TRANSACTION ON CONTROL AND MECHANICAL SYSTEMS, VOL. 2, NO. 7, PP. 337-349, JUL., 2013.

Actuation Technologies for Humanoid Robots with Facial Expressions (HRwFE) Yonas Tadesse

Abstract: Actuators are key components for creating Humanoid Robots with Facial Expressions (HRwFE). HRwFE are those robots capable of demonstrating expressions utilizing artificial muscles, deformable skin, motion control systems and artificial intelligence. The main objective of this paper is to briefly assess various actuation technologies: electromagnetic, pneumatic, piezoelectric, shape memory alloy, conducting polymer actuators in terms of various performance indices and comparing the properties with muscles in human head. Research in HRwFE are getting momentum and there exist a strong derive to build them because they can potentially be used for various applications including health care, military, industry and household. For all kinds of humanoid robots, the selection or synthesis of actuators that have similar properties to muscles in biological head is challenging. This paper focuses on the issues mentioned above and presents comparative assessment of the actuation technologies focusing on humanoid head. Keywords: Humanoid Robots, Facial Expressions, Actuators, Artificial Muscles, Design, Head, Neck, Face. 1

1.

INTRODUCTION

Humanoid robots, robots designed in the form of human, are getting particular attention due to their applications to solve societal needs. To this end, a number of researchers across disciplines attempted to design humanoids using various types of actuation technologies. If we think about the overall design process of humanoids, it involves the standard procedures of machine design where the selection of components to meet desired needs taking the major part. However, the design of humanoids is not similar to the standard machine design in general sense. It rather involves cognitive aspect, biological components that is to be mimicked, system integration of actuators and sensors, artificial intelligence and other aspects. The ideal model for the design and development of a humanoid robot is the biological human. One must understand anatomy of human: the biomechanics of human, the muscle structure, the nervous system and articulations of the system. Attempts have been made to mimic human focusing on, torso, head, face, hands and legs. If we observe human beings in general, the two legs in humans helped to walk and climb stairs, freeing the hand to do something else. Even, the two legs also help each other and allow the human

to stand by one leg and balance. A well-developed brain and the entire anatomical form of a human made human being to be the most successful in utilization of natural resources than any other species. Therefore, it is advantageous to build humanoid with similar structure to that of a human including the capability of demonstrating internal emotions. There are some groups that argue that humanoid should resemble like a machine and others argue that the more a human it looks, the more friendly communication between a humanoid and human be created. Humanoids development really cross boundaries of disciplines and even raises the philosophical question whether the progress in the area affects human beings adversely or favorably which might often be debatable. For us, we are more concerned on how to recreate the behavior, action, capability and appearance of human to the machine-like robot for better human-robot interactions. A few prominent researchers across the globe tossed the problem and addressed various aspects by developing prototypes. A fundamental question still remains in various aspects to fully realize these systems. This paper will discuss the current trend of humanoid robots development and the choice of actuation technologies to help the research in the area. The state of the art actuators and artificial muscles technologies that are suitable for the design and development of humanoid robots will be briefly discussed. The paper is organized in various sections. In the first section, introduction and the background of Humanoid Robots with Facial Expressions (HRwFE) will be discussed. Next, a thorough review of actuation technologies will be presented. Then, natural muscle characteristics will be discussed focusing muscles in the face, jaw and neck. In the last section, comparative analysis on actuation technologies and summary will be presented.

2.

HUMANOID ROBOTS WITH FACIAL EXPRESSIONS (HRWFE)

With the notion of assisting human beings with sociable communication between a machine like autonomous systems and human, humanoid robots that have capabilities of demonstrating facial expressions were developed in a number of universities and institutions.

Mechanical Engineering Department, Eric Jonsson School of Engineering and Computer Science, The University of Texas at Dallas (UTD), 800 West Campbell Rd., Richardson, TX75080,USA ([email protected]) RECEIVED: 19, APR., 2013; REVISED: 19, JUL., 2013; ACCEPTED: 22, JUL., 2013; PUBLISHED: 29, JUL., 2013.

