Dynamic modeling and control of a conveyance microrobotic system using active friction drive

Presents a new generation of compliant multidegree of freedom piezoelectric microconveyer for microobjects based on the cooperation of arrayed direct-drive micro standing-wave ultrasonic actuators (microSWUMs). Their operating driving principles based on active frictional contact forces offer direct-drive and low-speed characteristics at the microscale. The tradeoff, however, is the complexity of dynamic modeling and control to cope with the optimization of the intermittent friction drive mechanism. A method using an equivalent electromechanical circuit is proposed for estimating and analyzing the optimum driving force, including the dynamic electrical and the mechanical energy conversions. On the basis of the proposed method, the friction drive optimization of the microrobot is performed through the implementation of different controllers: 1) an electromagnetic-field-based preload controller ensuring optimal preload; 2) an open-loop piecewise-modulated controller for self locking and driving force control; and 3) a resonant frequency compensation. Finally, an experimental investigation has been performed on a prototype of ultrasonic microconveyer incorporating 48 arrayed microSWUMs whose overall dimensions are 47 /spl times/ 29 mm/sup 2/ in order to demonstrate the proposed optimized friction drive.     

一维纳米定位控制系统

研制了一套以数字信号处理（DSP）芯片为控制器的一维纳米定位控制系统。该系统主要由驱动平台、微型迈克尔逊干涉仪、DSP控制器等三个模块组成。其中驱动平台模块由线性滑轨、超声波马达HR4和驱动器AB2组成。HR4配合其专用驱动器AB2利用摩擦力来驱动侧面贴有陶瓷片的线性滑轨,微型迈克尔逊干涉仪用来感测滑轨的位移。基于BP神经网络的PID控制算法和迈克尔逊干涉仪的信号处理运算全部由DSP控制器完成。实验结果表明,该定位系统行程为20 mm,定位精度优于10 nm,重复定位标准偏差为7 nm。该定位系统具有系统架构简单,定位精度高等优点,可用于大行程高精度定位应用场合。     

Robust motion controller design for high-accuracy positioningsystems

This paper presents a controller structure for robust high speed and accuracy motion control systems. The overall control system consists of four elements: a friction compensator; a disturbance observer for the velocity loop; a position loop feedback controller; and a feedforward controller acting on the desired output. A parameter estimation technique coupled with friction compensation is used as the first step in the design process. The friction compensator is based on the experimental friction model and it compensates for unmodeled nonlinear friction. Stability of the closed-loop is provided by the feedback controller. The robust feedback controller based on the disturbance observer compensates for external disturbances and plant uncertainties. Precise tracking is achieved by the zero phase error tracking controller. Experimental results are presented to demonstrate performance improvement obtained by each element in the proposed robust control structure     

Design and Performance Tuning of Sliding-Mode Controller for High-Speed and High-Accuracy Positioning Systems in Disturbance Observer Framework

The tuning method of controllers can be used for effectively determining the overall performance of positioning systems. In particular, this method is highly effective in the case of high-speed and high-accuracy positioning systems. In this paper, a sliding-mode controller that uses one of the well-known approaches of robust control methodology is designed for high-speed positioning systems that require a high-accuracy performance. A performance-tuning method based on a disturbance observer (DOB) structure is also proposed. First, a generalized disturbance attenuation framework named robust internal-loop compensator (RIC) is introduced, and a sliding-mode controller based on a Lyapunov redesign is analyzed in the RIC framework. Then, the DOB properties of the sliding-mode controller are presented, and it is shown that the performance of the closed-loop system with a sliding-mode controller can be tuned up by using the structural characteristics of the DOB. These results make the design of an enhanced sliding-mode controller possible. Finally, the proposed algorithm is experimentally verified and discussed with two positioning systems. Experimental results show the effectiveness and the robustness of the proposed scheme.     

Observer-Based Ripple Force Compensation for Linear Hybrid Stepping Motor Drives

This paper describes our research on a force ripple compensation and closed-loop position control scheme using linear hybrid stepping motors (LHSMs) with significant thrust vibrations. In order to estimate unobservable force ripple components, we propose the Jacobian linearization observer that guarantees the convergence of state estimates into true states. For the precise control of velocity and position, an input-output feedback linearization controller is derived from a nonlinear position-dependent model of the LHSM based on elaborate reluctance network analysis. In addition, we discuss the separation principle used to separate the observer design from the controller design. Common problems associated with the force ripple, such as the positioning error, mechanical stress, and acoustic noise, are efficiently handled using the proposed active damping control scheme. Experimental results show that the positioning accuracy is significantly improved through a closed-loop control while restraining the thrust ripple.     

