2007_j_micro_system_a polymer based on flexible tactile sensor for both normal and shear load...

556 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 16, NO. 3, JUNE 2007

A Polymer-Based Flexible Tactile Sensor forBoth Normal and Shear Load Detections

and Its Application for RoboticsEun-Soo Hwang, Jung-hoon Seo, and Yong-Jun Kim, Member, IEEE

Abstract—This paper proposes and demonstrates a novel flexi-ble tactile sensor for both normal and shear load detections. Forthe realization of the sensor, polyimide and polydimethylsiloxaneare used as a substrate, which makes it flexible. Thin metal straingauges, which are incorporated into the polymer, are used formeasuring normal and shear loads. The salient feature of thistactile sensor is that it has no diaphragm-like structures. The unittactile cell characteristics are evaluated against normal and shearloads. The fabricated tactile sensor can measure normal loads ofup to 4 N, and the sensor output signals are saturated againstloads of more than 4 N. Shear loads can be detected by differentvoltage drops in strain gauges. The device has no fragile struc-tures; therefore, it can be used as a ground reaction force (GRF)sensor for balance control in humanoid robots. Four tactile unitsensors are assembled and placed in the four corners of the robot’ssole. By increasing bump dimensions, the tactile unit sensor canmeasure loads of up to 2 kgf. When loads are exerted on the sole,the GRF can be measured by these four sensors. The measuredforces can be used in the balance control of biped locomotionsystems. [2006-0111]

Index Terms—Ground reaction force (GRF), sensitive skin,strain gauge, tactile sensor.

I. INTRODUCTION

TACTILE sensors have shown promise in the use of manyapplications such as robotic systems [1]–[4], medical tools

for surgery [5], [6], and agriculture food processing industries[2], [7]. Emerging microelectromechanical systems (MEMS)technology has facilitated research in these areas. Many typesof MEMS-based tactile sensors have been developed [8]–[13].The three main types of materials that are used in tactile sensorsare silicon, polymer, and piezoelectric materials.

Manuscript received June 15, 2006; revised February 11, 2007. This workwas supported in part by the Ministry of Information and Communication,Korea, under the Information Technology Research Center support programthat is supervised by the Institute of Information Technology Assessment(IITA) under Grant IITA-2005-C1090-0502-0012, and in part by the KoreaScience and Engineering Foundation through the National Core Research Cen-ter for Nanomedical Technology under Grant R15-2004-024-00000-0. SubjectEditor C. Liu.

E.-S. Hwang and Y.-J. Kim are with the School of Mechanical Engineer-ing, Yonsei University, Seoul 120-749, Korea (e-mail: [email protected];[email protected]).

J. Seo was with the School of Mechanical Engineering, Yonsei University,Seoul 120-749, Korea. He is now with the Telecommunication and NetworkDivision, Samsung Electronics, Suwon 442-742, Korea (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2007.896716

Fig. 1. Schematic view of the proposed tactile sensor. The strain-sensitiveelements, i.e., the strain gauges, are embedded in a ductile polymer substratethat is used to measure the strains in the polymer substrate. The bump structurethat is used for load distribution is placed on top of the sensor surface.

In robotic systems, measurement of 3-D contact forces (i.e.,one normal force and two orthogonal shear forces) is critical fordetermining the full grasp force/torque and the object slipping.Without the ability to measure 3-D forces using such tactilesensors, the development of the robotic handling of fragileor irregular objects is limited [9], [10]. To measure 3-D con-tact forces, silicon-diaphragm structures are frequently used[8]–[11]; however, tactile sensors that are developed throughsilicon processes cannot be made flexible. In order to makethem flexible, the silicon process requires the incorporationof polyimide (PI) layers. Some research groups have usedPI layers as a connecting material between silicon-diaphragmsensors, while others have mounted silicon-diaphragm sensorson flexible printed circuit board substrates with a conductiveepoxy [10], [11]. These packaging processes, however, are verycomplex and reduce yield. In addition, silicon-diaphragm-basedtactile sensors are dominated by piezoresistive sensing methodsand require expensive equipment such as an ion implanter andlow-pressure chemical vapor deposition for their realization.Therefore, the development of silicon-diaphragm-based tactilesensors requires both complex and expensive processes.

