Search

Lithographic machine tools require both high motion accuracy and high motion flexibility. Projection based iterative learning control (P-ILC) is appealing for wafer stages to achieve two goals, simultaneously. P-ILC contains a nonparametric feedforward controller based on ILC, and a parametric feedforward controller with a projection step for feedforward tuning. In this paper, a set-membership based frequency-domain ILC algorithm (SM-F-ILC) is employed in the enhancing P-ILC scheme to improve the performance in the nonparametric feedforward control mode. SM-F-ILC can effectively compensate for repetitive errors, attenuate the nonrepetitive error accumulation and achieve fast convergence speed with model uncertainties. These superiorities also facilitate to improve the performance of P-ILC in the parametric feedforward control mode. The validity of the enhancing P-ILC scheme is demonstrated by experimental results. • The enhancing P-ILC scheme can achieve high motion accuracy and flexibility. • SM-F-ILC enables P-ILC high nonparametric and parametric feedforward performances. • The enhancing P-ILC scheme always contains a disturbance compensation scheme. [ABSTRACT FROM AUTHOR]

• There are nonlinear characteristics in the turntable servo system. • A controller based on deep deterministic policy gradient algorithm is proposed. • Experimentations of six control strategies in four conditions are compared. • The tracking accuracy and stability of turntable servo system are improved. In automotive manufacturing, brake disc balance accuracy is critical for braking system reliability. The tracking accuracy of the balancing machine's turntable servo system directly influences production efficiency and disc balance. To enhance turntable servo control in position and velocity tracking, this paper proposes a fractional order proportional integral derivative (FOPID) controller using a deep deterministic policy gradient (DDPG) algorithm inspired by deep reinforcement learning (DRL). A dynamic model of the servo system is developed to support the design of the DDPG FOPID control strategy. Anti-interference and anti-noise experiments are conducted to compare control strategies including fuzzy logic (Fuzzy), genetic algorithm (GA) PID, particle swarm optimization (PSO) PID, Q-learning PID, DDPG PID and DDPG FOPID through the physical experimental platform of the turntable servo system. Experimental results demonstrate that the DDPG FOPID strategy offers superior robustness and tracking performance, suggesting its potential to advance intelligent control methods in automotive manufacturing. [ABSTRACT FROM AUTHOR]

The increasing adoption of exotic and hard to cut materials in high-end applications makes electric discharge machining (EDM) process more relevant than ever. In particular, wire EDM variant (WEDM) deserves attention due to its ability to cut the workpiece's surroundings without completely evaporating the unused portion of the rare and expensive exotic materials. However, WEDM is a stochastic process that still lacks complete mathematical models and is driven by a myriad of parameters that will make its control a complex task. In WEDM, whenever the wire is fed too fast, the process will be interrupted not only by short circuits, but the wire electrode is most likely to break, jeopardising process efficiency in many ways. Currently, most of the literature has focused efficiency efforts in proposing a trade-off among parameters without a complete correlation with servo and control strategies. Therefore, this review aims to find and analyse the state-of-the-art approaches for controlling the Wire EDM process. A systematic literature review is conducted to identify and provide an adequate understanding of the best control strategies, fundamental parameters, and role. With no previous studies found, the present review focuses exclusively on servo control. First, it provides a comprehensive literature framework covering four decades, to next glimpse future developments of WEDM control strategies. Potential adoption of the findings can result in a deeper understanding of control variables and a broader and more efficient concept for WEDM servo feed control. • Wire EDM lacks complete mathematical models. • Fuzzy control strategy can deliver adaptive control solution for ill-defined processes. • Integrate vibration within control strategies is a crucial factor to advance in WEDM efficiency. • Current strategies are not suitable for different types of pulse generators. [ABSTRACT FROM AUTHOR]

• We integrate rotation axis calibration and servo control to register point clouds. • We propose the circle of deviation constraint for robust feature matching/rejection. • Alternating optimization divides, linearlizes and accelerates the objective function. • The proposed method outperformed other BA or ICP-based registration methods. A fast and accurate algorithm is presented to register scans from an RGB-D camera, which rotates and scans the entire scene in an automated fashion on a pan-tilt platform. The proposed algorithm, Axis Bound Registration , exploits the movement of the camera that is bound by the two rotation axes of pan-tilt servos, so as to realize fast and accurate registration of acquired point clouds. The rotation parameters, including the rotation axes, pan-tilt transformations and the servo control mechanism, are calibrated beforehand. Subsequently, fast global registration can be performed during online operation with transformation matrices formed by the calibrated rotation axes and angles. In local registration, features are extracted and matched between two scenes. For robust registration, false-positive correspondences are rejected based on the distances of pre-oriented keypoint pairs, namely the circle of deviation constraint. Then, a more accurate registration can be achieved by minimizing the residual distances between correspondence pairs, while estimated transformations are bound to the rotation axes. Results of comparative experiments validate that the proposed method outperforms state-of-the-art algorithms of various approaches based on camera calibration, global registration, and simultaneous-localization-and-mapping in terms of root-mean-square error and computation time. [ABSTRACT FROM AUTHOR]

