Modified Extensible Lattice Sequence (MELS)

A lattice sequence is a quasi-random sequence, or low discrepancy sequence, designed to equally spread out points in a space by minimizing clumps and empty spaces. This property makes lattice sequences an excellent space filling DOE scheme. This DOE type also has the property of extensibility, which means the method can take an existing set of data in a space, and add more data points to provide equal coverage; although with MELS it is optimal to extend on to MELS data. The number of runs is specified by the user.

Usability Characteristics

•	Use for exploring the entire design space and creating fitting functions to the exact output responses. It is the recommended default space filling scheme.

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To get a good quality fitting function, a minimum number of runs should be evaluated. (N+1)(N+2)/2 runs are needed to fit a second order polynomial, assuming that most output responses are close to a second order polynomial within the commonly used input variable ranges of -+10%. An additional number of runs equal to 10% is recommended to provide redundancy, which results in more reliable post-processing. As a result, this equation is recommend to calculate the number of runs needed or a minimum of 1.1*(N+1)(N+2)/2 runs.

•	Add existing data to the inclusion matrix to use the extensibility feature. While any data can be used as an inclusion, the best performance can be expected when the inclusion is an existing data set from a MELS DOE.

•	Supports input variable constraints.

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When building a MELS DOE with the intention of using it as a Validation matrix, the resulting Validation matrix could be a subset of the MELS based input matrix due to the extensible property of MELS. To prevent this from happening, change the Random Seed setting of the Validation matrix to be a number larger than the number of runs in the Input data before building it.

Settings

In the Specifications step, you can change the following settings from the Settings tab.

Parameter

Default

Range

Description

Number of runs

> 0 integer

Number of new designs to be evaluated.

Random Seed

Integer

0 to 10000

Controls the repeatability of runs, depending on the way the sequence of random numbers is generated.

0	random (non-repeatable)
> 0	triggers a new sequence of pseudo-random numbers, repeatable if the same number is specified.

Use Inclusion

false

true or false

The use of an inclusion matrix will trigger the DOE to be extensible as it tries to fill in the space already covered by the existing data set.