Designing compliant mechanisms composed of shape memory alloy and actuated by induction heating

Shape Memory Alloys (SMAs) are a type of smart materials that reacts mechanically to heat. Due to their complex behavior, they are often used in a simple geometry such as wires. This constrains the output displacement of the alloy to a simple linear contraction of the wire. When a different output displacement is desired, the SMA is coupled to a mechanism that transforms the motion, degrading the compactness of the whole system. To alleviate such issues, we propose fabricating, directly in SMA, a compliant mechanism that performs complex output motions and thus improves integration. The first part of this paper presents a method to design such systems. When coupled with a bias-spring and a heating system, these mechanisms form a full actuator. The conventional heating system relies on Joules losses coming from direct electric conduction through the alloy. However, now that the SMA has a complex shape, passing a current through it becomes an arduous task requiring multiple electrodes making the system cumbersome and deteriorating its integrability. Magnetic induction heating is proposed to tackle this limitation, heating the mechanism without contact and conserving a compact actuator.

Synthesis of Novel Integrated Actuators Powered by Shape Memory Alloys

Sean Thomas

In the modern age of miniaturisation, Smart Materials, a type of material that reacts mechanically to a certain stimulus, have become an integral part of this revolution. Among these materials, Shape Memory Alloys (SMAs), which have the highest volumetric work density, are the ideal candidate in creating lightweight and miniature actuators. These alloys, after being deformed, are able to revert back to their original shape when exposed to heat. This exotic behaviour has allowed them to be the core active component in a plethora of applications such as grippers, bio-mimetic robots and surgical instruments.Despite their high work density, their implementation comes with some challenges. While the Shape Memory Effect (SME), the ability to recover strain when a thermal load is applied, is a remarkable behaviour, it is also a complex and multi-physical one. This complicates their design and makes it difficult to predict their behaviour. Furthermore, these alloys are only able to recover strain when deformed at low temperatures. This implies that a biasing element is required to exploit these materials in reversible actuators. Due to these limitations, the work density of SMAs, when implemented as actuators in robotic systems, are often much lower than their theoretical maximum.In this thesis, the various types of SMA actuator implementations from different applications are examined to understand the design requirements and subsystems that are necessary to build an actuator. A holistic approach is, then, used to construct a design methodology to create highly integrated actuators in the hopes of preventing the work density degradation present in traditional SMA-based systems. Here, in this work, the identified subsystems of the SMA actuator are combined to serve as a multi-functional element in the novel integrated actuator.The work employs different strategies to integrate the SMA actuator into robotic systems while also proposing adapted sizing methodologies. The resulting SMA-powered system can be sized to be lightweight, compact and dynamic. Various case studies are presented that utilise the proposed holistic design approach and sizing methodologies to serve as a proof-of-concept and to validate the methodology.In this work, compliant and flexure-based mechanisms are exploited to exclude the need for a dedicated biasing element and create lightweight SMA-powered grippers to demonstrate the advantages of the proposed methodology. Additionally, utilising the mechanical behaviour of the SME, a lightweight mechanically-controlled crawling robot is designed and implemented. Furthermore, with the help of topology optimization and kirigami-inspired design, the work details the creation of compliant SMA structures that allow the material to generate multiple outputs while remaining compact and easy to assemble. Lastly, a novel bistable gripper is designed and sized to experimentally validate the design methodologies proposed in this work.The work demonstrates the different areas in which the degradation of the work density occur in traditional SMA systems. In this regard, design methodologies accompanied with sizing strategies are proposed that allows the creation of lightweight, high bandwidth and integrated SMA-based robotic systems. The results in this thesis, reveal the extraordinary value of SMAs in creating lightweight robotic systems and presents various strategies to allow the further integration of the alloy within the system.

EPFL2022

Compact fast-steering tip/tilt laser scanner for high power material processing applications

