# Haptic Soft-Keyboard for Tablet-Sized Touchscreens
## 3 Materials and Methods
### Perception Study to Estimate the Sensitivity to Time Difference of Tactile Key Click
#### 3.2.3 Procedures  
A two-interval forced choice (2 IFC) one-up one-down adaptive procedure [20] was used for the experiment to estimate the discrimination thresholds of the tactile key click effect simulated by a piezoelectric actuator designed in this study. Participants were tested with two conditions, haptics alone (H) or haptics combined with audible sound (HA). On each trial, for the H (or the HA) condition, the participants put their index finger on the tablet touchscreen (Samsung Galaxy Tab) and felt the key click effect simulated by the piezoelectric actuator driven by the stimulus signal (a 500 Hz square wave signal). For the HA condition, the participants perceived the key click effect with the same square wave (500 Hz) through both the touchscreen and the headphone. The sound was generated with a sound play function implemented in the microprocessor. The reference stimulus was presented with a 2 ms square wave, while the test stimulus was controlled by a time interval (\(t\)), as seen in Figure 3. The reference and the test stimuli were randomly displayed to avoid subjects' expectation errors. The participant's task was to indicate which one was felt as two pulses. According to the one-up one-down adaptive rule, the value of \(t\) in the test pulse was increased after an incorrect response and decreased after a correct response. The initial \(t\) value was chosen to be large enough (15 ms) so that the two peaks of a test pulse train were clearly felt to the participant. The value of \(t\) then decreased or increased by a  
Figure 4: Experimental setup of a perception study measuring temporal discrimination thresholds on a 10.1 inch touchscreen. The participant passively felt tactile feedback on the touchscreen and then responded to the provided stimulus.  
Figure 3: Two stimuli, (**a**) test and (**b**) reference, simulating a key click effect used for the psychophysical experiment. The time interval (\(t\)) between two peaks was controlled during the experiment.  
step size of 6 dB depending on the participant's responses. After the initial three reversals, the value of \(t\) changed by 2 dB. The initial larger change in \(t\) was necessary for fast convergence, whereas the later smaller change improved the accuracy of threshold estimates. Each adaptive series was terminated after eight reversals at the smaller step size. The participants were comfortably seated in front of the touchscreen with a keypad, as shown in Figure 4. They wore headphones to block any sound from the equipment for the H condition. Initial training was provided, where a series of stimuli were presented to familiarize the participants with the tactile key click effect on the touchscreen and the two experimental conditions (H and HA). The training time varied from participant to participant and averaged 10 min. Each participant was tested six times per condition, resulting in a total of twelve adaptive series per participant. It took between forty minutes to one hour for each participant to complete all the 12 series.