An intelligent microactuator robust against disturbance using electro-rheological fluid

Sensors and Actuators A 175 (2012) 101–107

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

An intelligent microactuator robust against disturbance using electro-rheological fluid Kazuhiro Yoshida a,∗ , Kazuhito Kamiyama b , Joon-wan Kim a , Shinichi Yokota a a b

Precision and Intelligence Laboratory, Tokyo Institute of Technology, R2-42, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan Graduate School, Tokyo Institute of Technology, R2-42, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan

a r t i c l e

i n f o

Article history: Received 7 December 2010 Received in revised form 13 September 2011 Accepted 27 December 2011 Available online 3 January 2012 Keywords: Microactuator Intelligent actuator ERF (electro-rheological fluid) Functional fluids Position feedback ER valve

a b s t r a c t The paper presents a novel intelligent ER microactuator with inherent position feedback mechanism. The actuator consists of a variable ER valve and an upstream restrictor. The variable ER valve is composed of a pair of movable and fixed parallel plate electrodes with variable gap length and is supplied with an ERF (electro-rheological fluid) whose apparent viscosity changes according to the applied electric field. The movement of the movable electrode is used as the output displacement. By applying voltage to the variable ER valve, its pressure drop increases due to the ERF’s apparent viscosity increase, so that the electrode gap length, i.e., the output displacement increases. Also, the actuator can suppress the displacement due to disturbance force with the inherent position feedback mechanism without additional sensors and controllers. The mechanism utilizes the pressure drop change of the variable ER valve with the changes of the electrode gap length and the ERF’s apparent viscosity due to the disturbance force. The feature of the proposed actuator is robustness against disturbance with simple and compact structure. In this study, the structure and working principle were revealed and the mathematical model was derived. An actuator prototype having effective working part with 14 mm diameter was fabricated and the static and dynamic characteristics were experimentally clarified. The actuator stiffness was proved to be 4.5 times higher than the actuator without the inherent position feedback mechanism. Furthermore, a mechanism to magnify the output displacement using the control pressure was proposed and the validity was confirmed through experiments. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Servo actuators for position control systems need position sensors to measure the controlled variables and controllers. However, sensors and controllers require large size, which becomes a serious problem especially for microactuators. Hence, actuators having inherent position sensors and controllers have been required and they are called the intelligent actuators [1]. As functional components applicable to intelligent actuators, actuators having inherent position sensors have been developed. For example, there are commercially available electric motors having built-in rotary encoders as rotational displacement sensors. For welfare robots and so on, Kure et al. proposed and developed a servo system using an intelligent FMA (flexible microactuator) having a flexible displacement sensor [2]. The sensor is conductive paste deposited on the measured surface and changes its electric resistance according to the displacement. Hirose et al. proposed and developed a shape

∗ Corresponding author. Tel.: +81 45 924 5011; fax: +81 45 924 5977. E-mail addresses: [email protected] (K. Yoshida), [email protected] (J.-w. Kim), [email protected] (S. Yokota). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.12.049

memory alloy servo actuator for an active endoscope, which utilizes the electric resistance change due to the phase transition of the shape memory alloy as a displacement sensor function [3]. Toki Corporation manufactures commercially available shape memory alloy servo actuators using their electric resistance changes as displacement sensors, which are applicable to radio-controlled model planes [4]. Some other smart materials such as piezoelectric materials [5], giant magnetostrictive materials [6], ionic polymer–metal composites (IPMCs) [7] also have functions of not only actuation but also sensing and they can be applied to intelligent actuators. For multi-DOF systems such as robot hands, snake like robots, and so on, the signal connection among actuators, position sensors and a centralized controller is a serious problem. Hence, intelligent actuators are required. For example, Boulenger et al. developed an intelligent linear actuator for robot hands and so on, which transforms rotation of a DC motor to linear motion through a screw and nut mechanism and has force and position sensors and a controller [8]. Faudzi et al. proposed and developed an intelligent pneumatic cylinder for a distributed physical human–machine interaction device such as an intelligent chair tool [9]. The device has 36 intelligent pneumatic cylinders each of which has an optical encoder, a pressure sensor, an on/off valve and an electronic chip for control. In

