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A new tactile stimulator using a planar coil type actuator
Sensors and Actuators A 178 (2012) 209–216
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A new tactile stimulator using a planar coil type actuator Hyung-Sik Kim a , Mi-Hyun Choi a , Hong-Won Yeon a , Jae-Hoon Jun a , Jeong-Han Yi a , Jong-Rak Park b , Dae-Woon Lim c , Soon-Cheol Chung a,∗ a b c
Department of Biomedical Engineering, Research Institute of Biomedical Engineering, College of Biomedical & Health Science, Konkuk University, Chungju, South Korea Department of Photonic Engineering, Chosun University, Gwangju, South Korea Department of Information & Communication Engineering, Dongguk University, Seoul, South Korea
a r t i c l e
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Article history: Received 8 November 2011 Received in revised form 24 February 2012 Accepted 24 February 2012 Available online 5 March 2012 Keywords: Tactile stimulator Planar coil type actuator Mechanical method Electromagnetic phenomenon
a b s t r a c t In this study, we developed a new tactile stimulator using a planar coil type actuator. The newly developed system consists of three units, which are control unit, drive unit, and planar coil type actuator. The control unit of this simulator controls stimulation frequency, intensity, time, and channel. The drive unit amplifies the stimulation signal to drive the actuator. Mechanical tactile stimulation of vibration and normal indentation can be made by modulating the planar coil type actuator and the arrangement of the permanent magnet. Even though the developed stimulator is simple, it has a wide frequency range of 0–400 Hz with 40 levels, and intensity modulation of 256 levels. Stimulation intensity does not change due to frequency change. The transient response time is about 100 s. The advantages of this actuator also include safety and usefulness. The performance of this tactile actuator was tested using a dummy finger made of silicone rubber. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Many studies related to tactile sensation have been performed in brain and cognition science [1–4], neurophysiology [5], human factor [6–9], and human–computer interaction (HCI) [10,11]. The high accuracy and resolution of actuator are required to manipulate stimulation frequency, intensity, and time precisely. For tactile stimulation, a mechanical method is commonly used [12]. The mechanical method uses moving-coil actuators [6–9,13–17], motors [18–22], and piezoelectric elements [1,12,23–26]. The moving-coil method uses electromagnetic phenomena, and historically, has been large, complex and expensive [13]. As technology advances, smaller and cheaper systems were developed, but the stimulation intensity was not controllable and was small [14]. Recently, the VBW32 system (Audiological Engineering Corp., USA) was used to conduct studies related to alteration of blindness [8], attention and behavior control [9], reaction of tactile stimulation
∗ Corresponding author at: Department of Biomedical Engineering, Research Institute of Biomedical Engineering, College of Biomedical & Health Science, Konkuk University, 322 Danwol-dong, Chungju-si, Chungcheongbuk-do 380-701, South Korea. Tel.: +82 43 840 3759; fax: +82 43 851 0620. E-mail addresses: [email protected] (H.-S. Kim), [email protected] (M.-H. Choi), [email protected] (H.-W. Yeon), [email protected] (J.-H. Jun), [email protected] (J.-H. Yi), [email protected] (J.-R. Park), [email protected] (D.-W. Lim), [email protected] (S.-C. Chung). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2012.02.044
due to location of the head [7], tactile stimulation and short-term memory [6]. However, this system had limited applications with low stimulation intensity except 250 Hz [27]. The C-2 Tactor system (Engineering Acoustics, Inc., USA) had a response time above 30 ms with the limitation of low stimulation intensity except 200–300 Hz [27]. Typical motor methods use small vibration motors, RC servomotors, and DC motors. The small vibration motor was not useful because the magnitude and frequency of the intensity was uncontrollable [21]. Recently, a tactile stimulator with vibration motor was studied using E-Prime software with simultaneous visual and auditory stimulation. However, this method had long response time more than 30 ms and difficulties to control the stimulation magnitude and frequency [22]. In general, methods using an RC servomotor had a very complicated structure, were large, had a small usable frequency range (under 25 Hz) [18], and had a limited number of channels [19]. Methods using a DC motor were preferred because it was a simple design, compact and light weight, and the intensity of stimulation was large. However, the usable frequency range (under 10 Hz) was small [20], and the response time was about 50 ms, which was longer than that of the moving-coil method. The vibration stimulator was developed using a piezoelectric element, but the intensity of stimulation was small, the stimulator was difficult to attach, and the magnitude of stimulation varied with frequency [1]. This type of stimulation is not linear, and hysteresis occurs. However, it does have the advantage of
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local stimulation, a controlled input signal, and elongation [23]. However, since high voltage (greater than 60–200 V) is needed to generate vibration, it is dangerous to use with humans [10,25,26]. The current mechanical tactile methods have complicated structure (e.g. moving-coil method), narrow frequency band (e.g. moving-coil and motor methods), and difficulty to control stimulation frequency, intensity, and time precisely. Also, the intensity of these methods is weak and varied due to different frequencies (e.g. piezoelectric method). Moreover, the starting and ending response can be long and these methods use high voltage with a limited number of channels. In this study, we developed a new planar coil type actuator to overcome the weaknesses of the current actuators and exhibit a broad application range. 2. Methods Fig. 1 shows the schematic of the newly developed tactile stimulator that can be operated with PC and E-Prime software. It can also be operated manually. E-Prime is a well-known software that is used as a visual and auditory stimulation tool in psychology, neuroscience, and cognitive and behavioral sciences [9,22]. This tactile stimulator consists of three parts: the control unit, drive unit, and planar coil type actuator. 