Servo Errors Research Articles

Estimation of Pointing Errors of Large Radio Telescopes under Solar Radiation Based on Digital Twin

The pointing accuracy of large radio telescopes is affected by antenna structure errors, installation errors, servo errors, gravity, and varying environmental factors. Although an on-line pointing model had been developed in the past for several large radio telescopes to correct these effects, it is also valuable to explore new ways to correct these effects. Therefore, in order to estimate the dynamic pointing error of large radio telescopes under solar radiation, a new way based on digital twin (DT) is used in this paper. Digital models of the Wuqing 70-m radio telescope (WRT70) are created based on the DT operational framework. Finally, the reliability of the WRT70’s DT system is verified, and the dynamic pointing error of WRT70 (staying at the position of the elevation angle of 90° and the azimuth angle of 180°) on two sunny days in different seasons is estimated by the created DT system. The research results show that the pointing error of large radio telescopes under solar radiation fluctuates greatly and the seasonal differences are very striking. The maximum elevation pointing error of WRT70 under solar radiation around the winter solstice is over 40 arcsec, which is equal to 4/11 of the beam width for WRT70 operating at 8 GHz. In this paper, it is feasible to estimate the time-varying pointing errors of large radio telescopes at rest under solar radiation by constructing a DT system, but it is not effective to estimate the dynamic pointing errors of large radio telescopes in operation. It is expected to establish a dynamic pointing error model and calibrate the dynamic pointing errors of large radio telescopes in operation by DT technology in the future.

High-precision servo control design and optimization for dicing semiconductor wafer

The dicing of semiconductor wafers is an essential component for fabricating electronic circuits. A servo control approach with high-precision and stability was designed to enhance the positioning accuracy and stability for dicing semiconductor wafers. The principal control and dicing processes were studied. The High Definition closed-loop control algorithm was designed, and the control parameters were optimized. The control performance was enhanced using the PID + Feedforward + NOTCH filter control algorithm. The error detection of linear and rotation axes was designed to compensate servo axis errors by using the laser interferometer and control card. After error compensation, the positioning accuracy and repeatability of the Y-axis were 2.031 μm and 1.889 μm. That of the C-axis were 0.00423°, and 0.00347°. The dicing experiment revealed the maximum chipping width was <10 μm and the dicing channel width was <42 μm. The designed servo control system ensured the high-precision and stable dicing of semiconductor chips.

An Intelligent Control Method for Servo Motor Based on Reinforcement Learning

Servo motors play an important role in automation equipment and have been used in several manufacturing fields. However, the commonly used control methods need their parameters to be set manually, which is rather difficult, and this means that these methods generally cannot adapt to changes in operation conditions. Therefore, in this study, we propose an intelligent control method for a servo motor based on reinforcement learning and that can train an agent to produce a duty cycle according to the servo error between the current state and the target speed or torque. The proposed method can adjust its control strategy online to reduce the servo error caused by a change in operation conditions. We verify its performance on three different servo motors and control tasks. The experimental results show that the proposed method can achieve smaller servo errors than others in most cases.

Noise-induced servo errors in optical clocks utilizing Rabi interrogation

We show that in optical clocks based on Rabi interrogation, both laser-frequency and magnetic-field flicker () noise with zero mean can lead to servo errors at the 10−18 level if the negative-detuning (red) and positive-detuning (blue) sides of the transition are always probed in the same order. This is due to the strong correlations of flicker noise in combination with an imbalance in the response of the servo discriminator to positive and negative differential frequency noise between the red- and blue-side probing. This imbalance is particularly large for a normalized discriminator. We derive an analytical expression for the servo error based on the correlation function of the laser-frequency or magnetic-field noise and compare it to numerical servo simulations to demonstrate how the error depends on the noise level, servo parameters, and probing sequence. We also show that the servo error can be avoided by normalizing the discriminator with a moving mean or by reversing the red/blue probing order for every second servo cycle.

Prediction Model of Three-Dimensional Machined Potassium Dihydrogen Phosphate Surfaces Based on a Dynamic Response Machining System.

