with two phase-shifted interference signals in an absolute surface encoder by using a mode-locked femtosecond laser. A series of discrete absolute positions of the scale grating is obtained from a series of peak wavelengths of the spectrum of the +1st- or -1st-order diffracted beam. The two beams at a specific wavelength λi interfere with each other to generate an incremental interference signal for high-resolution displacement measurement over a small interpolation range around the corresponding discrete absolute position xi. In the previous design of the optical head, the two beams were guided by optical fibers into a fiber coupler for the interference. This fiber optics design was simple and stable but could not identify the moving direction of small displacement within each interpolation range because only one interferential signal could be generated. The aim of this study is to develop a new design of the optical head, where two interference signals with a phase difference of π/2 are generated. For this purpose, free-space optics, instead of fiber optics, is adopted in the new optical head. Experiments are conducted to confirm the generation of the two phase-shifted interference signals. A Lissajous figure is plotted to verify the phase difference between the two signals.

On-Machine Calibration of Pitch Deviations of a Linear Scale Grating by Using a Differential Angle Sensor

A differential angle sensor is newly developed to calibrate the pitch deviations of a linear scale grating with a nominal pitch of 1.6 µm on an ultra-precision lathe. The angle sensor is composed of two angle detection units based on the laser autocollimation method. A collimated laser beam with a diameter of 1 mm, which is output from a laser diode with a wavelength of 685 nm, is projected onto the linear scale grating. The positive and the negative first-order diffracted beams from the scale are received by the two angle detection units, respectively. The X-slide of the ultra-precision lathe is employed to generate the necessary scanning motion for the calibration. Based on the fact that the pitch deviations will cause changes in the positive and the negative first-order diffraction angles, which are equal in magnitude and opposite in sign, the pitch deviations can be obtained from the differential output of the angle sensor. The tilt error motion of the X-slide, which is a major error factor in on-machine calibration, can also be removed in the differential output. The robustness of the developed angle sensor for on-machine calibration has been confirmed by testing the basic performances of the sensor on the machine tool. The feasibility of the on-machine calibration result of pitch deviations has been verified through comparing with the off-machine calibration result.

Anti-fogging technique for protection of optical grating scales

The combined technique consisting of CF4/O2 plasma nanotexturing of the glass surface and the deposition of the SiOx containing DLC film on the top of the grating scale followed by O2 plasma treatment was designed for the long-lasting protection of the optical grating scales from fogging. Hierarchical nanotexturing increased the average surface roughness Ra from 0.8 nm to 5.1 nm and considerably decreased the wetting angle from 11° to <1° leading to the superhydrophilic silicate glass surface. XPS analysis of the DLC film before and after O2 plasma treatment shows that even short exposure to O2 plasma produces oxidized silicon on the surface of the DLC film. O2 plasma treatment time of 180 s leads to the increase of the carbon CC peak (sp2 hybridization) intensity from 15% to 32%, and a decrease of the carbon C-C peak (sp3 hybridization) intensity from 67% to 45%. The wetting angle of the plasma nanotextured and DLC-coated optical grating scale retains superhydrophilic properties and optical transmittance after long-term storage. Real-time fogging testing confirms the fast appearance of a continuous transparent water film on the surface of the optical grating scale at the beginning of water condensation process. The proposed technique ensures long-lasting operation of the encoder with the optical grating scale below dew point temperature.

Improving the optical subdivision ability of a grating interferometer via double-row reverse blazed gratings

In this paper, a grating interferometer with higher optical subdivision ability is presented based on a reasonable configuration for blazed gratings. The proposed grating interferometer is characterized by the inverse side-by-side coupling of two blazed gratings. Because blazed grating has great diffractive efficiency at a specific diffraction order, this configuration allows for high optical subdivision by increasing diffraction times on the grating's surface. The operating principle of the proposed method is described in detail, and the corresponding experiment is carried out to evaluate its availability. The results show that 16 times optical subdivision can be realized, which means that we can achieve higher measurement resolution and accuracy for grating interferometers. It is relatively rare to employ blazed gratings as scale gratings to quantify displacement, and we anticipate that the construction of double-row reverse blazed gratings will establish the groundwork for further exploration of the grating interferometer's precision.