To reduce the cost of tactile sensors, a polymer-MEMS-based process is used. Various polymer-based tactile sensorshave been reported in [12]–[15]. Basically, two kinds of sensingschemes, i.e., resistive and capacitive, are used for polymer-based tactile sensors. The capacitive sensing method is veryeffective for measuring normal loads; however, it is very diffi-cult to use when measuring shear loads. Therefore, it cannot beused practically for 3-D load detections. Furthermore, capac-itive polymer-based tactile sensors have a narrow operational

1057-7157/$25.00 © 2007 IEEE

HWANG et al.: POLYMER-BASED FLEXIBLE TACTILE SENSOR FOR BOTH NORMAL AND SHEAR LOAD DETECTIONS 557

Fig. 2. FEM analysis models and results: (a) dimensions of a silicon-diaphragm model, (b) dimensions of the proposed tactile-sensor model, (c) FEM strainanalysis result for the silicon-diaphragm model, and (d) FEM strain analysis result for the proposed tactile-sensor model.

range due to the ductility of the polymers. Several groupsuse piezoelectric polymer films; however, this scheme also hasdifficulties in measuring shear loads [17].

Some tactile sensors use optical fibers with polymer struc-tures [15], [16]. This method requires additive measurement in-struments such as charge-coupled devices, cameras, and opticalsources or broadband wave sources. In this sensing scheme,extra equipment is needed, which is not practical in househelping and silver-mate robots. Therefore, the development ofa low-cost flexible tactile sensor that can be used for measuringthe 3-D forces with a wide operational range is still a matter ofconcern to many researchers.

This paper demonstrates a novel low-cost flexible tactilesensor that is used for measuring both normal and shear loads.For the realization of this sensor, PI and polydimethylsiloxane(PDMS) are used to make a flexible substrate, and thin-filmmetal strain gauges are used for measuring the 3-D forces. Thesalient feature of the proposed tactile sensor is that it has nodiaphragm-like structures. Sensitivity is traded for strength anddurability. This paper discusses in detail the sensing principlesand fabrication of the proposed device, and also introduces anapplication of the proposed tactile sensor.

II. SENSING PRINCIPLE AND SENSOR STRUCTURE

When a substrate is subjected to surface traction, it experi-ences stresses for maintaining force equilibrium. The stressesresult in strains according to Hooke’s law [18]. In many cases,thin diaphragm structures are used to magnify these strains.Our approach uses the deformation of a polymer substrateitself, which replaces the diaphragm structure in a rigid siliconsubstrate. Fig. 1 shows a schematic view of the proposed tactilesensor. The strain sensitive elements, i.e., the strain gauges,

Fig. 3. Normal and shear load sensing principles. (a) In the case of a normalload, both strain gauges are subjected to tensile stress. (b) In the case of a shearload, one strain gauge is subjected to tensile stress, and the other is subjected tocompressive stress.

have been embedded into a ductile polymer substrate. A thin-film metal film is used for the strain gauges. Both the polymerand the thin metal film make the sensor flexible. A bumpstructure is placed on top of the sensor surface to facilitate loaddistribution. When an external force is applied to the device, thethin metal film structure and the polymer incur a deformationthat causes changes in the electric resistance. The changes inresistance corresponding to the applied force can be measured.