Performance tuning of robust motion controllers for high-accuracy positioning systems

This paper presents a structural design method of robust motion controllers for high-accuracy positioning systems, which makes it possible to tune the performance of the whole closed-loop system systematically. First, a stabilizing control input is designed based on Lyapunov redesign for the system in the presence of uncertainty and disturbance. And adopting the internal model following control, robust internal-loop compensator (RIC) is proposed. By using the structural characteristics of the RIC, disturbance attenuation properties and the performance of the closed-loop system determined by the variation of controller gains are analyzed. Next, in order to design a robust motion controller for a high performance positioning system, dual RIC structure is proposed and it is shown that if the synthesis of the robust motion control law is performed in the RIC framework, the robust property of RIC can be naturally implanted in the feedback controller. The proposed structural design of robust motion controller provides a systematic approach to the problem of robust stability and performance requirement in the face of uncertainty. Furthermore, by allowing the tradeoffs between robust stability and performance to be quantified in a simple fashion, it can illuminate systematic design procedure of the robust motion controllers. Finally, the proposed method is verified through simulation and the performance is evaluated by experiments using a high-accuracy positioning system.     

Accurate position control of a pneumatic actuator using on/offsolenoid valves

The development of a fast, accurate, and inexpensive position-controlled pneumatic actuator that may be applied to a variety of practical positioning applications is described. A novel pulse width modulation (PWM) valve pulsing algorithm allows on/off solenoid valves to be used in place of costly servo valves. The open-loop characteristic is shown both theoretically and experimentally to be near symmetrical. A comparison of the open- and closed-loop responses of standard PWM techniques and that of the novel PWM technique shows that there has been a significant improvement in the control. A linear process model is obtained from experimental data using system identification. A proportional integral derivative controller with added friction compensation and position feedforward has been successfully implemented. A worst case steady-state accuracy of 0.21 mm was achieved with a rise time of 180 ms for step inputs from 0.11 to 64 mm. Following errors to 64-mm S-curve profiles were less than 2.0 mm. The controller is robust to a sixfold increase in the system mass. The actuator's overall performance is comparable to that achieved by other researchers using servo valves     

Enhancement of tracking ability in piezoceramic actuators subject to dynamic excitation conditions

An operator representing the inverse dynamics of hysteretic effects inherent to piezoceramic actuators is used to enhance the tracking accuracy of a piezoceramic-driven positioning system when subject to dynamic reference input signals covering a wide frequency range. An open-loop tracking controller and a closed-loop tracking controller are developed based on the new inverse algorithm and are experimentally shown to achieve high-accuracy tracking control.     

A new set-point controller with bounded torques for robotmanipulators

In this paper, we propose a simple controller for set-point control of robot manipulators. The structure of this controller is composed by a saturated proportional-saturated derivative feedback plus gravity compensation. Such a control scheme has two practical features. First, for all desired joint positions, this controller delivers torques inside prescribed limits according to the actuator capability and second, the steady-state position errors owing to static friction can be arbitrarily reduced. In the case of absence of friction, we show global asymptotic stability of the closed-loop system. The performance of the proposed controller is illustrated via experiments on a two-degrees-of-freedom (2-DOF) direct-drive robot system     

Time delay control with state feedback for azimuth motion of thefrictionless positioning device

A time delay controller with state feedback is proposed for azimuth motion control of the frictionless positioning device which is subject to the variations of inertia in the presence of measurement noise. The time delay controller, which is combined with a low-pass filter to attenuate the effect of measurement noise, ensures the asymptotic stability of the closed-loop system. It is found that the low-pass filter tends to increase the robustness in the design of the time delay controller, as well as the gain and phase margins of the closed-loop system. Numerical and experimental results support that the proposed controller guarantees a good tracking performance, irrespective of the variation of inertia and the presence of measurement noise     

Observer-based robust controller design and realization of a gantry stage

A precise positioning operation is typically required for gantry systems in applications such as drop-on-demand (DOD) printing processes, precision metrology, and circuit assembly. This work presents experimental results from studies of a disturbance observer (DOB) based variable structure controller (VSC) for a gantry stage. For DOB-based controllers, a nominal model is needed; however, obtaining the nominal model is difficult for systems with friction. A pseudo-random binary signal (PRBS) is utilized to identify the linear nominal model of the gantry stage with friction. To compensate for friction effects, a filtered-VSC is studied to increase robustness and compensate for modeling uncertainties and external disturbances. Experimental results demonstrate the effectiveness of the proposed robust control structure.     

