Soft-Landing Control of Short-Stroke Reluctance Actuators

Los actuadores de reluctancia se utilizan ampliamente debido a sus altas densidades de fuerza y baja disipación de calor. En particular, los actuadores de reluctancia simples de una sola bobina de carrera corta, como los relés electromecánicos y las electroválvulas, son la mejor opción para operaciones de conmutación de encendido y apagado en muchas aplicaciones debido a su bajo coste, tamaño y masa. Sin embargo, un inconveniente importante es el fuerte impacto al final de cada conmutación, que provoca rebotes, desgaste mecánico y ruido acústico. Son fenómenos muy indeseables que restan valor a las ventajas evidentes de estos actuadores y limitan su rango de aplicaciones potenciales.Esta tesis se centra en el desarrollo y estudio de soluciones de control de aterrizaje suave para actuadores de reluctancia de carrera corta, con el objetivo de minimizar sus velocidades de impacto. Es importante indicar que la eficiencia de dichos dispositivos se produce a costa de serios retos teóricos y prácticos en cuanto a su control, por ejemplo, dinámicas rápidas, híbridas y altamente no lineales, fenómenos electromagnéticos complejos, variabilidad entre unidades y falta de medidas de posición durante el movimiento.El punto de partida es la modelización del sistema, teniendo en cuenta sus subsistemas interconectados eléctricos, magnéticos y mecánicos. El objetivo principal de los modelos es servir para el desarrollo de métodos de control y estimación. Por lo tanto, se trata de modelos de parámetros concentrados expresados como representaciones del espacio de estados. Se especifican diferentes fenómenos electromagnéticos, con especial atención a la histéresis magnética. Se proponen dos tipos de modelos de diferente complejidad según se incorpore o se desprecie el fenómeno de la histéresis magnética.El primer enfoque para el control del aterrizaje suave es el diseño óptimo de las trayectorias de posición y sus correspondientes señales de entrada. La propuesta tiene en cuenta la incertidumbre en la posición del contacto y, por tanto, las soluciones obtenidas son más robustas. Mientras que las señales de entrada generadas son eficaces para las estrategias de control en lazo abierto, las trayectorias de posición generadas pueden utilizarse controles de prealimentación o de retroalimentación.Para mejorar la robustez de los controladores de lazo abierto, también proponemos una estrategia run-to-run que adapta iterativamente las señales de entrada. En concreto, está diseñada para trabajar conjuntamente con un controlador de prealimentación basado en las mencionadas trayectorias de posición construidas de forma óptima. Para el algoritmo de aprendizaje ciclo a ciclo, se elige una técnica de optimización, se ajusta y se compara con dos alternativas.Otro enfoque explorado es el control de retroalimentación para el seguimiento de trayectorias predefinidas de posición. La solución propuesta es un controlador estrictamente conmutativo en modo deslizante. Está enfocado en la simplicidad para facilitar su implementación, al tiempo que se tiene en cuenta la dinámica híbrida. Los análisis teóricos y simulados demuestran que el aterrizaje suave es posible con tasas de muestreo razonables.Los controladores de retroalimentación y otros controladores de seguimiento requieren mediciones o estimaciones precisas de la posición. Como la medición de la posición raramente es práctica, parte de la investigación se dedica al diseño de estimadores de estado. La principal propuesta es un suavizador Rauch-Tung-Striebel ampliado para sistemas no lineales, que incluye varias ideas nuevas relacionadas con el modelo discreto, las entradas y las salidas. Los análisis simulados demuestran que el efecto combinado de las nuevas adiciones da lugar a mucho mejores estimaciones de la posición.Reluctance actuators are widely used due to their high force densities and low heat dissipation. In particular, simple short-stroke single-coil reluctance actuators, such as electromechanical relays and solenoid valves, are the best choice for on-off switching operations in many applications because of their low cost, size and mass. However, a major drawback is the strong impact at the end of each commutation, which provokes bouncing, mechanical wear and acoustic noise. They are very undesirable phenomena that detract from the evident advantages of these actuators and limit their range of potential applications. This thesis focuses on the development and study of soft-landing control solutions for short-stroke reluctance actuators, aiming at minimizing their impact velocities. It is important to indicate that the efficiency of the aforementioned devices comes at the cost of serious theoretical and practical challenges regarding their control, e.g., fast, hybrid and highly nonlinear dynamics, complex electromagnetic phenomena, unit-to-unit variability and lack of position measurements during motion. The starting point is the system modeling, accounting for their interconnected electrical, magnetic and mechanical subsystems. The main purpose of the models is to be used for the development of control and estimation methods. Therefore, they are lumped-parameter models expressed as state-space representations. Different electromagnetic phenomena are specified, with special attention to the magnetic hysteresis. Two model types of different complexities are proposed depending on whether the magnetic hysteresis phenomenon is incorporated or neglected. The first approach for soft-landing control is the optimal design of position trajectories and their corresponding input signals. The proposal considers uncertainty in the contact position, and hence, the obtained solutions are more robust. While the generated input signals are effective for open-loop control strategies, the generated position trajectories can be used in feedforward or feedback control. In order to improve the robustness of open-loop controllers, we also propose a run-to-run strategy that iteratively adapts the input signals. Specifically, it is designed to work in conjunction with a feedforward controller based on the aforementioned optimally constructed position trajectories. For the cycle-to-cycle learning algorithm, an optimization technique is chosen, adjusted and compared to two alternatives. Another explored approach is feedback control for tracking predefined position trajectories. The proposed solution is a purely switching sliding-mode controller. The focus is on simplicity to facilitate its implementation, while also taking into account the hybrid dynamics. Theoretical and simulated analyses show that soft landing is achievable with reasonable sampling rates. Feedback and other tracking controllers require accurate measurements or position estimations. As measuring the position is rarely practical, part of the research is devoted to the design of state estimators. The main proposal is an extended Rauch–Tung–Striebel smoother, which includes several new ideas regarding the discrete model, the inputs and the outputs. Simulated analyses demonstrate that the combined effect of the novel additions results in much better position estimations.<br /