The magnitude of the strain in a diaphragm structure isdetermined by its displacement at the center and its thickness,while the magnitude of the strain in our proposed structure isdetermined through bump dimensions and the ductility of thepolymer substrate. A finite-element method (FEM) analysis isused to compare the magnitude of the strains in a polymersubstrate to that of the strains in the silicon diaphragm whenboth are subjected to the same force. The dimensions of the


Fig. 4. Fabrication steps for the realization of the proposed device: (a) strain-gauge patterns on PI, (b) second PI layer coating and via hole etching,(c) interconnecting one end by electroplating, (d) third PI layer coating and via hole etching, (e) interconnecting the other end, and (f) bump-structure patterning.

silicon-diaphragm structure are illustrated in Fig. 2(a), andthose of the proposed tactile sensor are illustrated in Fig. 2(b).For the FEM analysis, the material properties of silicon are165 GPa for Young’s modulus and 0.22 for Poisson ratio. Theproperties of the polymer substrate are 2.6 GPa and 0.35 forYoung’s modulus and Poisson ratio, respectively. The analysisresults are shown in Fig. 2(c) and (d). In the case of the silicondiaphragm, the maximum strain is about ten times greater thanthat of the polymer substrate. However, it should be noticedthat the strains in the polymer substrate are large enough tobe measured with a metal strain gauge. Therefore, the FEManalysis results confirm that it is possible to measure externalload using polymer and strain gauges even though they have nodeformable diaphragmlike structures.

Most silicon-diaphragm-based tactile sensors use doped sili-con piezoresistors as the strain-gauge material because dopedsilicon has a very high gauge factor in comparison to othermaterials. In our work, doped silicon is not compatible withpolymer substrates; therefore, we can use only thin-film metalas the strain-gauge material. The sensitivity of the proposedtactile sensor is expected to be less than that of the silicon-diaphragm tactile sensor due to the relatively small strains inthe polymer substrate and the low gauge factor of thin-filmmetal gauges. However, our sensor is expected to have a wideoperational pressure range, and it should be noted that a tactilesensor must serve as a front barrier to chemical and mechanicalcontact as well as a source of physical contact information[14]. Therefore, a polymer substrate and thin-film metal straingauges together is a good candidate for robot skin.

The normal and shear load sensing principles of the proposedsensor are described in Fig. 3. Strain gauges are embedded atthe center of a ductile polymer substrate. When a normal loadis applied on the surface of the sensor, the substrate deforms.This deformation induces equal strains on both strain gauges,as shown in Fig. 3(a). When a shear load is applied, as shown in

Fig. 3(b), one strain gauge experiences tensile strains, while theother experiences compressive strains. This difference resultsin different measurable voltage drops across each strain gauge,which allows the shear load to be determined. An unknown loadcan be found by superposition of these two cases [18]. Fromthese sensing principles, we designed the tactile-sensor unit cellto consist of four strain gauges for shear load detection in thex- and y-directions.

III. FABRICATION AND RESULT

Fig. 4 shows the simplified fabrication steps for the realiza-tion of a flexible tactile sensor. The fabrication starts with a 4-insilicon wafer that has a 1-µm-thick oxide on it. A PI precursor(PI2611, HDMicrosystems) is spin coated at 1000 r/min andcured at 300 ◦C in a convection oven. This layer is named thefirst PI layer. Then, a Cu–Ni metal layer is deposited througha thermal evaporation method and patterned by a wet-etchingprocess. Fig. 4(a) shows the patterned strain-gauge array on thehard-cured PI film. Again, the PI (second PI layer) is coatedand cured. The same parameters as those used in the first PIlayer are used in this step. Now, via holes are opened in O2

plasma through a reactive ion etching (RIE) method. Aluminum(Al) is used as a masking layer. After via hole formation, aseed layer is deposited. Now, a positive photoresist (AZ-7220)is spin coated and patterned for selective electroplating. Afterthis step, Cu electroplating is carried out for the contact padsand interconnects at one end of the strain gauge, as shown inFig. 4(c). Thereafter, the third PI layer is coated. Now, the sameRIE (via openings) and electroplating processes are employedfor forming contact pads and interconnects at the other endof the strain gauge, as shown in Fig. 4(d) and (e). After theformation of contact pads and interconnects, SU8 (MicroChem,SU8-2050) bump structures (1 × 1 mm2) are formed for loaddistribution, as shown Fig. 4(f). The bump height is 50 µm.


