Fast servo control and vibration suppression based on optical ZPETC and ZPNF

This paper proposes a new high speed robust control system based on both zero phase error tracking (ZPET) control system and disturbance observer for an optical disk recording system. In optical disk recording systems (for example CD-R), the servo system must have a precise response whose error is less than 0.1 [/spl mu/m] for a reference input signal with an amplitude of 100 [/spl mu/m]. Moreover, the servo system can detect only the error, i.e. the position difference between the optical spot and the track to be followed. For this purpose, the servo system proposed in this paper estimates the reference input signal from the error, and realizes the high speed robust control based on ZPET control. Numerical simulation results point out that the proposed system has a quick and robust response.

A trajectory control strategy for a nonminimum phase system is proposed. A continuous-time version of the Zero Phase Error Tracking Controller (ZPETC), which is a well-known discrete-time feedforward controller, is considered. In the continuous-time case, the overall transfer function consisting of the ZPETC and the closed-loop plant exhibits high-pass filter characteristics. This introduces serious gain errors between the desired and actual output if the desired output is made directly as the ZPETC’s input. This paper proposes the use of a specially designed sinusoidal trajectory to compensate for the gain errors. The sinusoidal trajectory imparts a synergic effect to tracking performance when combined with the continuous ZPETC. Continuous ZPETC with sinusoidal trajectory is evaluated successfully by applying to a nonminimum phase plant, single link flexible arm.

The paper shows the influence of the location of unstable zeros on the tracking performance of feedforward prefilters. Unstable zeros are divided into a number of classes. It is shown that existing feedforward prefilters (Zero Phase Error Tracking Control (ZPETC), E-filter, Extended Bandwidth ZPETC, ...) perform well for two classes, but fail for a particular class of unstable zeros. For this class, a characteristic frequency, fc, exists such that the induced gain error attenuates all frequencies of the reference trajectory f ≤ fc and amplifies frequencies f > fc. Hence, it is impossible to freely select the tracking bandwidth. Therefore, an optimal feedforward prefilter for discrete time nonminimum phase systems is presented to deal with this class of unstable zeros. As in the ZPETC method, the prefilter compensates for unstable zeros in the inverse system model, retains the zero phase property, and introduces small gain errors. But in addition, the design minimizes a cost function for which a least square solution is found. A frequency and time domain analysis shows the superior performance of the presented optimal prefilter design even for trajectory with high frequency components.

Dynamic feed-forward control of a parallel kinematic machine

Research of Compound Controller for Flight Simulator with Disturbance Observer

The combined contour control strategy of H_inf velocity feedback control, zero phase error tracking control (ZPETC) and normal cross-coupled control is proposed. It reduces the influences of load interference, mechanical time delay and parameter mismatch between two axes on the contour accuracy of XY table directly driven by two permanent magnet linear synchronous motors (PMLSM). H_inf velocity feedback control has the advantages of fast responding and suppressing interference. ZPETC implements precise tracking based on zero-pole and phase cancellations. With the contour error as the direct controlled variable, normal cross-coupled control improves the contour accuracy by real-time compensation between two axes. Compared with the conventional method, the contour error is decreased much and the controller design is simplified. Simulation results show that the designed control system possesses good tracking performance, strong robustness and high contour accuracy.

A control strategy, which combines predictive control with zero phase error tracking controller (ZPETC) and time delay scheme, is developed to overcome positioning and tracking difficulties for transmission mechanism with friction. Time delay control is used to cancel disturbances and potential nonlinearities. Predictive control uses future and past information to maintain servo performance, regulator performance and robust stability under the influence of disturbance and measurement noise. ZPETC is included to improve the overall system bandwidth. A cycloidal velocity profile is used to generate smooth reference motion to avoid saturating the actuator. Realistic numerical simulations that considered the effect of sampling, quantization and friction were performed to investigate the effectiveness of the proposed control scheme. Encouraging transient response and steady-state control performance were observed in the results of positioning control simulation of a one-dimensional transmission mechanism.

This paper considers a design problem of discrete-time preview feedforward controllers such that the gain characteristics of the overall system is within an arbitrarily specified bound subject to the zero phase error condition for a plant having nonminimum phase zeros. In order to solve this problem, a feedforward controller termed Optimal-Feedforward Controller with Zero Phase Error Tracking Controller (Optimal-FCZPETC) is introduced. With this controller, the phase characteristics of the overall system is zero for all frequencies and the maximum value of the gap between the gain of the overall system and unity, which is ideal gain characteristics, is minimized under given preview steps. The choice of the preview steps is an unsolved problem. In this paper, we investigate the Optimal-FCZPETC from the viewpoint of the preview steps. The contribution is to give explicitly the minimum value of the maximum gap between the gain of the overall system and unity for given preview steps and to show that the minimum value can be made arbitrarily small as the preview steps increase. As a result, a simple algorithm is proposed to find the minimum preview steps such that the gain characteristics of the overall system is within an arbitrarily specified bound. The effectiveness is shown by simulation results.

