A cable definition is a container for a number of parameter, point, and segment definitions used to construct a cable system. The Generic Cable properties, in conjunction with the Dynamics Properties, can be customized to adjust for thickness and density of the cable, material, points, as well as other parameters related to cable behaviors.

Definition

Define the following parameters:

• Inputs
  • Active: Indicates whether the cable system is enabled. When the cable is active, it is part of the simulation, when inactive, it is not part of the simulation and has no cost on the simulation.
  • Break Now: Selecting this box breaks the cable at the moment it is pressed in runtime.
  • Break Closest To: The part of the cable closest a point in the space defined by its X, Y, and Z values specified here will break when Break Now is selected.
  • Point Spooling on First (Last) Extremity Velocity: Sets the velocity of the cable's spooling (m/s). A negative value means spooling in and a positive value means spooling out. This field only appears when you select the Enable Point Spooling on First (Last) Extremity box in the Advanced Flexible Definition section of the Properties panel.
• Outputs
  • Total Length: This is the total length of the cable. If the cable is under tension, its total length will increase by a given elongation which depends on the stiffness of the cable and the tension in the cable. The total length of the cable will also change as the cable spools out/in from a Spooling point or a winch.
  • Elongation: The elongation is the difference between the total length of the cable when there is no tension (rest length) and the current total length of the cable under tension.
  • Max Tension: The maximum tension in the cable. Each segment is evaluated separately and the maximum value is displayed.
  • Max Torsion: The maximum torsion in the cable. Each segment is evaluated separately and the maximum absolute value is displayed.
•  Parameters
  • Param Definition
    • Material Name: The material for the cable collision geometries.
    • Collision Geometry Type: Selects the type of collision geometry from the drop-down list. A given segment might overwrite this value.
    • Geometric Radius: The radius of the selected collision geometry.
    • Compression Max Force: Maximum compression force the cable can withstand before it starts compressing (in newtons). This is mostly used to simulate chains. When set to a low value each section of a Flexible Segment will be able to move between two limits (Almost equivalent to a VxCylindrical with limits.)
    • External Damping Enabled: Activates a linear speed drag force on the cable.
    • External Damping: Scaling factor on the drag coefficient. A value of one simulates the effect of air drag.
    • Geometric Stiffness Enabled: Enables an adaptive technique to stabilize cables. See Cable Geometric Stiffness.
    • Geometric Stiffness Stability Threshold: Controls when damping is applied based on a stability analysis. This is a positive scalar, typically in the range (0.0, 2.0). A value of 0.0 will always apply some damping, and increasing it raises the threshold used in the stability analysis.
    • Geometric Stiffness Scale: Scales the computation of the geometric stiffness, which increases the frequency of damping being used to stabilize the simulation as constraint forces grow. A default of 1.0 is sufficient for most cases.
    • Strap Angle: Determines the angle of the principal orthogonal axis of the cable.
  • Point Definitions: See Creating a Point Definition.
  • Segment Definitions
    • Collision Geometry Type: Selects the type of collision geometry from the drop-down list. If defined, the collision geometry type here overrides the one set in the Param Definition for this segment.
    • Support Torsion: Select this box to indicate that the cable supports torsion and springs back to a torsion-free position.
    • Attachment Type at Start: Only available if the Point Definition at the start of the segment is an Attachment. This sets the type of attachment between the cable and the part. The available types are:
      • Ball and Socket: The cable can rotate freely in 3D at the attachment point .
      • Fixed: The cable is fixed and cannot bend at the attachment.
      • Flexible: The attachment between the cable and the part has the same bending stiffness, bending damping, etc. as the cable. When this type of attachment is selected, you will be able to select the direction of the cable. Only valid for flexible segment. The values are given in the local reference frame of the part at that point.
    • Attachment Type at End: Only available if the Point Definition at the start of the segment is an Attachment. Same available types as the Start.
    • Fixed Point-Point Distance: This option is for optimization and should only be selected if you know that the distance between the two end points of the segment cannot change and will always remain the same. Use with caution.
    • Fluid Definition:
      • Drag Coefficient: Value used to represent the resistance of an object in a fluid environment, such as water or air. To have drag on the cable, it must have a defined Collision Geometry.
      • Buoyancy Force Scale: A multiplication factor used to adjust the buoyancy force.
    • Flexible Definition:
      • Preferred Section Length: Sets the preferred length of all sections.
        • If the cable cannot spool, each section will be of equal length and as close as possible to the preferred section length.
        • If the cable can spool, the middle section will be made of sections of the preferred length, and the remainder will be divided into two sections at both ends of the spool.
        • If the distance between the end points is less than the preferred length, the two sections will share the distance.
      • Adaptive Definition
        • Active Joint Budget: Defines the percentage of points used in the cable to simulate bending. The higher the percentage, the more accurate the simulation but it uses more system resources.
        • Uniform Subdivisions: Number of uniformly distributed active joints in the cable.
        • Advanced Definition
          • Maximum Active Joint Count: The maximum number of active joints in the cable.
          • Maximum Motion Sample Count: Defines the number of sample taken in cable region with inactive joints to estimate the region's joint motion. A higher value results in more accurate cable motion simulation at the cost of system resources.
          • Minimum Angular/Linear Joint Acceleration/Velocity: A cut-off value used during joint motion estimation to determine whether a cable joint is relevant for the cable dynamics.
          • Angular/Linear Joint Acceleration/Velocity Coefficient: A weighting factor used during joint motion estimation to determine if a cable joint is relevant for the cable dynamics.
          • First Boundary Active Joints: The number of joints in the flexible cable that will not be adaptive at the first extremity.
          • Last Boundary Active Joints: The number of joints in the flexible cable that will not be adaptive at the last extremity.
      • Advanced Flexible Definition
        • Max Elongation Percentage: Sets the maximum length of a section, relative to the Preferred Section Length, before the section merges or splits.
        • Max Number of Sections: The maximum number of sections a cable can have before exhibiting abnormal behavior.
        • Hard Limits Enabled: If checked, a section of a cable will not be able to compress or extend.
        • Self-Collision: If enabled, cable sections will be prevented from penetrating each other; otherwise collisions between cable sections are ignored and the cable can pass through itself.
        • Solver Group: Select the solver group this segment will be a part of.
        • Segment Type: Specify the type of this segment. A segment needs to have the same type as the one specified in the Grabbing Tool extension to enable the possibility to be grabbed.
        • Enable Point Spooling on First (Last) Extremity: Check this box to enable point spooling. Use point spooling when you want a cable source without using a winch. This feature is only available for the extremity of a flexible cable segment. Also, the Point Type must be Attachment Point (not Winch).
          Once you select this box, the Point Spooling on First (Last) Extremity Velocity field appears in the Inputs section of this Properties panel.
      • Points: A Bezier curve is created from these control points, helping to define the shape of the cable segment. The Size field determines how many control points are used and the three fields below the Size field determine each point's X, Y, and Z values.

