Precise position control of shape memory alloy actuator using inverse hysteresis model and model reference adaptive control system

Position control of Shape Memory Alloy (SMA) actuators has been a challenging topic during the last years due to their nonlinearities in the governing physical equations as well as their hysteresis behaviors. Using the inverse of phenomenological hysteresis model in order to compensate the input–output hysteresis behavior of these actuators shows the effectiveness of this approach. In this paper, in order to control the tip deflection of a large deformation flexible beam actuated by an SMA actuator wire, a feedforward–feedback controller is proposed. The feedforward part of the proposed control system, maps the beam deflection into SMA temperature, is based on the inverse of the generalized Prandtl–Ishlinskii model. An adaptive model reference temperature control system is cascaded to the inverse hysteresis model in order to estimate the SMA electrical current for tracking the reference signal. In addition, a closed-loop proportional–integral controller with position feedback is added to the feedforward controller to increase the accuracy as well as eliminate the steady state error in position control process. Experimental results indicate that the proposed controller has great accuracy in tracking some square wave signals. It is also experimentally shown that the suggested controller has precise tracking performance in presence of environmental disturbances.     

Control of McKibben pneumatic muscles for a power-assist,lower-limb orthosis

In this research, a power-assist, lower-limb orthosis is developed to help the elderly or people suffering sports injuries walk or climb stairs. In the pneumatic muscle used for actuation, it is found that hysteresis phenomenon exists during the inflation–deflation process and such a phenomenon deteriorates the control performance. In order to eliminate the influence of hysteresis on the control system, a hysteresis model is constructed and used to devise an inverse control for feedforward compensation. The inverse control is combined with loop transfer recovery (LTR) feedback control to achieve better tracking performance. Moreover, bumpless switching compensators are also incorporated into the combined control system to ensure smooth switching between different phases of operation. To verify that the developed orthosis can effectively accomplish the assistive function, a human subject wearing the orthosis is asked to walk and to climb stairs. Experiments indicate that the orthosis is indeed helpful in assisting human locomotion.     

Adaptive control for enhancing tracking performances of flexible tendon–sheath mechanism in natural orifice transluminal endoscopic surgery (NOTES)

Tendon–sheath mechanism (TSM) has inherent advantages in the development of flexible robotic systems because of its simplicity, safety, flexibility, and ease of transmission. However, the control of TSM is challenging due to the presence of nonlinearities, namely friction, backlash-like hysteresis and the time-varying configuration of the TSM during its operations. Existing studies of TSM found in the literature only address tendon transmission under the assumption of fixed configuration and a complex inverse model of backlash is required. In order to flexibly use the system in a wider range of applications, the aforementioned nonlinear effects have to be characterized for the purpose of compensation. In this paper, we endeavor to address these issues by presenting a series of controller strategies, namely a feedforward control scheme under the assumption of known backlash-like hysteresis profile, and an adaptive control scheme to characterize the nonlinearities with unknown backlash hysteresis and uncertainties. The proposed control schemes do not require information of the tendon–sheath configurations, which is challenging to obtain in practice, in the compensation structures. In the absence of output position feedback, a simple direct inverse model-based feedforward has been used that efficiently reduce the tracking errors. The feedforward compensation does not require any complex algorithm for the inverse model. In the presence of output position feedback, a nonlinear adaptive controller has been developed to enhance the tracking performances of the TSM regardless of the random change in the tendon–sheath configurations during compensation. In addition, exact values of the model parameters are not required. They are estimated online during the operations. A dedicated experimental setup is introduced to validate the proposed control approaches. The results show that the proposed control schemes significantly improve the tracking performances for the TSM in the presence of uncertainties and time-varying configurations during the operations. There is a significant decrease of 0.0158 rad² (before compensation) to smaller value of 0.0012 rad² (use feedforward control) and 8.2815 × 10⁻⁵ rad² (use nonlinear controller) after compensation.     