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this paper, a novel intelligent actuator having sensor and controller inside is proposed using an ERF (electro-rheological fluid). The ERFs change remarkably and reversibly their apparent viscosity in millisecond order time when subjected to electric fields. The ERFs have two types: the particle and the homogeneous types [10]. The particle type ERF increases its apparent viscosity with the particle clusters due to the electric field. The rheological property is of the Bingham fluids with variable yield stress. A nematic liquid crystal, which is one of the homogeneous ERFs, increases its apparent viscosity with its rod-like molecules rotation due to the electric field. The rheological property is of the Newtonian fluids with variable viscosity [11]. The homogeneous ERF has no particles in it, so it can flow in narrow channels without clogging. Hence, the homogeneous ERF can be used in microchannels with low control voltage. In this study, a nematic liquid crystal is used. The ERF’s apparent viscosity and flow between a pair of parallel electrodes can be controlled with the applied voltage without mechanical sliding parts. Hence, the ERFs have been investigated to apply to ER dampers and ER brakes [12], ER clutches [13], ER valves [11,14,15] and ER actuators [10,16,17]. The ER actuators have two types; one is a combination of an ER clutch and an electric motor, and the other is a combination of an ER valve and a fluid power actuator. The former has been developed for force display devices [10] and so on. The ER clutch consists of a pair of parallel rotational electrodes and an ERF between them. It controls the transmission torque through the ERF with the applied voltage. Due to the low inertia of the output shaft of the ER clutch, the features of the actuator are high response and hence high performance of force display in virtual reality devices. Also, as the maximum rotational velocity is restricted to the constant one of the electric motor, this type ER actuator is a safe actuator applicable to human interface devices such as rehabilitation devices. The latter utilizes an ER valve which controls the ERF flow through a pair of fixed parallel electrodes with the applied voltage [11,14,15]. Yoshida et al. proposed and fabricated an ER microvalve mainly made of silicon using MEMS technologies and combined it with a polyimide-diaphragm microactuator [16]. Kim et al. fabricated flexible ER microactuator using flexible ER microvalve mainly made of a photoresist SU-8 using MEMS technologies [17]. The feature of such an ER microactuator is simple and miniaturizable structure. Furthermore, utilizing the functionality of the ERF, a simple and compact intelligent actuator will be realized in micro size. In this study, the latter type ER actuator was investigated. In this paper, by using the potential of ER devices mentioned above, a novel intelligent ER microactuator is proposed and developed, which can not only control the output displacement and force with the applied voltage but also suppress the displacement due to disturbance force using the inherent position feedback mechanism without additional sensors and controllers. The proposed actuator is called the DC (disturbance compensation) type ER microactuator. First, the structure and the working principle of the DC type ER microactuator are revealed and the mathematical model is derived. Second, a prototype is fabricated and the characteristics are experimentally investigated. Finally, a mechanism to magnify the output displacement is proposed and tested.

2. Proposition of DC type ER microactuator 2.1. DC type ER microactuator Fig. 1 shows a schematic of the DC type ER microactuator. The actuator consists of a variable ER valve and an upstream restrictor. The variable ER valve is composed of a pair of movable and fixed parallel plate electrodes with variable gap length. The electrodes

Fig. 1. Proposed DC type ER microactuator.

are disks and the ERF flows from the outside inlet ports to the central outlet port on the fixed electrode. The movable electrode is supported by an elastic element such as a bellows and moves in the vertical direction with keeping it parallel to the fixed electrode. An ERF is supplied as the working fluid at pressure Ps . When the applied voltage v increases, the control pressure pc increases due to the apparent viscosity increase of the ERF, so that the output displacement x increases. On the other hand, when the applied voltage v decreases, the output displacement x decreases. When disturbance force to increase the gap length is applied to the actuator, the electric field strength between the electrodes decreases for the constant applied voltage v, so that the apparent viscosity of the ERF decreases. The flow resistance of the variable ER valve decreases also with larger gap length. As a result, the control pressure pc and the output force f decrease. On the other hand, when disturbance force to decrease the gap length is applied to the actuator, the output force f increases. Those reactions mean that the actuator can suppress the displacement due to disturbance force using the variable ER valve as an inherent position sensor. The proposed DC type ER microactuator features robustness against disturbance with simple and compact structure without additional sensors and controllers. 2.2. Theoretical investigations Assuming the concentrate flow between disk electrodes to be steady isotropic flow at low velocity, the force fp due to pressure pc was derived as follows (see Appendix A): fp = Apc ,

