In the last twenty years, many tactile sensor devices have been presented, exploiting several physical phenomena as transduction modes [2�C4,8,9]. However, most of them do not satisfy completely the specific requirements of in-hand manipulation, being too bulky to be used without sacrificing dexterity or because they are fragile, rigid, slow or lack some fundamental characteristics. For this reason, it is not possible to choose a standard system like CCD or CMOS optical arrays used for the sense of sight. Moreover, tactile sensors get their information through physical interaction, this brings about problems of robustness to withstand several impacts and abrasions, and of compliance, to conform the device to the robot surface guaranteeing an adequate friction for handling tools securely [2].
The solutions presented in the literature for the fabrication of tactile sensors are innumerable, so that an in-depth classification based on task, site, transduction method and mechanical properties is necessary to organize and select the interested field [2�C4]. The present review is concentrated mostly on the last two classifications, i.e., transduction method and mechanical properties. Considering the mechanical properties, tactile sensors can be classified as rigid, flexible, compliant, conformable, stretchable, etc. Depending on the final application, the choice of these characteristics is fundamental for obtaining a perfect bonding and uniform coverage of the robot surface, and most of all for preventing damage and abrasion during the utilization.
The other classification is made with regard to the physical nature of the transduction method. Thus, tactile devices can be divided into piezoelectric [10,11], optical [12,13], magnetic [14,15], ultrasonic [16,17], resistive and capacitive [18�C21]. With the first four solutions it is possible to obtain extremely high sensitivity and elevated spatial resolution, however most of these devices require a large pay load, are expensive and complex to fabricate, difficult to reproduce, and have reduced flexibility. Therefore they can result unsuitable for integration on a robot hand or body. In contrast, capacitive and resistive approaches guarantee wide working ranges, low cost and power consumption, and the use of simple read-out electronics.
Most of them combine mechanical flexibility Dacomitinib and resistance, providing a better integration and a primary protection from external overpressure, shock and vibrations. For these reasons, both capacitive and resistive approaches are certainly the most investigated among all the solutions. Moreover the majority of the commercial tactile sensors exploit these transduction mechanisms because of the lower cost and easiness of fabrication together with the basic electronics needed for the read-out operation.