Comparison of Spatial and Temporal ... - Semantic Scholar

The 18th IEEE International Symposium on Robot and Human Interactive Communication Toyama, Japan, Sept. 27-Oct. 2, 2009

TuIAH.5

Comparison of spatial and temporal characteristic between reflection-type tactile sensor and human cutaneous sensation Satoshi Saga, Masashi Konyo and Koichiro Deguchi

rubber. The stress concentration occurs on the sensing units. In case of human the sensing units (receptors) has similar hardness with skin. So the stress concentration does not occur too much. The ideal tactile sensor should have similar stress concentration with human. Further, the strain gauges, used as the sensing units, can measure high frequency signals, though, the units cannot measure distributed information. If we want to measure the tactile information of human we have to use human-like tactile sensor. This is partly because touch itself will make the sensor deformed, and the deformation of the sensor also implys the contact information. So the cutaneous sensation of real human can be fully recorded when the ideal tactile sensor whose physical property is almost the same with human is used. So, how can we realize the deformation of the sensor which is similar to human skin? Here we can use optical sensing method and computer vision. That is, by using silicone rubber in order to construct similar hardness to human skin and by using optical remote sensing we develop a tactile sensor which is similar to human skin. You can find that this silicone rubber and optical sensing system is effective for creating human skin like tactile sensor. Kamiyama, et al. [3] or our reflection-type tactile sensor [4] have been researched these types of tactile sensors. Although these sensors cannot measure high frequency signals because they use imaging sensors for distribution sensing. Here we consider Shinoda’s argument [5]. Shinoda discussed based on temporal response or physical characteristics of human cutaneous sensation of skin. He mentioned the importance exist not only on the temporal or spatial sensing resolution but also the physical characteristic itself. Then we aim human like tactile sensor and re-design the reflectiontype tactile sensor. In this paper especially we compare the frequency characteristic and the filtering effect of component materials between the sensor and the human cutaneous phenomenon.

Abstract— Today there are many tactile sensors in the world. Though there is few sensor whose characteristics are similar to cutaneous sensation of human. If we want to measure the tactile information of human we have to use human-like tactile sensor. Here we have developed a reflection-type tactile sensor by using imaging device. This sensor employs a simple total reflection characteristic, so it has many degrees of freedom of selecting sensing units, and designing layer structures. This paper describes the design of the sensing unit and the structure of layers which has similar characteristics of human. First we compare the temporal frequency of the proposed sensor and human skin. Second we estimate the required spatial density of sensing units by simulation. Then we compare the total ability of proposed reflection-type sensor and cutaneous sensation of human.

I. I NTRODUCTION In recent years, many tactile sensors have been developed with the advancement in robotics. For example, there are sensors that measure the contact state or force distribution. Several tactile sensors are available in the market: some of them include a force sensor with 6 degrees of freedom that can measure the force at one point. Another sensor can measure the distribution of the contact state or one that can measure the force distribution (FlexiForce, Nitta Inc.). Though there is neither function of multi degrees of freedom sensing, such as distribution of force vectors, nor the elasticity of deformation of sensor itself in contact state. The tactile phenomena of human cannot be realized. The disadvantage of these distribution–type force sensors is that the number of sensor units with dense of wiring is excessive. Each sensor unit is arranged in close proximity to the measurement surface in order to allow a small sensor to be individually distributed, and the wiring that gathers information from a unit is also individually wired. Therefore, a sensor itself cannot avoid deterioration due to the stress of repeated measurement; further, the assembly of the wiring is complicated. Howe and Cutkosky [1] developed a sensor made of rubber skin with movement. Zhang, et al. [2] also developed a moving sensor which is made of silicone rubber. The movement realize the spatial resolution and the rubber realize the similar physical characteristic with human. Though the hardness of the material itself is similar to human skin, the implanted sensing units have far harder material than silicone

