Dynamic feed-forward control of a parallel kinematic machine

In this paper, a new scheme based on an optical zero phase error tracking controller (ZPETC) and zero phase error notch filter (ZPNF) is proposed. ZPETC is a method of feed-forward controller with effects that tracking speed of a position servo accelerated and system stability guaranteed. As the bandwidth of system includes the characteristic of gain and phase, ZPETC improves phase bandwidth greatly but the performance of gain bandwidth deteriorates slightly. So that a gain correction controller (GC)was introduce ahead of ZPETC to compensate the deterioration of the gain. Generally, ZPNF is adopted in two-mass system. ZPNF is a method of non-casual filter with effects of vibration eliminating for position servo on its load end and has less phase lag when compared with a normal notch filter. In proposed scheme, ZPETC and ZPNF are cascaded ahead of the position control loop. Simulation results on a two-mass PMSM system indicates that, a combination ZPETC and ZPNF improves both the tracking and positioning performance for a position servo system.

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.

Parallel manipulators with less than six degrees of freedom (DOF) have been increasingly used in high-speed hybrid machine tools. The structural features of parallel manipulators are dynamic, a characteristic that is particularly significant when these manipulators are used in high-speed machine tools. However, normal kinematic control method cannot satisfy the requirements of the control system. Many researchers use model-based dynamic control methods, such as the dynamic feedforward control method. However, these methods are rarely used in hybrid machine tools because of the complex dynamic model of the parallel manipulator. In order to study the dynamic control method of parallel manipulators, the dynamic feedforward control method is used in the dynamic control system of a 3-PSP (prismatic-spherical-prismatic) 3-DOF spatial parallel manipulator used as a spindle head in a high-speed hybrid machine tool. Using kinematic analysis as basis and the Newton-Euler method, we derive the dynamic model of the parallel manipulator. Furthermore, a model-based dynamic feedforward control system consisting of both kinematic control and dynamic control subsystems is established. The dynamic control subsystem consists of two modules. One is used to eliminate the influence of the dynamic characteristics of high-speed movement, and the other is used to eliminate the dynamic disturbances in the milling process. Finally, the simulation model of the dynamic feedforward control system of the 3-PSP parallel manipulator is constructed in Matlab/Simulink. The simulations of the control system eliminating the influence of the dynamic characteristics and dynamic disturbances are conducted. A comparative study between the simulations and the normal kinematic control method is also presented.The simulations prove that the dynamic feedforward control method effectively eliminates the influence of the dynamic disturbances and dynamic characteristics of the parallel manipulator on high-speed machine tools, and significantly improves the trajectory accuracy. This is the first attempt to introduce the dynamic feedfordward control method into the 3-PSP spatial parallel manipulator whose dynamic model is complex and provides a study basis for the real-time dynamic control of the high-speed hybrid machine tools.

Study of helicopter autorotation landing following engine failure based on a six-degree-of-freedom rigid-body dynamic model

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.

Dynamics and contouring control of a 3-DoF parallel kinematics 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.

Recently, the size of digital content has continued to increase. In order to cope, manufacturers of digital storage media have tried to increase capacity by narrowing the track pitch of these digital storage media. In addition, recent advancements in technology have resulted in a new optical disk has a rotation speed of over 15,000 rpm. Considering all these factors, we believe that a high-precision tracking control system is needed for optical disks. Consequently, this paper proposes a new robust double feedforward control (RDFFC) system for high-precision control. The proposed system is constructed by using two error prediction-based zero phase error tracking (ZPET) control systems, and includes an error-based disturbance observer (EDOB) that uses notch filter to suppress non-periodic disturbances. The proposed control system is designed for DDU-1000 for DVD. The experimental results confirm that our proposed system is practicable and effective.

Due to the closed-loop mechanism, the rigid body dynamic model of the parallel kinematic machine (PKM) is very complex and difficult to be used in the real-time control system. Under the uniform motion condition, this paper presents a study on the analysis and simplification of the rigid body dynamic model for a 6-UPS PKM. Firstly, the dynamic model is developed by Newton-Euler approach and decomposed into fifteen subitems. According to the working situation, the dynamic characteristics of the subitems are analyzed by the means of numerical simulation. Finally, the simplifying strategies of the dynamic model are presented and verified. It is proved by the verification results that the simplifying strategies can improve the computational efficiency greatly and only bring small simplifying error.

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.

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.

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.

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 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.