Fig. 5. Optical photographs of the fabricated flexible tactile sensor. (a) 8 × 8tactile sensor (with a size of 35 × 35 mm2 and a thickness of 70 µm). (b) Oneunit cell consists of four strain gauges for normal and shear load detections inthe x–y-directions. (c) Sensor indicating its flexibility.

Finally, the sensor array is released from the silicon wafer byetching out the silicon dioxide layer.

Fig. 5(a) shows an optical photograph of the fabricatedflexible tactile sensor (which has a size of 35 × 35 mm2 anda thickness of 70 µm). It has 8 × 8 tactile cells, each consistingof four strain gauges for detecting both normal and shear loadsin the x–y directions. Fig. 5(b) shows a close-up view of oneunit cell that consists of four strain gauges. The flexibility ofthe fabricated tactile sensor has been indicated in Fig. 5(c) byfolding it using fingertips.

The fabricated tactile sensor is attached on a ductile PDMS(Sylgard 184, Dow Corning) substrate using a double-sidedadhesive Kapton (Dupont) film that is 80 µm thick. This PDMSsubstrate plays a significant role in tactile sensing by transform-ing the load into strains. Loads are applied on a unit cell bya load cell. The applied loads are monitored using a load-cellindicator. The voltage drops across each resistor in one unit cellare measured by an analog-to-digital converter (ADC) using aWheatstone bridge configuration and a voltage amplifier. Thecircuit consists of a microcontroller, which has 10-bit ADCs,multiplexers for addressing the 8 × 8 sensors, operational

Fig. 6. Measurement results: (a) output voltages versus normal loads,(b) output voltage versus shear loads, and (c) 2-D image test result when pressedby a ring-shaped object (100 gf).

amplifiers for voltage amplifications, and an RS232C chip fortransmitting the signal to a computer.

The unit-cell characteristics were evaluated against normaland shear loads. Fig. 6(a) displays the normal load measure-ment result; the voltage drops across the strain gauges increase


Fig. 7. (a) Cross-sectional view of the GRF sensor and (b) overall structure ofthe GRF sensor module.

Fig. 8. Performance test of the GRF sensor: (a) graphs of the four tactile unitsensors’ output voltages versus the applied normal loads before assembling and(b) graphs of the four sensors’ output signals versus the applied normal loadsafter assembling them to the sole with front-barrier structures.

as the applied normal loads increase. The output signal ofthe sensor becomes saturated when the applied load is morethan 4 N. Since it has a strong structure, it can tolerateoverloads without breakdown. The operational load range iswider (0–4 N) than that of a diaphragm-based tactile sensor.

Fig. 9. (a) Parts of the GRF sensor module and (b) and (c) the assembled GRFsensor module.

Fig. 10. Cause of 25% variance in the GRF sensor test. (a) A normal loadis applied exactly on the bump; the sensor can measure the load. (b) Abiased normal load is shown; the sensor’s output has variations due to stressconcentrations in one area.

A pure shear load is applied using an adhesive tape by settingup the sensor vertically with a rigid object. The shear load testresults are shown in Fig. 6(b). It can be clearly observed thatthe voltage drop across one strain gauge increases while thevoltage drop across the other decreases. However, the voltagechange due to shear loads is much smaller than the voltagedrops due to normal loads. This fact makes it difficult todiscriminate between shear and normal load signals when theyboth exist. A 2-D image test was performed with the 8 × 8flexible tactile sensor. A ring-shaped object with a 100-gf loadis applied to the tactile sensor and the resulting image is shownin Fig. 6(c).

IV. APPLICATION TO A HUMANOID ROBOT AS A

GROUND REACTION FORCE (GRF) SENSOR

The proposed tactile sensor has three strong points. First,it has a wide operational pressure range. Second, since it hasno fragile structures, it can tolerate overpressure. Finally, it isextremely thin. These facts enable the sensor to be used forbalance control as a GRF sensor in the sole of a humanoidrobot foot.