Highlights • A servo-controlled heliostat has been designed and built for atmospheric studies. • It enables to track the Sun for several hours with a great stability. • The spectrometer can be placed at a long distance from the heliostat (>10 m). • The instrument was validated by recording atmospheric spectra by FTIR spectrometry. Abstract The spectrometry is an important tool for atmospheric studies. In particular, passive spectrometric analysis implies associating a spectrometer with a heliostat. Indeed the heliostat enables to compensate for the Earth rotation relative to the Sun. In this way the sunlight can be collected at any time in order to detect atmospheric gases. Unfortunately, the instruments described in the literature need to be placed close to the spectrometer (< 5 m) that is insufficient for the layout of our laboratory. So a servo-controlled heliostat, which is able to track the Sun with a great stability for several hours and which can be placed at a big distance from the spectrometer (> 10 m), has been designed and built. [ABSTRACT FROM AUTHOR]

In this paper, we explore the design and evaluation method of the three-degrees-of-freedom double objective-lenses microsuspension actuators to guide the steps of design, evaluation, measurement and verification of the actuator. Further, we apply a dynamic model of the actuator in time domain and a simplified servo control mathematical model based on the second Lagrange equation. Given that the traditional microsuspension actuator design method is always ambiguous, we propose an design and evaluation method to guide the design process when there are multiple complex parameters. According to the design and evaluation method, the prototype is designed and accomplished, the dynamic characteristics of the prototype are measured, and the control simulation is carried out to verify the double objective-lenses microsuspension actuators could be controlled to compensates micro servo error of optical storage system timely and quickly. Results demonstrate that the design and evaluation method can effectively guide the design process of the microsuspension actuator. Also, the control system based on a lag-lead controller verifies the servo performance of double objective-lenses microsuspension actuator. [Display omitted] • A design and evaluation method of double objective-lenses microsuspension actuators. • A four-degrees-of-freedom dynamic model. • A simplified servo control mathematical model. • Physical prototype experiment and control system simulation. [ABSTRACT FROM AUTHOR]

Servo system is widely used in NC machines and its performance directly determines the precision of the machines. In most situations, the control structure for the servo system usually contains a cascaded P-PI feedback controller and a feedforward controller. This paper focuses on the feedforward controller parameters tuning to improve the servo performance. The feedforward controller consists of a model inversion and a parameterized disturbance model. Its parameters are tuned iteratively using the last cycle motion results. This method has the good extrapolation capability to the references and the performance improvement capacity. Moreover, it is easy to implement in real machines due to the simplicity and thus is of interest to control engineers. Experiments are carried out on an industrial prototype system. The results show that the proposed tuning method can improve the servo performance rapidly and the references are not required to keep the same during the tuning process. [ABSTRACT FROM AUTHOR]

To guarantee the safety and efficient performance of the power plant, a robust controller for the boiler–turbine unit is needed. In this paper, a robust adaptive sliding mode controller (RASMC) is proposed to control a nonlinear multi-input multi-output (MIMO) model of industrial boiler–turbine unit, in the presence of unknown bounded uncertainties and external disturbances. To overcome the coupled nonlinearities and investigate the zero dynamics, input–output linearization is performed, and then the new decoupled inputs are derived. To tackle the uncertainties and external disturbances, appropriate adaption laws are introduced. For constructing the RASMC, suitable sliding surface is considered. To guarantee the sliding motion occurrence, appropriate control laws are constructed. Then the robustness and stability of the proposed RASMC is proved via Lyapunov stability theory. To compare the performance of the purposed RASMC with traditional control schemes, a type-I servo controller is designed. To evaluate the performance of the proposed control schemes, simulation studies on nonlinear MIMO dynamic system in the presence of high frequency bounded uncertainties and external disturbances are conducted and compared. Comparison of the results reveals the superiority of proposed RASMC over the traditional control schemes. RAMSC acts efficiently in disturbance rejection and keeping the system behavior in desirable tracking objectives, without the existence of unstable quasi-periodic solutions. [ABSTRACT FROM AUTHOR]

