Design Process and Modeling

The System Engineering Method

Mission Authority > Start

  1. Customer[1] Expectations (Project Objectives and Mission Profile)
  2. High Level Requirements (Level 1 Program/Project)
  3. Functional and Logical decompositions (Project WBS)[2]
  4. Trade Studies and Iterative Design Loop
    1. Form Creative Design Solution (System PBS)
    2. Define Level 2 System and Subsystem Requirements
    3. Make Hardware and/or Software Model(s) and Perform Experiments
    4. Organize and Analyze Data
    5. Does Functional & Performance Analysis show design will meet Functional Design and concept of operations (ConOps) Requirements?
    6. If additional detail need, Repeat Process
  5. Select a preferred design
    1. Does the system work[3] (performance)?
    2. Is the system achievable within cost and schedule constraints?
    3. If the answer is no, adjust Customer’s Expectations (Step 1) and start again.
  6. Communicate Results (PDR and CDR)
  7. Preparing presentations (PDR and CDR)
  8. Reports, plans, and specifications. (Project Planning)
  9. Implement the design. (Project Implementation)
[1] NASA introduces the term Stakeholders at this time, a term that encompasses both the customer and individuals directly or indirectly effected by the project. Due to the introductory nature of this course, I will simply use the term customer.  
[2] See Week 1 Job Descriptions
[3] This includes determining if the system is safe and reliable.

The Design Process

Design evolves through analysis and synthesis.

  • Webster’s definition of design: to conceive and plan out in the mind; to build, create, fashion, execute, or construct according to a plan
  • analysis: to divide a complex whole into its parts or elements; separating or distinguishing the component parts of something (as a substance, a process, a situation) so as to discover its true nature or inner relationship
  • synthesis: the composition or combination of parts or elements so as to form a whole

  • Design is an iterative process where the engineer must analyze the design (i.e., break apart, deconstruct), identify areas of greatest uncertainty, study (rapid prototype) possible solutions, and along the way eliminate poor or unsuitable solutions.

  • Once the parts of a design are understood, the design can be synthesized (i.e., put back together, reconstructed) and the design studied as a whole (i.e., at the system level).
  • Designs are evaluated based on the mission objectives and requirements.

Design Process – Iteration I

Design Process – Iteration II

Design Process – Iteration III

Design Process – Techniques

  • There are many techniques an engineer might use to determine if an idea has promise.
    • Draw a preliminary sketch of the design
    • Make a back of the envelope calculation
    • Conduct a trade-off study
    • Model the System

Design Process – Model the System

  • To facilitate the design process, engineers often rely on models.
  • A model simplifies a system or process so that it may be better studied, understood, and used in a design.
  • There are three common models used in engineering:
    • Mathematical
    • Computer Simulation
    • Physical Models
      • Full-scale Prototypes
      • Scale Models

Mathematical Models

  • Mathematical models usually consist of one or more equations that describe a physical system.
  • Many physical systems can be described by mathematical models. Such models can be based on scientific theories or laws that have stood the test of time, or they may be based on empirical data from experiments or observations.
  • In order to construct these mathematical models, simplifying assumptions are often made (e.g., the model system as an nth order constant coefficient linear differential equation).
  • Mathematical models are usually employed for simple systems. The difficulty in deriving the equations for complex systems outweighs their usefulness.

Computer simulation models

  • Computer simulation models allow engineers to examine complex systems.
  • These models typically incorporate many empirically1 based mathematical models as part of the total simulation model.
  • From these empirically based models a computer program is written.
  • This computer model is then subjected to many different simulated operating conditions.
  • Simulation programs used in EE400D include Solidworks, LTSpice, MATLAB

Physical models

  • Physical models have long been used by engineers to understand complex systems. They probably represent the oldest method of structural design.
  • Physical models have the advantage in that they allow an engineer to study a device, structure, or system with little or no prior knowledge of its behavior or need to make simplifying assumptions.
  • Full scale models are sometimes built, but most often they have been scaled down anywhere from 1:4 to 1:48.
  • Examples of studies made with physical models include:
    • Dispersion of pollutants throughout a lake.
    • Behavior of waves within a harbor.
    • Underwater performance of submarines of different shapes.
    • Performance of aircraft by using wind tunnels to simulate various flight conditions

Prototypes and Scale Models

  • Many designers use full-scale prototypes to test the operation of the design.
    • The prototype then helps the designer identify any weak areas of the design and hopefully how to improve upon them.
    • No idea should be discarded solely on the basis of one prototype or one test. Many great designs have been discarded prematurely and many working prototypes have failed to give acceptable products.

Indirect evaluation can be used as well, to evaluate a design. Scale models can be used to test aircraft design at a fraction of the cost of building a prototype.

  • Computer simulations and mathematical models may not be accurate enough to allow understanding of all the complexities of component interference or turbulence, but they still may be used to approximate the design of the first scale model for wind tunnel testing.