Exploring Fingers’ Limitation of Texture Density Perception on Ultrasonic Haptic Displays
Summary (2 min read)
1 Introduction
- Current commercial touchscreen devices rarely provide a compelling haptic feedback to human fingers despite the use of touch as input; haptic feedback is typically limited to vibration.
- Alongside vibrotactile actuation, two techniques, electrostatic-vibration [2, 18] and electroadhesion [22] use electrostatic force generated, respectively, by applying a voltage to the screen surface or by applying DC excitation of the tactile display.
- In all of the mentioned studies only one finger (index in most cases) for texture perception of tactile surfaces has been examined; the authors are aware of no work that has contrasted finger sensitivity, nor any work that explores single versus multi-finger sensitivity.
- The authors explore the limitation of individual human fingers and different hands on texture density perception in the case of two waveform types for ultrasonicbased haptic displays.
2 Experiment
- The authors carried out a psychophysical experiment to explore the limitations of touch perception of different finger types (index, middle, etc.) in dynamic active touch.
- The authors investigated both single and multi-finger tactile explorations of sinusoidal and square-wave textures on ultrasonic-based tactile displays.
- Texture is defined as the sequence of periodic haptic feedback effects generated by a specific type of signal waveform (such as square or sine) and accordingly its specific value of spatial period and amplitude.
- The experiment conformed to the principles of the Declaration of Helsinki and a general explanation of the experimental task was given to each participant before beginning the experimental procedure.
2.1 Participants
- By design, all of the participants were right-handed.
- The total experiment time was 50-60 minutes for each participant.
- Participants wore active noise-cancelling headphones (Panasonic RP-DJS200, Japan) during the experiment, while Gaussian white noise was played at a comfortable listening level in order to prevent potential interference from external auditory cues.
2.2 Experimental set-up
- The authors used an enhanced visual-tactile actuator (E-ViTa), a tactile feedback display based on ultrasonic vibrations for haptic rendering [26] .
- E-ViTa is developed on a Banana Pi, a single-board computer (Shenzhen LeMaker Technology Co. Ltd, China) with a 1 GHz ARM Cortex-A7, dual-core CPU and 1 GB of RAM working in parallel with STM32f4 microcontroller (STMicroelectronics, France).
- The communication between the microcontroller and the single board computer is provided via the Serial Peripheral Interface (SPI) bus at 10 kHz.
- This singleboard computer is connected to a 12.5cm capacitive touchscreen (Banana-LCD 5"-TS, MAREL, China) for detecting the user's finger position on the display with a sampling frequency of 62 Hz.
- A power electronic circuit converts a 12V DC voltage source into an AC voltage, controlled in amplitude and frequency and supplied to the piezoelectric cells.
2.3 General procedure
- A one-up-one-down staircase procedure (adaptive procedure) with fixed step sizes, commonly used in psychophysics [24, 17] was used in their investigation.
- The stimuli consisted of textures with sinusoidal and square wave gratings, which were tested on all fingers of both hands.
- Sample of data collected from a single participant using 1-up 1-down staircase procedure, also known as Fig. 2.
- Turnover points are marked with red color.
- The participants were free to explore the surface as long as they wanted.
3.1 One-finger exploration
- The median values of the individual fingers' 50% perceptual thresholds for discriminating from the 100 µm reference spatial period ranged between 150 µm and 300 µm for the sinusoidal grating and from 200 µm to 350 µm for the square grating.
- Index finger was found to be more sensitive than middle finger (Wilcoxon matched-pairs signed rank test: N=60, W=594, p < 0.0001) in all conditions.
- The authors then compared them with a Wilcoxon matchedpairs signed rank test.
- 50 % psychophysical thresholds for all finger types across both types of gratings were averaged for each participants.
- The same procedure was performed to compare the psychophysical thresholds (median±IQR) between both types of gratings.
3.2 Multi-finger exploration
- Participants performed the same task by exploring the actuated surface simultaneously with four fingers (except thumb).
- The results from multi-finger exploration showed similar trends to the exploration with one finger (Fig. 6 ).
- For both, the right and left hand, the authors compared the sensitivity of the index finger, which is the exploring finger in most studies on tactile perception and was also found to be the most sensitive in their experiments (Fig. 7 ).
