EE/CpE322 Lecture 10. Bruce McNair Based on Chapter 11 Engineering Design: A Project-Based Introduction (the 3 rd ed.), by C.L. Dym and P.

Transcription

1 EE/CpE322 Lecture 10 Bruce McNair Based on Chapter 11 Engineering Design: A Project-Based Introduction (the 3 rd ed.), by C.L. Dym and P. Little

2 A Model of the Design Process

3 Design for X Design projects involve not only engineers Manufacturing experts, marketing and sales professionals, reliability experts, cost accounts, lawyers Such team are concerned with understanding and optimizing the product under development for its entire life Design, development, manufacturing, marketing, distribution, use, and eventually disposal Design for manufacturing and assembly is an important issue -- how to get high quality products to market quickly.

4 Design for Manufacturing Design for manufacturing (DFM) is based on minimizing costs of production and/or time to market for a product, while maintaining an appropriate level of quality One basic methodology consists of six steps: Estimate the manufacturing costs for a given design alternative Reduce the cost of the components where possible Reduce the cost of assembly Reduce the cost of supporting production Consider the effects of DFM on other objectives If the results are not acceptable, revise the design and try again

5 Design for Assembly Design for assembly (DFA) is related to DFM, but focuses specifically on the aspect of manufacturing in which the artifact or system is put together, i.e., assembled. There are a number of design choices that can impact assembly such as: Limiting the number of components to the fewest that are essential to the working of the finished product. Using standard fasteners and integrating fasteners into the product itself. Designing the product to have a base component on which other components can be located. Designing the product to have components that facilitate retrieval and assembly Designing the product and its component parts to maximize accessibility, both for manufacture and during repairs.

6 Design for Affordability Design for affordability, or engineering economics, is important in almost any project. The time value of money Money obtained sooner is more valuable than money obtained later Money spent sooner is more valuable than money spent later This is because the money owed in the future is subject to opportunity costs and risks. Opportunity cost is the term used to describe that the money spent now could have been used for something else (or at least put in a bank to earn interest). Risk is the notion that the money obtained in the future may be less useful, due to inflation or other factors, or may not be paid at all if the debtor s situation changes.

7 Design for Affordability (cont d) To address these concerns, economists have developed the notion of discounting. 1 PV = FV 1 r + where PV is the present value of the costs, FV is the future, r is the discount rate (e.g. 10% annually), and t is the time period (e.g. 5 years) A central notion in engineering economics is that we want to consider the full set of life cycle costs for a design, not merely the initial costs. Because we are often comparing designs that are not identical in their time frames, many engineers use Equivalent Uniform Annual Cost (EUAC) to do this t

8 Design for Affordability (cont d) Estimating costs is a key skill in understanding the life cycle costs of a design. Labor costs - wages directly paid to workers, social security costs, insurance and other benefits (e.g., retirement plans), etc. Material costs - costs of all the parts, and any inventory that is lost in the production process. Material. costs may be affected by volume discounts. Overhead costs - executive salaries, warehouse or factory costs, administrative services, etc. In many companies, overhead can constitute as much as % of the direct labor expenditure. Profit for various stakeholders - most companies want to make money on their products.

9 Design for Reliability Design for reliability is important for most products. Engineers often consider two related concepts, reliability and maintainability. Reliability is defined as the probability that an item will perform its function under stated conditions of use and maintenance for a stated measure of the variate (time, distance, etc.). Maintainability is the probability that a failed component or system will be restored or repaired to a specific condition within a period of time when maintenance is performed within prescribed procedures.

10 Design for Reliability (cont d) In practice, we think reliability in terms of probability of failure. Types of failures include in-service failure, incidental failure, and catastrophic failure. Designers are often concerned with the Mean Time Between Failures (MTBF). To reduce the consequences of failure, engineers often introduce redundancy into systems, using parallel parts. Series systems are like chains - when the weakest link fails, the system goes down. Parallel systems are like cables - all the parts in parallel must fail or the system will work. Economics often drives the choice. Engineers determine the probability and costs of failures and design parallel systems when failure is costly (in dollar or life terms).

11 Design for Sustainability Engineers have an ethical obligation to consider the environmental consequences of the things they design. This is often done by first determining the types of impacts, and then estimating the overall size and scope of the impacts. Environmental impacts of designed systems or artifacts is in terms of: Air quality Water quality and consumption Energy demands Waste streams

12 Design for Sustainability (cont d) Global warming is an important issue to be considered in the design process. Engineers will need to be prepared to incorporate new skills and techniques to compute the carbon footprint of their designs, and look for ways to reduce them. Lower carbon footprint can be an attractive and marketable design attribute Environmental life cycle assessment has three essential steps: Inventory analysis list all the inputs and outputs as well as any intermediate products associated with the design Impact analysis determine all the effects on the environment and measure or estimate the magnitude of the effects Improvement analysis determine the needs and opportunities to address adverse effects found in the first two steps

13 Design for Quality Quality is the concept which unites all the -ilities. Quality can be defined as fitness for use. Among the tools used to incorporate quality into design are: flowcharting statistical process control benchmarking quality function deployment (QFD) QFD refers to a graphical way of organizing all the major relationships among stakeholder interests, desired design attributes, measures and metrics, targets, and current products or designs.

14 Design for Quality (cont d)

15 Design for Quality (cont d)