Python Module: cmdTorqueBodyToTorqueAtSite

Executive Summary

Converts a commanded body-frame torque into a TorqueAtSiteMsgPayload expressed in a site frame, for driving a MuJoCo torque actuator at that site.

Message Connection Descriptions

The following diagram and table list the module input and output messages.

Module I/O Messages
Msg Variable Name	Msg Type	Description
cmdTorqueInMsg	CmdTorqueBodyMsgPayload	Input commanded torque `torqueRequestBody` [N m], expressed in body frame B.
torqueOutMsg	TorqueAtSiteMsgPayload	Output torque `torque_S` [N m], expressed in site frame S.

Module Assumptions and Limitations

The output is \({}^{S}\boldsymbol{L} = [SB]\,{}^{B}\boldsymbol{L}\), where dcm_SB is a constant body-to-site rotation that defaults to identity (aligned frames). The configured matrix is copied on assignment; subsequent changes to the caller’s array do not affect the module. The torque is treated as a pure couple, so the site position does not affect the output. Reset() reports an error if cmdTorqueInMsg is not linked. Otherwise, it publishes zero torque with the reset timestamp and the module’s writer ID.

User Guide

The following setup assumes that scene is an MJScene already added to the simulation, its XML defines a site named hubSite, and mrpControl is a configured C Module: mrpFeedback controller scheduled on an FSW task. Here the site frame S is aligned with the hub body frame B, so the default identity rotation is sufficient. Configure the connections and schedule the adapter before calling InitializeSimulation():

from Basilisk.simulation import cmdTorqueBodyToTorqueAtSite

torqueActuator = scene.addTorqueActuator("hubTorqueAct", "hubSite")
torqueBridge = cmdTorqueBodyToTorqueAtSite.CmdTorqueBodyToTorqueAtSite()
torqueBridge.ModelTag = "torqueBridge"
torqueBridge.cmdTorqueInMsg.subscribeTo(mrpControl.cmdTorqueOutMsg)
scene.AddModelToDynamicsTask(torqueBridge)
torqueActuator.torqueInMsg.subscribeTo(torqueBridge.torqueOutMsg)

Adding the adapter to the scene’s dynamics task makes it convert the latest FSW torque command during dynamics evaluation. Connecting messages alone does not schedule a module. See scenarioFlexiblePanelMuJoCo for a complete simulation using this setup.

For a site with a fixed orientation relative to the body, assign a proper rotation matrix to torqueBridge.dcm_SB before initialization. Its rows are the site axes expressed in body coordinates, so it maps body components to site components. For example, for site axes Sx = By, Sy = -Bx, and Sz = Bz:

torqueBridge.dcm_SB = [
    [0.0, 1.0, 0.0],
    [-1.0, 0.0, 0.0],
    [0.0, 0.0, 1.0],
]  # [-]

The matrix must match the site’s orientation in the scene; assigning it only configures the conversion and does not rotate the site itself. A site on a moving appendage requires a time-varying transformation, which this constant rotation adapter does not compute.

class cmdTorqueBodyToTorqueAtSite.CmdTorqueBodyToTorqueAtSite(*args, **kwargs)[source]

Bases: SysModel

Convert a commanded body-frame torque into a torque expressed in a site frame.

Variables:

cmdTorqueInMsg – Commanded torque input message, expressed in body frame B.
torqueOutMsg – Torque output message, expressed in site frame S.

The output is torque_S = dcm_SB @ torqueRequestBody. dcm_SB defaults to identity, which corresponds to a site frame aligned with the body frame.

property dcm_SB: Direction cosine matrix mapping body-frame vectors into the site frame

validateInputMessages()[source]: Raise BasiliskError if a required input message is not linked.