2.1. Control unit The control unit controls stimulation frequency (0–400 Hz, 40 levels), intensity (0–7 V, 256 levels), time (100 s to s), and 5 channels (Fig. 1). The main processor, AVR-type ATMEGA128 processor (Atmel, USA), is a general purpose low-power 8-bit microprocessor and works at a 4.5–5.5 V operating voltage with several capabilities, such as an eight-channel 10-bit analog-to-digital converter (ADC), serial peripheral interface (SPI), two 8-bit and 16-bit timer/counters and 0–16 MHz speed grade. The stimulation signal of sinusoidal wave type was generated with frequency modulation by 12-bit digital-to-analog converter (DAC) AD7545A (Analog Device, USA). The signal generated by MATLAB (Mathwork Inc., USA) had no loss since digital data was saved in the internal memory of the microprocessor. The processor with 16-bit timer/counter was used to modulate frequency in a range of 0–400 Hz by 10 Hz increments (40 levels). The intensity was controlled with a 256-position digital potentiometer AD5290 (Analog Device, USA) by 256 levels. The transient response time of starting and ending stimulation is affected by inductance (L) and resistance (R). The planar coil type actuator has an inductance of 5 H and DC resistance of 3.2 resulting in the time constant (L/R) of 1.6 s. Therefore, the transient response time ideally could be controlled at the microsecond scale. Five stimulation channels were controlled using the Analog Multiplexers/DemultiplexersMC14051B (ON Semiconductor, USA). The frequency, intensity, time, and channels can be controlled and monitored manually using a character liquid crystal display (LCD) and switches at the control unit. These parameters can also be controlled by the PC and E-Prime software. The E-Basic script code of E-Prime controls the stimulation parameters and the parameters and trigger signals are delivered through the LPT1 port. 2.2. Drive unit The drive unit operates the planar coil type actuator in response to commands from the control unit (Fig. 1). The stimulation signals were sent to the planar coil actuator by a 20 W audio power amplifier LM1875 (National semiconductor, USA). To compensate
the loss of signal by “on resistance” of Analog Multiplexer in control unit and the resistance change due to pattern length of the printed circuit board (PCB), the multi-turn variable resistor was used in the input port of the audio power amplifier. The control unit and the drive unit can be assembled with a total dimension of 160 mm × 130 mm × 40 mm (W × L × H). 2.3. Planar coil type actuator The planar coil type actuator has been developed using planar technology that constructs the planar solenoid by coiling on the PCB. The advantages of planar technology over the conventional coil type are the following: (1) excellent repeatability and thermal characteristics; (2) durable; (3) humidity resistance; (4) various shapes; and (5) light-weight. Fig. 2 shows the basic principle and actual shape of the planar coil type actuator. The current (I) flows under the magnetic field (B) through the closed loop “abcd”. The resulting torque = m × B (m = I × S, m: magnetic moment, S: the area of the closed loop). The developed tactile stimulator uses the torque () of the planar coil actuator. The torque () can be changed by the magnetic field (B), the area of the closed loop (S), the current (I), and the distance (d) between the permanent magnet and the actuator (Fig. 3). Fig. 2(c) shows the multiple rectangular patterns on PCB to increase the area (S), the number of turn on the planar solenoid, and the torque (). The direction and intensity of torque were controlled by modulating the current (I) of the sinusoidal wave. The planar type actuator was designed by OrCad Layout (Cadence Design Systems Inc., USA). 2.4. Experiments The silicone rubber dummy finger with 20’s adult finger size was manufactured to test the tactile system (Fig. 4). Fig. 3 shows the two types of tactile actuators used in the experiments, TYPE-I and TYPEII. In TYPE-I, the actuator was placed on the permanent magnet over the distance (d). The vibration was applied to the dummy finger. In TYPE-II, the actuator was fixed in the transparent acrylic material with 0.25 mm nylon monofilament and the permanent magnet was positioned with distance d = 10 mm from the actuator. The normal indentation was applied to the dummy finger. The movement of the actuator was monitored by MMA7260Q (Freescale Semiconductor, USA) 3-axis accelerometer (Fig. 3, accelerometer 1) and the stimulation on the dummy finger was measured by the other one inserted into the dummy finger (Figs. 3 and 4, accelerometer 2). TDA3044B and TDS2014 (Tektronix, USA), 4 channel digital oscilloscope, monitored the accelerometer 1 and accelerometer 2 with the sampling rate of 10,000 samples/s and 2500 samples/s, respectively. 3. Results The performance of the planar actuator was evaluated based on data from accelerometer 1 and 2. Fig. 5 shows the raw data and the power spectral density of the x, y, and z axes with 250 Hz in TYPE-I. The frequency of the actuator (accelerometer 1) and the dummy finger (accelerometer 2) were the same and in-phase. The intensity of accelerometer 2 in the dummy finger was smaller than that of accelerometer 1 because of the damping effect of silicone rubber in the dummy finger. The amplitude in the y direction was larger than in the x and z direction in accelerometer 1 because the magnetic field is in the z direction, the current is in the x direction, and the resulting perpendicular force is in the y direction. However, the amplitude in the y direction was small in accelerometer 2 since the vice to fix the plastic bar of the dummy finger limited the vibration in the y direction (Fig. 4).
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Fig. 1. Schematic of the new tactile stimulator system using a planar coil type actuator.
Fig. 2. (a) and (b) Mechanism of the planar coil type actuator (B: external magnetic field, L: coil length, D: coil width, I: current, Ft : force on torque, F: force on “bc” and “ad”, m: magnetic moment). (c) The shape of planar coil type actuator (line width: 0.25 mm, turns: 13, thickness: 0.2 mm, material: FR-4).
Fig. 3. The experimental setup. TYPE-I: vibration stimulation, TYPE-II: normal indentation stimulation (d: distance between permanent magnet and planar coil type actuator).
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Fig. 4. Silicone rubber dummy finger and position of accelerometer 2.
Fig. 6 shows the results using 250 Hz in TYPE-II. The stimulation in TYPE-II was the normal indention in the z direction, so the amplitude here was dominant. The power spectral density was expressed by doubling the original data in accelerometer 2 since the original intensity was small. Figs. 5 and 6 show the acceleration of the accelerometer 1 and 2 and the frequency was 250 Hz in both TYPE-I and TYPE-II for comparison under the same condition.