  ⎧ D22 − D12  ⎪ 2 ⎪ A= D2 − , ⎪ ⎪ 4 2 ln(D2 /D1 ) ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

pc =

˛(E)

(X0 + x)

6 ˛ = ln 

3

D 2

D1

(1)

q, ,

where pc is the control pressure, q is the flow rate, A is the effective sectional area, D1 is the central outlet port diameter, D2 is the diameter of the fixed electrode effective area inside the inlet ports, X0 is the minimum gap length between the electrodes with a spacer, x is the displacement, and (E) is the apparent viscosity of the ERF at the electric field strength E. The minimum value of (E) without electric field is the base viscosity 0 and the ratio of the maximum to the minimum values of (E) is the ER effect index ER [16]. Based on the force balance among the force due to pressure fp , the spring force and the output force, the maximum output force

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Fig. 3. Block diagram of the DC type ER microactuator.

displacement is maximum, the parameters K1 and K2 were obtained as follows:

⎧ 2 ⎪ ⎨ K1 = 3˛ER 0 Ps R(X0 + xmax ) , 2

Fig. 2. Analyzed characteristics of the DC type ER microactuator.

⎪ ⎩

fmax for the maximum output displacement xmax was derived as follows: fmax =

˛0 APs RX03 (ER − 1) (˛0 + RX03 )(˛ER 0 + RX03 )

,

The actuator was proved to have a position feedback loop and the actuator stiffness Ka was derived as follows: F = K + AK1 . X

(2) 3. Fabrication and characterization of DC type ER microactuator prototype 3.1. Fabricated DC type ER microactuator prototype

K(˛0 + RX03 )

where R is the flow resistance of the upstream restrictor, K and L are the spring constant and the initial stretched length of the bellows, ˇ is a constant, Ps is the supply pressure, and qmax is the maximum supply flow rate. Based on the derived mathematical model, the maximum performance of the actuator was evaluated for the maximum output displacement xmax . The evaluated values were the maximum output force fmax and a maximum energy index. The maximum energy index is defined as xmax × fmax which is thought to be proportional to the maximum output energy. Fig. 2 shows the analyzed results assuming D1 = 1.0 mm, D2 = 8.8 mm, ˇ = 1.1, 0 = 24 mPa s, ER = 5, Ps = 200 kPa and qmax = 0.1 cm3 /s. It was found that the maximum energy index xmax × fmax becomes maximum at xmax = 41 ␮m. The output force variation ıf due to the control pressure variation ıpc and the displacement variation ıx can be expressed as follows: 1 (−ıf + Aıpc ). K

(3)

Based on the derived results in Section 2.2, the actuator prototype was designed and fabricated. Fig. 4 shows schematics of the fabricated variable ER valve. The size was 37 mm diameter and 7.5 mm thick and the effective working part was 14 mm diameter and 2 mm thick. The disk electrodes were divided by three parts with independent hydraulic circuits and the slant of the movable electrode was suppressed. The movable electrode was supported with a silicone rubber sheet instead of a bellows. The homogeneous ERF used here was a commercially available nematic liquid crystal (MLC-6457000, Merck Ltd., Japan) whose base viscosity 0 = 24 mPa s and ER effect index ER = 5.4 [16]. 3.2. Experiments The static characteristics were measured with supply pressure Ps = 200 kPa. The output displacement x under no load was measured with a laser displacement meter (measurement range ±5 mm, resolution 0.05 ␮m) and the control pressure pc (an average

Also, based on Eq. (1), with linearization at a driving point, the following equations were derived: pc =

˛(E)

Ps

(X0 + x)3 ((˛(E))/(X0 + x)3 ) + R

,

∴ ıpc = −K1 ıx + K2 ıE, (4)

where,

⎧ 3˛(E)Ps R(X0 + x)2 ⎪ ⎪ K1 = , ⎪ 2 ⎨ {˛(E) + R(X + x)3 } 0

⎪ ˛Ps R(X0 + x)3 ⎪ ⎪ ⎩ K2 =

˛(E) + R(X0 + x)3



(7)

Eq. (7) means the actuator has the actuator stiffness higher than the stiffness K without the position feedback loop and the actuator can suppress the displacement due to disturbance force.

