Introduction to Kinematic Analysis

Kinematic Software Vendors

 

Kinematic and Dynamic Analysis Software

"Motion study" is a catch-all term for simulating and analyzing the movement of mechanical assemblies and mechanisms. Traditionally, motion studies have been divided into two categories: kinematics and dynamics. Kinematics is the study of motion without regard to forces that cause it; dynamics is the study of motions that result from forces.

Kinematics is a simpler task and may be adequate for many moving-parts applications. Essentially, such simulations show where all of the parts in an assembly are in time as it goes through a cycle. This technology is useful for simulating steady-state motion (with no accelerations) as well as in evaluating motions for interference purposes, such as assembly sequences of complex assemblies. Many basic kinematic packages, however, go a step further to provide "reaction forces," that is, forces that result from the motion.

Dynamics simulation is more involved in that the problem needs to be further defined and more data comes as a result of accounting for forces. But, dynamics are often required to more accurately simulate the real motion of a mechanical system.

Generally speaking, kinematic simulations help evaluate form, while dynamic simulations assists in analyzing function.

Traditionally, kinematics and dynamics have followed the classic analysis software method of preprocessing (preparing the data), solving (running the solution algorithms, which involve the solution of simultaneous equations), and postprocessing (analyzing the results). Even though today's programs are much more interactive, most follow this basic process since it is a logical way to attack the problem, and most solvers are a separate program.

One of the reasons that solid modeling is becoming so popular is that it sets the stage for so many applications. You can practically create working drawings automatically, realistically render models that look like the real thing, and generate physical models from rapid prototyping equipment. Similarly, studying the motion of moving mechanisms and assemblies is fast becoming almost a "free" byproduct of solid modeling, helping engineers to:

• Interferences can be shown in a variety of ways. Most systems will provide color feedback, say, by turning parts that experience interferences red. More useful, however, are systems that turn the interference volume into a separate piece of geometry, which can then be used to modify the parts to eliminate the interference. Indeed, this ability to detect and fix interferences without switching between software is one of the primary benefits of integrating motion simulation and CAD.

• Trace functions provide more insight into motion. The motion of a particular joint or point on a part can be plotted out in 3D space as a line or a 3D surface. Or, the system can "leave" copies of the geometry at specified intervals. Such functions can also provide an envelope of movement that can be then used to design housings or ensure clearances.

• Motion data such as forces, accelerations, velocities, and exact locations of joints or points on geometry can usually be extracted as well, although such capabilities are more applicable to dynamics simulations rather than kinematic studies. Some systems allow users to attach "instruments" onto their models to make it simple to specify what results they like to see.

• Plots and graphs of such data are most commonly used since the values vary overtime and are much more meaningful than a single value at any given time. Most packages provide a plethora of plotting and graphing functions. One especially useful capability for studying design alternatives is to plot the results of two different simulations on the same graph. Such data can also help designers determine the sizes of motors, actuators, springs, and other mechanism components.

• Forces that result from the motion are of particular interest since they can be used as loads (or at least to calculate them) for structural analysis of individual members. Typically, the highest load for a cycle is used to perform a linear static finite-element analysis (FEA) of critical individual components in a mechanism. Integration of solid modeling, motion simulation, and FEA software can greatly streamline this process, especially important when studying design alternatives, where many analyses are required.

Performing these tasks with the aid of software is nothing new--specialized programs for such analyses have been used for years by engineers with a critical need for such technology for projects such as suspension design. But, doing all of these tasks inside your CAD system on an everyday basis is just now catching on as solid modelers are being tightly linked to motion simulation software. Indeed, some feel that such capabilities will become integral to solid modeling and thus a part of almost every engineer's tool box.