Moustapha Hafez

The common configuration for deflecting laser beams in two dimensions is based on two single-axis galvanometric scanners placed perpendicular to each other. Over recent years, these fast scanners have proved to be both very reliable and very accurate. They are widely available in the marketplace and have wide-ranging technical specifications. However, such a configuration is rather bulky and can not satisfy the needs of industry which seeks compact solutions for an easy integration of these scanning heads into their production lines. In addition, two separate mirrors leads to optical aberrations when these scanning heads are used in conjunction with scanning lenses, which are specially designed for a single entrance pupil. Therefore, a compact fast-steering tip/tilt laser scanner with a single mirror would act as a key component in diverse applications, e.g. in material processing, astronomy, intersatellite laser communications, imaging systems, bio-medical and ophthalmologic applications. The compact two-axis tilt laser scanner presented in this work can perform a variety of functions such as image tracking, beam stabilization and alignment, line-of-sight pointing, and scene scanning. Designing such a scanner required a multi-disciplinary approach involving optics, mechanics, electromagnetics, sensors and control theories. The scanner is composed of one single mirror with a large active area and has a single point of rotation for the two axes of tilt. Moreover, the center of gravity coincides with the center of rotation to reach optimum dynamic behavior. The scanner's compact dimensions allow an easy integration in the various optical systems. Furthermore, the mirror is easily interchangeable which gives greater flexibility in the choice of the laser source. Another major advantage of the proposed design is its ability to be scaled down to miniaturized versions. The scaling depends mainly on the active surface of the mirror, which is generally specified by the application. The larger the mirror, the higher its inertia and the larger are the actuators in order to guarantee high performance. Two pairs of push-pull linear electromagnetic actuators are used to drive the mirror of the scanner. The mirror suspension is based on a sliding bearing composed of a cone-ball contact with optimized friction and wear behavior. A magnetic pre-load force is used to hold the mirror and to give sufficient rotational stiffness. Furthermore, a position transducer based on a photodetector and a miniature laser pen is integrated in the module and is used for closed loop feedback positioning. The mirror can be tilted around both axes by ± 61mrad (± 3.5°) with an accuracy better than 50μrad. Moreover, a differential resolution in the order of 2.5μrad, and settling times for maximum tip and tilt deflections of 10ms and 14ms respectively are achieved. The overall volume of the scanner is 30 x 40 x 50 mm3, and its total weight does not exceed 90g. Design, simulations and experimental investigations were carried out in this work in order to optimize the different components of the tip/tilt scanner. These components are: the motors, the bearing system, the position transducer and the feedback control loop. An analytical model that allows an optimization of the geometrical parameters for an electromagnetic actuator based on rectangular coils and magnets is proposed. This model proved to be very reliable and can be extended to the design of a variety of electromagnetic driving systems. In addition, an extensive analysis of the beam path distortions generated when a single mirror is pivoted around two axes is covered. Furthermore, correcting equations used to compensate for the systematic errors resulting from this configuration are proposed. The tip/tilt scanner is suited to a variety of applications in industrial material laser processing. These applications are not just restricted to conventional processes like welding, cutting, drilling, and marking, but also to new technologies such as micro-manipulation of parts by laser. In this new approach, bending and shrinkage of the material result due to laser irradiation, allowing fast adjustment and positioning of micro-mechanical and opto-mechanical components with sub-micron accuracy. Furthermore, local annealing of shape memory alloys, leading to stiffness variation in monolithic parts such as micro-grippers, represents a very innovative application in smart materials. On the other hand, the scanner is also a key component in bio-medical and ophthalmologic applications. The first measurements carried out with the scanner for optical tomography of the retina appeared very promising. Furthermore, an optical tweezers set-up has been realized using the scanner, allowing easy and flexible manipulation of laser beam trapped cells and molecules.

EPFL2000

A Novel 4-DOF Origami Grasper With an SMA-Actuation System for Minimally Invasive Surgery

Marco Salerno

Minimally invasive surgery (MIS) is one of the most challenging techniques for robot designers due to the limited size of access points, the high miniaturization level, and the dexterity needed for performing surgical tasks. Conversely, only a few microfabrication technologies are currently available for developing such small-sized systems, which allow safe operations in human bodies. In order to match these challenges in MIS, both design and integration of actuation systems should proceed in parallel with an identification of most effective transmission mechanisms and kinematics. In this paper, an origami parallel module that generates two rotations and one translation is integrated with a twisting module and a compliant gripper to form a novel four-degree-offreedom grasper. The rotational motion leads to the pitch and yaw motion of the gripper, while the translational motion is converted to a roll motion of the gripper via the twisting module that is stacked on top of the parallel module. In light of plane-symmetric properties of the origami structure in the parallel module, both inverse and forward kinematics are resolved with a geometric approach, revealing a unique joint space and a kinematic mapping of the parallel module, leading to the design of two sets of on-board actuation systems. During the analysis, bending motion of a central spring and static properties of the compliant gripper are modeled using finite-element methods. The structure of the twisting module for motion transmission of the grasper is designed and fabricated using origami folding techniques. Gripping forces of the compliant gripper are evaluated in experimental tests. Further analyses of the system performance are addressed in accordance with the scaling ratio of miniaturization and the scalability of the system is demonstrated by a millimeter-sized origami parallel module produced by the smart composite microstructure fabrication process.

Ieee-Inst Electrical Electronics Engineers Inc2016