To achieve fast and flexible walking, a thorough design of the mechatronical system is essential. All components of the robot must be seen as tightly coupled parts of a highly integrated system.The humanoid robot LOLA is 180 cm tall and weighs approximately 55 kg. Its physical dimensions are based on anthropometric data. Fig. 1 shows a photograph of the robot. The distinguishing characteristics of LOLA are the redundant kinematic structure with 7-DoF legs, an extremely lightweight construction and a modular joint design using brushless motors. The mass distribution of the leg apparatus is improved to achieve good dynamic performance.
One of the most important questions regarding hardware design is how to choose the robot's kinematic structure in order to enable natural, stable and fast walking. Simulations and experiments have shown that additional redundant DoFs enable more natural and flexible walking patterns and extend the abilities of the robot in general. Fig. 2 shows the kinematic configuration with 25 actuated DoFs: the legs have 7 DoFs each, the pelvis has 2 and each arm 3 DoFs. A 3-DoF stereo camera head with pan and tilt axes and adjustable camera convergence angle is currently under development.
Besides the kinematic structure, further design goals can be defined to improve leg dynamics: (1) sufficient mechanical stiffness, (2) high center of mass, and (3) low moments of inertia of the leg links. To improve leg dynamics, new kinematic structures are developed for the knee and ankle joints, where heavier component parts (e.g. motors) are located close to the hip joint axis.
Since the robot's weight has a strong influence on global system dynamics, lightweight construction is of great importance. Our approach is to design major structural components as investment castings made from aluminum. In order to meet the weight and stiffness targets, design proposals are created by topology optimization. However, lightness of construction must be balanced with the demand for powerful drives necessary in order to achieve the desired torques and speeds at the required bandwidth. Therefore, actuator performance is increased carefully, using state-of-the-art motor, gear and sensor technology with high power density.
LOLA is controlled by an on-board PC mounted on the upper body and several local controllers carrying out low-level tasks, such as link position and velocity control, and sensor data processing. Joint controllers, sensors and the on-board PC form an intelligent sensor-actuator network with central controller. The new decentral components increase the system's performance from a technological point of view: similar to hierarchical structures in biological systems, sensor data are preprocessed decentrally and only relevant information forwarded to the central controller. Gait generation and stabilization run on the on-board electronics system without any support from outside except for power supply. An external PC is used only for monitoring purposes and to give basic operating commands if the robot is not connected to the vision system. Because of the high computational demand, image data processing is done on an external PC cluster.