An efficient dynamical model of reluctance actuators with flux fringing and magnetic hysteresis

This paper presents an efficient and accurate dynamical model of reluctance actuators, suitable for prediction and control applications. It is a hybrid lumped-parameter state-space model that takes into account the mechanical and electromagnetic dynamics, including eddy currents, flux fringing, magnetic hysteresis and saturation. Special emphasis is placed on the hysteresis model, which is based on the Jiles–Atherton theory. The novel parts of the model – the gap reluctance expression and the modified Jiles–Atherton hysteresis model – are identified, showing that the simulated results fit very well the experimental data. Furthermore, its potential application for control is exemplified with a feedback strategy, in which the design of the controller and observer are based on the proposed dynamical model

Probability-Based Optimal Control Design for Soft Landing of Short-Stroke Actuators

The impact forces during switching operations of short-stroke actuators may cause bouncing, audible noise, and mechanical wear. The application of soft-landing control strategies to these devices aims at minimizing the impact velocities of their moving components to ultimately improve their lifetime and performance. In this brief, a novel approach for soft-landing trajectory planning, including probability functions, is proposed for optimal control of the actuators. The main contribution of the proposal is that it considers the uncertainty in the contact position, and hence, the obtained trajectories are more robust against system uncertainties. The problem is formulated as an optimal control problem and transformed into a two-point boundary value problem for its numerical resolution. Simulated and experimental tests have been performed using a dynamic model and a commercial short-stroke solenoid valve. The results show a significant improvement in the expected velocities and accelerations at contact with respect to past solutions in which the contact position is assumed to be perfectly known

Model-free sliding-mode controller for soft landing of reluctance actuators

Some electromagnetic actuators suffer from high velocity impacts during non-controlled switching operations, which cause contact bouncing, mechanical wear, and acoustic noise. Soft-landing control strategies aim at minimizing the impact velocities of these devices to improve their performance. This paper presents a sliding-mode controller for soft landing of single-coil reluctance actuators. It is a switching model-free controller, which results in a very simple implementation. A generalized dynamical hybrid model of an actuator is utilized for deriving the robustness condition, based on the Lyapunov theory. Then, the condition is evaluated for a dynamical model, based on a commercial device, and several reference trajectories. Finally, the controller performance is validated through simulations. The effect of the sampling rate on the resulting impact velocities is also analyzed

Rauch-Tung-Striebel Smoother for Position Estimation of Short-Stroke Reluctance Actuators

This article presents a novel state estimator for short-stroke reluctance actuators, intended for soft-landing control applications in which the position cannot be measured in real time. One of the most important contributions regards the system modeling for the estimator. The discrete state of the hybrid system is treated as an input. Moreover, the model is simplified to facilitate the identification of parameters and the implementation of the estimator. Thus, auxiliary variables are added to the state vector in order to indirectly account for modeling errors. Another important contribution is the state estimation approach. It is based on the Rauch–Tung–Striebel fixed-interval smoother, which allows refining past data from later observations. Numerous simulations are performed to analyze and compare the proposal and several alternatives. In addition, experimental testing is presented to evaluate and validate the estimator. As the simulated and experimental analyses demonstrate, the combined effect of the novel additions results in significantly smaller estimation errors of position and velocity