Real-time electro-hydraulic hybrid system for structural testing subjected to vibration and force loading

Real-time electro-hydraulic hybrid system (REHS) with shaking table and force loading simulator is an essential experimental facility for evaluating structural performance subjected to simultaneously vibration excitation and force loading. The key feature of this paper is combination of a feedforward force controller including modified force inverse model compensator (MFIMC) and velocity feedforward compensator (VFFC) with an internal model control (IMC) to compensate the surplus force disturbance caused by active motion of shaking table and to obtain high fidelity force loading tracking performance. An acceleration tracking controller is also designed with modified acceleration inverse model compensator (MAIMC) to extend the acceleration tracking frequency bandwidth and to improve the acceleration tracking performance. The acceleration/force closed-loop transfer function model and their inverse model are identified and designed by multi-step recursive extended least squares (RELS) algorithm and zero magnitude error tracking controller (ZMETC) technology respectively because the identified transfer function model of the acceleration and force loading closed-loop systems may be a nonminimum-phase (NMP) system and their inverse model are instable. An acceleration and force modeling error compensator (MEC) are utilized in MFIMC and MAIMC to minimize the effect of the inaccuracy of identified model and designed inverse model. Experimental results obtained on a real uniaxial REHS with xPC rapid prototyping technology clearly demonstrate the benefit of the proposed compensation method.     

Dual-stage repetitive control with Prandtl–Ishlinskii hysteresis inversion for piezo-based nanopositioning

The positioning performance of piezo-based nanopositioning systems is limited by dynamic and hysteresis effects in the piezoactuator. Herein, a high-performance, dual-stage repetitive controller (dual-RC) with a feedforward hysteresis compensator is proposed for tracking periodic trajectories, such as the scanning-type motion, in nanopositioning systems. Firstly, a discrete-time dual-RC is created by cascading a conventional RC with an odd-harmonic RC. The favorable gain characteristics of the dual-RC coincide with the odd harmonics of the scanning-type periodic reference trajectory, thus offering good robustness and low tracking error. Secondly, a new inverse-hysteresis compensator is developed based on the Prandtl–Ishlinskii hysteresis model. The structure of the inverse model mimics the structure of the forward model, where the parameters of the inverse model can be easily identified from measured input–output data. Finally, the controllers are applied to a custom-designed high-speed nanopositioner, and simulations and experimental results are provided to illustrate the performance improvement of the proposed control scheme compared to industry-standard PID control and conventional RC. High-speed positioning results (tracking of triangle scan trajectories) at rates of 1 kHz, 1.5 kHz, and 2 kHz are shown. Compared to a conventional RC, the tracking error of the dual-RC is 48% lower at 1 kHz and 33% lower at 2 kHz scanning frequency. It is also shown that by compensating for hysteresis, the performance of the RC system designed based on the linear dynamics can be enhanced.     

Adaptive positioning control for a LPMSM drive based on adapted inverse model and robust disturbance observer

The adaptive robust positioning control for a linear permanent magnet synchronous motor drive based on adapted inverse model and robust disturbance observer is studied in this paper. First, a model following two-degrees-of-freedom controller consisting of a command feedforward controller (FFC) and a feedback controller (FBC) is developed. According to the estimated motor drive dynamic model and the given position tracking response, the inner speed controller is first designed. Then, the transfer function of FFC is found based on the inverse model of inner speed closed-loop and the chosen reference model. The practically unrealizable problem possessed by traditional feedforward control is avoided by the proposed FFC. As to the FBC, it is quantitatively designed using reduced plant model to meet the specified load force regulation control specifications. In dealing with the robust control, a disturbance observer based robust control scheme and a parameter identifier are developed. The key parameters in the robust control scheme are designed considering the effect of system dead-time. The identification mechanism is devised to obtain the parameter uncertainties from the observed disturbance signal. Then by online adapting the parameters set in the FFC according to the identified parameters, the nonideal disturbance observer based robust control can be corrected to yield very close model following position tracking control. Meanwhile, the regulation control performance is also further improved by the robust control. In the proposed identification scheme, the effect of a nonideal differentiator in the accuracy of identification results is taken into account, and the compromise between performance, stability, and control effort limit is also considered in the whole proposed control scheme.     