Reduction of Crosstalk Errors in a Surface Encoder Having a Long Z-Directional Measuring Range

A modified two-axis surface encoder is proposed to separately measure both the in-plane displacement and the Z-directional out-of-plane displacement with minor crosstalk errors. The surface encoder is composed of a scale grating and a small-sized sensor head. In the modified surface encoder, the measurement laser beam from the sensor head is designed to be projected onto the scale grating at a right angle. For measurement of the X- and Y-directional in-plane scale displacement, the positive and negative first-order diffracted beams from the scale grating are superimposed on each other in the sensor head, producing interference signals. On the other hand, the Z-directional out-of-plane scale displacement is measured based on the principle of a Michelson-type interferometer. To avoid the influence of reflection from the middle area of the transparent grating, which causes periodic crosstalk errors in the previous research, a specially fabricated transparent grating with a hole in the middle is employed in the newly designed optical system. A prototype sensor head is constructed, and basic performances of the modified surface encoder are tested by experiments.

Weak reflectivity measurements of large-mode-area FBGs by a scale composed of a fiber Fabry-Perot interferometer.

In a previous study, we proposed a measuring method for the reflectivity of weak-reflection large-mode-area fiber Bragg gratings by using scale gratings. We experimentally found that the interference between two scale fiber Bragg gratings (FBGs) is beneficial for increasing reflectivity scales, which can improve the measurement accuracy. Therefore, in this study, we designed and fabricated FBG-based Fabry-Perot cavities (FBG-FP) in single-mode fibers by two inscription methods, namely ultraviolet (UV) laser exposure and femtosecond-laser direct writing. Then, a large-mode-area double-clad (LMA-DC) FBG of weak reflectivity was measured by these two scales, and the experimental results show that the Bragg resonance reflectivity is less than 4.28% and 1.14% ∼ 2.28%, respectively. This method of measuring the weak grating reflectivity based on FBG-FP scales is convenient, efficient, and accurate. It is also worth mentioning that the method of femtosecond-laser direct writing eliminates the period limitation of the phase mask, thereby expanding the measurement wavelength range of FBGs. In the future, with the improvement of fiber grating fabrication technology, it is expected that more accurate results can be obtained.

Design and Testing of a Compact Optical Angle Sensor for Pitch Deviation Measurement of a Scale Grating with a Small Angle of Diffraction

The design and testing of different optical heads were performed to evaluate the pitch deviation of a diffraction scale grating with a small diffraction angle. Based on the proposed pitch deviation evaluation method employing optical angle sensors based on laser autocollimation, a modified optical head with position-sensitive detectors (PSDs) is first designed and constructed by following the conventional optical configuration. Owing to the small angle of diffraction of the first-order diffracted beams, the modified optical head has a large working distance, resulting in poor sensor stability. Therefore, a novel and compact optical head employing a pair of small prisms is designed and developed to shorten the working distance of the optical head. An additional modification was also made to the developed compact optical head in such a way that collimator objectives (COs) in the laser autocollimation units are removed to improve the sensor sensitivity. Experimental comparisons were conducted using the three types of optical heads to verify the feasibility of the developed optical angle sensor with PSDs.

Method for measuring reflectivity of weak reflection large-mode-area fiber Bragg gratings using scale gratings.

Accurate fundamental-mode (FM) reflectivity measurements of FM weak reflection large-mode-area (LMA) FBGs constitute a challenging problem owing to high-order modes (HMs). In this paper, we propose a novel measurement method that uses scale gratings fabricated in a single-mode fiber. The weak reflectivity of the measured FBGs was achieved by comparing the peak reflection resonance with that of the scale gratings. The measured minimum reflectivity of the scale grating was 1.3%, giving the measurement accuracy. The accuracy can be improved further by increasing the number of reflectivity scales. A Fabry-Perot-interferometer-based scale grating was proposed, designed, and fabricated using a chirped phase mask to achieve soaring scale numbers. The minimum reflectivity of the scales decreased by 0.37%. Several LMA double-cladding FBGs with weak reflectivity were measured. The results show that this novel measurement method is convenient and efficient, does not depend on the transmission spectrum of the grating, and can circumvent the influence of HM. It is anticipated that weak reflectivity can be measured more accurately by improving grating fabrication technology.