Abstract: Frequency modulation spectroscopy (FMS) is most commonly used for trace gas detection, but its sensitivity is usually limited by the background noise attributed to residual amplitude modulation (RAM). In this paper, comprehensive theoretical analyses of FMS with and without RAM are presented. Then the measurements of RAM are experimentally verified utilizing a fiber electro-optic modulator based FMS setup. In order to reduce the RAM, a novel servo control method with its correction signal obtained from a polarization detection scheme was proposed and pure FMS signals of C2H2 gas at wavenumber of 6544.4419cm−1 are obtained. Furtherly, long-term measurements of the dispersion background signal with feedback loop on and off are performed to evaluate the stability of the instrument. Finally, this servo control method is extended into the application of Pound–Drever–Hall frequency stabilization technique to improve the locking accuracy and stability. [Copyright &y& Elsevier]

Abstract: Servos (as commonly used in radio controlled vehicles and small scale robotics) are DC actuators which use a potentiometer to provide built-in feedback to localise an actuator arm. To minimise jerky control movements, many servo controllers include velocity and acceleration control which operate using a defined velocity profile resulting in the Pulse Width Modulation (PWM) control value being changed over time. On power-up the initial position of the servo arm is unknown to external controller—resulting in the arm moving to the starting value at maximum speed (which may be mechanically hazardous). In this paper two different techniques for performing a soft-start for servos are described and evaluated, namely: voltage profiling and intermittent drive. The different techniques are implemented and evaluated using four different servos based on measured velocity profiles, current consumption, and peak torque. [Copyright &y& Elsevier]

Abstract: This paper deals with position control of a single degree of freedom (sdof) manipulator actuated by Pneumatic Artificial Muscle (PAM) actuator. Though the compliance characteristics of these actuators are desirable, their inherent non-linearities due to compressibility of air and non-linear load–contraction characteristics make them difficult to model and design of controller becomes very complex. To overcome this, a novel method is hereby proposed to establish an accurate empirical model for the system. The quasi-static characteristics of the PAM are first modelled followed by the dynamic characteristics through spectral analysis. For cost competitiveness, the present work incorporates inexpensive on–off switching valves operated using a novel model based proportional pressure regulator developed using a modified PWM algorithm. Finally, a conventional PID position controller is extended using the empirical PAM model and an inner pressure regulation loop to achieve trajectory tracking of a sdof manipulator. Results for pressure regulation and position control confirm the effectiveness of the adopted approach in achieving performance comparable to the state-of-the-art results at a reduced control complexity and total set-up cost. [Copyright &y& Elsevier]

Abstract: Design and implementation of servo controller for a hydraulic servo system with a load of large inertia, low stiffness and multi-stage resonance is considered. To achieve a set of desired requirements, this paper is divided into three parts of work. First, notch filters and peak filters extracted from the general phase stabilized controller are applied in order to restrain the resonance peak of the load. Second, a disturbance observer in the feedback path is used for rejecting low-frequency disturbance and high-frequency noises. Third, as the complete inversion of the closed-loop dynamics at the whole frequency band does not necessarily give the best performance, low-pass filter, proportional derivative (PD) compensation, and lead correction are adopted in the feedforward path. The work proposed here overcomes the challenge of improving amplitude margin and phase margin while eliminating resonance simultaneously. The effectiveness of the combined control is investigated through simulation and experiment results, which show that the control strategies raised here successfully complete resonance rejection and also give ideal performances that meet the indices. [Copyright &y& Elsevier]

Abstract: Motion synchronization of servo systems carries great importance in various precision manufacturing operations. For double-sided (duplex) milling of thin plates, in particular, the cutting tools must engage with both sides of workpiece in a precisely synchronized manner to avoid forced vibrations. A continuous sliding mode controller (SMC) is developed to deal with the problem of position synchronization for dual spindle servo systems. Synchronization error is defined as the differential position error between the two servo systems that follow same reference motion command. Hence, the developed continuous SMC penalizes 3 error states; namely the individual servo positioning errors, and the synchronization error between them. As a result, individual trajectory tracking as well as motion coupling objective is introduced. The final control law is implemented in the form of a PID controller in real time, and double-sided milling experiments are performed to verify its effectiveness. [Copyright &y& Elsevier]

