PRODUCTS & TECHNOLOGY: MULTIBODY

Simpler is better — that’s whatwe’ve all been told. The more compli-cated something is, the more waysthere are for it to break. This seems logical and is something we shouldconsider as we invent new machines.The challenge is that simple machinesdo simple things and often can only do one thing well. A simple bottleopener, for instance, probably isn’tthe best tool for anything other thanopening bottles, but it does what it was designed to do. Complicatedmachines — both mechanical and bio-logical — have more parts, and oftencan be used to do more than one thing.As an example, the adult human bodytypically has 206 bones and can beused for all kinds of things from openingbottles to competing in triathlons.Inventing machines that can do a varietyof things requires that the machineshave multiple parts that work together,preferably without failing. Simulationtools in the product portfolio fromANSYS help make designing usefulmachines easier and faster, as well asmore fun.

JointsWhen machines were simpler,

there were fewer options, and multipleparts could be connected in mech-anical software from ANSYS only using shared nodes, beam elements,coupling, constraint equations andnode-to-node contact. These methods

www.ansys.comANSYS Advantage • Volume II, Issue 2, 20082020

Multibody Dynamics:Rigid, Flexible andEverything in BetweenAdvances in simulation solutions for machine features accommodate more complex designs.By Steve Pilz, Product Manager, ANSYS, Inc.

Images:motorcycle © iStockphoto.com/Paul Griffin bicycle © iStockphoto.com/Artsem Martysiok

As simulationcapabilities grow,an engineer’s ability to simulatemore complexmachines increases.

Joints were first releasedwith the COMBIN7 element,

which was used to model onlypinned, or revolute, joints. AtANSYS 10.0, major advances to jointtechnology were made via theMPC184 element, which could beused to model multiple joint types,such as those that are translational,cylindrical, spherical, slot, universal,general or fixed. Joint elements are particularly interesting to thoseinvolved with the design of multiple-part machines because they can beused to enable large rotations andtranslations between parts at a verylow computational expense. To illus-trate the potential computationalsavings of using joints, a metal hingeis used as an example. (Figure 1.)

were adequate formany years, buteventually generalsurface contact wasreleased to addressthe limitations. With thisnew functionality, parts undergoinglarge rotations, deformations, sticking, sliding and a host of other real-worldbehaviors could be modeled.

General surface contact becamepopular and widely used. It alsobecame more robust and efficient witheach successive ANSYS release formechanical applications and is nowconsidered mature, proven technology.One problem with the widespread useof general surface contact, however, is that sometimes it is more than isrequired. The relatively new capabilityto connect parts via joints has somepotentially huge advantages that canbe applied to many situations.

PRODUCTS & TECHNOLOGY: MULTIBODY

There are many ways to set up a model for a metal hinge, but thetwo used in this investigation are a traditional general surface contactapproach and a revolute joint approach (Figure 1). To simplify, the partsare set to be rigid so that problem size changes can be compared more easily. For each approach, a single CPU laptop is used to run the simulations.

In the general surface contact approach, to enable rotational free-doms but constrain all translationals except one, three contact surfacesare required (Figure 2) and one remote displacement, which rotates the hinge 90 degrees counter-clockwise. Using a few user-definedmesh specifications for surface contact size (body and edge sizing), theproblem consisted of 7,188 elements (Figure 3) and took 2,249 seconds to solve.

By changing from a general surface contact approach to a revolutejoint–based approach, there are three rigid parts and two joints connecting those parts to each other at the hinge: one revolute jointbetween the ear and the pin, and one fixed joint between the base and thepin. The pin could be suppressed since it won’t perform any function onceit is replaced with a revolute joint, but it is included in the model to makethe run-time comparison equivalent with the general surface contactapproach. The total problem size, as expected, is far smaller, uses only 14elements (Figure 4) and requires a solution time of only 1.625 seconds.

So what have we learned? First, if detailed contact information at the hinge pin is unimportant, it is a lot more efficient to replace thousands of contact elements with a single revolute joint element.

Doing that, the model can be solved in a fraction of the time it took tosolve without the use of joints. Second, as can be seen from the elementlisting in Figure 4, even in a model in which contact surfaces are notspecified, there are still contact elements — which come from use ofthe joint or MPC184 element — but far fewer of them.

TYPE NUMBER NAME

1 1 MASS21

2 1 MASS21 3 1 MASS21 4 1 CONTA176 5 1 TARGE170 6 180 CONTA174 8 1 TARGE170 9 178 CONTA174 10 180 CONTA174 11 178 TARGE170 12 576 CONTA174 13 1 CONTA176 14 1 TARGE170 15 832 CONTA174 16 1408 CONTA174 17 1408 TARGE170 18 288 CONTA174 19 832 CONTA174 20 288 CONTA174 21 832 TARGE170

TYPE NUMBER NAME

1 1 MASS21

2 1 MASS21

3 1 MASS21

4 1 CONTA176

5 1 TARGE170

6 2 CONTA176

7 1 TARGE170

8 1 CONTA176

9 1 TARGE170

10 2 CONTA176

11 1 TARGE170

12 1 MPC184

Figure 1. Hinge model Figure 2. For this hinge model, general surface contact joints are used in three locations.First, where the ear meets the base, frictionless surfaces prevent translation along the axis of the pin and still allow rotation of the ear and base against each other at the joint.Second, bonded surfaces between the pin and the base prevent the pin from spinning ortranslating relative to the base. Then lastly, frictionless surfaces between the ear and thepin allow the ear to rotate freely about the pin.