To comprehensively obtain the effect of the machining process on the three-dimensional surface topography of machined potassium dihydrogen phosphate crystals, a dynamic response model of a machining system was built to calculate the dynamic displacement variables in the different processing directions. This model includes almost all processing factors, such as cutting parameters, environment vibration, radial and axial runout of the spindle, cutting tool parameters, material parameters, guide way error, fast tool servo and lubrication condition errors, etc. Compared with the experimental results, the three-dimensional topographies and two-dimensional profiles of the simulation surfaces were nearly consistent with those of experimental machined surfaces. As the simulation shows, the cutting parameters, axial runout of the spindle, and the output noise of the fast tool servo can respectively impact the main, low, and high frequencies of the machined surface topography. The main frequency of all the simulated and experimental surfaces in this study was 0.0138 μm-1. The low and high frequencies of the simulation surfaces had slight differences, about 0.003 μm-1 from those of the experimental surfaces. The simulation model, based on dynamic response, can accurately predict the entire machining process and three-dimensional topographies of machined potassium dihydrogen phosphate surfaces.

A linear hybrid model for enhanced servo error pre-compensation of feed drives with unmodeled nonlinear dynamics

Servo error pre-compensation (SEP) is commonly used to improve the accuracy of feed drives. Existing SEP approaches often involve the use of physics-based linear models (e.g., transfer functions) to predict servo errors, but suffer from inaccuracies due to unmodeled nonlinear dynamics in feed drives. This paper proposes a linear hybrid model for SEP that combines physics-based and data-driven linear models. The proposed model is shown to approximate nonlinearities unmodeled in physics-based linear models. In experiments on a precision feed drive, the proposed hybrid model improves the accuracy of servo error prediction by up to 38% compared to a physics-based model.

Dynamic model identification for CNC machine tool feed drives from in-process signals for virtual process planning

Virtual process planning offers advantages in terms of predicting servo and contouring errors ahead of time, and taking corrective action by modifying the program or CNC parameters. Successful application of virtual process planning requires accurate models that capture the dynamics of feed drives. Identification of such models is typically time consuming.This paper presents a pole search method used in conjunction with least squares (LS) projection technique for identifying virtual machine tool drives. Compared to earlier research published in rapid identification, and the generic system identification method (i.e., a two-stage instrumental variable method), the parameter convergence has been improved significantly. This is achieved by reducing the number of unknown variables solved during LS estimation from eight to four (corresponding to each candidate pole triplet), and by avoiding the use of noisy position measurements and their time derivatives in the LS matrix pseudo-inversion. As a result, virtual feed drive models can be constructed using only in-process gathered data, without interrupting a machine tool's production for dedicated identification tests. Effectiveness of the new method is demonstrated in simulations and experimental case studies on two different machine tools, a gear grinding machine, and a 5-axis machining center. With the new method, servo errors can be predicted during the process planning stage to within 1-2% of closeness of their actual (experimental) values.

Modal Analysis, Metrology, and Error Budgeting of a Precision Motion Stage

In this study, a precision motion stage, whose design utilizes a single shaft supported from the bottom by an air bearing and voice coil actuators in complementary double configuration, is evaluated for its dynamic properties, motion accuracy, and potential machining force response, through modal testing, laser interferometric metrology, and spectral analysis, respectively. Modal testing is carried out using two independent methods, which are both based on impact hammer testing. Results are compared with each other and with the predicted natural frequencies based on design calculations. Laser interferometry has been used with varying optics to measure the geometric errors of motion. Laser interferometry results are merged with measured servo errors, estimated thermal errors, and the predicted dynamic response to machining forces, to compile the error budget. Overall accuracy of the stage is calculated as peak-to-valley 5.7 μm with a 2.3 μm non-repeatable part. The accuracy measured is in line with design calculations which incorporated the accuracy grade of the encoder scale and the dimensional tolerances of structural components. The source of the non-repeatable errors remains mostly equivocal, as they fall in the range of random errors of measurement in laser interferometry like alterations of the laser wavelength due to air turbulence.

