Precision Motion Systems Research Articles

An Adjoint Based Data-Driven Inverse-Free Iterative Feedforward Control Method

Abstract Feedforward control is widely used in control systems since it can achieve high tracking performance by effectively compensating for known disturbances before they affect the system. For traditional feedforward control methods, the performance improvement highly depends on the model quality of the system model and the accuracy of the model-inversion. However, on one hand, in practice, the modeling error is inevitable, especially for precision motion systems with complex dynamics, on the other hand, the non-minimum phase (NPM) systems often lead to a problem that plant inversion is unstable and non- causal. Therefore, this paper proposes an approach that combines the bene?ts of data-driven feedforward control and the gradient descent method. This integrated approach aims to address challenges related to laborious model identi?cation and unstable plant inversion simultaneously. The main idea is to replace the model with dedicated experiments on the system and to avoid calculating plant inversion by applying the gradient descent method to the learning process. The simulation and experimental results show that the algorithm can achieve the optimal point-to-point tracking performance without relying on model-inversion.

Data-driven reference trajectory optimization for precision motion systems

We propose a data-driven optimization-based pre-compensation method to improve the contour tracking performance of precision motion stages by modifying the reference trajectory and without modifying any built-in low-level controllers. The position of the precision motion stage is predicted with data-driven models, a linear low-fidelity model is used to optimize traversal time, by changing the path velocity and acceleration profiles then a non-linear high-fidelity model is used to refine the previously found time-optimal solution. We experimentally demonstrate that the proposed method is capable of simultaneously improving the productivity and accuracy of a high precision motion stage. Given the data-based nature of the models, the proposed method can easily be adapted to a wide family of precision motion systems.

Iterative Control Tuning: With Application to MIMO Precision Motion Systems

Iterative Control Tuning: With Application to MIMO Precision Motion Systems

Generation Mechanism and Decoupling Strategy of Coupling Effect in Maglev Planar Motor

Magnetically levitated (maglev) planar motor is a promising precision motion system in industry. In order to address the issue of coupling effect in maglev planar motor and achieve satisfying precision motion performance, a novel decoupling strategy based on generation mechanism of coupling effect is proposed in this article. Detailed modeling process is first provided including wrench modeling, coordinates transformation, and control allocation. Further theoretical analysis proves that coupling effect caused by certain factors can be observed by specific phenomena which can be utilized as a criterion for calibration. An effective synthesized decoupling strategy is then proposed. In comparative experimental investigation, the practical phenomena are consistent with the prediction by the proposed theoretical analysis. After applying the proposed decoupling strategy, the tracking errors in the affected axes are enormously reduced and the coupling effect is eliminated to a great extent. Achieved by simply observing specific phenomena, the proposed decoupling method for maglev planar motor is actually a meaningful supplement to existing baseline to further calibrate certain factors that are not considered in previous methods.

Design and Analysis of an Active Swing Decoupling Compliant Mechanism With Multiple Co-Directional Input Branches

AbstractIn this work, we propose a novel bio-inspired swing decoupling mechanism supporting high precision motion systems, which is composed of multiple co-directional input branches with a rigid swing unit and an anti-rotational guiding unit. By actively adjusting the input displacements, the decoupling mechanism can switch between the swing and translational modes, where the parasitic rotations can be significantly suppressed by the anti-rotational guiding unit. With this, fully decoupled X and Y linear motions are obtained in the presence of co-directional input branches. A theoretical model of the decoupling mechanism is also established to accurately describe the decoupling behavior, which is verified by finite element simulations. A prototype of the proposed swing decoupling mechanism is fabricated and instrumented with comprehensive experimental apparatus, where the experimental results effectively validate the excellent decoupling performance and demonstrate good potentials to precision engineering applications.

