Yik Ren Teo

Discrete-time repetitive control with model-less FIR filter inversion for high performance nanopositioning

Repetitive control (RC) is used to track and reject periodic exogenous signals. RC achieves tracking by incorporating a model of a periodic signal in the feedback path, which provides infinite loop-gain at the harmonic frequencies of the periodic signal. To improve robustness, the periodic signal model is bandwidth limited, and to improve the performance, an inverse plant response filter is used. This filter can either be an infinite impulse response (IIR) filter or a finite impulse response (FIR) filter. The accuracy of the filter typically determines the allowable bandwidth of the periodic signal model, and it is therefore desirable to obtain the most accurate inverse possible. In this paper a model-less method for synthesizing an FIR filter for the inverse response is presented, and it is compared to the common approach of using an inverse model-based IIR filter. An experimental comparison of the two approaches is presented, and it is demonstrated that the two methods produce identical results, but where the model-less FIR filter approach has the added benefit of avoiding the modeling effort needed to obtain the IIR filter.

Optimal Integral Force Feedback and Structured PI Tracking Control: Application for High Speed Confocal Microscopy.

Abstract In this paper, an improvement to Integral Force Feedback (IFF) damping control is proposed. The main limitation of Integral Force Feedback is that the maximum modal damping depends on the systems parameters. Hence, for some system achievable damping is insignificant. The proposed improvement allows any arbitrary damping ratio to be achieved for a system by introducing a new feed-through term in the system. To achieve displacement tracking, one technique is to enclose the system in an integral feedback loop. However, the bandwidth is limited due to the effects of an additional pole in the damping loop. The proposed Structured PI controller is parameterised so that it contains a zero that cancel the additional pole. Experiment was conducted on a commercial objective lens positioner. The results show an exceptional tracking and damping performance and the systems insensitivity to changes in resonance frequency. The maximum bandwidth achievable with a commercial PID controller is 26.1 Hz. In contrast, with the proposed method, the bandwidth is increased to 255 Hz.

A Simplified Method for Discrete-Time Repetitive Control Using Model-Less Finite Impulse Response Filter Inversion

Repetitive control (RC) achieves tracking and rejection of periodic exogenous signals by incorporating a model of a periodic signal in the feedback path. To improve the performance, an inverse plant response filter (IPRF) is used. To improve robustness, the periodic signal model is bandwidth-limited. This limitation is largely dependent on the accuracy of the IPRF. A new method is presented for synthesizing the IPRF for discrete-time RC. The method produces filters in a simpler and more consistent manner than existing bestpractice methods available in the literature, as the only variable involved is the selection of a windowing function. It is also more efficient in terms of memory and computational complexity than existing methods. Experimental results for a nanopositioning stage show that the proposed method yields the same or better tracking performance compared to existing methods. [DOI: 10.1115/1.4033274]

A New Repetitive Control Scheme Based on Non-Causal FIR Filters

Repetitive Control (RC) is a popular technique for tracking periodic signals and rejecting periodic disturbances. Repetitive control achieves accurate tracking of periodic trajectories by incorporating a periodic signal generator within the feedback loop. The periodic signal generator provides an infinite loop-gain at the harmonic frequencies of the reference signal. However, this scheme cannot be used in isolation due to challenges with stability and robustness. The stability and robustness can be improved by incorporating appropriate filters. However, there is a trade-off between robustness and tracking performance. The current state-of-the art is to implement plant inversion and include phase compensators to improve the high-frequency tracking performance. In this work, a RC controller is combined with a non-causal FIR filter to improve the tracking performance without the requirement for phase compensators or plant inversion. The performance of the proposed RC design is demonstrated on a piezoelectric positioner.

Regulation and integral control of an underactuated robotic system using IDA-PBC with dynamic extension

This paper proposes a method for design of a set-point regulation controller with integral action for an underactuated robotic system. The robot is described as a port-Hamiltonian system, and the control design is based on a coordinate transformation and a dynamic extension. Both the change of coordinates and the dynamic extension add extra degrees of freedom that facilitate the solution of the matching equation associated with interconnection and damping assignment passivity-based control designs (IDA-PBC). The stability of the controlled system is proved using the closed loop Hamiltonian as a Lyapunov candidate function. The performance of the proposed controller is shown in simulation.

Low-Order Damping and Tracking Control for Scanning Probe Systems

This article describes an improvement to integral resonance damping control (IRC) for reference tracking applications such as Scanning Probe Microscopy and nanofabrication. It is demonstrated that IRC control introduces a low-frequency pole into the tracking loop which is detrimental for performance. In this work, the location of this pole is found analytically using Cardanos method then compensated by parameterizing the tracking controller accordingly. This approach maximizes the closed-loop bandwidth whilst being robust to changes in the resonance frequencies. The refined IRC controller is comprehensively compared to other low-order methods in a practical environment.

Robust speed tracking control of synchronous motors using immersion and invariance

This paper presents a novel control strategy for velocity tracking of Permanent Magnet Synchronous Machines (PMSM). The model of the machine is considered within the port-Hamiltonian framework and a control is designed using concepts of immersion and invariance (I&I) recently developed in the literature. The proposed controller ensures internal stability and output regulation, and it forces integral action on non-passive outputs.

A review of scanning methods and control implications for scanning probe microscopy

In a scanning probe microscope (SPM), the image is obtained by scanning a sample relative to a physical probe which captures the topography. The conventional scanning method is a raster pattern where a triangular or sawtooth waveform is applied to the x-axis while the y-axis position is incremented. In recent years the need for higher imaging speeds has motivated the investigation of novel scanning trajectories such as spiral, cycloid, and Lissajous patterns. Although the benefits of these approaches has been described in the literature, the lack of a comprehensive comparison makes the actual performance gains, if any, unclear. This article evaluates the efficacy of current scanning methods by comparing the required scan rate, signal bandwidth, sampling frequency, and probe velocity. In addition, the control implications of each method are also discussed qualitatively. Based on the prescribed performance metrics, the sinusoidal raster method significantly outperforms the raster, spiral, and Lissajous methods whilst being the simplest to implement. Therefore, it is concluded that the most suitable currently known method for high-speed or video-rate AFM imaging is sinusoidal raster scanning.

Model-less FIR repetitive control with consideration of uncertainty

Repetitive control (RC) is used to track and reject periodic exogenous signals by including a model of a periodic signal in the feedback path. The performance of RC can be improved by also including an. The accuracy of this filter is the main limitation to the RC bandwidth. The bandwidth is typically limited with a robustness filter - often a low-pass filter which attenuates the model at high-frequencies where the model-mismatch often occurs. In this paper, two robustness filter designs are compared. The first design is a brick-wall low-pass filter commonly used in the literature. The second design is based on the uncertainty between the inverse plant response filter and the measured response of the system. Experimental results demonstrate that the proposed method outperforms the traditional brick-wall filter approach.

Active damping control using optimal Integral Force Feedback

This article shows an improvement to Integral Force Feedback (IFF) for active damping control of precision mechanisms. The benefits of IFF include robustness, guaranteed stability and simplicity. However, the damping performance depends on the stiffness of the system; hence, some systems cannot be adequately controlled. In this article, an extension to the classical integral force feedback control scheme is proposed. The new method achieves arbitrary damping for any mechanical system by introducing a feed-through term in the system.