˛0 APs R{ER X03 − (X0 + xmax )3 } ⎪ ⎪ ⎪ K=

, ⎪ ⎪ xmax (˛0 + RX03 ) ˛ER 0 + R(X0 + xmax )3 ⎪ ⎪ ⎪ ⎪ ⎪ ˛0 APs ⎪ ⎩L = ,

∴ ıx =

(6)

K2 = 0.

Ka = −

⎧ ˇxmax ⎪ X0 = , ⎪ 1/3 ⎪ ⎪ ER − 1 ⎪ ⎪ ⎪ ⎪ Ps ˛0 ⎪ ⎪ ⎪ ⎨ R = qmax − (X0 + xmax )3 ,

ıf = Aıpc − Kıx,

{˛ER 0 + R(X0 + xmax )3 }

(5)

d(E) . dE

The block diagram was obtained as in Fig. 3. The variables in capital letters are the Laplace transforms. When the output

Fig. 4. Fabricated variable ER valve of the DC type ER microactuator prototype.

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Fig. 6. Measured step responses of the DC type ER microactuator prototype.

4. Proposition of displacement magnification unit using control pressure Fig. 5. Measured static characteristics of the DC type ER microactuator prototype.

4.1. Proposed displacement magnification unit of three inlet port pressures) was measured with semiconductor type pressure transducers (measurement range 0–100 kPa). To eliminate the residual air bubbles in the actuator, the actuator was filled with ERF under vacuum condition [11]. Fig. 5(top) shows the measured results. The origin of the output displacement x is the value without supply pressure and voltage. It was found that the output displacement x has nonlinearity, however the hysteresis is small. The output displacement range was 52 ␮m. The output displacement x at v = 0 was designed to be zero, but the realized value was not zero due to the initial stretched length error of the silicone rubber sheet. The output force f was measured through the deflection of a brass beam (50 mm × 10 mm × 1.4 mm, spring constant 7.6 kN/m) for the output displacement x to be the value with zero voltage. Fig. 5(bottom) shows the measured results. The maximum output force fmax = 1.5 N was obtained. Based on the measured values and the mathematical model in Section 2.2, the parameters were identified. The solid lines in Fig. 5 show the simulated results of the output displacement x, the control pressure pc and the output force f. It was ascertained that the results agree well with the measured values, although the parameter ˛ and the effective sectional area A are 1.1 and 0.57 times larger than the original parameters, respectively. The differences between the identified and the analyzed parameters are due to the anisotropic flow from the three elliptic inlet holes with 67◦ angle width. The difference between the measured and the simulated output force f is thought be caused by the same reason. The actuator stiffness at the maximum output displacement was measured using the beam mentioned above at the applied voltage v = 800 V. As a result, Ka = 16 kN/m was obtained. The value is 4.5 times higher than the value without the position feedback loop and the validity of the proposed actuator was confirmed. The value of the mathematical model was 14 kN/m which agrees with the measured value. Fig. 6 shows the measured step responses of the actuator prototype. The identified time constants of the first order lag responses are also shown in Fig. 6. The step down responses were higher than the step up like the ER microvalves [15], which is due to the mechanism difference; for step up, the liquid crystal molecules form domains with the electric field; for step down, the domains collapse with the flow. As for the effect of the applied voltage amplitude, the step down responses had few differences but the step up responses decreased with higher voltage amplitude, which is like the ER microvalves [15]. Based on the obtained time constants, the bandwidth of 4 Hz or more was confirmed.

