Performance-driven control of nano-motion systems

The performance of high-precision mechatronic systems is subject to ever increasing demands regarding speed and accuracy. To meet these demands, new actuator drivers, sensor signal processing and control algorithms have to be derived. The state-of-the-art scientific developments in these research directions can significantly improve the performance of high-precision systems. However, translation of the scientific developments to usable technology is often non-trivial. To improve the performance of high-precision systems and to bridge the gap between science and technology, a performance-driven control approach has been developed. First, the main performance limiting factor (PLF) is identified. Then, a model-based compensation method is developed for the identified PLF. Experimental validation shows the performance improvement and reveals the next PLF to which the same procedure is applied. The compensation method can relate to the actuator driver, the sensor system or the control algorithm. In this thesis, the focus is on nano-motion systems that are driven by piezo actuators and/or use encoder sensors. Nano-motion systems are defined as the class of systems that require velocities ranging from nanometers per second to millimeters per second with a (sub)nanometer resolution. The main PLFs of such systems are the actuator driver, hysteresis, stick-slip effects, repetitive disturbances, coupling between degrees-of-freedom (DOFs), geometric nonlinearities and quantization errors. The developed approach is applied to three illustrative experimental cases that exhibit the above mentioned PLFs. The cases include a nano-motion stage driven by a walking piezo actuator, a metrological AFM and an encoder system. The contributions of this thesis relate to modeling, actuation driver development, control synthesis and encoder sensor signal processing. In particular, dynamic models are derived of the bimorph piezo legs of the walking piezo actuator and of the nano-motion stage with the walking piezo actuator containing the switching actuation principle, stick-slip effects and contact dynamics. Subsequently, a model-based optimization is performed to obtain optimal drive waveforms for a constant stage velocity. Both the walking piezo actuator and the AFM case exhibit repetitive disturbances with a non-constant period-time, for which dedicated repetitive control methods are developed. Furthermore, control algorithms have been developed to cope with the present coupling between and hysteresis in the different axes of the AFM. Finally, sensor signal processing algorithms have been developed to cope with the quantization effects and encoder imperfections in optical incremental encoders. The application of the performance-driven control approach to the different cases shows that the different identified PLFs can be successfully modeled and compensated for. The experiments show that the performance-driven control approach can largely improve the performance of nano-motion systems with piezo actuators and/or encoder sensors

A novel practical control approach for rate independent hysteretic systems

A disturbance rejection based control approach, active disturbance rejection control (ADRC), is proposed for hysteretic systems with unknown characteristics. It is an appealing alternative to hysteresis compensation because it does not require a detailed model of hysteresis, by treating the nonlinear hysteresis as a common disturbance and actively rejecting it. The stability characteristic of the ADRC is analyzed. It is shown that, in the face of the inherent dynamic uncertainties, the estimation and closed-loop tracking errors of ADRC are bounded, with their bounds monotonously decreasing with the observer and controller bandwidths, respectively. Simulation results on a typical hysteretic system further demonstrate the effectiveness of the proposed approach

Advanced Control of Piezoelectric Actuators.

168 p.A lo largo de las últimas décadas, la ingeniería de precisión ha tenido un papel importante como tecnología puntera donde la tendencia a la reducción de tamaño de las herramientas industriales ha sido clave. Los procesos industriales comenzaron a demandar precisión en el rango de nanómetros a micrómetros. Pese a que los actuadores convencionales no pueden reducirse lo suficiente ni lograr tal exactitud, los actuadores piezoeléctricos son una tecnología innovadora en este campo y su rendimiento aún está en estudio en la comunidad científica. Los actuadores piezoeléctricos se usan comúnmente en micro y nanomecatrónica para aplicaciones de posicionamiento debido a su alta resolución y fuerza de actuación (pueden llegar a soportar fuerzas de hasta 100 Newtons) en comparación con su tamaño. Todas estas características también se pueden combinar con una actuación rápida y rigidez, según los requisitos de la aplicación. Por lo tanto, con estas características, los actuadores piezoeléctricos pueden ser utilizados en una amplia variedad de aplicaciones industriales. Los efectos negativos, como la fluencia, vibraciones y la histéresis, se estudian comúnmente para mejorar el rendimiento cuando se requiere una alta precisión. Uno de los efectos que más reduce el rendimiento de los PEA es la histéresis. Esto se produce especialmente cuando el actuador está en una aplicación de guiado, por lo que la histéresis puede inducir errores que pueden alcanzar un valor de hasta 22%. Este fenómeno no lineal se puede definir como un efecto generado por la combinación de acciones mecánicas y eléctricas que depende de estados previos. La histéresis se puede reducir principalmente mediante dos estrategias: rediseño de materiales o algoritmos de control tipo feedback. El rediseño de material comprende varias desventajas por lo que el motivo principal de esta tesis está enfocado al diseño de algoritmos de control para reducir la histéresis. El objetivo principal de esta tesis es el desarrollo de estrategias de control avanzadas que puedan mejorar la precisión de seguimiento de los actuadores piezoeléctricos comerciale

