96 A Survey of Iterative Learning Control Al earning-based method for high-performance tracking control

A VCM (voice coil motor) is disclosed, the VCM including: a rotor including a cylindrical bobbin for accommodating a lens and protruded at a bottom end with a boss, and a coil block arranged at a periphery of the bobbin; a stator including a magnet facing the coil block and a yoke fixing the magnet; and an elastic member including a first elastic member formed with a through hole coupled to the boss of the bobbin and a second elastic member coupled to an upper end facing the bottom end of the bobbin; wherein the boss is formed with a disengagement prevention unit preventing the first elastic member from being disengaged from the boss, and the first elastic member is formed with a coupling unit contacting a joint where the disengagement prevention unit and the coupling unit meet.

Voice Coil Motor

This study investigated robust yaw moment control for motion stabilization in four-wheel electric vehicles. A two-degree-of-freedom direct yaw moment control scheme is proposed. An engineering weighting function with embedded engineering specifications is included in the proposed approach. The controller was synthesized and its corresponding properties were studied. A rear-wheel drive in-wheel motor electric vehicle was employed for a practical evaluation of the scheme. Because of the robust control framework, the presented system can overcome model uncertainties, side wind disturbances, and parameter variation problems. Experiments were conducted to illustrate the feasibility and effectiveness of the proposed controller.

Robust Yaw Stability Control for In-Wheel Motor Electric Vehicles

Magnetically levitated planar motor is a new-generation motion device in modern precision industry, while its advanced motion controller design is still of main concern. In this paper, a learning adaptive robust control (LARC) motion controller is proposed for a magnetically levitated planar motor developed in our laboratory to achieve good tracking performance. The planar motor consists of a Halbach permanent magnetic array as the stator, and a levitated platen containing four groups of three-phase windings as the mover. Based on the Lorentz force law, the mover placed in the magnetic field is subjected to vertical force for levitation and horizonal force for planar motion through dynamics decoupling and current allocation. An LARC control scheme containing adaptive robust control (ARC) term and iterative learning control (ILC) term in a serial structure is then proposed for the magnetically levitated planar motor to achieve high-performance tracking even there exist parametric variations and uncertain disturbances. Comparative experiments between traditional lead, ARC, ILC, and the proposed LARC are carried out on the planar motor to track sinusoidal, point-to-point, and planar circular motions, respectively. The experimental results consistently validate that the proposed LARC control strategy outperforms other controllers much, and possesses not only good transient/steady-state tracking performance but also parametric adaptation ability and uncertain disturbance robustness. The proposed scheme actually provides a practically effective technique for motion control of magnetically levitated planar motors in industrial applications.

Performance-Oriented Precision LARC Tracking Motion Control of a Magnetically Levitated Planar Motor With Comparative Experiments

Any industry processes, aerospace systems, transportation systems, power grid systems, etc. are becoming more and more complex. Modeling the accurate physical models for these plants using first principles may be impossible or infeasible. Even if their physical models are available, they may be too complex to be tractable for controller design, system monitoring, performance evaluation, etc. However, huge amounts of measured process data can easily be collected due to the well developed information technology, both in the form of stored historical data from prior measurements and on-line data in real time during process runs. Thus, it would be very significant if we can make full use of those on-line or off-line process data to directly design controller, predict and assess system states, evaluate performance, make decisions, perform real-time optimization, and conduct fault diagnosis. For this reason, the establishment and development of data-driven control (DDC) theory and methodology are the urgent issues both in theory and applications.

Data-Driven Control and Learning Systems

In precision motion control, well-designed feedforward control can effectively compensate the reference-induced tracking error. To achieve excellent tracking performance such as nanometer accuracy regardless of reference variations, an integrated model-data-based zero-phase error tracking feedforward control (ZPETFC) strategy is synthesized for precision motion systems with complex and nonminimum phase (NMP) dynamics. The feedforward controller comprises a conventional ZPETFC controller and a gain compensation filter structured with symmetric finite impulse response (FIR) filter. Especially, the conventional ZPETFC is predesigned based on the plant model, and consequently, the feedforward controller is parameterized by the gain compensation filter coefficients, which results in excellent capacity for approximating the inverse behavior of the complex and NMP dynamics. In order to compensate the modeling error in the conventional ZPETFC design and improve the tracking performance, a data-based instrumental-variable method with impulse response experiment is developed to obtain the optimal parameter vector under the existence of noise and disturbances. Furthermore, the ridge estimate method using singular value decomposition is employed to guarantee a fast convergent iteration in the case of ill-conditioned Hessian matrix. The proposed ZPETFC strategy enables a convex optimization procedure with the inherent stability in the iterative tuning process, and is finally implemented on a developed ultraprecision wafer stage. Comparative experimental results demonstrate that the strategy is insensitive to reference variations in comparison with iterative learning control, and outperforms preexisting model-based ZPETFC and data-based FIR feedforward control.