Fig. 7 shows the power spectral density of accelerometer 2 in the z direction with 4 stimulation frequencies (100, 200, 300, and 400 Hz) and 4 stimulation intensities in TYPE-I and TYPE-II. The intensity correlated well with the frequency and the various stimulation intensities could be obtained. Stimulation intensity did not change due to frequency change. Even though 2nd harmonic property was observed at 200 Hz, we could neglect it since the difference
Fig. 5. Raw data and power spectral density measured from accelerometer 1 and 2 with 250 Hz in TYPE-I. (Ax : acceleration signal and power spectral density in the x-axis, Ay : acceleration signal and power spectral density in the y-axis, Az : acceleration signal and power spectral density in the z-axis.)
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Fig. 6. Raw data and power spectral density measured from accelerometer 1 and 2 with 250 Hz in TYPE-II. (The power spectral density was expressed by doubling the original data in accelerometer 2.) (Ax : acceleration signal and power spectral density in the x-axis, Ay : acceleration signal and power spectral density in the y-axis, Az : acceleration signal and power spectral density in the z-axis).
with the fundamental frequency was at least −18 dB. Table 1 shows more detail data from accelerometer 2 in the z direction in TYPE-I and TYPE-II with 9 frequency levels and 10 intensity levels. Fig. 8 shows the displacement calculated by the numerical method (trapezoidal numerical integration algorithm) using MATLAB (Mathworks Inc., USA) by double integration of acceleration signals obtained from accelerometer 2. The displacement was proportional to the amplitude of drive output voltage. The square wave has been used as inputs into the planar coil type actuator and the outputs from the accelerometer 1 in the z direction in TYPE-II are shown in Fig. 9. Theoretically, the transient response time is 1.6 s, but it has been measured 100 s. This may be caused by the relatively slow response time (<1 ms max.) of the accelerometer (MMA7260Q) and the weight of the accelerometer
1 (weight = 0.242 g) that may affect on the movement of the planar coil actuator (weight = 0.352 g).
4. Discussion The new tactile stimulator consists of only three parts (control unit, drive unit, and planar coil type actuator) and has a wide frequency range of 0–400 Hzwith 40 levels and stimulation intensities of 256 levels with stimulation times from 100 s to s. Also, stimulation intensity does not change due to frequency change. The stronger stimulation can be achieved by controlling the material characteristic of the permanent magnet and the distance between the magnet and the actuator.
Table 1 The acceleration of the accelerometer 2 in the z direction with nine different frequencies and ten different intensities in TYPE-I (left) and TYPE-II (right). Intensity
Frequency 20
20 50 75 100 125 150 175 200 225 250
0.33 0.81 1.37 1.72 2.15 2.52 3.08 3.65 3.92 4.25
50 0.11 0.44 0.82 1.36 1.95 2.35 2.84 3.15 3.55 3.88
0.36 0.80 1.36 1.74 2.16 2.53 3.10 3.66 3.94 4.29
100 0.12 0.45 0.88 1.36 1.99 2.35 2.84 3.16 3.55 3.89
Unit: frequency (Hz) and acceleration (G).
0.36 0.82 1.36 1.74 2.17 2.55 3.08 3.66 3.93 4.33
150 0.15 0.49 0.82 1.36 2.02 2.36 2.91 3.22 3.55 3.88
0.36 0.82 1.36 1.76 2.21 2.52 3.10 3.66 3.92 4.32
200 0.16 0.54 0.88 1.41 2.02 2.37 2.91 3.17 3.59 3.88
0.37 0.84 1.41 1.80 2.20 2.56 3.11 3.71 3.99 4.35
250 0.15 0.53 0.88 1.44 2.02 2.37 2.95 3.17 3.60 3.91
0.38 0.91 1.41 1.79 2.22 2.61 3.19 3.72 3.99 4.41
300 0.14 0.51 0.84 1.44 2.07 2.37 2.94 3.31 3.6 4.01
0.36 0.93 1.37 1.75 2.18 2.54 3.24 3.66 3.98 4.39
350 0.18 0.59 0.92 1.51 2.11 2.44 2.88 3.31 3.66 4.00
0.38 0.86 1.37 1.72 2.22 2.56 3.12 3.65 3.94 4.30
400 0.19 0.55 0.86 1.48 2.11 2.40 2.88 3.30 3.59 3.92
0.37 0.82 1.37 1.73 2.17 2.54 3.12 3.65 3.96 4.28
0.14 0.55 0.88 1.48 2.08 2.40 2.91 3.30 3.59 3.92
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Fig. 7. The power spectral density of the accelerometer 2 in the z direction with four different frequencies and intensities in TYPE-I and TYPE-II (Az : power spectral density in the z-axis).
The transient response time of the new actuator is 1.6 s theoretically, but it has been measured 100 s because the weight of the accelerometer 1 may slow down the movement of the planar coil actuator. Compared to the transient time of the conventional actuator with a few milliseconds, that of the new actuator with 100 s is still much faster. The new actuator can be operated at low voltage and the safety function to prevent over flow of current is included in LM1875
audio power amplifier. The coil pattern of the planar coil type actuator is insulated by solder mask. Therefore, the safety of the system can be achieved. This actuator is hard to deform with temperature and humidity since the planar coil has been imprinted on the PCB with an etching process. Compared to the conventional wounded coil type actuators, it is stable, repeatable, simple, easy to fabricate. Since the planar coil can be molded into various shapes, the stimulation place and area can be easily modified. Two types of stimulation, vibration (TYPE-I) and normal indentation (TYPE-II), can be made
1200 TYPE-II (X TYPE (X-axis) axis) TYPE-I (Y-axis) TYPE-I (Z-axis) TYPE-II (Z-axis)
3 2.5
Accelerometer 1 Output
800
2 600
1.5
AZ (G)
Displacement
1000
400
1 0.5
Square wave Input
200
0 0
-0.5 0
1
2
3
4
Drive voltage (Vpeakk )
5
6
100µs
7
Fig. 8. The relation between the displacement and the output signal from the drive unit. The displacement was calculated with the trapezoidal numerical integration algorithm by the double integration of acceleration signals (Vpeak : the peak output voltage).
-1
0
1
2
3
4
Time (Sec)
5
6
7 -4
x 10
Fig. 9. The transient response time of the planar coil type actuator by square wave input (output of accelerometer 1 in the z direction in TYPE-II).