Absolute frequency measurement of the 88Sr+ clock transition using a GPS link to the SI second

We report the results of a recent measurement of the absolute frequency of the – transition of the ion. The optical frequency was measured against the international atomic time realization of the SI second on the geoid as obtained by frequency transfer using a global positioning system link and the precise point positioning technique. The measurement campaign yielded more than 100 h of frequency data. It was performed with improvements to the stability and accuracy of the single-ion clock compared to the last measurement made in 2012. The single ion clock uncertainty is evaluated at when contributions from acousto-optic modulator frequency chirps and servo errors are taken into account. The stability of the ion clock is at 1 s averaging, a factor of three better than in the previous measurement. The results from the two measurement campaigns are in good agreement. The uncertainty of the measurement, primarily from the link to the SI second, is 0.75 Hz (). The frequency measured for the S–D clock transition of is Hz.

Visual Servo Regulation of Wheeled Mobile Robots With an Uncalibrated Onboard Camera

In this paper, a visual servo regulation strategy is designed for an uncalibrated camera system mounted on a wheeled mobile robot subject to nonholonomic motion constraint, which can drive the mobile robot to the target pose with exponential convergence. Unlike existing methods, the proposed approach can work well without the camera intrinsic parameters being known in advance. Specifically, a novel fundamental matrix-based algorithm is first proposed to rotate the robot to point toward the desired position, with the camera intrinsic parameters estimated simultaneously by employing the fundamental matrix and a projection homography matrix. Subsequently, by utilizing the obtained camera intrinsic parameters, a straight-line motion controller is developed to drive the robot to the desired position, with the orientation of the robot always facing the target position. Another pure rotation controller is finally adopted to correct the orientation error. The exponentially convergent properties of the visual servo errors are proven with mathematical analysis. The performance of the proposed uncalibrated visual servo regulation method is further validated by both simulation and experimental results.

Effect of servo and geometric errors of tilting-rotary tables on volumetric errors in five-axis machine tools

The tilting-rotary table becomes a standard accessory for five-axis machine tools. An approach for volumetric errors evaluation taking into account servo and geometric errors of the tilting-rotary table is proposed in this paper. A simple machining model of volumetric circles is used to evaluate volumetric errors due to servo errors of the tilting-rotary table. A kinematic error model is used to predict the volumetric errors resulting from geometric errors associated with the tilting-rotary table. Then effects of the two error sources are added to predict the total volumetric errors. A test part obtained by improving the cone frustum specified in NAS979 is presented to validate this method. Cases studies with cutting experiments are carried out on a commercial five-axis machine tool. The results show that the proposed model is effective to evaluate the effect of servo and geometric errors of the tilting-rotary table on volumetric errors in the five-axis machine tool.

Reconfigurable data driven virtual machine tool: Geometric error modeling and evaluation

Standards communities are developing robust machining and instrumented performance tests to evaluate the accuracy of 5-axis machining centers through the direct and indirect measurement of errors affecting the simultaneous motion of all five axes. The combined effect of the numerous geometric errors and servo errors complicates diagnostic analysis of the measurement results and estimation of the individual errors. To better understand the effect of the individual errors on the measurement results, we developed a reconfigurable five-axis data driven virtual machine tool (DDVMT) error simulator. The DDVMT is a generalized model that incorporates machine tool information models, geometric error models, controller models, and (standardized) machine tool metrology test methods and analyses into one modeling scheme. This paper describes the data driven virtual machine tool and demonstrates its ability to simulate the effects of geometric errors on multi-axis performance tests.