A fast transient tracking differentiator for high-precision motion systems

High-quality velocity signal plays an important role in fast and precise motion systems, but most commercially available motion systems are not commonly equipped with velocity sensors; hence, full access to the system states is impossible. A fast transient tracking differentiator is proposed in this article for obtaining the noise-less time derivative from a noisy measurement. The fast transient tracking differentiator is conceived within the framework of tracking differentiator methodology and is accomplished by integrating a power-like function with a smooth hyperbolic sine function. Tracking differentiator theory and Lyapunov direct method are employed to show the global asymptotic convergence. Advantages of the proposed fast transient tracking differentiator include the easy implementation with intuitive structure and high computational efficiency and faster transient and higher noise attenuation for both small and large initial estimation errors. The proposed fast transient tracking differentiator can be applied to a large class of motion systems by cascading to a robust output feedback proportional–derivative control for fast and high-precision positioning with position measurement only. Numerical simulation and real-time experimental comparisons demonstrate that the proposed approach provides an easygoing model-free output feedback solution for fast and accurate velocity reconstruction and high-performance positioning of uncertain motion systems with position measurement only.

Nonlinearity Compensation and High-Frequency Flexibility Suppression Based RIC Method for Precision Motion Control Systems

For precision motion systems widely applied in industrial manufacturing equipments, it is critical to achieve both high trajectory tracking accuracy and superior disturbance rejection ability. In this article, a novel nonlinearity compensation and high-frequency flexibility suppression based real-time iterative compensation (RIC) method is proposed to achieve excellent tracking performance in practice. The unexpected nonlinearity and high-frequency flexible mode of the plant, which limits the achievable control performance, is first compensated and suppressed. Subsequently, a RIC method based on accurate linear prediction model is proposed to further reduce the tracking error by adding a compensation term to the initial reference trajectory. The trajectory compensation idea of RIC is comparative to remarkable iterative learning control (ILC), but the proposed RIC can online generate and adjust the compensation term during real-time motion without abundant offline iteration trials in ILC. This mechanism significantly enhances the robustness to trajectory variations and external disturbances. Comparative experiments carried out on a ball-screw-driven precision motion stage with full-closed loop position feedback validate the effectiveness and superiority of the proposed method for various trajectory tracking tasks. The proposed method outperforms ILC on tracking performance, and possesses the robustness to various disturbances and reference variations, which leads to industrial application significance.

A Modeling Method for a 6-SPS Perpendicular Parallel Micro-Manipulation Robot Considering the Motion in Multiple Nonfunctional Directions and Nonlinear Hysteresis

Abstract Precision motion systems that call for an ultrahigh resolution are widely used in ultrahigh-precision engineering applications. In this article, a 6-SPS perpendicular compliant parallel micro-manipulation robot is presented. It is difficult to establish the mapping relationship between the applied voltage and end-effector pose due to the existence of compliant joints and piezoelectric actuators. A modeling method for a 6-SPS perpendicular compliant parallel micro-manipulation robot considering the motion in nonfunctional directions and hysteresis effect is proposed to reflect the characteristic from input voltage to output pose. The motion of the spherical compliant joint is regarded as a spatial six degrees-of-freedom mechanism. This method considers the motion errors in multiple nonfunctional directions of the compliant joints. The internal force and torque are also taken into account based on the spatial force and moment balance condition. The slow, fast dynamic modes, and hysteresis nonlinearity behavior are described using a series and parallel hybrid model structures. The kinematic model and compliance model without considering the motion in nonfunctional directions are conducted for comparison. Simulation results show that the presented modeling method has higher modeling accuracy. Finally, experiments on a 6DOF compliant parallel micro-manipulation robot reveal the fine modeling performance of the developed method for the precision motion systems in the presence of intrinsic hysteresis effect and motion error of compliant joints.

Development of a Flexure Mechanism for High-Precision Applications in Scanning Applications

The utilization of the Planar XY Flexural Mechanism is extensive in precision motion systems, where it induces relative motion between a fixed support and a motion stage through the utilization of material flexibility. In contrast to rigid link mechanisms, this mechanism presents noteworthy advantages, including zero backlash, frictionless motion, and high-order repeatability, all within a considerably more compact form. Notably, flexure mechanisms are characterized by their construction from a single monolith. This research primarily concentrates on modeling the flexural process to achieve precise scanning across a broader range at increased speeds. In the assessment of the motion stage's static deflection, Finite Element Analysis (FEA) is employed in the static analysis. Subsequently, the mechanism undergoes activation using a weight pan and weights, with displacement being monitored through a Dial Gauge Indicator. The experimental setup encompasses the flexural mechanism, Dial Gauge, Weight Pan and Weights, Pulley, String, Small metal strip, and Optical Bread Board. A comparison of experimental and analytical findings reveals minimal variation, affirming the effectiveness of the flexural mechanism in delivering precise motion for high-precision applications.

