HingedRigidBodyStateEffector parameters description not clear

[AmolikaSoni]

I am trying to simulate a scenario for solar panel deployment. I am using "HingedRigidBodyStateEffector" for this. I am not able to completely understand the meaning of the following variables:

    double thetaInit;                //!< [rad] Initial hinged rigid body angle
    double thetaDotInit;             //!< [rad/s] Initial hinged rigid body angle rate
    double thetaRef;                  //!< [rad] hinged rigid body reference angle
    double thetaDotRef;               //!< [rad/s] hinged rigid body reference angle rate

Here, what exactly is 'theta' in reference to? What is this angle in reference to? Is it about the X axis, Y axis, Z axis or some about some other vector? Also, how and where is it defined about which axis the deployment will occur?

In the example file, scenarioDeplyingPanel.py, what is the purpose of startTheta in HingedBodyLinearProfiler? should it be same as thetaInit in HingedRigidBodyStateEffector

[schaubh]

The module documentation points to some papers that define the frame in more details. Theta is defined as a 2nd axis rotation relative to the hinge frame. The reference angle is defined in the 2nd paper in the documentation

[AmolikaSoni]

Is there a specific reason to define 2nd axis as axis of rotation relative to the hinge frame?

[schaubh]

The 2nd frame was a natural selection with the original development scenario. You can always define your hinge frame such that this is the case. Note that hingedRigidBodyEffectror does have some assumptions such as this rotation axis, the panel mass distribution is balanced, etc. The newer SpinningBodyOneDOFEffector (see http://hanspeterschaub.info/basilisk/Documentation/simulation/dynamics/spinningBodies/spinningBodiesOneDOF/spinningBodyOneDOFStateEffector.html) is a more general solution where to rotation axis can be set to be any axis, and the mass distributions are kept as general as possible. There is also a 2-DOF version of this effectors. There were papers published on these effectors that discuss their benefits (http://hanspeterschaub.info/Papers/VazCarneiro2023.pdf).

[AmolikaSoni]

This was helpful. Thank you for the response.