Creating Generic Cables

A generic cable type in Vortex® Studio is a cable that can be customized to create a specialized cable to fit a simulation.

A cable definition is a container for a number of parameter, point, and segment definitions used to construct a cable system. Parameter definitions determine certain physical or visual aspects of the cable in general, but these general settings can be overwritten on each (full) segment.

This procedure explains how to create and configure a cable definition. Depending on what type of cable, the configurable properties will change appropriately.

If you do not have a mechanism file containing these parts, you can follow the instructions in Building Assemblies from a 3D Model and Building Parts from a 3D Model to import a 3D model that represents the equipment you want to use with the cable (for example, a crane) and then create parts for it (for example, winches, pulleys, booms, rings, and/or loads) as needed.

To add a generic cable, complete the following:

1. In the Toolbox, select Cable Systems.
   
2. Select Generic Cable and drag it into the 3D View, or onto the desired mechanism in the Explorer panel.
3. In the Explorer panel, under Generic Cable, click Dynamics Generic Cable. The Properties panel opens on the right.
4. For a generic cable, in the Properties view, in the Point Definition header, add the amount of points that are required for the cable. You need to define points for at least two places on your cable: a starting point (for example, where it will attach to your mechanism's winch) and an endpoint (where it will interface with a ring on a hook). Before you begin defining your points, make sure you consider all the points you will need to make in order to create the desired cable, particularly because you need to create points in a specific order: START, MID1, MID2, MID3, END, etc.