3D Positioning Method for Pineapple Eyes Based on Multiangle Image Stereo-Matching

Currently, pineapple processing is a primarily manual task, with high labor costs and low operational efficiency. The ability to precisely detect and locate pineapple eyes is critical to achieving automated pineapple eye removal. In this paper, machine vision and automatic control technology are used to build a pineapple eye recognition and positioning test platform, using the YOLOv5l target detection algorithm to quickly identify pineapple eye images. A 3D localization algorithm based on multiangle image matching is used to obtain the 3D position information of pineapple eyes, and the CNC precision motion system is used to pierce the probe into each pineapple eye to verify the effect of the recognition and positioning algorithm. The recognition experimental results demonstrate that the mAP reached 98%, and the average time required to detect one pineapple eye image was 0.015 s. According to the probe test results, the average deviation between the actual center of the pineapple eye and the penetration position of the probe was 1.01 mm, the maximum was 2.17 mm, and the root mean square value was 1.09 mm, which meets the positioning accuracy requirements in actual pineapple eye-removal operations.

Dual-Loop Iterative Learning Control With Application to an Ultraprecision Wafer Stage

Iterative learning control (ILC) enables high performance for motion systems executing repetitive tasks. The robustness filter of ILC enhances the robustness w.r.t. model uncertainties and disturbances, but results in that the repetitive error cannot be eliminated. In this article, a dual-loop ILC (DILC) approach is proposed for precision motion systems to explicitly address the design tradeoff of standard ILC between robustness and tracking performance. In the proposed DILC approach, the standard ILC is paralleled with an additional feedforward signal. When ILC converges, the additional feedforward signal is updated by the converged total feedforward signal, and then, the ILC begins a new iteration. As a result, the nonzero asymptotic error caused by the robustness filter is eliminated by adding an iterative action over the feedforward signal onto ILC. Comparative simulation and experimental results confirm that, compared to ILC, the proposed DILC can significantly enhance the tracking performance without the sacrifice of robustness w.r.t. model uncertainties and disturbances. Application to an ultraprecision wafer stage illustrates that the proposed DILC decreases the peak values of moving average and moving standard deviation of the tracking error by 52.7% and 43.9%, respectively.

Aerosol Jet Printing of 3D Pillar Arrays from Photopolymer Ink.

An aerosol jet printing (AJP) printing head built on top of precise motion systems can provide positioning deviation down to 3 μm, printing areas as large as 20 cm × 20 cm × 30 cm, and five-axis freedom of movement. Typical uses of AJP are 2D printing on complex or flexible substrates, primarily for applications in printed electronics. Nearly all commercially available AJP inks for 2D printing are designed and optimized to reach desired electronic properties. In this work, we explore AJP for the 3D printing of free-standing pillar arrays. We utilize aryl epoxy photopolymer as ink coupled with a cross-linking “on the fly” technique. Pillar structures 550 μm in height and with a diameter of 50 μm were 3D printed. Pillar structures were characterized via scanning electron microscopy, where the morphology, number of printed layers and side effects of the AJP technique were investigated. Satellite droplets and over-spray seem to be unavoidable for structures smaller than 70 μm. Nevertheless, reactive ion etching (RIE) as a post-processing step can mitigate AJP side effects. AJP-RIE together with photopolymer-based ink can be promising for the 3D printing of microstructures, offering fast and maskless manufacturing without wet chemistry development and heat treatment post-processing.

