Fuzzy logic copies this feature of human decision making using levels of possibility in a number of uncertain (or Fuzzy) categories. For example, think about the coupled tanks system (see Elke's white paper on Coupled Tanks systems on the download page of www.control-systems-principles.co.uk for full details) in which the object is to adjust the input voltage, u, to the pump motor (Figure 1) so that the level in tank 2 is held at a steady value. the measured output is the level in tank 2, denoted by the signal y2. my colleague Elke would apply a PI controller to this system as fast you could say 'the hills are alive with the sound of music'. But if the exact level is not important then why not use a simpler controller? For example, a common sense controller could use the following fuzzy control rules: