Designing with Optimization
This section provides an overview of using optimization in the computer aided design process enhanced by Working Model FEA.
Designing with optimization is an efficient method to obtain improved feasible designs early in the design process of a part. This technology holds promise to enhance assembly-based design optimization. Further information on optimization may be found here.
Designing with optimization may be divided into the following steps:
- Define the design space and boundary conditions (CAD, Working Model FEA)
- Determine a design concept (Working Model Concept)
- Define the parametric geometry model (CAD)
- Validate the design performance (Working Model FEA)
- Fine-tune the design (Working Model Shape)
- Validate the final design performance (Working Model FEA)
1. Define the design space and boundary conditions
Defining the design space may be as simple has determining a bounding box which encapsulates the region in space where the part will exist. To do so, the designer must know how the part will function as well as any space constraints. Defining the design space includes selecting the material the part will be constructed from.
Defining the boundary conditions is the most crucial and ambiguous step in the design process. The designer is required to know how the part will interact with other parts and then define where and how the part is loaded and restrained from moving.
The CAD system is used to define the design space. Working Model FEA is used to apply loads and restraints to the design space.
2. Determine a design concept
The design concept is simply an idea. The design concept is a portion of the design space that performs the functions and satisfies the intentions determined in Step 1. Every design literally has an infinite number of design concepts. The challenge of this step is to determine a concept, or idea, that works best under other design constraints. These design constraints may be weight, manufacturing, cost, stress, displacement, frequency, vibration.
Working Model Concept incorporates the output from Step 1 with advanced optimization technology to determine a design concept and provide an idea for the designer. The advantages of this technology are enormous. It provides efficient design ideas that would not have been considered by the designer.
Note: When creating the finite element mesh (a mathematical representation of the geometry) for Working Model Concept, it is best to use a small element size. Though this will require more time to perform the concept optimization, the results will provide greater detail.
3. Define the parametric geometry model
The result of Working Model Concept is a design idea. This idea is conveyed in the form of a new mesh which may be traced, or simply used as a visual guide (in VRML format), when creating the parametric geometry model in the CAD system.
Moving from idea to geometry is not a simple task (though it is greatly simplified when starting with an idea from Working Model Concept). This is when the designer uses his expertise and CAD skills to create a parametric geometry model that represents the design concept and continues to satisfy the design criteria for the part.
4. Validate the design performance
Verifying the performance of a part ranges from detailed (and expensive) prototyping and physical testing to computer aided engineering technology such as motion simulation and finite element analysis (FEA). At the core of Working Model FEA is the ability to simulate the part in a defined working environment. Similar to Step 1, the designer defines how the part is loaded and restrained. Working Model aides the designer in determining loads on parts in an assembly.
The Design Doctor feature of Working Model FEA will help the designer validate the design. If the design does not satisfy the established criteria (the stress is too high, there is too much frequency) or it satisfies the criteria inefficiently (the stress is much less than the allowable), then Working Model Shape should be used to fine tune the design.
Note: When creating the finite element mesh for Working Model FEA, care should be given when selecting the size of the elements. Though larger elements are faster, they typically have larger errors. It is generally wise to select a small element size (perhaps the same size used for Working Model Concept) for more accurate results.
5. Fine-tune the design
Working Model Shape is used to make a bad design good, and a good design better. Shape optimization uses the dimensions of parametric geometry as design variables to alter the design. In doing so, it strives to obtain an objective function (such as minimize weight) while satisfying design constraints on simulation responses (such as stress, displacement, frequency, and buckling load factor).
Working Model Shape provides new values for the dimensions used as design variables (along with other diagnostic information). Working Model Shape will also automatically update the geometry in the CAD system. This feature provides the designer an easy method to apply geometry changes and immediately see the results.
Shape optimization is very sensitive to the inputs. If it is unable to converge to an optimal design, consider reducing the number of design variables, changing the mesh, or even changing the initial design. Furthermore, shape optimization is restricted to a single design concept. It may not be used to produce topology changes (such as edges or faces that are created or destroyed when a dimension changes) nor does it support discrete parameters (such as the number of holes).
Note: When creating the finite element mesh for Working Model Shape, it is best to use a large element size. The reason for this is for performance and to avoid mesh distortion, especially around curves. Shape optimization stretches and squeezes the elements which may cause excessive distortion and invalid results. To overcome errors that will be introduced by using larger elements, adjust the design constraints. For example, if the maximum stress is 35 ksi with a small element size and 25 ksi with a larger element size, then a stress constraint should be reduced in a similar manner (a 60 ksi stress constraint would be reduced to 60*25/35 = 43 ksi).
6. Validate the final design performance
If a larger element size was used to fine-tune the design with Working Model Shape, the new design will need to be validated again to insure the design criteria is still satisfied. This is a good idea even if the same element size was used for shape optimization because poor element quality may have been introduced into a mesh that has been stretched and squeezed.
Create a new mesh using the same element size that was used in Step 4. After performing the simulation, verify the results. If the design criteria is not satisfied, further use of Working Model Shape may be needed in the problem areas.