# Haptic Soft-Keyboard for Tablet-Sized Touchscreens
## 1 Introduction  
These days, the soft keyboard is one of the most used input methods in devices from mobile phones to tablets. Researchers have been working to invent or develop advanced typing methods for mobile touchscreens for over a decade. Most of the efforts were put forth to propose new layouts of key characters by analyzing users' typing behaviors or to improve typing productivity using add-on methods such as typing error correction and autocomplete. However, most of the frustration users feel is due to there being no real key click feedback when typing on the touchscreen. For this, phone users tend to use synchronized vibrations generated from a low-cost linear motor when they type. Nevertheless, most of the users are not satisfied with the built-in vibration feedback of mobile phones, since it feels like buzzing rather than physical key pressing feedback. For larger touchscreens, such as 10.1 inch tablets, there is no commercial tactile soft keyboard available yet. Therefore, there is a need to develop a haptic soft keyboard technology that imitates the physical keyboard for larger touchscreens of mobile devices, including tablets. To this end, piezoelectric actuators that have fast response time (10 to 50 us) and strong deformation (even up to a few hundred millimeters when high voltage is applied) were introduced to be mounted inside touchscreens [1; 2]. Despite the strength of piezoelectric actuators, there is little known about how to use them for implementing tactile key click effects on commercial tablet-sized touchscreens, because piezoelectric actuators generally require very high voltage to be deformed. For instance, single layered piezoelectric actuators are operated at 100 to 200 volts (peak to peak). The required voltage can be reduced by designing multilayered piezoelectric actuators; however, due to the capacitance, the input current should be increased proportional to the number of internal layers. Concerning the need forstronger tactile feedback for larger mobile devices such as tablets, multilayer piezoelectric actuators are preferred to implement tactile feedback on touchscreens. To provide tactile key click effects on the entire touchscreen of a 10.1 inch tablet, multiple piezoelectric actuators should be embedded and controlled accurately using a mobile battery. However, driving multiple piezoelectric actuators at the same time on the touchscreen of a tablet is a challenging issue. The first prototype of a haptic soft keyboard using four piezoelectric actuators was developed with two commercial mobile devices (a Windows phone and Microsoft Surface Pro tablet) by [PERSON] and [PERSON] [1]. They used a portable piezoelectric driver that can drive up to four multilayered piezoelectric actuators at the same time. For the haptic soft keyboard of the tablet, they developed the haptic soft keyboard and implemented an add-on interface for tactile feedback to the touchscreen. A user study measuring the performance of key typing on the touchscreen for three feedback conditions (visual only, visual-aural, and visual-haptic) was conducted, and the results showed improved key typing performance with haptic feedback in terms of accuracy, speed, and usability. They showed how to utilize multiple piezoelectric actuators to simulate a seemingly real key click effect on a larger touchscreen. However, there was a 10 ms time delay to get tactile feedback, the multilayer piezoelectric actuators were too thick, and the piezo driver circuit was too big to be embedded. In a following study, [PERSON] developed a standalone haptic soft keyboard that can be simply added to existing tablet devices regardless of operating systems [2]. In the present study, we present a new approach introducing an optimal design for a multilayered piezoelectric actuator that meets the need of a haptic soft keyboard for 10.1 inch tablets, implementing stronger tactile feedback by utilizing human sensitivity to time differences obtained by a psychophysical experiment and equalizing the force distribution of the touchscreen.