Fabrication and Characterization of Array Tactile Actuator Based on Cellulose Acetate

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셀룰로오스 아세테이트 기반 어레이 촉각 액추에이터의 제작 및 특성평가

김현찬¹, 윤성률², 고현우¹, 김재환^1,

Hyun-Chan Kim¹, Sungryl Yun², Hyun-U Ko¹, and Jaehwan Kim^1,

1 인하대학교 기계공학과 (Department of Mechanical Engineering, Inha University) 2 한국전자통신연구원 투명소자 및 UX 창의연구실 (Transparent Transducer and UX Creative Research Center, Electronics and Telecommunications Research Institute)

Corresponding author: [email protected], Tel: +82-32-874-7325 Manuscript received: 2015.3.26. / Revised: 2015.6.17. / Accepted: 2015.6.30.

This paper reports the enhanced fabrication and characterization of a 3 × 3 array tactile actuator composed of cellulose acetate. The array tactile actuator, with dimensions of 15 × 15 × 1 mm³, consists of 9 pillar-supported cells made from a cellulose-acetate molding. The fabrication process and performance test along with the results for the suggested actuator are explained. To improve the cell-array fabrication, a laser cut was adopted after the molding process. The displacement of the unit cell increased the input voltage and frequency. Various top masses are added onto the actuator to mimic the touch force, and the acceleration of the actuator is measured under actuation. When 2 kV is applied to the actuator, the maximum acceleration is 0.64 g, which is above the vibrotactile threshold. The actuation mechanism is associated with the electrostatic force between the top and bottom electrodes.

KEYWORDS: Array tactile actuator (어레이 촉각 작동기), Cellulose acetate (셀룰로오스 아세테이트), Molding technique (몰딩 테크닉), Vibrotactile threshold (진동촉각 임계값)

1. Introduction

Touch sense gives versatile information from environment to human. Haptic technology produces virtual touch sense on haptic interface. So, the haptic technology is essential for virtual simulation systems which are useful for simulators, remote work systems for extremely dangerous or distant such as nuclear reactors, space stations, underwater explorations, mine excavations

and remote surgery. Nowadays, although great efforts have been paid to develop haptic technology, the haptic technology does not fully satisfy the human’s desire for mimicking our intrinsic perception. Thus, it is important to improve current status of haptic feedback technology to meet the desire.¹ There are two primary haptic senses in human: kinesthetic and tactile sensations. Tactile sensation is the sensory data obtained from receptors of skin, and kinesthetic sensation is the data obtained __________

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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through receptors of joints and muscles. For virtual reality simulators, kinesthetic actuators can hardly be inserted into wearable type haptic devices including haptic glove due to their size and weight. So, tactile sensation can be important for effective perception, and a lot of tactile actuators have been innovated and used. In order to create a variety of tactile sensation in wearable type haptic devices, it is necessary to consider vibrotactile actuators that can vibrate in a wide frequency range.²

The first commercialized vibrotactile actuator is an eccentric motor.^3,4 As the motor rotates, eccentric mass produces acceleration and the entire vibrotactile unit vibrates. Thus, the vibrotactile unit transmits a vibratory stimulus to a user’s finger or hand. Although the eccentric motor generates vibration in the frequency range of 80 - 250 Hz, it hinders a variety of vibrotactile sensation. Furthermore, its response time is too late for conveying vibrotactile sensation to a user in real time. In order to improve the response time of the eccentric motors, a linear resonance actuator was developed.⁵ Although its response time is fast enough to create vibrotactile effect, the frequency bandwidth is limited to near its resonance frequency. For producing vibrations with a wide frequency range from a small device, piezoceramic actuators were developed.^6,7 By carefully designing piezoceramic thin film actuators and integrating to mobile devices, this technology provided realistic possibility for haptic and multimodal design with considerable latitude. However, it is not easy to selectively stimulate mechanoreceptors with the piezoceramic actuators because its vibration is useful at their resonance frequencies. Instead of piezoceramic actuators, piezopolymers such as polyvinylidene fluoride (PVDF) and dielectric elastomers were studied for vibrotactile actuators.^8-10

