Force/tactile sensor for robotic applications

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Abstract

The paper describes the detailed design and the prototype characterization of a novel tactile sensor1 for robotic applications. The sensor is based on a two-layer structure, i.e. a printed circuit board with optoelectronic components below a deformable silicon layer with a suitably designed geometry. The mechanical structure of the sensor has been optimized in terms of geometry and material physical properties to provide the sensor with different capabilities. The first capability is to work as a six-axis force/torque sensor; additionally, the sensor can be used as a tactile sensor providing a spatially distributed information exploited to estimate the geometry of the contact with a stiff external object. An analytical physical model and a complete experimental characterization of the sensor are presented.

Introduction

Since the early days of robotics, researchers have recognized that equipping a robot with many sensors is a way to confer them autonomy sufficient to perform tasks in unstructured environments. This sensory system should provide information to the robot about physical properties of different nature. Among these properties, when manipulation tasks are considered, the sense of touch is of paramount importance. Tactile sense is used by humans to grasp and manipulate objects avoiding slippage, or to blindly operate in an dynamic environment. An artificial tactile sensor, by mimicking the human touch, should possess the capability to measure both dynamic and geometric quantities, i.e. contact forces and torques as well as spatial and geometrical information about the contacting surfaces. Each of these may be measured either as an average quantity for some part of the robot or as a spatially resolved, distributed quantity across a contact area [1]. A definition of tactile sensor is given by Lee and Nichols [2]: a device or system that can measure a given property of an object or contact event through physical contact between the sensor and the object. The one above is probably the best, and at the same time the broadest definition of a tactile sensor.

Most of the tactile sensors already developed are constituted by an array of sensing elements, called taxels,2 integrated into the fingertips of a manipulator end effector. Already in 1982 Harmon [3], defined a first set of tactile sensor design parameters, making a list which is still widely used by researchers today. These features are absolutely general and application dependent, thus not definitive. Other very important design parameters may be found in [4], which is more focused on the feature of an entire sensing skin and on the integration of such a skin on a complex manipulator structure. A recent and comprehensive review on features that tactile sensors should possess and technologies used to realize them can be found in [5]. Very few commercial devices are currently available, even tough many technologies have been proposed in the scientific literature to build tactile sensors. This is mainly due to high manufacturing complexity and cost. In particular, prototypes of tactile sensors that use the following different technologies can be found: resistive [6], [7], [8], [9], [10], [11], piezoelectric [12], [13], capacitive [14], [15], magnetic [16], [17] and optoelectronic [17], [18], [19], [20], [21]. With reference to the optoelectronic technology, that exploits the electromagnetic properties of light, more details are described in the following since the prototype presented in this paper is based on this technology. Widely used sensors are based on Fibre Bragg Gratings (FBG). Typical examples are the two sensors discussed in [18], which exploit the relationship between the variations of the FBG wavelength and the external force applied to the FBG. The sensors based on optical fibres are expensive and difficult to integrate into complex robotic structures (e.g. anthropomorphic hands, robotic arms) because of the bending losses that occur in the fibres routing. Other sensors are based on scattering by small or big particles (compared to the wavelength used) and make use of highly scattering materials such as foams. An example of how foams can be used for tactile sensing technology can be found in [19], where the urethane foam presents a cavity, whose dimensions vary according to the external force applied. When the cavity is compressed, the scattered energy density varies, and by detecting this variation, it is possible to reconstruct the external force magnitude. The main drawback of this sensor type is related to the stochastic nature of the scattering phenomenon [22], therefore the measurement process is characterized by a lack of repeatability. The prototypes proposed in [20], [21] use CCD cameras to measure tactile images. These solutions involve large volumes, weights and costs that complicate the integration into robotic hands. In [17] a sensor prototype with two different measuring systems is presented. One of these uses an LED and four phototransistors to measure the deformation in the centre of an elastic dome and then these deformation measurements are experimentally related to the external vertical force applied to the dome. In the proposed configuration, the sensor works as a simple force sensor with a soft interface.

The force/tactile sensor proposed in this paper exploits the thorough study based on Finite Element (FE) modelling conducted in [23] where the working principle has been presented for the first time. There, only a simplified prototype with limited sensing capabilities was tested with the aim of showing only the feasibility of the approach, while the main focus was on the mechanical characterization and optimization of the device.

