Programming and Units.... 2

[Qshake]

All

I would appreciate any advice on how to solve the attached problem with units.

Before I tried to use units in this MathCAD sheet, I was able to evaluate the function, Vu(x) for the maximum and minimum as shown in the attached sheet. Now, having defined units for all the variables necessary to calculate Vu(x) I cannot get anything. The MathCAD sheet outlines the Vx:=Vu(x) as all red, meaning undefined.

I've tried a lot of things to no avail. I can see from the MathCAD website that units can be used in programming.

I appreciate any help.

Thanks,
Q

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[davidbeach]

Probably ran into SUC, which sucks. SUC stands for static unit checking and is one of the worst new "features" of versions later than 11.2a.

 

[IRstuff]

Is it possible to post the actual worksheet, preferrably in M11 format?

On the face of it, the only thing I can see at all is that your indexed x is actually defined as 0ft..48.75ft², since you have Ltot*ft in the definition of x.

TTFN

FAQ731-376

 

[Qshake]

IRstuff - thanks for the clarification on the ft^2. I had been trying so many things that I've left a few ends open.

If I can back save to M11 I will repost tomorrow.

Thanks again.

Q

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Qshake]

BTW, I also found out via examples on the MathCAD website that when using units with plots or graphs, the units will default to the base units. I didn't know that either beforehand. I just assumed if you had a variable in kips, it plotted in kips and not pounds.

And for my example, I see that there is an issue between pound-mass and pound-force. SO to read the correct values from the graphs I produced you have to divide Vu(x) by 32,200 to get an answer in kips.

I suppose somewhere in the beginning of my definitions for units I didn't distinguish between mass and force. But then I did try to rely on the MathCAD base units.....

I appreciate the tips.

Regards,
Qshake

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[IRstuff]

The workaround of choice, if such a thing makes any sense, is to divide the quantities being graphed by the units desired, i.e., a force function divided by lbf. This effectively normalizes the display graph to the desired units, and avoids any transcription errors with conversion factors.

TTFN

FAQ731-376

 

[zdas04]

Like IRSTUFF said, if you put TOTAL_FORCE/kcs in your y-axis (for example) you get the right magnitude line with the benefit of it being self-documenting.

David

 

[pstuckey]

You can't have units in a vector subscript. V[x, and M[x both require x to be unitless. You could divide by ft, or define x as unitless initially then multiply by ft where necessary.

Peter

 

[Qshake]

pstuckey, et al -

I owe you all an update (perhaps just to help you or others in the future).

Where I defined the range, x:=0..Ltot, I now know that when using units you must use the format, x:= 0ft, 1.0ft..Ltot, where Ltot has units of feet. In this case, the 1.0ft is the increment of the step. Once I found that out, the Ltot was then defined, x was defined and condequently Vu(x) was defined.

Then came the matter of what pstuckey notes. I had to divide the subscript, x in Vx by ft in order for it to be unitless. Once completed the whole set up works just as it did.

Whew! I love the program but like many programs wish there were more clear error messages.

Thanks to all for your help and directions.

Q



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[jghrist]

I think it is clearer to first define a range variable, like n:=0..N-1, where N is the number of elements.
Then define the vector x[n:=n*ft

 

[UcfSE]

Does your series of "if"'s require an "otherwise" at the end? I don't see one.