Constraints

[killes]

From reading the Doc, I understand that we can use e.g. DistanceConstraint as in pendulum2Dconstraint.py to define the distance between ground and the weight.

How would I implement a distance that is constant along a given path?

Imagine some object obstructing the path of the string as shown here:

https://sites.pitt.edu/~vincentm/OLLI-WebPhysSciFi/BlockedPendDiscuss.html

[killes]

I guess in that case I could split up the string and have constrains on both of them.

However, this isn't really what I want to do.

I need something where the two parts of the string are of non-constant length but the sum of both is constant. Ie a string running over a pulley with two weights and the weights are moving up and down.

[jgerstmayr]

Regarding "I need something where the two parts of the string are of non-constant length but the sum of both is constant.", there is also the ConnectorReevingSystemSprings which keeps the total length of a string constant (except for an elastic deformation which can be kept small in case of high stiffness).

[jgerstmayr]

Constraining something onto a given path is a sliding joint. This requires to parameterize the path, and there are three options:

1. the path is prescribed (this could be done by moving the ground over time object using a PreStepUserFunction)
2. the path is rigid: this works similarly as a contact to rigid surfaces; in 2D, one could use ANCFCable2D elements which are fixed
3. the path is flexible: this is what is available for ANCFCable2D elements

What you describe in your reference can be realized with ANCFCable2D elements in combination (with possibly a set of) ObjectContactCircleCable2D or ObjectContactFrictionCircleCable2D, checkout the example ANCFcontactCircle.py which should work well to start with.

[jgerstmayr]

Another option: use two masses. In the intermediate point of the string, add a very small mass (e.g. 1e-6), and add two distance constraints. Then, add a simple contact between the intermediate mass and the ground, e.g., using GenericContact (or using some switching mechanism, but this is harder to get done without experience).

[killes]

Thank you! I will look into the various options.