ISSN: 2345-234X

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Humanoid robots with facial expression can potentially be used to replace repetitive tasks of human. They can be used in schools as information personnel. They can be used as a media journalist that can read text easily showing some gestures like human. They can be used to assist users in a front desk as a robot in charge. They are also proven to be a therapeutic treatment for children with Autism. A number of benefits of humanoids with facial expressions will emerge in our society from elderly to infants care. The actual developments of these machines require understanding biological human muscle structure and how they mobilize the muscles to create facial expressions. It was shown in literatures that 268 voluntary muscles are responsible for creating facial expression in humans. Three primary muscle types are employed: (1) linear muscles which share common anchor points, (2) sheet muscles that run parallel to each other and activated together, and (3) sphincter muscles that contract at center point [1-2]. To replicate all the voluntary muscles on humanoid is a challenging issue and none of the humanoid heads developed so far duplicate all the facial muscle of biological head. Certain techniques are also required to replicate human facial expressions to humanoids using existing actuation technologies. For example, Facial Action Coding System (FACS) proposed by Ekman and Friesan in 1978, provides the correlation between facial deformation and the muscles involved. FACS defines 46 action units related to the anchor points pertaining to expression-related muscles [3]. Additionally, FACS explains 20 action units for some head movements and eye gazes. FACS was implemented on graphic workstation and is being used in development of talking head for animations and video games. The implementation of FACS in humanoid faces is dependent on platform of choice including the size of the robot head, the actuators type, the level of complexity, and therefore, require continuous assessment of actuation technologies. Some of the HRwFE prototypes developed are Albert HUBO [4-5], a collaborative effort between Korean Advanced Institute of Science and Technology, Automation and Robotics Research Institute and Hanson Robotics, USA. SAYA[6], Replee Q1 and Q2[7], from Japan; Robota[8], and ROMAN[9] from Europe. Albert-HUBO utilizes servo motors for face actuation (39 DOF) [4-5]. Lilly is also servo motor based HRwFE with embedded piezoelectric sensors in the skin focusing more on human-robot interaction (16 DOF) [10-11]. SAYA utilized Mckibben actuators (19 DOF)[6]. Repliee Q1 (31 DOF) and Q2 (42 DOF) are both full body humanoid robot with capability to show facial and hand motion using pneumatic actuators [7]. Applications of humanoids such as flute playing [12], as receptionist [13-14] and patient care[15] were described in the literature. A detailed description of challenges in developing facial expressive humanoid head with DC motor, shape memory alloy and McKibben has been provided in reference [16]. Shape memory alloy (SMA) actuators were also utilized for

human-size head with 210 mm height and 2.1 kg weight [17]. A number of realistic facial expressions and human-like movements were demonstrated. Piezoelectric motor based humanoid face has also been reported in [18], which had the drawback of complex driving circuit. The application of dielectric elastomer for artificial facial muscle has been presented by other researchers [19].

(a) [13]

(b) [15]

(c) [16]

(d) [16]

(e) [1-2]

(f)[54]

Fig. 1. Humanoid robots with Facial expressions (HRwFE) (a) piezoelectric, (b) servomotors with piezo sensors, (c) polymer based, (d) shape memory alloy based, (e) high DOF with servo motors and (f) realistic baby humanoid with servo motors.[(e) and (f) Courtesy of Hanson Robotics Inc.]

