Third Joint Eurohaptics Conference and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems Salt Lake City, UT, USA, March 18-20, 2009
Development of A Miniature Pin-Array Tactile Module using Elastic and Electromagnetic Force for Mobile Devices Tae-Heon Yang*, Sang-Youn Kim†, Chong Hui Kim‡, Dong-Soo Kwon* and Wayne J. Book** *
Human-Robot Interaction Center, KAIST, Korea †
‡
Interaction Lab. KUT Univ., Korea
Agency for Defense Development, Korea **Georgia Institute of Technology, USA with a motion sensor [2]. The hardware platform enables a user to sense vibrotactile feedback according to scrolling and/or tilting motion. Although the haptic sensation by vibrotactile actuators improves usability and/or immersion in mobile devices, the vibrotactile actuators can hardly generate detailed texture and/or small scale shape. In order to overcome the limitation, there have been attempts to develop pin-array type tactile actuating systems which can selectively stimulate human’s mechanoreceptors by independently actuating the array of pins. C. R. Wagner et al. developed a tactile actuation system which used servo motors to operate a mechanical pin-array device [3]. S.Y. Kim et al. developed a compact pin-array type of a tactile display unit using piezoelectric bimorphs and attached it to a PHANToMTM haptic device [4]. R.Velazquez et al. presented a pin-array tactile actuating system with an SMA (shape memory alloy) coil and a permanent magnet [5]. Even if there have been fruitful research works for developing various pin-array tactile displays, it is not easy to embed them into mobile devices due to the size and the large power consumption of the tactile module. Therefore, for mobile devices, the tactile module should be designed with consideration of its size, power consumption, weight, and performance (pin’s stroke, output force, and working frequency). For haptic feedback in mobile devices, this paper presents a new small-size tactile actuator which consumes low power. Moreover, this paper suggests a miniature tactile module using the proposed tactile actuators.
ABSTRACT In these days, tactile sensation using a vibration motor is recently receiving attention for immersive interaction with mobile devices. However, the vibration motor is not enough to provide detailed texture or small-scale shape. In order to generate various and exiting tactile sensation, this paper presents a new tactile actuator with a solenoid, a permanent magnet and an elastic spring. This paper also proposes a miniature tactile module with the proposed actuators. On constructing a miniature tactile module, we separate the elastic springs in the actuators into several layers to minimize the contactor's gap without decreasing the performance of the tactile module. We conduct experiments to investigate each contactor's output force and the frequency response of the proposed tactile module. Each contactor can generate enough output force to stimulate human’s mechanoreceptors. Moreover, since the contactors are actuated in a wide range of frequency, the proposed tactile module can generate various tactile sensations. KEYWORDS: Tactile Actuator, Solenoid, Tactile Module, Haptics, Mobile Device. INDEX TERMS: Tactile devices and display, System design and analysis, Design and manufacturing applications 1
INTRODUCTION
Haptic feedback is recently regarded as one of the dominant factors to improve immersive interaction with mobile devices because the size of a visual display unit is not enough to provide dynamic and exciting feeling to users. Users can communicate and/or interact with their mobile device efficiently by adding haptic information to auditory and visual information. Among actuators conveying haptic feedback, vibration motors have been widely used to deliver haptic information to users in their mobile device, because vibration motors are already applied in many commercial mobile devices. A. Chang et al. developed a mobile system (ComTouch) for providing vibrotactile feedback coupled with auditory information [1]. The ComTouch allows rich communication among users by converting hand pressure into vibrational intensity. I. Oakley et al. developed a hardware platform in which a user provides his/her command to a device
2
*
Yuseong-gu, Daejeon
[email protected] Cheonan-si, Chungcheongnam-do
[email protected] ‡ Yuseong-gu, Daejeon
[email protected] * Yuseong-gu, Daejeon
[email protected] **Atlanta GA
[email protected] †
978-1-4244-3858-7/09/$25.00 ©2009 IEEE
DESIGN OF A NEW MINIATURE TACTILE ACTUATOR
A solenoid actuator not only creates enough force and amplitude to stimulate a human skin, but also generates enough wide range of frequency to convey abundant tactile sensation. In those reasons, various attempts have been made to develop pin-array tactile displays with the solenoid actuators [6][7][8]. However, it is not easy to reduce the size of tactile displays with the array of solenoids without decreasing the magnitude of stimulating force. The reason is that the actuating force of a solenoid considerably depends on its size. Therefore, to minimize the size of the solenoid actuator, we added an elastic spring to provide additional elastic returning force. In other words, we combined the elastic returning force by the elastic spring and the electromagnetic force between a solenoid and a permanent magnet. Fig. 1 shows the proposed tactile actuator using the solenoid and the elastic spring. The proposed new actuator is composed of an elastic spring, a contactor, a permanent magnet, and a solenoid. The contactor is adhered to the center of the elastic spring and the permanent magnet is attached to the end of the contactor. A solenoid is placed with tiny gap to the permanent magnet.
