Simplified models such as the inverted pendulum model are often used in humanoid robot control because the full dynamics model of humanoid robots is too complex to design a controller. These models are usually derived from simple mechanical systems that represent the essential properties of the robot dynamics. This method for deriving simplified models is a manual process that heavily relies on the controller developer's intuition. Moreover, mapping the state and input between the original and simplified models requires model-specific code. This paper describes a general method for systematically obtaining simplified models of humanoid robots. We demonstrate an application of derived models to humanoid robot balance control using linear quadratic regulators.