Scale was another factor to consider in humanoid development. A small humanoid robot that could be used for disabled children education was reported in [8], relying on the mechanics of upper body. The authors presented the design of a 23 DOF system, including 3 DOF spine, 7 DOF arms, 3 DOF eyes and a 3 DOF neck. There are many potential applications of small scale baby robot including child education, entertainment, and medical studies. Baby

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robots will help in understanding of human anatomy for medical personnel and provide support to the ongoing research in prosthesis. A comprehensive study that describes the development of a small scale baby humanoid robot including architecture of the head with structural and computational modeling, analysis on selection of appropriate muscle-like actuator were discussed in [20]. Basically, the facially expressive baby robot was 600 mm tall, with a head size of 140mm height, 90mm width and 110mm depth. For such small scale, contractile actuators such as shape memory alloys (SMAs) are appropriate choice for artificial facial muscles. SMA actuators are low profile, their overall size is small (~100 μm diameter), provide strain up to 4% and blocking stress greater than 200 MPa [21]. The high power to mass ratio of SMAs is advantageous in scaling up the total size and reducing the weight of humanoid head. Another benefit of SMAs is they do not generate acoustic and electromagnetic noise that might interfere with voice recognition system. In addition, SMAs are available commercially in large quantities. But SMAs are not the perfect actuators; they have some problems which will be discussed in the later sections. Figure 1 is an illustration of the HRwFE that were developing using various actuation technologies from our group or collaborators. Fig 1(a) is a humanoid robot actuated by piezoelectric motors; Fig 1(b) is based on servo motors, with embedded piezoelectric sensors that was used as sensors in the face; Fig1(c) is a polymer based humanoid head that was not fully developed but proposed as one of the better replacement of the existing humanoids; and Fig.1 (d) is a small scale baby HRwFR developed using shape memory alloy actuators. Fig.1 (e) is the highly expressive humanoid robot that was driven by several servo motors and Fig.1 (f) is a lovely baby HRwFE that was demonstrated recently by Hanson Robotics Inc.

3.

TYPES AND PROPERTIES OF MUSCLES IN HUMAN HEAD

It is advantageous to look into the detail of human anatomy to mimic the natural muscles and replicate in humanoids. This requires studying the muscles in the face, neck and jaw. A. Facial Muscles The natural muscles in the face are divided into four regions: the scalp, the eyelid, the nasal and the mouth as shown in Fig. 2(a). These muscles are activated by cranial nerve VII (Fig. 2(b)), the facial nerve which originates in the brainstem. Control of the facial musculature includes spontaneous control by the pons, and voluntary movements by the facial area of the motor cortex. The upper and lower face musculatures are controlled separately by specific brainstem nuclei. Skeletal muscles in the face are composed of fibers organized into motor units (MUs). The fibers can be classified as fast or slow twitch. Limited data on the fiber type found within facial muscles indicates that the fibers tend to be fast twitch. MUs contract when activated by their

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αMN (alpha motor units), and relax when the activation is removed. To effect facial expressions, the muscles work in a complex arrangement that is not readily described by a single muscle vector. This inherent complexity suggests that MUs within several muscles are activated simultaneously or in a complex pattern to generate forces to create an expression[22]. B. Jaw Muscles Human jaws are the critical element for facial expressions because the opening and closing of the jaw bring about a drastic change in the shape of the face. Human jaw consists of upper and lower sections known as maxillary and mandible bone. Jaw muscles include masseter, temporalis, medial pterygoid, and lateral pterygoid muscles which are used to elevate mandible. Lateral pterygoid and digastrics muscles are responsible for opening the mandible. The biomechanics of the mandible and the temporomandibular joint is complex. The natural arrangement of the joint and mandible geometry allows the mandible to move in six degrees of freedom. In most human being, lines drawn from the contact points of the mandibular central incisors to the condyle on either side form an equilateral triangle with 4 inches (102 mm) side length and known as Bonwill triangle. C. Neck Muscles Human neck has several vertebra discs, ligaments and muscles which help to hold, flex, and extend the head. There are two important muscles in the neck, known as trapezius and sternocleidomastoid muscles. The trapezius is a broad flat muscle which covers most of the back side of neck. The sternocleidomastoid muscle is a large thick bundle which is easily seen in front of the neck. This muscle travels diagonally from its origin behind the ears to its insertion on the sternum (breastbone). The sternocleidomastoid muscles divide the neck into two regions, anterior and posterior region. The posterior cervical compartment of biological neck includes cervical vertebrae and several deep muscles of the neck. Biological neck consists of seven cervical vertebrae. These vertebrae are connected with series of ligaments and joints (disc). Much of the extension and flexion movement of the head occur about the first cervical joint (C1), also known as atlas. In addition, due to the oval structure of the first cervical vertebra, only a forward and backward head rocking motion is allowed at this joint. The range of motion of this joint is approximated as the nodding gesture. The cervical vertebrae are stacked upon one another. The moveable joint has a fluid (synovial fluid) and it is through these joints that much of the neck movement is permitted. The seven cervical vertebrae of biological neck have different degree of freedom. The first one has one DOF and the remaining six have three DOFs each, but the angular motion of each is small. The overall range of motion is high because the neck is a long serial chain. In order to attain similar motion, a ball joint seems to be adequate but designing a drive mechanism for a ball joint is complicated. Rather, a simple kinematic

