Recently I’ve been working on several modeling examples for an upcoming workshop. Creating a simulation model for any component presents many challenges, starting with figuring out exactly what you need to model. Naturally, the answer to this question determines not only what information the model can teach you about the system, but also, in general, how difficult the model will be to create.
For the workshop I’m developing, I needed a model for an electric seat, one that moves back and forth along a predetermined path – the mechanical portion of a mechatronic system; think of an electric seat in an automobile. There are several viable approaches for modeling devices like my electric seat including using equations, building blocks, or component-based modeling using components that represent physical devices in the system. I chose the latter, building my electric seat model using models that represent actual components of the system. While this approach is not new – electrical system designers have used it for years, modeling systems using models of real world devices like resistors, capacitors, diodes, and transistors – it is relatively new to mechanical modeling simply because libraries of mechanical simulation models are usually not readily available. Multi-physics modeling languages like VHDL-AMS make such libraries possible.
While electric seats can be a bit complicated in practice, modeling the mechanism that moves the seat back and forth is conceptually quite simple. There are three components: a leadscrew, a mechanical stop, and a mass – for my model, the motor is separate from the seat mechanism. The leadscrew translates the motor’s circular motion into linear movement; the mechanical stop limits the seats movement in both forward and reverse directions; the mass models the mass of the seat plus its occupant. With this simple description as the specification for my model, I used models from SystemVision’s mechanical library to create the following schematic:
Model of the mechanical portion of an electric car seat mechanism
And then I ran a couple of quick simulations to make sure my seat model worked:
Results of seat position simulation
The bottom waveform shows the voltage applied to the motor — 0 to 12.5 VDC. The top waveform shows the position of the seat as it travels along the leadscrew — 0 to 240 mm. Note that even though voltage is continuously applied to the motor, the seat movement stops at 240 mm, the limit set by the mechanical stop model.
While this is a simple example, it illustrates the mechanical system modeling possibilities if you have a library of basic mechanical device models.
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