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Elastic Spring Contactor
Elastic Returning Force
Permanent Magnet
1
Magnetic Attractive Force
Solenoid Steel core Coil
First Elastic Plate
3 2
4
Second Elastic Plate
(a)
Third Elastic Plate
Contactor Protrusion
Fourth Elastic Plate
Elastic Returning Force Electromagnetic Repulsive Force
5 4 3 1 2
Figure 2. Structure of Multilayered Elastic Spring and Assembling Procedure
Fig. 2 also shows the assembling procedure of the nine actuators with the multilayered elastic springs. In Fig. 2, the contactor 2, which is attached to the elastic spring 2 of the second elastic plate, moves up and down passing through a gap between the elastic springs 1 and 3 in the first elastic plate, and also passing through the third and fourth elastic plates. The contactor 4, which is adhered to the elastic spring 4 of the third elastic plate, moves up and down passing through a gap between the elastic springs 3 and 5 in the first elastic plate, and also transiting through the second and fourth elastic plates. Likewise, each contactor passes through the different elastic springs and plates. While this assembling procedure enlarges the size of the elastic springs, the contactors’ gap is minimized. Therefore, in the nine pin-array structure, the stimulating force and the stroke are increased with the small contactor’s gap.
(b) Figure 1. Design and Working Principle of a New Tactile Actuator (a)Current Off State (b) Current In State
Since the core of the solenoid is made by a steel alloy, the permanent magnet connected to the elastic spring through the contactor is attracted to the solenoid steel core causing the elastic spring deformed without current input as shown in Fig. 1(a). The deformation of the elastic spring generates elastic returning force. When electric current flows into the solenoid, repulsive force between the solenoid and the permanent magnet is created (see Fig. 2(b)). Due to the elastic returning force and the electromagnetic repulsive force, the contactor strongly rises up and stimulates a human operator’s finger pad. Since the permanent magnet is weakly attached on the steel core due to the elastic returning force, impulse current input is only needed to generate strong and fast returning actuation. Therefore, the proposed tactile actuator considerably consumes low power, and produces predominant performances (output force, working frequency and response rate). 3
Elastic Spring
3.2
Design of New Miniature Tactile Module using The Proposed Tactile Actuators Fig. 3 shows the disassembled schematic of a new miniature tactile module using elastic and electromagnetic force. The tactile module is composed of a touch plate, four different elastic plates (plates with elastic springs), five spacers, nine contactors with a permanent magnet, separators, and the solenoids with the solenoid holder. The touch plate is a part where a human operator places his/her finger on it to feel the tactile sensation. The spacers are inserted among the elastic plates to remove interference among the elastic springs in different layers. Each contactor with a permanent magnet is grasped by the center hole of the corresponding elastic spring in the elastic plate. The separators enable the contactors to actuate vertically, because it eliminates the interaction among the magnets. The solenoids are arranged by the solenoid holder and they generate the electromagnetic repulsive force by interaction with the permanent magnets.
