Variable transmission_interface::HW_IF_ABSOLUTE_POSITION

Variable Documentation

constexpr auto transmission_interface::HW_IF_ABSOLUTE_POSITION = "absolute_position"

Implementation of a differential transmission.

Implementation of a simple reducer transmission.

This transmission relates two actuators and two joints through a differential mechanism, as illustrated below.

../_images/differential_transmission.png

where:

  • \( x \), \( \dot{x} \) and \( \tau \) are position, velocity and effort variables, respectively.

  • Subindices \( _a \) and \( _j \) are used to represent actuator-space and joint-space variables, respectively.

  • \( x_{off}\) represents the offset between motor and joint zeros, expressed in joint position coordinates (cf. SimpleTransmission class documentation for a more detailed description of this variable).

  • \( n \) represents a transmission ratio. Reducers/amplifiers are allowed on both the actuator and joint sides (depicted as timing belts in the figure). A transmission ratio can take any real value except zero. In particular:

    • If its absolute value is greater than one, it’s a velocity reducer / effort amplifier, while if its absolute value lies in \( (0, 1) \) it’s a velocity amplifier / effort reducer.

    • Negative values represent a direction flip, ie. input and output move in opposite directions.

    • Important: Use transmission ratio signs to match this class’ convention of positive actuator/joint directions with a given mechanical design, as they will in general not match.

This transmission relates one actuator and one joint through a reductor (or amplifier). Timing belts and gears are examples of this transmission type, and are illustrated below.../_images/simple_transmission.png

where:

  • \( x \), \( \dot{x} \) and \( \tau \) are position, velocity and effort variables, respectively.

  • Subindices \( _a \) and \( _j \) are used to represent actuator-space and joint-space variables, respectively.

  • \( x_{off}\) represents the offset between motor and joint zeros, expressed in joint position coordinates.

  • \( n \) is the transmission ratio, and can be computed as the ratio between the output and input pulley radii for the timing belt; or the ratio between output and input teeth for the gear system. The transmission ratio can take any real value except zero. In particular:

    • If its absolute value is greater than one, it’s a velocity reducer / effort amplifier, while if its absolute value lies in \( (0, 1) \) it’s a velocity amplifier / effort reducer.

    • Negative values represent a direction flip, ie. actuator and joint move in opposite directions. For example, in timing belts actuator and joint move in the same direction, while in single-stage gear systems actuator and joint move in opposite directions.

Note

This implementation currently assumes a specific layout for location of the actuators and joint axes which is common in robotic mechanisms. Please file an enhancement ticket if your use case does not adhere to this layout.