Dynamics and control of a spatial disorientation trainer

Modern combat aircraft can fly at unusual orientations. The spatial disorientation trainer (SDT) examines a pilot's ability to recognise these orientations, to adapt to unusual positions and to persuade the pilot to believe in the aircraft instruments for orientation and not in his own senses. The SDT is designed as a four-degree-of-freedom (4DoF) manipulator with rotational axes. Through rotations about these axes, different orientations can be achieved; different acceleration forces acting on the pilot can also be simulated. In this paper, a control algorithm of an SDT that improves the quality and safety of the SDT motion while improving position accuracy and reducing servo errors is proposed. This control algorithm uses approximate inverse dynamics based on the recursive Newton–Euler algorithm, which accounts for the motors present in the system; it calculates motor torques, as well as the forces and moments acting on the SDT links based on the achievable velocities and accelerations of the robot links. This algorithm enables accurate dimensioning of the axes bearings and links as well. The maximum possible accelerations of the SDT links are calculated in each interpolation period based on the total moments of inertia for the axes of rotation of these links, mutual influences of the link accelerations on each other, and motor capabilities. The forces, moments and torques that act on the SDT links obtained with the suggested algorithm have lower magnitude and smoother profile. In this study, the forces and angular velocities that act on the simulator pilot in the SDT are calculated along with the roll and pitch angles of the gondola for these forces.

FPD 공정용 Glass 이송 시스템을 위한 자기부상 EMS의 개발

In recent semiconductor and FPD (Flat Panel Display) manufacturing processes, high clean-class delivery operation is required more and more for short working time and better product quality. Traditionally SLIM (Single-sided Linear Induction Motor) is widely used in the liner drive applications because of its simplicity in the rail structure. A magnetically levitated (Maglev) unmanned vehicle with SLIM traction, which is powered by a CPS (Contactless Power Supply) can be a high precision delivery solution for this industry. In this paper unmanned FPD-carrying vehicle, which can levitate without contacting the rail structure, is suggested for high clean-class FPD delivery applications. It can be more acceptable for the complex facilities composed with many processes which require longer rails, because of simple rail structure. The test setup consists of a test vehicle and a rounded rail, in which the vehicle can load and unload products at arbitrary position commanded through wireless communications of host computer. The experimental results show that the suggested vehicle and rail have reasonable traction servo and robust electromagnetic suspensions without any contact. The resolution of point servo errors in the SLIM traction system is accomplished under 1mm. The maximum gap error is <TEX>${\pm}0.25mm$</TEX> with nominal air gap length of 4.0mm in the electromagnetic suspensions. This type of automated delivery vehicle is expected to have significant role in the clean delivery like FPD glass delivery.

Dynamic and geometric error assessment of an XYC axis subset on five-axis high-speed machine tools using programmed end point constraint measurements

This paper presents a technique for assessing the volumetric errors on a five-axis machine tool for motion involving two linear axes and one rotary axis at selected feed rates using data from two sources. The first source of data is obtained through a programmed end point constraint procedure with measurement of the 3D volumetric positioning errors between a point on the tool holder and another fixed to the machine table reference frame. The tests involve maintaining the nominal coincidence of these two points whilst exercising the three axes. The second source of data is the position feedback signal from the encoder provided by the machine controller. Tests were carried out at low and high feed rates to evaluate the effect of geometric and dynamic errors. Polynomial functions are used to represent and then predict the geometric errors. The predicted geometric errors are then added to the dynamic errors provided by the servo errors from position feedback signals and propagated to the tool centre point and are compared with the measured volumetric errors. It shows that the influence of the geometric errors are dominant at low feed, whereas the effects of the servo errors of the linear axes become dominant as the feed increases, reaching 80% of the total error at a feed of 10,000 mm/min.

Development of an integrated look-ahead dynamics-based NURBS interpolator for high precision machinery

Methodologies for planning motion trajectory of parametric interpolation such as non-uniform rational B-spline (NURBS) curves have been proposed in the past. However, most of the algorithms were developed based on the constraints of feedrate, acceleration/deceleration (acc/dec), jerk, and chord errors. The errors caused by servo dynamics were rarely included in the design process. This paper proposes an integrated look-ahead dynamics-based (ILD) algorithm which considers geometric and servo errors simultaneously. The ILD consists of three different modules: a sharp corner detection module, a jerk-limited module, and a dynamics module. The sharp corner detection module identifies sharp corners of a curve and then divides the curve into small segments. The jerk-limited module plans the feedrate profile of each segment according to the constraints of feedrate, acc/dec, jerk, and chord errors. To ensure that the contour errors are bounded within the specified value, the dynamics module further modifies the feedrate profile based on the derived contour error equation. Simulations and experiments are performed to validate the ILD algorithm. It is shown that the ILD approach improves tracking and contour accuracies significantly compared to adaptive-feedrate and curvature-feedrate algorithms.