DESIGN OF A MINIATURE TACTILE MODULE
3.1 Assembling Procedure of Tactile Actuators In the proposed tactile actuator, the stimulating force and the contactors’ stroke are proportional to the width and the thickness of an elastic spring respectively. As the thickness and the width of the elastic spring increase, the elastic returning force and the contactor’s stroke become larger. Therefore, we should increase the thickness and the width of the elastic spring for better performance. However, if the size of the elastic spring becomes larger, the gap between a contactor and its neighbor which should be as close as possible for better tactile sensation becomes farther. Thus, to increase the size of the each elastic spring without increasing the contactors’ gap, we adopted a multilayer structure as shown in Fig. 2. The elastic springs are positioned in different elastic plates.
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sum of the elastic returning force and the electromagnetic repulsive force is larger than the magnetic attractive force, the contactor with a magnet moves upwards to stimulate a finger tip (See Fig. 1). The elastic returning force is about 5mN when the elastic spring is maximally stretched (0.2 mm) and the magnetic attractive force was measured to about 7 mN. Therefore, the electromagnetic repulsive force should be larger than about 2mN to push out the permanent magnet upward considering the elastic returning force and the magnetic attractive force
Solenoid Part (Solenoid Array + Solenoid Holder)
Permanent Magnet Contactor
Solenoid Array Separator
2.8mm 5mm
Spacer Elastic Plate ( Plate with Elastic Springs)
Solenoid Holder
Touch Plate Figure 3. Disassembled Schematic of Tactile Module
Figure 5. Array of Nine-Miniature Solenoids
The Pacinian Corpuscle is the highly sensitive mechanoreceptor over wide range of frequency. [9] It seems to be a good approach to design the contactor's gap to stimulate the Pacinian Corpuscle. The length of the Pacinian Corpuscle is 3~4mm in adult's finger. [10] The greatest long axis and largest transverse diameter of the Pacinian Corpuscle is 3.5 mm and 4.84mm each. [11] Considering the size of the Pacinian Corpuscle, the contactor's gap of the proposed tactile module was determined as 3mm.
A permanent magnet is attached to the end of a corresponding contactor which is adhered to an elastic spring. Since this permanent magnet interacts with the corresponding solenoid, the diameter of a solenoid has to be smaller than the gap (3mm) between a contactor and its neighbor contactor’s gap. In our system, we fixed the diameter of each solenoid as 2.8mm. With the fixed diameter, the height of the solenoid was simulated using FEMM (Finite Element Method Magnetics). From the simulation result, the height of the solenoid was determined as 5mm to generate repulsive force, larger than 2mN, as shown in Fig. 5 (0.06mm diameter wires and about 1200 turns). As simulated, we manufactured the miniature solenoids by a commercial precision machine work (wire-cutting). The nine-miniature solenoids were arranged by a solenoid holder.
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FABRICATION OF THE PROPOSED TACTILE MODULE
4.1 Design and Fabrication of Elastic Spring Parts The elastic spring was simulated to investigate that the desired force is generated at given deformation using a 3D analysis tool (COSMOSXpress). The material was decided as a SUS 304 (Steel Use Stainless, type 304) because it is light and it has a good elastic characteristic. We set the size of the elastic spring as 4.5X4.5mm considering the contactor’s gap and the multilayered structure. From the simulation result, we decided the width of the elastic spring’s beam as 0.5mm, and the thickness of the elastic spring as 0.15mm. The spring is elastically deformed up to 0.2mm under 5 mN load.
m m
mm 8.5
0.5mm
4.5mm
4.5mm
6mm
15
m m
First Elastic Plate
Fabrication of Nine Pin - Array Tactile Module
15
6mm
4.3
Second Elastic Plate
Figure 6. Developed Pin-Array tactile Module
Third Elastic Fourth Elastic Plate Plate
The Fig.6 shows a new miniature pin-array tactile module with the proposed tactile actuators. The total size of the proposed tactile module is 15mm X 15mm X 8.5mm and its weight is only 8g. The contactor’s gap (distance from the center of a contactor to the contactor of its neighbor) is 3.0mm and its diameter is 0.5 mm. Each actuator can be independently actuated with 0.2mm stroke and with a wide working frequency range. The proposed tactile module consisting of nine tactile actuators consumes considerably small power. When one actuator moves at 1Hz, its power consumption is 0.16W when the 5V input is applied to the module. In the case of actuating at 340Hz, the power consumption is 0.39W (5V input). In addition, the response time of the proposed tactile module is on the order of millisecond.
