Sensors and Actuators A 222 (2015) 130–139 Contents lists available at ScienceDirect Sensors and Actuators A: Physical j ourna l ho me page: www.elsevier.com/locate/sna ACUPAD: A track-pad device based on a piezoelectric bimorph Nicola Lamberti a,∗ , Giosuè Caliano b , Alessandro Stuart Savoia b a Dipartimento d’Ingegneria Industriale, Università di Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (SA), Italy b Dipartimento di Ingegneria, Università di Roma Tre, Via della Vasca Navale, 84, 00146 Roma, Italy a r t i c l e i n f o Article history: Received 5 March 2014 Received in revised form 24 October 2014 Accepted 28 October 2014 Available online 6 November 2014 Keywords: Trackpad Piezoelectric bimorph Tactile sensor a b s t r a c t A track-pad is a pointing device, featuring a tactile sensor, able to translate the motion and position of a user’s finger, or a stylus, to a relative position on a screen. In this paper a piezoelectric tactile sensor for track-pad applications is proposed; the active element of the device is a cheap piezoceramic bimorph disk, widely used in buzzers and telephone receivers, clamped all around its border. The device operating principle is the following: when a stylus is positioned on the bimorph surface, the displacement field of the contact point is modified, with a consequent variation of the device electrical input impedance; the stylus position can be therefore related to the impedance variation. The system was analyzed by FEM, obtaining a clear dependence of the device characteristic frequencies on the stylus radial position, while by moving the stylus along the angle, a clear variation of impedance values is obtained. A device prototype was realized and FEM results were experimentally confirmed, validating the proposed device performance. © 2014 Elsevier B.V. All rights reserved. 1. Introduction In recent years, the advent of portable personal digital assis- tants (PDAs), smartphones, and tablet PCs, has made necessary to use multi-touch or stylus-based input devices that are perceived as being more useful for mobile applications [1–3]. These input devices exchange informations between man and machine and must function as position encoders; the major considerations, handling the device two dimensional positional information, are the location of the input surface, with respect to the display, and the mechanism by which a user indicates selected positions on the device surface. In order to use at the same time the input device and the display, there must be a direct mapping of positions from the two surface devices. For the human user, this relationship is con- siderably simplified if the two surfaces are coincident; this assumes a one-to-one mapping scale and requires a transparent input sur- face. Another possibility is that the two surfaces are in different physical locations; for example, in a smartphone the input device can be positioned on the back shell, by making available the use of the device with just one hand. In this last case the user can observe the mapping of his finger position, in relation to the desired item or position, and can accordingly modify his selection. ∗ Corresponding author. Tel.: +39 089 964305. E-mail address: nlamberti@unisa.it (N. Lamberti). The first hand-held touch technologies were resistive soft films that accepted both finger touch and stylus based interactions [4]; this multi-modal input technology must comply with contradic- tory design constraints requiring both sensitivity to low-pressure interactions, associated with finger touch, and high-pressure inter- actions coming from point-like forces imposed by passive stylus tips. The detection of light and swift movements are limited by the low immunity to scratches and the hard-to-avoid force activation threshold. An alternative method for sensing a space and time variable touch pressure by means of a network of capacitors was already available almost three decades ago, but did not allow precise pen use [5]. Recently Apple Inc. (Cupertino, CA, USA) proposed a surface capacitive network with a charge transfer measuring tech- nique, able to develop multi-touch interactions [6]; with the strong growth of projected capacitive technology, they were able to obtain 2-D multi-touch images by means of so called finger touch tech- nologies, often with pre-touch detection [7,8]. Multitouch capacitive technology is not the ultimate paradigm and there is still a need for a precise input stylus; for example, the company Wacom (Wacom Co., Ltd., Tokyo, Japan) entered in the smartphone market with a smart inductive stylus that, also by using the near touch detection, is able to convert pressure into additional writing parameters used for example to obtain thicker drawing lines [9]. For this device, using air as the propagation medium, the inclination of the stylus is a problem. http://dx.doi.org/10.1016/j.sna.2014.10.031 0924-4247/© 2014 Elsevier B.V. All rights reserved.