An Integrated Model-Data-Based Zero-Phase Error Tracking Feedforward Control Strategy With Application to an Ultraprecision Wafer Stage

Wafer stage is an important mechatronic unit of industrial lithography tool for manufacturing integrated circuits. This paper presents a novel state/model-free variable-gain discrete sliding mode control (DSMC) to suppress the unmolded position-dependent dynamics and disturbances in the nanopositioning wafer stage. The proposed DSMC is essentially composed of feedforward control term, linear feedback control term, and nonlinear switching control term, which can be designed separately. The gain of the switching control term is meaningfully designed to be variable to balance the tradeoff between the robustness and the chattering. Data-driven parameter optimization approach is employed to achieve the optimal controller parameters of the typically nonlinear controller, where off-line parameter updating is iteratively carried out based on the input/output data to minimize a predefined objective function. This scheme facilitates a rapid implementation without either a parameter model or a state observer, and excellent tracking performance with the optimal controller parameters. Moreover, the closed-loop stability is analyzed, and the proposed DSMC is finally implemented on an ultraprecision wafer stage developed in our lab. Comparative experimental results demonstrate that it not only achieves nanoscale tracking accuracy but also possesses promising robustness to position-dependent dynamics and disturbances.

State/Model-Free Variable-Gain Discrete Sliding Mode Control for an Ultraprecision Wafer Stage

The invention relates to a displacement measuring system of a heterodyne grating interferometer, which comprises a reading head, a measuring grating and an electronic signal processing component, and displacement measurement is realized by the displacement measuring system on the basis of theories of grating diffraction, optical doppler effect and optical beat frequency. The reading head comprises a dual frequency laser generator, an interferometer and a signal conversion unit, wherein dual frequency laser is generated by the dual frequency laser generator and divided into reference light and measuring light by a polarization spectroscope, the measuring light enters the measuring grating to generate positive and negative first-order diffraction, a beat signal which contains displacement information in two directions is formed by diffracted light and the reference light in a photodetection unit, and linear displacement output is realized through signal processing. According to the measuring system, sub-nanometer and even higher resolution and precision can be realized, horizontal large stroke displacement and vertical displacement can be simultaneously measured. The measuring system has the advantages of insensitivity to environment, high measuring precision, small size, light weight and the like, and when the measuring system is used as an ultraprecise platform position measuring system of a photoetching machine, the comprehensive performance of a platform can be improved.

Displacement measuring system of heterodyne grating interferometer

Iterative feedback tuning (IFT) enables the data-based optimization of feedback controller parameters without the plant model and disturbance model. However, the traditional IFT suffers from a generally slow convergence rate and requires multiple experiments in each iteration, which severely limit its applicability for precision motion industry. In this paper, a new framework for IFT with its focus on the practical applicability is synthesized. Specifically, in order to improve the convergence rate, a novel two-loop iterative algorithm of IFT is proposed by introducing a weighted gradient of the performance criterion. This algorithm seeks to directly minimize the performance criterion instead of its linear approximation in each iteration. Furthermore, an unbiased estimate method of the auxiliary variables is developed based on the impulse response experiment. These guarantee the proposed approach high convergence rate with less experiments required per iteration. Comparative simulation and experimental results demonstrate that the proposed approach converges much faster than the traditional IFT, and outperforms the model-based approach in terms of the tracking performance. The ease of implementation and effectiveness makes the proposed approach highly suitable for industrial applications.

Convergence Rate Oriented Iterative Feedback Tuning With Application to an Ultraprecision Wafer Stage

The invention discloses a voice coil motor relating to an ultraprecise motion positioning device and comprising an upper permanent magnet assembly, a lower permanent magnet assembly and one or two winding assemblies arranged between the upper and the lower permanent magnet assemblies. The voice coil motor is technically characterized in that the degree of freedom of the voice coil motor moving along the Y-axis direction is increased; when a motor rotor moves along the Y-axis direction, the effective coil area between the permanent magnet assemblies is unchangeable, and the motor rotor has a Lorentz resultant force of zero along the Y-axis direction and an unchangeable Lorentz force along the X-axis direction. The invention meets the motion requirements that the rotor of the voice coil motor finely moves in the X direction and the motor rotor can move for a long stroke along the Y-axis direction, increases working space of the motor, and solves the problem of limited motion of the rotor along the Y-axis direction in the traditional voice coil motor structure.

Haihua Mu

Papers

An Integrated Model-Data-Based Zero-Phase Error Tracking Feedforward Control Strategy With Application to an Ultraprecision Wafer Stage

State/Model-Free Variable-Gain Discrete Sliding Mode Control for an Ultraprecision Wafer Stage

Displacement measuring system of heterodyne grating interferometer

Convergence Rate Oriented Iterative Feedback Tuning With Application to an Ultraprecision Wafer Stage

Voice coil motor