Design of a perfect-tracking soft-landing controller for electromagnetic switching devices

Electromagnetic switching devices such as electromechanical relays and solenoid valves suffer from impacts and mechanical wear when they are activated using a constant-voltage policy. This paper presents a new control approach that aims at achieving soft landing in these devices, i.e., a movement without neither impacts nor bouncing. The hybrid nonlinear dynamics of the system is firstly described taking into account the limited range of motion that characterizes this class of devices. Then, the nonlinear expression of the control law is derived and a method to design a soft-landing reference trajectory is proposed. It is shown that, when certain conditions are met, the design methodology presented in the paper results in a controller that achieves perfect tracking of the reference trajectory and, hence, soft landing is accomplished. The theoretical analysis is validated by simulation using a dynamical model of a specific switching device

Run-to-run adaptive nonlinear feedforward control of electromechanical switching devices

Feedforward control can greatly improve the response time and control accuracy of any mechatronic system. However, in order to compensate for the effects of modeling errors or disturbances, it is imperative that this type of control works in conjunction with some form of feedback. In this paper, we present a new adaptive feedforward control scheme for electromechanical systems in which real-time measurements or estimates of the position and its derivatives are not technically or economically feasible. This is the case, for example, of commercial electromechanical switching devices such as solenoid actuators. Our proposal consists of two blocks: on the one hand, a feedforward controller based on differential flatness theory; on the other, an iterative adaptation law that exploits the repetitive operation of these devices to modify the controller parameters cycle by cycle. As shown, this law can be fed with any available measurement of the system, with the only requirement that it can be processed and converted into an indicator of the performance of any given operation. Simulated and experimental results show that our proposal is effective in dealing with a long-standing control problem in electromechanics: the soft-landing control of electromechanical switching devices

Nonlinear Bounded State Estimation for Sensorless Control of an Electromagnetic Device

This paper presents a novel nonlinear state observer with discrete-time measurements for estimating the plunger position of linear travel solenoid valves. The observer is an unscented Kalman filter (UKF) for nonlinear systems that iteratively calculates an estimated mean and covariance of the state. It is based on a basic lumped parameter model, which contributes to the computational efficiency of the observer and facilitates its implementation. The magnetic reluctance is modeled taking into account the magnetic saturation and is partly defined by data obtained from finite element analysis (FEA). Boundary constraints are added to the estimated position to prevent it from surpassing its physical limits. Different tests performed with simulated and experimental data show that the estimations are accurate and robust to noise and model inaccuracies. Besides, although the observer has been developed for a specific device, the method can be easily extended to other electromechanical systems in which the position needs to be estimated

On the Stability of Electromechanical Switching Devices: A Study of Hysteretic Switching Behaviors

Electromagnetic relays and solenoid actuators are commercial devices that generally exhibit bistable behavior. In fact, this is the reason why they are extensively used to switch between two possible configurations in electrical, pneumatic, or hydraulic circuits, among others. Although the state of the art is extensive on modeling, estimation, and control of these electromechanical systems, there are very few works that focus on analysis aspects. In this paper, we present an equilibrium and stability analysis whose main goal is to provide insight into such bistable behavior. The study is based on a hybrid dynamical model of the system also presented in the paper. This model is used to obtain analytic expressions that relate the physical parameters to the switching conditions. The results are extensively discussed and possible practical applications are also proposed. Finally, experimental results with a real device are used for validation of the theoretical analysis and also for illustrating one of the possible practical uses

An audio-based iterative controller for soft landing of electromechanical relays

Electromechanical relays and contactors suffer from strong collisions at the end of the switching operations. This causes several undesirable phenomena, such as clicking, mechanical wear and contact bounce. Thus, there is great interest in mitigating these switching impacts while keeping the advantageous features of these devices. This paper proposes a complete control strategy for soft landing. The control structure includes three main components. The first one is a real-time flux-tracking feedback controller, which presents several advantages over voltage or current control. The second one is a feedforward controller, which computes the flux reference signal based on a proposed dynamical model and the desired position trajectory for the switching operations. Lastly, the third control component is a learning-type run-to-run adaptation law that iteratively adapts the model parameters based on an audio signal. It exploits the repetitive nature of these devices in order to circumvent modeling discrepancies due to unit-to-unit variability or small changes between operations. The effectiveness of the proposed control is demonstrated through various experiments