There are two types of methods for controlling the balanceof bipedal locomotion systems. One method pays attention tosupporting leg exchanges. The biped system is very unstable;by exchanging the supporting legs before completely becoming


Fig. 11. Measurement of the GRF with the GRF sensor module.

flat to the ground, the locomotion is dynamically maintained.The other follows the zero-moment point (ZMP) criterion. ZMPis a point on level ground where the torque generated by bothinertial and gravitational forces becomes zero. If the ZMP existsunder the foot, the locomotion system does not tumble. Thus,the desired motions are planned, so that the ZMP criterion issatisfied; then, the controller is required to realize such motions[19]. Problems occur when the system walks on an unevensurface. In this situation, it is essential to measure the real-timeGRF to maintain balance control. Our objective is to measurethe GRF in order to keep the ZMP under the foot.

In this application, the weight of our humanoid robot is about7 kg. In walking situations, one sole of the foot supports all 7 kgof the body weight. To measure these loads, we used four tactileunit sensors for each sole. Thus, it is required that each unitsensor measure a normal load of up to 2 kgf. To satisfy thisrequirement, we changed the dimensions of the bump on top ofthe sensor surface that is used for load distribution. When thebump dimensions increase, the stresses in the polymer substratedecrease since the larger area is subjected to the load.

The GRF sensor structure is identical to the proposed tactilesensor, except for the dimensions of the strain gauges and thebumps. We designed the bump dimensions to be 4 mm (width),4 mm (length), and 2 mm (height). The strain-gauge dimensionswere also modified according to the bump dimensions. Thesensing mechanism and the overall structure are illustrated inFig. 7(a) and (b). The GRFs are measured at the four corners ofthe sole plate, as illustrated in Fig. 7(b).

We evaluated the performance of the four unit sensors againstnormal loads before assembling them into the sole plate;Fig. 8(a) shows the results. The four unit sensors show similarperformances and can measure up to 2.2-kgf loads with onlysmall errors. These data were programmed into the microcon-troller chip for load calibration. The assembled sensor moduleis shown in Fig. 9. Since the ground plane may be uneven, front-barrier structures have been placed under the bump. The bumpis pressed by these front-barrier structures, and the deformationof the polymer is detected by strain gauges. Once again, the

performance of each of the unit sensors was evaluated afterassembling four unit sensors into the sole of the foot. Normalloads were applied on the front-barrier structures. The changein the sensor output voltage was converted to the magnitude ofthe normal loads in the microcontroller, and the converted datawere sent to a personal computer. Fig. 8(b) shows the evaluationresults for each sensor. Each sensor can measure a load of up to2.0 kgf within a 25% error.

The 25% variance between the four sensors on the robotfoot was caused by the assembled parts with the front-barrierstructures. When loads were exerted at the center of the bumpstructure, as shown in Fig. 10(a), the four sensors exhibitedsimilar characteristics, as shown in Fig. 9(a). When the front-barrier structure was pressed with biased normal loads, asshown in Fig. 10(b), the stresses were concentrated on onestrain gauge and caused the measured voltage change to becomelarger. The measured voltage change became smaller whenthe stress concentration occurred in the area without a straingauge. There are two methods to overcome this biased loadingeffect. One is to increase the number of strain gauges aroundthe bump structure in each sensor and to average the measuredvoltage drops. This, however, makes the fabrication process andthe electrical routing difficult. The other method is to makethe bump shape hemispherical to prevent stress concentrationbeneath the bump.