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joint 2R or 3R can provide satisfactory mechanism to mimic overall neck movement [11]. Table 1 summarizes the characteristics of natural muscles found in human head.

Actuator technologies for artificial muscles have been previously reviewed by Madden et al. [28], Kornbluh et al. [33] and Huber et al [34]. The actuators can be categorized depending upon the mechanism, and environmental operating conditions (dry or wet). The common actuation technologies include electromagnetic, electrostrictive, dielectric elastomer, liquid crystal elastomer, conducting polymer, carbon nanotube, ionic polymer metal composite, shape memory alloys (thermally activated, ferromagnetic), molecular and pneumatic actuators. A. Piezoelectric Actuators

(a)

(b)

[55]

[56]

©chastity/123rf.com

Courtesy of Patrick J. Lynch

(c)

(d)

[57]

Courtesy thexodirectory.com

Courtesy of earnestholistichealth.com

[58]

Fig. 2. Human head and neck muscles :(a) Facial muscles, (b) facial nerves, (c) jaw muscles and (d) neck muscles and structures. TABLE 1 CHARACTERISTICS OF HUMAN MUSCLES [23-31]

Parameter Fiber diameter

Work output

Value  10-100 µm (skeletal muscle)  28-45 µm ( facial muscle)  29-65 mm (facial)  Up to 100 mm, (facial)  230-610 µm2 (facial, cross sectional )  0.35 MPa ( skeletal)  0.007-0.8 MPa (general )  40 % (skeletal)  1-100 % (general )  0.18 – 40.57 [J/Kg]

Power output

 9-284 W/Kg

Strain rate Frequency

Life cycle

 5 %/s  5-30 Hz (slow), 30-65 Hz (fast twitch)  10-20 Hz (slow ),30-50 Hz (fast twitch)  2-173 Hz ( general )  >109 cycles (skeletal)

Efficiency

 35

Fiber length Fiber area Stress generation Strain

4.

The application piezoelectric actuators are limited in robotics as compared to others. Basically, piezoelectric actuators operate under the principle of reverse piezoelectric effect. The reverse piezoelectric effect is the ability of a piezoceramic to generate strain under the application of electric field. Usually the strain is small, but can be amplified by a certain mechanism such as moonie and cymbal design. A cymbal can provide higher displacement, stable under load and easier to fabricate. Also, the piezoelectric principles can be applied to create a rotary motor that serve as an actuator. A piezoelectric motor operates with a unique principle of simultaneous excitation of bending and longitudinal mode on a piezoceramic. The superposed excitation of the modes can create an elliptical motion on a piezoelectric element which can then be utilized to drive a linear stage and obtain unlimited linear/rotary motion. Ultrasonic motors have been used to a design of humanoid neck [35], and a facial expression in humanoid head Albert [18]. Piezoelectric actuators have fast response time, high stress and high resolution in displacement. These actuators can be easily miniaturized and still provide excellent control over the motion. Other advantages of the piezoelectric motor include: (i) it can store large energy (ii) no electromagnetic noise generation (iii) high efficiency, (iv) nonflammable, and (v) safe in overload and short circuit condition. Compared to magnetostrictive actuators and shape memory alloys, piezoelectric maintain stable operation over a prolonged period of time. However, the strain output is limited below 2%. B. Electromagnetic

