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gravity_compensation.md

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May 2022

Gravity Compensation

The Force Dimension SDK documentation describes gravity compensation as follows:

To prevent user fatigue and to increase accuracy during manipulation, Force Dimension haptic devices features gravity compensation. When gravity compensation is enabled, the weights of the arms and of the end-effector are taken into account and a vertical force is dynamically applied to the end-effector on top of the user command. Please note that gravity compensation is computed on the host computer, and therefore only gets applied whenever a force command is sent to the device by the application.

The Force Dimension ROS2 node allows for the configuration of gravity compensation via ROS2 parameters.

Parameters

Gravity compensation is controlled via two parameters: gravity_compensation and effector_mass_kg. The gravity_compensation parameter is a Boolean value that simply determines whether or not gravity compensation is enabled. The effector_mass_kg defines the mass of the robotic end-effector, in kilograms. This value is used by the Force Dimension SDK to compute a compensatory force.1 The default value is reported by the Force Dimension node during startup configuration, and is recorded to the text log. The default value for the Novint Falcon is 0.190. The default value for the delta.3 is 0.279.

The following is a sample YAML configuration file for setting the gravity compensation parameters:

/robot/force_dimension:
  ros__parameters:
    gravity_compensation: true
    effector_mass_kg: 0.279

Both parameters trigger an update of the robot's configuration immediately, but the forces applied to the end effector do not change until a new command is issued. This is worth emphasizing: After a change in parameters, the compensatory forces applied to the robotic end effector are updated only after the next force command is issued to the robot.

Gravity compensation is most effective when the effector_mass_kg parameter is close to the true mass of the robotic endpoint effector. When the parameter setting is slightly less than the true value, the robot can be expected to descend at a slower pace than it would without compensation. When the parameter setting is slightly higher than the true value, the effector will rise. Identifying the optimal parameter value may require some trial-and-error calibration, if the mass of the effector is uncertain.

Testing and calibration

In a configured ROS2 environment, run the Force Dimension node:

ros2 run force_dimension node

During initialization, the node will report the configured effector mass. For this example, a mass of 0.190 kg will be assumed.

In a second configured environment, simulate reduced-gravity by increasing the effector mass by 25% (0.190 * 1.25 = 0.2375):

ros2 param set /robot/force_dimension effector_mass_kg 0.2375

In the first ROS2 environment, the Force Dimension should report this parameter change to the log. However, the parameter change has no effect until a force command is delivered to the robot. To trigger the gravity compensation update, send a force command:

ros2 topic pub --once /robot/command/force geometry_msgs/msg/Vector3 \
  "{x: 0.0, y: 0.0, z: 0.0}"

At this point, the robot can be considered to be operating within a simulated physical environment, in which gravity has been reduced by 25%. Depending on the robot, and how it is set up, this might cause the end effector to rise toward the top of the workspace. If this does not happen, then increase the effector mass parameter by small increments -- re-issuing the force command for each parameter change -- until the end effector is observed to move upward.

To confirm the effect, disable gravity compensation:

ros2 param set /robot/force_dimension gravity_compensation false

For this parameter change to take effect, a force command must be issued to the robot, as before. Be prepared to catch the end effector when the change takes effect. With gravity compensation disabled, the force of gravity will typically cause the robotic effector to drop.

Footnotes

  1. The effector mass is set via the dhdSetEffectorMass function in the Force Dimension SDK. Gravity compensation is enabled via the dhdSetGravityCompensation function. ↩