音圈电机激光扫描器的线性扫描控制

建立了音圈电机驱动的激光扫描器的模型并进行了参数辨识。由于音圈电机驱动的激光扫描器反馈回路的高频噪声较大,利用光电精密跟踪的传统方法,如改进型PID控制算法,以及理想闭环特性(最佳二阶特性)的逆向设计方法,系统并不能稳定工作,易振荡。分析了造成振荡的原因,采用前馈复合控制方法,避开了振荡因子同时又减小了稳态误差,解决了稳态误差和振荡的矛盾。结合反馈回路噪声大的特点,将滤波器加入反馈回路中,使激光扫描器不仅能稳定工作,消除了扫描时候的抖动,同时减少了线性扫描时候的磁滞现象。     

An adaptive controller for a direct-drive SCARA robot

A direct adaptive controller for trajectory tracking of high-speed robots such as a direct-drive SCARA robot is presented. In this robot, nonlinear effects due to centrifugal, Coriolis, and inertial forces are more important than friction and gravity forces, unlike most industrial robots. The control law of the adaptive scheme consists of a PD regulator plus feedforward compensation of full dynamics. The feedforward terms are adjusted by an adaptation law so that the steady-state position errors are zero. With this adaptive controller, the joint acceleration measurement is not required and no inversion of the estimated mass matrix is involved. The tracking performances of the controller applied to a two-degree-of-freedom SCARA is illustrated by a real-time implementation based on a single-chip digital signal processor (DSP)     

Position control of shape memory alloy actuator based on the generalized Prandtl–Ishlinskii inverse model

Hysteresis and significant nonlinearities in the behavior of Shape Memory Alloy (SMA) actuators encumber effective utilization of these actuator. Due to these effects, the position control of SMA actuators has been a great challenge in recent years. Literature review of the research conducted in this area shows that using the inverse of the phenomenological hysteresis models can compensate the hysteresis of these actuators effectively. But, inverting some of these models, such as Preisach model, is numerically a complex task. However, the generalized Prandtl–Ishlinskii model is analytically invertible, and therefore can be implemented conveniently as a feedforward controller for compensating hysteresis nonlinearities effects in SMA actuators. In this paper a feedforward–feedback controller is used to control the tip deflection of a large deflected flexible beam actuated by an SMA actuator wire. The feedforward part of the control system is based on the generalized Prandtl–Ishlinskii inverse model while a conventional proportional–integral feedback controller is added to the feedforward controller to increase the accuracy together with eliminating the steady state error in position control process. Experimental results show that the proposed controller performs well in terms of achieving small overshoot and undershoot for square wave tracking as well as small tracking errors for sinusoidal trajectory. It has also great capability for tracking hysteresis minor loops.     

机械臂变指数趋近律滑模控制律设计

针对机械臂滑模控制中存在抖振的问题,采用改进趋近律的思想来设计滑模控制律,以达到有效抑制抖振的目的。在对滑模控制的特点和常用的指数趋近律进行分析的基础上,提出了一种变指数趋近律,并对其趋近性能进行了分析;结合机械臂动力学模型和改进的趋近律设计了相应的滑模控制策略,对其控制效果进行了验证。仿真结果表明,该控制策略不仅有效地抑制了系统的抖振,而且保证了机械臂系统对期望轨迹的快速跟踪性,进而提高了机械臂的工作性能。     

