SolidWorks

# Physical Simulation?

31 Jan, 2003

The ability to dynamically review the interaction between assembly components using SolidWorks software has been enhanced with the addition of Physical Simulation. Physical Simulation extends the capabilities of the Physical Dynamics functionality. Physical Dynamics allows assembly components to move and dynamically interact with one another when they collide. Physical Simulation allows you to assign elements (such as gravity, motors, springs) to an assembly component to create the simulation. With the latter, you don't need to manually grab the assembly component and move or rotate it according to its function within the assembly.

Both Physical Simulation and Physical Dynamics allow for the movement of assembly components along the under-constrained degrees of freedom. For example, in Figure 1, the gears are under-constrained along the axis of rotation. This allows the assembly component to turn around its axis. SolidWorks enables assembly components to dynamically interact with one another. When one gear touches another, both will rotate. This is the basis for Physical Simulation. The simulation can also be set up to stop when an interference or minimum clearance is violated.

 Figure 1. This chain assembly illustrates the realistic reaction of the chain to the gear when rotation simulation is applied.

Each assembly component has six degrees of freedom: up/down (+/- y-axis), in/out (+/- z-axis), and right/left (+/- x-axis) as referenced by the default axes. When an assembly is fully constrained, it can't be moved. There are times when an assembly component should be defined using either a parametric constraint (such as distance, angle, or length) or be left under-constrained to evaluate the movement of the assembly component. One of the advantages of creating a 3D solid and an assembly is you can use the assembly to check fit, form, and function. This does not have to be done with a static assembly. If an assembly can move, it should be defined in order to create a virtual prototype of the final product. Even though an assembly component won't interfere at one state (for instance, closed), it does not mean there won't be interferences at another state (for instance, open). Physical Simulation and Physical Dynamics allow the designer to analyze inferences across the entire range of motion.

Simulation vs. Dynamics

In many cases, Physical Simulation has advantages over Physical Dynamics.

It is easier to set up the simulation. By using the simulation elements provided, it makes the set up and editing process more straightforward.

Some of the simulation elements would be difficult to portray using Physical Dynamics. For example, gravity can't be simulated using Physical Dynamics.

Each of the simulation elements has assignable attributes. For instance, users can specify the strength of gravity or the velocity of a rotary motor. These attributes can't be created or defined with Physical Dynamics.

The simulation objects can be viewed and defined within the FeatureManager design tree, which makes them easy to change.

The simulation can be saved for later playback. If assembly components are moved, replaced, or deleted, the simulation can be rerun.

Setting Up a Simulation Session

To set up a simulation session, you can follow these simple steps:

• Constrain the assembly components
• Leave the desired assembly-motion degrees of freedom under-constrained
• Record the simulation
• Playback and review the simulation

Definable Elements

When creating a simulation, there are various elements you can define.

Gravity pulls components in a specified direction. Note that gravity observes the following rules: all components will fall at the same rate, regardless of mass; only one gravity element can be defined per assembly; and the motion created by a linear or rotary motor will override the gravity component.

Linear Motor moves components in a specified direction. Linear and rotary motor elements observe the following rules: motors move a component in a specified direction (they are not forces); users should not add more than one motor to the same assembly component; mass does not affect the motor's strength; and motion defined by motors will supercede the motion defined by gravity or a spring.

Rotary Motor rotates components around a specified axis.

Spring moves components in a specified direction. When used with Linear Motor, Springs behave as follows: they apply force to an assembly component using a free length and a spring constant; higher spring constant moves a component faster; mass is taken into account for a spring; and motion created by a spring will stop at its free length.

Recording and Playing

After the assembly has been created and the simulation elements added, it's time to record the simulation. When recording, users can't change the assembly by using the Move or Rotate Component tools. The movement is created by the way the assembly is constrained (mated) and the attributes of the simulation elements. This animation is then saved within the assembly. If assembly components are modified (moved, deleted, and so on) the simulation will need to be re-recorded.

You can use the Simulation Toolbar, shown in Figure 2, to define the simulation elements or record and playback the simulation.

 Figure 2. The Simulation Toolbar allows a user to define elements and record or playback a simulation.

Ball and Ramp

As shown in Figure 3, the ball is only constrained within the assembly so that it stays on the center of the track. Gravity is added as a simulation element. When the simulation is replayed, the ball falls down the track and off the bottom rail. Only assembly components affect the simulation; reference geometry (planes) and surfaces do not alter the simulation.

Bike Chain

As shown in Figure 1, the chain and gear are constrained along a plane that keeps the chain and gear lined up along the plane of rotation. The gear is constrained so that it aligns with the mounting block and can turn freely around its axis. The last chain link is mated to the gear so that they both remain concentric. A rotary motor is defined to turn the gear clockwise.

 Figure 3. This ball-and-ramp scenario illustrates the impact of gravity.

When the simulation is replayed, the chain moves to follow the gear. Gravity is not added to this simulation because it will cause the chain to fall before the gear starts to turn. To complete a more accurate simulation, however, the other end of the chain needs to be attached to the other gear, and gravity should be added.

Conclusion

SolidWorks' ability to define simulation elements directly within an assembly can be used to create better, more meaningful assembly motion studies. Errors caught within this stage of the design are easier and cheaper to correct.

While Physical Simulation offers many of the features of a dynamic-motion package, there are applications where a product such as COSMOSMotion may be a better fit. Engineers and designers who need to define additional physical attributes (such as forces, displacement, and others), require engineering test data based on the analysis, or may require a tighter link between motion analysis and finite element analysis (FEA). In these cases, users may want to investigate the differences between a standalone motion-analysis package and SolidWorks software.

# About the Author: Greg Jankowski

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