The sensor is based on the use of optoelectronic technologies and it aims to overcome most of the problems encountered in the works cited above, mainly: difficulty of the integration into small spaces, high costs, repeatability and complex conditioning electronics. The sensor has different capabilities, i.e. it can measure the six components of the force and torque vectors applied to it, and it can be used as a tactile sensor providing a spatial and geometrical information about the contact with a stiff external object. In fact, an approximated analytical model of the physical contact is derived, that is usefully exploited to extract information on the contact geometry from the sensor signals. Experimental characterization results are presented to both validate the model and to show how the sensor, with a proper algorithm, can be used to provide a complete characterization of the contact between the sensor and a stiff external object.

Section snippets

Sensor concept

To realize all the sensor capabilities as stated in the introduction, a deformable elastic layer is positioned above a matrix of sensible points (the taxels) so as to transduce the force and torque vectors into deformations which are then measured by the sensible points as explained in the following subsection. Furthermore, the signals provided by the taxels, which are spatially distributed below the deformable layer, constitute a spatially distributed information that will also allow to

Prototype characterization as force/torque sensor

The objective of this section is to show all the potentiality of the presented prototype sensor and a calibration procedure necessary to use it as a six-axes force/torque sensor. The characterization of the sensor has been made in the hypothesis that the contact surface can be approximated by a plane with an high stiffness with respect to the deformable layer. The hypothesis that the contact surface is a plane can be considered verified each time the external object has a curvature radius

Prototype characterization as tactile sensor

As said, the proposed prototype can be also used as tactile sensor, estimating not only force and torque components as described in Section 3, but also the contact geometry. In some applications, e.g. complex robotic manipulation tasks, the availability to the control system of an estimate of contact plane position and orientation together with the interaction forces exchanged by the sensor and the external object are fundamental to successfully execute the task. To obtain this information from

Conclusions

The paper described the detailed design and the experimental characterization of a novel tactile sensor for robotic applications based on optoelectronic technology. The sensor mechanical structure has been designed so as to allow the device to work as a six-axis force/torque sensor. An analytical physical model has been derived to let the sensor act as a tactile sensor for estimation of the geometry of the contact with a stiff external object. A complete calibration procedure has been presented

Acknowledgements

The research leading to these results has been partly funded by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant agreement no. 216239 (DEXMART project) and partly by the Italian Ministry of University and Scientific Research (PRIN 2009) under the national project ROCOCO.

Giuseppe De Maria was born in Napoli, Italy, on December 1948. In 1973 he received the Laurea degree in Electronic Engineering from the University of Naples. He was associate professor of Automatic Control at the University of Naples “Federico II”. Since 1992 he is full professor of Automatic Control at the Faculty of Engineering of the Second University of Naples. His research interests include robust control, control of mechanical systems, industrial and advanced robotics, control of

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    Giuseppe De Maria was born in Napoli, Italy, on December 1948. In 1973 he received the Laurea degree in Electronic Engineering from the University of Naples. He was associate professor of Automatic Control at the University of Naples “Federico II”. Since 1992 he is full professor of Automatic Control at the Faculty of Engineering of the Second University of Naples. His research interests include robust control, control of mechanical systems, industrial and advanced robotics, control of aerospace and aeronautical systems, active noise and vibration control of flexible structures. Now his research interests are focused on the control of smart materials, in particular piezoceramics, magnetostrictive, with the aim to realize artificial muscles. Concerning this field of research he is responsible of National and European research contracts. He is work-package leader of the DEMART project in the 7th Framework Programme of European Community.

    Ciro Natale received the Laurea degree and the Research Doctorate degree in Electronic Engineering from the University of Naples in 1995 and 2000, respectively. From 2000 to 2004 he has been Research Associate at the Second University of Naples, where he currently holds the position of Associate Professor of Robotics. From November 1998 to April 1999 he was a Visiting Scholar at the German Aerospace centre in Oberpfaffenhofen, Germany. His research interests on robotics include modelling and control of industrial manipulators, force and visual control, cooperative robots. In the aeronautics application sector, his research activities are focused on modelling and control of flexible structures, active noise and vibration control, identification and control of smart materials. He has published more than 70 journal and conference papers as well as authored and co-authored monographs on robotics and control of flexible structures. He currently serves as Associate Editor of the IEEE Trans. on Control Systems Technology.

    Salvatore Pirozzi was born in Napoli, Italy, on April 21st, 1977. He received the Laurea and the Research Doctorate degree in Electronic Engineering from the Second University of Naples, Aversa, Italy, in 2001 and 2004, respectively. From 2008 he is a Assistant Professor at the Second University of Naples. His research interests include modelling and control of smart actuators for advanced feedback control systems and design of innovative sensors for robotics applications, as well as identification and control of vibrating systems. He published more than 30 international journal and conference papers and he is co-author of the book “Active Control of Flexible Structures”, published by Springer.

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    Euopean patent pending, Application no. EP11425148.1.

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