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One point worth stressing is that the iCub platform isdesigned to serve as a research tool for embodied cognition,visuomotor coordination, and development. Hence, the projectconsortium includes teams of engineers as well as psychol-ogists, neuroscientists and biologists. The nal mechanicaldesign will be made available to researchers worldwide andreleased under a General Public License (GPL).II. MECHANISM SPECIFICATIONSIn this section we summarize some of the human anatomicaldata considered in the design and the assumptions we madeto transform these speci cations to our robot.A. Anatomical Data1) Neck: The human neck has a very complex muscular andskeletal system, with more than twenty muscles ([7], [8], [9],[10]) and ten bones ([7], [11]). The head has more complexityif we consider the mouth, the eyes and the facial expressions.The neck is constituted by seven vertebrae and the atlasthat supports the skull. The vertebrae can be modeled as ex-ible springs giving exion/extension and adduction/abductionmotion. The atlas bone gives the possibility of rotating thehead and an upper exion/extension movement. All thesebones are actuated by muscles in a differential way. For everymotion, agonistic and antagonistic muscles are used, eachhaving a exion, rotation and abduction function, e.g. thesternocleidomastoid muscle. Some of the muscles begin inthe spine while others continue through in the neck.The neck kinematic model has been the object of studiesin human biomechanics, for the analysis of injuries caused byimpacts, sports training, etc. The most standard model of thehuman neck has four degrees of freedom ([12], [13], [14]).2) Eyes: Each human eye has six muscles. As it is aglobe inside a socket three motions can be considered, abduc-tion/adduction, elevation/depression and rotation. The muscleshave combined actions to achieve these motions, as describedin Table I. Each eye is completely independent of the other.Muscle ActionSuperior rectus Elevates, adducts, and rotates eyeball mediallyInferior rectus Depresses, adducts, and rotates eyeball laterallyLateral rectus Abducts eyeballMedial rectus Adducts eyeballSuperior oblique Abducts, depresses, and medially rotates eyeballInferior oblique Abducts, elevates, and laterally rotates eyeballTABLE IEYE MUSCLES ACTION. (FROM www.eyegk.com)The human oculomotor system combines several basicmovements: saccades, smooth pursuit, vergence, vestibulo-ocular re ex, optokinetic re ex, microsaccades and accom-modation.Saccadic and smooth pursuit eye movements occur whenthe eyes pursue an object. During smooth pursuit, the eye triesto match the (angular) speed of the tracked target, usually atrelatively low speeds (up to 30o=s). Saccadic eye movementsare high speed jumping movements, in the range of a fewhundreds degrees per second.
The saccadic eye movementoccurs when the eye ball movement is not able to pursue anobject or when the human searches outside of the view.Apparently, the role of the microsaccadic movements isto ensure that the photo-sensitive cells are stimulated in apersistent manner. The accommodation re ex is responsiblefor focusing. In the arti cial system, these two behaviors are implemented at the camera level. The vestibulo-ocularbehavior is responsible for stabilizing the image when thehead makes very fast motions, relying on estimates of angularacceleration from the vestibular system.Table II presents the range of movements for each joint,[15] and the eye/neck speed for human adults during saccadicmovements with 2.5, 5, 10, 40, 60 degrees amplitude. Thesaccade speed increases with the motion amplitude. Hence,the speed during small amplitude saccades resemble thoseof smooth pursuit. These data also show how the effort ispided amongst eye and neck degrees of freedom when someredundancy exists (e.g. eye and neck pan movements). Thisinformation has been used for the design of the iCub headspeci cations.Adult values Range Velocity AccelerationHead weight: 4.5 5Kg [0] [0=s] [0=s2]min max min maxeyes pan 90 166 850 16000 82000tilt 80neck pan 110 23 352 330 3300tilt 90swing 80Neck/eye (pan) ratio 14% 41 % 2% 4%TABLE IIANTHROPOMORPHIC DATA ([16], [15]), SHOWING THE MOTIONAMPLITUDE AND SPEED FOR SOME DEGREES OF FREEDOM IN THE HUMANEYE/NECK SYSTEM.B. Robot Head Speci cationsThe initial