Interaction control of an industrial manipulator using LPV techniques

This paper presents the design and implementation of a hybrid force/motion control scheme on a six-degrees-of-freedom robotic manipulator employing a gain-scheduled linear parameter-varying (LPV) controller. A nonlinear dynamic model of the manipulator is obtained and the unknown parameters are estimated. The manipulator is decomposed into an inner and a wrist submodel, and a practical way is proposed to investigate the coupling between them. The motion control part of the hybrid controller which is the main focus of this paper is formed by a combination of an LPV controller and a model-based inverse dynamics controller for the inner submodel and the wrist joints, respectively. A quasi-LPV model with a reduced number of scheduling parameters is derived for the inner submodel, and a polytopic LPV gain-scheduled controller is synthesized in a two-degrees-of-freedom structure including feedback and feedforward parts, which is augmented by a friction compensation term. A PD controller with a feedforward path is designed to control the interaction force. The proposed hybrid force/motion scheme is implemented on the 6-DOF CRS A465 robotic manipulator to perform a writing task. Comparison of the results with those of a hybrid force/motion controller with a plain model-based inverse dynamics motion control and the same force control shows that the proposed controller improves the position tracking performance significantly.     

High-bandwidth tracking control of piezo-actuated nanopositioning stages using closed-loop input shaper

An integrated control strategy for piezo-actuated nanopositioning stages is proposed in this paper. The aim is to achieve high-speed and high-precision tracking control of nanopositioning stages. For this purpose, a direct inverse compensation method is firstly applied to eliminate the hysteresis nonlinearity without involving inverse model calculation. Then, an inside-the-loop input shaper is designed to suppress the vibration of the compensated system. A Smith predictor is introduced to prevent the potential closed-loop instability caused by the time delay of the inside-the-loop input shaper. Finally, a high-gain feedback controller is employed to handle the disturbances and modeling errors. To demonstrate the effectiveness of the proposed control method, comparative experiments are carried out on a piezoelectric actuated stage. The results show that the proposed control approach increases the tracking bandwidth of the stage from 22.6 Hz to 510 Hz.     

MIMO multirate feedforward controller design with selection of input multiplicities and intersample behavior analysis

Inversion-based feedforward control is a basic method of tracking controls. The aim of this paper is to design MIMO multirate feedforward controller that improves continuous-time tracking performance in MIMO LTI systems considering not only on-sample but also intersample behavior. Several types of MIMO multirate feedforward controllers are designed and evaluated in terms of the 2-norm of the control inputs. The approach is compared with a conventional MIMO single-rate feedforward controller in simulations. The approach improves the intersample behavior through the optimal selection of input multiplicities with MIMO multirate system inversion.     

压电驱动器建模与控制技术研究

该文对压电驱动器建模与控制技术进行了研究。通过二阶系统描述了压电驱动器的线性动态特性。基于Preisach迟滞模型,提出了全新的简化模型,避免了Preisach模型中复杂的双重积分运算。通过前馈与反馈控制技术相结合,为压电驱动器设计了一个综合控制器。在前馈控制器中,通过Preisach模型描述逆迟滞环,有效解决了Preisach模型不可逆的问题。通过比例、积分、微分(PID)反馈控制器,有效提高了系统鲁棒性,并有效补偿了前馈控制器中模型不精确带来的误差。最终,通过实验分别验证了模型的有效性及控制器的控制作用。     

基于率相关迟滞模型的压电微动平台前馈控制

为了降低压电微动平台的动态迟滞误差对平台定位精度的影响,该文设计了基于率相关迟滞逆模型的前馈控制器对其进行迟滞补偿。首先,在对平台受力分析和运动分析的基础上建立平台的动力学模型;其次,在经典Prandtl-Ishilinskii(PI)模型的基础上加以改造,得到Modified Prandtl-Ishilinskii(MPI)模型,并将MPI模型与平台的线性动力学模型串联,得到分离式率相关MPI模型,进而基于率相关MPI逆模型建立平台的前馈控制器;最后,对所设计的控制器进行阶跃响应和正弦轨迹跟踪实验。实验结果表明,所设计的控制器具有较好的定位精度与跟踪性能,可以有效地补偿压电微动平台的动态迟滞误差。     

