2. Fundamentals
2.1. Definitions. Consider the rotational inverted pendulum
mounted to a DC motor as shown in Figure 1. The DC motor
is used to apply a torque ô1 to Arm 1. The link between Arm
1 and Arm 2 is not actuated but free to rotate. The two arms
have lengths L1 and L2. The arms have masses m1 and m2
which are located at l1 and l2, respectively, which are the
lengths from the point of rotation of the arm to its center
of mass. The arms have inertia tensors J1 and J2 (about the
centre of mass of the arm). Each rotational joint is viscously
damped with damping coefficients b1 and b2, where b1 is
the damping provided by the motor bearings, and b2 is the
damping arising from the pin coupling between Arm 1 and
Arm 2.
A right hand coordinate system has been used to define
the inputs, states, and the Cartesian coordinate systems 1 and
2. The coordinate axes of Arm 1 and Arm 2 are the principal
axes, such that the inertia tensors are diagonal of the form