Aesthetic Design and Development of Humanoid Legged Robot

Mathew Schwartz,1 Soonwook Hwang,2 Yisoo Lee,2 Jongseok Won,2 Sanghyun Kim,2 and Jaeheung Park1,2

Abstract— This paper presents a new full sized humanoidleg robot that combines aesthetics and design theory withpractical research goals in robotics. The research goal of therobot is to create human-like and compliant motion in multiplecontact situations through the use of torque controlled joints.Low gear ratio and direct connections are used at each jointfor low-friction and back-drivability but without explicit jointtorque sensors. On the other hand, in creating human-likemotions, not only technical specifications but also aestheticdesign is important as the same performance of the robotcan be perceived very differently depending on the design.The aesthetic design is, in this paper, achieved by the robotdesign process using an integrated design and frame throughmulti-axis CNC machining. The unique integration of the frameand design also drastically reduces parts and complexity ofassembly for easy maintenance. In this paper, the design processand features are presented with range of motion, weight,and key aesthetic decisions. Compliant motion capability isdemonstrated by experimental results.

I. INTRODUCTION

A well designed object simultaneously takes into accountthe aesthetics, functionality, durability, and usability. Inrobotic design, this means a design should account for visualmeaning (aesthetics), the goal of the robot (functionality),how long it will last (durability), and how researchers willinteract with the robot (usability). The robot presented inthis paper is the result of these aspects considered together(Fig. 1).

There are many reasons for good aesthetic design withinthe humanoids field. The most beneficial is within socialrobots in which the appearance of the robot directly affectsa humans’ perception of its capability. In addition to thesocial aspect, both compliant motion and visual cues of therobots’ function are critical for safety when humans androbots coexist. In research of humanoid movement, at aminimum, robotic design must provide functionality of thestructure and usability in the way of accessing vital partseasily. The weight and durability of the robot are difficultto balance and require thorough planning in regards tomaterial and electronic selection, assembly, and proportion.As components wear or must be modified, the ability toquickly access vital electronics is necessary.

Aesthetics have an important role within the field ofmotion control. Challenges such as speed and diverse terrain

*This work was supported by the Advanced Institutes of ConvergenceTechnology(Grant AICT-2012-P3-21)

1Advanced Institutes of Convergence Technology, Seoul National Uni-versity, Republic of Korea umcadop@gmail.com

2Graduate School of Convergence Science and Technology, SeoulNational University, Republic of Korea. Jaeheung Park is the correspondingauthor. {jbs4104,howcan11,js1der,ggory15,park73}@snu.ac.kr

Fig. 1. DYROS (DYnamic RObotic System) Humanoid Leg Robot.

can be easily quantified, however, it remains difficult toquantify the human perception of how well the robot moves.In this aspect, the qualitative performance of a humanoids’walking style can be closely related to the research of socialrobotics. How much a humanoid looks like a human whilewalking can depend on the aesthetic design of the legs.

In the field of design, it is known that by just changingthe aesthetic of an object, the perceived usability will change[1]. Paolo Dario suggests in the field of personal robotics thatthe performance of a robot can be evaluated in the same wayas a home appliance [2]. As industrial robots have been de-signed largely for manufacturing facilities and with minimumthought of integration with humans, the knowledge gainedin these areas about design cannot be directly transferred[3]. In addition, [4] suggests the social robots which lookmechanical are not designed as a commercial product andinstead are designed for research. It is important to note thatthere may not be one specific design that succeeds aboveall in every category. As found in [5], the overall qualityof aesthetic design is more important than the closenessto anthropomorphic appearance. Similarly, [6] has found adifference in the acceptance of a robots’ design based on thetask the robot is performing.

2014 14th IEEE-RAS International Conference onHumanoid Robots (Humanoids)November 18-20, 2014. Madrid, Spain

978-1-4799-7174-9/14/$31.00 ©2014 IEEE 13

Advancements made in manufacturing, computer model-ing, material properties, and electronics over the past 40years have enabled researchers to develop more compactand able robots, such as the case of Honda P2 to P3 [7].However, while current CAD/CAM techniques allow formore complex three-dimensional parts, very few humanoidshave taken advantage of machining techniques beyond thethird axis. Many research robots use two-dimensional partsto assemble a three-dimensional structure or have used aminimum amount of three-dimensional machining and putcasings on the robot for a desired aesthetic. However, evenwith casings the robots rarely have an aesthetic design thatmoves beyond rectangular shapes.

