Excel Solver | Why does Simplex LP not work on this model?

First, in order to use the Simplex engine, all your equations must be linear. This means that you cannot multiply, for instance, a binary variable and a continuous/integer one (I haven't checked if this happens in your model)..

Second, Simplex will not work with SUMIF, or any other IF function, as you can check here.

Building a linear model may be hard, but don't give up.

HTH,

Francesco

I have checked your workbook, and didn't find any problem. There are a couple of SUMIFs that can be easily changed to simple sums. I have also simplified a little your setup.

With your original data, OpenSolver can easily give you a solution. Playing with inputs (setting ZD.03 <= 1 instead of = 1, or relaxing the binary constraints) I got reasonable results. Yet, Solver still complained about the model being non linear, without a single hint about what's wrong: the linearity report highlights every constraint.

After rebuilding the model with inputs/1000 (as given), Solver gladly reported the solution I'm attaching. I think it may have been a problem of scaling, but it's a wild guess. Anyhow, it's good practice keeping figures as small as you can.

HTH,

Francesco

Have you checked cells K23:K25? I haven't really improved your model, just rearranged the layout to be sure it was linear.

If you divide by 1000 your inputs and change the SUMIFs, Solver works fine with your original workbook (and gives you the same solution, of course).

HTH,

Francesco

After some test, I confirm this is a problem of scaling, and it is a known "feature" as reported here.

With the original workbook, Solver complains about the model being non linear not right at the start, but after 13 iterations. This behavior is connected only to Capacity and Initial demand data, changing the magnitude of other inputs has no effect.

Rescaling inputs is the easiest solution, another one is setting a higher tolerance for constraint precision, such as 0.001.

HTH,

Francesco

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