5. For each point created, you must select the point type and the part associated with it. Under Point Type, select a type from the drop down menu. Click the ellipses inside the part parameter and click the part to be associated with the point.

   Select the type before selecting the part to have Vortex Studio automatically set parameters.

6. You can adjust the Radius, Density, Material Name and any other of the parameters that appear under the Param Definition section in order to customize how the cable behaves and interacts with other elements in the simulation.

   Materials are case-sensitive, so it is very important that you type the name of the material exactly as it appears in the Materials Table.
   At runtime, if Vortex Studio cannot find the material matching the name you typed, then it will use the default material instead.
7. In the Generic Cable Properties page, on the Collision Geometry Type drop-down list, select the geometry shape to use for generating contacts. For example:
   • Sphere generates a sphere collision geometry at the intersection of each part.
   • Capsule creates a capsule collision geometry on a segment. With flexible segments, several capsules are created to follow the flexible segment shape.

You can further change the way the cable looks by customizing the cable in the Explorer panel and clicking the Graphics Cable extension. You can also use stiffness and damping calculator to customize the cable.In the Properties view, under the Segment Definitions header, you must make sure that the Flexible check box is unchecked to create a umbilical subsea cable.

Dynamics Properties

In Vortex® 2020a, Cable Systems includes an extension tool to define the Vortex cable system's physical parameters stiffness and damping.

This extension allows the user to set parameters that can be obtained from existing cable manufacturers. From these values, the extension computes a set of stiffness/damping outputs that reflect the physical properties of the cable. Any changes to one of these input parameters will trigger an update of the cable system parameters. The output values can be viewed in the Outputs section of the Dynamics Properties. These values are automatically applied to the corresponding cable system parameters.

The Dynamics Properties extension is automatically created with any new cable system. The tool can be used during runtime to change the cable parameters during simulation.

In the Explorer panel click the Dynamics Properties extension; then adjust the parameters provided by the cable manufacturer in the Properties panel.

Inputs

The set of input parameters used to calculate the cable's physical output parameters are listed below:

The Relative Elongation and the Load are used to compute the axial stiffness of the cable system. A smaller Relative Elongation and an higher Load will result in a higher axial stiffness.

The Young's modulus, the Cable Diameter and the Number Of Wires are used to compute the bending and the torsion stiffness. The higher the Young's modulus, the higher the bending stiffness will be. Increasing the Cable Diameter will increase the bending stiffness as well, while an increase in the Number of Wires will make the cable more flexible. The Number of Wires can be set to 1, in which case the cable will behave like a rod of a given material with corresponding large Young's modulus value.

Scaling Factors

In addition to the inputs described above, the Dynamics Properties extension allows you to fine tune the Outputs values by modifying each of them by a scaling factor.

By default, each scaling factor is set to one. For example, if the torsion stiffness of the cable is a too high, a scaling factor of 0.9 will have the effect of decreasing the torsion strength by 10%. Scaling allows the user to make adjustments to the cable behaviour.

Breakable Parameters

While not contributing to the Outputs values, you can determine when the cable will break through these Breakable Parameters values:

• Enable Breakage: Specifies whether the cable will break when the tension surpasses the Maximum Tension value.
• Break After X Count: Determines the cumulative number of frames where the tension is greater than Maximum Tension after which the cable will break.
• Maximum Tension: Specifies the maximum tension supported by the cable system before the cable breaks.

Snapback Parameters

The Snapback feature is an effect that is applied on top of the cable or loop group breakage feature. It simulates the result of the quick release of energy that was stored in the cable at the time of its breaking. This extension provides parameters to model the unpredictable effect of a snapping cable.