The full GRF on the robot’s foot was also measured. Loadsof 2, 3, and 4 kgf were applied at the geometrical center ofthe robot-foot plate, as shown in Fig. 11(a)–(c), respectively. Ineach case, the GRF sensor module could measure the overallload on the robot-foot plate. The measured results are shownin Fig. 12(a)–(c). When the 2-kgf loads were exerted at thegeometrical center, the sensor outputs were expected to be thesame as the 500-gf loads at each sensor. However, the sensoroutputs showed variations due to the biased loading effect onthe front-barrier structures. When distributed 4-kgf loads wereapplied to the plate, the sensor was also able to detect the loads,as shown in Figs. 11(d) and 12(d). When the loads were appliedto the front and rear parts of the robot sole plate, the GRF


Fig. 12. Results of the GRF measurement. The measured forces (with slash marks) are compared to ideal sensors.

sensor module could detect the center of the pressure using themeasured GRF from each sensor, as shown in Figs. 11(e) and(f), and 12(e) and (f). These measured data can be used for thebalance control of bipedal locomotion systems.

V. CONCLUSION

A novel flexible tactile-sensor module for both normal andshear load detections has been proposed and demonstrated. PIand PDMS were used as the flexible substrate for the sensor.Strains that are induced in the polymer substrate can be detected

by using strain gauges that are incorporated in the polymer.The measurement results show that a flexible tactile sensor haslower sensitivity and a wider operational pressure range thanconventional silicon-membrane-based tactile sensors. The shearload can also be easily detected using a simple measurementcircuit.

Since the proposed tactile sensor has no fragile structures,the sensor can be applied to a GRF sensor. Each tactile unitsensor can measure loads of up to 2 kgf by increasing the bumpdimensions. Using four tactile unit sensors, the GRF can bemeasured.


The proposed device has the advantages of high flexibil-ity and a wide operational pressure range. However, it hasrelatively low sensitivity and is inaccurate for small loads.The promising aspect of the tactile sensor is the simultaneousdetection of normal and shear loads with a simple structure.

REFERENCES

[1] V. J. Lumelsky, M. S. Shur, and S. Wagner, “Sensitive skin,” IEEE SensorsJ., vol. 1, no. 1, pp. 41–51, Jun. 2001.

[2] M. H. Lee, “Tactile sensing: New directions, new challenges,” Int. J. Rob.Res., vol. 19, no. 7, pp. 636–643, Jul. 2000.

[3] D. Yamada, T. Maeno, and Y. Yamada, “Artificial finger skin having ridgesand distributed tactile sensors used for grasp force control,” J. Robot.Mechatron., vol. 14, no. 2, pp. 140–146, Apr. 2002.

[4] M. H. Lee and H. R. Nicholls, “Tactile sensing for mechatronics—A stateof the art survey,” Mechatronics, vol. 9, no. 1, pp. 1–31, Feb. 1999.

[5] K. J. Rebello, “Applications of MEMS in surgery,” Proc. IEEE, vol. 92,no. 1, pp. 43–55, Jan. 2004.

[6] B. Preising, T. C. Hsia, and B. Mittelstadt, “A literature review: Robotsin medicine,” IEEE Eng. Med. Biol. Mag., vol. 10, no. 2, pp. 13–22,Jun. 1991.

[7] P. N. Brett and R. S. Stone, “A tactile sensing technique for automaticgripping of compact shaped non-rigid materials,” in IEE Colloq. Intell.Autom. for Process. Non-Rigid Products, 1994, pp. 1–5.

[8] L. Wang and D. J. Beebe, “A silicon-based shear force sensor: Devel-opment and characterization,” Sens. Actuators A, Phys., vol. 84, no. 1,pp. 33–44, Aug. 2000.

[9] L. Wang and D. J. Beebe, “Silicon-based tactile sensor for finger-mountedapplications,” IEEE Trans. Biomed. Eng., vol. 45, no. 2, pp. 151–159,Feb. 1998.

[10] J. H. Shan, M. Mei, L. Sun, D. Y. Kong, Z. Y. Zhang, L. Ni, M. Meng,and J. R. Chu, “The design and fabrication of a flexible three-dimensionalforce sensor skin,” in Proc. IEEE/RSJ Int. Conf. Intell. Robots and Syst.,Edmonton, AB, Canada, Aug. 2–6, 2005, pp. 1818–1823.