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by modulating the planar coil type actuator and the arrangement of the permanent magnet. The different types of input waves are possible including sinusoidal, square, triangular, and saw-tooth waves for tactile stimulation. The system is easily expandable to multiple channels. Thus, the new system is very useful with a broad application range. The dummy finger made of silicone rubber (Young’s modulus = 1.4 MPa) has the different mechanical property to the human finger (human stratum corneum, Young’s modulus = 0.42–0.85 MPa) [28,29]. The amount of tactile stimulation could be different in actual human finger. Our study shows the pattern and trend of transferred stimulation into the dummy finger. The advanced dummy finger with similar mechanical property of human finger is required for future study. The intensity is limited by the weight and elasticity of the subject in TYPE-I and by the distance between the subject and the planar coil actuator in TYPE-II. That limitation is common for all other actuators as well. However, the new actuator can calibrate the intensity precisely with the characteristic of the permanent magnet, the arrangement distance, the shape and size of the planar coil, and the current applied on the drive unit. 5. Conclusions In this study, a new tactile simulator was developed using a planar coil type actuator. The developed actuator transfers electromagnetic phenomenon to mechanical force and has several advantages compared with conventional mechanical methods that use moving-coil actuators, motors, and piezoelectric elements. It has a wide frequency range of 0–400 Hz with 40 levels, and intensity modulation of 256 levels. Stimulation intensity does not change due to frequency change. The transient response time is about 100 s. Also, the advantages of this actuator include simplicity and safety. Therefore, this actuator can be used to study tactile perception and cognition with high accuracy in a wide variety of applications. Acknowledgements This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2011-0027920). References [1] G.S. Harrington, C.T. Wright, J.H.D. Ill, A new vibrotactile stimulator for functional MRI, Hum. Brain Mapp. 10 (2000) 140–145. [2] V. Jousmaki, N. Nishitani, R. Hari, A brush stimulator for functional brain imaging, Clin. Neurophysiol. 118 (2007) 2620–2624. [3] C. Dresel, A. Parzinger, C. Rimpau, C. Zimmer, A.O. Ceballos-Baumann, B. Haslinger, A new device for tactile stimulation during fMRI, Neuroimage 39 (2008) 1094–1103. [4] H.A. Allen, G.W. Humpherys, Direct tactile stimulation of dorsal occipitotemporal cortex in a visual agnostic, Curr. Biol. 19 (2009) 1044–1049. [5] S.M. Golaszewski, F. Zschiegner, C.M. Siedentopf, J. Unterrainer, R.A. Sweeney, W. Eisnerf, S. Steinleitner, F.M. Mottaghyg, S. Felver, A new pneumatic vibrator for functional magnetic resonance imaging of the human sensorimotor cortex, Neurosci. Lett. 324 (2002) 125–128. [6] A. Gallace, H.Z. Tan, P. Haggard, C. Spence, Short term memory for tactile stimuli, Brain Res. 1190 (2007) 132–142. [7] C. Ho, C. Spence, Head orientation biases tactile localization, Brain Res. 1144 (2007) 236–241. [8] M. Auvray, A. Gallace, J.H. O’Brien, H.Z. Tan, C. Spence, Tactile and visual distractors induce change blindness for tactile stimuli presented on the fingertips, Brain Res. 1213 (2008) 111–119. [9] C. Ho, V. Santangelo, C. Spence, Multisensory warning signals: when spatial correspondence matters, Exp. Brain Res. 195 (2009) 261–272. [10] V. Hayward, O.R. Astley, M. Cruz-Hernandez, D. Grant, G. Robles-De-La-Torre, Haptic interfaces and devices, Sens. Rev. 24 (2004) 16–29. [11] S.K. Kim, G.H. Park, S.H. Yim, S.M. Choi, S.J. Choi, Gesture-recognizing hand-held interface with vibrotactile feedback for 3D interaction, IEEE Trans. Consum. Electr. 55 (2009) 1169–1177.
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Biographies Hyung-Sik Kim received the MS in Biomedical Engineering from Konkuk University, in 2007. He is currently working toward the PhD degree in Biomedical Engineering. His research areas are medical electronics and embedded systems. Mi-Hyun Choi received the MS in Biomedical Engineering from Konkuk University, in 2009. She is currently working toward the PhD degree in Biomedical Engineering. Her research areas are brain imaging and physiological signal. Hong-Won Yeon received the BS in Biomedical Engineering from Konkuk University, in 2011. He is currently working toward the MS degree in Biomedical Engineering. His research areas are brain imaging and physiological signal. Jae-Hoon Jun received the PhD degree in Biomedical Engineering from Texas A&M University, College Station, in 2001. He is a professor in the Dept. of Biomedical Engineering at Konkuk University, Korea. His research interests include optical sensing, photothermal therapy, tissue characterization, biomechanics, and T-ray imaging. Jeong-Han Yi received the PhD degree in Electronic Engineering from the Korea Advanced Institute of Science and Technology (KAIST), Korea, in 1992. He is a professor in the Dept. of Biomedical Engineering at Konkuk University, Korea. His research interests are magnetic resonance imaging, medical imaging system, FES and Magnetic stimulation. Jong-Rak Park received the PhD degree in Physics from the Korea Advanced Institute of Science and Technology (KAIST), Korea, in 2000. After working three years for LG and Samsung as a Senior Research Engineer, he joined the Dept. of Photonic Engineering, Chosun University as a faculty member in 2003. His work is focused on fundamentals and applications of laser optics and applications of optical techniques to manufacturing processes of semiconductor and display devices.
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Dae-Woon Lim received the PhD degree in Electrical Engineering and Computer Science from Seoul National University, Korea, 2006. He is an assistant professor in the Dept. of Information and Communication Engineering at Dongguk University, Korea. His research interests are in the area of signal processing, wireless communications, and channel coding.
