Current over-stressing small DC motors to evaluate performance limits of electromechanical actuators for haptic applications

Actuators for haptic devices tend to have a different set of requirements in comparison to many other engineering applications. Small permanent magnet DC electric motors are commonly used as actuators in haptic devices and, in operation, tend to spend a significant period of time in a `stalled' condition where they are attempting to oppose an applied force. Ideally a haptic actuator together with its power amplifier exchange energy reversibly with the mechanical loads. However this is not feasible at room temperature and to achieve good force performance results in energy loss as heat in the motor windings. This paper identifies the relationship between heat loss and force generation in haptic electromagnetic actuators. The work then presents results on current over-stressing of small DC motors so as to understand the risks of demagnetisation against thermal damage to the armature. Results indicate that it should be possible to apply short current over-stresses to commercial DC permanent magnet motors to increase end point force. Also by paying careful attention to heat dissipation in the design of small permanent magnet actuators motors, it should be possible to improve the overall performance of actuators for haptic applications

Modelling the air-gap field strength of electric machines to improve performance of haptic mechanisms

The air-gap of electro-magnetic (EM) actuators determines key operating parameters such as their ability to generate force. In haptic devices these parameters are not optimised for the conditions typically seen in operation and include the heat produced in the air-gap, the volume of the air-gap, and the intensity and direction of the magnetic field. The relationship between these parameters is complex thus design decisions are difficult to make. This paper considers the role of the radial magnetic field in cylindrical electric motors, a type often used in haptic devices. Two models are derived and compared with experimental measurements. The first model is a closed form solution, the second is a classic Poisson solution to Ampere's equation. These models are shown to be valid for making more general design decisions in relation to haptic actuators, and in particular allow an evaluation of the trade off between the volume of the air-gap, the resulting radial magnetic field and hence heat generated and the resulting forces

Enabling wearable soft tactile displays with dielectric elastomer actuators

PhDTouch is one of the less exploited sensory channels in human machine interactions. While the introduction of the tactile feedback would improve the user experience in several fields, such as training for medical operators, teleoperation, computer aided design and 3D model exploration, no interfaces able to mimic accurately and realistically the tactile feeling produced by the contact with a real soft object are currently available. Devices able to simulate the contact with soft bodies, such as the human organs, might improve the experience. The existing commercially available tactile displays consist of complex mechanisms that limit their portability. Moreover, no devices are able to provide tactile stimuli via a soft interface that can also modulate the contact area with the finger pad, which is required to realistically mimic the contact with soft bodies, as needed for example in systems aimed at simulating interactions with virtual biological tissues or in robot-assisted minimally invasive surgery. The aim of this thesis is to develop such a wearable tactile display based on the dielectric elastomer actuators (DEAs). DEAs are a class of materials that respond to an electric field producing a deformation. In particular, in this thesis, the tactile element consists of a so-called hydrostatically coupled dielectric elastomer actuator (HC-DEAs). HC-DEAs rely on an incompressible fluid that hydrostatically couples a DEA-based active part to a passive part interfaced to the user. The display was also tested within a closed-loop configuration consisting of a hand tracking system and a custom made virtual environment. This proof of concept system allowed for a validation of the abilities of the display. Mechanical and psychophysical tests were performed in order to assess the ability of the system to provide tactile stimuli that can be distinguished by the users. Also, the miniaturisation of the HC-DEA was investigated for applications in refreshable Braille displays or arrays of tactile elements for tactile maps

Sensorless Position Control of Piezoelectric Ultrasonic Motors:a Mechatronic Design Approach

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

AN EVALUATION OF THE TRAVELING WAVE ULTRASONIC MOTOR FOR FORCE FEEDBACK APPLICATIONS

The traveling wave ultrasonic motor is considered for use in haptic devices where a certain input-output relation is desired between the applied force and the resulting motion. Historically, DC motors have been the standard choice for this purpose. Owing to its unique characteristics, the ultrasonic motors have been considered an attractive alternative. However, there are some limitations when using the ultrasonic motor for force-feedback applications. In particular, direct torque control is difficult, and the motor can only supply torque in the direction of motion. To accommodate these limitations we developed an indirect control approach. The experimental results demonstrate that the model reference control method was able to approximate a second order spring-damper system

