Optimal Design and Sensorless Position Control of a Piezoelectric Motor Integrated into a Mechatronic Cylinder Lock

Electronic door lock cylinders need some sort of mechanical actuator to toggle the locking state between open and closed. The main requirements are high security standards, low power consumption, reasonable production costs, and facile integration of the actuator into the mechatronic system. Existing mechatronic locking devices are actuated by DC motors. Complex security setups are necessary to prevent the motor to be moved when not supplied or to impede magnetic picking. Piezoelectric ultrasonic motors (PUM) respond better to these challenges. The presented work adresses a design methodology whose goal is to optimally integrate a PUM into an existing mechatronic system. By doing so, we optimise the motor, the drive electronics and the position control method, while the interdependency of all three subsystems is the gist of the optimisation approach. With respect to the drive electronics, resonance matching is realised with a LLCC resonant converter and a resonance frequency tracking controller. A direct digital synthesis based drive signal regulator allows for reducing the overall size of drive and control electronics. The locking position is detected indirectly by motional admittance modulation, obviating the need for a position sensor.

Sensorless Position Control of Piezoelectric Ultrasonic Motors

Markus Flückiger

This dissertation considers mechatronic systems driven by piezoelectric ultrasonic motors (PUM). The focus is set on optimal system design and sensorless position control. Mechatronic industry faces the challenge to deliver ever more efficient and reliable products while being confronted to increasingly short time to market demands and economic constraints driven by competition. Although optimal design strategies are applied to master this challenge, they do not entirely respond to the given circumstances, as often only local criteria are optimised. In order to obtain a globally optimal solution, the many subfunctions of a mechatronic system and their models must be interrelated and evaluated concurrently from the very beginning of the design process. In this context PUM have been used increasingly during the last decade for various positioning applications in the field of mechatronic systems, laboratory equipment, and consumer electronics where their performances are superior to conventional electromechanical drive systems based on DC or BLDC motors. The position of the mobile component must be controlled. In some cases open-loop control is a solution, but more often than not sensors are used as feedback device in closed-loop control. Sensors are expensive, large in size and add fragile hardware to the device that compromises its reliability. Thus, not only the superior performance is not fully exploited but also the economical feasibility of the PUM drive system is jeopardised. Replacing sensors by advanced control techniques is an approach to these problems that is well established in the field of BLDC motors. Those sensorless control strategies are not directly transferrable, because of the fundamentally different working principles of PUM. Hence, the research of sensorless closed-loop position control techniques applicable to PUM and their validation with digitally controlled functional models is the very topic of this thesis. We propose a dedicated design methodology to this statement of the problem. A core model of the mechatronic system is conceived as general and simple as possible. It then develops for the different interrelated views reflecting the mechanical, electromechanical, drive electronic, sensorial and digital control functions of the global system. Each one becoming more specific and detailed in this process, the different views still enable mutual constraint adjustments and the dynamic integration of results from the other views during the design process. Starting with the stator of the PUM, a view describes the mechanical displacement. An electric equivalent model is written such that power input from the drive electronics is related to the mechanical energy transmitted to the mechanics. The resulting differential equations are solved by the finite element method (FEM). Position feedback configurations in the mobile part of the PUM are modelled analytically in order to be implemented in digital control and their electrical implications are updated to the stator model. In this way, sensors do not necessarily materialise physically any more, but are distributed among the mechanical configuration, the drive electronics and the digital controller. With respect to the sensor data, the controller is not simply receiving finalised data on the measured system parameter, but rather implements the sensor itself in software. Finally, the position detection performance obtained with the aforementioned design methodology was evaluated with the example of mechatronic locking devices actuated by custom-made as well as OEM motors. Functional models of motors, electronics and digital controllers were used to identify the limits of the proposed methods, and suggestions for further research were deduced. These results contribute to the development of robust sensorless position controllers for PUM.

EPFL2010

Modélisation et commande du moteur piézoélectrique à onde progressive

Matteo Bullo

Piezoelectric motors are resonant vibromotors. They represent a new actuator generation in the field of servo-drives. In particular, the travelling wave ultrasonic motor presents a high torque at low speed, a zero speed torque without feeding, low sensitivity to electromagnetic disturbances as well as being a more compact solution if compared to conventional electromagnetic motors. Much researches has been performed by others to determine an analytical model based on the identification of an electromagnetic equivalent circuit or on exploitation of a theoretical model based on numerical approaches, which use finite elements methods. While leading to satisfactory analysis, these modeling methods can hardly be exploited in the design of control algorithms. Indeed, they require considerable processing resources to generate and visualize the results. For this reason, we introduce in this thesis, an analytical model that is easily adaptable to operational applications and control techniques. The proposed analytical model has been validated by comparing measured characteristics with those obtained in simulations, which was possible thanks to the realization of a modular test bench. The travelling wave ultrasonic motor is characterized by strong non-linearity. It also depends highly on the wear state of the materials, which is difficult to model, and on the contact surface between stator and rotor. In addition, the mechanical resonance frequency experiences drift due to the variations of temperature. These considerations of strong non-linearities and parameter sensitivities of the motor represent a challenge for the study and design of an efficient and robust control strategy. We introduce with this thesis a new control approach that guarantees a closed loop response which is independent of the motor operating point. Moreover, the proposed control method allows to avoid the discontinuities typically present with this type of actuator with a very reasonnable hardware requierments. Finally, an important extension in the product range of the piezoelectric actuators is proposed in the last part of this thesis. It acts to develop an fMRI (functional Magnetic Resonance Imaging) compatible haptic interface with one degree of freedom. The use of a robotic interface in conjunction with an fMRI environment would enable neuroscientists to investigate the brain mechanism used to perform tasks with arbitrary dynamics, and could become a critical tool in neuroscience and rehabilitaiton. There is, however, a major problem for robot working within an fMRI environment : conventional actuators and materials interfere with the strong permanent magnetic field and the fast switching magnetic field gradients. Consequently, non-ferromagnetic materials must be used to avoid forces on the device itself, that can compromise its performance and may result in hazardous conditions for the patient or the medical staff. In addition, the materials should be non-conducting to avoid the generation of eddy currents. The travelling wave ultrasonic motor was used because it provides benefits compared to the conventional electromagnetic actuators. Non-ferromagnetic piezoelectric ceramic material is used and as a result motor operation is not affected by the presence of the strong magnetic fields ecountered in the clinical scanners.

EPFL2005

Optimization of a Single Phase Ultrasonic Linear Motor

José Fernandez Lopez, Markus Flückiger, Yves Perriard

Piezoelectric ultrasonic motors are superior to electromagnetic micromotors, because their efficiency remains theoretically constant during miniaturization. However, the still relatively recent technology has a considerable unexploited optimization potential. Numerical structural analysis by the means of the finite element method (FEM) is a common approach for dimensioning piezoelectric motors. Consequently, there is a need for efficient optimization procedures fitted to the FEM simulation. We developped a dedicated design methodology to first well understand the influence of the geometrical parameters on the movement of the motor. The parameters with the strongest influence on the objective function, the vibration amplitude of the resonator, were used in a following optimization stage. The operation of the optimized motor was proofed on a test bench. Interferometrical measurements validated quantitatively the FEM model along with the suggested design methodology.