Figure 4. Construction of Elastic Spring
Fig.4 shows the design of the four kinds of the elastic plates. The first elastic plate has four elastic springs, the fourth one has one elastic spring, and the others have two springs. The elastic plates were manufactured by a commercial precision machine work (wire-cutting). 4.2 Design and Fabrication of Solenoid Parts An elastic returning force is generated when the elastic spring is deformed due to a magnetic attractive force between the magnet connected to the elastic spring and the solenoid steel core. If the
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PERFORMANCE OF THE MINIATURE TACTILE MODULE
end of the contactor and it interacts with the corresponding solenoid. The proposed tactile actuators were arranged and modularized to construct a miniature tactile module. We minimized the contactors’ gap which is a dominant factor for providing tactile sensation by adopting a multilayered structure without decreasing performance (working frequency, output force and stroke). The developed tactile module is small and consumes low power. Therefore, the presented tactile module offers the possibility to incorporate it into mobile devices. If our tactile actuator and the tactile module are incorporated into mobile devices, users will be provided immersive and realistic feeling during interacting with the mobile devices.
Since the proposed tactile module is miniature and low power consuming system, it may produce small output force to stimulate human skin. Therefore, we measured output force whether the output force is larger than minimum activation force for stimulating human’s mechanoreceptor. The human's vibrotactile threshold has the highest value, about 40µm, in static stimulus. [12] The mechanical impedance of the skin to normal displacement at the fingerpad is 0.09 mN/µm. [13] Therefore, we can calculate the minimum activation force to stimulate the human's mechanoreceptors is 3.6mN by multiplying the vibrotactile threshold and the mechanical impedance. To investigate the contactor’s output force, a load cell (BCL-3L) was placed on the tactile module through its extended contactor. When the contactor of the tactile module hits the extended contactor, the indicator (NT-501A) displays the force. We measured the force six times and took the average. The measured output force of a contactor was 5mN which is about one point five-times larger than the minimum activation force (3.6.mN). This result shows that each contactor provides enough force to stimulate the human’s mechanoreceptor. There are four major mechanoreceptors (Meissner corpuscle, Merkel’s disk, Ruffini ending, and Pacinian corpuscle) in the human glabrous skin. [14] The frequency bandwidth is a dominant factor to stimulate the skin, because each mechanoreceptor has own working frequency in the range of 0 Hz to 300 Hz. Therefore, we measured the amplitude of the proposed tactile actuator as a function of vibration frequency. The Laser Doppler Vibrometer (LDV) were used to measure the amplitude by adjusting the frequency. In this experiment, we measured the amplitude when the square wave input is inserted to the developed tactile actuator.
