## 2 Materials and Methods
### Design of the Fuzzy-Control Anti-Cybersickness Intelligent System (FCACIS)
#### 2.2.1 Fuzzy Sets
\(\widetilde{T}_{3}\) is defined as an exposure time that is too long (Long), which has a significant effect on cybersickness. All of these functions are constructed as trapezoidal membership functions. Accordingly, the defined domain of exposure time is [0 min, 60 min], where \"0 min\" indicates that the induction of cybersickness is very low and \"60 min\" indicates that the induction of cybersickness is very high; 8 min, 20 min, and 30 min are used as segment points. Figure 3: Depth perception ability assessment: The user judges the distance between the two pillars that appear in the window and uses the control keys “\(\uparrow\)” and “\(\downarrow\)” to adjust their front and rear positions for confirmation. When the positions of the two pillars appear equidistant, the confirm button is pressed. (a) shows that the left pillar is closer to the subject; (b) shows that the right pillar is closer to the subject. The top image is the image shown to the subject, and the bottom image shows the actual locations. Movement speeds fuzzy set  
According to the research of [PERSON] et al. (2001), as the movement speed of the virtual screen gradually increases from 3 m/s, the symptoms of cybersickness gradually increase [29]. When the speed is 10 m/s, the symptoms of cybersickness are the most intense. Therefore, for the movement speed fuzzy set \(\mu Vy\) (\(v\)), three membership functions are defined: \(\tilde{V_{1}}\), \(\tilde{V_{2}}\), and \(\tilde{V_{3}}\). \(\tilde{V_{1}}\) is defined as slow movement (Slow), which has a slight effect on cybersickness. \(\tilde{V_{2}}\) is defined as slightly faster movement (Medium), which has a moderate impact on cybersickness. \(\tilde{V_{3}}\) is defined as fast movement (Quick), which has a significant effect on cybersickness. All of these functions are constructed as trapezoidal membership functions. Accordingly, the defined domain of movement speed is [0 m/s, 20 m/s], where \"0 m/s\" indicates that the induction of cybersickness is very low and \"20 m/s\" indicates that the induction of cybersickness is very high; 3 m/s, 6.5 m/s, and 10 m/s are taken as segment points. Fuzzy set of angular rotation velocities  
[PERSON] and [PERSON] (2020) reported that when the angular rotation velocity in a virtual environment is less than 15\({}^{\circ}\)/s, the symptoms of cybersickness in subjects are not obvious [16]. As the angular rotation velocity increases, the symptoms of cybersickness gradually increase. When the angular rotation velocity exceeds 65\({}^{\circ}\)/s, the symptoms of cybersickness become obvious and severe. Therefore, for the angular rotation velocity fuzzy set \(\mu A_{z}\) (\(a\)), three membership functions are defined: \(\widetilde{A_{1}}\), \(\widetilde{A_{2}}\), and \(\widetilde{A_{3}}\). \(\widetilde{A_{1}}\) is defined as slow angular rotation (Slow), which has a slight effect on cybersickness. \(\widetilde{A_{2}}\) is defined as slightly faster angular rotation (Medium), which has a moderate impact on cybersickness. \(\widetilde{A_{3}}\) is defined as fast angular rotation (Quick), which has a significant impact on cybersickness. All of these functions are constructed as trapezoidal membership functions. Accordingly, the defined domain of angular rotation velocity is [0\({}^{\circ}\)/s, 360\({}^{\circ}\)/s], where \"0\({}^{\circ}\)/s\" indicates that the induction of cybersickness is very low and \"360\({}^{\circ}\)/s\" indicates that the induction of cybersickness is very high; 15\({}^{\circ}\)/s, 40\({}^{\circ}\)/s, and 65\({}^{\circ}\)/s are used as segment points. The anti-cybersickness inference output signal for fuzzy control, the CSSV, can be obtained through a fuzzy inference engine. According to previous research, when the SSQ score is less than 7.5, nausea symptoms are rare [37]; when the score is between 7.5 and 22.5, at least one symptom of nausea will occur; and when the score is greater than 22.5, nausea symptoms will worsen rapidly.