Influence of design parameters on the effectiveness of friction isolators in mitigating pre-motion friction in mechanical bearings

Mechanical bearings (i.e., sliding and rolling bearings) are widely used for motion guidance in precision positioning stages due to their low cost, high off-axis stiffness and vacuum compatibility. However, mechanical-bearing-guided stages suffer from the presence of pre-motion (i.e., pre-sliding/pre-rolling) friction which adversely affects their positioning speed and motion precision. Friction isolator has been proposed as a low-cost and robust method for mitigating the undesirable effects of pre-motion friction. It has been experimentally demonstrated that a stage with friction isolator achieves significantly reduced settling times and motion errors in point-to-point positioning and tracking applications, respectively. This paper investigates the influence of design parameters on the effectiveness of friction isolators. Experiments are carried out using three isolators with different parameters to evaluate the settling time and in-position stability during point-to-point motions, and the accuracy and robustness of feedforward friction compensation during circular tracking motions. The experimental investigation is generalized through numerical simulation and frequency domain analysis using a simple model of a friction-isolated stage under the influence of pre-motion friction. Generalized design guidelines are provided based on the observed tradeoffs between different performance metrics (e.g., precision, speed).     

A relay-based method for servo performance improvement

This paper proposes a relay-based performance-improving method for servo mechanism systems. The method first utilizes a relay-based feedback technique to identify the model parameters and the Coulomb friction value. Then, based on the identified results, a control algorithm, which consists of a feedforward controller, a time-delay compensator and a sliding mode controller, is designed. The feedforward controller and the time-delay compensator are used to compensate the system dynamics and the external disturbances respectively. Their parameters are decided directly according to the identified values. The sliding mode controller is to stabilize the system, two of whose parameters are one-to-one mapping to the closed-loop characteristic roots. Thus, this method avoids the complicated parameters tuning process, which is attractive in practice to the control engineers. Experimental studies on a linear-motor-driven table illustrate that the proposed method is capable of improving the servo performance greatly and canceling the external disturbances effectively.     

光电跟踪自抗扰控制技术研究

为了实现对快速运动目标高精度跟踪,对光电跟踪系统中的自抗扰控制技术进行了研究。根据速度闭环传递函数,利用三阶非线性扩张观测器估计系统状态变量,实现对不确定性因素的补偿,通过改变位置环被控对象传递函数提高系统的跟踪精度。对系统进行仿真与实验研究,分析自抗扰控制对光电跟踪系统动态和稳态性能的影响,与PI控制相对比,结果表明:对于高速运动目标,利用自抗扰控制技术可以将系统的跟踪精度提高7倍左右;对于低速运动目标,由于摩擦和系统噪声的影响,系统的跟踪精度仅提高了4倍左右。若在控制回路中引入相位超前环节,可以将系统的超调量降低40%,进一步改善了系统的动态性能,该技术的实现对于高精度跟踪控制的研究具有重要的应用价值。     

Controlled aspiration and positioning of biological cells in a micropipette

Manipulating single cells with a micropipette is the oldest, yet still a widely used technique. This paper discusses the aspiration of a single cell into a micropipette and positioning the cell accurately to a target position inside the micropipette. Due to the small volume of a single cell (picoliter) and nonlinear dynamics involved, these tasks have high skill requirements and are labor intensive in manual operation that is solely based on trial and error and has high failure rates. We present automated techniques in this paper for achieving these tasks via computer vision microscopy and closed-loop motion control. Computer vision algorithms were developed to detect and track a single cell outside and inside a micropipette for automated single-cell aspiration. A closed-loop robust controller integrating the dynamics of cell motion was designed to accurately and efficiently position the cell to a target position inside the micropipette. The system achieved high success rates of 98% for cell detection and 97% for cell tracking (n = 100). The automated system also demonstrated its capability of aspirating a single cell into a micropipette within 2 s (versus 10 s by highly skilled operators) and accurately positioning the cell inside the micropipette within 8 s (versus 25 s by highly skilled operators).     

Inversion-free force tracking control of piezoelectric actuators using fast finite-time integral terminal sliding-mode

The major hurdles to control the force created by piezoelectric actuators (PEAs) are originated from its strong nonlinear behaviors which include hysteresis, creep, and vibration dynamics. To achieve an accurate, fast and robust force tracking performance without using complicated modeling and parameter identification of PEAs, this paper presents a practical direct force control scheme. The proposed controller is based on two core approaches: 1) fast finite-time integral terminal sliding mode (FFI-TSM) which allows fast convergence and high accuracy to the closed-loop system without control chattering; and 2) an inverse-model-free compensation, named force-based time-delayed estimation (FBTDE) which offers significant robustness with minimum use of plant dynamics information. The finite-time stability of the overall closed-loop system is proven through the Lyapunov’s method. The proposed force tracking controller is implemented on the PEA system driving a variable physical damping actuator mechanism. The overall accuracy, convergence speed, and robustness of the proposed controller are validated under various experimental scenarios. Comparative experimental results are particularly presented to verify the effectiveness of the FFI-TSM term and the FBTDE term.     