Recently, cellulose was re-discovered as a smart material, termed as electroactive paper (EAPap).^11,12 Cellulose, the most abundant renewable material in nature, has merits in terms of light weight, low price, biocompatible and biodegradable characteristics. Thus, it is advantageous to use cellulose as an active material for tactile actuators. Piezoelectric behavior of the EAPap actuators was studied for a possible vibrotactile actuator.¹³ Using a thin EAPap film, stacked and unimorph EAPap actuators were prepared and their performance was

investigated for possible application of vibrotactile actuators. Although the EAPap actuators required low actuation voltage, their deformation was not enough for tactile devices. A film-type vibrotactile actuators based on cellulose acetate (CA) were made.^14-16 The CA film, which is formed via an esterification process reacting hydroxyl groups in cellulose with acetic anhydride,¹⁷ is optically transparent and it possesses high dielectric constant applicable for electrostatic type actuator. Taking these advantages of the CA film, vibrotactile actuators that have pillar-supported cantilever beams with 50 × 25 mm² and 25

× 25 mm² were made and showed their maximum acceleration of 0.32 g and 0.23 g, respectively (g=9.8m/s²).

However, these actuators are too big to be used for wearable haptic devices. They should be fit into a small area at which fingers are located. Furthermore, they need to be arrayed to render the texture of a target object.

This paper aims at developing the 3 × 3 array tactile actuator with CA films. The array should be fit into 15 × 15 mm² and 1 mm thickness. Also, the actuator should be able to produce enough acceleration force for haptic applications. Thus, the design and fabrication process are critical to meet the stringent requirements. Especially, fabrication issues are illustrated in this paper and we adopt a molding technique of CA to meet the size requirement. CA is an optically transparent and easily soluble in acetone, which is useful for fabricating film type transparent actuators. For further assemble and partitioning the array cells, a laser cut was used.

Experiment for performance evaluation of the actuator is explained.

2. Experiment

2.1 Preparation of CA Protrusion onto a FPCB Fig. 1(a) shows schematically illustrated fabrication steps of the CA protrusion. A photo-resist (PR) mold was fabricated on a silicon wafer by using a photo-lithography process for a Gersteltec GM1075 SU-8 photoresist. A 10:1 mixture of polydimethylsiloxane (PDMS) elastomer (Sylgard®184A) to cross-linker (Sylgard®184B) was drop-cast on the mold, degassed in a vacuum chamber for 30 min and cured in a heating oven at 60 ℃ for 3 hours.

After peeling off the PDMS structure from the PR mold, it was fixed on an O2 plasma treated glass plate. The

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PDMS structure with multiple cuboid trenches was used as a master mold to form a three-dimensional CA structure with rectangular protrusions (500 μm wide and 4.5 mm long) with a consistent height of 200 μm. A cellulose acetate (CA) solution (CA powders: 15 wt%, solvent: pure acetone) was spin-cast on the PDMS master mold and cured at room condition (25 ℃). The solidified CA structure with the master mold was attached onto a CA layer encapsulating electrode patterns of a flexible printed circuit board (FPCB) using a thin CA coating as an adhesive layer.

2.2 Fabrication of 3×3 Array Actuator

As shown in Fig. 1(b), an active CA layer was fabricated with the continuative molding described in Section 2.1. The CA layer is designed to be multiple unit layers interconnected with thin CA linkages, which are supporting components to prevent the unit layers from being undesirably positioned during elimination of PDMS master mold from the CA layer. A compliant electrode pattern including an electrical circuitry was formed on each center area (3.5 mm wide and 4.5 mm long) at the top surface of the unit layers through spray- coating of silver nanowires (AgNWs, average diameter:

25±5 nm, average length: 35±5 μm) dispersed in isopropyl alcohol under a metal template. The use of volatile solvent enables the AgNWs to form a stable conductive network without a significant aggregation among them. The resulting electrode patterns are mechanically robust against repetitive deformation due to the benefits from flexible nature and high electrical conductivity (sheet resistance: as low as 1.27 Ω/sq) of the AgNWs.

After transferring a liquid bridging material (a mixture of CA solution and N,N-dimethylacetamide (DMAc)) onto the surface of the CA protrusions, the CA layer was placed onto the protrusions and then both components were unified through a solidification process of the CA/DMAc coating. Since DMAc has relatively higher boiling point than acetone, the use of DMAc as an additive solvent for the CA solution can be profitable to secure processing time required for alignment of both components.

Finally, the PDMS master mold and the CA linkages are simultaneously removed from the CA layer geometry.

Electrical leads are constructed to independently operate each pixel with electric voltage. After assembling the top

side and the pillar part, a laser was used to cut each cell (Fig. 1(c)). This laser cut will enhance the quality of the fabricated array actuator so as to improve the performance.

Fig. 2 shows the prepared array actuator with 9 cells in 3

× 3 matrix (size of each pixel: 4.5 × 4.5 mm²).