Robots such as Saika-4 [8] and KHR-2 [9] used al-most entirely two-dimensional manufacturing. KHR-3 [10]continued to use two-dimensional manufacturing, with theexception of one part, while using coverings to create adesired aesthetic, which remained relatively flat. Bonobo [11]used a variety of three dimensional manufacturing techniqueswhich allowed the plastic coverings to act as support forthe frame. However, in the aesthetic aspect, from front andside views maintains a relatively planar design. One of themost arbitrarily curved robots has been the adaptation of theKUKA-DLR-Lightweight-Robots into humanoid legs [12].While this robot has few rectangular segments, the originaldesign was not for a humanoid leg.

Although there is a wide range of robot designs, it isnot feasible to describe them all. Some, such as [13] aretoo small and limited to discuss manufacturing challenges.Others, such as Petman [14] demonstrate an impressiveanthropomorphic humanoid that fits almost all aspects of therobot into a standard human size. However, as the intentionof Petman was for the use of textiles, it is difficult to comparethe robot casings designed for testing the textiles with designcasings of existing robots.

Full scale human proportions were considered alongsidethe motor selection. Through a CAD/RP/CAM process theaesthetic design of the robot was achieved through the frame,eliminating the need for casings. Multi-axis machining wasused in order to reduce structural components and createa unique and minimalistic design. The closeness to ananthropomorphic figure is not necessarily the gold standardand as such DYROS Humanoid was not designed to replicatea human but to reference the proportions. The aesthetics areimportant to the goal and was approached by an interdisci-plinary team of industrial design and engineering.

On the other hand, our robot is developed to create com-pliant and human-like motion in multi-contact situations. Thetorque-controlled robots can have an advantage in creatingthese motions over position-controlled robots. While jointtorque sensors can be used to create the torque-controlledjoints as in [12], [15], [16], we choose not to use thejoint torque sensors but to have low gear ratio and directconnection of motors to joints. This can provide low frictionand back-drivability at joints so that the motor torquesclosely match the joint torques after gear reduction.

This paper presents the design process and performance

Fig. 2. Diagram of the link lengths used in the robot based off of anaverage Korean female. The lengths are adjusted slightly for simplicity indesign and control. The joint order is listed on the left.

metrics of the design in terms of weight, strength, rangeof motion, visual cues, and accessibility. The experimentalresults demonstrate the performance of torque-controlledjoints during gravity compensation.

II. METHODOLOGY

A. Technical Concept

The robot is designed as a torque-controlled robot with 12DOF. A low reduction ratio of 50:1 is used for the motorswhich are directly connected to the joints. This gear ratioand direct connection are to provide minimum friction, aswell as providing good back-drivability for compliant motioncontrol without using joint torque sensors. A small motor sizeis chosen in order to maintain a thin leg. While the currentrobot consists only of legs, the technical specifications werechosen to include a full size upper body as well. The jointconnections are, in order: Hip yaw, Hip roll, Hip pitch, Kneepitch, Ankle pitch, Ankle roll.

B. Proportions

As the robot was developed in South Korea, the averageKorean female proportions [17] were taken as the baselinefor creating the full scale legs. Link lengths and joint orderlocations are seen in Fig. 2. A larger distance between thetwo legs was used in order to avoid collisions between thelegs as it is one of the most effective and easiest ways toovercome these collisions. There are two legs with 6 degreesof freedom each, 3 axis of rotation in the hip joint, 1 in theknee, and 2 in the ankle.

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C. Electronics

As a starting point for motor selection we based oursimulation robot on MAHRU [18]. The robot was simulatedin the physics based simulation software RoboticsLab [19].Contact consistent whole-body control framework was usedfor the robot control in the simulation [20]. The simulatedrobot weight was 71.295 kg: 40.245kg for the lower bodyand 30.05kg for the upper body.

Both squat motion and walking motion were simulated.The squat motion was controlled up to a 141 degree bend ofthe knee joint. Squatting time was simulated at 1 second.Table I shows the results of the squat simulation. The

TABLE ISQUAT MOTION SIMULATION RESULTS

Joint Peak torque(Nm)

Peak velocity(rad/sec)

RMS torque(Nm)

RMS velocity(rad/sec)

1 0 0 0 02 0 0 0 03 27.030 2.594 11.205 0.8324 182.816 4.916 85.094 2.4915 24.027 3.286 10.053 1.1356 0 0 0 0

forward walking motion was controlled with a speed upto 0.3m/sec. In the walking simulation the COM position,foot position and orientation, and trunk orientation werecontrolled in the task space. The double support time was0.3sec, single support time was 0.7sec, and the stride lengthwas 0.1, 0.2, and 0.3 meters, of which the largest RMS valuein all scenarios was taken. Simulation results of the walkingmotion are seen in Table II.

