Simulation based experiments of traveling-plane-wave-actuator miropumps and microswimmers

A biologically-inspired micropropulsion method is presented by constructing a series of finite element computational fluid dynamics models for time irreversible inextensible wave propagation method in viscous medium. First, micropump models encompassing fully submerged and anchored waving inextensible film mounted inside a microchannel are analyzed to attain flow, hydraulic power consumption and efficiency plots with respect to parameterized design variables via both 2D and 3D models. Each model is governed by incompressible isothermal Stokes and Navier-Stokes equations respectively and conservation of mass, integrated with deforming mesh employing arbitrary Lagrangian Eulerian method. Next, propulsion velocity, power consumption and efficiency plots of a fully submerged free microswimmer utilizing a wave propagating tail inside a viscous environment is analyzed with respect to parameterized design variables via 3D models governed by incompressible isothermal Navier-Stokes equations and conservation of mass, integrated with deforming mesh employing arbitrary Lagrangian Eulerian Method. All resultant swimmer motions are modeled directly incorporating with stress interactions between surrounding viscous fluid and swimmer surfaces. It is demonstrated that net forward thrust can be harvested from this interaction. Numerical results are compared with the asymptotical results to analytical studies mainly carried out by Sir Taylor (1951), Katz (1974) and Childress (1981) based on mainly 2D assumptions. It is observed that there exists a strong agreement between earlier results and numerical results besides from wavelength parameter which illustrates slight deviation in power consumption characteristics due to the effects introduced by the existence of third dimension

