# Design and Implementation of a Semantic Information Expression Device Based on Vibrotactile Coding
## 2 Hardware Design
### Vibration Coding Design and Experimentation
#### 2.2.2 Vibration Position Recognition Accuracy Test  
In order to help the user to judge the vibration position and to improve the accuracy of the user's identification of vibration positions, comparative experiments were carried out. A comparison experiment was carried out using one vibration intensity and two vibration intensities (one strong and one weak) for six dot positions, and the final vibration intensity scheme adopted was determined by the subjects' accuracy in identifying the vibration positions.  
In the experiment, a total of 24 vibration motors of 3 \(\times\) 8 were selected, and their vibration modes were set as one-point vibration, two-point vibration, three-point full vibration, and full non-vibration, and the permutations and combinations were carried out. Eight columns of motors were divided into (a), (b), and (c) three large groups and eight groups, as shown in Figure 5. The vibration of each group was touch-read with the index finger during the experiment.  
Figure 4: Mean values of ratings of vibratory and numbness perception for different duty cycles. The horizontal coordinate represents the duty cycle (indicating vibration intensity), which was chosen to range from 0.2 to 0.6 for the test. The vertical coordinate represents the average of the subjects’ ratings for the five vibration intensities.  
Ten subjects wore headphones and blindfolds to exclude interference (the experimental scenario is shown in Figure 6). The experiment was conducted with one vibration intensity, which was set by the experimenter to 0.3. Eight sets of vibrations were presented to the subjects sequentially, then in reverse sequence, and finally randomly in a disrupted sequence. Subjects touch-read and then recorded the points, a total of 10 people \(\times\) (8 groups in order + 8 groups in reverse order + 8 groups random) = 240 experiments. Every time a touch-reading was performed, the subject needed to rest their fingers for 30 s to avoid numbness of the fingers that would affect the results of the experiment. The two vibration intensity experiments were conducted in the same way, except that the vibration intensity was adjusted to 0.2 for Rows 1 and 3, and 0.3 for Row 2 (similar to Figure 7b). The vibration position recognition accuracy of the two experiments are shown in Figure 7a.  
Figure 5: Eight vibration modes. Black dots indicate vibration, white dots indicate no vibration. (**a**) One-point vibration. Divided into three groups. (**b**) Two-point vibration. Divided into three groups. (**c**) Three-point full vibration and full non-vibration. Divided into two groups.  
Figure 6: Vibration position recognition accuracy test. Subjects would wear headphones and blindfolds while touch-reading the device. At the end of the touch-read, blindly note down the information read.  
Figure 7: (**a**) Vibration position recognition accuracy for two experiments. The horizontal coordinate represents the 8 vibration modes (seen in Figure 5), and each vibration mode represents a group; the vertical coordinate represents the subject’s touch-reading accuracy for the 8 vibration modes. (**b**) Two types of vibration intensities. Rows 1 and 3 are strong vibration and Row 2 is weak vibration.  
When a Braille cell has only one vibration intensity, the average recognition accuracy is only 72.50%, while the average recognition accuracy reaches 93.75% when two vibration intensities are strong and weak. In Group 8, where all three points were not vibrating, the results should have been at a 100% touch-read accuracy, but the results were biased by the numbness of the fingers. In addition, in the experiment, it was found that the tactile sensitivity of the fingertip was the highest, while the vibration felt in the joint part of the finger would transfer the vibration sensation to the fingertip, and the tactile sensitivity of the position between the fingertip and the joint was low. As a result, two vibration intensities were selected for the encoding scheme of this device, as shown in Figure 7b.