To extend application fields of the DC type ER microactuators, the output displacement range is required to enlarge. However, passive type displacement magnification units such as a lever mechanism have a problem of small output force. In this study, a displacement magnification unit utilizing the control pressure pc called the DMU was proposed as shown in Fig. 7. The DMU is a fluid power actuator like a bellows that is attached on the movable electrode and is connected to the control pressure pc port in the hydraulic circuit. The output displacement xa is the sum of the movable electrode displacement x and the bellows displacement xm . When the applied voltage v increases, the control pressure pc increases as in Section 2.1. So the gap length of the electrodes, the length of the DMU and hence the output displacement xa increase. On the other hand, when the applied voltage v decreases, the output displacement xa decreases. Hence, the output displacement xa can be controlled with the applied voltage v. When disturbance force to increase the output displacement xa is applied for the constant applied voltage v, the flow resistance of the variable ER valve decreases due to the large gap length and low electric field strength. So the control pressure pc and hence the output force fa decrease. On the other hand, when disturbance force to decrease the output displacement xa is applied, the output force fa increases. Hence, the actuator with DMU can suppress the displacement due to disturbance force.

Fig. 7. Proposed DC type ER microactuator with DMU.

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Fig. 8. Block diagram of the DC type ER microactuator with DMU.

Fig. 8 shows the block diagram of the actuator with DMU. Xm , Am and Km are the Laplace transform of the displacement, the effective sectional area and the spring constant of the bellows of the DMU. When the output displacement xa is maximum, Eq. (6) is satisfied. As can be seen in Fig. 8, the block diagram of the actuator with DMU has a position feedback loop. The output displacement xa under no load, the output force fa with zero displacement and the actuator stiffness Kam at the maximum output displacement were derived as follows: xa =

fa =



1+

Am K AKm

Fig. 10. Measured static characteristics of the DC type ER microactuator prototype with DMU.



xe ,

(8)

⎧

(A − Am )K ⎪ ⎨ 1− fe (A ≥ Am ) (K + Km )

⎪ ⎩ Am fe (A < Am )

,

(9)

A

Kam =

(K + AK1 )Km , K + Km + (A − Am )K1

(10)

where xe and fe are the output displacement and the output force of the actuator without DMU. Eqs. (8) and (9) show that the DMU can magnify the output displacement with small or zero loss of the output force with adequate parameters. 4.2. Experiments Fig. 9 shows a schematic of the fabricated DC type ER microactuator prototype with DMU. The DMU was 18 mm diameter and 2.7 mm thick and the effective working part was 14 mm diameter and 2 mm thick. The dimensions of the variable ER valve were the same as the one in Fig. 4. For the DMU to be thin structure, a diaphragm using silicone rubber sheet was used instead of a bellows. The static characteristics of the actuator prototype with DMU were measured using the same method as in Section 3.2. Fig. 10(top) shows the measured output displacement xa under

Fig. 11. Measured step responses of the DC type ER microactuator prototype with DMU.

no load, which is relative value to the one with zero voltage. Nonlinearity due to the characteristics of the ERF was found, however the hysteresis was small. The maximum output displacement range was 203 ␮m, which is 3.9 times larger than the actuator prototype without DMU. Fig. 10(bottom) shows the measured output force fa with zero displacement. The maximum output force 1.8 N was obtained, which is 1.2 times higher than the actuator prototype without DMU. The measured actuator stiffness at the maximum output displacement with the applied voltage v = 800 V was 6.3 kN/m, which is 39% of the actuator prototype without DMU due to the displacement magnification. Fig. 11 shows the measured step responses of the actuator prototype with DMU. Due to flow saturation, the responses were lower than those without DMU. Based on the identified time constants of the first order lag responses, the bandwidth was 0.2 Hz. 5. Conclusions In order to realize an intelligent microactuator robust against disturbance without additional sensors and controllers, the DC type ER microactuator was proposed and the basic investigations were conducted. The main results are summarized as follows:

Fig. 9. Fabricated DC type ER microactuator prototype with DMU.