TABLE IIWALKING MOTION SIMULATION RESULTS

Joint Peak torque(Nm)

Peak velocity(rad/sec)

RMS torque(Nm)

RMS velocity(rad/sec)

1 53.944 0.811 16.869 0.1872 91.258 0.497 46.224 0.2523 178.823 2.536 68.671 1.1244 86.952 2.145 48.540 0.8735 114.165 2.043 30.220 0.7806 24.255 0.628 6.562 0.231

The motors are chosen as the ones in Table III such that thepeak torque in squat and walking simulation is approximatelytwo times the continuous torque and less than the peak torqueof each motor. All joints have a Kollmorgen motor andharmonic gear.

The upper body consists of a computer with an Intel I7-630m processor and 4Gbyte DDR3 RAM. The computerhas additional safety features to endure vibration while alsobeing compact. The computer runs roboticsLab as a realtimecontrol software. An AS5145 absolute encoder is connectedto the joint link and a RMB incremental encoder is connectedto the motor. In order to control each motor at the same time,the Elmo gold solo whistle digital servo drive was selected asthe motor driver. EtherCAT is used for fast communicationbetween the motor drive and computer. The robot has two

ATI DAQ MINI85 FT sensors located above the foot andbelow the ankle motor. An IMU 3DM-GX3-25 is located inthe upper housing with the computer.

D. Design Concept

After simulation to find the required motors, a simplis-tic box model was created with the proper placementsof the motors. A common method for humanoid designis in maintaining this relatively box-like shape and usinglightweight plastic casings to create the desired aesthetic.DYROS Humanoid was designed to integrate the frame andthe design. The exclusion of coverings and an open framedesign allow for more airflow in cooling the electronics.A combination of curved cylinders and plates are used tocreate a unique aesthetic while informing the observer ofthe intended human-like movement. Structural front platesact as a design component as well as a secondary heatsink.Through time invested in the multi-dimensional parts, boththe aesthetic and structural components can be unified.Additionally, the assembly time, complexity of the robot,and maintenance difficulty are reduced.

Fig. 3. Left: Initial design of upper link showing high stresses and specificweak points. Right: Revised design with thicker cylinder and integratedconnection points.

E. Stress Analysis

After completing an initial design, a CAD model wascreated and basic structural analysis was run in ANSYS. Astress analysis with a payload of 30kg as a static load withthe material set as AL7075 (yield strength of 503MPa) ina locked upright position gives a basic structural analysis.The largest stress recorded at that level was 3.1956MPa.To account for higher loads during walking, a static uprightanalysis with a 150kg load gives a safety factor of 5. Inthis scenario, the highest stress was 10.005MPa. The initialframe design passes with a safety ratio of 50.275. This safetyratio was higher than needed and allowed for modificationsto the structure and design. Fig. 3 shows the weak points of

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TABLE IIIJOINT AND MOTOR SPECIFICATIONS

Joint Hip Yaw (1) Hip Roll (2) Hip Pitch (3) Knee Pitch (4) Ankle Pitch (5) Ankle Roll (6)Max Cont. Output Power (W) 300 364 427 427 427 209Reduction Ratio 50 50 50 50 50 50Cont. Torque after reduction(Nm) 42.8 61.0 77.0 77.0 77.0 21.45Peak Torque after reduction(Nm) 152.0 231.0 307.5 307.5 307.5 76.5Speed @ 48V after reduction(rad/sec) 7.92 5.24 6.13 6.13 6.13 15.73

Fig. 4. Left: The first design iteration with two rotations showing an intersection of the link connections. Right: The revised design with a larger rangeof motion.

the design and the modification. The second design iterationunified the connection of the cylinders to evenly distributeloads and strengthened weak areas. The second design witha 30kg load showed a 5.7015MPa stress and the 150kg loadgave a 16.618MPa stress. The final safety factor is 30.268,above any future estimates.

