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Parameters

A number of ROS2 parameters can be used to configure the operation and behavior of the Force Dimension node. These parameters can be modified programmatically, via configuration files, on the command line at startup, or via ros2 param calls.

Sampling interval

The sampling frequency defines how frequently the node updates. The ROS2 parameter that determines the sampling frequency is sample_interval_s. This parameter takes floating point values, in units of seconds.

In order to ensure smooth haptics and stable application of forces, the Force Dimension robots should operate at a frequency in the kilohertz range.1 The recommended update rate is 2000 Hz (corresponding to a parameter value of sample_interval_s: 0.0005).

Feedback sample decimation

Reporting feedback and storing data at a high sampling frequency can result in busy transmission channels and large / cumbersome data files. For this reason, the Force Dimension node accommodates decimation of message publication on the feedback topics. The parameters that determine the decimation factor are contained within the feedback_sample_decimation parameter namespace.2

Each parameter consists of an integer that represents a decimation factor for each feedback variable. This integer determines the number of samples or update cycles between each published message. That is, a message is published each time the sample counter can be evenly-divided by this factor. If the parameter value is 1, then no decimation occurs. If the parameter value is less than 1, then no messages are published at all (i.e., the feedback channel / topic is disabled).

As an example, consider the ROS topic that publishes the position of the robotic end-effector (e.g., /robot/feedback/position). If the sampling / update rate of the node is 2000 Hz (i.e., sample_interval_s: 0.0005), then a decimation factor of 50 (i.e., feedback_sample_decimation.position: 50) would result in position feedback published at 40 Hz.

Hardware switch

The disable_hardware ROS2 parameter is a boolean value that determines whether or not Force Dimension SDK calls are passed through. If the parameter value is true, then no SDK calls are made, and interaction with any robots is therefore disabled. The node continues to operate and publish data, even in the absence of calls to the SDK. This can be useful for testing and / or debugging, in cases where a robot is not needed or not available.

Button emulation

It is possible to treat the gripper of Force Dimension robots that lack buttons as though it were a button. See the dhdEmulateButton function of the Force Dimension haptics SDK for further details. The gripper.emulate_button ROS2 parameter toggles this capability. Button emulation is enabled or disabled only during the activation process, so that changing the parameter while the Force Dimension node is active will have no effect.

Sample configuration file

ROS2 parameters can be dumped and loaded to/from a YAML configuration file. Here is a sample configuration file for the Force Dimension node:

/robot/force_dimension:
  ros__parameters:
    sample_interval_s: 0.0005
    feedback_sample_decimation:
      position: 50
      button: 50
      gripper_gap: 50
      gripper_angle: 50
      velocity: 200
    disable_hardware: false
    gripper:
      emulate_button: false
    gravity_compensation: true
    effector_mass_kg: 0.279

Footnotes

  1. Reference requested. ↩

  2. ROS2 parameter namespaces do not seem to be well-documented. Parameter namespace prefixes are separated from parameter names by a period, and all parameters in a namespace can be retrieved at once. In YAML configuration files, parameter namespaces are specified via indentation. ↩