The robot’s transfer function (motor + linkages) continually changes due to the nonlinearities in the robot.
* These nonlinearities include changing inertial loads, coupling between joints, changes in gravitational torque, gear backlash, shaft eccentricity, mass imbalance, inherent vibrations and friction.

The transfer function of a robot joint/linkage system changes with
configuration due to varying inertial and gravitational loads. Unless the control system corrects for these non-linear dynamics, the
response and stability of the joint controllers change with configuration.

The dynamic effects which have the most impact on control loop stability are due to changing mass or configuration.

* The torque required to balance gravitational load changes as the configuration of the manipulator changes.

* The inertias of the robot linkages, as seen by an actuator, change rapidly as the configuration changes.
* The inertia changes whenever an object is picked up or put down.

Increasing the inertia, reduces the open-loop gain (Ko)and shifts the left pole (- α) towards the origin with the result that a critically damped system becomes underdamped.

The two poles that were located together on the real axis, in the closed-loop transfer function, move apart and become complex. Increasing the friction moves the open-loop pole away from the origin, and the closed-loop response is overdamped.