# An Extended Functional Design Approach Based on Working Principle and Property and Its Application
## 4 The Extended Functional Design Approach  
In this section, the extended functional design approach is proposed, as shown in Figure 5, which follows some representations of the FBS model [4,7]. Generally, the approach includes five processes (1-5) and integrates two design strategies. Specifically, strategy 1 is represented by process 1 (from function to structure), which could be realized via most of the existing functional design approaches discussed in Section 2. Meanwhile, strategy 2 is represented by processes 2, 3, and 4 (if necessary, one can also include process 5), which is characterized by the integration of the working principle and the property to realize the transformation from the desired function to the possible structure. Process 1--Design reuse process, from function to structure (F-S):  
This is a recalling and reusing process to find a structure solution to the desired function by function-oriented searching, a structure analogy, a bio-inspired analogy, etc. This process could be completed using most of the existing functional design approaches discussed in Section 2. Therefore, it will not be described in detail in this paper. Process 2--Mapping process, from function to working principle (F-WP):  
This is a divergent process to find the available working principles for the desired function. Some functions cannot be realized by direct design reuse or adaptation, which makes it necessary to consider how to generate a new structure solution, based on a working principle. There may be a one-to-many mapping relationship between a function and the working principles, and several working principles that have the possibility to achieve the desired goal may be found. For example, the function \"change position\" may correspond to working principles, including \"thermal expansion and contraction\", \"Hooke's law\", \"hydraulic transmission\", \"pneumatic transmission\", \"electromagnetic effect\", etc. For each  
Figure 5: The extended functional design approach, based on the working principle (WP) and property (P). trial, only one working principle should be selected as a step into the next process. If the selected working principle does not result in a satisfactory structure, then the designers can return to this process and select a different candidate working principle. Process 3--Analysis process, from working principle to property (WP-P):  
This is an analytical process to derive the appropriate properties, according to the selected working principle in Process 2. The working principle only has a chance to realize the desired function when it is applied to the correct property. For example, the thermal expansion and contraction principle is closely related to the thermal expansion property, which can be quantified by the thermal expansion coefficient. The larger the value of the coefficient, the more sensitive the material is to a temperature rise, and the more obvious the volume expansion. The working principle of a bimetallic strip in a common bimetal thermostat is the thermal expansion and contraction principle; metal sheets with different thermal expansion coefficients are deformed and displaced to connect or disconnect the circuit, thereby controlling the circuit. Process 4--Synthesis process, from property to structure (P-S):  
This is a synthesis process. After identifying the property, it is necessary to consider how to integrate this property into a complete structure. The identified property is necessary for realizing the desired function; a complete structure must have some other auxiliary properties. Referring to the bimetal mentioned above, in addition to its thermal expansion property, designers must consider other properties that are also needed. Process 5--Reformulation process, from structure to working principle (S-WP):  
This is an iterative process. Because Process 2 is a divergent process, there may be many available working principles for a function, and only one candidate working principle can be selected at a time.