# Haptic Soft-Keyboard for Tablet-Sized Touchscreens
## 4 Results  
Three evaluation experiments were conducted to verify the piezoelectric actuators designed with optimum parameters (three layers and the layer ratio) and the haptic discrimination threshold and to measure typing performance with the tactile feedback system. A piezoelectric actuator manufactured  
Figure 7: Accelerations distributed on a 10.1 inch commercial touchscreen (Samsung Galaxy Tab); non-uniform tactile feedback distribution (**a**) and the globally equalized distribution by our proposed algorithm (**b**). Note that the redder area means the stronger tactile feedback, and black circles are points where accelerations were measured. based on the final design was tested on a 10.1 inch touchscreen under a laboratory environment. The strength of vibro-tactile feedback (acceleration) on a commercial touchscreen was measured as increasing input voltage to a piezoelectric actuator. Accelerations were measured with two conditions, free stroke on a detached touchscreen and after embedding the actuator in a 10.1 inch tablet. For the free stroke condition, the piezo actuator was glued on a separated touch glass (see Figure 8), and for the embedding condition, the actuator was attached to the bezel space between the LCD panel and the touchscreen of a tablet. Then, an accelerometer (KISTLER 8688A50) was placed on the touchscreen to measure accelerations as the input voltage (a square wave of 500 Hz) of the piezo actuator was incrementally altered. The input signal to the piezo was controlled by a function generator (Agilent 33120). Figure 8 shows the accelerations measured with the two conditions (before and after embedding actuators). The results showed that, for the embedding condition, the strength of tactile feedback was reduced by 27.3% due to piezoelectric actuators' compression by the process of reassembly of the tablet device. The second experiment was conducted to verify whether the two-peak tactile pulse made users feel better about the touchscreen. For this experiment, ten participants (five males and five females, age range 21-29, average age 25.5 years old) who had never experienced haptic interfaces took part in the experiment. The experiment setup was the same as shown in Figure 4, but the value of \(t\) of the test stimulus in Figure 3a was set to 3 ms, which was much smaller than the estimated discrimination threshold (\(3.919\pm 0.393\)) to ensure imperceptibility. The test and the reference stimuli were randomly displayed to avoid subjects' expectation errors. No sound feedback was provided during the experiment. The participant's task was to indicate which one felt better as a key click. Each participant was tested ten times for both stimuli, resulting in a total of ten trials per participant. It took about five minutes for each participant to complete all of the trials. The result was that 81% of participant responses were that the two-peak pulse was better, but there was a variation from person to person due to different sensitivities. The last experiment was conducted to evaluate typing performance (efficiency, speed, and accuracy) with the developed tactile touchscreen for a 10.1 inch tablet device. For the experiment, a commercial QWERTY keyboard was implemented on the Android platform of a tablet device (uPad), as shown in Figure 9. The developed system had a latency of 1.87 ms from sensing touch using the Android touch event listener to tactile feedback. A two-peak tactile pulse (\(t\) = 3 ms) was used for tactile key click feedback, and the signal was amplified to 65 Vpp by the amplifier (TI DRV 2667, gain 38.4 dB). The tactile feedback was synchronized with the key-down event provided from the Android platform.