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ABSTRACT: Developing global policies for humanoid robots using dynamic programming is difficult because they have many degrees of freedom. We present a formalism whereby a value function for a humanoid robot can be approximated using the known value functions of similar systems. These similar systems can include approximate models of the robot with reduced dimensionality or trajectories derived from human motion capture data. Once an approximate value function is known, a local controller is used to compute control signals. The approximate value function provides information about the global strategies that should be used to solve the task. The local controller provides complementary information about the robots dynamics. We present an implementation of this strategy and simulation results generated by this implementation.
Humanoid Robots, 2007 7th IEEE-RAS International Conference on; 01/2008
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ABSTRACT: This paper proposes a gain switching algorithm for joint position control of a hydraulic humanoid robot. Accurate position control of the lower body is one of the basic requirements for robust balance and walking control. Joint position control is more difficult for hydraulic robots than it is for electric robots because of a slower actuator time constant and the back-drivability of hydraulic joints. Backdrivability causes external forces and torques to have a large effect on the position of the joints. External ground reaction forces therefore prevent a simple proportional-derivative (PD) controller from realizing accurate joint position control. We propose a state feedback controller for joint position control of the lower body, define three modes of state feedback gains, and switch the gains according to the zero moment point (ZMP) using linear interpolation. The performance of the algorithm is evaluated with a dynamic simulation of a hydraulic humanoid.
Humanoid Robots, 2007 7th IEEE-RAS International Conference on; 01/2008
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ABSTRACT: In human walking, the swing leg moves backward just prior to ground contact, i.e. the relative angle between the thighs is
decreasing. We hypothesize that this swing leg retraction may have a positive effect on gait stability, because similar effects
have been reported in passive dynamic walking models, in running models, and in robot juggling. For this study, we use a simple
inverted pendulum model for the stance leg. The swing leg is assumed to accurately follow a time-based trajectory. The model
walks down a shallow slope for energy input which is balanced by the impact losses at heel strike. With this model we show
that a mild retraction speed indeed improves stability, while gaits without a retraction phase (the swing leg keeps moving
forward) are consistently unstable. By walking with shorter steps or on a steeper slope, the range of stable retraction speeds
increases, suggesting a better robustness. An optimization of the swing leg trajectory of a more realistic model also consistently
comes up with a retraction phase, and indeed our prototype demonstrates a retraction phase as well. The conclusions of this
paper are twofold; (1) use a mild swing leg retraction speed for better stability, and (2) walking faster is easier.
07/2007: pages 427-443;
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ABSTRACT: Biomechanical models of human standing balance in the sagittal plane typically treat the two ankle joints as a single degree of freedom. They describe the sum of the torques produced by the ankles, but do not predict what the contribution of each ankle will be. Similarly, balance algorithms for bipedal robots control the location of the overall center of pressure, but do not consider the individual centers of pressure under each foot. We present theoretical and experimental results showing an optimal solution to the problem of producing a single desired torque with redundant actuators, resulting in alignment of the individual centers of pressure under each foot. This produces a feedback gain structure not addressed in the biomechanics literature and a balance controller that is potentially more robust to unexpected changes in the region of support. We show that the feedback gain matrix of this controller has an unexpected structure - large off-axis integral gain elements indicate that the ankle torque that equalize the position of the center of pressure is determined primarily by information from the other foot. We also demonstrate controllers based on this design using the Sarcos Primus hydraulic biped.
Humanoid Robots, 2006 6th IEEE-RAS International Conference on; 01/2007
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ABSTRACT: We are interested in adding actuation to passive dynamic walkers to enable them to control their velocity. We control velocity by using dynamic programming to design control laws for each desired velocity. We consider three cases: a simulated planar compass gait walker, a simulated 3D compass gait walker with roll dynamics, and a simulated planar compass gait walker with a torso. Each of the walkers have massless legs. The actions include foot placement, ankle torque, and desired torso orientation. We use Poincare sections to define the state of the model, and thus choose a new action once per footstep. The optimization criterion is based on the effort of swinging the limbs, applying torques, and maintaining the desired velocity. By generating control laws at different desired velocities and then selecting the appropriate control law we are able to control velocity in each of these walkers, and smoothly transition between different velocities. Our results also indicate how complex nonlinear control laws can be approximated by gain-scheduled linear control laws.
Humanoid Robots, 2006 6th IEEE-RAS International Conference on; 01/2007
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ABSTRACT: We describe three bipedal robots that are designed and controlled based on principles learned from the gaits of passive dynamic walking robots. This paper explains the common control structure and design procedure used to determine the mechanical and control parameters of each robot. We present this work in the context of three robots: Denise, the Delft pneumatic biped, R1, a highly backdrivable electric biped, and R2, a hydraulic biped. This work illustrates the application of passive dynamic principles to powered systems with significant control authority
Humanoid Robots, 2005 5th IEEE-RAS International Conference on; 02/2005
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ABSTRACT: In human walking, the swing leg moves backward just prior to ground contact, i.e. the relative angle between the thighs is decreasing. We hypothesized that this swing leg retraction may have a positive effect on gait stability, because similar effects have been reported in passive dynamic walking models, in running models, and in robot juggling. For this study, we use a simple inverted pendulum model for the stance leg. The swing leg is assumed to accurately follow a time-based trajectory. The model walks down a shallow slope for energy input which is balanced by the impact losses at heel strike. With this model we show that a mild retraction speed indeed improves stability, while gaits without a retraction phase (the swing leg keeps moving forward) are consistently unstable. By walking with shorter steps or on a steeper slope, the range of stable retraction speeds increases, suggesting a better robustness. The conclusions of this paper are therefore two-fold; (1) use a mild swing leg retraction speed for better stability, and (2) walking faster is easier
Humanoid Robots, 2005 5th IEEE-RAS International Conference on; 02/2005
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[show abstract]
[hide abstract]
ABSTRACT: In human walking, the swing leg moves backward just prior to ground contact, i.e. the relative angle between the thighs is decreasing. We hypothesized that this swing leg retraction may have a positive effect on gait stability, because similar effects have been reported in passive dynamic walking models, in running models, and in robot juggling. For this study, we use a simple inverted pendulum model for the stance leg. The swing leg is assumed to accurately follow a time-based trajectory. The model walks down a shallow slope for energy input which is balanced by the impact losses at heel strike. With this model we show that a mild retraction speed indeed improves stability, while gaits without a retraction phase (the swing leg keeps moving forward) are consistently unstable. By walking with shorter steps or on a steeper slope, the range of stable retraction speeds increases, suggesting a better robustness. The conclusions of this paper are therefore twofold; (1) use a mild swing leg retraction speed for better stability, and (2) walking faster is easier.