(1) The DC type ER microactuator with inherent position feedback mechanism was proposed, which uses a variable ER valve and

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has robustness against disturbance with simple and compact structure without additional sensors and controllers. The basic characteristics were theoretically analyzed. (2) A DC type ER microactuator prototype whose effective working part was 14 mm diameter and 2 mm thick was fabricated. The static and dynamic characteristics were experimentally clarified and the actuator stiffness 4.5 times higher than the actuator without the inherent position feedback mechanism was realized. Also, the validity of the mathematical model was confirmed. (3) A displacement magnification unit (DMU) using the control pressure was proposed and the basic characteristics of the DC type ER microactuator prototype with DMU were experimentally clarified. The validity of the DMU was confirmed. Acknowledgements A part of the research was supported by Grant-in-Aid for Scientific Research in Priority Areas, No. 16078205 of the Ministry of Education, Culture, Sports, Science and Technology of Japan. Appendix A. Derivation of Eq. (1) The concentrate flow between disk electrodes is assumed to be steady isotropic flow at low velocity as shown in Fig. A.1, where, D1 is the central outlet port diameter, D2 is the diameter of the fixed electrode effective area inside the inlet ports, X0 is the minimum gap length between the electrodes with a spacer, x is the displacement, pc is the control pressure, and q is the flow rate. The parallel annular plates with r + dr in outer diameter and r in inner diameter is modeled as a parallel rectangular plates with 2r in width, X0 + x in height, and dr in length. So, the differential pressure dp between r and r + dr is derived as follows: dp =

6(E) dr r(X0 + x)3

q,

(A.1)

where (E) is the apparent viscosity of the ERF at the electric field strength E. Based on Eq. (A.1), the pressure p(r) is derived as follows:



r

p(r) =

dp = r=D1 /2

6(E) (X0 + x)

ln 3

 2r D1

q.

(A.2)

Based on Eq. (A.2), the control pressure pc and the force fp due to pressure pc are derived as follows: pc = p



D 2

2

=

˛(E) (X0 + x)3

q,

(A.3)

D2 /2

fp =

p(r)2r dr = Apc , D1 /2

(A.4)

where,

⎧ D 6 2 ⎪ ⎪ ⎨ ˛ =  ln D1 ,   D22 − D12  ⎪ 2 ⎪ ⎩ A = 4 D2 − 2 ln(D /D ) . 2

(A.5)

1

References [1] K. Suzumori, S. Wakimoto, Intelligent actuators for mechatronics with multidegrees of freedom – making mechatronic systems simple, smart and reliable, in: T. Higuchi, K. Suzumori (Eds.), Next-Generation Actuators Leading Breakthroughs, Springer-Verlag, London, 2010, pp. 165–176. [2] K. Kure, T. Kanda, K. Suzumori, S. Wakimoto, Flexible displacement sensor using injected conductive paste, Sensors and Actuators A 143 (2) (2008) 272–278. [3] S. Hirose, K. Ikuta, M. Tsukamoto, Development of a shape memory alloy actuator. Measurement of material characteristics and development of active endoscopes, Advanced Robotics 4 (1) (1990) 3–27. [4] http://www.toki.co.jp/biometal/english/contents.php. [5] P. Muralt, R.G. Polcawich, S. Trolier-McKinstry, Piezoelectric thin films for sensors, actuators, and energy harvesting, MRS Bulletin 34 (9) (2009) 658–664. [6] A.G. Olabi, A. Grunwald, Design and application of magnetostrictive materials, Materials and Design 29 (2) (2008) 469–483. [7] M. Shahinpoor, Y. Bar-Cohen, J.O. Simpson, J. Smith, Ionic polymer-metal composites (IPMCs) as biomimetic sensors, actuators and artificial muscles – a review, Smart Materials and Structures 7 (6) (1998) R15–R30. [8] Y. Boulenger, E. Krämer, H. Liu, N. Seitz, G. Hirzinger, An intelligent linear actuator and its control system, in: Proc. 2001 IEEE/ASME Int. Conf. on Advanced Intelligent Mechatronics, 2001, pp. 521–526. [9] A.A.M. Faudzi, K. Suzumori, S. Wakimoto, Development of an intelligent pneumatic cylinder for distributed physical human–machine interaction, Advanced Robotics 23 (1–2) (2009) 203–225. [10] J. Furusho, M. Sakaguchi, New actuators using ER fluid and their applications to force display devices in virtual reality and medical treatments, International Journal of Modern Physics B 13 (14–16) (1999) 2151–2159. [11] M. De Volder, K. Yoshida, S. Yokota, D. Reynaerts, The use of liquid crystals as electrorheological fluids in microsystems: model and measurements, Journal of Micromechanics and Microengineering 16 (3) (2006) 612–619. [12] For example: A. Khanicheh, D. Mintzopoulos, B. Weinberg, A.A. Tzika, C. Mavroidis, Evaluation of electrorheological fluid dampers for applications at 3-T MRI environment, IEEE/ASME Transactions on Mechatronics 13 (3) (2008) 286–294. [13] For example: K.P. Tan, R. Stanway, W.A. Bullough, Validation of dynamic torque response of an electrorheological (ER) clutch, Mechanical Systems and Signal Processing 20 (2) (2006) 463–492. [14] M. Kohl, Fluidic actuation by electrorheological microdevices, Mechatronics 10 (4–5) (2000) 583–594. [15] K. Yoshida, M. Kikuchi, J.-H. Park, S. Yokota, Fabrication of micro electrorheological valves (ER valves) by micromachining and experiments, Sensors and Actuators A 95 (2–3) (2002) 227–233. [16] K. Yoshida, J.-H. Park, H. Yano, S. Yokota, S. Yun, Study of valve-integrated microactuator using homogeneous electro-rheological fluid, Sensors and Materials 17 (3) (2005) 97–112. [17] J.-w. Kim, K. Yoshida, K. Kouda, S. Yokota, A flexible electro-rheological microvalve (FERV) based on SU-8 cantilever structures and its application to microactuators, Sensors and Actuators A 156 (2) (2009) 366–372.