Modelling and Evaluation of Piezoelectric Actuators for Wearable Neck Rehabilitation Devices

Neck pain is the most common neck musculoskeletal disorder, and the fourth leading cause of healthy years lost due to disability in the world. Due to the need of hands-on physical therapy and Canada’s aging population, access to treatment will become highly constrained. Wearable devices that allow at-home rehabilitation address this future limitation. However, few have emerged from the laboratory setting because they are limited by the use of conventional actuators. An overlooked type of actuation technology is that of piezoelectric actuators, more specifically, travelling wave ultrasonic motors (TWUM). In this work, a clear procedure that outlines how the required parameters within the hybrid TWUM model can be identified, as well as an assessment of the use of TWUMs within wearable devices for the neck, is presented. The procedure includes custom testing setups that were designed to identify the stator motion parameters, and the Coulomb coefficient of friction. The accuracy of the determined parameters were confirmed when the angular velocity of the hybrid model at different duty cycles was compared to the real TWUM being modelled, producing a coefficient of determination of 0.974. The model was then used to create a position control system that controlled the joints of a virtual robotic manipulator that modelled the neck. The manipulator exhibited a maximum absolute mean error of only 0.0289 m when simulating the required trajectories of range of motion exercises. This performance, in addition to the exemplary traits TWUMs express, demonstrate their potential to advance the field of wearable mechatronic devices

Haptics Rendering and Applications

There has been significant progress in haptic technologies but the incorporation of haptics into virtual environments is still in its infancy. A wide range of the new society's human activities including communication, education, art, entertainment, commerce and science would forever change if we learned how to capture, manipulate and reproduce haptic sensory stimuli that are nearly indistinguishable from reality. For the field to move forward, many commercial and technological barriers need to be overcome. By rendering how objects feel through haptic technology, we communicate information that might reflect a desire to speak a physically- based language that has never been explored before. Due to constant improvement in haptics technology and increasing levels of research into and development of haptics-related algorithms, protocols and devices, there is a belief that haptics technology has a promising future

Advanced Mobile Robotics: Volume 3

Mobile robotics is a challenging field with great potential. It covers disciplines including electrical engineering, mechanical engineering, computer science, cognitive science, and social science. It is essential to the design of automated robots, in combination with artificial intelligence, vision, and sensor technologies. Mobile robots are widely used for surveillance, guidance, transportation and entertainment tasks, as well as medical applications. This Special Issue intends to concentrate on recent developments concerning mobile robots and the research surrounding them to enhance studies on the fundamental problems observed in the robots. Various multidisciplinary approaches and integrative contributions including navigation, learning and adaptation, networked system, biologically inspired robots and cognitive methods are welcome contributions to this Special Issue, both from a research and an application perspective

