Integrated approach for auto-adaptive synchronous motor winding

The intrinsic characteristic of a permanent magnet synchronous motor is defined by its internal dimensions, its material properties, as well as the power supply used in conjunction with the motor. When an extended operating range is desired but the electronics are limited, motor manufacturers face a problem. Furthermore, when different operating points are needed -e.g. a low torque with a high speed as well as a large torque at a lower speed- the used drive system is oversized so that all operating points are included in the motor's characteristic. Therefore, this thesis aims to find a solution to obtain an extended operating range while considering the use of limited electronics. In addition, the proposed solution must be embeddable into the motor.

A survey of possible solutions allowing the increase of the motor's intrinsic characteristic is presented. Among them, the dynamic winding reconfiguration is chosen because it allows an increase in torque and in speed while having restrictions on the electronics. This reconfiguration is performed by using switches.

As the connections between the coils of the winding will be reshaped, coils can be placed in parallel. Depending on the geometrical distribution of these coils, circulating currents may appear, which in turn will be generating unwanted losses. Therefore, a study of the spatial distribution of the turns is undertaken demonstrating that these currents can be significantly reduced without adding complexity to the winding manufacturing.

Considering the integration of the switching system and the alternating current that will run through it, the MEMS (Micro Electro-Mechanical System) relays are favored because of their ease of use, their galvanic separation when they are open, but especially thanks to their separation between the control command and the signal, which removes all the floating voltage issues. In order to reduce the contact resistance, the design of the relay is based on an innovative system of double springs combined with variation of the flexural rigidity of the contact structure. In addition, the influence of microfabrications tolerances is considered in the design. A novel manufacturing process flow has been specifically developed to manufacture the relay in a cleanroom environment. The prototypes obtained and the mechanical and electrical characterizations performed on them confirmed the feasibility of the design and the manufacturing methodology.

From then on, the control allowing the winding reconfiguration is studied. The latter focuses on slotless permanent magnet motors. Their low inductance implies rapid current variations and by extension rapid torque variations as well. The proposed methodology explores the transient regime from one configuration to another and is based on a fine control of current amplitude and the commutation angle. Numerical simulations and experimental results confirm the accuracy of the proposed approach.

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

Multi-Finger Haptic Devices Integrating Miniature Short-Stroke Actuators

Grégory Savioz

The omnipresence of electronic devices in our everyday life goes together with a trend that makes us always more immersed during their utilization. By immersion, we mean that during the development of a new product, it is more and more required to stimulate several senses of the user so as to make the product more attractive. The sense of touch does not escape the rule and is more and more considered. Definitely democratized by its integration in smart phones with touchscreens, the haptic feedback allows enhancing the human-machine interactions in many ways. For instance by improving the comfort of use of a button through the modification of its force feedback. It can also offer an interactive experience during the manipulation of digital information and even improve the communication, particularly through the internet and for blind people, with the introduction of non-verbal signals. For these reasons, the present thesis focuses on the conception of multi-finger haptic devices, a new kind of peripherals integrating multiple actuators and capable of providing a fully programmable force feedback to the user's fingers. A global methodology is presented, outlining the different constituents necessary for their conception: actuator, sensor, control, communication and software user interface. Then, generic tools corresponding to the two first elements are presented. An accurate modeling of miniature electromagnetic short-stroke actuators is made possible thanks to the combination of 3D finite element modeling (FEM) and design of experiments (DOE). The non-usual behavior of magnetic flux lines in miniature actuators with relatively large airgaps imposes to avoid simplified analytical models and to use the reliable results of finite elements. The long computation times required by 3D FEM are balanced by the use of selective DOE making the modeling methodology easily adaptable, rapid and accurate. The parametrical model of the force provided by the modeling methodology is then integrated in a full parametrical setup allowing for the optimization of the actuator force using a conventional algorithm. The advantage of the parametrical optimization is that complementary non-linear constraints such as weight and temperature can be added, making the model multi-physic. Then, several original position measurement techniques using existing sensors are developed including a low-cost custom single-photointerrupter sensor allowing for direction discrimination for fast-prototyping and a hybrid sensing method using tiny Hall sensors and taking advantage of the leaks of the main actuator magnet. Two innovative self-sensing methods are then presented, allowing for the measurement of the mover position of linear short-stroke actuators. The first solution estimates the position of the coil by measuring the acceleration through the back emf. However in this case, a constant acceleration is required, which strongly restrains the application scope. The second solution allows for a real-time measurement of the position thanks to a passive oscillating RLC circuit influenced by the variation of the coil impedance. All the solutions presented are low-cost, compact and require few computation resources. Finally, in order to illustrate the methodology proposed along the thesis, several prototypes are fabricated, giving an overview of the possibilities offered by multi-finger haptic devices. A haptic numeric pad is notably used in an experiment made in collaboration with the University Service of Child and Adolescent Psychiatry in Lausanne with the aim of improving the impaired emotional processing of psychotic adolescents. Moreover, the successful identification of several touch sensations on the same haptic pad lays the first stones of a new tactile language.

EPFL2012

Small scale very high speed slotless permanent magnet motors

Guillaume Paul-André Burnand

This research work has been triggered by an industrial project aimed at modelling and manufacturing a small-scale very high speed motor. Very high speed electrical motors offer the main advantage of reducing the size of a system. Indeed, at constant output mechanical power, increasing the rotational speed enables to decrease the electromagnetic torque, hence the size of the motor. When a system already possesses very high speed devices, it becomes attractive to combine them with an electrical machine. Combined with very high speed, the miniaturisation of motors benefits limited weight and space applications. Used as hand tools, they provide solutions for otorhinolaryngology or dental surgery. Because they can be freely arranged in the airgap, slotless windings represent a substantial capacity for enhancement. Thanks to new manufacturing technologies, the design of windings can be rethought. Different shapes and topologies can be embraced leading to improved performances for electrical motors. The aim of this thesis is consequently twofold: to give multiphysics models enabling the design and the optimisation of small scale very high speed permanent magnet synchronous motors, and to propose an innovative solution for a new type of slotless winding topology entailing the performances of electrical motors. By essence, very high speed motors are pushed to their limits: they experience high mechanical stresses in the materials of the rotor and the bearings, are subject to critical speeds, operate sometimes at high temperature and harsh environments and, due to high electrical frequencies, produce significant iron and winding losses. As a result, robust and reliable multiphysics models have to be established. In this thesis, a very complete set of both mechanical and electromagnetic analytical models is presented. Analytical models are preferred to numerical models as they bring more insight in understanding physical phenomena occurring in the motors and require less computational effort for optimisation. All the aforementioned analytical models are used in the context of the design and the optimisation of small scale very high speed slotless permanent magnet motors with ball bearings. With an appropriate optimisation algorithm, fixed constraints on the maximal size and the operating point, optimal designs are obtained for several rotational speeds. In particular, an original contribution comes from the comparison of Litz-wire and rectangular wires as well as hollow and solid magnet rotors used in small-scale very high speed slotless motors. In order to experimentally validate the models, a 400 krpm 12.7 mm diameter prototype is manufactured and tested up to 475 krpm. In addition, it demonstrates the feasibility of miniaturised very high speed motors. An astute experimental method is implemented in order to separate the losses components related to the rotation of the rotor. Therefore, every model can be obtained and/or validated independently. Finally, the research work emphasis on the modelling and the optimisation of a novel slotless winding topology. By challenging the traditional way of manufacturing windings, the use of nonconstant wire sections and an optimised shape enables the performances of the winding, and thereby the motor, to be significantly improved. A prototype of the new winding topology is manufactured and leads to the validation of the theoretical model.

EPFL2020