Up to now, fuzzy controllers with only one input and one output of the fuzzy system have been considered. The same arguments with respect to the degrees of freedom of a fuzzy system are also valid in the case of multiple inputs. A graphical representation of the characteristic is not as easy as in the 2D cases of section 17.2.2. Moreover, for the case of two inputs, a 3D representation is possible. For a fuzzy controller with a fuzzy system having two inputs and and one output one gets a band of characteristics in a 2D discrete representation, or a 3D representation with the output over the two inputs, as shown in Figure 17.12 for a PD-type of fuzzy controller.
A fuzzy controller is typically a classical controller using nonlinear characteristics. But the design and parametrisation is entirely different.