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2019,  1 (2):   176 - 184   Published Date:2019-4-20

DOI: 10.3724/SP.J.2096-5796.2018.0014
1 Introduction2 Actuator evaluation 2.1 Equipment selection 2.2 Equipment evaluation 2.3 Selection of installation location 3 Psychophysical experiments 3.1 Experiment 1-immersion change in roller coaster scence 3.1.1   Participants 3.1.2   Stimuli and procedure 3.1.3   Results and analyze 3.2 Expriment 2-immersion change in explosion scene 3.2.1   Participants 3.2.2   Stimuli and procedure 3.2.3   Results and analyze 4 Conclusions

Abstract

Due to the inherent shortcomings of the hardware, the immersion of visual interaction between the user and the virtual reality (VR) equipment is greatly reduced. In this paper, effects of eye-around vibration haptics on improving the VR immersion were studied. The vibration was generated by flexible vibrators whose performance was evaluated by a laser vibrometer. Fitting the vibrators on the human eye area at different positions and derived by different waveforms and frequencies of the input signal, the effects of vibration on the human vision and comfort of the users were verified. Then, with the selected input signals and fitting locations, different kinds of vibration were applied on the eye area cooperating with virtual reality images or videos to evaluate the changes of immersion. Research results provide references to the modeling of eye tactile feedback and the design of relevant tactile device in improving the VR immersion.

Content

1 Introduction
With the rapid development of computer graphics and other technologies, the virtual reality (VR) has attracted more and more attention and achieved outstanding fruits. On the market, there are mainly three types of VR hardware equipment at present. The first one is the PC head-mounted device (HMD) and the representative is Oculus Rift[1]. This device is with high immersion characteristic, but the user’s mobile range will be limited by the length of the cable and the price is expensive[2]. The second one is the Mobile VR. Typical devices are Google Cardboard[3], Samsung's Gear VR[4] and Mojing[5]. Although the user’s experience of Mobile VR is lower than PC-equipment HMD, it has some remarkable characteristics such as low cost, easy to carry, and easy to develop applications by developers. Thus, it becomes widely popular among the consumers and developers, which led to the rapid development of the VR market[6,7]. The third one is the new VR equipment with the integration of AR technology such as Microsoft Hololens[8] and Magic Leap[9], which are lightweight and with strong endurance[10,11]. Due to the pursuit of lightweight effect, it requires a miniaturized computing hardware which is one of the difficulties in development.
The common deficiency in the using of all the above-mentioned VR equipment is that the users are bound in a confined space[12]. At the same time, due to the limited vision imaging, the low computer rendering frequency and the low display resolution, the immersion of visual interaction between the user and the equipment is greatly reduced. In order to enhance the immersive sense of virtual reality, the head-mounted display (HMD) detects the head shaking, tilting and rolling direction to provide stereoscopic visual immersion. Moreover, it can be equipped with data glove to enable users to control the virtual reality space at a harsh environment for enhancing the user perception[13].
However, there are several shortcomings in response time and accuracy on head and hand tracking. The proportion of people wearing VR glasses who report nausea is high[14]. Adverse effects such as dizziness, carsickness or discomfort are widely cited as the common side-effects of HMD use, especially for the first-time users. The vertigo sensation mainly comes from the fact that users do not anticipate the VR environment in the process of use, and the view suddenly moves irregularly, so that the vertigo symptom can be regarded as the result of sensory mismatch[15]. To solve such problems, researchers have developed many ways to reduce the vertigo. The entry method is a common technique that allows users to suddenly jump from one place to another to ease the dizziness of moving. However, such movements have negative effects on the experience and may increase the directional obstacle, so this technology are not enough to improve the immersion sense in the VR system[16]. Leap Motion is a new technology which provides input through gesture tracking. It realizes naked hand interaction in 3D environment[17]. Walking in place (WIP) technology enables users to walk in one place and use body posture recognition devices such as Microsoft Kinect to control avatar movement and rotation[18]. The double-handed gesture interaction (DHGI) method, combined with the LM technology, enables users to move through tracking simple hand gestures. Although the response speed is low, it is more comfortable and intuitive[19].
In this paper, the less used eye tactile feedback is applied to improve the immersion of virtual reality. Tactile feedback is refers to the use of computer and specific force rendering algorithm to calculate the interaction force between the user and the virtual environment, then use tactile/force feedback device to generate force on the user. By applying a vibration stimulus around the eye, this paper studies the role of vibration in improving the VR immersion feeling.
The vibration was generated by a flexible vibrator. A laser vibrometer was used to study the vibration characteristics of the vibrator. Fitting the vibrator on the human eye area at different positions and under different waveforms and frequencies of the input signal, the effects of vibration on the human vision and comfort of the users were verified. Finally, with the selected input signals and fitting positions, different kinds of vibrations were applied on the eye area cooperating with virtual reality images or videos to test the changes of immersion. The research results makes contributions to the modeling of eye tactile feedback and the design of relevant tactile device.
2 Actuator evaluation
2.1 Equipment selection
The Haptuator Planar vibrator (@Tactile Labs Inc. Canada) was selected as the actuator[20] (Figure 1). It has the advantages of high precision, fast response and wide frequency band. What’s more, its surface is flexible to contact to skin. Table 1 presents its parameters. The assorted haptu-Quad amplifier was used to amplify the input signal to drive the vibrator (Figure 1). It has an input channel which can be directly connected to audio device such as mobile phone and computer, etc. The vibration amplitude can vary according to the tone strength of audio.
Vibrator parameters[20]
Parameter Name Value Unit
Model Number MR-HPL-12126-A /
Nominal Dimension 12×12×6 mm
Net Weight 1.8 g
Peak Acceleration 1V input, 85Hz with 5g extra load (20g total)

