Digital systems
Nonlinearity

Systems can contain significant nonlinearities. Some of them were presented in previous text, such as limitation (saturation), system asymmetry or a general function dependency (e.g. quadratic, sigmoidal or square root). Other examples are a relay characteristic, hysteresis, multiple hysteresis, dead zone, Coulomb friction, etc. The characteristic of saturation is very common at actuators (controller output) – for example, a controller computes desired heating water temperature to 250 °C, but physical limit is up to 100 °C, e.g. 80 °C. In that case, an output value is clamped to the limit. The system can contain multiple nonlinearities, e.g. the asymetry together with square root dependency in case of hydraulic system. Another nonlinearities in hydraulic systems can be caused e.g. by varying tank shape.

Fig. 8.13: Characterics for typical
Fig. 8.14: Hydraulic nonlinear one capacitor system