- For the right hand, the authors did not observe a significant difference between the two exploratory techniques for any of the two types of gratings (Wilcoxon matched-pairs signed rank test: N=15, W=39, p = 0.09 for the sinusoidal grating and N=15, W=17, p = 0.30 for the square grating).
4 Conclusion and perspective
- In this paper the authors investigated the effects and limitations of different human fingers for texture density perception for both single and multi-finger exploratory techniques and for both sine and square wave gratings of right-handed participants by leveraging ultrasonic vibration.
- Furthermore, their findings showed that there was no significant difference between the sensitivity rate of the index finger (the most sensitive single-finger type) and multi-finger tactile exploration for users' dominant hand.
- Given these results, the authors hypothesize the index finger has a major impact on the overall multi-finger sensitivity of the user's dominant hand for both types of gratings.
- Funded by European ERDF grants (IRCICA, CPER MAUVE) and ANR funding agency.
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References
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"Exploring Fingers’ Limitation of Te..." refers methods in this paper
...1: Structure of the E-Vita ultrasonic based haptic display used in our experiment [26]...
[...]
...We used an enhanced visual-tactile actuator (E-ViTa), a tactile feedback display based on ultrasonic vibrations for haptic rendering [26]....
[...]
27 citations
"Exploring Fingers’ Limitation of Te..." refers background in this paper
...As well, a potential use of different sensory thresholds of finger types, which we found in our study, is to leverage these sensitivities to novel finger identification techniques on tactile displays [8, 19] in order to allow users to perform different interaction tasks....
[...]
11 citations
"Exploring Fingers’ Limitation of Te..." refers background in this paper
...As well, a potential use of different sensory thresholds of finger types, which we found in our study, is to leverage these sensitivities to novel finger identification techniques on tactile displays [8, 19] in order to allow users to perform different interaction tasks....
[...]
11 citations
"Exploring Fingers’ Limitation of Te..." refers background in this paper
...[9, 11] investigated the tactile perception of transient changes of different frictional signals on ultrasonic based haptic devices....
[...]
10 citations
"Exploring Fingers’ Limitation of Te..." refers background in this paper
...[14] studied the limitation of tactile elements for texture perception and how to optimize interaction performance of end users through the perception of different haptic effects [15], as well as how tactile and auditory signals can be combined to enhance the user’s spatial perception in musical...
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Frequently Asked Questions (9)
Q2. What are the future works in "Exploring fingers’ limitation of texture density perception on ultrasonic haptic displays" ?
In the future, the authors would like to investigate if these results generalize to lefthanded users, which would confirm the importance of hand dominance. As well, a potential use of different sensory thresholds of finger types, which the authors found in their study, is to leverage these sensitivities to novel finger identification techniques on tactile displays [ 8, 19 ] in order to allow users to perform different interaction tasks.
Q3. What is the reason for the differences in sensitivity?
While hand dominance is most likely the driving force behind variable sensitivity, it is possible that perceptual differences could result from left-right physiological differences.
Q4. What is the advantage of the 1-up-1-down staircase procedure?
The 1-up-1-down staircase procedure offers the compelling advantage of reducing the total time of their experiment, since the authors investigate a high number of trials and conditions for each participant.
Q5. What are the sensitive fingers for perceiving differences in spatial textures?
Their results indicate that the index and the thumb are the most sensitive fingers for perceiving differences in spatial textures for both sine and square virtual gratings; the little finger, followed by the ring, is the least sensitive for texture perception in one-finger exploration for the two types of gratings.
Q6. What is the effect of the enhanced perception sensitivity?
This enhanced perception sensitivity permits the dominant hand to control exploratory motion in order to optimize the consistency of tactile feedback across fingers.
Q7. What is the common type of feedback used on touchscreens?
Current commercial touchscreen devices rarely provide a compelling haptic feedback to human fingers despite the use of touch as input; haptic feedback is typically limited to vibration.
Q8. What was the sensitive finger in the experiment?
The little finger was the least sensitive, i.e., had the highest perceptual threshold level in all conditions, and the ring finger was the second least sensitive in all conditions.
Q9. What is the significance of the results?
In terms of implications, for designers, these results provide guidance on the need to vary haptic stimuli depending on whether the stimulus is designed for the dominant or non-dominant hand, depending on whether it is designed for the index finger or for any finger, and depending on whether it is to be a single-finger of multi-finger interaction.