[11] F. Jiang, G.-B. Lee, Y.-C. Tai, and C.-M. Ho, “A flexible micromachined-based shear-stress sensor array and its applications to separation-pointdetection,” Sens. Actuators A, Phys., vol. 79, no. 3, pp. 194–203,Feb. 2000.

[12] H.-K. Lee, S.-I. Chang, S.-J. Kim, K.-S. Yun, E. Yoon, and K.-H. Kim,“A modular expandable tactile sensor using flexible polymer,” in Proc.Int. Conf. Micro Electro Mech. Syst., Miami, FL, Jan. 30–Feb. 3, 2005,pp. 642–645.

[13] R. F. Santos, R. F. Rocha, S. Lanceros-Mendez, C. Santos, andJ. G. Rocha, “3 axis capacitive tactile sensor,” in Proc. IEEE ISIE,Dubrovnik, Croatia, Jun. 20–23, 2005, pp. 1539–1544.

[14] J. Engel, J. Chen, and C. Liu, “Development of polyimide flexible tac-tile sensor skin,” J. Micromech. Microeng., vol. 13, no. 3, pp. 359–366,Feb. 2003.

[15] J.-S. Heo, J.-H. Chung, and J.-J. Lee, “Tactile sensor arrays usingfiber Bragg grating sensors,” Sens. Actuators A, Phys., vol. 126, no. 2,pp. 312–327, Feb. 2006.

[16] M. Ohka, H. Kobayashi, and Y. Mitsuya, “Sensing characteristics ofan optical three-axis tactile sensor mounted on a multi-fingered robotichand,” in Proc. IEEE/RSJ Int. Conf. Intell. Robots and Syst., Edmonton,AB, Canada, Aug. 2–6, 2005, pp. 493–498.

[17] E. S. Kolesar and C. S. Dyson, “Object imaging with a piezo-electric robotic tactile sensor,” J. Microelectromech. Syst., vol. 4, no. 2,pp. 87–96, Jun. 1995.

[18] K. L. Johnson, Contact Mechanics, 4th ed. Cambridge, U.K.: CambridgeUniv. Press, 1994, ch. 3.

[19] S. Ito and H. Kawasaki, “A standing posture control based on groundreaction force,” in Proc. IEEE/RSJ Int. Conf. Intell. Robots and Syst.,Nov. 2000, vol. 2, pp. 1340–1345.

Eun-Soo Hwang received the B.S. and M.S. degreesin mechanical engineering from Yonsei University,Seoul, Korea, in 2001 and 2003, respectively. Heis currently working toward the Ph.D. degree inthe Microsystems Laboratory, School of MechanicalEngineering, Yonsei University.

His research interests include fabrication andpackaging processes for flexible sensors, flexiblesensor arrays for sensitive skin, and sensor networksfor health monitoring using flexible sensors.

Jung-hoon Seo received the B.S. and M.S. degreesin mechanical engineering from Yonsei University,Seoul, Korea in 2005 and 2007, respectively.

He is currently with the Telecommunication andNetwork Division, Samsung Electronics, Suwon,Korea, as an Assistant Engineer.

Yong-Jun Kim (M’01) received the B.Eng. degree inelectrical engineering from Yonsei University, Seoul,Korea, in 1987, and the Ph.D. degree in electricalengineering from Georgia Institute of Technology,Atlanta, in 1997. His thesis work involved the ap-plication of polymer/metal multilayers to MEMS.

From 1996 to 2000, he was with Samsung Elec-tronics Company as a Senior Engineer and ProjectLeader, where he worked on electronic packagingand various MEMS devices. In 2000, he joined thefaculty of Yonsei University as a Professor in the

School of Mechanical Engineering. His current research interests include gen-eral micromachining, bio and environmental sensors, RF MEMS, and flexibleelectronic packaging.