## 2 Characteristics and Requirements  
Design requirements are based on non-technical needs and expectations of a user, and they are often formulated in non-technical terms. Engineers and researchers identify those requirements in a design process and describe them using mathematical models. Figure 1 shows a scheme for interpreting and classifying user design requirements into specific topics that can be modeled mathematically. It also introduces the concept of strict and soft design requirements according to user description. This classification is not unique and must be adapted to specific characteristics of design problems by considering fundamental aspects of engineering, as pointed out in [17]. In an initial design stage, engineers will focus on solving strict design requirements related to user needs. Therefore, these requirements must be formulated properly so that a design procedure guarantees that they can be fulfilled. Strict design requirements will be selected to produce a feasible device that can be manufactured. Once it is feasible to be built, it also must be capable of doing the task that a user requires. For this reason, the first classification on the left side of Figure 1 is for obtaining a feasible and working device. A first approach to formulate strict design requirements is by using inequality constraints. This approach can be applied directly to design parameters to bound them within practical upper and lower limits or indirectly by defining constraint functions in terms of the design parameters. In some cases, it is crucial to consider strong constraints to describe well-defined design prescriptions, and they can be formulated as equality constraints. For example, some mechanical aspects that are frequently described using equality or inequality constraints are related to stress, displacement, workspace, and mobility conditions. On the other hand, soft design requirements in the right side of the scheme presented in Figure 1, are related to user expectations that are based on the system's desired behavior or performance. These requirements can be adjusted not only according to different applications but also they can change as the development of the design process evolves. In some cases, different user expectations are dependent of each other, and as a consequence, they can be described using a single expression. However, in general, these requirements can compromise with each other. This characteristic means that an improvement of one user expectation can negatively affect another user expectation. For this reason, a formulation for soft design requirements must be flexible enough to consider the trade-off among different user expectations. Soft design requirements can be formulated using functions in terms of design parameters to evaluate desired criteria and constraints. It is possible to calculate a performance index for each selected criterion and to compare the results from all the computed solutions to select the best one. Some of the soft design requirements can be related to user expectations as for example reduction of energy consumption, increase of system performance, reduction of number of components, increase of safety, reduction of used materials, simplification of manufacturing process, and improvement of design cost, production, and operation. From this context, the herein presented Mechatronic Concurrent Design procedure is based on a formulation of a design problem as a constrained multi-objective optimization problem. This approach can consider conveniently multidisciplinary aspects to provide a synergistic combination of solutions for the involved design requirements. User needs and expectations must guide a design process in Mechatronics. However, the influence of the user's expectations can be limited by a set of constraints coming from a specific design problem.