II. C ONDITIONS OF HUMAN - LIKE TACTILE SENSOR Here we consider Shinoda’s argument. Shinoda discussed about ideal tactile sensor based on temporal response or physical characteristics of human cutaneous sensation of skin. He mentioned the following 5 conditions for the sensor. 1) Spatial resolution: Less than 1 mm 2) Temporal resolution: Over 1 kHz 3) Measuring force of each point as 3 dimensional vector 4) Sensing range: Over 16 bit 5) Physical characteristics with human skin (elasticity, shape, friction characteristic)

MSR IJARC Fellow & Tohoku University, 6-6-01 Aoba, Aramaki-aza, Aoba-ku, Sendai, Japan, [email protected] Tohoku University, 6-6-01 Aoba, Aramaki-aza, Aoba-ku, Sendai, Japan,

[email protected] Tohoku University, 6-6-01 Aoba, Aramaki-aza, Aoba-ku, Sendai, Japan,

[email protected]

978-1-4244-5081-7/09/$26.00 ©2009 IEEE

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These 1–4 conditions are determined by the sensing units itself or their communications zone, so the conditions can be cleared by selecting adequate sensing unit and designing adequate system. However the last condition is the most difficult one. This is because the cutaneous sensation is a touching sensation and the contact deforms sensor itself. This is the most different point between visual / auditory and cutaneous sensor. The cutaneous sensation induces contact and deformation between the contact object and sensor itself. The rubbing movement affects the friction or shape of the sensor itself. So the most important things are to create sensing units, which is similar response to human receptors, and sensor material itself, which are similar physical characteristics to human skin.

Fig. 2.

alternates patterns and the PDs alternates imaging sensor like web camera. Additionally the LEDs and PDs are also outside of the contact part. According to this fabrication, the sensor is still free from stress concentration. In this case, we consider the possibility of utilizing a high-speed measurement method by a LED and a PD (Fig. 3). Here we measure high frequency vibration by using this sensor with high speed sensing method. First we input from 10 Hz to 1 kHz sinusoid signal to audio speaker and contact the speaker to the tactile sensor. Then we compare the measured speaker deformation and PD output (Fig. 3 (right)). Example output results of 200 Hz and 500 Hz signal is shown in Fig. 4. This experiment shows the sensor can measure and evaluate 500 Hz vibration to the maximum. Additionally the sensitivity (PD output voltage [V]/deformation shift [mm]) in each frequency input is plotted on the Fig. 5. Fig. 4 shows the higher input frequency becomes, the smaller the output amplitude becomes. On the other hand fig. 5 has small variance. This is because the construction setup of LED and PD position is not so rigid. Although the lack of rigidity, fig. 5 shows the sensitivity rate toward the frequency does not change so large. This shows the sensing stability of the proposed sensing method.

III. F REQUENCY CHARACTERISTICS OF SENSING UNITS A. Experiment for high frequency response The reflection-type sensor comprises a reflective surface and an imaging device (Fig. 1). The reflective surface is made of transparent elastic body. This body has around 1.5 of refractive index. Owing to the difference of refractive index between the body and air, the surface of the body has total reflection characteristic in some angles. By using transparent elastic body and flexible reflective surface, this sensor can employ the principle of “optical lever,” which can enhance very small deformation. Furthermore, by using entire reflection image, we can measure the two dimensional distribution of enhanced deformation. The use of imaging sensor decreases the wiring problems to the minimum. If the surface of the contact part is pressed down, the reflective surface is deformed, and according to the “optical lever,” the reflection image is deformd drastically. Reflective surface ( based on Snell ' s law )

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One of the important point of this sensor is the simplicity of the contact part. The contact part is now made of transparent silicone rubber and acryl frame. The camera and image pattern exists outside of contact part. Thus the equipments outside of the contact part such as camera do not affect the deformation of silicone rubber. This is a difference of the reflection-type tactile sensor from other implanted tactile sensors. We can combine the Light Emission Diodes (LEDs) and Photo Diodes (PDs) as a simple component instead of camera and employ faster sensing method(Fig. 2). The LEDs

