Challenges in Legged Locomotion

This talk was made for a general audience

Transcript

1. Vertical hopper compositions for preflexive and feedback-stabilized
   quadrupedal bounding, pacing, pronking, and trotting
   [Full and Koditschek (1999)]
   “Templates and anchors…”
   [De and Koditschek (IJRR, 2018)]
   Simple compositional controller for running gaits
   Provable coordination Robustness to
   parametric uncertainty
   Bio-inspired: animals exhibit
   similar task-level reduction
   

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2. Input-decoupled anchoring
   • Can find reduced coordinates 𝑟 𝑞 s.t.
   𝑢𝑎𝑛𝑐ℎ
   does not appear in ሷ
   𝑟
   • 𝑟 𝑞 is ~ virtual leg pos
   • SLIP dynamics are exactly embedded!
   A new kind of anchoring
   Input-decoupled anchoring with actuated
   IP template behavior
   Floating torso model 𝑥 ∈ 𝑆𝐸(2)
   Conventional
   anchoring
   Input-
   decoupled
   anchoring
   Invariant+attracting pitch-stable manifold (conventional anchoring/ZD)
   [Full & Kod (1999)] [Westervelt et al (2007)]
   

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3. Challenges in legged locomotion: mechanics
   [Seok et. al. (2015)]
   GR Vision 60 v4
   Tesla
   Model S
   Ghost
   Vision 60
   Specific joint torque
   (Nm/kg)
   0.41 6.0 (15x)
   Specific peak power
   (W/kg)
   250 1,305 (5x)
   Specific energy
   (Wh/kg)
   40 30+ (similar)
   Control freq (Hz) 100 1,000 (10x)
   High force and power
   • The actuators (not the structure) need to support
   vehicle weight (including actuators)
   • Reciprocating swing legs need high peak power
   Specialized actuator requirements
   • High gear ratios result in high reflected actuator inertia
   • Harsh ground impact
   • High power required to swing legs
   • Control systems require very high actuator bandwidth
   for robustness/agility
   • Series-elastic limits bandwidth [Wensing (2017)]
   Efficiency fundamentally limited
   Can be seen in animals and robots
   

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4. Challenges in legged locomotion: control
   • Air/water are soft, ground is hard
   • Stiff actuators that work in the factory and for
   aerial robots don’t work with legs
   • Managing interaction force requires force control
   bandwidth in the KHz range, precise detection of
   external forces, and specialized algorithms
   Impedance control
   [Posa et al (2014)]
   Air/water are predictable, whereas every single step
   for a legged robot contains unexpected contacts
   System dimension/underactuation
   Dimension Actuators
   Quadruped 36 12
   Wheeled/tracked diff
   drive (vel control)
   3 2
   Wheeled/tracked fast 6 2
   Quadrotor 12 4
   Managing contacts
   • The robot body can only be controlled via
   ground contact
   • Even state of the art trajectory optimization
   cannot navigate the millions of possible
   contact sequences in anywhere near real time
   Environment complexity
   Even a single misstep/missed contact out of
   thousands can cause total failure
   Robustness requirements
   

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