Abstract: Pseudo-derivative feedback with feed-forward gain (PDFF) combines the advantages of proportional-integral (PI) and pseudo-derivative feedback (PDF) controllers. However, PDFF responds more slowly to a command than does PI. To increase the speed of response of the PDFF controller, this work presents a PDFF with moving average errors control. A low-pass IIR filter path for errors compensation that accelerates the slow response is added to a PDFF control loop. A fuzzy inferencer is utilized to adjust the feed-forward gain and integral gain of the PDFF controller to allow closed-loop poles of the transfer function to be properly placed to improve load torque disturbance rejection capability. Simulated and experimental results reveal that the response and load disturbance rejection ability of the fuzzy PDFF-IIR controller are better than those of the traditional PDFF controller. [Copyright &y& Elsevier]

Abstract: Rotor position detection is important for motor servo system design. In general, there are two kinds of methods to obtain the position information in real time, sensor or sensorless methods. The sensor methods use position sensors such as optical encoder. This will greatly increase the cost of the system, and the sensor with high precision is difficult to be installed. On the other hand, the sensorless method can reduce the cost, but the reliability and complexity of the algorithm is still the problems. In this paper, a new low-cost integrated position feedback unit, which is composed of the integrated position sensor, signal processing hardware and software, is described. The sensor is easy to manufacture and has better precision with the help of signal processing circuit and software based on DSP. The sensor can obtain absolute rotating angle using inductance detection method, and it is originally designed and used for a 2-phase hybrid stepping motor position servo system. The integrated sensor and the proper control strategy make the system become a low-cost high-performance position servo system. Even though the feedback unit is originally designed for a 2-phase hybrid stepping motor, the same unit also can be used with other types of motors. [ABSTRACT FROM AUTHOR]

Abstract: Digital servo control was introduced in mass production hard disk drives (HDD) in 1986. Since then various kinds of digital control technologies have been applied to HDD servo systems, and the performance has been improved dramatically. A 3.5-in. HDD capacity, which was 251MB in 1988, is 2000,000MB (2.0TB) in 2009. Faster access (high quality motion control), more precise positioning (nanometer scale control), and more robustness against disturbances and plant fluctuations are required to meet the increasing demand for larger capacity and higher performance HDDs. The field of HDD servo technology provides an example of techniques for micro or nanometer accuracy. Although the application aspects may be different in other fields requiring high positioning accuracy, the methods used in HDDs are general enough that they can be transferred. This paper is not intended to act as a review paper, but presents several fundamental ideas of precision positioning servo control, especially ones derived from the history of HDD servo control. [Copyright &y& Elsevier]

Abstract: This paper is concerned with the rejection of multiple narrowband disturbances in hard disk drives (HDDs). Inspired by a control blending idea, the multi-frequency disturbance rejection is formulated as a blending control problem. Each disturbance rejection is accomplished by using the optimal control method. Based on all optimal controllers, the blending technique is applied to yield a single controller which is capable of achieving rejection of all disturbances. Rejections of two and three disturbances for a 1.8-inch HDD VCM actuator are taken as application examples in the paper. Simulation and experimental results show that the ultimate controller results in a simultaneous attenuation of disturbances with frequencies higher or lower than the closed-loop system bandwidth. Moreover, the method turns out to be able to lift phase and thus prevent phase margin loss when it is used to deal with disturbances near bandwidth. [Copyright &y& Elsevier]

Abstract: This paper takes into consideration the rejection of narrow-band disturbances at two frequencies higher than servo bandwidth in order to obtain good positioning accuracy for a micro-actuator. A phase stabilized feedback controller combined with a disturbance observer structure is applied to achieve the rejection of the two disturbances. A high frequency bandpass filter is adopted in the disturbance observer in order to reject the high frequency disturbance, and simultaneously the stability of the feedback control loop is remained when the disturbance observer is added on to the external stabilized controller. The effectiveness of the combined control is investigated through simulation and experiment results, which show that this control scheme can reduce position error significantly and is better than using phase stabilized feedback control only or proportional-integral-derivative (PID) controller with notch filters. [Copyright &y& Elsevier]

Abstract: This paper is concerned with disturbance rejection for data storage systems which applies microactuator to expand its servo bandwidth. An approach based on the generalized Kalman–Yakubovic–Popov (KYP) lemma together with the Youla parameterization is proposed which allows us to suppress disturbances of particular frequencies within or beyond the servo bandwidth via convex optimization. The rejection of narrowband high-frequency disturbances is achieved by using this approach. Simulation together with implementation results demonstrate the simplicity and effectiveness of the design method as compared with the traditional frequency weighting method. [Copyright &y& Elsevier]

