A flexible sensor technology for the distributed measurement of interaction pressure

We present a sensor technology for the measure of the physical human-robot interaction pressure developed in the last years at Scuola Superiore Sant'Anna. The system is composed of flexible matrices of opto-electronic sensors covered by a soft silicone cover. This sensory system is completely modular and scalable, allowing one to cover areas of any sizes and shapes, and to measure different pressure ranges. In this work we present the main application areas for this technology. A first generation of the system was used to monitor human-robot interaction in upper- (NEUROExos; Scuola Superiore Sant'Anna) and lower-limb (LOPES; University of Twente) exoskeletons for rehabilitation. A second generation, with increased resolution and wireless connection, was used to develop a pressure-sensitive foot insole and an improved human-robot interaction measurement systems. The experimental characterization of the latter system along with its validation on three healthy subjects is presented here for the first time. A perspective on future uses and development of the technology is finally drafted

SUAS: A Novel Soft Underwater Artificial Skin with Capacitive Transducers and Hyperelastic Membrane

The paper presents physical modeling, design, simulations, and experimentation on a novel Soft Underwater Artificial Skin (SUAS) used as tactile sensor. The SUAS functions as an electrostatic capacitive sensor, and it is composed of a hyperelastic membrane used as external cover and oil inside it used to compensate the marine pressure. Simulation has been performed studying and modeling the behavior of the external interface of the SUAS in contact with external concentrated loads in marine environment. Experiments on the external and internal components of the SUAS have been done using two different conductive layers in oil. A first prototype has been realized using a 3D printer. The results of the paper underline how the soft materials permit better adhesion of the conductive layer to the transducers of the SUAS obtaining higher capacitance. The results here presented confirmed the first hypotheses presented in a last work and opened new ways in the large-scale underwater tactile sensor design and development. The investigations are performed in collaboration with a national Italian project named MARIS, regarding the possible extension to the underwater field of the technologies developed within the European project ROBOSKIN

Force-Torque Sensing in Robotics

Being able to perform dynamic motions repeatably and reliably is an active research topic. The present thesis aims to contribute to this by improving the accuracy of force-torque sensing in robots. It focuses primarily on six axis force-torque sensors, although other sources of force-torque sensing are explored. Force sensing technologies, calibration procedures of these sensors and the use of force-torque sensing in robotics are described with the aim to familiarize the reader with the problem to solve. The problem is tackled in two ways: improving the accuracy of six axis force-torque sensors and exploring the use of tactile sensor arrays as force-torque sensors. The contributions of this thesis are : the development of the Model Based In situ calibration method for improving measurements of sensors already mounted on robots and the improvement in performance of the robot as a consequence; the design of a calibration device to improve the reliability and speed of calibration; and the improvement of force sensing information of a capacitive tactile array and its use on a robot as force-torque information source. The developed algorithms were tested on the humanoid robotic platform iCub

Polyvinylidene fluoride - based MEMS tactile sensor for minimally invasive surgery

Minimally invasive surgery (MIS) procedures have been growing rapidly for the past couple of decades. In MIS operations, endoscopic tools are inserted through a small incision on human's body. Although these procedures have many advantages such as fast recovery time, minimum damage to human body and reduced post operative complications, it does not provide any tactile feedback to the surgeon. This thesis reports on design, finite element analysis, fabrication and testing of a micromachined piezoelectric endoscopic tactile sensor. Similar to the commercial endoscopic graspers, the sensor is teeth like in order to grasp slippery tissues. It consists of three layers; the first layer is a silicon layer of teeth shapes on the top and two supports at the bottom forming a thin plate and a U-Channel. The second layer is a patterned Polyvinylidene Fluoride (PVDF) film, and the third layer is a supporting Plexiglas. The patterned PVDF film was placed on the middle between the other two layers. When a concentric load is applied to the sensor, the magnitude and the position of the applied load are obtained from the outputs of the sensing elements which are sandwiched between the silicon supports and the Plexiglas. In addition, when a soft object/tissue is place on the sensor and load is applied the degree of the softness/compliance of the object is obtained from the outputs from the middle PVDF sensing elements, which are glued to the back of the thin silicon plate. The outputs are related to the deformation of the silicon plate which related to the contacting object softness. The sensor has high sensitivity and high dynamic range as a result it can potentially detect a small dynamic load such as a pulse load as well as a high load such as a firm grasping of a tissue by an endoscopic grasper. The entire surface of the tactile sensor is also active, which is an advantage in detecting the precise position of the applied point load on the grasper. The finite element analysis and experimental results are in close agreement with each other. The sensor can potentially be integrated with the gasper of a commercially available endoscopic graspe

Piezoelectric Transducers Based on Aluminum Nitride and Polyimide for Tactile Applications

