5.1
General Design Process:
The steps listed below can describe the general design process:
- Develop a set of specifications to formulate a clear and precise problem statement.
- Propose a possible set of top-level designs to solve problem.
- Decide on the most feasible or promising design.
- Develop solution(s) (build circuits or create computer simulations for testing).
- Optimize critical parameters of the design(s).
- Test design(s) against specifications.
- If design performance is not satisfactory, return to steps 2 through 5 (depending on the amount redesign necessary to achieve satisfactory performance).
- If design performance is satisfactory, complete documentation on the design process, technical description of critical components, and performance analysis.
The preproposal involves completing steps 1 through 3 and creating a time table for accomplishing steps 4 through 8.
The final documentation should be complete enough for someone else to clearly see how the design’s performance compares with the criterion in the problem statement, repeat your design, and verify your performance results.
Top-Down Design:
The top-down method develops a design by initially proposing a solution in terms of function block diagrams. This first step is called the top level. Then each functional block in the top level is broken down into more detailed function blocks to create the second level. Once the function blocks are detailed enough so that a circuit to perform the function is easily determined, the process is finished.
Example: Build a hearing aid with the following specifications .....
The necessary functions (convert sound waves to electric signal, amplify signal, filter signal, convert amplified andfiltered signal back into sound waves). This suggests the following top-level design.
Specifications for the function and input-output characteristics must be developed for each block.
At the second-level design each of above functional block are broken down into finer detail. For example, assume the transfer function of the filter block is specified. The top-level design for the filter (second level for the entire design) can be specified as (parallel approach - something in the specifications may suggest this approach):
BPF1 - The purpose of band-pass filter 1 is to help shape the transfer function in the frequency range .... so at f=40 a 4 dB gain exists, and f= ....
(Do this for each block at this level).
The top-level design for the filter (second level for the entire design) can also be specified as (series approach - something in the specifications may suggest this approach):
Low pass- The purpose of the low pass filter to shape the frequencies in the range ... so that at f = 15000 the gain is 30 dB and at ....
(Do this for the rest of the filters).
Integration issues: Since each filter effects the entire frequency range, you also need to discuss how, for example, the high-pass and band-reject filters may affect the low-pass filter’s range and how you might use this or minimize its effect. Discuss this in general terms.
Time Tables:
Once the top-level diagrams have been completed, work can be assigned to determine the actual circuits, integrate them at the top level, optimize performance, test against specifications, redesign, and complete the final report.
The most efficient way to assign the work is to have as many parallel tasks going on as possible. If someone is forced to do nothing until some else finishes his or her task, the work should be redistributed. Rather than have someone in the group remain in active for an extended time, you can at least assign several people to the same task, or work independently on the same thing and take the best result.
Regular group meeting should be scheduled to keep tabs on the progress, to educate each other on your findings, and make adjustments if necessary (almost always necessary).
You should schedule tocomplete and test the design one week before the due date, so the last week can be devoted to minor modification (if needed) and completing the final report.
Engineering Notebooks:
It is important to develop organized work habits. As tasks become more complex, it is more difficult to remember results determined along the way. Writing down your work helps you to:
- focus on the task at hand and organize your thoughts
- verify what you have accomplished and when
- summarize the important results for future reference
- record work useful for debugging and completing final documentation
The entries must be dated and chronological - no appendices. If errors are made, indicate so in your next entry and correct for them. Do not tear out pages that are in error. Loose pages can be attached to pages in your notebook, if work was done on different paper or printed out from a computer.
Specifications and Other Criteria (Problem Statement):
Unlike homework and test problems, design problems are typically under-specified. This implies there can be many solutions. It becomes difficult to judge a design in terms of right and wrong, but instead the judgment is in more vague terms such as bad, good, and better (best is often difficult to prove even for simple problems).
The given specifications should be organized at the beginning of the project and stated in such a way that they can be measured when the final design is tested. Sometimes the specifications are given in terms of an error (or deviation from a target specification) that should be minimized (or goodness maximized) or bounds on the error that should be satisfied.
The problem statement should be as short as possible, while clearly stating all the specified criteria,along with some additional criteria if necessary to limitpossibilities or to help in creating an accurate way of computing the error.