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| Conditional Equations Posted by Wayne McClelland to the ICCON Bulletin Board, 3-August-1998
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As we know, with I-DEAS it is easy to relate one dimension to one or more other dimensions, either within the same part or within an assembly. But sometimes we have a need to represent some "conditional" relationship or discontinuous function. For example:
"if the length of part A is less than 50 mm then the width of part B equals 7 mm otherwise the width of part B = 9 mm"
It turns out there is a little-known area of I-DEAS called "Part Equations"(in the Dimensions icon stack) that does exactly what we want. This brief article will summarize how this is done using a customer example disguised as a "baby crib".
Specifically our design problem is stated as:
 | the assembly is made up of a "base", "uprights", and the upper "rail" |
| the overall length (TotalX) and width (TotalY) are the driving parameters |
| the number of uprights in the X direction (nx) is a conditional function of TotalX and the number of uprights in the Y direction (ny) is a conditional function of TotalY |
| the length and width of the base and rail are tied directly to TotalX and TotalY |
| we want the whole assembly to update when we change TotalX and/or TotalY including the proper selection of the number of uprights |
First you may want to download the model so that you can follow how one accomplishes the above with I-DEAS.
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One key to making this work is to understand that conditional functions can be defined within parts but not for assemblies. Therefore what we first do is to put all the uprights into one part. In this case we simply created a pattern of uprights (using TotalX and TotalY as driving dimensions) and then cutout the unwanted array elements in the center. For more elaborate situations you could model all the separate pieces and then "Add" them into one part. Again the key is that the conditional expressions belong to the part.
Before we examine the conditional expressions, let's just look at how the assembly is tied together with standard assembly equations.
| in our specific example, the overall base length (base_xdim) is actually TotalX plus the width of one upright (up_x) and similarly for the base width (base_zdim) |
| the rail dimensions (rail_x and rail_y) are matched to the base size and the rail section size (raildx, raildy) matched to the upright section (up_x, up_y) |
| assembly constraints are used to tie the uprights to the base and the rail to the uprights. |
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Now we get to the "conditional" aspect of our design. The Part Equations for the "uprights" part are:
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Here we see our desired conditions:
| the number of uprights in the x direction are either 3, 4, or 5 based upon the value of TotalX |
| the number of uprights in the y direction are either 2, 3, or 4 based upon the value of TotalY |
In our case, the parameters nx and ny had been given numeric values during creation of the initial pattern of uprights. As shown in the bottom portion of the above form, it is key that these parameters are set to "From Equation" which means that they become dependent variables defined by the part's overall equation network.
Let's see how the assembly behaves. If we do a Modify, pick the uprights, and bring up the Dimension Values form... we can now set a new value for "TotalX" to say 4000 mm and "TotalY" to say 6000 mm. Then hit the Update icon and watch the number of uprights in x and y change (graphic at the top of this article). Cool.
So in summary, what the Part Equations capability is allowing us to do is specify discontinuous expressions that graphically, in their most general form, look like the following.