The development of micro systems with smart sensing capabilities is paving the way to progresses in the technology for humanoid robotics. The importance of sensory feedback has been recognized the enabler of a high degree of autonomy for robotic systems. In tactile applications, it can be exploited not only to avoid objects slipping during their manipulation but also to allow safe interaction with humans and unknown objects and environments. In order to ensure the minimal deformation of an object during subtle manipulation tasks, information not only on contact forces between the object and fingers but also on contact geometry and contact friction characteristics has to be provided. Touch, unlike other senses, is a critical component that plays a fundamental role in dexterous manipulation capabilities and in the evaluation of objects properties such as type of material, shape, texture, stiffness, which is not easily possible by vision alone. Understanding of unstructured environments is made possible by touch through the determination of stress distribution in the surrounding area of physical contact. To this aim, tactile sensing and pressure detection systems should be integrated as an artificial tactile system. As illustrated in the Chapter I, the role of external stimuli detection in humans is provided by a great number of sensorial receptors: they are specialized endings whose structure and location in the skin determine their specific signal transmission characteristics. Especially, mechanoreceptors are specialized in the conversion of the mechanical deformations caused by force, vibration or slip on skin into electrical nerve impulses which are processed and encoded by the central nervous system. Highly miniaturized systems based on MEMS technology seem to imitate properly the large number of fast responsive mechanoreceptors present in human skin. Moreover, an artificial electronic skin should be lightweight, flexible, soft and wearable and it should be fabricated with compliant materials. In this respect a big challenge of bio-inspired technologies is the efficient application of flexible active materials to convert the mechanical pressure or stress into a usable electric signal (voltage or current). In the emerging field of soft active materials, able of large deformation, piezoelectrics have been recognized as a really promising and attractive material in both sensing and actuation applications. As outlined in Chapter II, there is a wide choice of materials and material forms (ceramics: PZT; polycrystalline films: ZnO, AlN; polymers and copolymers: PVDF, PVDF-TrFe) which are actively piezoelectric and exhibit features more or less attractive. Among them, aluminum nitride is a promising piezoelectric material for flexible technology. It has moderate piezoelectric coefficient, when available in c-axis oriented polycrystalline columnar structure, but, at same time, it exhibits low dielectric constant, high temperature stability, large band gap, large electrical resistivity, high breakdown voltage and low dielectric loss which make it suitable for transducers and high thermal conductivity which implies low thermal drifts. The high chemical stability allows AlN to be used in humid environments. Moreover, all the above properties and its deposition method make AlN compatible with CMOS technology. Exploiting the features of the AlN, three-dimensional AlN dome-shaped cells, embedded between two metal electrodes, are proposed in this thesis. They are fabricated on general purpose Kapton™ substrate, exploiting the flexibility of the polymer and the electrical stability of the semiconductor at the same time. As matter of fact, the crystalline layers release a compressive stress over the polymer, generating three-dimensional structures with reduced stiffness, compared to the semiconductor materials. In Chapter III, a mathematical model to calculate the residual stresses which arise because of mismatch in coefficient of thermal expansion between layers and because of mismatch in lattice constants between the substrate and the epitaxially grown films is adopted. The theoretical equation is then used to evaluate the dependence of geometrical features of the fabricated three-dimensional structures on compressive residual stress. Moreover, FEM simulations and theoretical models analysis are developed in order to qualitative explore the operation principle of curved membranes, which are labelled dome-shaped diaphragm transducers (DSDT), both as sensors and as piezo-actuators and for the related design optimization. For the reliability of the proposed device as a force/pressure sensor and piezo-actuator, an exhaustive electromechanical characterization of the devices is carried out. A complete description of the microfabrication processes is also provided. As shown in Chapter IV, standard microfabrication techniques are employed to fabricate the array of DSDTs. The overall microfabrication process involves deposition of metal and piezoelectric films, photolithography and plasma-based dry and wet etching to pattern thin films with the desired features. The DSDT devices are designed and developed according to FEM and theoretical analysis and following the typical requirements of force/pressure systems for tactile applications. Experimental analyses are also accomplished to extract the relationship between the compressive residual stress due to the aluminum nitride and the geometries of the devices. They reveal different deformations, proving the dependence of the geometrical features of the three-dimensional structures on residual stress. Moreover, electrical characterization is performed to determine capacitance and impedance of the DSDTs and to experimentally calculate the relative dielectric constant of sputtered AlN piezoelectric film. In order to investigate the mechanical behaviour of the curved circular transducers, a characterization of the flexural deflection modes of the DSDT membranes is carried out. The natural frequency of vibrations and the corresponding displacements are measured by a Laser Doppler Vibrometer when a suitable oscillating voltage, with known amplitude, is applied to drive the piezo-DSDTs. Finally, being developed for tactile sensing purpose, the proposed technology is tested in order to explore the electromechanical response of the device when impulsive dynamic and/or long static forces are applied. The study on the impulsive dynamic and long static stimuli detection is then performed by using an ad hoc setup measuring both the applied loading forces and the corresponding generated voltage and capacitance variation. These measurements allow a thorough test of the sensing abilities of the AlN-based DSDT cells. Finally, as stated in Chapter V, the proposed technology exhibits an improved electromechanical coupling with higher mechanical deformation per unit energy compared with the conventional plate structures, when the devices are used as piezo-actuator. On the other hand, it is well suited to realize large area tactile sensors for robotics applications, opening up new perspectives to the development of latest generation biomimetic sensors and allowing the design and the fabrication of miniaturized devices