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B. Comparison of frequency response between human and the sensor Here we consider the frequency response of human. Fig. 6 shows the structure of human skin. There are 4

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corresponds to the response and distribution of human SA and RA receptors, and the system using the LEDs and PDs corresponds to the response and distribution of human PA receptor (Fig. 8). Thus the ideal design of human-like tactile sensor not only leads to reflection-type tactile sensor using optical measurement, but also the two measurement methods correspond to response and distribution of human receptors in temporal and spatial resolution. The combination of these two methods enables the ideal human-like tactile sensor. Howe and Cutkosky [1] also proposed a tactile sensor which has spatial and temporal distribution by moving itself. Though our sensor enables the both distribution without movement. Additionally the movement may enhance the sensing distribution.

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mechanoreceptors under the human skin. These are called as Meissner’s corpuscles (RA), Merkel disks (SA), Pacinian corpuscle (PC) and Ruffini ending. These distributions are as follows; RA and SA are placed under each fingerprint, between epidermis and dermis. These receptors have dense distribution. On the other hand PC are placed under deep skin, and has sparse distribution. Additionally the response also discriminate these receptors. PC has higher response around 200 Hz vibration, RA has response around 20 – 40 Hz, and SA has response around several Hz order (Fig. 7, Freeman et al. [6]). The sensitivity of higher vibration will detect fast changing of state such as stick-slip phenomena. The observation research of the stick-slip phenomena suggest this [7], [8]. Nahvi et al. reported that 89 Hz of stick-slip vibration was measured between human finger and contact object. Konyo et al. reported over 100 Hz vibration was measured. Thus the importance of higher response of the sensor lies the detection of the change on contact phase.

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IV. C OMPARISON OF FILTERING EFFECT BETWEEN HUMAN AND THE SENSOR

Next we consider the function of layered structure in tactile sensing. The reflection-type tactile sensor has two layered silicone rubber structure. Tanaka et al. [9] also developed two layered tactile sensor. Additionally the human skin also has two layered structure with epidermis and dermis. Thus some sensors which acquire contact state have layered structures. Then we consider the meanings of these structures and design the suitable layer for reflection-type tactile sensor. The role of layered cutaneous structure of human is known as the concentrator of strain energy or shear strain to some special position, where the some mechanoreceptors exist [10], [11]. On the other hand, the tactile sensor researched by Tanaka et al. is designed for inverse aim. That is, the sensor uses layered structure in order to level the uniform stress distribution. Thus, by designing the layer structure these sensors have their own measurement characteristics. The reflection-type tactile sensor uses optical sensing method. So the importance is only the deformation of sensing surface. However we simulate the strain distribution of internal structure. By using the result we consider the structure of human skin, Tanaka’s sensor and the reflection-type tactile sensor. As a FEM simulator, we use ANSYS. This time we simulate with the 2 dimensional model. This means each figure shown later is section of each model.

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Fig. 7. Response of each receptor (adapted from Freeman & Johnson[6])

Then we re-consider the reflection-type tactile sensor. By using ordinal imaging sensor (e.g. CCD sensor with an optical lens) the sensor can measure high density spatial resolution, though, these imaging sensor’s capturing rate is 30 – 60 fps. With some camera which has higher framerate the sensor will become high framerate sensor. Though the system will become larger and more expensive one. On the other hand higher measuring method using LEDs and PDs can evaluate 500 Hz vibration to the maximum from the discussion of previous section. The sensing method does not use imaging device, so even the arrays of LEDs and PDs cannot enhance spatial resolution than the system using imaging devices. Here you can find the similarity between the characteristics of these sensing methods and those of human mechanoreceptors. That is, the system using the ordinal imaging sensor

A. Strain distribution of each sensor The Young’s modulus of silicone rubbers (epidermis / dermis) are set to 1.6 × 106 Pa, 0.08 × 106 Pa each (based on the data of real silicone rubber), and the Poisson’s