Spectrum Acquisition System Using Photoelectric Detection and Its Data Mining Simulation of Spectral Texture Features

Weak spectral signal detection technology is a new subject in the measurement engineering field. It obtains the characteristic information of the measured object based on the characteristics of light diffraction, interference, polarization and so on. Regarding light source entropy, the artificial light source of mercury argon lamp with a spectrum of 250 nm~918 nm is selected as the first research object. The grating monochromator with built-in grating scale lines (1200 g/mm, 600 g/mm) is selected as the light splitting device. Due to the weak spectral signal, the output signal is vulnerable to noise interference after a certain amount of photoelectric conversion. AD202 variable voltage coupling filtering technology is adopted. It means that in the signal isolation and amplification process, the AD202 variable voltage coupled filter amplifier chip is employed to design the current/voltage conversion circuit and the same direction proportional operation amplifier circuit. Meanwhile, a second-order low-pass filter circuit is designed based on the characteristics of the OP07 operational amplifier. For the data mining of multispectral image texture features, Contourlet transform is used, that is, the multispectral image texture features are fuzzy preprocessed to remove the adjacent range correlation knowledge. On this basis, the texture features of multispectral images are analyzed. In the experiment, Contourlet transform is adopted to transform multispectral images from spatial domain to frequency domain, which has the advantages of short extraction time, low cost, and high mining accuracy. A spectrum acquisition system based on photoelectric detection is designed. With chlorophyll as the research object, the sample solution to be tested has high photoelectric absorbance in the range of blue-violet light (400 nm~500 nm) and red-orange light (600 nm~700 nm).

A New Optical Configuration for the Surface Encoder with an Expanded Z-Directional Measuring Range.

This paper proposes a new optical configuration for a two-axis surface encoder that can measure the in-plane (X-axis) and out-of-plane (Z-axis) displacements of a positioning stage. The two-axis surface encoder is composed of a scale grating and a sensor head. A transparent grating is employed in the sensor head for measurement of the Z-directional displacement of the scale grating based on the Fizeau-type measurement method; a reference beam reflected from the transparent grating and the zeroth-order diffracted beam from the scale grating are superimposed to generate an interference signal. A pair of prisms and a beam splitter are also employed in the sensor head, so that the positive and negative first-order diffracted beams can be superimposed over a long working distance to generate an interference signal for measurement of the X-directional displacement of the scale grating. Focusing on the new, extended Z-directional measurement mechanism, proof-of-principle experiments were carried out to verify the feasibility of the proposed optical configuration for the surface encoder that can measure the uni-directional displacements of a scale grating along the X- and Z-axis. Experimental results from the developed optical configuration demonstrated the achievement of a Z-directional measuring range of ±1.5 mm.

A Self-Calibration Stitching Method for Pitch Deviation Evaluation of a Long-Range Linear Scale by Using a Fizeau Interferometer.

An interferometric self-calibration method for the evaluation of the pitch deviation of scale grating has been extended to evaluate the pitch deviation of the long-range type linear scale by utilizing the stitching interferometry technique. Following the previous work, in which the interferometric self-calibration method was proposed to assess the pitch deviation of the scale grating by combing the first-order diffracted beams from the grating, a stitching calibration method is proposed to enlarge the measurement range. Theoretical analysis is performed to realize the X-directional pitch deviation calibration of the long-range linear scale while reducing the second-order accumulation effect by canceling the influence of the reference flat error in the sub-apertures’ measurements. In this paper, the stitching interferometry theory is briefly reviewed, and theoretical equations of the X-directional pitch deviation stitching are derived for evaluation of the pitch deviation of the long-range linear scale. Followed by the simulation verification, some experiments with a linear scale of 105 mm length from a commercial interferential scanning-type optical encoder are conducted to verify the feasibility of the self-calibration stitching method for the calibration of the X-directional pitch deviation of the linear scale over its whole area.

