Design and performance of an active acoustic back cover based on piezoelectric elements

Due to the widespread use of portable devices, there is a strong focus on possible methods of interaction between man and the device, in order to exchange information. Capacitive touchscreens are commonly used on the front of smartphones or tablets; nevertheless, for some applications it can be useful to have a pointing solution on the back of the device, so that fingers do not cover the visible area. The solution proposed in this paper is to turn the plastic back cover of the device into a position encoder, by gluing some piezoelectric small plates, electrically connected in parallel, on its inner surface. Moving the finger on the back cover causes a variation of the electrical impedance of the structure, thus the desired position can be revealed and reported on the front screen. In this work, the performances of a trackpad based on piezoelectric elements are investigated by Finite Elements Analysis, and an automatic method for the recognition of the finger position is proposed. Experimental tests were also conducted on a prototype, in order to confirm the capabilities of the proposed device.