Control Architecture and Algorithms of the Anthropomorphic Biped Robot Bip2000
Christine Azevedo and the BIP team INRIA - 655 Avenue de l’Europe 38330 Montbonnot, France
ABSTRACT INRIA  and LMS  have designed and realized an anthropomorphic legged robot, BIP2000 (fig.1). A planar version achieves walking, and the whole robot is able to keep its balance on one foot while moving. The purpose of this paper is to present the principles and the architecture of the robot control we have used. After having presented the robotic system, and the software architecture, we will detail the principles of the robot control. We will finally present implementation issues and experimental results. ...view middle of the document...
Sagittal and Coronal Planes
1.2 Actuation and Transmission The actuators are brushless DC motors. Five joints are equipped with classical harmonic-drive gears (rotations z6,z7,z8,z9,z15 in fig.3). The other joints transmitters are screw-nuts with satellite rollers combined with rod-crank systems (fig.5). The use of this system gives high reversibility and variable mechanical advantage (ex. reduction ratio varies from 0.6 to 1 at the knee). These transmissions are arranged in parallel at the ankles and at the trunk/pelvis linkage (fig.6).
Fig5. Knee Transmission System
Fig6. Ankle Parallel Mechanism
1.3 Sensors Synchro-resolvers provide with the relative angular positions of the motor axes. In order to recover absolute angular position, potentiometers are mounted on every joint. The 3 strain gage-based force sensors located on each foot allow to recover the vertical component of the ground reaction force, two moments and the XY position of the center of pressure. We also use a two-axis inclinometer in order to know the direction of the gravity vector, and ultrasonic sensors on the legs for reconstructing the ground profile. 1.4 Computer Architecture The computer architecture and control algorithms were designed by INRIA. The payload in the trunk includes parts of commercial power units and control boards (VME/68040 25Mhz-based CPU). The real-time operating system is VxWorks and the control development is achieved under the ORCCAD environment  running on a Unix workstation (fig.7).
Fig7. System Architecture
Fig8. The Robot Controller ORCCAD
2. SOFTWARE CONTROL ARCHITECTURE The effectiveness of our control technique is based on the accurate modeling of the robot, the generation of adapted trajectories and the implementation via a dedicated robot controller. 2.1 Model We consider the robot with rigid links and 15 joints. The reference frame is attached to the foot which is in contact with the ground. Its dynamics can always be written under the Lagrangian form: && & & M(q)q + C(q,q)q + G(q) = Γ (1) where q is the set of joint variables (15 angular positions), M is the matrix of kinetic energy, C is the vector of Coriolis/centrifugal forces, G is the vector of potential-based forces, and Γ is the 15-dimensional actuation vector. In order to determinate each term of equation (1), we use an automatic generation tool  of the Lagrange dynamics of free-flying rigid tree robots with variable references. The robot is described in terms of Khalil Kleinfinger representation parameters. We compute three models, in case the robot is supported by its right foot, by its left foot or hung up. During the double support phase, we compute an intermediary model based on the 2 single support models (equ.3).
2.2 Reference trajectories generation A gait is generated by defining several positions of the robot in the space (postures) and interpolating between them to get the desired motion. Different elements are taken into account: the...