  Due to their compliant structure, industrial robots without
precision-enhancing measures are only to a limited extent suitable for
machining applications. Apart from structural, thermal and bearing
deformations, the main cause for compliant structure is backlash of
transmission drives. This paper proposes a method to improve trajectory
tracking accuracy by using secondary encoders and applying a feedback and a
flatness based feed forward control strategy. For this purpose, a novel
nonlinear, continuously differentiable dynamical model of a flexible robot
joint is presented. The robot joint is modeled as a two-mass oscillator with
pose-dependent inertia, nonlinear friction and nonlinear stiffness, including
backlash. A flatness based feed forward control is designed to improve the
guiding behaviour and a feedback controller, based on secondary encoders, is
implemented for disturbance compensation. Using Automatic Differentiation, the
nonlinear feed forward controller can be computed in a few microseconds online.
Finally, the proposed algorithms are evaluated in simulations and
experimentally on a real KUKA Quantec KR300 Ultra SE.