TABLE IVLINK WEIGHTS BEFORE AND AFTER DESIGN ITERATION

Part 1st Design 2nd DesignUpper Link Plate 583.7(g) 402.46(g)Upper Link Cylinder 516.78(g) 767.82(g)Lower Link Plate 670.32(g) 515.22(g)Lower Link Cylinder 411.97(g) 410.15(g)Total: 2182.77(g) 2095.65(g)

F. Design Iteration

The links were given constraints in SolidWorks to viewthe range of motion. While the initial design of the structureprovided the desired range of motion on each individualaxis, a rotation on more than one axis at the same timeshowed interference by the link connections. The range ofmotion was extended by modifying some connections to themotors (Fig. 4). In line with the stress analysis, some partswere modified for their aesthetics as well as structure. Thecylinders on the upper link were thickened from 18mm to22mm to provide a stronger and more balanced visual weightto the solid front plate. The upper link plate was then reducedfrom 10.71mm to 7.99mm thick. The lower link cylindersremained almost the same, while the front plate was also

reduced. Table IV shows the second iteration was stronger,more aesthetically consistent, and decreased in weight.

G. Manufacturing

Many approaches to manufacturing humanoids in regardsto both material and process have been used. While [9] and[10] use almost entirely two-dimensional manufacturing pro-cesses, others such as [11] uses a combination of CAD/CAMtechniques to create molds and plastic parts. [8] uses A2017for the frame and [21] uses casting to create magnesium alloylinks. Due to our designs’ free form shape, either multi-axismachining or various casting methods were required, suchas the investment casting done in [22]. However, the generalrule of investment casting tolerance starting at +/- 0.010”for a part dimension of 1” without secondary operations[23] is outside the required tolerance for this application.For the links, a heat treated aluminum alloy (AL7075) wasused with multi-axis machining to 0.002” tolerance. Thefinal parts were anodized for aesthetics and durability. Twodesigns were manufactured for the foot, one with a curvethat can allow the robot to walk in a heel-toe manner, andone that is flat for initial stages of research. The curved footwas machined out of stainless steel for strength under theload of the robot while the flat foot was machined with thesame AL7075 as the links. The upper body was designed totemporarily hold electronic components with the intention ofreplacing it in the future. For parts with text written, a waterjet cutter was used.

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Fig. 5. Side and front view of the robot design with red highlights showingthe tangent line of the front plate and cylinder to the motor connections andblue line showing the protrusion of the upper link vs. lower link.

Fig. 6. Visualization of color combinations for the robot.

III. RESULT

A. Overview

The final design of the robot consisted of smooth con-nections both physically and visually. The flat mountingplates that connect the links directly to the motor follow thetangential lines created by the curve of the front plate andrear cylinders. From both the front and perspective view, theframe creates a fully three dimensional shape. The robot legtakes design influence from the human as the thigh musclein a human is larger than the calf. In this robot, however, thedirect connections of the motors prevent the actual structurefrom achieving the same proportions. For example, the twomotors directly connected at the ankle joint make the anklelarger than the knee. While the three planar joints on the sideview are equal in size, a combination of the thicker cylinder

Fig. 7. Two bolts are needed to detach the sidebar and easily changeelectronics or fix broken wires.

on the upper link and the larger protrusion of the upper platecreate a similar aesthetic to human proportions. Fig. 5 showsthe visual tangents in red, as well as the curvature of the robotin different views.

Through 3D design programs multiple color combinationswere visualized before anodizing the final parts (Fig. 6). Thetwo toned colors, black and red, were selected to create focalpoints on the elements of most interest such as the curvedplates and rear cylinders. The color combination creates aunified look of these separate pieces. The black coloring wasapplied to the motor casings to detract from their size. Ina practical manner, the brighter red color on the links isimportant for the visual understanding of how the links aremoving through space during gait.

B. Parts

Motors of the joints are encased in units directly attachedto the frame by screws. The complex parts minimizedconnections required in assembling the legs. The upper linkstructure is made up of 8 pieces and 27 screws, while thebottom consists of 10 pieces and 32 screws. Additionalscrews are used in the connection of the structure to themotors and encoder casings.

The total weight of the robot with the flat aluminum footis currently 54.635kg. However, 15.84kg is the temporaryupper body. The lower body is 38.795kg, slightly under theestimated 40kg used in the motor selection and simulation.Of this weight, 26.112kg are the structural components whilethe rest is made up of screws and electronics, such as themotor, making the integrated frame and design 67% of thelower leg weight. The stainless steel feet are 1.555 kg whilethe flat version in aluminum is 0.318 kg.

