“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. Other closely related terms for the same types of studies are multibody dynamics, mechanical system simulation, and even virtual prototyping.
Kinematic analysis is a simpler task than dynamic analysis and is adequate for many applications involving moving parts. Kinematic simulations show the physical positions of all the parts in an assembly with respect to the time as it goes through a cycle. This technology is useful for simulating steady-state motion (with no acceleration), as well as for evaluating motion for interference purposes, such as assembly sequences of complex mechanical system. Many basic kinematic packages, however, go a step further by providing “reaction forces,” forces that result from the motion.
Dynamic simulation is more complex because the problem needs to be further defined and more data is needed to account for the forces. But dynamics are often required to accurately simulate the actual motion of a mechanical system. Generally, 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 programs follow this basic process since it is a logical way to solve the problem. Most solvers are available as independent software programs.
One of the reasons for the popularity of solid modeling is that it sets the stage for many applications. You can practically create working drawings automatically, rendering models that closely resemble the real objects and generating physical models from rapid prototyping equipment. Similarly, studying the motion of moving mechanisms and assemblies is rapidly becoming almost a “free” byproduct of solid modeling, helping engineers to do the following:
The basic output of motion studies are numerous, including animation, detecting interference, trace functions, basic motion data, and plots and graphs. Animated motions are the classic output of simple kinematic analyses. Initially, the designer uses simple animation as a visual evaluation of motion to see if it is what is desired. More sophisticated animations can show motion from critical angles or even inside of parts, a definite advantage over building and running a physical prototype.
The ability to detect and fix interferences without switching between software is one of the primary benefits of integrating motion simulation and CAD. Most systems provide color feedback, for example, by turning to red parts that experience interferences. 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.
Trace functions provide additional information about motion. The motion of a joint or a particular point on a part can be plotted in 3D as a line or surface. Or, the system can leave copies of the geometry at specified intervals. Such functions can provide an envelope of movement that can be used to design housings or ensure clearances.
Motion data, such as forces, accelerations, velocities, and the exact locations of joints or points on geometry can usually be extracted, although such capabilities are more applicable to dynamic simulations rather than kinematic studies. Some systems allow users to attach instruments to their models to simplify specifying what results they want to see.
Most packages provide a plethora of plotting and graphing functions. Plots and graphs are most commonly used because values vary over time and are more meaningful than a single value at any given time. An 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 size of motors, actuators, springs, and other mechanism components.
Forces that result from motion are of particular interest because 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 of 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.
Engineers have used specialized software programs for performing various analyses for years in projects such as automobile suspension design. Doing all of the tasks in a single CAD program is becoming routine as solid modelers are being tightly linked to motion simulation software.