speci cations for the iCub are quite demanding.The total head weight should not exceed 1.5 Kg and the size isthat of a 2 year old child. The head is 13.6 cm wide, 17cm longand 17.3cm deep. The neck is 7cm wide and 9cm long. Formodularity, we pide the head in the neck and eye subsystems.In order to guarantee a good representation of the humanmovements, the iCub head contains a total of 6 DOFs:neck pan, tilt and swing and eye pan (independent) and tilt(common) as shown in Figure 2. Facial may be includedin a later stage. We are studying the use of a minimal setof facial expressions (implying the smallest possible numberof motors or moving parts) to convey information about therobot's emotional status.Although the human neck has four dof, the upper neck ex-ion/extension was ignored because of space/weight limitations.For most tasks this motion is not necessary, since orientingthe eyes toward a scene point requires only two degrees offreedom. There is still some redundancy in the neck to avoidobstacles, occluders or to choose better viewpoints.The eyes cyclotorsion was ignored because it is not usefulfor control, and similar image rotations are easily produced bysoftware. The elevation/depression from both eyes is alwaysthe same in humans, in spite of the existence of independentmuscles. Similarly, a single actuator is used for the roboteyes elevation (tilt). Eye vergence is ensured by independentmotors. Figure 2 shows the nal chosen kinematics, that allowsall basic ocular movements.Fig. 2. Illustration of the Head degrees of freedom. There is a total of sixdegrees of freedom, three for the neck and three for the eye system (facialexpressions are not included).One of the most critical design steps is that of de ning thedesired velocities and accelerations for the various joints. Thiswill directly impact on the choice of motors and, therefore, onsize/weight constraints.Data regarding accelerations, velocities and joint range ofthe oculomotor system of human babies are not available,and very few studies exist in the literature of psychologyor physiology. This section explains how we determined theanthropomorphic data and speci cations for the iCub robothead. Overall, the iCub dimensions are those of a two-year-old human child, and it is supposed to perform tasks similarto those performed by human children.In Table II there are two key observations for the iCub headdesign. First, we will use the smaller range of saccadic speedsas a reference, since (i) these are adult data and children havesigni cantly smaller speeds and (ii) small amplitude saccadesare close to smooth pursuit movements, which are far morefrequent during the robots normal operation. Secondly, we willuse the ratio between neck/eye velocity (14% 41%) andacceleration (2% 4%) as an important design parameter.Using this information, and hypothesizing a trapezoidalmotion pro le for the eye movements (as axis control boardsusually specify), we can compute all joint accelerations, asshown in Table III.Max AccelerationRange Vel. Full range[0] [0=s] acc.[0=s2] Ttot [s] mean veleyes pan 90 180 1440 0.625 144tilt 80 160 1280 0.625 128neck pan 110 90 295 1.528 72tilt 90 73.6 241 1.528 59swing 80 65.5 214 1.528 52TABLE IIICOMPUTED SET OF ANGULAR SPEED AND ACCELERATION FOR THEVARIOUS DEGREES OF FREEDOM OF THE ROBOT CUB HEAD.We set the eye pan maximum speed to 180 0=s and assumed that the neck pan reaches half that speed. We further considerthat 20% of the time is used for acceleration and another20% for slowing down. The remaining part of the trajectoryis executed at maximum speed. In addition we considered thatthe neck tilt and swing dofs would take the same time as theneck pan to move its entire motion range. These speci cationswere used for the head design.III. MECHANICAL DESIGNThe Mechanical Design of the iCub head is pided in threemajor parts: Neck Mechanism, Eyes Mechanism and Cover(face). During the design process, we used the speci cationsderived previously and adopted the following desirable char-acteristics/criteria: DOFs., range of motion, joint speed and torque accordingto detailed speci cations. Compactness and weight, to meet all the desired speci -cations (< 1:5Kg), Modularity and simplicity of the structure to facilitatemaintenance and assembly, Self-contained to facilitate integration with the other partsof the robot,