The inputs to the fuzzy system are angle error e (if the reference is the upright position then the angle error becomes the displacement from the upright position) and the change in angle error c. The output from the fuzzy system is the command voltage u. The universe of discourse of the variables was normalized to cover a range of [- 1, 1] and scaling gains (ge, gc, gu) were used to normalize. To simplify the implementation of the controller a standard choice for the membership functions was used with five membership functions for the three fuzzy variables (meaning 25 = 52 rules in the rule base) and symmetric, 50% overlapping triangular shaped membership functions (Fig. 3(b)), meaning that only 4 (22) rules at most can be active at any given time. The rules were determined heuristically based on the knowledge of the plant and the resulting rule table is shown in Fig. 3(a).