Figure 3. Element description for hingejoint modeled with general surface contact

Figure 4. Element description for hingejoint modeled with a revolute joint

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Joints: General Surface Contact vs. Revolute Joint Approach

Pin

Base

Ear

PRODUCTS & TECHNOLOGY: MULTIBODY

ANSYS Rigid DynamicsThe ANSYS Rigid Dynamics

module, first released at Version 11.0,makes extensive use of joints for connecting parts. This is an ANSYSWorkbench add-on tool for users who have ANSYS Structural, ANSYSMechanical or ANSYS Multiphysicslicenses. The module enhances the

capability of those products by adding anexplicit solver that is tuned for solvingpurely rigid assemblies. As a result, it issignificantly faster than the implicit solverfor purely rigid transient dynamic simula-tions. The ANSYS Rigid Dynamicsmodule also has added interactive jointmanipulation and ANSYS WorkbenchSimulation interface options.

Interactive joint manipulation allowsthe user to solve a model essentially inreal time — the explicit solver producesa kinematic solution with part positionsand velocities — using the mouse todisplace the parts of the model. This tool is on the menu bar in the Con-nections folder. New Configure, Set andRevert buttons can be used to exercisea model that is connected via joints, seta configuration to use as a startingpoint or revert back to the original con-figuration as needed. In the case shownin Figure 5, before finding a solution, thehinge has been rotated a little morethan 46 degrees to verify that the jointis, in fact, behaving like a hinge.

The ANSYS Rigid Dynamics mod-ule is run using the same techniquesthat are used in ANSYS WorkbenchSimulation — attaching to the CAD orthe ANSYS DesignModeler model,using the model tree, populating theConnections folder and inserting NewAnalysis, for example.

The combination of the explicitRunge–Kutta time integration schemeand a dedicated rigid body formulationcreates a product that while limited toworking only with completely rigid parts,

Figure 5. Interactive joint manipulation is possible within the ANSYS Rigid Dynamics module, performed on a computerscreen by using the mouse to move the model.

Figure 6. Folding arms of John Deere agricultural sprayer model to be subjected to time–history loading Image courtesy Brenden L. Stephens, John Deere

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Angle=46.9715 Degrees

PRODUCTS & TECHNOLOGY: MULTIBODY

is extremely well suited to solvingmulti-jointed assemblies, such as thefolding arm agricultural assembly (Figure 6). This scheme is adept at handling complex time–history input(Figure 7) and is extremely fast com-pared to more traditional solvers. Solvetime, even for complex assemblies, istypically measured in seconds andminutes rather than in hours and days.One caveat worth mentioning is that,at release 11.0, parts need to beconnected with joints rather than contact when using the ANSYS RigidDynamics capability. If contact isrequired to accurately represent thepart interactions, flexible dynamicssimulation is required.

The ANSYS Rigid Dynamics toolshould be used first on any complex,multi–part assembly with connections.Fast solution times can help usersquickly find joint definition problems,inadequate boundary conditions, over-constraints and other problems. Withthe time saved, multiple design ideascan be analyzed in the same amount of

time that it previously would havetaken to simulate a single concept.

ANSYS Flexible DynamicsIs the ANSYS Rigid Dynamics

tool all that is needed to fully under-stand a prototype of a machine?What happens if the parts deform?Will they break? Will they fatigue andfail after a short time or only afterextreme use? If parts bend, twist andflex, will the machine still perform itsintended function?

The ANSYS Rigid Dynamicscapability, for all its strengths, doesn’t provide a complete pictureof a machine’s performance. In a thorough machine prototype inves-tigation, the next step is a flexibledynamics analysis, which allowssome or all of the machine’s parts tobehave as they would in the realworld — flexing, twisting and deform-ing. Flexible dynamics allows users toexamine parts to identify whetherthey are stiff and light, as they wouldbe if made from titanium, or heavy

and flexible, as they would be if madefrom rubber.

A more in-depth explanation of theuse of ANSYS Structural, ANSYSMechanical or ANSYS Multiphysicsproducts running flexible nonlineardynamics simulations is necessary todemonstrate the steps required to takean all-rigid dynamics model and turn itinto a partially or completely flexiblemodel. This translation from a rigid to aflexible model includes material assign-ment, meshing and solver setup.Without writing a spoiler to any futurearticles on this subject, this is remark-ably easy to do.

The simple machines have alreadybeen invented. We don’t really need amore efficient bottle opener. With theaddition of more realistic and fastermodeling solutions — achieved by com-bining the ANSYS Rigid Dynamicsmodule and ANSYS Structural, ANSYSMechanical or ANSYS Multiphysics software — complicated machines canbe less prone to failure and producefewer career-limiting disasters. ■

Figure 7. Time–history loading at six different geometric locations along the sprayer model in Figure 6Image courtesy of Brenden L. Stephens, John Deere

ANSYS Advantage • Volume II, Issue 2, 2008www.ansys.com 23

CH43 - Vert1 Displ FBK (Displacement)CH46 - Long1 Displ FBK (Displacement)

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CH45 - Vert3 Displ FBK (Displacement)CH48 - Lat2 Displ FBK (Displacement)

0. 11.5 23. 34.5 46. 57.5 69. 80.5 92. 103.5 115.

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