Motion System Design

Understanding motion simulation

In this installment:

• Simulation vs. other software
• Design verification
• Reverse engineering motion
• Modeling velocity, acceleration

When computer-aided engineering (CAE) methods became available for design work in the 1980s, finite element analysis (FEA) was the first to be widely adopted. Over the years, it's helped design engineers study the structural performance of new products, and replace costly prototype iterations with inexpensive computer simulations run on CAD models.

But now, mechanical products are increasingly complex and competition to bring designs quickly to market is more intense than ever. For these reasons, many engineers are using structural performance modeling with FEA as well as kinematics and dynamics simulation before building physical prototypes. So what exactly is motion simulation, and what specific problems can it solve?

Mechanism analysis and synthesis

Suppose an engineer is designing a trammel to trace different ellipses. Once mates are defined in CAD assemblies, they can be animated to show how individual components will move. Now, basic assembly animation can show relative motion of assembly components, but motion speed is irrelevant and timing is arbitrary. Finding velocities, accelerations, joint reactions, and power requirements of all involved components requires a more powerful tool, motion simulation.

In the case of our elliptic trammel example, only the motor speed, points to be traced, and required motion results need to be selected; the software does everything else automatically. It also uses material properties from the CAD parts to define mechanism component inertial properties, and translates CAD assembly mating conditions into kinematic joints. Then it formulates equations that describe the mechanism motion.

An added bonus is that results take little additional time to generate, because all the parameters needed for motion simulation are already defined in the CAD assembly, and need only to be transferred to the simulation program.

Numerical solvers in action

Unlike flexible structures studied with FEA, mechanisms are assemblies of components assumed rigid, with few degrees of freedom. To illustrate: When investigating an inverted slider's motion (an exercise commonly found in kinematics textbooks) the objective is to find the angular speed and acceleration of the rocking arm while the crank rotates at a constant speed. Several analytical methods can solve the problem, but students most frequently use a complex-numbers method.

However, solving such a problem by hand requires intensive calculations, and even with the help of computerized spreadsheets, it may take a few hours to construct velocity and acceleration plots. Then, if the geometry of the slider changes, the whole process must be repeated — making this an interesting assignment for undergraduate students, but completely impractical for real-life product development.

Motion simulation software can predict the motion of an inverted slider almost instantly, and check for interferences — which goes beyond CAD assembly animation capabilities. It conducts these interference checks in real time, and provides the exact spatial and time positions of all mechanism components as well as exact interfering volumes.

When geometry changes, the software updates all results in seconds, and those results pertaining to motion can be output graphically or tabulated in any format.

Now, it's true that engineers can represent simple mechanisms like trammels and inverted sliders as 2D mechanisms; it's difficult and time consuming, but the approach does provide analytical solution methods. In contrast, even the simplest 3D mechanisms have no established methods for analytical solution. This is where motion simulation really shines: Software can solve these problems in seconds — even if the mechanism includes many links, springs, dampers, and contact pairs.

Motion simulation also works in reverse: Product developers can use it for mechanism synthesis by converting trajectories of motion into CAD software, and using those to create new part geometry. For example, to design a cam to move a slider along a guide rail, motion simulation can generate a profile of that cam; then the engineer can define slider position as a function of time and trace slider movement on a rotating (blank cam) round plate, and finally send the trace path to CAD geometry to create the cam profile.

Designers can also use trajectories to verify the motion of industrial robots, test tool paths (to determine the most suitably sized robot), and establish power requirements without physical tests.

Another important application is simulation of motion induced by collisions. Even though certain assumptions must be made about the elasticity of such impacting bodies, motion simulation produces accurate results for mechanisms with components that may experience only temporary contact.

For more information, visit solidworks.com or motionsystemdesign.com. Look for the 2nd part of this series in an upcoming issue of MSD.

If an object can move without undergoing deformation, it has rigid-body motion, or a rigid-body mode. The presence of rigid-body motion classifies objects as mechanisms.

For example, a ball joint with an immovable base has three rigid-body motions because it can move in three independent directions, or rotations, without deformation. Three independent variables, also called degrees of freedom, describe the position of this mechanism.

A plate sliding on an immovable base plate has three rigid-body motions, because the sliding plate can translate in two directions and can rotate in one without experiencing any deformation. Again, three degrees of freedom describe the position of the mechanism.

A four-bar linkage, on the other hand, has one rigid-body motion. One independent variable (for example, the angular position of any link) describes the position of the entire mechanism. Note that depending on the detailed hinge design, hinge pins may have local rigid-body motions — that is, rotation about the pin axis and/or sliding along the pin axis.

All of these mechanisms may also have degrees of freedom called elastic modes, which result from deformation. In a four-bar linkage, for example, each individual link may perform some motion while also experiencing vibration. Modes of vibration require FEA analysis, not motion simulation.

Right the first time

Ward Machine Tool, Fowlerville, Mich., designs and manufactures custom lathe chucks for aluminum wheels, rotary actuators, and specialty machining fixtures. Ward's engineers use simulation for verifying whether or not a new design will work — before sending it to be manufactured. For example, the company developed and tested the dual-actuated/multi-range aluminum wheel lathe chuck shown here without testing any physical prototypes. Ward reports that use of SolidWorks and COSMOSMotion saved \$45,000 and reduced testing time to just 10% of the former build-and-test process.