Soon-Cheol Chung received the PhD degree in Electrical Engineering from the Korea Advanced Institute of Science and Technology (KAIST), Korea, in 1999. He is a professor in the Dept. of Biomedical Engineering at Konkuk University, Korea. His research interests are neuroscience, behavioral science, and biomechanics.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A new tactile stimulator using a planar coil type actuator Hyung-Sik Kim a , Mi-Hyun Choi a , Hong-Won Yeon a , Jae-Hoon Jun a , Jeong-Han Yi a , Jong-Rak Park b , Dae-Woon Lim c , Soon-Cheol Chung a,∗ a b c
Department of Biomedical Engineering, Research Institute of Biomedical Engineering, College of Biomedical & Health Science, Konkuk University, Chungju, South Korea Department of Photonic Engineering, Chosun University, Gwangju, South Korea Department of Information & Communication Engineering, Dongguk University, Seoul, South Korea
a r t i c l e
i n f o
Article history: Received 8 November 2011 Received in revised form 24 February 2012 Accepted 24 February 2012 Available online 5 March 2012 Keywords: Tactile stimulator Planar coil type actuator Mechanical method Electromagnetic phenomenon
a b s t r a c t In this study, we developed a new tactile stimulator using a planar coil type actuator. The newly developed system consists of three units, which are control unit, drive unit, and planar coil type actuator. The control unit of this simulator controls stimulation frequency, intensity, time, and channel. The drive unit amplifies the stimulation signal to drive the actuator. Mechanical tactile stimulation of vibration and normal indentation can be made by modulating the planar coil type actuator and the arrangement of the permanent magnet. Even though the developed stimulator is simple, it has a wide frequency range of 0–400 Hz with 40 levels, and intensity modulation of 256 levels. Stimulation intensity does not change due to frequency change. The transient response time is about 100 s. The advantages of this actuator also include safety and usefulness. The performance of this tactile actuator was tested using a dummy finger made of silicone rubber. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Many studies related to tactile sensation have been performed in brain and cognition science [1–4], neurophysiology [5], human factor [6–9], and human–computer interaction (HCI) [10,11]. The high accuracy and resolution of actuator are required to manipulate stimulation frequency, intensity, and time precisely. For tactile stimulation, a mechanical method is commonly used [12]. The mechanical method uses moving-coil actuators [6–9,13–17], motors [18–22], and piezoelectric elements [1,12,23–26]. The moving-coil method uses electromagnetic phenomena, and historically, has been large, complex and expensive [13]. As technology advances, smaller and cheaper systems were developed, but the stimulation intensity was not controllable and was small [14]. Recently, the VBW32 system (Audiological Engineering Corp., USA) was used to conduct studies related to alteration of blindness [8], attention and behavior control [9], reaction of tactile stimulation
∗ Corresponding author at: Department of Biomedical Engineering, Research Institute of Biomedical Engineering, College of Biomedical & Health Science, Konkuk University, 322 Danwol-dong, Chungju-si, Chungcheongbuk-do 380-701, South Korea. Tel.: +82 43 840 3759; fax: +82 43 851 0620. E-mail addresses: [email protected] (H.-S. Kim), [email protected] (M.-H. Choi), [email protected] (H.-W. Yeon), [email protected] (J.-H. Jun), [email protected] (J.-H. Yi), [email protected] (J.-R. Park), [email protected] (D.-W. Lim), [email protected] (S.-C. Chung). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2012.02.044
due to location of the head [7], tactile stimulation and short-term memory [6]. However, this system had limited applications with low stimulation intensity except 250 Hz [27]. The C-2 Tactor system (Engineering Acoustics, Inc., USA) had a response time above 30 ms with the limitation of low stimulation intensity except 200–300 Hz [27]. Typical motor methods use small vibration motors, RC servomotors, and DC motors. The small vibration motor was not useful because the magnitude and frequency of the intensity was uncontrollable [21]. Recently, a tactile stimulator with vibration motor was studied using E-Prime software with simultaneous visual and auditory stimulation. However, this method had long response time more than 30 ms and difficulties to control the stimulation magnitude and frequency [22]. In general, methods using an RC servomotor had a very complicated structure, were large, had a small usable frequency range (under 25 Hz) [18], and had a limited number of channels [19]. Methods using a DC motor were preferred because it was a simple design, compact and light weight, and the intensity of stimulation was large. However, the usable frequency range (under 10 Hz) was small [20], and the response time was about 50 ms, which was longer than that of the moving-coil method. The vibration stimulator was developed using a piezoelectric element, but the intensity of stimulation was small, the stimulator was difficult to attach, and the magnitude of stimulation varied with frequency [1]. This type of stimulation is not linear, and hysteresis occurs. However, it does have the advantage of
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local stimulation, a controlled input signal, and elongation [23]. However, since high voltage (greater than 60–200 V) is needed to generate vibration, it is dangerous to use with humans [10,25,26]. The current mechanical tactile methods have complicated structure (e.g. moving-coil method), narrow frequency band (e.g. moving-coil and motor methods), and difficulty to control stimulation frequency, intensity, and time precisely. Also, the intensity of these methods is weak and varied due to different frequencies (e.g. piezoelectric method). Moreover, the starting and ending response can be long and these methods use high voltage with a limited number of channels. In this study, we developed a new planar coil type actuator to overcome the weaknesses of the current actuators and exhibit a broad application range. 2. Methods Fig. 1 shows the schematic of the newly developed tactile stimulator that can be operated with PC and E-Prime software. It can also be operated manually. E-Prime is a well-known software that is used as a visual and auditory stimulation tool in psychology, neuroscience, and cognitive and behavioral sciences [9,22]. This tactile stimulator consists of three parts: the control unit, drive unit, and planar coil type actuator. 