Motion Simulation Cuts through System Development Uncertainty

That new prototype actuator mechanism is complete. Weeks of design work, machining intricate parts, and assembling a complex motion system are happily at an end. The innovative design checks out on paper, and in CAD. Its physical package looks sleek and compact, as well. Yet, upon testing the actuator it doesn't accelerate fast enough or deliver the required force at the end of its motion.

08/01/2001


That new prototype actuator mechanism is complete. Weeks of design work, machining intricate parts, and assembling a complex motion system are happily at an end. The innovative design checks out on paper, and in CAD. Its physical package looks sleek and compact, as well. Yet, upon testing the actuator it doesn't accelerate fast enough or deliver the required force at the end of its motion.

What was left out of the equation? Motion system simulation.

Software simulation tools for motion control give engineers and product developers a fast track to optimal design, help visualize and analyze complex designs involving motion—as well as other design constraints—and ensure that products will work as intended.

Know before you build

A bonus for using simulation is what can be learned before machining any parts or building a prototype. Designs can be refined and changes made more easily from "virtual experience." The result is a higher degree of reliability and lower product development costs.

Simulation is not exactly design software or CAD, but the related ability to create life-like representations of parts or assemblies in motion—even whole motion systems.

Simulation goes beyond animation. For motion systems, one distinction is that after the good-looking animation is completed, physical properties of components and assemblies must be checked for real-world loads and stresses. Load information becomes the output to an analysis package, most often finite-element analysis (FEA). Ability to import CAD models into simulation software is promoting wider use of these tools for motion-system simulation.

The role of simulation is to help engineers visualize 3D motion under realistic conditions, then enable design refinements through iterative evaluations in software without building a physical prototype. Also known as "virtual prototyping," this process avoids numerous costly physical tests and design rework, explains Tim Webb, senior manager of marketing communications at Mechanical Dynamics Inc. (MDI, Ann Arbor, Mich.). "Many manufacturing companies use virtual prototyping software, such as ADAMS [Automatic Dynamic Analysis of Mechanical Systems] from MDI, to perform more design trade-off studies upfront and reduce costly physical prototypes," says Mr. Webb.

ADAMS—considered by many as the premier mechanical system and motion simulation software—was developed by MDI, a pioneer in this software arena. With ADAMS, users can build a parametric 3D mechanical system on their terminals and link it to a controller designed in Matlab/Simulink (or another control package) to simulate nonlinear dynamic motion of the controlled system.

The virtual model incorporates realism. System components include mass properties, and physical forces such as gravity, friction, and contact. "Simulation results consist of realistic animation, as well as plots and tables of reaction forces, accelerations, velocities, and positions of all system parts and linkages," adds Mr. Webb. For more realistic simulation, finite element modal results can be integrated with the dynamic model to accurately predict flexibility of components and system modal vibrations.

ADAMS serves as the dynamic "solution engine" for the modeling/simulation option embedded in various CAD systems and desktop tools (see examples below). Dynamic Designer, another software from MDI, is based on ADAMS and comes in three flavors of increasingly sophisticated functions: Simply Motion, Motion, and Motion Professional. It uses drag/drop methods, pop-up menus, etc., similar to office software. Output of simulations to Microsoft Excel and video files are also among Dynamic Designer's capabilities.

Popular computing engine

To handle overall control system computations, Matlab, from the MathWorks (Natick, Mass.), is often used with mechanical system simulation software. It offers an environment for creating control algorithms and analyzing/displaying the simulated results graphically. Simulink, part of Matlab, provides various block diagrams with which users can graphically model and simulate linear, nonlinear, hybrid, and other dynamic control systems, and observe their behavior. References to Matlab/Simulink by various software suppliers cited here attest to its popularity as a basic computing engine.

Motion control is one of four major simulation types interwoven into MSC.visualNastran 4D (vN4D)—one of the latest products from MSC.Software (Santa Ana, Calif.)—states Jim Neuner, strategic partner manager for MSC. "We have a catch phrase for the vN4D product that goes: 'Draw It, Move It, Break It, Control It' . " says Mr. Neuner. "These encompass the four main types of simulation that 4D provides."