Magneto-Rheological Actuators for Human-Safe Robots: Modeling, Control, and Implementation

In recent years, research on physical human-robot interaction has received considerable attention. Research on this subject has led to the study of new control and actuation mechanisms for robots in order to achieve intrinsic safety. Naturally, intrinsic safety is only achievable in kinematic structures that exhibit low output impedance. Existing solutions for reducing impedance are commonly obtained at the expense of reduced performance, or significant increase in mechanical complexity. Achieving high performance while guaranteeing safety seems to be a challenging goal that necessitates new actuation technologies in future generations of human-safe robots. In this study, a novel two degrees-of-freedom safe manipulator is presented. The manipulator uses magneto-rheological fluid-based actuators. Magneto-rheological actuators offer low inertia-to-torque and mass-to-torque ratios which support their applications in human-friendly actuation. As a key element in the design of the manipulator, bi-directional actuation is attained by antagonistically coupling MR actuators at the joints. Antagonistically coupled MR actuators at the joints allow using a single motor to drive multiple joints. The motor is located at the base of the manipulator in order to further reduce the overall weight of the robot. Due to the unique characteristic of MR actuators, intrinsically safe actuation is achieved without compromising high quality actuation. Despite these advantages, modeling and control of MR actuators present some challenges. The antagonistic configuration of MR actuators may result in limit cycles in some cases when the actuator operates in the position control loop. To study the possibility of limit cycles, describing function method is employed to obtain the conditions under which limit cycles may occur in the operation of the system. Moreover, a connection between the amplitude and the frequency of the potential limit cycles and the system parameters is established to provide an insight into the design of the actuator as well as the controller. MR actuators require magnetic fields to control their output torques. The application of magnetic field however introduces hysteresis in the behaviors of MR actuators. To this effect, an adaptive model is developed to estimate the hysteretic behavior of the actuator. The effectiveness of the model is evaluated by comparing its results with those obtained using the Preisach model. These results are then extended to an adaptive control scheme in order to compensate for the effect of hysteresis. In both modeling and control, stability of proposed schemes are evaluated using Lyapunov method, and the effectiveness of the proposed methods are validated with experimental results

Dynamic modeling and bioinspired control of a walking piezoelectric motor

Szufnarowski F. Dynamic modeling and bioinspired control of a walking piezoelectric motor. Bielefeld: Universität Bielefeld; 2013.Piezoelectric motors have increasingly extended their field of applications during recent years. Improved material properties and manufacturing techniques have led to a variety of designs which can achieve theoretically unlimited displacements for moderate voltage levels while retaining a relatively high stiffness. In practical terms, this leads to stronger and faster motors which become a viable alternative to electromagnetic drives, especially if compact size and small weight are important. The piezoelectric motor considered in this work consists of four piezoelectric bender elements which can forward a ceramic bar by means of a frictional interaction. The drive elements can be compared to "legs" walking on a movable plane. The walking motor offers outstanding force generation capabilities for a motor of its size. Despite this fact, this motor has not been used in a force control scenario before and no motor models exist in the literature which can reproduce the effect of load on its performance. In this work, two dynamic motor models are developed to address the latter issue. Both of them faithfully reproduce the non-linear motor velocity decrease under load. The first model is based on an analytic approach and describes the low-level frictional interactions between the legs and the ceramic bar by means of several physically meaningful assumptions. This analytic model explains several non-linear phenomena in the operation of the walking motor within the full bandwidth of its rated operation. Non-linear influences due to the impact dynamics of the legs, ferroelectric hysteresis and friction are identified in the motor and new insights for an improved motor design as well as an improved motor-drive strategy gained. Moreover, the analytic model finds its application in a theoretical investigation of an alternative motor-drive strategy which is based on findings in insect walking. Specifically, it is shown that the performance of the motor can be improved by a half in terms of its force generation and doubled in terms of its maximal velocity, as compared to classical drive approaches, if the bioinspired drive strategy as proposed in this work is used. The second model is based on an experimental approach and system identification. Although less general, the second model is well-suited for a practical application in a force-control scenario. In particular, the experimental model is used in this work for the development of a load compensation strategy based on force feedback which restores the linearity of motor operation for moderate levels of loading. Based on the linearized motor model, a force controller is developed whose performance is evaluated both theoretically and experimentally. The developed force controller is also used in a bioinspired control scenario. Specifically, two walking motors together with their force controllers are employed in a 1-DOF antagonistic joint as force generators. The motors are supposed to partially mimic the functionality of a muscle based on the non-linear force-length relation as derived by Hill. A simple positioning task shows the feasibility of this kind of non-standard application of a piezoelectric motor. Beside the development of motor models and bioinspired control approaches, this work addresses the issue of drive-signal generation for the walking motor. Specifically, the development of motor-drive electronics is presented which supersedes the commercially available products due to its compactness and the possibility of waveform generation at much higher drive frequencies, above 50 kHz, as compared to the nominal limit of 3 kHz and commercial products. In this context, the possibility of motor operation at ultrasonic frequencies is discussed which would benefit the motor in terms of its speed and the absence of audible noises