COMMONLY USED ACTUATORS IN HUMANOID ROBOTS

As explained at the beginning of this paper, actuation technology is the critical factor that determines the capabilities and limitations of the humanoid robots.

This technology is widely used in robotic application due to its maturity (widely studied), ease of control, availability and a wide variety of size. Electromagnetic actuators could be AC or DC type depending on the excitation input. The principle of operation of these actuators is based on effect of Faraday's law. Such effects are utilized in a variety of ways to make electromagnetic based actuator. Many types of electromagnetic actuators exist including brush/brushless DC motor, AC motor, stepper motor, and RC servo. Most small motors can be controlled with variety of commercially available controllers which can generate pulse width modulation (PWM) signal. Variety of position sensors such as encoder, potentiometers, Hall Effect sensors are used for position

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feedback. The overall cost is cheap. For example, RC servo motor costs $20 or less, typical size of the motor is 30 x 12 x 30 mm (model HS81) and weighs 16 gm. Summary and typical values used in facial robotics are shown in table 2 (http://www.servocity.com/). TABLE 2 CHARACTERISTICS OF ELECTROMAGNETIC ACTUATORS (SERVOS MOTORS)

HS-81(micro servo) Dimension Weight Stall Torque (4.8/ 6.0V) Operating Speed at no load (4.8/6V) Current Drain (4.8V) Current Drain ( 6V) Power input[W] Price Efficiency of moving coil Frequency[Hz] Moving coil transducer frequency [Hz] Frequency of servos [Hz]

Dead band width Life cycle

29.8 x 12 x 29.6 mm 16.6gm 2.6 /3 kg.cm

HS-55 ( submicron servo) 22.8 x 11.6 x 24 mm 8 gm 1.1/1.3 kg-cm

0.11/0.09 sec/600

0.17 /0.14 sec/600

8.8mA/idle and 220mA no load. 9.1mA/idle and 280mA no load 1.68W

5.4mA/idle and 150mA no load 5.5mA/idle and 180mA no load 1.08W

$ 17.49 0.50-0.80

$13.99 [34]

1.- 2.0 50-400** 2x104- 5 x104

( AC servo motor)* [34]

-30-50(Analog servo) ***

- 300 to 400 (Digital servos) 8µsec 300,000**

*

By U.A.Bakshi, V.U.Bakshi Electrical Circuits And Machines. ** http://www.societyofrobots.com/actuators_servos.shtml *** http://pcbheaven.com/wikipages/How_RC_Servos_Works/

TABLE 3 CHARACTERISTICS OF PNEUMATIC ACTUATORS [34-38]

Parameter Geometry[mm] Force [N] Strain [%] Density[kg/m3] Stress [MPa] Efficiency[-] Frequency [Hz] Power density [W/m3] Applied Voltage[V] Life cycle

Value L= 1780,D= 70 mm 2500 35, 10-100 180-250 0.5-0.9 0.30-0.40 5x101– 3x102, 16 5x106 14,000

C. Pneumatic actuators Air muscle actuators are becoming important in the robotics since their introduction in 1980s. Pneumatic air muscles (PMAs) use compressed air to create contraction in their thin flexible reinforced membrane. PMAs usually appear as a pair of agonist and antagonist configuration to

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perform both contraction and relaxation. The advantages in selecting air muscles are flexibility, light weight, smooth movement, damped motion and high power to weight ratio compared to DC motor. Whereas, the drawbacks of PMAs are the requirement of extra compressor and solenoid valves to operate; difficulty in control especially for long tubes due to pressure drop; and noise generated due to pressure flow and the nonlinear response characteristics. Some properties of pneumatic muscle actuators are given in table 3.