2.2

21.6

G

m/s²

Maximum Input Voltage 2.0 Vms
2.2 Equipment evaluation
To demonstrate more working details of the vibrator under different stimulus signals, a laser vibrometer (@MTI Inc.) was used to measure the vibration characteristics. Evaluation setup was shown in Figure 2. The output port of a signal generator was connected with the input port of an amplifier to provide different input signals. A fixed laser probe perpendicular to the surface of the vibrator was used to measure the vibration amplitude and frequency.
The evaluation tested the vibration characteristics of the actuator from three aspects: (1) fix the voltage amplitude of input signal at 1Vpp, change the frequency and waveform to observe the relationship between the vibration amplitude and the frequency of different waveforms; (2) fix the frequency and amplitude of input signal, record the vibration amplitude value at intervals to verify whether the vibration amplitude will change with time; (3) change the voltage amplitude of the input signal at the same frequency to observe the relationship between the displacement and the input voltage. The recorded data was showed in Figure 3, Figure 4 and Figure 5.
Figure 3 presents that, at the frequency below 60Hz, the vibration amplitude curves were smooth; at the frequency higher than 60Hz, the amplitude increases rapidly with the increasing of frequency. Figure 4 presents that the working time has no great influence on the vibration quantity. Figure 5 presents that as the voltage of the input signal increased, the vibration amplitude under square input wave increased and that under sine input wave kept stablly.
It was noted that the value and the fluctuation of vibration amplitude under square input wave are larger than those under sine input wave. Hence, when in need of the steady vibration stimulation, using sine input wave is better; otherwise the using of square input wave is better.
The main significance of the measurements on the actuator in this section is to know the relationship between vibration amplitude of actuator and different input signals. In company with the preliminary analysis of the vibration model of the actuator through the above results, the subsequent experiments in Section 3 tried to find the most suitable vibration amplitudes corresponding to different applications.
2.3 Selection of installation location
In order to make the actuator generate different stimulation to human eyes, the effect of installation position on vision and subjective feeling was observed. As the mechanicoreceptor and nerves are rich distributed around the eyelid, five positions around the eyelid were selected: upper eyelid medial, upper eyelid lateral, upper eyelid middle, outer eye canthus and lower eyelid middle.
A medical PE tape was used to install the actuator at five eyelid positions (Figure 6). The results presented in Table 2 showed that different positions where the vibration applied have different effects on people’s vision and subjective feeling. The obtained experimental results will be applied in the later research on how to enhance the visual immersion sense by using different vibration stimuli at different positions at VR environment.
Effects of installation position
Installation Position Effects on vision Effects on subjective feeling
Lower eyelid middle No effect Relatively comfortable; slightly itchy
Outer eye canthus No effect Tension at the scalp caused by a foreign matter stimulus
Upper eyelid lateral Obvious visual dithering Scalp tension is existed but acceptable
Upper eyelid middle Violently visual dithering Strong discomfort; tension on the scalp; difficulty in opening the eye and vision blocking due to the mass and volume of the actuator
Upper eyelid medial Weak visual dithering There is a little bit of discomfort; it is worth noting that it can increase the blink rate
3 Psychophysical experiments
