This Client Server article is republished in its entirety from 2004 for reference purposes.
By Bentley Technical Support Group21 September 2004
Users who work with the 3D Feature Modeler in MicroStation V8 2004 Edition will have noticed the calculator-like symbol (Figure 1) in the 3D tools. They may even have wondered what that's all about, but not investigated further. That symbol is the Equation icon, and this article will explain its use in simple broad terms.
The process that uses these equations is known as variable driven modeling (VDM). Using VDM is similar to using parameter sets (see Dan Eng's excellent article, Using Parameter Sets, Client Server, July 14, 2004). While Eng discussed global variables, we are going to be looking at local variables in this article.
Using local variables, the ratios are defined by the properties of the slab that you define. Any setting with this symbol in MicroStation's Feature Modeler tools will let you assign a variable or an equation to that setting. For example, if you have a simple slab, you could set equations for it such that any changes made to its length and width would automatically correct the length/height/width ratio. In other words, the width would change proportionally if you decided to alter the length.
Why would you want to do this? There are many instances in which this would be helpful. For example, suppose you are designing a concrete culvert under a road. You could design it so that you could use your design again if you had a wider road to fit the culvert underneath. Another "for instance" would be if you had a metal angle bracket. You could associate the length to be a ratio of the thickness. If it were longer, the width would have to increase. Perhaps you have an injection-molded part with ribs along its length. The rib count could be made to increase proportionally as the plastic length increased. The list goes on!
To achieve this, equations can be assigned to all solids in the design model. If a variable is required for a particular solid, you can assign the variable to be local. Variables are changed automatically whenever a variable in the equations is altered. Equations need to be valid MicroStation expressions. They can be either algebraic expressions or trigonometric expressions (cos, sin, tan acos, sqrt, etc, etc.). MicroStation's mathematical capabilities can also be utilised (+, -, *, / etc, etc).
To summarize in practical terms, let's look at the concrete culvert example (Figure 2) I mentioned earlier.
We are now going to create a variable that will change the width of the culvert by a factor of two and extend the length by a fixed variable (say 400 units). To do this we call up the Modify Parametric Solid or Feature tool from the Modify Feature pallet. Refer to Figure 3.
Figure 3 Select the solid and notice that the editable parameters are highlighted (Figure 4). The Edit slab tool appears together with our little calculators at the sides of the length, width and height boxes. Note also that the H, L and W symbol appears in the center of the solid, helping us determine which direction is which.
That means for the width to be a ratio of the length by a factor of two, the following procedure applies. Select the Width equation icon, double click on Length to designate _SLAB1_WIDTH in the focus area and simply add "times two" so that the equation now reads _SLAB1_WIDTH*2. Click OK and the width will become 2x300L =600units (refer to Figure 5).
We now decide that the length has to be 400 units. This can be manually typed into the Length box rather than assigned to a variable, per Figure 5.
Figure 6 shows the completed culvert with the original alongside as a comparison.
Naturally, the example shown is rather simplistic for illustration purposes. You can make the slab be any variation you need using the mathematical expressions shown above. There are some others listed in the Variables and Equations topic of MicroStation's Help. You could, for instance make the slab to be two meters plus an eighth of its length and the height to be a quarter of its width.
For further reading on this and related topics, please look at MicroStation's on-line Help under the searches: "Feature Modelling Variables," "Variables and Equations," and "Working with Variables, Equations, and Constraints."
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