upkie::cpp::observers::WheelContact Class Reference

Observe contact between a given wheel and the floor. More...

#include <WheelContact.h>

Classes
struct	Parameters
	Observer parameters. More...

Public Member Functions
	WheelContact (const Parameters &params)
	Initialize observer. More...
void	observe (const double torque, const double velocity, const double dt) noexcept
	Process a new observation. More...
double	abs_acceleration () const noexcept
	Low-pass filtered absolute wheel acceleration, in [rad] / [s]²
double	abs_torque () const noexcept
	Low-pass filtered absolute wheel torque, in [N m].
bool	contact () const noexcept
	Current contact state.
double	inertia () const noexcept
	Apparent inertia I = |torque| / |acceleration| at the wheel.
void	reset_contact () noexcept
	Reset contact to false.

Detailed Description

Observe contact between a given wheel and the floor.

We use an acceleration-from-torque simple linear regressor with no affine term and online updates. There is a quick intro to the math in the following post on simple linear regression with online updates. The output of this regressor is an "inertia" (torque over acceleration slope) that is low during contact and large in the air (as then the wheel needs little torque to spin freely).

The observer is made more reliable by three hacks post-processing stages:

1. Max-passthrough filtering: We further filter the output inertia by a max-passthrough, to detect contact loss fast, which otherwise decays exponentially, so that we tend to assume contact remains if there is no contact loss for a while.
2. Touchdown and liftoff hysteresis: Filtering aside, the other memory in this observer comes from hysteresis. Contact is detected as soon as the contact inertia goes below the configured touchdown inertia, and lost when it spikes above the liftoff inertia. This allows a gray band of variations while the robot moves around in contact.
3. No decision when acceleration or torque is low: There is a second hysteresis around low filtered accelerations or low filtered torques. If filtered values are low then real values have been zero for a while, which is inconclusive: the robot could be standing perfectly upright rather than having lost contact. (Most likely, if it loses contact, these values won't stay zero for long anyway.) When this happens we just keep the memorized contact state and wait for more exciting data.

Constructor & Destructor Documentation

WheelContact()

upkie::cpp::observers::WheelContact::WheelContact ( const Parameters &  params )

explicit

Initialize observer.

Parameters

[in]

params

Observer parameters.

Member Function Documentation

observe()

void upkie::cpp::observers::WheelContact::observe ( const double  torque, const double  velocity, const double  dt  )

noexcept

Process a new observation.

Parameters

[in]	torque	New torque observation.
[in]	velocity	New velocity observation.
[in]	dt	Duration in seconds since last observation.

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The documentation for this class was generated from the following files:

• upkie/cpp/observers/WheelContact.h
• upkie/cpp/observers/WheelContact.cpp