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This research was supported by the 'SMEs Technology Innovation Program.' This work is also supported by grant No. (R01-2007000-20977-0) from the Basic Research Program of the Korea Science & Engineering Foundation. REFERENCES [1]
A. Chang, S. O'Modhrain, R. Jacob, E. Gunther, and H. Ishii, “ComTouch: Design of a Vibrotactile communication Device,” ACM Designing Interactive Systems Conference, pp 312-320, 2002. [2] I. Oakley, S. O’Modhrain, Tilt to Scroll: Evaluating a Motion Based Vibrotactile Mobile Interface, Proceedings of the First Joint Eurohaptics Conference and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems (Pisa, Italy, March 18-20, 2005), pp. 40-49. [3] C. R. Wagner, S. J. Lederman, and R. D. Howe, Design and Performance of a Tactile Shape Display Using RC Servomotors, The Electronic Journal of Haptics Research, Haptics-e, Vol. 3, No. 4, 2004. [4] S. Y. Kim, K.U. Kyung, J. Park, and D. S. Kwon , Real-time Areabased Haptic Rendering and the Augmented Tactile Display Device for a Palpation Simulator, Advanced Robotics, 21(9), pp. 961– 981 2007. [5] R.Velázquez, E.Pissaloux, M.Hafez and J. Szewczyk, A Low-Cost Highly-Portable Tactile Display Based on Shape Memory Alloy Micro-Actuators, VECIMS 2005 – IEEE, International Conference on Virtual Environments, Human-Computer Interfaces and Measurement Systems(Giardini Naxos, Italy, July 18-20, 2005) [6] S. F. Frisken-Gibson, P. Bach-Y-Rita, W. J. Tompkins, and J. G., Webster, A 64-Solenoid, Four-Level Fingertip Search Display for the Blind, IEEE Transactions on Biomedical Engineering, VOL. BME-34, NO. 12, 1987 [7] T. Fukuda, H. Morita, F. Arai, H. Ishihara and H. Matsuura, Micro Resonator Using Electromagnetic Actuator for Tactile Display, International Symposium on Micromechatronics and Human Science (Nagoya, Japan, October 5-8, 1997) [8] M. B. Khoudja, M. Hafez, J. M. Alexandre, A. Kheddar, and V. Moreau, VITAL: A New Low-Cost VIbro-TActiLe Display System, Proceedings of the 2004 IEEE International Conference on Robotics & Automation, (New Orleans. LA, April, 2004) [9] Ronald T. Verrillo, Psychophysics of Vibrotactile Stimulation, J. Acoustical Society of America, 77(1), 1985. [10] Cauna, N. and Mannan, G., The structure of human digital Pacinian corpuscles (Corpuscula Lamellosa) and its functional significance, J. Anat. 92, 1-25, 1985. [11] B. Stark, T. Carlstedt, R. G. Hallin and M. Risling, Distribution of Human Pacinian Corpuscle in The Hand, Journal of Hand Surgery, British and European Volume 23B: 3:370-372, 1998. [12] Ki-Uk Kyung, Minseung Ahn, Dong-Soo Kwon, Mandayam A. Srinivasan, Perceptual and Biomechanical Frequency Response of Human Skin:Implication for Design of Tactile Displays, Proceedings of the First Joint Eurohaptics Conference and Symposium on Haptic
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Figure 7. Frequency Response of the Tactile Actuator
Fig. 7 shows the result of the measured amplitude of the proposed tactile actuator according to the frequency. The maximum stroke of the proposed tactile actuator is about 50dB (200µm) within the frequency region below 340Hz. Therefore, the proposed tactile actuator can selectively stimulate the various human’s mechanoreceptors. 6
ACKNOWLEDGMENT
CONCLUSION
Even though a pin-array type is a powerful structure for generating tactile sensation, it is not easy to incorporate the module into mobile devices because of its size and power consumption. This paper proposed a miniature and low powerconsuming tactile actuator which provides enough working frequency, output force and amplitude to stimulate the human’s mechanoreceptors. This actuator consists of an elastic spring, a contactor, a permanent magnet and a solenoid. The contactor is grasped to the center of the elastic spring which provides the elastic returning force. The permanent magnet is attached to the
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Interfaces for Virtual Environment and Teleoperator Systems (Pisa, Italy, March 18-20, 2005) [13] James Biggs, Mandayam A. Srinivasan, Tangential Versus Normal Displacements of Skin: Relative Effectiveness for Producing Tactile Sensations, Proceedings of the 10th Symposium On Haptic Interfaces For Virtual Environment and Teleoperator Systems (Orlando, Florida, MARCH 24-27, 2002) [14] R.S. Johansson, and A.B. Vallbo, Tactile sensibility in the human hand: relative and absolute densities of four types of mechanoreceptive units in glabrous skin,” Journal of Physiology, Vol. 286, pp.283-300, 1979.
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