Fabrication of high precision grating patterns with a compliant nanomanipulator-based femtosecond laser direct writing system

Femtosecond laser direct writing (LDW) is an enabling technique in the fabrication of micro-/nano-scale devices. In a typical LDW system, the high precision motion system serves as the key component for micro-/nano-fabrication. In this paper, a cost-effective LDW system is proposed with a self-developed compliant nanomanipulator. The accuracy of the LDW system with the proposed nanomanipulator is validated through the fabrication of optical grating patterns with micro-scale feature sizes (<3 μm). The optical grating patterns are fabricated with different spacings, scan speeds and laser powers, and surface characterization and quantitative analysis of the samples are conducted. The characterization results demonstrate that the proposed LDW system is capable of controlling the spacing accuracy within 300 nm in a millimetre-level working range. Compared with the existing commercial laser direct writing system of sub-micrometer level motion accuracy, the proposed compliant nanomanipulator-based femtosecond laser direct writing system achieves comparable performances with much lower costs, and it has broad potential applications to sub-micron level laser direct writing fabrication.

Magnetic Levitation Technology for Precision Motion Systems: A Review and Future Perspectives

Precision motion systems are the core of a wide range of manufacturing equipment and scientific instruments, and their motion performance directly determines the quality and speed of the associated manufacturing or metrology processes. Magnetically levitated precision motion systems, where the moving target is supported by magnetic forces and without any mechanical contact, provide advantages of frictionless motion, vacuum compatibility, and contamination-free operation. These features endow the magnetic levitation technology with the capability to deliver excellent overall performance for precision positioning systems. Through decades of research and engineering efforts, significant advances have been made in the actuation, sensing, design, and control of magnetically levitated precision motion systems. This paper provides an introduction to the fundamentals of the feedback control, actuation, and sensing for the magnetic levitation technology, and provides a comprehensive literature review of various magnetically levitated precision positioning systems developed over the past three decades. The final part of this paper identifies several challenges in the design and control of today’s precision motion systems using magnetic levitation and provides an outlook on the possible directions for future research and development.

Real-Time Iterative Compensation Framework for Precision Mechatronic Motion Control Systems

With regard to precision/ultra-precision motion systems, it is important to achieve excellent tracking performance for various trajectory tracking tasks even under uncertain external disturbances. In this paper, to overcome the limitation of robustness to trajectory variations and external disturbances in offline feedforward compensation strategies such as iterative learning control (ILC), a novel real-time iterative compensation (RIC) control framework is proposed for precision motion systems without changing the inner closed-loop controller. Specifically, the RIC method can be divided into two parts, i.e., accurate model prediction and real-time iterative compensation. An accurate prediction model considering lumped disturbances is firstly established to predict tracking errors at future sampling times. In light of predicted errors, a feedforward compensation term is developed to modify the following reference trajectory by real-time iterative calculation. Both the prediction and compensation processes are finished in a real-time motion control sampling period. The stability and convergence of the entire control system after real-time iterative compensation is analyzed for different conditions. Various simulation results consistently demonstrate that the proposed RIC framework possesses satisfactory dynamic regulation capability, which contributes to high tracking accuracy comparable to ILC or even better and strong robustness.

Continuous reset element: Transient and steady-state analysis for precision motion systems

This paper addresses the main goal of using reset control in precision motion control systems, breaking of the well-known “Waterbed effect”. A new architecture for reset elements will be introduced which has a continuous output signal as opposed to conventional reset elements. A steady-state precision study is presented, showing the steady-state precision is preserved while the peak of sensitivity is reduced. The architecture is then used for a “Constant in Gain Lead in Phase” (CgLp) element and a numerical analysis on transient response shows a significant improvement in transient response. It is shown that by following the presented guideline for tuning, settling time can be reduced and at the same time a non-overshoot step response can be achieved. A practical example is presented to verify the results and also to show that the proposed element can achieve a complex-order behaviour.