Based on the evaluation of actuator and the effects of vibration on human eyes, this section investigates how to make VR experience more immersive by adding vibration on eye area under different input signals and different installation positions. The VR device used in these experiments is HTC vive (@HTC Inc.)[21]. The VR device consists of a head-mounted display, two single-hand controllers and a location-based system that can track both the display and the controller in space. It can provide higher frame rate, lower delay, larger field of vision and higher visual quality[22].
As shown in Figure 3, Figure 4, Figure 5 and Table Table,2, in order to improve the immersion, it’s necessary to use changing signals and vibration positions to obtain more appropriate feelings according to different scenes. For instance, at explosion scene, a strong and then a weak sine wave should be used to simulate a strong and then a weak vibration. At the scene of walking through the woods and the leaves hitting the face continuously, the actuator should be installed at upper eyelid medial to stimulate the constant blinking. At the scene of going down the steps, a low-frequency square wave can be used to make people feel a clear beat at the moment of downhill. In this paper, two experiments were carried out to study how to improve the VR immersion through the vibration on eye area.
3.1 Experiment 1-immersion change in roller coaster scence
In this experiment, VR immersion is enhanced by adjusting the installation position, waveform and frequency of the input signal. A slice of a roller coaster gliding from top to down rapidly in the video of VIVEPORT was taken as the scene and different input signals and installation locations were set. The input signal frequency was set to 5Hz, 30Hz and 60Hz, which across the vibration frequency within proper range. The psychological evaluation questionnaire with intensity of 1-9 levels was used to judge the immersion: 1-3 for bad immersion, 4-6 for good immersive, 7-9 for excellent immersion. The experiment design was orthogonal.
3.1.1   Participants
Ten volunteers (half males and half females) participated in this experiment. They were aged 20-28 and were undergraduate or graduate students.
3.1.2   Stimuli and procedure
The experiment setup was shown in Figure 7. The actuator was positioned on outer eye canthus, lower eyelid middle, and upper eyelid lateral of the participant, respectively. Then they wore the VR device.The actuator was turned on when the roller coaster started to fall and the participants felt the immersion. The broadcast lasted about one minute and the participants performed one trial in order to avoid the vertigo.
3.1.3   Results and analyze
The average participants' immersion was recorded, as shown in Table 3 and Table 4. In order to find the best combination and achieve the strongest VR immersion, a set of orthogonal experiments were designed with three variables: waveform, frequency and installation position.
Immersion degree in sine wave condition
Position Frequency (Hz)
5 30 60
Outer eye canthus 2 8 5
Lower eyelid middle 1 7 6
Upper eyelid lateral 1 5 3
Immersion degree in square wave condition
Position Frequency (Hz)
5 30 60
Outer eye canthus 2 8 5
Lower eyelid middle 1 7 6
Upper eyelid lateral 1 5 3
This experiment, which has one factor with two levels and two factors with three levels (Table 5), is a mixed levels orthogonal experiment. Consequently, a quasi-level method was applied in experiment design, in which the third level of factor A (sine wave) repeated in the experiment. Orthogonal analysis with interaction was conducted. The orthogonal analysis table L27(313) was selected, where columns 9 and 10 were empty. The experimental results were not presented here due to the limited paper length. Since there were multiple interaction columns, the variance analysis was conducted, as shown in Table 6.
Factor level
Level Waveform (A) Position (B) Frequency (C)
1 Sine wave Outer eye canthus 5
2 Square wave Lower eyelid middle 30
3 (Sine wave) Upper eyelid lateral 60
Variance analysis
Differences between the source SS df MS F Significance
A 3.63 1 3.63 12.25 *
B 19.85 2 9.93 33.50 **
A×B 0.15 2 0.07 0.25
C 65.41 2 32.70 110.38 **
A×C 36.59 2 18.30 61.75 **
B×C 5.70 4 1.43 4.81
e 1.63 4 0.41
e actual value 1.78 6 0.30
F0.05(2,6) 5.14 F0.01(2,6) 10.92
F0.05(1,6) 5.99 F0.01(1,6) 13.74
F0.05(4,6) 5.19 F0.01(4,6) 9.15
