Shamel Fahmi

• Most of WBCs fail to generalize beyond rigid terrain. Not terrain aware

• Locomotion over soft terrain is challenging because of the induced contact dynamics

• Soft Terrain + WBC is even worse, why?

Shamel Fahmi

• Mismatch between what the WBC “perceives” vs. what is actually happening

• WBC is assuming that the feet are stationary

Shamel Fahmi

• Drop the assumption of the rigid terrain. Use a more generic model that accounts for the terrain impedance 

• But that’s not enough, how do you know the impedance parameters of the terrain. 

• Hmm, estimate it?

Shamel Fahmi

Shamel Fahmi

Soft Terrain Adaptation aNd Compliance Estimation
(STANCE)

Shamel Fahmi

• # Motivation

• Standard Whole Body Control

• C3 Whole Body Control

• Terrain Compliance Estimation

• Results

• Conclusion and Future Work

Shamel Fahmi

• The Standard WBC (sWBC) framework:

  • Locomotion Planner (see [1], [2] for example)
  • State Estimator
  • WBC
  • Low-level Control

Shamel Fahmi

• Execute the planned trajectories of the body and swinging leg (a.k.a control tasks)

Goals:

• Keep the robot balanced

• Control the contact forces

• Respect the robot's dynamics and actuation limits

• Respect the contact constraints due to the interaction with the environment

• Maintain contact consistency

• Casted as an optimization problem

Goals:

• Solved via Quadratic Programming (QP)

• Mapped using inverse dynamics

1. Control Tasks Tracking

2. Physical Consistency

3. Joint & Torque Limits

4. Friction Constraints

5. Unilaterality Constraints

6. Stance Task

Shamel Fahmi

• # Motivation

• # Standard Whole Body Control

• C3 Whole Body Control

• Terrain Compliance Estimation

• Results

• Conclusion and Future Work

Shamel Fahmi

• Problem

• Over soft terrain, the stance feet are non-stationary

• The stance task does not hold anymore

• Compliant contact consistent (c3)

• Soft contact dynamics must be taken into account

1. Reformulate the stance task to include soft contact dynamics

2. Encode it in the WBOpt problem

Shamel Fahmi

1. Reformulate the stance task 

• Using the KV model:

  • 3D linear springs and dampers parallel and perpendicular to the contact point

  • It fits our QP formulation

  • Cartesian impedance

• c3 Stance Task

Shamel Fahmi

Shamel Fahmi

• How can we generalize beyond one type of terrain

• How can we do it online?

Shamel Fahmi

• # Motivation

• # Standard Whole Body Control

• # C3 Whole Body Control

• Terrain Compliance Estimation

• Results

• Conclusion and Future Work

Shamel Fahmi

• Objectives

  • Estimate the terrain parameters online

  • Based on the current measurements (states)

  • Decoupled from the C3WBC but uses the same contact model

Shamel Fahmi

Shamel Fahmi

Shamel Fahmi

• # Motivation

• # Standard Whole Body Control

• # C3 Whole Body Control

• # Terrain Compliance Estimation

• Results

• Conclusion and Future Work

Shamel Fahmi

Shamel Fahmi

Shamel Fahmi

Shamel Fahmi

Shamel Fahmi

• # Motivation

• # Standard Whole Body Control

• # C3 Whole Body Control

• # Terrain Compliance Estimation

• # Results

• Conclusion and Future Work

Shamel Fahmi

• Unlike previous work on WBC, we do not assume that the ground is rigid.

• Adapt online to any type of terrain compliance

• C3WBC

  • ​Extends the sWBC but incorporating soft contact dynamics

  • C3WBC can adapt to the given terrain compliance

• ​TCE

  • ​Estimate the terrain compliance online

  • Not computationally expensive

Shamel Fahmi

• With STANCE, HyQ can traverse and transition between multiple terrains with different compliance without pre-tuning.

• STANCE makes the locomotion strategy c3. It allowed HyQ to remain consistent with ground, and thus more robust in challenging and scenarios.

• Each leg independently senses and adapts to changes in terrain compliance.

Shamel Fahmi

• ​State Estimation

• Low level torque control

• Limitations of STANCE

Shamel Fahmi

Shamel Fahmi

Shamel Fahmi

Shamel Fahmi

• STANCE

  • Adapt online to any type of terrain compliance

  • C3WBC

    • ​Extends the sWBC but incorporating soft contact dynamics

    • C3WBC can adapt to the given terrain compliance

  • ​TCE

    • ​Estimate the terrain compliance online

    • Not computationally expensive