Bottom-Up Gold Filling in New Geometries and Yet Higher Aspect Ratio Gratings for Hard X-ray Interferometry

An extreme bottom-up filling variant of superconformal Au electrodeposition yielding void-free filling of recessed features is demonstrated with diffraction gratings composed of a two-dimensional patterned “chessboard” array of square vias of aspect ratio (depth/width) ≈ 23 as well as one-dimensional arrays of trenches having aspect ratios exceeding 50 and 65. Deposition on planar and patterned substrates is examined in several near-neutral x mol∙l−1 Na3Au(SO3)2 + 0.64 mol∙l−1 Na2SO3 electrolytes (x = [0.08, 0.16, 0.32]) containing ≈50 μmol∙l−1 Bi3+ additive. The electrolytes are similar to those used in earlier work, although the upper bound on Na3Au(SO3)2 concentration is twofold greater than previously described. Filling results are complemented by associated current and deposition charge transients whose features, particularly with well controlled pH, exhibit repeatable behaviors and timescales for incubation with passive deposition followed by bottom-up, void-free filling. While incompletely filled features can exhibit substantial via-to-via variation in fill height, self-passivation that follows complete bottom-up filling results in highly uniform filling profiles across the substrates. Visibility measurements capture the quality and uniformity of the as-formed wafer scale gratings. X-ray phase contrast imaging demonstrates their potential for imaging applications.

Self-calibration of a variable-line-spacing grating for an absolute optical encoder with a Fizeau interferometer

The principle of the self-calibration method for the evaluation of a planar scale grating having a constant pitch is extended to realize the evaluation of the pitch distribution of a planar scale grating having variable line spacings (VLSs) along the X- and Y-directions. In the conventional self-calibration method, the wavefronts in the zeroth-order diffracted beam and the first-order diffracted beams observed by a Fizeau interferometer arranged in the Littrow configuration were employed to evaluate the pitch deviation of a scale grating. The arithmetic operation with the wavefront data realizes the evaluation of the pitch deviation over a large area in a short time, while cancelling the influence of the out-of-flatness of a scale grating. Meanwhile, theoretical equations in the conventional self-calibration method cannot be directly applied to the evaluation of a VLS grating due to its unique properties of the pitch distribution. In this paper, major modifications are thus made to the conventional theoretical equations for deriving the pitch distribution of a VLS grating. To verify the performance of the newly proposed method, the pitch distribution of a VLS grating employed in a commercial planar absolute encoder is evaluated in experiments.

On-line angle self-correction strategy based on a cobweb-structured grating scale

Installation eccentricity of the grating scale is the main source of error for rotary optical encoders in practical applications. In this paper, an on-line angle self-correction strategy for rotary optical encoders is proposed. Firstly, the eccentricity error model is described, which explains the numerical relationship between the eccentricity error and the eccentric parameters including eccentric distance and eccentric angle. Then, a bioinspired cobweb-structured grating scale is adopted as the measurement standard of the rotary optical encoder. This grating scale allows the rotary optical encoder to sense the rotary angle and the radial displacement simultaneously. The eccentric parameters can be determined in real time from the rotary angle and the radial displacement information. Thus, the measured rotary angle can be on-line corrected by applying the eccentric parameters to the eccentricity error model. The feasibility and effectiveness of the proposed strategy have been confirmed from the experimental results.

Industrial tests of co-combustion of alternative fuel with hard coal in a stoker boiler

ABSTRACT This paper presents the results of industrial research on co-combustion of solid recovered fuel (SRF) with hard coal in a stoker boiler type WR-25. The share of SRF in the fuel mixture was 10%. During the co-combustion of SRF, no technological disturbances of the boiler were noted. The obtained SO2 and NOX emissions were comparable with coal combustion, but dust emissions increased. During the co-combustion of the coal mixture with 10% of alternative fuel, acceptable standards for co-incineration of waste were exceeded for NOx, dust, CO, HCl, HF, heavy metals, dioxins, and furans. The by-products of waste co-combustion with coal were non-hazardous waste. The obtained results constitute a very important contribution to the process of boiler retrofitting toward a waste co-incineration unit, and to meeting the legislative and environmental requirements. Implications: Due to some challenges related to waste storage and transportation, combustion in incineration plants and Waste-to-Energy plants is not possible. The adaptation (formal and technical) of medium scale boilers as co-incineration plants reveals high potential. Nevertheless, the lack of experience and investigations of waste co-combustion in real industrial scale grate boilers is observed. Thus, the implication of this article results consists of the investigations using industrial scale mechanical grate boiler (different from incineration type). Moreover, the investigations were carried out in a low-capacity boiler (~50% of nominal capacity). This novel experience is very important because reduced heat dissipation into the grid caused by high ambient temperatures occurs very frequently. These tests are valuable from the point of view of retrofitting the unit to obtain technological and emission parameters that would allow obtaining the status of a waste co-incinerating unit. The results of these investigations are addressed to power plant management board and engineering staff.

