General characteristics of the cord of a pulley system
Simscape / Multibody / Belts and Cables
The Belt-Cable Properties block sets the attributes of a pulley cord, among them its geometry and color, as well as its alignment constraints. For the purpose of visualization, the cord is treated as a one-dimensional line with color and opacity. Circular arc segments straddle the pulleys and straight line segments bridge the distances between the arcs.
By default, the cord can enter and exit a drum—pulley or spool—at an angle to its center plane. This angle can vary during simulation—for example, due to translation of the drum on a prismatic joint. While the contact point is always in the center plane of the drum, the drum can move when mounted on a joint. Use this strategy to model, for example, the winding of a cord on a spool and the fleet angle of the same (denoted θ in the figure).
The cord can also be constrained to enter and exit a drum in the center plane of that
drum. The assembly may still be three-dimensional, with different pulleys on different
planes, but only as long as the contact angles are each zero. Set the Drum
Belt-Cable Alignment block parameter to
Planar to enforce this constraint. (Because the constraint is
monitored, the simulation stops with an error if a contact angle should be other than
Other attributes are set automatically by modeling assumptions and calculations. Among the assumptions are those of a massless cord that is both inextensible and always taut. Length is determined at the moment of assembly and fixed thereafter. Its value is consistent with a cord that precisely spans the pulley system as arranged in its initial configuration.
That configuration depends on the starting positions of the various joints in a model. These are matched where feasible to the state targets specified in the joint blocks. It is possible to use those targets to set the length of the cord and to guide the placement of its ends—for example to apply an initial rotation to a spool or an initial translation to a load-bearing anchor.
The belt-cable port (P) identifies the cord characterized by this block. It matters little where in a belt-cable network the port connects. It is important, however, that each belt-cable network in a model contain one instance of this block. A belt-cable network is distinct from another if there is no belt-cable connection line between the two.
The visualization of the cord is by default on but it can be suppressed in the block.
If it is on, you can click any point on a cord to highlight its entire length. Look in
the tree view pane for the name of the Belt-Cable
Properties block associated with the selected cord—the block name is
identified there. Right-click the block name and select
Block if necessary to update the cord visualization properties.
P — Belt-cable network attachment point
Attachment point for the belt-cable network whose properties this block aims to specify.
Drum Belt-Cable Alignment — Type of alignment allowed between the cord and the center plane of a drum
Unrestricted (default) |
Type of alignment allowed between the cord and the center plane of a drum
(whether pulley or spool). Select
enable the cord to enter and exit the drum center plane at an angle. The
entry and exit angles can differ during simulation. Select
Monitored Planar to require that the cord
always enter and exit a drum in line with its center plane. If at any time
the contact angle differs from zero, the simulation then stops with an
Type — Means by which to visualize the cord
Pitch Line (default) |
Means by which to show the cord in a model visualization. The default setting corresponds to a pitch line that arcs around each pulley and spool at the pitch radii specified in their respective blocks. The line segments are circular in the ranges of contact between the cord and pulley or spool; they are straight in the distances between the arcs.