This study focuses on a piezo-actuated two-degrees-of-freedom (2-DOF) decoupled compliant micro-positioning stage driven by a novel compound bridge-type amplification mechanism embedded with Scott-Russell mechanism. According to the system identification, the transfer functions of two working directions are obtained. Both of the measured dynamic cross couplings between them are also lower than −40 dB, demonstrating that the single-input-single-output control strategy can be adopted reasonably. Based on the identified frequency response, a fractional order proportional-integral (FOPI) controller is designed for the 2-DOF compliant micro-positioning stage. Compared with the integer order proportional-integral (IOPI) controller using same parameters, the amplitude error of the FOPI controller is 2% lower than that of IOPI controller. Additionally, a fractional order zero phase error controller (FOZPETC) is designed by obtaining the inverse of the fractional order closed-loop system in a large frequency range. And the gain of the corrected control system is 1 in a wide frequency band to eliminate the phase error in the close-loop control system. Finally, using the compound control strategy of FOZPETC+FOPI, a series of trajectory tracking tests are carried out on the proposed novel 2-DOF compliant micro-positioning stage. All the experimental results prove that the designed controller can achieve good positioning and tracking performance of the micro-positioning stage, and confirm the effectiveness of the compound control strategy of FOZPETC+FOPI.

In this paper, a novel 2-DOF high-speed/high-accuracy robotic system is proposed, which consists of two permanent linear DC servomotors and a planar parallel mechanism. Analysis of robotic dynamical model verifies that load variation is violent. Due to the direct-drive principle of a linear motor, the load disturbance can have a significant effect on position accuracy. For this purpose, a discrete-time disturbance observer, feedback controller and ZPECT feedforward controller is presented. The disturbance observer, which is applied to the plant velocity loop, provides robustness for design of both the feedback controller and the feedforward controller. The feedback controller is intended to make the command following properties fast, and the feedforward term realized as a Zero Phase Error Tracking Controller improves tracking. In the experiment, robotic max acceleration is equal to 20m/s2, max velocity is more than 1 m/s, and resolution is 0.5 μm. While the tracking errors are less than 50 μm, the steady-state errors are less than 1 μm, and steady time is less then 42 ms. Experiment results show that: high-performance motion system can be obtained with this discrete-time controller, even in large load variance motion system.

Pulse-Width-Modulation with Zero Phase Error Tracking Controller System for Electrocardiogram Feature Extraction and Tracking from Virtual Patient

An optimal discrete controller is presented in this paper for contour error control using the standard linear quadratic (LQ) method, upon which the offset loss function and the control loss function were considered to attain a relatively good compromise. The weight coefficient for the control loss and the optimal discrete sampling period are garnered, then a zero phase error tracking controller (ZPETC) is designed with the disturbance observer (DOB) to improve the anti-interference capability and the robustness of the system. The proposed methods are capable of enhancing the stability and rapidity of the discrete controller and reducing the contour error of the system. The results may be valuable for various trajectory tracking control applications.

Iterative learning control (ILC) is an effective control tool to reduce systematic errors of the system that perform repetitive tasks. In this paper, based on the voice coil motor XY motion platform, we design a filter type ILC feedforward and feedback control structure to insure the robust stability and the tracking performance of the motion system. In the design of the control system, the concept like double closed loop control and zero phase error tracking control algorithm are being used. Both simulations and experiments show that with this control structure, a better dynamic and static performance is obtained, i.e., the maximum acceleration of the system up to 5g, maximum speed of 0.8m/s, after 4 times iterative learning, trajectory tracking error can be reduced to 0.003mm, High speed and high precision point to point control is realized. So ILC can be widely used in the field of motion system, especially for IC package, that repeatedly perform the same movement.

This paper is to develop an optimal electro-hydraulic dual-shaking table system with high waveform replication precision. The parameters of hydraulic cylinders, servo valves, hydraulic supply power and gravity balance system are designed and optimized in detail. To improve synchronization and tracking control precision, a hybrid control strategy is proposed. The cross-coupled control using a novel based on sliding mode control based on adaptive reaching law (ASMC), which can adaptively tune the parameters of sliding mode control (SMC), is proposed to reduce the synchronization error. To improve the tracking performance, the observer-based inverse control scheme combining the feed-forward inverse model controller and disturbance observer is proposed. The system model is identified applying the recursive least squares (RLS) algorithm and then the feed-forward inverse controller is designed based on zero phase error tracking controller (ZPETC) technique. To compensate disturbance and model errors, disturbance observer is used cooperating with the designed inverse controller. The combination of the novel ASMC cross-coupled controller and proposed observer-based inverse controller can improve the control precision noticeably. The dual-shaking table experiment system is built and various experiments are performed. The experimental results indicate that the developed system with the proposed hybrid control strategy is feasible and efficient and can reduce the tracking errors to 25% and synchronization error to 16% compared with traditional control schemes.

A set of shuttle training aircrafts (STAs) are used by NASA to train astronaut pilots to land the space shuttle after returning from orbit. The STA simulates the dynamics of the much larger and heavier space shuttle with the help of some hardware and software modifications. We present a STA controller that tracks the shuttle pitch trajectory. Our control system includes a feedforward controller based on zero phase error tracking control and a fuzzy proportional derivative (PD) feedback controller. Test results on a ground hardware simulator used at the Ellington Field of NASA Johnson Space Center show that the new controller significantly reduces pitch following errors between the shuttle and the STA, when compared to the existing controller.