Modeling wind-gust disturbances for the analysis of antenna pointing accuracy

This paper develops and compares three wind models used to simulate antenna pointing errors. The first model represents wind forces acting on the antenna dish, thus closely representing actual wind action. The second model represents wind gusts as torque disturbances applied at the antenna drives. The third model represents gusts acting at the antenna rate input. A wind filter introduced in this paper, makes the wind gusts of the third model equivalent to the gusts of the second model. The third model is applicable to the currently developed models of the antenna's closed loop systems. Simulink models of the 34-meter and 70-meter antennas' close-loop system were used to simulate antenna pointing errors in wind gusts. The paper shows that for the NASA 34-meter and for the 70-meter antennas, all three models give similar results, and the results are compatible with field measurements of the servo errors taken in wind.

A Study on Improvement of Circularity by Optimizing Tool Path of Boring

This paper deals with controlling approaching and escaping paths for the fixed circular machining pattern on NC machine tools. Recently accuracy of circular motion on NC machine tools has extremely improved. One of the reasons is that servo control gain became higher. Several servo errors caused by motor cogging or guide friction were reduced. The other reason is that the KGM measurement system has been developed in addition to the DBB. Different radius circles became able to be tested. However, there are two errors that do not appear on circularity measurement with the DBB or the KGM. The first one is servo error due to different acceleration between approaching path and shaping path in circular machining pattern. The error cannot be observed because circularity measurement pattern is different from circular machining pattern. The second one is tool deformation due to cutting force. Circularity measurement is done without cutting, so the measurement cannot observe the error. In this study, the errors are analyzed and a new path pattern is proposed to reduce the errors.

310 NC工作機械の輪郭運動誤差の解析(第1報) : コーナー部の運動誤差解析(OS10 加工計測・評価)

This paper deals with the analysis for motion error of the XY table of NC machine tools. When the table moves along corners, the overshoot or undershoot occurs due to the several mechanical and control factors. In order to clarify the mechanism, the simulation model of feedforward and feedback control block as well as mechanical block is used. By comparing the simulated and measured motion trajectory on an existing machine, it was found that the simulation could predict the motion errors at the corner, which can be interpreted into servo errors and mechanical errors.

LTO: A better format for mid-range tape

In the last two years, Linear Tape-Open® (LTO®) tape drives have become clear leaders in the mid-range tape marketplace. Tape drives designed to the LTO Ultrium® format were the first of the super drives to be shipped to mid-range tape customers. This paper describes some of the eclectic features designed into the LTO format. The technical emphasis is on aspects of the logical format that are new or different from preceding drives, though some aspects of the LTO roadmap and physical format are also discussed. The logical format comprises all of the data manipulations and organization involved in writing customer data to tape. This includes data compression to compact the data, appending of error-correction codes (ECCs) to protect the data, run-length-limited encoding of the ECC-encoded data, prepending headers to the encoded data to make it self-identifying on read-back, and storing of information about the data and the way it is stored in a cartridge memory module. Physical format aspects that are discussed include encoding data into the servo pattern and write shingling. Also discussed are the format-enabled aspects of drive functionality that have been improved over previous tape drive products, including enabling backward writing, elimination of problematic failure mechanisms, dynamic rewrite of defective data, handling servo errors without stopping tape, and enabling robust reading. Contrasts are made with previous products and competing products based on other format choices. Also discussed throughout is the way in which an eclectic format can be created by cooperation among three format-development companies.