A Calibration Scheme With Combination of the Optical Shaft Encoder and Laser Triangulation Sensor for Low-Frequency Angular Acceleration Rotary Table

A low-frequency angular acceleration rotary table (LFAART) is capable of sinusoidal motion around a fixed position and essential to verify dynamic characteristics of a gyroscope device. The table’s performance at different swinging frequencies and amplitudes needs to be calibrated to realize traceability. However, there has been little research on the LFAART calibration. Besides, the existing measurement methods of sinusoidal angular motion parameters are not adequate for the needs of the LFAART calibration in terms of measurement range, precision, and feasibility. This article presents a calibration scheme combining an optical shaft encoder and a laser triangulation sensor. When the rotation amplitude covers sufficient slots of the scale grating, the optical shaft encoder is applied, while the laser triangulation sensor is used for a tiny amplitude motion. An algorithm is proposed to accurately calculate the angular acceleration amplitude and frequency of the rotation utilizing sinusoidal phase-modulated signals of the optical shaft encoder output. Singular zero-crossings of the series are discarded, and intervals of reversing moments are recognized according to the signal derivative’s sign at remaining zero-crossings. Then intervals are divided into subintervals to obtain accurate reversing moments. Finally, angular displacements between two adjacent reversing moments are estimated based on the fringe counting method, and the amplitude and frequency are calculated. The measurement uncertainty analysis and experiment demonstrate that the calibration scheme, including the algorithm, possesses high accuracy and resolution, wide range, and good operability, which perfectly solves the rotation’s angular acceleration amplitude and frequency.

Thermal utilization of meat-and-bone meal using the rotary kiln pyrolyzer and the fluidized bed boiler – The performance of pilot-scale installation

Thermal utilization of meat-and-bone meal (MBM) is subject to stringent regulations that are meant to provide elimination of any potential pathogens. Incineration as well as other possible routes for thermal conversion of MBM are still at the research state. The universal technology was developed that allows to combust various types of waste organic materials, including animal waste, municipal solid waste and sludge, mixed at any ratio with different types of biomass. It provides the possibility to utilize the waste-and-biomass fuel blends of up to 90%wt of moisture content, while maintaining the allowable pollutant emissions and soil contamination. This regards mainly NOx, SO2, HCl and VOC. Contrary to the typical large scale grate boilers used for waste burning, the developed operating pilot-scale plant with a capacity of 12 MW offers the complete combustion of MBM, resulting in a flue gas which is proved to be free of flammable gaseous components and sooty particles in slag and fly ash. The thermal decomposition and combustion of waste using this technology ensures thermal conversion of chemical energy contained in waste and biomass. The efficiency of the prototype installation varied between 84.8 and 88.4% depending on the facility load.

An absolute surface encoder with a planar scale grating of variable periods

An absolute surface encoder for measurement of absolute in-plane position with a mode-locked femtosecond laser and a planar scale grating having variable periods along the two orthogonal directions is proposed. In the proposed method, the absolute in-plane position is measured through observing the spectra of the coupled groups of the positive and negative first-order diffracted mode-locked femtosecond laser emanated from a single point on the planar scale grating. Due to the dispersive effect of the planar scale grating having variable periods, each of the diffracted modes has a unique angle of diffraction associated with the absolute position information. Therefore, the absolute in-plane position can be detected by obtaining the peak frequencies in the optical spectra of the coupled positive and negative first-order diffracted beams received by dual detector units. The dual-detector configuration of the optical head also contributes to improving the robustness of the absolute position measurement against the angular error motion of the planar scale grating. Furthermore, interference signals capable of being used for the improvement of the absolute position measurement can be generated under the dual detector configuration. Results of the numerical calculations for the theoretical verification of the proposed method, fabrication of a scale grating having a variable period (VLS grating) with interference lithography, design and development of the optical head, as well as the results of basic experiments by using the developed optical head and the VLS grating, are reported.