mc_external_forces_observer

An mc_rtc observer that estimates external joint torques acting on a robot and optionally feeds them back to the QP controller as setExternalTorques.

Requirements

• mc_rtc

Installation

The recommended way is via the mc_rtc superbuild. For a standalone build, ensure mc_rtc is installed, then:

cmake -B build && cmake --build build && cmake --install build

Estimation methods

Two strategies are available via the estimation_method key:

Method	Description	Latency
ForceSensorBased	Projects FT sensor wrenches into joint space: τ_ext = Σ Jᵀ w	None
MomentumObserver	Integrates the generalised momentum residual; FT sensors are fed as known inputs so only unmeasured forces incur gain-related latency	Proportional to 1/K

Configuration

All keys are optional and fall back to the defaults shown below.

ObserverPipelines:
  - name: MainPipeline
    gui: true
    log: true
    observers:
      # Required before ExternalForcesObserver when using MomentumObserver
      - type: Encoder
        velocity: encoderVelocities      # encoderFiniteDifferences also works

      # Required for floating-base robots (tested: BodySensor in sim, Tilt on hardware)
      - type: BodySensor
        update: true
        bodySensor: FloatingBase

      - type: ExternalForcesObserver
        robot: <robot_name>              # defaults to the main robot
        updateRobot: <robot_name>        # robot whose torques are updated; defaults to robot
        estimation_method: MomentumObserver # or ForceSensorBased
        is_active: true                  # false = estimate and log only, no feedback to QP
        use_forces_from_ft_sensors: true # include FT sensor wrenches in the estimate
        # MomentumObserver-only keys:
        residual_gain: 10.0              # observer gain K; higher = faster response, more noise
        torque_source_type: CommandedTorque  # JointTorqueMeasurement preferred when available

Floating-base robots: add an Encoder observer and a floating-base observer (e.g. BodySensor or Tilt) before ExternalForcesObserver in the pipeline.

Torque source types

Value	Notes
CommandedTorque	Uses QP output
JointTorqueMeasurement	Uses joint-side sensors directly; most accurate; rotor inertia is removed from the inertia matrix
CurrentMeasurement	Not yet implemented; falls back to CommandedTorque
MotorTorqueMeasurement	Uses motor-side sensors directly

Datastore interface

Other controllers or plugins can interact with the observer at runtime:

Key	Type	Description
EF_Estimator::isActive	bool	Whether feedback is currently applied
EF_Estimator::toggleActive	void()	Enable/disable feedback
EF_Estimator::setGain	void(double)	Set gain K and reset observer state
EF_Estimator::getGain	double	Read current gain
EF_Estimator::isUsingFTSensorMeasurements	bool	FT fusion status
EF_Estimator::toggleFTSensorMeasurements	void()	Toggle FT fusion