Design and analysis of an SLPT-based CCFL driver

In this paper, a single-layer piezoelectric-transformer (SLPT)-based driver is realized for driving a cold-cathode fluorescent lamp (CCFL). First, a half-bridge resonant inverter is adopted for driving the SLPT and the CCFL to achieve zero-voltage-switching (ZVS) effect. In addition, a PQ-plane-design-oriented approach is presented for determining the power circuit parameters. Second, a feedback controller is proposed to match the power circuit control requirement. The feedback controller provides the proper switching frequency for the drive to be operated at the most efficient frequency. In addition, functions of dimming control and no-load protection are also available from the controller. Third, a small-signal model is derived and the closed-loop stability analysis is made to guarantee the stable tracking of the command signal of the controller. Finally, a hardware prototype is also constructed to verify the effectiveness of the proposed driver.     

Design and control of a brushless DC limited-angle torque motor with its application to fuel control of small-scale gas turbine engines

This paper presents techniques for design and control of a brushless direct-current (DC) limited-angle torque motor (LATM) with its application to fuel control of gas turbine engines. Given the desired specifications, a two-pole brushless DC LATM with a toroidally wound armature is designed using selected ferromagnetic material and rare-earth permanent magnets; its electromagnetic characteristics is then computationally found and well tuned using the finite element method (FEM) in order to ensure whether the design meets the performance specifications. To achieve the simple and inexpensive semi-closed-loop fuel control, a robust position controller (including a proportional-integral-derivative (PID) controller with a prefilter) is synthesized for providing the required positioning performance for the developed motor, thereby achieving an inexpensive semi-closed-loop fuel control. A closed-loop fuel controller associated with the proposed position controller and a flow meter is then proposed based on multi-loop control structure in order to achieve required linear input–output relationship. All the proposed fuel control laws were implemented using a stand-alone single-chip digital signal processor (DSP). Experimental results are conducted to show the efficacy and usefulness of the developed limited-angle torque motor with its application to an experimental gas turbine fuel control test platform.     

An approach to inverse nonlinear control using neural networks

In this paper, we present a strategy for controlling a class of nonlinear dynamical systems using techniques based on neural networks. The proposed strategy essentially exploits the property of neural networks in being able to approximate arbitrary nonlinear maps when suitable learning strategies are applied. For the closed-loop control, such a network is used in conjunction with a technique of inverse nonlinear control to form what we call an inverse nonlinear controller using neural networks, abbreviated as the INC/NN controller. Properties of the controller are discussed, and it is shown that the proposed INC/NN controller allows the closed-loop error dynamics to be specified directly through a set of controller gains. Extensions of the basic INC/NN controller to incorporate integral control action, to higher order systems, and to a class of nonlinear multi-input multi-output dynamical systems are also indicated. Finally, results of some real-time experiments in applying the INC/NN controller to a position control system which has inherent nonlinearities are presented.     

Analysis and control of a 6 DOF maglev positioning system with characteristics of end-effects and eddy current damping

Magnetic levitation technology has become the great candidate to provide ultraprecision motion in vacuum environment due to its characteristics, e.g., non-contact, frictionless, and unlimited stroke, etc. This paper presents the design, modeling, analysis, and control of a 6 Degrees-Of-Freedom (DOF) maglev positioning system, and the proposed system is able to achieve a planar motion of 50 mm × 50 mm while the levitation height is up to 4 mm. In this work, the model of eddy current damping for moving Halbach Permanent Magnet (PM) array is analytically established to predict the damping force in operating of the maglev positioning system. Furthermore, the magnetic field end-effect is analyzed in the maglev system, where two guidelines are provided to ideally avoid the model error due to end-effect during the design of the maglev system. To control the maglev positioning system to achieve good planar tracking performance, the system identification is carried out for x and y-axes, and a simple PID controller is designed and optimized according to the specifications characterizing on the closed-loop performance of the maglev system. Finally, the experiments are conducted on the maglev prototype to demonstrate the positioning performance, and the results show that the maximal static positioning error is less than 200 nm with the Root Mean Square Error (RMSE) around 60 nm.