Building Flex Body Models - Kxcad.net

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8 Adams/Flex Connecting Flexible Bodies to Your Model Once you have your flexible body imported into Adams/View, you need to make it part of your model by attaching it to rigid bodies or other flexible bodies and applying forces to it as appropriate. You can also replace existing rigid bodies with flexible bodies. Learn more about how to make flexible bodies essential components of your model: • About Connecting Flexible Bodies • Limitations on Applying Elements to Flexible Bodies • Using Massless Links • Adding Markers to Flexible Bodies About Connecting Flexible Bodies When you initially import a modal neutral file (MNF) containing a flexible body, Adams/Flex places the flexible body at the global coordinate system origin. The flexible body is not attached to any other part in your model nor are its movements constrained. Using any of the features of Adams/View, you can connect the flexible body to elements in your model, apply forces to it, and attach points and markers to it. There are some limitations, however, and these are explained in Limitations on Applying Elements to Flexible Bodies. As you connect flexible and rigid bodies, you will often find it useful to create dummy parts, called massless links, to help you create your model. Also, the connection of some modeling elements is only supported through massless links so Adams/View often creates massless links between the bodies to facilitate the creation of the model. For more information, see Using Massless Links. If you haven't already done so, you may want to go through the first tutorial, Integrating Flexible Bodies into Your Adams Model, of Getting Started Using Adams/Flex, to get some ideas for how to connect flexible and rigid bodies. Limitations on Applying Elements to Flexible Bodies Currently, there are a few limitations to the elements you can use with flexible bodies. Most of these limitations are based on the version of Adams/Solver that you are using, either the FORTRAN or C++ version. Most of the limitations are removed if you use Adams/Solver (C++) as the solver executable. Learn more about the limitations: • Flexible Connector Limitations in Adams/Solver (FORTRAN) • Force Limitations in Adams/Solver (FORTRAN) • Force Limitations in both Adams/Solver (FORTRAN) and (C++) • Joint Limitations in Adams/Solver (FORTRAN) • Table of Supported Forces and Joints
Flexible Connector Limitations in Adams/Solver (FORTRAN) Building Flex Body Models The following Adams/View flexible connectors have not been implemented for use with flexible bodies in Adams/Solver (FORTRAN): • Beams • Bushings • Field elements • Torsion spring (Rotational spring-damper elements) In each case, the flexible connectors can indirectly act on the flexible body through a dummy part, called a massless link. If one of these elements is connected directly to the flexible body, Adams/Solver (FORTRAN) automatically introduces a massless link for you. Learn about Using Massless Links. Force Limitations in Adams/Solver (FORTRAN) Like the rotational spring damper, you cannot directly attach a rotational single-component force to a flexible body in Adams/Solver (FORTRAN). You need to use a masslink to connect the force to the flexible body. Learn about using massless links. Force Limitations in both Adams/Solver (FORTRAN) and (C++) You cannot apply the following forces to a flexible body when the flexible body is the reaction body: • Six-component general force vector • Three-component force vector • Three-component torque vector Again, you can avoid this limitation by attaching massless links to your flexible body, which makes the massless link the reaction body. It is important, however, to understand the consequences of making the massless link the reaction body. For example, consider a rigid sphere rolling on a flexible surface. Ideally, you would model the contact using a force vector element with the I marker on the bottom of the sphere and a floating marker on the flexible surface. Because a point of contact on a flexible body must be a set of nodes determined a priori, you cannot define a floating marker on a flexible body. Therefore, the flexible body cannot be the reaction body. As a workaround, assume that you attach a massless link on some remote location on the flexible surface and define the floating marker on the massless link. Adams/Solver would simulate the model without difficulty, but the effect you modeled is now different from the one you intended to model. Rather than modeling a point force sliding over the flexible surface, you modeled a varying point torque acting on the flexible surface at the location of the massless link. For this reason, Adams/Solver does not automatically introduce a massless link like it does when attaching an unsupported force to a flexible body. When the flexible body has a large number of modes (more than 40), the computational cost can be considerable. In these cases, you will find it more expedient to attach a massless link at the location of the flexible body marker so that the function expression depends on part states rather than the higher number of flexible body states. 9
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