Innovative technologies for the actuation of space manipulators

In this work, innovative technologies for the actuation of space robotic systems are investigated as possible alternative to traditional motors. The research activity focused on double-cone Dielectric Elastomer Actuators (DEAs). The most notable results achieved are predictive models for the static and dynamic performances estimation of the mentioned devices and experimental validation of both single actuators and a robotic arm prototype. The general objective of the thesis is to evaluate innovative actuation technologies for space robotics; the main expected output of the research is the feasibility proof of a robotic space system based on low-TRL (Technology Readiness Level) devices. This objective is achieved by fulfilling two secondary goals: - development of models to predict the actuator performances and validation of ready-to-use design tools; - experimental evaluation of a multi-body manipulator prototype in laboratory environment. The motivation on which this work is based, comes from the wide interest on robotics that recently grew among the space community. A large variety of space missions can benefit from the implementation of automated systems reducing risks, costs, delays and errors deriving from human interaction (i.e. astronauts or ground operators) with space vehicles and structures. On-Orbit Servicing (OOS) missions, in particular, are based on robotic servicing vehicles that perform complex tasks on client objects enabling unprecedented scenarios of improved accessibility to space. Future effective and efficient exploitation of space is strongly dependent on the development of key technologies to support existing and planned orbital assets, aiming to extend spacecraft operational life and to boost mission flexibility. Investigation on innovative actuation technologies is critical to improve space robotics performances and enable new applications. The TRL advancement of young technologies is at the basis of the development of new systems. To date, a considerable number of relevant applications of robotics have been operated in space; main tasks include assembly of complex structures, manipulation of client vehicles and support to astronauts activities. Five human operated manipulators have equipped the Space Shuttle or the International Space Station (ISS), along with a variety of other experimental demonstrators; three examples of humanoid robotic astronauts have been tested and reached different levels of development; a wide range of autonomous demonstrative OOS missions have been conceived and designed, are currently under development or, in some cases, have been flown with success; several planetary probes and (partially) autonomous rovers have been operated on the surface of extraterrestrial bodies like the Moon or Mars. These missions and others constitute the solid background on which this work is based and consolidate the motivation behind the research. The past and present trend in the space sector is to seek improved capabilities, flexibility and autonomy of vehicles, assigning a prominent role to robotics as a key enabling technology. By far the most common actuators in space systems are conventional DC drives like stepper motors and brushless motors: the first are used in robotic arms for control simplicity and positioning accuracy, the second are the standard option in reaction wheels. In some cases brushed DC motors (in sealed or planetary environment) and, less often, voice coil motors have been used. Innovative technologies, like smart materials, are rarely adopted mainly due to reliability and heritage reasons. In general, the space community is very conservative and new technologies have to be proven fail safe and robust, and, for this reason, well-known solutions are often preferred. Nevertheless, implementation examples of smart technologies in space exist and they performed particularly well in off-nominal conditions, where traditional solutions show limitations. It is worth mentioning the most notable: piezo-electric actuators and motors, used in micro-positioning and precision pointing; shape memory devices, employed in release mechanisms; bimetallic actuators, implemented in single-shot systems and thermal control; Electro-Active Polymers (EAPs). The latter have not been extensively employed in space systems yet, although interest is growing around them on the basis of the appealing capabilities proved in many laboratory tests. A wide choice of alternative EAP materials and configurations have been proposed, with ample performance ranges. Dielectric Elastomer Actuators are a promising branch of EAPs family, whose space TRL is currently 2-3. Dielectric Elastomers are arguably the best performing EAPs and, for this reason, very appealing. DEAs have been selected to be investigated in this work for three main reasons: - good compromise performances in terms of stroke/deformation, force/torque and time response; - interesting characteristics like low mass and low power consumption, possibility to improve performances through design flexibility and modularity, multi-DoF configurations, simple manufacturing process, low costs, solid state actuation (no friction), self-sensing capability; - highly innovative technology with low TRL. Double-cone actuators are selected for their flexibility and multi-DoF architecture. An example mission scenario is conceived and simulated in order to determine preliminary requirements for the robotic system and the single actuator. An Active Debris Removal (ADR) mission is selected as a key OOS application of robotic systems. In the considered scenario a large piece of debris (1400 kg) is captured by a small spacecraft by means of a multi-DoF manipulator. The debris is spinning with respect to the servicing spacecraft which is equipped with a robotic arm composed by a variable number of joints (1-3). The capture interface is rigid and guarantees the mechanical connection between the manipulator and the client object. Several simulations are performed with different initial conditions and capture strategies, including the options of a rigidly controlled or free flying spacecraft. The requirements have been defined in terms of forces/torques and rotations at the robot joints. The maximum angular deflection required to the entire robotic arm is 90 deg; torque and forces are strongly dependent on the initial debris (relative) angular momentum, thus it is possible to relax the joint requirement imposing stricter constraints to the target selection or relative navigation system of the servicer. The double-cone DE actuator is based on two circular, pre-stretched membranes of elastomer coated with compliant electrodes on both sides. By applying high voltage to the electrodes, electrostatic forces squeeze the membrane reducing its thickness and, consequently, expanding the material in the plane. Such material deformation is exploited to displace the actuator shaft. Multiple DoF are obtained by selecting a proper electrode layout; a 2-DoF (one rotational and one translational) configuration is selected in view of the proposed robotic application. On the basis of the results available in literature, the commercial polyacrylic elastomer called 3M VHB 49XX is chosen. Proper electromechanical models are identified for the mentioned polymer. Once a set of geometrical and manufacturing parameters are defined, numerical simulations based on literature as well as newly developed FEM models are performed in order to collect a large number of performance data. Interpolating relations are obtained from the collected data and allow to estimate the steady-state performances of the actuator. Torque/force and rotation/stroke are proportional to the squared value of applied high-voltage. The mentioned relations allow to compute the gain to which squared voltage has to be multiplied to estimate the desired quantity. The mean error on estimations is 6.1% for angular rotation, 10.6% for torque, 22.5% for linear stroke and 11.8% for force. A different approach is adopted to model the dynamic behavior of DEAs: transfer function (TF) based models are developed from time dependent data collected through long term tests. The elastomeric material adopted in the device manufacturing shows a relevant viscoelastic behavior that considerably affects the time response of actuators. The TF approach is chosen to simplify the estimation of the transient behavior of DEAs and to provide a practical design tool for robotic applications. The prediction capabilities of TF models are evaluated by comparison with experimental step response. The mean error on the 70% rise time is 15% for angular rotation, 9.5% for torque, 14% for linear stroke and 14% for force; the mean error on amplitude for t > t_r is 4% for angular rotation, 4% for torque, 9% for linear stroke and 11% for force. The developed models, both static and dynamic, are suitable for the implementation of control algorithms and, consequently, for robotic applications. The capability to control the actuator is experimentally proven by testing Single Input / Single Output compensators to actuate both DoF independently. Laboratory tests are conducted in order to evaluate the step response of double-cone actuators. Good accordance is obtained between the simulated and the experimentally measured time response with errors compatible with the prediction inaccuracies of the mentioned models. Finally, a multi-body application of double-cone actuators is designed, manufactured and tested along with a proper control algorithm. The robotic arm is composed by two double-cone DEAs mounted in series. Each actuator has two DoFs and the manipulator moves in the horizontal plane. Two degrees of kinematic redundancy are achieved in the manipulator by controlling only the in-plane position of the end-effector. The arm prototype is suspended by an inextensible cable that reduces the effects of gravity on the motion. The experimental task is the tracking of simple linear and arc trajectories. A vision system monitors the position of the end-effector (optical marker) and feeds the position information to a control computer that commands the voltage actuation to the joints through a properly designed control algorithm. The kinematic redundancy is exploited by the controller to optimize the end-effector trajectory to achieve a given objective: several control schemes with alternative optimization functions are designed and simulated numerically in order to select the best performing option. The chosen control algorithm aims at the minimization of joint variables in order to reduce the risk of actuators saturation. The system performs well and the maximum position error norm is 6.4% of total path length for linear trajectory and 6.8% for arc trajectory