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ratio are set to 0.48 for each based on the measurement. We prepare the two layered model whose total depth to be 11.0 mm. The depth between epidermis and dermis is changed for each model. Then we press and shear the model with small cylinder whose diameter is 0.35 mm and the Young’s modulus is 2.0 × 1020 Pa, and we record the surface deformation and internal equivalent von-Mises elastic strain. We placed 4 sensor probes at the bottom of the sensor. Friction coefficient is set to be 0.3. Prepared depth are as follows; • (a) Epidermis: 1.50 mm dermis: 9.50 mm • (b) Epidermis: 4.25 mm, dermis: 6.75 mm • (c) Epidermis: 5.50 mm, dermis: 5.50 mm

fingerprint is set to be 0.1 mm, the pitch of fingerprint is set to be 0.35 mm, and the height of papillae dermis is set to be 0.24 mm. The Young’s modulus of epidermis, dermis, hypodermis is set to be 0.136 × 106 Pa, 0.08 × 106 Pa, 0.04 × 106 Pa, each. The Poisson’s ratio is set to be 0.48 [10]. We prepare the two layered model whose total depth to be 11.0 mm. The depth between epidermis and dermis is changed for each model. Then we press the model with small cylinder whose diameter is 0.35 mm and the Young’s modulus is 2.0 × 1020 Pa, and we record the surface deformation and internal equivalent von-Mises elastic strain. We placed 12 sensor probes at each layer of the cutaneous model. Friction coefficient is set to be 0.3. Prepared depth are as follows; Fig. 10 shows each position of Merkel disks (SA), Meissner’s corpuscles (RA), Pacinian corpuscle (PA). • (A) Epidermis: 1.00 mm, dermis: 10.00 mm • (B) Epidermis: 3.00 mm, dermis: 8.00 mm • (C) Epidermis: 4.95 mm, dermis: 6.05 mm C. Simulation results and discussion 1) Sensor model: From the simulation result of press and shear movement(Fig. 9), according to the thickness of the epidermis rubber, the strain is diffused at deep inside the structure. The difference of the graph is smaller in thicker epidermis condition. Tanaka’s two layer structure uses this feature. That is, the sensor uses layered structure in order to level the uniform stress distribution. This stress distribution is suitable for the sensing unit which can measure planer deformation. Here we consider the layer structure for reflection-type tactile sensor. The reflection-type tactile sensor measures surface deformation by using change of reflection image. Thus the more angle changes the more the reflection image changes. The simulation result shows that the thinner the epidermis rubber becomes the more the change of reflection image is localized. This result says that the thinner epidermis rubber draws the better output for reflection-type tactile sensor is acquired. Additionally, the strain shifts toward the shear direction in deep inside the structure at thicker epidermis. Simultaneously the thicker the epidermis rubber becomes, the clearer the asymmetry of the deformation of the surface becomes. That is, if there is the same deformation toward the sensor, the difference between the shapes of pressing movement and shear movement becomes larger in thicker model (c) than (a). 2) Model of human skin: Next we consider simulation results of human skin model. From the simulation result of press and shear movement (Fig. 10), according to the thickness of the epidermis rubber, the strain is diffused at deep inside the structure. This result is similar to sensor model. Additionally, the strain occurs not only just under the contact point, between papillae dermis but also adjacent point toward shear direction. This result corresponds to Sato et al.’s discussion [12]. They only discuss the shear strain, so they mentioned the absence of strain jus under the contact point.