The available parameters are:

• Enable Snapback: Selecting this box turns the Snapback feature on.
• Snapback Force Scale: Specifies a multiplying value that determines the magnitude of the snapback force. The magnitude of the force is equal to the tension that exists in the cable at the breaking instant multiplied by this value.
• Force Direction Mode: Selects the direction of the snapback force. The choices are:
  • Random: The direction is randomly chosen in the tension plane.
  • Target: The direction goes from the breaking point to the Target Offset.
  • Target Projected: Similar to the Target option, but the force's vector is projected on the Tension Plane.
• Target Offset: Sets the local position offset for the target, relative to the Target Reference Frame. See below.
• Target Reference Frame: Selects any object to serve as the reference frame for Target Offset. See below.

As seen in the image above, the Tension vector runs along the cable, while the Tension Plane is represented as a circle perpendicular to the Tension vector. The center of the circle is located at the point where the cable is about to break. The gray arrow indicates the world space position of the target, defined as the Target Offset expressed in the reference frame of Target Reference Frame, which can be any object. "Direction to Target" is a vector from the breaking position to the Target position. The snapback force will be along that direction in the case where Force Direction Mode is set to Target. When the mode is set to Target Projected, the final direction of the snapback force is represented as the vector labeled "Direction to Target Projected". This corresponds to the vector "Direction to Target "projected on the Tension Plane.

If Force Direction Mode is set to Random, the snapback force will be in a random direction on the Tension Plane.

Creating a Point Definition

A point definitions list is like a container that holds a list of point definitions that comprise a cable.

Each point definition also stores information about itself, such as its type (winch, pulley, etc.), and which part on the mechanism it is associated with, etc. Each time a point definition is added, at least one segment definition is automatically added at the same time in order to provide a way to determine certain physical or visual aspects of that specific cable segment.

This procedure explains how to define a point definition on a cable and how to configure its adjacent segment(s).

1. From the Parameters section of the Properties Panel, locate the Point Definitions section.
   

   Initially, no point definitions appear until you add them, and no segment definitions will appear until you have defined at least one point.

2. Increase the value in the size numeric control until you have specified the total number of points you want to define.

   As you change the size value, you can see a new section appearing with the number as the section label.
   

3. From the Point Type drop-down list, select which type the point should be, keeping in mind that if you choose Winch or Attachment Point, you are defining an end point; and if you choose Pulley or Ring, you are defining a mid point. An Undefined Point type is not considered in the definition of the Cable System creation.

   For the Winch and Pulley types, the part must have a cylinder as a collision geometry and a hinge constraint in order to get proper cable behavior.

   As soon as you select a point type, the following parameters appear:

   • For winches and pulleys you get Radius and Inverse Wrapping.
   • For rings you get Relative Primary Axis.

4. Click the Browse button in the Part reference box.
   

   The Select Part dialog box appears.

5. From the Explorer Panel, locate the part that corresponds to this point and click on it.

   The name of the part now appears in the Select Part dialog box.

6. Set the offset: Offset with respect to the center of mass of the part where the cable will be attached. The offset is specified in the local frame of the part.
   
   Since for a winch and a pulley the cable should be at a radius distance from the axis of rotation, only offset along the axis of rotation will be taken into account. In the following image, even if the x offset is 3 meters, the small orange dot is still at radius distance from the axis of rotation. Only the z offset of 1 meter is taken into consideration.
   
7. Angle: The angle in degree for the cable around the winch. This setting is only available for the winch. This value helps determine how much cable will be wrapped around the winch at design time, and find the end point of the cable on the winch. At runtime, the cable can unwrap from the winch but the end of the cable will stay at the same spot.
8. 
9. Radius: The radius of the winch/pulley. Deduced from the collision geometries of the assigned part but can be overwritten by the user. The radius is taken from the first cylinder or capsule in the list of geometries.

10. Click the Confirm button on the Select Part dialog box.

    The name of the part now appears in the parameter box.

    You can customize the behavior and appearance of the cable at this point by modifying any of the other parameters in this definition.

11. Repeat these steps for every point you need to add and configure.

    As soon as you have added at least two points to your cable definition and changed the Point Type to anything other than Undefined, the Editor automatically creates a new segment definition too. See Modifying Segment Definitions for information on how to set up each segment.