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LabsHumanoids & Human Centred MechatronicsProjects ■ The Lower Body of iCub

The Lower Body of the Child Humanoid robot ‘iCub’

Anthropomorphic design, natural and adaptive locomotion and human like behavior and performance are some of the intrinsic features that have driven the rapid growth of humanoid robots during the past decade. The body development of such a humanoid platform that has the physical capacity of a human being poses many challenges from the mechatronic point of view. These must be addressed in a methodical and concurrent manner in order to co-ordinate and integrate the various components that form the full and complete mechatronic structure.

  • The 'iCub' platform has as its aim the replication of the physical and cognitive abilities of a 3½ year old baby. This "baby" robot will act in a cognitive scenario, performing the tasks useful to learning, interacting with the environment and humans.
  • The small (104cm tall), compact size (<23kg and fitting within the volume of a child) and high number (53) of degrees of freedom combined with the OPEN approach for research in cognitive development form fundamental differences from the many excellent humanoids already developed.

The assembly of the lower body
Within the Department of Advanced Robotics, the mechanical design and realization of the lower body of the 'iCub' child humanoid was evolved from the original design back in 2006 to its current version which is shown in Figure 1 together with an overview of lower body kinematics and location of the degrees of freedom. From the kinematic perspective the lower body includes the lower torso (housing the waist module) and the two leg assemblies. The height of the new 'iCub' lower body from the foot to the waist is 671mm, with a maximum width and depth (at the hips) of 176mm and 110mm respectively. The total lower body weight is 12.7kg with each leg weighing approximately 4.35kg and the waist section including the hip flexion motors weighing 4kg.

Figure 1 Kinematic configuration and the mechanical assembly of the "iCub" lower body.


The actuation group

The actuation solution adopted for the new lower body is the same with that adopted in the first prototype using a combination of a harmonic drives(CSG series, 100:1 ratio for all joints) and a Kollmorgen RBE series brushless frameless motor (BLM).

Figure 2 View of the iCub motor/gearbox actuator group.


iCub Waist

The iCub's waist was realized using a mechanism where the torque and power of the two actuators used for the upper body pitch and yaw motions is transferred to these two motions using a cable based differential mechanism as seen in Figure 3.

Figure 3 The mechanical design of the differential 3 D.O.F iCub waist.


iCub Leg
The leg has a more modular structure allowing for easy assembly and maintenance. In general the leg has an anthropomorphic kinematic form with 6DOF consisting of; the hip, the thigh with the knee joint, the calf with the ankle joint and the foot, Figure 4.

Figure 4 The mechanical realization of the "iCub" leg modules.

iCub Lower Body Mechanical Design


Coming next
Work towards the development of the new generation of the 'iCub' robot is already in progress. To enhance the iCub body we are currently investigating different mechanical configurations of compliant actuator components. They will be employed to upgrade and finally turn the 'iCub' robotic platform into the first humanoid robot with a full active/passive compliant body. The new 'iCub' will demonstrate superior robustness and high adaptability which will permit a wider range of safe experimentation and interaction scenarios.


  1. N.G.Tsagarakis, G.Metta, G.Sandini, D.Vernon, R.Beira, F.Becchi, L.Righetti, J.S.Victor, A.J. Ijspeert, M.C.Carrozza and D.G.Caldwell, "iCub -The Design and Realization of an Open Humanoid Platform for Cognitive and Neuroscience Research", International Journal of Advanced Robotics, Special Issue on "Robotic platforms for Research in Neuroscience", Vol.21, No.10, pp, 1151-1175, 2007.
  2. N.G. Tsagarakis, F. Becchi, L. Righetti, A. Ijspeert, and D.G. Caldwell. "Lower body realization of the baby humanoid - 'iCub'", Proc. of IEEE IROS pp. 3616-3622, 2007.
  3. N.G. Tsagarakis, M.D Sinclair, F. Becchi, Metta G, G Sandini, and D.G Caldwell, 'Lower body design of the 'iCub' a human-baby like crawling robot' , 6th IEEE-RAS Humanoids, pp. 450-455, 2006.
  4. W.Hinijosa, N.G.Tsagarakis, Martin Singlair, D.G.Caldwell, "Performance Assessment of a 3 DOF Differential Based Waist joint for the iCub Baby Humanoid Robot", IEEE 15th International Symposium on Robot and Human Interactive Communication, (RO-MAN), pp 195-201, 2006.

Last Updated on Tuesday, 06 September 2011 10:47


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