D. Shape memory alloy This actuation technology is made of an alloy of metals such as nickel-titanium, copper-zinc, and silver-cadmium alloys at certain compositions. Shape memory alloys are smart material that exhibit shape memory effect and superelastic effect. Shape memory effect is the recovery of original shape from deformed shape with residual strain through thermal cycling. The material remembers its original shape and return back to its shape when heated. The advantages of SMAs include: (i) low profile typically, 100-500 µm in diameter, (ii) high force to weight ratio, (iii) uses simple current drive, and (iv)the operation is silent. Despite these facts, SMAs have limited application due to their low operational frequency and narrow bandwidth. The limitation is due to the time required for heating and cooling of the actuators. The maximum operating frequency of SMA is less than 10 Hz. In this regards, characterization of SMA’s up to 100Hz showed detectable force response and determined a firsts order transfer function of SMA[39]. In [21], a 100 micron SMA were able to operate at 0.55 Hz without losing the force generation capacity and strain. Toki Corp described that BMF 100 Bimetal can operate at 3 Hz, (http://www.toki.co.jp/). Bergamasco et al., 1989, illustrated that fluid cooling a 0.5 mm SMA operated at 2 HZ[40]. Howe et al, showed pneumatic cooling a 75 micron and 30 mm long operated around 6-7 Hz frequency[41]. The response time of SMA actuator is also dependent on preloading stress, loading condition and amplitude of activation potential. Commercially available SMA exhibits various behaviors. For instance, comparing Flexinol (Dynalloy Inc.) and Biometal fiber (Toki Corporation), Flexinol force generation rate is of the order of 1 – 7 MPa/s in the pre-stress range of 0 – 160 MPa whereas for Biometal fiber the force generation rate is 10 – 80 MPa/sec in the pre-stress range of 0 – 320 MPa[21]. Modeling and control of SMAs are not that easy as compared to electromagnetic actuators. The properties of the actuator are dependent on fraction of martensite and vary as the internal temperature states changes in time. Even, the fraction martensite is a function of four transition temperatures which can be obtained from kinetic law. The four characteristic temperature, martensite start, martensite finish, austenite start, and Austenite finish are dependent on the stress level. The behaviour of SMAs can be described by the enhanced phenomenological model[42] which are dependent upon the stress, temperature and the rate form of stress and temperature in the SMA.

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TABLE 4 CHARACTERISTICS OF SHAPE MEMORY ALLOY ACTUATORS Commercially available SMA

Flexinol (Dynalloy, Inc.) Sample1 Sample 2

Diameter[ µm] Length [mm] Max Force [gm] Max Stress[MPa] Recommended power [W/m] Current [mA] Voltage per length [V/m ] Power input (Pi =V*I) [W] Resistance[Ω/m] Operating frequency without losing the force generation & strain[Hz] Max frequency[Hz] Power output [mW] (Po= x*f*F) Power output [mW] (Po= x*f*F)@ max freq. Transformation temperature [0C] Density [gm/cm3] Weight [gm] Strain [% ] Specific heat [Cal/mol ℃] Life Category Bandwidth Frequency [Hz]

D = 100, D = 127 L = 100 L = 100 F = 150 F = 230 σ =187 σ =178 P = 4.86 P = 4.4 I = 180 I = 250 V = 27 V = 17.6 Pi1= 0.486 Pi2= 0.44 R = 150 R = 70 f = 0.18 , f = 0.25, 6 -7 *, 100 ** Po1= 1.1 Po2= 2.3 Po1= 41.2 Po2= 63.1 90 ρ = 6.45 W1=0.005 W2=0.008 3- 5 6-8 Ni-Ti alloy 1.32 rad/sec (0.21 Hz) 2x10-2 – 7x100

permittivity of free space and the square of the electric field.

