How to fix the end point of the rod in a determined position?

[Edmund-Wang0546]

I hope to get the curve of a Flexible rod with 2 fixed ends, like:

The start point could be fixed by setting 'start' in straight_rod class and adding 'OneEndFixedBC' with 'constrained_position_idx=(0,)'. But how to fix the end point of the rod in a determined position?

[armantekinalp]

You can use GeneralConstrain and pass the index you want to constrain. https://github.com/GazzolaLab/PyElastica/blob/master/elastica/boundary_conditions.py#L262

[Edmund-Wang0546]

Thank you. This is a very simple example. We just hope to verify the deformation of a flexible rod. For example, the rod's length is 1.2, start element is fixed at (0,0,0) and end element is fixed at (1,0,0). These would never change during the simulation. Thus, no motion is generated during the simulation. I just hope to see the curve of the rod.

[armantekinalp]

Do you know the rod node positions? or do you expect PyElastica to compute rod node positions based on start, end position and total length? If it is the second case this is not possible because there can be many different deformations can happen. I would suggest to start with a straight rod and apply a force in -x direction at the tip to move rod end from (1.2, 0, 0) to (1,0,0).

[cuicw]

I also encountered a similar problem. I input the curve configuration of the flexible rod as a position into CosseratRod.straight _ rod, and the specific input is as follows : position = positions1.T,, but the following error pops up :
AssertionError :
Not equal to tolerance rtol = 1e-07, atol = 2.22045e-12
Rod normal and tangent are not perpendicular to each other !
Mismatched elements : 2 / 3 ( 66.7 % )
Max absolute difference : 0.57071637
Max relative difference : inf
x : array ( [ -4.471247e-001, -5.707164e-001, 1.750243e-306 ] )
y : array ( 0 )

[cuicw]

I already know the curve configuration of the flexible rod ( initial point, end point and rod length ), and the position information of each node is also calculated and discretized by the theoretical formula

[armantekinalp]

This is a different issue. Please open a new issue.

Short answer: Provided rod normal is not perpendicular to the rod tangents. Make sure you provide correct normal vector. If you rod does not lie on a plane such that there is not a single normal vector, then you should pass directors to the rod initializer. See the documentation .