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Robustness, to resist the efforts suffered during its work-ing period use of standard mechanical componentsIn the following sections we describe the various possibili-ties considered for each component.A. Neck MechanismFor the Neck Mechanism, we have analyzed, developed andprototyped 3 different solutions (Figure 3).Fig. 3. Three alternative solutions for neck mechanism1) Spring Neck: Inspired by the exibility of the humanneck, one design concept included a spring, Figure 4, actuatedwith 3 cables separated 120o apart, producing a sphericalmotion of the head. Unfortunately, the center of rotation isvery dif cult to estimate because it depends on the springsize, elasticity and on the topology of the actuators. A nalmotor is included on top, assuming the function of atlas, forthe head pan. Another problem with this design is that motorswould have to be placed in the robot chest, jeopardizing thedesign modularity.Fig. 4. Prototype of robot head with spring-based neck.2) Parallel Neck: To be modular and self-contained, thehead structure must support a large number of mechanicaland electrical components. On the other hand, high torquemotors are required to drive the cameras, in particular toachieve the velocity of saccadic eye movements. So, to satisfyboth (con icting) requirements, an interesting solution forthe robot neck structure is based on a parallel mechanism.Parallel mechanisms have remarkable characteristics such ashigh precision, high load capacity, high rigidity, interior spacefor cabling and very easy solutions for the inverse kinematics.Also, since all motors are xed on the base, the inertia of themoving part is relatively small.Fig. 5. CAD Model of the Parallel Neck and prototypeIn this architecture, the platform and the base are joined bya passive spherical pair. The platform orientation is controlledby three legs of type UPS (U, P and S stand for universal joint,prismatic pair and spherical pair, respectively), the prismaticpair being the actuated joint. This parallel neck has theadvantage of being a three dof mechanism (spherical motion)and has the drawback of having reduced workspace becauseof the passive spherical pair [17], [18], [19].For such a small size neck (9cm 7cm), we had todesign our own linear actuators, with the mechanism movedby three Linear Ball Screw Actuators (Fig. 6). Comparing with other solutions, this type of actuators offers a goodcompromise between size, controllability and load capacity.The mechanical structure of these components is shown inFigure 6. One of the main disadvantages for this concept isthat it is very dif cult to avoid the interference between thevarious parts, for such large movements (motion range).
Also,the achieved velocities are slightly smaller than the originalspeci cations.Fig. 6. Ball Screw Linear actuator3) Serial Neck: The third solution tested was of a serialmanipulator (Figure 7) with three degrees of freedom, placedin a con guration that best represents human neck movements.We used DC micromotors (Faulhaber [20]) with planetarygearheads. In spite of its simplicity, the mechanism is veryrobust, easy to control and highly performing, meeting allthe speci cations. For these reasons, this was the nal choiceadopted for the iCub head neck.Fig. 7. CAD Model of Serial MechanismOne major requirement of this system is the resilience todamage of the robot units since the learning (cognition, under-standing, and behavior) of the robot will involve potentiallythe many falls and accidents experienced by any childlearning to cope with the world. This may be particularlycritical for the delicate head and neck mechanisms.To overcome this problem, each joint in the serial neckmechanism uses an overload clutch system (Figure 8). EachOverload Clutch System is essentially composed by a drivenFig. 8. Clutch based overload protection system.component, which is xed to the rotating part of the mecha-nism, a nut, a belleville spring and a clamp device, xed tothe motor shaft. When the belleville spring is compressed bythe nut, the driven component is smashed against the clampdevice, producing enough friction to transmit the movementof the joint. In an overload situation, this friction will not beenough and the driven component will slide, protecting themotor gearbox from the impact.The overload clutch system increases the robustness ofthe mechanism, giving it the possibility to fall on the oorand suffer different kind of impacts and efforts during itsinteraction with the external world.B. Eyes MechanismThe eyes mechanism has three degrees of freedom. Botheyes can pan (independently) and tilt (simultaneously). Thepan movement is driven by a belt system, with the motorbehind the eye ball. The eyes (common) tilt movement isactuated by a belt system placed in the middle of the twoeyes. Each belt system has a tension adjustment mechanism.Fig. 9. eye mechanismFor the necessary acceleration and speed, we have chosenFaulhaber DC micromotors, equipped with optical encodersand planetary gearheads. The nal available torque and speedare comfortably larger than the required speci cations.C. External CoverBecause this robot is designed to be engaged in socialinteraction, one of the main concerns in the design is the understanding of which facial features dimensions contributemore to the communication with humans. This research isimportant for the elds of human-computer interaction andthe impact of design on this eld has to be well understood.General dimensions of the different parts of the robot andthe total number of facial features are some examples thatin uence heavily the perception of human-ness in robots. Theexternal cover must also ensure the protection of the headmechanisms, absorbing the external efforts, suffered by therobot during operation. Figure 10 shows the rst prototype ofthe iCub face, where a toy-like concept was selected for thedesign.Fig. 10. Preliminary Design of the iCub face, integrated with the mechanism.IV. SENSORS AND ELECTRONICSAs an open physical platform for embodied research, thatcan be used by the research community from different typesof science elds (like physiology, cognitive robotics andperceptual science), the iCub mechatronics system cannot bevery complex. So, in order to guarantee easy assembly andmaintenance procedures, the mechanical system architectureis also completely modular, in such a way that we can removeand replace a certain module, without having to disassemblethe entire structure. Figure 11 shows the head with theintegration of the electronics/sensors.Fig. 11. Modular Architecture of the SystemTo allow the robot to interact with other people and to haveall desired behavior several sensors were applied. For vision,the main sensory modality, two DragonFly cameras [21] withVGA resolution and 30 fps speed. These cameras are very easyto integrate because the CCD sensor is mounted on a remotehead, connected to the electronics with a exible cable. In thisway, the sensor head is mounted in the ocular globe, while theelectronics are xed to a non-moving part of the eye-system.The inertial sensor is very important to have the vestibulo-ocular re ex and to detect the overall posture of the body. Wehave selected the from MTi sensor, from Xsens TechnologiesB.V. Several microphones are installed around the head to beable to locate the sound source of people in the surroundingarea.All motor control boards will be specially designed to tin the size constraints of the robot. They are all integratedin the head and connect to the remote computer with CAN-bus. To measure the head position (kinesthetic information),the motors have magnetic encoders, for calibration purposesand noting that the protection system drift in case of overloadcondition, absolute position sensors were applied to each neckjoint.Fig.
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