Abstract: We present a new MEMS-based milli-mirror for precise tracking in high-density optical disk drives (ODDs). The device consists of a torsionally suspended mirror plate, one pair of torsion springs, which support the mirror plate and offer a restoring torque, and two pairs of electrodes attached to the mirror plate and glass substrate. The dimensions of mirror plate and torsion springs were determined so that a 5V dc bias ±4.5V ac drive voltage would provide the mirror with ±0.02° rotation to transmit laser beam spot on spinning disk. The MEMS-based milli-mirror was successfully fabricated using MEMS technology. Displacement–voltage linearization scheme was implemented by differential voltage driving. The static and dynamic performances of mirror prototype, such as capacitance versus driving voltage, rotation angle versus driving voltage, and resonant frequency were characterized and compared well with the simulation solutions. The mechanical resonant frequency of the mirror is expected to be high enough to satisfy the requirement of the servo bandwidth of precise tracking-control in high-density blue-laser optical disk drive. [Copyright &y& Elsevier]

Robust gap-control is applied to compensate for electrode wear in an electric discharge scanning (ED-Scanning) process. This control compensates for the wear without reference to the wear ratio of the electrodes. An auxiliary gap-control loop in the vertical direction can compensate for the wear of the electrode, according to the discharge voltage when the electrode is driven along the two horizontal axes. A proportional controller for a gap-control system with maximal robustness is designed and robustly analyzed to override the non-linear and time-varying feedback. The simulated and practical responses of such a robust gap-control system yield favorable performances. The controller can maintain the location of the discharge on the vertical axis and fix the depth of the removal to that of a particular layer. The practical scanning process reveals that the dimension of a hole can be guaranteed only if the data concerning the depth of removal under specific discharge conditions and electrode diameters are applied. [Copyright &y& Elsevier]

This paper studies the dynamic behavior of Cylindrical Ultrasonic Motor (CUSM) under the external excitation of two sinusoidal voltage signals. Such a study is useful for designing effective servo control scheme for the CUSM. In particular, it is important to understand how the output speed is related to the three possible control inputs—voltage amplitude, driving frequency and phase difference of the two excitation voltage signals. In this paper, the Timoshenko beam model and mode-summation method are used to analyze the dynamic response of the lead zirconate titanate (PZT) stator tube of the CUSM under external excitation. Then, the input–output relations between the speed output and control inputs are established. Finally, the validity of this theoretical analysis is verified experimentally. [Copyright &y& Elsevier]

Linear motors offer several advantages over their rotary counterparts in many precision manufacturing applications requiring linear motion; linear motors can achieve a much higher speed and have the potential of gaining a higher load positioning accuracy due to the elimination of mechanical transmission mechanisms. However, these advantages are obtained at the expense of added difficulties in controlling such a system. Specifically, linear motors are more sensitive to disturbances and parameter variations. Furthermore, certain types of linear motors such as the iron core are subject to significant nonlinear effects due to periodic cogging force and force ripple. To address all these issues, the recently proposed adaptive robust control (ARC) strategy is applied and a discontinuous projection-based ARC controller is constructed. In particular, based on the special structures of various periodic nonlinear forces, design models consisting of known basis functions with unknown weights are used to approximate those unknown nonlinear forces. On-line parameter adaptation is then utilized to reduce the effect of various parametric uncertainties such as unknown weights, inertia, and motor parameters while certain robust control laws are used to handle the uncompensated uncertain nonlinearities effectively for high performance. The resulting ARC controller achieves a guaranteed transient performance and a guaranteed final tracking accuracy in the presence of both parametric uncertainties and uncertain nonlinearities. In addition, in the presence of parametric uncertainties, the controller achieves asymptotic output tracking. Extensive simulation results are shown to illustrate the effectiveness of the proposed algorithm. [Copyright &y& Elsevier]

• A new multi-function servo control dynamic shear apparatus is developed. • It can perform the servo control in both vertical and horizontal directions. • It can also preform the coupled shear-flow test. In this paper, we report a new multi-function servo control dynamic shear test apparatus, DJZ-500. This apparatus consists of five parts, including power system, loading frame, servo control unit, cooling system and PC console. The normal load and shear load are measured by load cells with an accuracy of 0.1 kN; normal and shear displacement are measured by LVDTs with an accuracy of 0.1 μm. DJZ-500 has the dynamic and static loading modules, which can perform the force and displacement servo control experiments in both vertical and horizontal directions. In addition, the dynamic loading modules can execute the custom waveforms, which can be utilized to investigate the seismic response of rock and soil masses. We presented six sets of representative tests on sands to demonstrate these functions. Moreover, DJZ-500 can also preform the coupled shear-flow test with the pumping system to investigate the flow behavior in sheared rock fractures. This device makes up for the deficiency of the existing equipment, and promotes the study of frictional properties of geomaterials. [ABSTRACT FROM AUTHOR]