Meaning of parameters in JointSpringForceField

[Altero0101]

Hello everyone,

I’ve been working with JointSpringForceField for a while now, but I am still wondering what is the physical meaning of few of the parameters in that Force Field when defining a spring. From the API Documentation I have understood the meaning of some of them, but still I have some doubts. Here are the parameters according to the component explorer:

– Index1 and Index2 identify the indices of the MechanicalObjects to link. The order has an influence on how the response of the system is computed?
– FREE_AXIS is a vector of 6 booleans that activate/deactivate the translation and the rotation with respect to the three main axes;
– KS_T 100 200 are respectively the soft and hard translational stiffnesses. What is the difference between the soft and hard one?
– KS_R rotational stiffnesses defined as KS_T;
– What is the physical meaning of blocKsr?
– What is the physical meaning of Kd?
– R_LIM_X 0 2 are the minimum and maximum angle of activity of the spring (?). How are these angles measured?
– R_LIM_Y and R_LIM_Z are like R_LIM_X;
– L init spring x, L init spring y and L init spring z represent the initial position offsets of the spring (rest configuration);
– Rot init spring represents a quaternion of the rotational offset (rest configuation);

I’ve done many tests with this Force Field and also with other alternatives and combinations, but still this Force Field seems to be the perfect solution for my scene. In my case it is working well, but maybe there are some parameters I can adjust to improve the performances of my scene.

Any info about that is useful and appreciated.

Thank you,
Lorenzo

[hugtalbot]

hey @Altero0101

Sorry for keeping you waiting, but I am not big user of the JointSpringForceField .. therefore I was waiting for others to give a hint.
But I'll take the lead!

• the two first indices (m1 and m2, aka Index1 and Index2) correspond to the point IDs of one rigid object to be connected by a JointSpring. Switching them should not affect the result
• FREE_AXIS defines the axis where the movements is free (translation in x,y,z and rotation in x,y,z)
• KS_T defines the stiffness soft or hard, respectively if the translation dof is free (i.e. FREE_AXIS is 1) or not (i.e. FREE_AXIS is 0)
• KS_R is indeed the rotational stiffness
• blocKsr seems to be defined as "stiffness to apply on axis where the rotations are bloqued" and is supposed to be equal to the hardStiffnessRot divided by 100
• Kd is a damping factor (creating a spring-damper system)
• R_LIM_* corresponds to the limit angles which can be defined on rotation axis X, Y and Z. In your statement R_LIM_X 0 2 yes, it was the minimum and maximum angle of activity of the spring along X. The unit is in radian (I discovered it!)
• you got it right for both L init and Rot init

I hope this helps.

[hugtalbot]

PS : the API documentation can also help : https://www.sofa-framework.org/api/master/sofa/html/classsofa_1_1component_1_1solidmechanics_1_1spring_1_1_joint_spring.html#a8105e795f0a35efba5ee1c61efaefaa0

[Altero0101]

Hey @hugtalbot

thank you for your reply, now it is clear. Indeed, that is really helpful, thanks again.