Analog controller based on sliding mode control for piezoelectric actuators

Today, the digital implementation of the controllers is mainly preferred from reprogrammability point of view. Many important control problems can be effectively solved using a digital architecture in conjunction with analog-to-digital (ADC) and/or digital-to-analog conversion (DAC). Digital solutions offer two very attractive advantages: (1)-promise to shorten design cycles, and (2)-provide the freedom to reprogram the design in simple ways. This ease-of-change stands in sharp contrast to the great effort required to redesign a typical hard-wired analog implementation. However, depending on the complexity of the plant and the degrees of freedom (DOF) to be controlled, digital implementation of an algorithm may be demanding due to the high computational power requirement to run in real time. The necessity for the acquisition of the analog signals on the other hand requires ADC and DAC conversions that compel extra conditions on the system. Hence, multi-DOF systems may require either diminish in the systems operation frequency or additional hardware to run the algorithm in parallel for each DOF. This work aims to develop an analog motion controller for single input single output (SISO) plants of complex nature. As the control algorithm, Sliding Mode Control (SMC) like the well known robust nonlinear controller is selected as a design framework. Originally designed as a system motion for dynamic systems whose essential open-loop behavior can be sufficiently modeled with ordinary differential equations, Sliding Mode Control (SMC) is one of the effective nonlinear robust control approaches that provide system invariance to uncertainties once the sliding mode motion is enforced in the system. An important aspect of sliding mode is the discontinuous nature of the control action, which switches between two values to move the system motion on so-called “sliding mode” that exist in a manifold and therefore often referred as variable structure control (VSC). The resulting feedback system is called variable structure system (VSS). The position tracking of the piezoelectric actuators (PEA) is selected as the test bed for the designed system. Piezoelectricity, the ability of the material to become strained due to an electric field, gives the possibility to user those materials as actuator in sub-micrometer domain for a range of applications. Piezoelectric effect is a crystalline effect, and therefore, piezoelectric actuators do not suffer from “stick slip” effect mainly caused by the friction between elements of a mechanical system. This property theoretically offers an unlimited resolution, and therefore piezoelectric actuators are already used in many applications to provide sub-micrometer resolution. Still the achievable resolution in practice can be limited by a number of other factors such as the piezo control amplifier (electronic noise), sensor (resolution, noise and mounting precision) and control electronics (noise and sensitivity to EMI). As a result of this work, we are aiming an analog controller for SISO systems and by the use of this controller, improvement on the tracking performance for the plant we are studying and decrease on the possible computational load on digital controllers is targeted

Development of a Compact Piezoworm Actuator For Mr Guided Medical Procedures

In this research, a novel piezoelectric actuator was developed to operate safely inside the magnetic resonance imaging (MRI) machine. The actuator based on novel design that generates linear and rotary motion simultaneously for higher needle insertion accuracy. One of the research main objectives is to aid in the selection of suitable materials for actuators used in this challenging environment. Usually only nonmagnetic materials are used in this extremely high magnetic environment. These materials are classified as MRI compatible materials and are selected to avoid hazardous conditions and image quality degradation. But unfortunately many inert materials to the magnetic field do not possess desirable mechanical properties in terms of hardness, stiffness and strength and much of the available data for MRI compatible materials are scattered throughout the literature and often too device specific . Furthermore, the fact that significant heating is experienced by some of these devices due to the scanner’s variable magnetic fields makes it difficult to draw general conclusions to support the choice of suitable material and typically these choices are based on a trial-and-error with extensive time required for prototype development and MRI testing of such devices. This research provides a quantitative comparison of several engineering materials in the MRI environment and comparison to theoretical behavior which should aid designers/engineers to estimate the MRI compatible material performance before the expensive step of construction and testing. This work focuses specifically on the effects in the MRI due to the material susceptibility, namely forces, torques, image artifacts and induced heating