C. Accessibility

An important feature of the design is the easily accessiblemotor drivers as the electronics and wires are likely todegrade over time. The side bar is held in place by twoscrews as seen in Fig. 7. The removal of these two screwsprovides easy access to the motor drivers for the link.Additionally, the use of multi-axis machining allowed for

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Fig. 8. Schematic of upper link. The connection for the heatsink is partof the front plate. Three bolts hold the heatsink and electronics in place.

Fig. 9. Top: Flat foot for early research annotated with bolts, f/t sensor, andsteel plate. Bottom: Curved design for future research annotated with sideholes for mounting a plate. Right: Top view of curved foot design showing8 holes for quick changing of foot design.

the design to integrate the mounting bar for the heatsink.This bar reduced the number of extra brackets required tosecure the electronics. Both the thigh and shin have threeholes in which the electronic assembly is easily attached anddetached (Fig. 8). By optimizing the connections, the entirefront panel is able to act as a secondary heatsink for theelectronics.

The foot is attached to a steel plate that separates the footand the FT sensor. By removing the 6 bolts, the foot is easilyswitched. As the curved foot is designed for heel-toe rollingin the future, two threaded holes are available for mountinga flat plate to aid in stability at early stages of research.Fig. 9 shows the two designs and the configuration of theFT sensor, foot, and steel plate.

D. Range of Motion

The range of motion desired was that of a normal human.In the physical robot, the limiting factors for much of therange of motion is in the wiring and interference with theadjacent leg. Table V shows the structural range of motionas measured through computer modeling programs and theactual range of motion after assembly accounting for wire

interference as well as the adjacent leg.

TABLE VJOINT LIMITATIONS OF STRUCTURE AND ACTUAL

Joint Structural (Computer) Actual (Physical)Minimum Maximum Minimum Maximum

HipY(1) −∞ ∞ -47.3 46.5R(2) -109.95 109.95 -33.4 111P(3) -104.69 35.91 -103.1 31.6

Knee P(4) -101.74 130.94 -21.5 121.1

Ankle P(5) -89.43 40.01 -83.6 35.3R(6) -43.12 106.21 -42.5 67.8

E. ExperimentsThe back-drivability and compliant motion are demon-

strated by the experiment of gravity compensation. The robotstands on the right foot compensating for its own weight inFig. 10 (a).

(a) (b) (c)

Fig. 10. Snapshot of the experiment.

Then, a person held the left foot and moved it to a certainposition approximately at 1 second and then moved it backto the original position approximately at 7.5 seconds asshown in Fig. 10 (b) and (c). During the experiment, anotherperson held the body of the robot as there was no balancingcontroller, only gravity compensation. Fig. 11 shows theplots of data during the experiment. The values of x, y,z represent the position of the left foot and the measuredforce in Cartesian coordinates are denoted by Fx, Fy, andFz, respectively. The x, y and z directions correspond tothe Ventral, Lateral, and Cranial directions of the robot. Theforce sensors are used only to measure how much forces areapplied during the movement by the person. It was not usedfor force control.

From the experimental data in Fig. 11, it can be notedthat the required force to move the foot was from 10 to 40N. These values are related to static friction of the joints.As soon as the robot started to move, the joints were back-drivable and compliant to the movement of the person so thatthe person could move the left foot as desired. This resultdemonstrates the performance of compliant motion duringgravity compensation without using joint torque sensors.

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Fig. 11. Experimental data showing the force and foot location of themotors during gravity compensation.

IV. CONCLUSION

This paper presented a new humanoid design that utilizedmulti-axis machining to create an easily accessible robotwith an integrated design and frame. The robot is built withmotors directly connected to the joint with a low gear ratio.This configuration allows for compliant motion and goodback-drivability without using joint torque sensors. These aredemonstrated by the experimental results of gravity compen-sation. More complex multi-contact compliant motions are tobe implemented in the future.

The current design is for indoor robotics research focusingon control. However, with the current research into materialsand nano properties, we imagine a time when the openframe can still be applicable by combining hydrophobictreatments to the electronics and breathable fabric over thelinks to protect from water and dust. With the curvature ofthe frame, such an application would maintain the free-formaesthetic created in this robot. While this paper presents thelower body of a humanoid, the upper body is planned to bedeveloped as well.

ACKNOWLEDGMENT

The authors acknowledge U3 Robotics for their assistancein manufacturing and electronics.

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