Concentrating just on motion-specific functions within vN4D, "Move It" supplies kinematic and dynamic simulation, including interference detection, friction forces, and collision models. "Control It" focuses on inverse kinematics, enforced motion, and connects to Simulink from the MathWorks. ["Break It" simulates stresses using FEA technology.]

"Motion control capabilities come from a combination of features built into vN4D, and an open system architecture that allows linking to other programs and calculations," says Mr. Neuner. These features include a complete function language, allowing users to define motor torque, actuator position, or body position as a function of other (or combination of) system variables.

Inverse kinematics is a feature that permits defining the position or motion of a mechanism over time, while measuring physical values on its joints, motors, actuators, and other elements. "Inverse" refers to a complex method for finding motions of intermediate elements in a linked structure from known motions of other key links. Table-driven input enables defining motion drivers (position, motor and actuator parameters, velocities, etc.) via tabular data that can come from an Excel or text file.

Connectivity, FEA

Connecting to other programs is just as important in simulation software. Mr. Neuner attributes MSC.visualNastran 4D's connectivity to its "open" system architecture. Examples include connecting to Microsoft Excel—via the bi-directional link available with the table-driven input—and to other programs using the OLE Automation API (on-line embedding, application program interface) that comes with vN4D software.

The latest version of vN4D also connects to Matlab/Simulink and other analysis software. Generic I/O blocks in the bi-directional link created by MSC permit interactive tracking of positions, gains, and other dynamic system settings.

Other motion-related features of vN4D include proximity and interference "meters" that a user can insert to detect the closest approach between moving parts in an assembly and check for any interference. The software also enables dynamic motion and FEA simulation in one package.

Formerly known under the Working Model name, visualNastran is available in several configurations. Version 4D is a family of products exclusively for the desktop.

Algor Inc. (Pittsburgh, Pa.) emphasizes time-to-market issues that simulation tools can help solve. According to Mark Decker, Algor senior engineer, products have evolved to where "mid-range, competitively priced" motion and FEA-based simulation tools are available, but with a difference. Today's simulation tools, combined with finite element analysis capability, automate many FEA tasks that would otherwise take many hours. Moreover, extensive FEA experience or a Ph.D. in engineering is not needed.

Mechanical Event Simulation (MES) software from Algor allows the virtual simulation of impacts, drop tests, crash tests, and mechanism assemblies. Unlike pure kinematic solutions that assume rigid body motion, MES accounts for the elasticity of real components based on geometric and material properties. The software also handles inertial effects.

"Compared to the typical FEA process, MES reduces the overall design cycle by minimizing the need for downstream prototyping, advanced analysis, and testing. It also promotes concurrent engineering," states Mr. Decker. "The future of computer-aided engineering lies in this ability to accurately portray a product's intended behavior through integration of motion simulation, stress analysis, and multiphysics analysis."

Interference detection, more

SolidWorks Corp.'s (Concord, Mass.) main software product is SolidWorks 2001, which has a number of attributes for motion system simulation. Its 3D solid modeling methods help analyze parts of an assembly in motion. For example, the software is commonly used in machine design, according to Aaron Kelly, SolidWorks product manager, to check pick-and-place machine motions.

Interference detection is a useful function in simulating motion. SolidWorks 2001 has both "static" and "dynamic" detection capabilities. Dynamic collision detection finds potential crashes between parts of a machine or mechanism going through its motions. Dynamic clearance works with preset tolerances to find how close parts actually come to each other.

For more specific motion visualization involving rotation, sectioning, and exploded parts of assemblies, there is SolidWorks Animator, an add-on to the main product that performs the above functions automatically. Mr. Kelly highlights Animator for:

  • Design reviews, to simulate virtual machines running through their paces;

  • Dynamic enhancement of sales and marketing presentations; and

  • Documentation for assembling and servicing a machine.

"Motion simulation and control gets a lot of attention at UGS," says Keith Perrin, analysis product manager, "because so many of our customers' products involve elements that move in some way." UGS is the new streamlined name of Unigraphics Solutions Inc. (Cypress, Calif.), initiated in early 2001.