Electroactive Materials for Applications in the Field of Wearable Technologies

The objective of this PhD thesis is to present the most performing EAP-based materials, technologies and devices developed by our lab (Ch.4, 5 and 6) also in collaboration with other research groups (Ch.1 and 2) for sensing, actuating and energy harvesting, with reference to their already demonstrated or potential applicability to electronic textiles and wearable technologies in general. Over the last decade great strides have been made in the field of wearable technology: thanks to new discoveries in materials science and miniaturized electronics, tissues and "smart" devices for monitoring vital parameters, rehabilitation and tele-assistance were born. However, a complete and self-powered system, able to exchange information with the external environment, to generate power using the usual movements of the human body (walking, work, sport) and to drive wearable devices, is not yet available on the market and it would find a considerable number of applications (monitoring physiological parameters for athletes and special forces officers in emergency situations, etc.). After a first survey of the state of the art concerning the so-called "smart materials” and technologies currently available for " wearable " activities, the work has developed on three major directives consisting in: energy generation and storage, sensing and actuation. Energy generation and storage. An experimental study, conducted mainly during the first year of PhD, has identified possible candidate materials (piezoelectric PVDF, electret PP) for the energy harvesting and subsequent generation of power from movement and gestures by exploiting the piezoelectric properties of selected materials. These materials have been either found on the market or processed in laboratory. In collaboration with the University of Pavia, a circuit for the storage of electric charges generated was made. Both the commercial materials and those obtained in laboratory were electromechanically tested and the generation of electric charges has been used to develop a demonstrator generator-LED embedded in a shoe. Sensing. During the first and second year, different sensor configurations of "dry" piezoelectric PVDF sensors were tested for the monitoring of vital parameters (heart and breathing rate). Such sensors, prepared in collaboration with the University of Lodz (TUL, Poland), our partners in the PROETEX European project (6th FP 2006-2009), were woven into fabrics to be easily integrated into clothing, and their response was studied. Signal intensities comparable to those of common 3M medical electrodes have been observed. A further development of these materials should be turn to reduce noise, while a computational study might deal with the signal filtering and elimination of motion artifacts. Along with the study of piezoelectric sensors mentioned above, during the third PhD year the production and characterization of dielectric elastomers for sensing applications (artificial skin) was developed too, in collaboration with the Genoa DIST (Dipartimento di Informatica, Sistemistica e Telematica). Such elastomers, characterised by high dielectric constants and restrained compressive elastic moduli, were develop in order to act as dielectric medium in piezocapacitive sensing devices. The obtained materials will be used as artificial skin in robotic systems. Actuating. In parallel with the two lines described above, the activity was concentrated, throughout the period of PhD, on the development of new dielectric elastomer actuators, to be used as high dielectric constant, low elastic modulus and, especially, low electric driving fields devices so that they can be used once inserted inside the clothing (simplified prototype actuators able to change the porosity / texture of different textiles were developed during the first year of activity for the FLEXIFUNBAR European project (6th FP 2005-2008)). The "blend" approach has been privileged over the "composite" approach, previously studied in the master thesis, and has led to promising results both from the applicative point of view, with an increase in the electromechanical performance, and on a fundamental level, for the implications emerging from the interaction between different phases in the study of dielectric response of partially heterogeneous systems. Electromechanical encouraging results were then obtained during the second year of activity with the development of silicone/polyurethane (SI/PU) blends prepared by appropriate volume fractions. Further improvements have also been achieved during the third year of doctoral studies, when it was introduced in the same mixtures a third component, the conjugated polymer poly-(3-hexylthiophene-2,5-dyil) (P3HT), already used by our group for its high polarizability in order to increase the dielectric constant of silicon actuators. The obtained samples, dielectrically, mechanically and electromechanically tested, showed that the conjugated polymer leads to a further significant increase in the electromechanical response of the blend only when added at levels of 1 wt%. This polymer shows, in fact, a certain influence on the microscopic distribution of the SI and PU "phases" in the blend. This effect is maximized for the 1 wt% concentration at which the presence of interfaces is maximized and thus a larger surface polarization, combined with the characteristic high polarizability of P3HT, leads to dielectric constant and strain further implementations. Similar increases in performance, compared to pure components, were also found in mixtures prepared using other polyurethanes and silicones adopting, when necessary, appropriate steps to modify the kinetics of reaction (addition of solvents). The results obtained with this "blending" approach are supported by the Intephase Theory (IT), recently introduced to complete the well known Effective Medium Theory (EMT) which, although applicable to a variety of particle composite structures, is not suitable to describe the behaviour of systems where the presence of an interphase between filler and matrix is significant. The EFT demonstrates that border regions, showing dielectric characteristics different from those of the starting components, can strongly influence the system performance. Through theoretical and experimental evidence, in fact, it is known that, while the inner parts of the matrix polymer chains are able to adopt a configuration that minimizes spontaneous conformational energy, at the interface they are linked or otherwise conditioned in their movements, giving rise to a region where the electrical properties (in some cases also thermal and mechanical) are different from those of both the pure material composing the mixture. During the third year, the production and characterization of elastomeric foams with dielectric properties suitable for sensing (artificial skin) and actuating applications were also developed. The electromechanical performance of these polyurethane-based foams, after appropriate polarization under very high electric fields (Corona poling), were compared with those of two commercial products, which were also subjected to corona poling. Studies have been conducted also on the life of the induced polarization produced by poling in the foam and on the influence of electric field exposure time on the final response of the material. The slightly positive results obtained in terms of increased dielectric constants and strains have opened a new line of activity that represents an innovation in the field of dielectric elastomers, that is the preparation of elastomeric foams with electret properties