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B. Strain distribution of human skin Next we model the human skin. Some simulation results are known [10], [11]. They simulated the finger structure with or without the existence of epidermal ridges or papillae dermis. Here we simulate the finger structure with the change on thickness of epidermis. That is, we consider whether the thickness of epidermis is important for temporal or spatial information of sensing or not. The ridge line height of

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However equivalent von-Mises strain exists from just under the contact point toward the shear movement direction. These two result shows that the two layered structure causes each strain toward SA, RA, PC. Comparing each graphs of Fig. 10, we evaluate the effect of changing thickness of epidermis. Here we compare the strain at each receptor and strain distribution. In condition (A), there exist two or three times large strain of RA position than that of SA’s. At RA and SA position there exist strain distribution not only just under the contact point but also adjacent point toward shear direction. The distribution is more localized than condition (B) at RA and SA position. The strain is normalized at each PC position. In condition (B), each strain at SA, RA, and PC is almost the same. At RA and SA position there exist strain distribution not only just under the contact point but also adjacent point toward shear direction. The strain is normalized at each PC position. In condition (C), the strain at RA, SA and PC is almost the same with each other. The strain are normalized at every SA, RA and PC positions. Here the distribution density of each receptor is known as follows; RA and SA exist around every papillae dermis. They have dense distribution. On the other hand PC has more sparse distribution than two other receptors. This fact draws that adequate thickness of epidermis and dermis make the strain distribution suitable for distribution density of each receptor. 3) Comparison result and discussion: Based on the discussion of previous section, we re-consider about the reflection-type tactile sensor. The reflection-type tactile sensor can enhance its spatial resolution by using imaging device such as web camera and can enhance its temporal resolution by using simple optical device such as LEDs and PDs [13]. This construction is similar to spatial resolution of RA and SA, or similar to temporal resolution of PC (Fig. 8). SA has broad spatial resolution, RA has middle spatial resolution, and PC has low spatial resolution. On the other hand temporal resolution of each receptor is low, middle, high resolution, respectively. Next, imaging device has broad spatial resolution and middle temporal resolution. combination of LED and PD system has low spatial resolution and broad temporal resolution. That is, though the expansion of each field is a little different, the roles of RA and SA can be played by imaging device, and the role of PC can be played by LEDs and PDs. In the model of human skin, the thicker epidermis becomes, the more adequate distribution of strain for PC can be acquired. On the contrary, the thinner epidermis becomes, the more adequate distribution of strain for SA, RA can be acquired. In reflection-type tactile sensor, simple optical device induce sparse spatial resolution, so the detection of small displacement becomes difficult. So if we want to detect small displacement, we have to enlarge the deformation region at the expense of spatial resolution (Fig. 11). In order to realize this, we have to thicken the epidermis rubber based on the result of Fig. 9. On the other hand the thinner the epidermis rubber becomes, high spatial resolution sensing can be realized.

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Fig. 10. Sliding result of (A),(B),(C); Each condition has 3 graphs with shallow/middle/deep position data

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Fig. 11.

Based on these results, adequate thickness of epidermis and dermis make the strain distribution suitable for distribution density of each receptor. Furthermore, sensing methods of reflection-type tactile sensor and the human skin are far different from each other, though, the resolution aims and the layer structures coincide with each other. This result draws that in the designing of the human-like tactile sensor, the aim for the structure of human skin is suitable for reflection-type tactile sensor. As a future work we plan to design two sensing method as one sensor simultaneously. Furthermore we want to realize human-like tactile sensor which has sufficient spatial and temporal resolution.

Difference of region size effects the lighting

Based on these discussions, comparison between simulation results of reflection-type tactile sensor and the human skin model, these sensing methods are far different from each other, though, the resolution aims and the layer structures coincide with each other (Table I).

ACKNOWLEDGEMENT This work is partially supported by KAKENHI (19860012), Grant-in-Aid for Young Scientists (Startup).

