Simulating for Mechatronics

Brian MacCleery
National Instruments Corp.
Austin, Tex.

Though the term mechatronics was coined back in 1969, it is still tough to find easy-to-use electromechanical design tools that enable design optimization across engineering disciplines. Few machine designers today have the resources to createa virtual prototype simulation that captures the dynamic interplay between mechanical, electrical, and controlsystem components. At the same time, the design process has only become more complex with the wider adoption of modern electronic controls and servodrives.

The trend toward complexity is expected to continue. Recent interviews with machine designers in North America and Europe revealed 29 of 30 companies expected their design process to become more complicated in the years ahead. To address this added complexity, system-level tools increasingly abstract away complexity but do so in a way that provides high-fidelity simulation models. These models enable creation of a virtual prototype early in the design process.

An ideal system for virtual machine prototyping should account for the dynamics of mechanical transmissions and payloads, electrical motors and drives, and such embedded control-software tasks as motion trajectories and PID tuning. Just as important, the tools must fit naturally into the design process and not require a team of specialists.

Fortunately, design-tool vendors increasingly realize that no one company can address this span of tasks alone. The result is better links between mechanical simulation, electrical simulation, and control design software. For example, National Instruments Corp. and SolidWorks recently collaborated to improve mechatronics-oriented design by posting online a pre-release version of their codeveloped NI LabViewSolidWorks Mechatronics Toolkit.

The project is still in its early stages and the toolkit is only for lead-users interested in evaluating the technology. But the goal of the project is ambitious; to make virtual-machine prototyping a reality by bridging the gap between 3D mechanical CAD and motionsystem design tools.

SIMULATION MADE EASY
To eliminate the need for math gurus on staff, the CosmosMotion simulation engine takes advantage of such existing information in 3D CAD models as part mates and mass properties to create a high-fidelity mechanical dynamics simulation. In the LabView graphical-programming environment, NI Motion Assistant technology is used to simplify the design of multiaxis motion trajectories consisting of straight-line, arc, or contour moves with specified velocity and acceleration constraints.

Under the hood, an ActiveX interface links the tools together. Clicking the run button in LabView generates the motion trajectory data, downloads it to CosmosMotion, and starts the simulation. After the mechanical simulation completes, you can optionally bring the torque and velocity results back into LabView for an electrical simulation to validate the selection of motor and drive components.

The technology under development promises to let engineers design and validate motion profiles, detect collisions, and simulate the mechanical dynamics of their machines (including mass and friction effects). The same environment will help estimate machine cycle time, verify component selections for motors, drives, and mechanical transmissions, and evaluate engineering trade-offs among mechanical, electrical, control, and embedded systems.

Using the toolkit, designers will be able to build complex motion profiles containing a series of sequential or concurrent move operations. For each move operation you can specify trapezoidal or S-curve profiles and apply velocity, acceleration, deceleration, and jerk constraints. The alpha version of the toolkit currently supports 2D coordinated-motion profiles and an unlimited number of uncoordinated motion axes. Each axis of motion in LabView maps to a constrained joint in CosmosMotion and is applied as a displacement-versustime array.

For visualization purposes, the 3D CAD model can be animated using the motion control profiles and timing/sequencing logic. Of course, mechanical animations have been available in CAD for some time. The key difference is that the simulations now can take place under the control of the same type of industrial-grade motion-control processing engine that will run on the physical machine. This simplifies the design of complex multiaxis coordinated-motion profiles and enables a much higher fidelity validation of the machine design.

The collision-detection feature in CosmosMotion allows validation of motion-profile designs using the actual 3D CAD model. Designers can now evaluate the need for interlock logic to prevent collisions, optimize motion profiles to minimize dead time, evaluate what-if scenarios, and test control-system logic without risking damage to a physical machine. After the machine has been deployed to the field, collision detection can also be used to validate new motion profiles, reducing the risk of unplanned downtime from programming mistakes.

All in all, a simulation that includes the actual motion-profile constraints and the mechanical dynamics helps in the accurate estimate of cycle time and machine throughput. LabView indicates the duration in seconds for the motion profile at the end of each simulation.

MOTOR AND DRIVE SIZING
Motor torque and velocity needs of a system depend on the acceleration qualities of the motion profile, the mechanicaltransmission components, and the payload. To view the torque and velocity profile charts for motion profiles, one need only right-click on the constrained joint in CosmosMotion after the simulation run is complete.

CosmosMotion models can include couplers that translate rotary motion into linear motion to simulate transmissions such as ball screws. Mechanical simulations can also account for mechanical dynamic effects such as payload mass, friction, and gravity. These simulations can help determine whether velocity and acceleration constraints are, in fact, feasible and confirm that trade-offs made during design are the right ones.

CosmosMotion uses the material properties in SolidWorks models to automatically calculate the mass of solid bodies and to help calculate the friction between materials such as acrylic and steel. Overall, the information about motion profile kinematics and mechanical dynamics that the toolkit provides helps more accurately select appropriate motors, drives and transmission components.

MAKE CONTACT
Design & Assembly Concepts Inc., design2assembly.com/
National Instruments Corp., ni.com/mechatronics
SolidWorks, solidworks.com