2.1. Control unit The control unit controls stimulation frequency (0–400 Hz, 40 levels), intensity (0–7 V, 256 levels), time (100 s to s), and 5 channels (Fig. 1). The main processor, AVR-type ATMEGA128 processor (Atmel, USA), is a general purpose low-power 8-bit microprocessor and works at a 4.5–5.5 V operating voltage with several capabilities, such as an eight-channel 10-bit analog-to-digital converter (ADC), serial peripheral interface (SPI), two 8-bit and 16-bit timer/counters and 0–16 MHz speed grade. The stimulation signal of sinusoidal wave type was generated with frequency modulation by 12-bit digital-to-analog converter (DAC) AD7545A (Analog Device, USA). The signal generated by MATLAB (Mathwork Inc., USA) had no loss since digital data was saved in the internal memory of the microprocessor. The processor with 16-bit timer/counter was used to modulate frequency in a range of 0–400 Hz by 10 Hz increments (40 levels). The intensity was controlled with a 256-position digital potentiometer AD5290 (Analog Device, USA) by 256 levels. The transient response time of starting and ending stimulation is affected by inductance (L) and resistance (R). The planar coil type actuator has an inductance of 5 H and DC resistance of 3.2 resulting in the time constant (L/R) of 1.6 s. Therefore, the transient response time ideally could be controlled at the microsecond scale. Five stimulation channels were controlled using the Analog Multiplexers/DemultiplexersMC14051B (ON Semiconductor, USA). The frequency, intensity, time, and channels can be controlled and monitored manually using a character liquid crystal display (LCD) and switches at the control unit. These parameters can also be controlled by the PC and E-Prime software. The E-Basic script code of E-Prime controls the stimulation parameters and the parameters and trigger signals are delivered through the LPT1 port. 2.2. Drive unit The drive unit operates the planar coil type actuator in response to commands from the control unit (Fig. 1). The stimulation signals were sent to the planar coil actuator by a 20 W audio power amplifier LM1875 (National semiconductor, USA). To compensate
the loss of signal by “on resistance” of Analog Multiplexer in control unit and the resistance change due to pattern length of the printed circuit board (PCB), the multi-turn variable resistor was used in the input port of the audio power amplifier. The control unit and the drive unit can be assembled with a total dimension of 160 mm × 130 mm × 40 mm (W × L × H). 2.3. Planar coil type actuator The planar coil type actuator has been developed using planar technology that constructs the planar solenoid by coiling on the PCB. The advantages of planar technology over the conventional coil type are the following: (1) excellent repeatability and thermal characteristics; (2) durable; (3) humidity resistance; (4) various shapes; and (5) light-weight. Fig. 2 shows the basic principle and actual shape of the planar coil type actuator. The current (I) flows under the magnetic field (B) through the closed loop “abcd”. The resulting torque = m × B (m = I × S, m: magnetic moment, S: the area of the closed loop). The developed tactile stimulator uses the torque () of the planar coil actuator. The torque () can be changed by the magnetic field (B), the area of the closed loop (S), the current (I), and the distance (d) between the permanent magnet and the actuator (Fig. 3). Fig. 2(c) shows the multiple rectangular patterns on PCB to increase the area (S), the number of turn on the planar solenoid, and the torque (). The direction and intensity of torque were controlled by modulating the current (I) of the sinusoidal wave. The planar type actuator was designed by OrCad Layout (Cadence Design Systems Inc., USA). 2.4. Experiments The silicone rubber dummy finger with 20’s adult finger size was manufactured to test the tactile system (Fig. 4). Fig. 3 shows the two types of tactile actuators used in the experiments, TYPE-I and TYPEII. In TYPE-I, the actuator was placed on the permanent magnet over the distance (d). The vibration was applied to the dummy finger. In TYPE-II, the actuator was fixed in the transparent acrylic material with 0.25 mm nylon monofilament and the permanent magnet was positioned with distance d = 10 mm from the actuator. The normal indentation was applied to the dummy finger. The movement of the actuator was monitored by MMA7260Q (Freescale Semiconductor, USA) 3-axis accelerometer (Fig. 3, accelerometer 1) and the stimulation on the dummy finger was measured by the other one inserted into the dummy finger (Figs. 3 and 4, accelerometer 2). TDA3044B and TDS2014 (Tektronix, USA), 4 channel digital oscilloscope, monitored the accelerometer 1 and accelerometer 2 with the sampling rate of 10,000 samples/s and 2500 samples/s, respectively. 3. Results The performance of the planar actuator was evaluated based on data from accelerometer 1 and 2. Fig. 5 shows the raw data and the power spectral density of the x, y, and z axes with 250 Hz in TYPE-I. The frequency of the actuator (accelerometer 1) and the dummy finger (accelerometer 2) were the same and in-phase. The intensity of accelerometer 2 in the dummy finger was smaller than that of accelerometer 1 because of the damping effect of silicone rubber in the dummy finger. The amplitude in the y direction was larger than in the x and z direction in accelerometer 1 because the magnetic field is in the z direction, the current is in the x direction, and the resulting perpendicular force is in the y direction. However, the amplitude in the y direction was small in accelerometer 2 since the vice to fix the plastic bar of the dummy finger limited the vibration in the y direction (Fig. 4).
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Fig. 1. Schematic of the new tactile stimulator system using a planar coil type actuator.
Fig. 2. (a) and (b) Mechanism of the planar coil type actuator (B: external magnetic field, L: coil length, D: coil width, I: current, Ft : force on torque, F: force on “bc” and “ad”, m: magnetic moment). (c) The shape of planar coil type actuator (line width: 0.25 mm, turns: 13, thickness: 0.2 mm, material: FR-4).
Fig. 3. The experimental setup. TYPE-I: vibration stimulation, TYPE-II: normal indentation stimulation (d: distance between permanent magnet and planar coil type actuator).
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Fig. 4. Silicone rubber dummy finger and position of accelerometer 2.
Fig. 6 shows the results using 250 Hz in TYPE-II. The stimulation in TYPE-II was the normal indention in the z direction, so the amplitude here was dominant. The power spectral density was expressed by doubling the original data in accelerometer 2 since the original intensity was small. Figs. 5 and 6 show the acceleration of the accelerometer 1 and 2 and the frequency was 250 Hz in both TYPE-I and TYPE-II for comparison under the same condition.
