Threads of reasoning: a case study

In the design of technology intensive products like copiers, wafer steppers and televisions, one searches for a product that satisfies the product requirements as well as the business drivers. The main need in an early design phase is to bring structure in the typical chaos of uncertainty and the huge amount of realization options present. Potential realization choices all have advantages and disadvantages, which cause tensions and conflicts. The earlier the (essential) conflicts and tensions are identified, the better it is. Turning them from implicit to explicit helps the system architect in making the trade-off consciously or at least in selecting the most important tensions and conflicts that require further in-depth investigation. In this respect we demonstrate the effectiveness of a technique called "threads of reasoning". The illustrative case study is the design of the paper flow control (sensors, actuators, control architecture, etc.) in a high-volume copier/printer.