Fig. 1 Schematic of fabrication processes for array tactile actuator; (a) CA protrusion (b) Active layer (c) Assemble of the CA protrusions with the active layer

Fig. 2 Schematic of array tactile actuator; (a) Planar view of whole structure (b) Details in components constructing a unit cell

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2.3 Test Setup for Performance Evaluation For performance evaluation of the CA tactile- actuators, acceleration and deformation profile of the actuators were respectively measured by using an Ometron VS100 laser doppler vibrometer and an EM4SYS LSV110D laser scanning vibrometer during electrically induced actuation using an Agilent 33210A function generator interconnected to a high voltage 20/20A Trek amplifier. Particularly, for acceleration measurement, different mass ranging from 5 to 15 grams was individually loaded on the upper side of an actuator among nine units and a platform configured to a mass on a wedge-shaped sponge was placed on bottom side of the actuators in order to establish a free vibration condition (Fig. 3). Notes that total mass loading on both sides of the actuator is fixed to 100 grams in order to set vibrotactile test condition similarly to mobile devices.

3. Results and Discussion

The array actuator with nine unit cells was prepared by using a molding technique with an assembling technique based on a liquid bridging material.

Performance of the array actuator was evaluated by measuring the acceleration and deformation profile under electrical actuation. As an operating principle illustrated in Fig. 4, the array actuator is capable of producing a bending deformation to vertical direction responding to an electrostatic attraction force induced by an electric voltage applied to the electrodes. Since each cell in the 3×3 matrix is designed to allow independent operation, the actuator can provide localized and programmable vibrotactile responses. The response of the cells changes with the applied electric voltage and operating frequency.

Fig. 5 shows displacement profiles of a unit cell among the array actuator. At a constant voltage of 1.5 kV, the displacement increases as high as 2.3 μm with frequency in human sensitive frequency range, 20 ~ 500 Hz (Fig. 5(a)). The increasing trend is associated with that a fundamental resonance frequency (1.5 kHz) of the actuators is much higher than the frequencies available for vibrotactile responses. At a constant frequency of 200 Hz, higher electric voltage leads to larger displacement, which reaches to 1.2 μm at 2 kV (Fig. 5(b)). The voltage dependent responses follow the electrostatic model since

the displacement tends to increase in proportional to E² in whole operation conditions. The overall displacement is higher than a reported displacement threshold for vibrotactile response.² Fig. 6 shows a visualized image mapping the displacements, which are measured at a whole surface of an active CA layer during operation of the unit cell actuator with 500 Hz at 1.5 kV. The displacement becomes higher as the points moves from the boundaries fixed with a couple of CA protrusions to central area (Fig. 6). A curve plotting the displacement profiles at the center-line forms axisymmetric parabolic shape.

Fig. 3 Schematic of acceleration measurement system

(a) Voltage off (b) Voltage on Fig. 4 Actuation mechanism of array tactile actuator

(a) (b) Fig. 5 Performance of the tactile actuator; (a)

Frequency dependent displacements at a constant voltage of 1.5 kV with DC offset of 0.75 kV (b) Input voltage dependent displacements at a constant frequency of 200 Hz

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For the acceleration measurements, five different masses in a range of 5 and 15 gram are individually placed on the unit CA tactile-actuator. Fig. 7 shows the loading mass dependent acceleration profile with frequency. As the loading mass becomes smaller, the amplitude of the acceleration increases as high as 0.64 g and the resonance frequency moves to higher band. For the loading conditions, the overall amplitude of acceleration at each resonance frequency is higher than a reported vibrotactile threshold of 0.06 g in the frequency range (10~500 Hz).¹⁸

As shown in Fig. 8, the acceleration response with respect to the sinusoidal voltage signal is fairly reversible with a small time delay (< 1 ms) during cyclic operations at a human sensitive frequency under loading a mass of 5 g. The results suggest that the array actuator is capable of producing consistent and perceivable vibrotactile responses at wide frequency range when a fingertip is in contact with the actuator.

4. Conclusions

The paper report development of CA based array tactile actuator capable for immersive virtual reality systems. 3×3 cells of the array tactile actuator were fabricated by molding technique and laser cut.

Electrostatic force generates vibration of pillars supported cantilever structures on each cell. Displacement shows increasing trend as increasing the frequency and also the input voltage. Acceleration with top mass imitating touch force is above 10 times of reported vibrotactile threshold as 0.06 g, suggesting that the array tactile actuator is capable of producing a perceivable vibrotactile response when a fingertip is in contact with the actuator.

ACKNOWLEDGEMENTS

This research was supported by National Research Foundation of Korea (NRF-2013M3C1A30595867), South Korea.

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