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Pro/Engineer   May 2003   Tip-of-the-Month

Sign Changes and other Relations Issues

 The Desired Product
Figure 1  -- The desired result
(A relation driven "Pin" pattern.)

Relations are awesome!  They provide an array of solutions for numerous modeling scenarios and control.  There are, of course, some funny issues, and I ran into one the other day that proved interesting.  Here is the problem, and the solution.

The Problem:

... How to make relations work with negative values with pattern tables?  In this case, it was desired to have the some of the "pins" symmetric (right side to left side) and some positioned independently, so a pattern table was used.  (Figure 2.)

To assure symmetry, a relation was created, but an error message was given (Figure 3).  What does that mean?  I tried several things with the $, but nothing.  Finally, I called PTC.

Pattern Table    Relations Error Message
Figure 2  -- Pattern table for "PINS".
Figure 3  -- Relations Error Message.

The Solution:

The answer is a $ must precede a value that MIGHT be assigned a negative number (ie. $d21=-d4).  When I tried this previously, I got another error message, which seemed to reflect the same condition, but in fact, it was complaining about its own error message.

TIP:  When you get an error message, you have to erase it or you will get more errors.  (Too bad it can't figure out its own error messages.)

 A Pin OFF
Figure 4  -- A pin on the wrong side
(A result of very strange sign control.)

A Second Problem:

The relations don't always work as you would expect.  Writing the equation d24=d21 would normally assign d24 the same value as d21.  So, you'd think that if d21 were negative, then d24 would also be negative.  Not always in Pro/E. 

(I wonder if this is the new math that my mother always complained about.)  The result is shown in Figure 4.

The Second Solution:

According to PTC Tech Support, there is no way to predict the sign of a value, or what will work in an equation until you've tried it.  (Who programmed this stuff anyway?)

PTC's advise is "Trial and Error".  That, of course, makes for some interesting models (and lots of failures) because you just never know what will happen.  (See Figure 5.)

You just never know! 
Figure 5  -- You never know what you'll get!

The Final Solution:

The resolution to this problem is up in the air.  They tell me they can't file an SPR (bug report) because it works as it is supposed to.?.?  (But they can't tell you how it's supposed to work.)  I guess this is another of those laughable reflections of the pitiful lack of forethought that has plagued Pro/Engineer.

They tell me there are other issues with pattern tables and relations, but would not go into detail.  So, if you delve in, keep a smile on your face and have some fun.  You may be in for an amusing ride.


See you Next Month!
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