Fig. 7 shows the power spectral density of accelerometer 2 in the z direction with 4 stimulation frequencies (100, 200, 300, and 400 Hz) and 4 stimulation intensities in TYPE-I and TYPE-II. The intensity correlated well with the frequency and the various stimulation intensities could be obtained. Stimulation intensity did not change due to frequency change. Even though 2nd harmonic property was observed at 200 Hz, we could neglect it since the difference
Fig. 5. Raw data and power spectral density measured from accelerometer 1 and 2 with 250 Hz in TYPE-I. (Ax : acceleration signal and power spectral density in the x-axis, Ay : acceleration signal and power spectral density in the y-axis, Az : acceleration signal and power spectral density in the z-axis.)
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Fig. 6. Raw data and power spectral density measured from accelerometer 1 and 2 with 250 Hz in TYPE-II. (The power spectral density was expressed by doubling the original data in accelerometer 2.) (Ax : acceleration signal and power spectral density in the x-axis, Ay : acceleration signal and power spectral density in the y-axis, Az : acceleration signal and power spectral density in the z-axis).
with the fundamental frequency was at least −18 dB. Table 1 shows more detail data from accelerometer 2 in the z direction in TYPE-I and TYPE-II with 9 frequency levels and 10 intensity levels. Fig. 8 shows the displacement calculated by the numerical method (trapezoidal numerical integration algorithm) using MATLAB (Mathworks Inc., USA) by double integration of acceleration signals obtained from accelerometer 2. The displacement was proportional to the amplitude of drive output voltage. The square wave has been used as inputs into the planar coil type actuator and the outputs from the accelerometer 1 in the z direction in TYPE-II are shown in Fig. 9. Theoretically, the transient response time is 1.6 s, but it has been measured 100 s. This may be caused by the relatively slow response time (<1 ms max.) of the accelerometer (MMA7260Q) and the weight of the accelerometer
1 (weight = 0.242 g) that may affect on the movement of the planar coil actuator (weight = 0.352 g).
4. Discussion The new tactile stimulator consists of only three parts (control unit, drive unit, and planar coil type actuator) and has a wide frequency range of 0–400 Hzwith 40 levels and stimulation intensities of 256 levels with stimulation times from 100 s to s. Also, stimulation intensity does not change due to frequency change. The stronger stimulation can be achieved by controlling the material characteristic of the permanent magnet and the distance between the magnet and the actuator.
Table 1 The acceleration of the accelerometer 2 in the z direction with nine different frequencies and ten different intensities in TYPE-I (left) and TYPE-II (right). Intensity
Frequency 20
20 50 75 100 125 150 175 200 225 250
0.33 0.81 1.37 1.72 2.15 2.52 3.08 3.65 3.92 4.25
50 0.11 0.44 0.82 1.36 1.95 2.35 2.84 3.15 3.55 3.88
0.36 0.80 1.36 1.74 2.16 2.53 3.10 3.66 3.94 4.29
100 0.12 0.45 0.88 1.36 1.99 2.35 2.84 3.16 3.55 3.89
Unit: frequency (Hz) and acceleration (G).
0.36 0.82 1.36 1.74 2.17 2.55 3.08 3.66 3.93 4.33
150 0.15 0.49 0.82 1.36 2.02 2.36 2.91 3.22 3.55 3.88
0.36 0.82 1.36 1.76 2.21 2.52 3.10 3.66 3.92 4.32
200 0.16 0.54 0.88 1.41 2.02 2.37 2.91 3.17 3.59 3.88
0.37 0.84 1.41 1.80 2.20 2.56 3.11 3.71 3.99 4.35
250 0.15 0.53 0.88 1.44 2.02 2.37 2.95 3.17 3.60 3.91
0.38 0.91 1.41 1.79 2.22 2.61 3.19 3.72 3.99 4.41
300 0.14 0.51 0.84 1.44 2.07 2.37 2.94 3.31 3.6 4.01
0.36 0.93 1.37 1.75 2.18 2.54 3.24 3.66 3.98 4.39
350 0.18 0.59 0.92 1.51 2.11 2.44 2.88 3.31 3.66 4.00
0.38 0.86 1.37 1.72 2.22 2.56 3.12 3.65 3.94 4.30
400 0.19 0.55 0.86 1.48 2.11 2.40 2.88 3.30 3.59 3.92
0.37 0.82 1.37 1.73 2.17 2.54 3.12 3.65 3.96 4.28
0.14 0.55 0.88 1.48 2.08 2.40 2.91 3.30 3.59 3.92
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Fig. 7. The power spectral density of the accelerometer 2 in the z direction with four different frequencies and intensities in TYPE-I and TYPE-II (Az : power spectral density in the z-axis).
The transient response time of the new actuator is 1.6 s theoretically, but it has been measured 100 s because the weight of the accelerometer 1 may slow down the movement of the planar coil actuator. Compared to the transient time of the conventional actuator with a few milliseconds, that of the new actuator with 100 s is still much faster. The new actuator can be operated at low voltage and the safety function to prevent over flow of current is included in LM1875
audio power amplifier. The coil pattern of the planar coil type actuator is insulated by solder mask. Therefore, the safety of the system can be achieved. This actuator is hard to deform with temperature and humidity since the planar coil has been imprinted on the PCB with an etching process. Compared to the conventional wounded coil type actuators, it is stable, repeatable, simple, easy to fabricate. Since the planar coil can be molded into various shapes, the stimulation place and area can be easily modified. Two types of stimulation, vibration (TYPE-I) and normal indentation (TYPE-II), can be made
1200 TYPE-II (X TYPE (X-axis) axis) TYPE-I (Y-axis) TYPE-I (Z-axis) TYPE-II (Z-axis)
3 2.5
Accelerometer 1 Output
800
2 600
1.5
AZ (G)
Displacement
1000
400
1 0.5
Square wave Input
200
0 0
-0.5 0
1
2
3
4
Drive voltage (Vpeakk )
5
6
100µs
7
Fig. 8. The relation between the displacement and the output signal from the drive unit. The displacement was calculated with the trapezoidal numerical integration algorithm by the double integration of acceleration signals (Vpeak : the peak output voltage).
-1
0
1
2
3
4
Time (Sec)
5
6
7 -4
x 10
Fig. 9. The transient response time of the planar coil type actuator by square wave input (output of accelerometer 1 in the z direction in TYPE-II).