The critical value F0.05 (2,6)=5.14, F0.01 (2,6)=10.92 for the given significance level α=0.01 revealed that the factor B, factor C and factor AxC have a very significant impact on the test results. Therefore, the optimal choice of factor A should be based on factor C. The level collocation of A and C were listed at Table Table,7, and finally, the optimal combination can be determined as C3A1B1.
Level collocation of factor A and C
A1 A2
C1 1.33 4
C2 4.67 1.67
C3 6.67 4.67
Optimal solution C3A1B1
3.2 Expriment 2-immersion change in explosion scene
As the Haptuator Planar vibrator has the ability to change its vibration according to the sound level of input audio, this experiment has built an explosion scene with a bang audio, and then the vibrator generated a varying vibration along with the explosive audio. The effects of vibration on VR immersion in this scene were observed.
3.2.1   Participants
Five volunteers (three males and two females) participated in this experiment. They were aged 20-23 and were undergraduate students.
3.2.2   Stimuli and procedure
Figure 8 shows the explosion scene established in Unity3D. It can explode in the place where the mouse clicks. If the explosion happens near or on the wall, it will have an explosive impact on it. There is a physical collision between the bricks; hence the sound was mixed by the sound of the explosion and the squeezed sound by the collapse of the bricks.
The driver of Haptuator Planar was connected to the headset interface of VR equipment. The participants wore the VR equipment and described the level of immersion under different conditions.
3.2.3   Results and analyze
The data of each subject and mean value were recorded in Table 8. It can be seen from the results that when the installation positions were the upper eyelid lateral, the Lower eyelid middle and the outer eye can thus, there was a good immersion achieved compared with no wearing of a vibrator. In this experiment, according to the reports of the subjects, when the vibrator was installed at the upper lateral eyelid, there were sight jitter and blurred vision caused by the vibration, which enhances the explosion shock, in turn, strengthens the immersion. In addition, the vibration applied at three positions brought the scalp tension, similar to the explosion stress feeling in the real scene. It's a small pity that, during the experimental process, the vibrator produced noises at the same time due to the characteristics of the vibrator, and the vibration intensity was not significantly changed. The vibrator has space for improvement.
Immersion levels at different installation positions
Participant number Installation position
No wearing Upper eyelid lateral Lower eyelid middle Outer eye canthus
1 3 7 5 6
2 3 7 7 6
3 4 8 6 5
4 3 7 6 7
5 4 6 7 6
Mean value 3.4 7 6.2 6
4 Conclusions
This paper has studied the effects of eye tactile feedback on VR immersion. Some conclusions are obtained: (1) to make shaking effect on the sight, the vibration should be applied at the upper eyelid lateral and with the sine wave input at about 30Hz; (2) the sine wave input with high frequency gives the subjective feeling of continuous vibration to the users, which is suitable for the continuous impact at VR scene. Whereas the square wave input with low frequency gives users a sense of throbbing irritation, which is suitable for special VR scenes such as passing through the potholed ground and so on, or to stimulate the blink of eye; (3) through the feedback of participants, when have a vibration on the eye area, it can slightly relieve the vertigo during the watching of VR movies.
Eye tactile feedback has great potential to improve VR’s user experience. Through the vibration stimulation, it could make the VR more immersive, relieve the visual fatigue of the user when using VR, extend the using time, and protect the eye-sight. The future research can be studied from following aspects: finding the ways to stimulate the eye at multi-points, and each point cooperate with each other at different times having different vibration modes; the input signal types, parameters and ranges can be further studied to enhance the sense of immersion.

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