TABLE I D IFFERENCE AND INDIFFERENCE BETWEEN REFLECTION SENSOR AND

[1] R. D. Howe and M. R. Cutkosky. Dynamic tactile sensing: perception of fine surface features withstress rate sensing. IEEE Transactions on Robotics and Automation, 9(2):140–151, Apr. 1993. [2] Yuhua Zhang, Yuka Mukaibo, and Takashi Maeno. A multi-purpose tactile sensor inspired by human finger for texture and tissue stiffness detection,. In Proceedings of 2006 IEEE International Conference on Robotics and Biomimetics, pages 159–164, 2006. [3] Kazuto Kamiyama, Kevin Vlack, Hiroyuki Kajimoto, Naoki Kawakami, and Susumu Tachi. Vision-based sensor for real-time measuring of surface traction fields. IEEE Computer Graphics & Applications Magazine, 25(1):68–75, 2005. [4] Satoshi Saga, Hiroyuki Kajimoto, and Susumu Tachi. High-resolution tactile sensor using the deformation of a reflection image. Sensor Review, 27:35–42, 2007. [5] Hiroyuki Shinoda. Development of robotics skin. In Application and foundation of cutaneous technology, pages 29–33. The Japan Society of Mechanical Engineerings, 2008. [6] Alan W. Freeman and Kenneth O. Johnson. A model accounting for effects of vibratory amplitude on responses of cutaneous mechanoreceptors in macaque monkey. Journal of Physiology, 323:43–64, 1982. [7] A. Nahvi, J.M. Hollerbach, and D.D. Nelson R. Freier. Display of friction in virtual environments based on human finger pad characteristics. In Proc. Symp. on Haptic Interfaces, ASME International Mechanical Engineering Congress and Exposition, 1998. [8] Masashi Konyo, Hiroshi Yamada, Shogo Okamoto, and Satoshi Tadokoro. Alternative display of friction represented by tactile stimulation without tangential force. In Haptics: Perception, Devices and Scenarios (Proceedings of 6th International Conference, EuroHaptics 2008), pages 619–629, 2008. [9] Yoshihiro Tanaka, Daisuke Kinoshita, and Hideo Fujimoto. Tiny protrusion detecting sensor with force sensing resistor film. In Proceedings of the 25th Anuual Conferenceof the Robotics Society of Japan, 2007. [10] Takashi Maeno, Kazumi Kobayashi, and Nobutoshi Yamazaki. Relationship between the structure of human finger tissue and the location of tactile receptors. Bulletin of JSME International Journal, 41(1):94– 100, 1998. [11] G.J. Gerling. The sampling position within, not the undulating geometry of, fingertip skin microstructure may amplify the sensation of edges. In 14th Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2006, 2006. [12] Takashi Sato, Hiroyuki Kajimoto, and Susumu Tachi. The role of skin structure in human tactile sensing. In Proceedings of the 2003 JSME Conference on Robotics and Mechatoronics, page 54, 2003. [13] Satoshi Saga, Satoshi Tadokoro, and Susumu Tachi. Dynamic conditions of reflection-type tactile sensor. In Haptics: Perception, Devices and Scenarios (Proceedings of 6th International Conference, EuroHaptics 2008), pages 464–473, 2008.

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V. C ONCLUSIONS This paper describes the importance of sensing material itself based on Shinoda’s conditions. Especially the stress concentration induced by the implanted sensing unit inside the elastic construction of the sensor, and we propose the optical sensing method for tactile sensing. Then we propose the sensing method using LEDs and PDs for the sensing system instead of ordinal imaging device. Through the experiment with the system we show the high frequency sensing system can evaluate 500 Hz vibration to the maximum. Then we mention the spatial distribution of mechanoreceptors and temporal response of the receptors of human skin, and discuss about the similarity of sensing method between human skin and the sensor. Next in order to consider the meanings of two layered structure for tactile sensing, we create simple sensor model and human skin model, and through simulation we examine the effect of difference between thicknesses of epidermis and dermis. In the each model the result shows the thicker epidermis diffuse the strain at deep inside the structure. Additionally the shear movement induces the shift of strain distribution toward shear direction. In the sensor model the thicker the epidermis becomes, the change of surface shape between press movement and shear movement becomes clearer.

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