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by modulating the planar coil type actuator and the arrangement of the permanent magnet. The different types of input waves are possible including sinusoidal, square, triangular, and saw-tooth waves for tactile stimulation. The system is easily expandable to multiple channels. Thus, the new system is very useful with a broad application range. The dummy finger made of silicone rubber (Young’s modulus = 1.4 MPa) has the different mechanical property to the human finger (human stratum corneum, Young’s modulus = 0.42–0.85 MPa) [28,29]. The amount of tactile stimulation could be different in actual human finger. Our study shows the pattern and trend of transferred stimulation into the dummy finger. The advanced dummy finger with similar mechanical property of human finger is required for future study. The intensity is limited by the weight and elasticity of the subject in TYPE-I and by the distance between the subject and the planar coil actuator in TYPE-II. That limitation is common for all other actuators as well. However, the new actuator can calibrate the intensity precisely with the characteristic of the permanent magnet, the arrangement distance, the shape and size of the planar coil, and the current applied on the drive unit. 5. Conclusions In this study, a new tactile simulator was developed using a planar coil type actuator. The developed actuator transfers electromagnetic phenomenon to mechanical force and has several advantages compared with conventional mechanical methods that use moving-coil actuators, motors, and piezoelectric elements. It has a wide frequency range of 0–400 Hz with 40 levels, and intensity modulation of 256 levels. Stimulation intensity does not change due to frequency change. The transient response time is about 100 s. Also, the advantages of this actuator include simplicity and safety. Therefore, this actuator can be used to study tactile perception and cognition with high accuracy in a wide variety of applications. Acknowledgements This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2011-0027920). References [1] G.S. Harrington, C.T. Wright, J.H.D. Ill, A new vibrotactile stimulator for functional MRI, Hum. Brain Mapp. 10 (2000) 140–145. [2] V. Jousmaki, N. Nishitani, R. Hari, A brush stimulator for functional brain imaging, Clin. Neurophysiol. 118 (2007) 2620–2624. [3] C. Dresel, A. Parzinger, C. Rimpau, C. Zimmer, A.O. Ceballos-Baumann, B. Haslinger, A new device for tactile stimulation during fMRI, Neuroimage 39 (2008) 1094–1103. [4] H.A. Allen, G.W. Humpherys, Direct tactile stimulation of dorsal occipitotemporal cortex in a visual agnostic, Curr. Biol. 19 (2009) 1044–1049. [5] S.M. Golaszewski, F. Zschiegner, C.M. Siedentopf, J. Unterrainer, R.A. Sweeney, W. Eisnerf, S. Steinleitner, F.M. Mottaghyg, S. Felver, A new pneumatic vibrator for functional magnetic resonance imaging of the human sensorimotor cortex, Neurosci. Lett. 324 (2002) 125–128. [6] A. Gallace, H.Z. Tan, P. Haggard, C. Spence, Short term memory for tactile stimuli, Brain Res. 1190 (2007) 132–142. [7] C. Ho, C. Spence, Head orientation biases tactile localization, Brain Res. 1144 (2007) 236–241. [8] M. Auvray, A. Gallace, J.H. O’Brien, H.Z. Tan, C. Spence, Tactile and visual distractors induce change blindness for tactile stimuli presented on the fingertips, Brain Res. 1213 (2008) 111–119. [9] C. Ho, V. Santangelo, C. Spence, Multisensory warning signals: when spatial correspondence matters, Exp. Brain Res. 195 (2009) 261–272. [10] V. Hayward, O.R. Astley, M. Cruz-Hernandez, D. Grant, G. Robles-De-La-Torre, Haptic interfaces and devices, Sens. Rev. 24 (2004) 16–29. [11] S.K. Kim, G.H. Park, S.H. Yim, S.M. Choi, S.J. Choi, Gesture-recognizing hand-held interface with vibrotactile feedback for 3D interaction, IEEE Trans. Consum. Electr. 55 (2009) 1169–1177.
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Biographies Hyung-Sik Kim received the MS in Biomedical Engineering from Konkuk University, in 2007. He is currently working toward the PhD degree in Biomedical Engineering. His research areas are medical electronics and embedded systems. Mi-Hyun Choi received the MS in Biomedical Engineering from Konkuk University, in 2009. She is currently working toward the PhD degree in Biomedical Engineering. Her research areas are brain imaging and physiological signal. Hong-Won Yeon received the BS in Biomedical Engineering from Konkuk University, in 2011. He is currently working toward the MS degree in Biomedical Engineering. His research areas are brain imaging and physiological signal. Jae-Hoon Jun received the PhD degree in Biomedical Engineering from Texas A&M University, College Station, in 2001. He is a professor in the Dept. of Biomedical Engineering at Konkuk University, Korea. His research interests include optical sensing, photothermal therapy, tissue characterization, biomechanics, and T-ray imaging. Jeong-Han Yi received the PhD degree in Electronic Engineering from the Korea Advanced Institute of Science and Technology (KAIST), Korea, in 1992. He is a professor in the Dept. of Biomedical Engineering at Konkuk University, Korea. His research interests are magnetic resonance imaging, medical imaging system, FES and Magnetic stimulation. Jong-Rak Park received the PhD degree in Physics from the Korea Advanced Institute of Science and Technology (KAIST), Korea, in 2000. After working three years for LG and Samsung as a Senior Research Engineer, he joined the Dept. of Photonic Engineering, Chosun University as a faculty member in 2003. His work is focused on fundamentals and applications of laser optics and applications of optical techniques to manufacturing processes of semiconductor and display devices.
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Dae-Woon Lim received the PhD degree in Electrical Engineering and Computer Science from Seoul National University, Korea, 2006. He is an assistant professor in the Dept. of Information and Communication Engineering at Dongguk University, Korea. His research interests are in the area of signal processing, wireless communications, and channel coding.
Soon-Cheol Chung received the PhD degree in Electrical Engineering from the Korea Advanced Institute of Science and Technology (KAIST), Korea, in 1999. He is a professor in the Dept. of Biomedical Engineering at Konkuk University, Korea. His research interests are neuroscience, behavioral science, and biomechanics.