A novel AQM algorithm based on feedforward model predictive control

Developing feedforward model predictive controller as an active queue management (AQM) scheme is studied in this paper. MPC is an advanced control strategy for AQM. However, the conventional MPC is usually an implementable form of feedback MPC. In this paper, a feedforward and feedback optimal control law is presented. It is a clean, easily implementable, version of model predictive control that incorporates feedforward. Firstly, we use the nominal fluid model to design the feedforward control input so that the output tracks the given queue length with small error. Furthermore, in order to achieve robust performance and to reject the (unmeasured) disturbance, the feedback component is designed. In particular, a disturbance observer is incorporated into the prediction output in standard feedback MPC. This framework can significantly improve performance in the presence of measurement noise and certain types of model uncertainty. Finally, the simulation results show the effectiveness of FF‐AQM algorithm.     

Control of High-Speed Solid-Rotor Synchronous Reluctance Motor/Generator for Flywheel-Based Uninterruptible Power Supplies

A hybrid controller, consisting of a model-based feedforward controller and a proportional–integral feedback compensator, for a solid-rotor synchronous reluctance motor/generator in a high-speed flywheel-based uninterruptible power supply application is proposed in this paper. The feedforward controller takes most of the control output of the current regulator based on the machine model, and the PI controllers compensate the possible inaccuracies of the model to improve the performance and robustness of the complete control system. The machine current tracking error caused by parameter inaccuracy in the model-based controller is mathematically analyzed and utilized to dynamically compensate the estimated flux linkage to eliminate the steady-state error in current regulation. Stability analysis is also presented, and it can be seen that the regulation performance and robustness of the system are improved by the proposed hybrid controller. Simulation and experimental results consisting of a flywheel energy storage system validates the performance of the controller.      

永磁同步电机伺服系统自抗扰控制器设计

对永磁同步电机(PMSM)调速系统中的时变输入提出具有更高跟踪精确度的改进型自抗扰控制策略.传统的自抗扰控制主要针对阶跃信号进行快速和无静差追踪,对时变信号存在较大的跟踪误差,使自抗扰控制的应用受限.文中对稳态误差的存在原因进行了理论分析,进而设计带有微分前馈和并联线性扩张状态观测器(P-LESO)的改进型转速自抗扰控...     

Random vibration test control of inverter-fed electrodynamic shaker

The random vibration control of an inverter-fed electrodynamic shaker is presented in this paper. First, the dynamic model of the shaker is found and a current-controlled pulsewidth modulation inverter is designed and implemented. The feedback controller is augmented with a command feedforward controller and a disturbance feedforward controller to let the armature exciting current have low harmonic content and possess excellent waveform tracking performance. Then, an acceleration controller and its random vibration command are arranged. In the proposed acceleration control scheme, a command feedforward controller and a robust disturbance feedforward controller are also employed to let the shaker have close random acceleration command waveform tracking control performance, and the performance be insensitive to the system parameter variations. It follows that the acceleration control with desired frequency response in a vibration test could be achieved through properly setting the command signal. The effectiveness of the proposed control scheme is verified by simulation and measured results     

Friction Compensation of an $XY$ Feed Table Using Friction-Model-Based Feedforward and an Inverse-Model-Based Disturbance Observer

Uncompensated friction forces compromise the positioning and tracking accuracy of motion systems. A unique tracking error known as quadrant glitch is the result of complex nonlinear friction behavior at motion reversal or near-zero velocity. Linear-feedback control strategies such as PID, cascade P/PI, or state-feedback control have to be extended with model- and nonmodel-based friction-compensation strategies to acquire sufficiently high path and tracking accuracy. This paper analyzes and validates experimentally three different friction-compensation strategies for a linear motor-based xy feed drive of a high-speed milling machine: (1) friction-model-based feedforward; (2) an inverse-model-based disturbance observer; and (3) the combination of both techniques. The friction models considered are as follows: a simple static-friction model and the recently developed generalized Maxwell-slip (GMS) model. GMS friction-model-based feedforward combined with disturbance observer almost completely eliminates the radial tracking error and quadrant glitches.