Effects Of Pulsation Frequency On Trailing Edge Plasma Actuators For Flight Control

This thesis details the aerodynamic testing of a dielectric barrier discharge (DBD) plasma actuator operating over a separation step created at the trailing edge of a modified NACA 0012 aerofoil. The work focuses specifically on the use of pulsed or interrupted plasma actuation as opposed to continuously driven actuation, to increase the change in the lift produced by activating the system. The behaviour of the actuation system is characterised in a lamina flow regime at a Reynolds number of 1.33 x 105 using force balance measurements. At zero incidence the actuator produced a peak change in CL of approximately 0.015. However, this result is sensitive to changes in the interruption frequency of the plasma, by changing the plasma drive waveform the system was able to produce both positive and negative changes in lift. A relationship was identified between the change in CL produced and the ratio of the plasma interruption frequency to the natural vortex shedding frequency. This effect was investigated using both time averaged particle image velocimetry (PIV) and instantaneous phase locked PIV images captured in sequence throughout the plasma interruption cycle. The phase locked images showed how variation in the pulsation frequency was able to produce bi-directional actuation by either constructively or destructively interfering with the vortex formation from the back of the separation step. This interference in turn altered the level of separation which was occurring, altering the degree of upwash in the wake and therefore the lift generated by the aerofoil. PIV images were also gathered for device operation at a Reynolds number of 2.3 x 104; this produced a much higher ratio of DBD jet energy to that of the freestream. These conditions showed modified actuator behaviour due to the increased authority over the flow. However, the data still showed a strong interdependence on the reinforcement or destruction of the vortex street by the actuator interruption. Furthermore, work was undertaken to develop an actuator topology based on thin metallised films along with a dielectric which was hardened against the chemical and electrical stresses present in a functioning DBD device. The failure mechanisms of metallised film actuators were investigated, and actuators with lifetimes exceeding 8 hours were demonstrated. A manufacture method for a silicon polymer (PDMS) – Kapton® laminate is detailed; this is shown to be highly resistant to both electrical breakdown and chemical attack by the oxygen plasma

Smart actuation and sensing for meso-scale surgical robotic systems

This dissertation presents the development of meso-scale surgical robotics based on smart actuation and sensing for minimally invasive surgery (MIS). By replacing conventional straight tools by steerable surgical robots, surgical outcomes can potentially be improved due to more precise, stable, and flexible manipulation. Since bending and torsion are the two fundamental motion forms required by surgical tools to complete general surgical procedures, compact torsion and bending modules, both integrated with intrinsic sensors for motion feedback, have been developed based on shape memory alloy (SMA). The developed actuation and sensing techniques have been applied on a robot for neurosurgical intracerebral hemorrhage evacuation (NICHE) and a steerable catheter for atrial fibrillation (AFib) treatment. The NICHE robot consists of a straight stem, an SMA torsion module, and an SMA bending module as a distal bending tip. By synchronizing the motion of the stem, the bending module, and the torsion module, the robot is capable of tip articulation within the brain to remove hemorrhage effectively through suction and electrocauterization. In addition, a skull-mounted robotic headframe has been developed based on a Stewart platform to manipulate the NICHE robot. The robotic catheter is developed by integrating multiple SMA bending modules with flexible braid reinforced tubing. Polymer 3D-printing is used to fabricate all the structural components due to its relatively low cost, short fabrication period, and capability of fabricating complicated structures with high accuracy. The developed surgical robotic systems have been thoroughly evaluated using phantom or cadaver models under computed tomography (CT) and/or magnetic resonance imaging (MRI) guidance. The imaging-guided experimental studies showed that the developed robotic systems consisting of smart actuation and sensing were compatible with CT and MR imaging.Ph.D

