Engineering Optimization

34Troubleshooting Optimizers

34.1 Introduction

This chapter provides a collection of perspectives and insights that may help with identifying issues and solving them when applying optimization.

34.2 DV Values Do Not Change

If a DV value does not change as iterations progress, and if the DV keeps the initial value, then this is a signal of any of a number of undesired aspects. It could be that the coding for the optimizer does not include the DV that is not changed. Perhaps the variable name is misspelled. It could be that the OF calculation is independent of that DV (either because coding erroneously omits it from the OF calculation or because it is irrelevant to the OF or because the OF is effectively insensitive to the DV). It could be that the initial value of the DV is on a constraint or in a flat spot. Don’t accept DV* results if any DV does not change with iteration. Something is wrong. Investigate the surface topology. Check the coding, logic, and constraints.

34.3 Multiple DV^* Values for the Same OF^* Value

Again, there could be several reasons that distinctly different DV* sets give the same OF* value:

• Discretization: The objective function may have flat spots w.r.t. DV value. This could be the result of an aspect that discretizes the continuum DV to integer or other finite values. Here, small changes in the continuum DV will lead to the same discretized value for the OF calculation, and at the optimum the continuum DV will have a range of values. You can let the ...