UGS' software product set, Unigraphics, focuses on motion at various levels, explains Mr. Perrin. The specific Unigraphics product that handles motion simulation is Scenario for Motion+. It includes static, dynamic, and kinematic motion simulation of multi-body systems, and combines Scenario with ADAMS software from Mechanical Dynamics Inc., a UGS Alliance Partner.

With Scenario's simulation paradigm, users can make efficient "what-if" evaluations and test their designs, checking and refining multiple design "scenarios" to the one with the best performance. Various motion-specific objects—such as joints, springs, dampers, motion drivers, and compliant bushings—are used to create and evaluate the design model or virtual prototype. All motion objects are associative to the underlying geometry, that is, objects update automatically as alternatives are evaluated, according to UGS.

Links to other analysis software within Scenario (structural, vibration, thermal FEA, etc.) are included, as well as direct linking to ADAMS, Matlab, and Simulink.

Solid Edge, another product of UGS, integrates motion-analysis capability into the assembly-modeling level. A "wizard" interface guides users through building kinematic analysis models, defining motion parameters and inputs, and executing the simulation. Automatic building of accurate motion analysis models from Solid Edge assemblies takes place through Simply Motion, a kinematics analysis and motion simulation package from MDI. "The package produces detailed animations of assemblies through their full range of motion, with interference detection that assists in identifying and eliminating problems and improving performance," adds Mr. Perrin.

Simulation on the web?

Future possibilities for designers and engineers include the option to run motion simulation and analysis software over the Internet. This is not a huge stretch from sharing other design information on the web.

Some of the latest Microsoft Windows-based products allow exchange of graphic and animation files online. For example, visualNastran from MSC.Software is web accessible, and a menu selection in MDI's Dynamic Designer/ Motion allows posting of animations on an internal web site for viewing by remote users.

However, business issues rather than technological ones will decide the extent of online activity. The question of how much design data can and should be moved onto the web, and various security and licensing issues, needs to be resolved. (See more in an Online Extra to this article at www.controleng.com.)

As in other areas of software, the migration of products away from older operating systems (Unix and others) creates wider applications and more user interest. The newest Microsoft Windows-based tools have brought motion system simulation to the desktop.




System simulation should be everywhere

Nearly every automation and machinery system involves motion that can benefit from software simulation. The gamut of applications runs from robotic motion analyses and articulation of heavy machinery to medical devices, industrial equipment (assembly lines, actuators, positioning mechanisms, etc.) and aerospace systems. Complex and sophisticated applications have the most to gain from motion simulation methods because of their inherent risk from less experience and unknown factors.

Carco Electronics (Menlo Park, Calif.), a manufacturer of motion simulators for aerospace and defense markets, applies MSC.Software's (Santa Ana, Calif.) visualNastran Desktop for near life-like simulation of its products. In a five-axis missile flight motion simulator for the U.S. Air Force Research Laboratory, several parts move independently (in close proximity) under computer-guided random motion.

Potential collisions must be avoided. visualNastran (vN) improved on a CAD-only software approach to perform this function. In case of a collision, a sharp spike is displayed on one of vN's software "force meters." Visualization aids in making the design changes needed. Carco reports 5-10% project cost savings with use of simulation, after cost of the analysis is included.

High Frequency Motion Simulator (HFMS), developed by Dearborn Gage Co. (Garden City, Mich.), is a similar five-axis machine that provides six degrees of freedom control needed for accurate simulation of defensive missile systems (three axes for missile motions; two axes for target motion and sensor tracking). With up to 1,000-Hz bandwidth capability, HFMS also overcomes response limits of older hydraulic simulators. MSC's visualNastran 4D helped accelerate analysis and design of HFMS. The model building process was cut from weeks to hours, according to Dearborn Gage, a leading manufacturer of precision gaging and inertial test systems.

UGS (Cypress, Calif.) mentions the role of its Scenario for Motion software in the design evolution of MK16 ejection seats developed by Martin-Baker (Denham, Buckinghamshire, U.K.). Scenario—part of UGS' Unigraphics software product set—helps simulate the highly dynamic behavior of ejection seat deployment mechanisms.

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