Biographies Kazuhiro Yoshida received his B.E. degree in electrical engineering from Yokohama National University, Japan in 1984 and the M.E. and Ph.D. degrees in control engineering from Tokyo Institute of Technology, Japan in 1986 and 1989, respectively. From 1989 to 1996, he was a Research Associate at the Precision and Intelligence Laboratory, Tokyo Institute of Technology, Japan, where he is currently an Associate Professor. His research interests include micromachines using fluid power, functional fluids applications, and fluid power systems. He is a member of the Japan Society of Mechanical Engineers, the Japan Fluid Power System Society, IEEE, etc. Kazuhito Kamiyama received his B.E. degree in mechanical engineering from Waseda University, Japan in 2006 and the M.E. degree in mechano-micro engineering from Tokyo Institute of Technology, Japan in 2008. His research interest is on microactuators and ERFs.

Fig. A.1. Concentrate flow between disk electrodes.

Joon-wan Kim was born in Seoul, Korea in 1974. He attended the Pohang University of Science and Technology in the Department of Mechanical Engineering and received his degree of Bachelor of Science in 1999. He as a research student joined the Department of Precision Engineering, Graduate School of Engineering, the University of Tokyo in 1999 and entered the master course in 2000. He was awarded honor researches and received the degree of Master of Science in 2002. He gained his Ph.D. at the same department in 2005. From 2002 to 2005, he was also a Junior Research Associate at Materials Fabrication Laboratory, RIKEN. Since 2005, he has been an assistant professor at Precision and Intelligence Laboratory in

K. Yoshida et al. / Sensors and Actuators A 175 (2012) 101–107 Tokyo Institute of Technology. His current interests are micromachining and MEMS for functional fluid devices. Shinichi Yokota received his Ph.D. degree in mechanical engineering from Tokyo Institute of Technology in 1982. He was with the Research Laboratory of Precision Machinery and Electronics at Tokyo Institute of Technology as a Research Associate

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from 1975 to 1986. Since 1986, he has been with the Precision and Intelligent Laboratory also at Tokyo Institute of Technology, where he is currently a Full Professor, a president of the Japan Fluid Power System Society and a fellow of the Japan Society of Mechanical Engineers. His research interest is on micro actuators using functional fluids, fluid power mechatronics.