Introduction:
With careful use of the principles of Design for Sustainability (DFS), a company can keep a product fully functional and fielded for a much longer time. The best time to extend a product's lifetime is at the very beginning, at the design stage. It is here that it is most beneficial because it is the place where changes have the least impact on the total production cost. This article will outline some of the more important considerations of DFS.

I. Systems Level Approach:
The systems level, or Top-Down, approach is ideal for implementation of DFS in that each step in the development process progresses down the design hierarchy. By utilizing the Top-Down approach, the system is first described by major function or system, as in a block diagram. Once each major function or system has been thoroughly defined, all of the sub-systems, which are required to handle all of the inputs and outputs that define that particular block of the system, can then be defined. The use of Commercial-Off-The-Shelf (COTS) components aids in this approach as the sub-systems have already been designed and manufactured, so the engineer only has to make the interconnects. This approach lends itself to the next consideration: Modularity.

II. Modularity:
The three main tenets of the modularity approach are: ease of manufacturability, ease of upgrade, and ease of repair. By making a product modular in design, only one particular portion of the system may need to be upgraded at any given time. This increases the sustainability by reducing the re-design cycle. Modularity reduces time and cost to repair an item as the Line Replaceable Unit (LRU) becomes a part of the whole unit, instead of the unit itself. The third advantage to modularity is the ease of manufacturability. This advantage lies in the fact that a product can be designed to have many options that the customer can choose from without having to make custom modifications. A base unit, which would only need to be designed once, could accommodate any number of device options. Each option can then be designed independently and if changes to that design are needed, only that particular module is affected. An example of such a unit would be a universal power supply. Universal power supplies are those power supplies that have an input voltage and frequency range. Some universal power supplies also have the ability to be configured with different outputs. This is an ideal situation as a company's product can be upgraded at any time and used anywhere in the world. Economically, it is more advantageous to use a modular approach for product development because the costs of sustainability are greatly reduced.

III. Standard Parts:
The use of exotic, custom parts is one of the main causes of the shortening of the lifetime of a product. This is due to the fact that these custom parts are typically only made for short periods of time and are then discontinued. Using more standard parts and components increases the sustainability of a product as they would be more readily available. Using parts that are second sourced is also a good way to increase sustainability because there are more suppliers that make that part.

IV. Standard Busses and Interfaces:
The use of industry standard busses and interfaces allows for the incorporation of COTS components into a company's product. This allows for greater sustainability as industry standard busses and interfaces have been around for many years, ex: RS-232, RS-485, MIL-STD-1553, VME, CPCI, etc.

V. Hardware vs. Software Implementation:
The critical consideration, whether to implement a function in hardware or software, plays an important part in sustainability. For complex calculations and the ability to easily change the desired output, software is more beneficial. However, the cost of maintaining a software based system is potentially higher than a hardware solution because the code needs to be supported and maintained. To minimize these costs, a company can use a top-down, modular approach to code generation. This allows changes to be made quickly, and due to the modularity, only partial compilation is necessary. In addition, writing code that is processor independent makes porting the code to a different processor simple. Also, good, clear documentation of the source code is essential for long term sustainability. For those systems that are less complex, a hardware solution may be the best choice. A hardware implemented function may require more cost up front in the selection of stable, low-drift components so that tuning needs to be done only once. With no code to maintain, the hardware solution can be designed in a modular fashion that will make changing the functionality easier and increase sustainability.

VI. Associated Costs:
The costs associated with DFS are often called Life-Cycle costs. These costs come in five different categories: design, production, maintenance, logistics, and disposal. The design costs are only a small fraction of the Life-Cycle costs. Once a product has been designed, it can be built in quantity. Then, that product needs to be supported by both maintenance and logistics. The logistics costs can be very high if there are a lot of components in the device. Each of these components needs to be ordered and stocked.

There are also disposal costs. These costs can be minimal for simple systems, or can be very expensive for more complex systems. Another critical aspect of costs is in which cycle a change occurs; the further along in the process, the more the cost. Therefore, it is critical to catch any problems as early in the process as is possible. The cost of a unit's sustainability is lowest when problems are discovered early. The best way to make sure that problems get taken care of early is the subject of the next consideration: Inter-Disciplinary Communications.

VII. Inter-Disciplinary Communications:
One of the most important aspects of DFS is inter-disciplinary communications. It does a company no good to design a great product if it can not be manufactured. For maximum sustainability, it is best that all departments communicate with each other and meet face-to-face during the critical design stages so that there will be no confusion as to what the critical issues are. A good method of increasing inter-disciplinary communications is the building of Product Development Teams. These teams have representatives from each of the departments who stay in constant contact and let the rest of the team know of issues as soon as they arise.

Example:
An example of a system that has been designed for sustainability is LINK-16 (see figure 1). This is a military communications system that utilizes many different modes: Ground-Air, Air-Air, Air-Sea, etc. The main reason that the LINK-16 system is highly sustainable is that it is modular in nature, and was designed with that in mind.

Link 16 is designed to work with existing Army, Navy, and Air Force communications systems as well as to incorporate new systems. As each of the Armed Forces evolves, its communications needs will most assuredly change as well. If a branch of the Armed Forces wants to deploy a new communications system, it can do so by designing a new module for the LINK-16 system as well as re-fielding its existing systems to lower echelon elements, thus extending the sustainability of the overall system far into the future.

When a company exercises good DFS principles, it can be assured that its products will be sustained long into the future. If you have any questions, or would like advice on implementing Design for Sustainability practices, please call the EMPF Helpline at 610-362-1290