Merge Request Options

As described in the Massless Cable Technical Documentation, the massless cable segments allow a single constraint to model the cable for a series of massless cable segments. Since the massless constraint does not impart torque to pulleys within this series of segments, the performance of a cable system consisting of massless segments can be improved by merging the pulleys into the base parts to which they are hinged, thereby removing the corresponding hinge constraint from the system.

Since these base parts may be themselves further merged in content, it is possible for multiple points in the cable definition to be constrained by adding a single representative part to the massless constraint. Thus, even though a single massless constraint is limited to constraining 6 parts, if the representative parts belong to a larger system of merged parts, the effective number of parts constrained may be arbitrarily large.

By default, which pulleys are merged and the representative parts of a collection of merged parts that are actually constrained by the massless constraint are chosen automatically. However, the mechanism designer may want finer control over the dynamic state of the pulleys in a run of massless segments and the representative parts added to the massless constraint. This may be because some dynamic information from a pulley is desired, e.g. the angular velocity of the pulley, or the force on it, or it may be so that the designer can control which parts are contained within a given massless constraint.

To this end, a number of merge request options are exposed in the point definition container for pulleys, rings, and attachment points.

Note that these options are merely requests, as some cable topologies require pulleys to be dynamic. For example, a pulley that ends the massless constraint of one run of massless segments and starts another must be dynamic. In this case, whatever merging request option specified by the user will be ignored.

Pulley Merge Request Options

Open

Dynamic

The pulley remains dynamic. This is useful if a Python script requires some dynamic pulley output, such as the torque applied to the part. This option can also be used to end a run of massless segments corresponding to a single massless constraint early, in case continuing the run would lead to an unsupported topology, such as ending a run of massless segments on a ring.

Use Part

The part corresponding to the pulley is used. Selecting this option will make the pulley a part in the underlying massless constraint. This option makes the system equivalent to a rigid cable system that admits internal pulleys. This could be useful for controlling exactly which parts are contained in the constraint.

Use Base

The part to which the pulley is hinged is used. This is useful if the merging topology may change dynamically. For example, there may be a crane with two head blocks, and the head blocks may have attachment points enabling them to merge together. If the head blocks begin the simulation merged together, then this option should be chosen so that the part in the massless constraint is that specific head block, allowing them to be unmerged during simulation without changing the parts involved in the underlying massless constraint.

Use Root

The merge root of the merge tree to which the pulley belongs is used. This may coincide with the base part, but will generally be another part representative of the groups of merged parts. This is the default option in the automatic merging code logic.

Ring and Attachment Merge Request Options

Open

Since rings and attachment points do not have an associated hinge constraint, they cannot be merged to remove a constraint.

However, it is still possible to optimize the parts in the massless constraint by the choice of representative part. Thus, only the Use Part (which again adds the part specified in the point definition to the constraint), and Use Merge Root (which again adds a representative part for the group of merged parts), options are available for rings and attachment points.

The Use Merge Root option is selected by default in order to maximally optimize the massless constraint.