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Capture Point: A Step toward Humanoid Push Recovery Jerry Pratt 1, John Carff 1, Sergey Drakunov 1, Ambarish Goswami 2 1 Florida Institute for Human and Machine Cognition 2 Honda Research Institute Humanoids 2006 December 6, 2006

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Capture Point: A Step toward Humanoid Push Recovery

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Some Push Recovery Approaches Replan trajectories. Solve Constrained Optimization Problem. Machine Learning. Heuristics based on intuition and simple models.

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Outline Push Recovery Overview Our Approach to Push Recovery. Simulation Examples. Ongoing and Future Work.

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Importance of Push Recovery Bipedal robots in human environments: –Bumping into objects. –Incidental contact when walking down a sidewalk. –Tripping over cluttered floors. –Contact during sports. –Intentional pushes. –Method of human input interface. Understanding and Assisting Humans –Falls are a major cause of injury.

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Theoretical and Practical Difficulties of Push Recovery Non-linear dynamics Multi-variable dynamics Limited foot-ground interaction Hybrid dynamics (dynamics are both continuous and change discretely during steps) Quick detection of push required. Fast reaction speed required. Relatively large actuator power required.

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Human Push Recovery Strategies Move the Center of Pressure, predominately through ankle torques. Accelerate Angular momentum by lunging and windmilling. [Video]Video Take a step. [Video]Video Combinations. [Video1, Video2]Video1Video2

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Why these Strategies Work Broomstick (Inverted Pendulum) Analogy Tightrope Walker Analogy

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Using Angular Momentum effectively increases the size of your footprint [Popovic, Goswami, Herr IJRR2005]

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Outline Push Recovery Overview Our Approach to Push Recovery. Simulation Examples. Ongoing and Future Work.

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Capture Points and Capture Regions (Quick Definition) Capture Point: Point that the biped can step to and stop in one step without falling down. Capture Region: Set of all Capture Points. F

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Balance Strategy 1: Center of Pressure Kinematic Workspace Of Swing Leg Support Foot Capture Region

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Balance Strategy 2: Accelerate Angular Inertia (Windmill or Lunge) Kinematic Workspace Of Swing Leg Support Foot Capture Region

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Balance Strategy 3: Take a Step Kinematic Workspace Of Swing Leg Support Foot Capture Region

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Balance Strategies 4,5,6: Multiple Steps, Run, or Fall Kinematic Workspace Of Swing Leg Support Foot Capture Region

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How to Compute Capture Points? Simple Models with Closed-Form Solutions. Numerical Search Learning

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Computing the Capture Point for the Linear Inverted Pendulum (Kajita and Tani 1991) ModelKajita and Tani 1991

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Computing Capture Points for the Linear Inverted Pendulum plus Flywheel Model

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Deriving Linear Inverted Pendulum Plus Flywheel Dynamics using Similar Triangles Mg z0z0 X- /Mg FxFx

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Computing Capture Points for the Linear Inverted Pendulum plus Flywheel Model Flywheel is torque-limited due to motors. Flywheel is position limited to model humanoid upper body. –Greatest effect the flywheel can have is through a bang-bang torque profile so that the flywheel accelerates and decelerates as quickly as possible, stopping at its maximum or minimum angle limit.

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Computing Capture Points for the Linear Inverted Pendulum plus Flywheel Model Bang-bang torque profile Solve for T R1 and T R2 given initial and final states of the flywheel. Since dynamics are linear and torque profile has Laplace Transform, everything can be computed in closed form.

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State Trajectories

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Projection of Phase Portrait

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Dynamic Evolution of Capture Points Using Linear Inverted Pendulum Model, dynamic evolution can be computed in closed form.

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Outline Push Recovery Overview Our Approach to Push Recovery. Simulation Examples. Ongoing and Future Work.

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Push Recovery from Impulsive Push

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Stopping in one step by lunging

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Applying Linear Inverted Pendulum based Capture Point to 12 dof 3D model

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Stepping Stones by guiding the Capture Point to the Desired Stepping Point

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Take Home Message 1 Precise foot placement is not necessary for push recovery, but good foot placement is. –If any point of the foot is placed inside the Capture Region, the humanoid can stop. –Larger feet and/or more angular momentum increase robustness to poor foot placement.

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Take Home Message 2 Simple Models can be Useful! –Understanding of the fundamental principles. –Control Algorithm Development.

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Future Work Apply to a real humanoid. Derive closed form calculations of Capture Points for arbitrary CoM height trajectory. Numerically compute Capture Regions for complex models and compare to simple models. Extend to persistent pushes. Expand techniques to other aspects of walking: –Dynamic Turning. –Rough Terrain.

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Capture Point: A Step toward Humanoid Push Recovery Jerry Pratt 1, John Carff 1, Sergey Drakunov 1, Ambarish Goswami 2 1 Florida Institute for Human and Machine Cognition 2 Honda Research Institute Humanoids 2006 December 6, 2006

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