Biometal fiber BMF100 (Toki Corp.) D = 100 L= 100 F = 70 † σ =87.4 P = 5.4 I = 200 V = 27 Pi3= 0.54

TABLE 5 CHARACTERISTICS OF DIELECTRIC ELASTOMER ACTUATORS [28] Silicone 3.2 120 1100 80%max 15% typical 12% , 1400 Hz

Stress [MPa] Strain [-] Density[kg/m3] Efficiency frequency Frequency max BW, -3dB Peak power [W/kg] Continuous power [W/kg] Life

R= 135 f = 0.55 3 Po3= 8.3 Po3= 47.0 70 ρ = 6.37 W3=0.005 4 6.1 -8 106 < Ni-Ti alloy

[34]

†

The output the vendor specifies (The force generated at any level of initial stress). It was also experimentally measured[21]. * Howe et al , pneumatic cooling, 75 µm diameter and 30 mm long, 900C transition temperature , 6-7 Hz frequency[41]. ** Teh and Featherstone, Flexinol diameter 75, 100 and 125 µm, up to 100 Hz and the response was around -35 db[39]. *** It was mentioned that efficiency of SMA is less than 0.1 % but fuel power could reach 3%. For efficiency, it is better to use hot fluid than electric heating [43]. In other scenario, the maximum SMA efficiency was described as 10%[44].

E. Electroactive Polymers (EAP) Electro active polymer is a class of smart material that includes polymer as the main constituent element. Conducting polymer (conjugate polymer), ionic polymer metal composite (IPMC)[45], dielectric elastomer, carbon nanotube (CNT)[46] are categorized under EAP. Their operation is either electronic or ionic charge transfer that results in volume change of the materials. 1) Dielectric elastomer Dielectric elastomer is an electroactive polymer made out of polymer film sandwiched between two electrodes. When potential is applied on the top and bottom electrode, electrostatic force induces a change in dimension. Maxwell’s governing equations are commonly utilized to model dielectric elastomer actuators. Essentially, the stress generated is proportional to the relative dielectric constant,

500

VHB 7.7 380 960 90 max 30 typical 2.3% 100 Hz, modified >50 kHz 10, 100 3600 @ 1.6MPa load 400

>107 @5% strain 106 @10% strain

>107 @5% strain 106 @50% strain

>50 kHz 1400 5000 @ 300kPa load

2) Polypyrrole actuator The principle of operation of polypyrrole conducting polymer actuator is based on diffusion of ions in and out of the polymer when a potential is applied between the actuator terminal and a counter electrode. The chemical redox reaction allows the flow ions and results in a volume change of the polymer. The actuator can be synthesized from a pyrrole monomer in the presence of large dopant ion using electrochemical deposition. Conducting polymer based artificial muscles were studied by researchers due to the large strain, low operating voltage ( 0.9 (12)

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DISCUSSION

It is important to consider the performance indices before selection of appropriate actuation technology for humanoids. Pneumatic and electromagnetic actuators have been widely used to develop humanoid body parts. What matters most is the main objective of using the particular actuation technology. If actuating a large mass is the prime objective, at higher speed and with good efficiency pneumatic are preferred. However, the efficiency, calculation of pneumatic actuator doesn’t take into account the compressor. The problem is that many authors do not present all parameters of the actuator and is difficult to present comparative table. In the case of HRwFE development SMA, conducting polymers, RC servo motors, piezoelectric, dielectric elastomers and pneumatics have been considered. The issues with these technologies are discussed below. SMA actuators is a promising candidate for facial muscles because of low profile, high force to weight ratio, simple current drive, and silent operation. But the disadvantages are low operational frequency, narrow

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bandwidth and high magnitude of driving current. Conducting polymer based actuators provide large strains (>20%) at low driving voltages (