Nonlinear friction compensation methods for an in-parallel actuated 6-DOF manipulator

Presents simple but effective nonlinear friction compensation methods for a 3-PRPS (prismatic-revolute-prismatic-spherical joints) in-parallel 6-DOF manipulator. It is well known that an integral action in the feedback controller can remove the steady-state error due to constant disturbance. Thus, the integral-action can compensate coulomb friction, which is the DC-component of the total friction and viscous friction in some degrees. When the direction of position command reverses, the integrator output can not change the sign of its output instantaneously, due to friction at zero velocity, i.e. stiction resulting tracking errors. To overcome this problem we attempt to reverse the sign of the integrator output as the sign of velocity changes. The effectiveness of this approach is demonstrated by experiments on a 3-PRPS in-parallel manipulator. Also discussed are disturbance observer and velocity observer approaches for friction compensation