Effective partitioning of the multiple technologies present in a mechatronic system is another significant predictor of project success. Subsystem partitioning begins with a common-sense breakdown of the design, using Figure 1 as a highlevel framework. To the degree possible, separate out mechanical subsystems from electrical subsystems, and the same with controls and software. From there, subsystems can further be broken down into subcategories beneath the high-level distinctions, including, for example, digital, analog, and mixed-signal electronics; divisions in mechanical subsystems; and breaking out overlapping areas (e.g., sensors and actuators) as additional subsystems.
Next, subsystems can be assigned to specific job functions and design groups, and input/output requirements can begin to be defined at the boundary crossings between subsystems.2 Figure 2 shows the partitioning process, moving from functional design through implementation.
With this framework in place, the design and analysis can begin for each subsystem — later to be combined and analyzed as a complete system.
Simulation and Virtual Prototyping
In contrast to physical prototyping, virtual prototyping and system simulation allows a system to be tested as it is being designed, and provides access to its innermost workings at every phase of the design process (this is difficult or impossible with physical prototypes). Moreover, simulation provides for analysis of the impact of component tolerances on overall system performance, which is out of the question with physical prototypes.
When employed early in the design process, simulation provides an environment in which a system can be tuned and optimized, and critical insights can be gained, even before components are available and before hardware can be built. After the basic design is locked down, simulation can again be em - ployed to verify intended system operation, varying parameters statistically in ways that would otherwise be impossible with physical prototypes.
Subsystem and Component Modeling
In order to create a model for a system, each subsystem and component in the real system needs to have a corresponding model. These models are then stitched together (as would be their physical counterparts) to create the overall system model. Using the Department of Defense-initiated VHDLAMS modeling standard (IEEE 1076.1), system integration can begin before physical hardware is available, including embedded software or any other domain that can be described using algebraic or differential equations.
To be specific, VHDL-AMS allows expression of simultaneous, nonlinear differential and algebraic equations in any model; the model creator need only express the equations and let the simulator solve them in time or frequency domain. Domain knowledge from any engineering discipline can be encapsulated in reusable libraries that are accessible by any member of the design team.
The art of creating these models, and knowing exactly what to model and why, are keys to successful simulation. Some modeling include:
- Which system-performance characteristics are critical, and which can be ignored without affecting results?
- Does a model already exist?
- Can an existing model be modified?
- What component data is available?
Several software simulators exist for simulating mechatronic designs (such as SystemVision from Mentor Graphics). These simulators support VHDL-AMS, SPICE, and embedded C code in providing an environment in which mechanical, electrical, software, and systems engineers can collaborate using common models and a common modeling environment3. In conjunction with proper mechatronic system-design training, careful interdiscipline communication, and deliberate system partitioning, simulation technology can play a key role in mechatronic project success.
This article was written by Bill Hargin, Director of Product Marketing, System-Level Engineering Division, Mentor Graphics Corporation, Wilsonville, OR. For more information, click here.
- Aberdeen Group, System Design: New Product Development for Mechatronics, Boston, MA, January 2008. (www.aberdeen.com)
- Scott Cooper, Mentor Graphics Corp., Design Team Collaboration within a System Modeling and Analysis Environment, 2004. (www.mentor.com/systemvision)
- Ashenden, G. Peterson, D. Teegarden, The System Designer’s Guide to VHDL-AMS: Analog, Mixed-Signal and Mixed-Technology Modeling. San Francisco: Morgan Kaufman Publishers, September 2002. (www.mkp. com/vhdl-ams)