Identification of a Class of Precision Motion Systems with Uncertain Hysteretic Nonlinearities

In this paper, we propose a nonrecursive identification algorithm for a class of precision motion systems with uncertain hysteresis nonlinearities. This class of precision motion systems can be modeled by a Hammerstein system with a hysteresis nonlinearity. The hysteresis nonlinearity can be symmetric, asymmetric, rate-independent, or rate-dependent. The proposed identification algorithm is a two-stage algorithm that identifies the hysteresis nonlinearity and the linear system using measurements of the input and output of the Hammerstein system. The first stage of the algorithm uses a delayed impulse signal with simple least squares to identify the linear part of the Hammerstein system. The delayed impulse signal is shown to be persistently exciting of a sufficient order for the considered model structure. The second stage of the algorithm uses a sinusoidal input and the identified linear model obtained from the first stage to construct an estimate of the unknown intermediate signal, which represents the output of the hysteresis nonlinearity. Finally, a model of the hysteresis nonlinearity is obtained using the sinusoidal input and the constructed estimate of the intermediate signal. The identification algorithm is demonstrated on an experimental setup that includes a piezoelectric actuator, which exhibits a hysteresis nonlinearity and badly damped dynamics.

Online Iterative Learning Compensation Method Based on Model Prediction for Trajectory Tracking Control Systems

In this article, to guarantee the good tracking performance of the precision motion system for various tracking tasks, an online iterative learning compensation method is proposed for closed-loop motion control systems. The prediction model is based on the closed-loop model of the linear second-order system with a proportional-integral-derivative controller, and an estimation term is added to deal with the influence of slow-varying uncertain disturbances. On the basis of the accurate state prediction, the dynamical feedforward compensation can be obtained, which suppresses the tracking error caused by the dynamical lag. Furthermore, in order to simultaneously compensate the errors caused by nonlinear factors such as uncertain disturbances and to guarantee the smoothness of the compensated trajectory, the optimal compensation gain is determined through online iterative calculation. The online iterative approach is similar to iterative learning control, but does not require several offline iterations of a repeating trajectory. Comparative experiments are carried out on an industrial motion stage. Various experimental results consistently demonstrate that the proposed compensation scheme can achieve the tracking accuracy comparable to iterative learning, while maintaining the robustness to trajectory changes and uncertain disturbances without reoffline iteration.

Assembly deviation modelling to predict and trace the geometric accuracy of the precision motion system of a CNC machine tool

Geometric accuracy is the direct embodiment of the assembly deviation of a precision motion system. This paper proposes a novel methodology for predicting the geometric accuracy induced by assembly deviation of a precision motion system and tracing the sensitivity of the assembly deviation with respect to geometric accuracy terms. The small displacement torsor is first characterized as the periodic function of the assembly deviation and dimension parameters of the components of ball-screw feeding system, the vector constraint relationship of the parallel assembly of the feeding system is established, and a spatial propagate model of the assembly deviation for the feeding system is then constructed. A random balance design and Fourier amplitude sensitivity test method are introduced to trace the effect of each assembly feature on the geometric accuracy. The experiments were performed on a single-axis feeding system to verify the effectiveness of the proposed method. This method has the advantage that the mapping relationship between the assembly deviation and geometric accuracy with respect to the instructed position is quantified, and the assembly and adjustment is pertinent to the traceability analysis. The model has the potential to accurately allocate and maintain the machining centre.

Multi-objective optimization of a reluctance actuator for precision motion applications

Recent research and assessments have shown that the reluctance actuator is a promising candidate to drive the next generation precision motion system of the lithography machine in the semiconductor industry. This paper proposes an multi-objective optimization algorithm for an optimal design of E-core reluctance actuator considering the requirements of the precision motion system. The objectives of optimization include maximizing the output magnetic force, minimizing the mass and the time constant of the actuator, and driving the actuator stiffness value to zero. First, the characteristics of the reluctance actuator are expressed in terms of the design variables of the reluctance actuator. After that, the optimization algorithm using the Grid search method and Pareto optimal approach is proposed to obtain the optimal design of the reluctance actuator. Finally, the optimal design of the reluctance actuator is implemented in a precision motion system example. The simulation results are obtained using Finite Element Analysis in COMSOL to check the saturation of magnetic flux in the reluctance actuator. The dynamic response of reluctance actuator motion system is obtained in the time domain for different inputs to verify the optimal design of the reluctance actuator.