Towards rapid 3D direct manufacture of biomechanical microstructures

The field of stereolithography has developed rapidly over the last 20 years, and commercially available systems currently have sufficient resolution for use in microengineering applications. However, they have not as yet been fully exploited in this field. This thesis investigates the possible microengineering applications of microstereolithography systems, specifically in the areas of active microfluidic devices and microneedles. The fields of micropumps and microvalves, stereolithography and microneedles are reviewed, and a variety of test builds were fabricated using the EnvisionTEC Perfactory Mini Multi-Lens stereolithography system in order to define its capabilities. A number of microneedle geometries were considered. This number was narrowed down using finite element modelling, before another simulation was used to optimise these structures. 9 × 9 arrays of 400 μm tall, 300 μm base diameter microneedles were subjected to mechanical testing. Per needle failure forces of 0.263 and 0.243 N were recorded for the selected geometries, stepped cone and inverted trumpet. The 90 μm needle tips were subjected to between 30 and 32 MPa of pressure at their failure point - more than 10 times the required pressure to puncture average human skin. A range of monolithic micropumps were produced with integrated 4 mm diameter single-layer 70 μm-thick membranes used as the basis for a reciprocating displacement operating principle. The membranes were tested using an oscillating pneumatic actuation, and were found reliable (>1,000,000 cycles) up to 2.0 PSIG. Pneumatic single-membrane nozzle/diffuser rectified devices produced flow rates of up to 1,000 μl/min with backpressures of up to 375 Pa. Another device rectified using active membrane valves was found to self-prime, and produced backpressures of up to 4.9 kPa. These devices and structures show great promise for inclusion in complex, fully integrated and active microfluidic systems fabricated using microstereolithography alone, with implications for both cost of manufacture and lead time

Soft Robotics: Design for Simplicity, Performance, and Robustness of Robots for Interaction with Humans.

This thesis deals with the design possibilities concerning the next generation of advanced Robots. Aim of the work is to study, analyse and realise artificial systems that are essentially simple, performing and robust and can live and coexist with humans. The main design guideline followed in doing so is the Soft Robotics Approach, that implies the design of systems with intrinsic mechanical compliance in their architecture. The first part of the thesis addresses design of new soft robotics actuators, or robotic muscles. At the beginning are provided information about what a robotic muscle is and what is needed to realise it. A possible classification of these systems is analysed and some criteria useful for their comparison are explained. After, a set of functional specifications and parameters is identified and defined, to characterise a specific subset of this kind of actuators, called Variable Stiffness Actuators. The selected parameters converge in a data-sheet that easily defines performance and abilities of the robotic system. A complete strategy for the design and realisation of this kind of system is provided, which takes into account their me- chanical morphology and architecture. As consequence of this, some new actuators are developed, validated and employed in the execution of complex experimental tasks. In particular the actuator VSA-Cube and its add-on, a Variable Damper, are developed as the main com- ponents of a robotics low-cost platform, called VSA-CubeBot, that v can be used as an exploratory platform for multi degrees of freedom experiments. Experimental validations and mathematical models of the system employed in multi degrees of freedom tasks (bimanual as- sembly and drawing on an uneven surface), are reported. The second part of the thesis is about the design of multi fingered hands for robots. In this part of the work the Pisa-IIT SoftHand is introduced. It is a novel robot hand prototype designed with the purpose of being as easily usable, robust and simple as an industrial gripper, while exhibiting a level of grasping versatility and an aspect comparable to that of the human hand. In the thesis the main theo- retical tool used to enable such simplification, i.e. the neuroscience– based notion of soft synergies, are briefly reviewed. The approach proposed rests on ideas coming from underactuated hand design. A synthesis method to realize a desired set of soft synergies through the principled design of adaptive underactuated mechanisms, which is called the method of adaptive synergies, is discussed. This ap- proach leads to the design of hands accommodating in principle an arbitrary number of soft synergies, as demonstrated in grasping and manipulation simulations and experiments with a prototype. As a particular instance of application of the method of adaptive syner- gies, the Pisa–IIT SoftHand is then described in detail. The design and implementation of the prototype hand are shown and its effec- tiveness demonstrated through grasping experiments. Finally, control of the Pisa/IIT Hand is considered. Few different control strategies are adopted, including an experimental setup with the use of surface Electromyographic signals