Dynamic simulations of an industrial robotic manipulator with joint friction and joint elasticity

Manipulators for laser welding need to be capable of tracking a specified seam with an accuracy in the order of 0.1 mm at speeds exceeding 100 mm/s. A framework for realistic dynamic simulations has been set up to study the applicability of industrial robotic manipulators for such tasks. These simulations describe the closed loop system of the mechanical robot arm and the (digital) controller. Unfortunately, high accuracy simulations usually require large computer loads. In earlier papers we discussed a so-called perturbation method [1] with subsequent application of modal techniques [2]. This proved to be an accurate and computationally efficient tool for simulations of the controlled trajectory motion of manipulators with flexible links. We also illustrated the use of the method for the motion of a rigid manipulator with friction [3]. In the present paper simulations of a manipulator with both joint friction and joint elasticity are considered. Parameters for mass and friction properties of an industrial robotic manipulator are taken from previous work. Joint elasticity is modelled using joint elements with three elastic degrees of freedom in each joint: one degree of freedom in the driving direction and two bending directions. Compliances are estimated from the first eigenfrequencies of the robot. At first, a nonlinear finite element method is used [4] in which the motion of flexible manipulator mechanisms is described by relative degrees of freedom, which are actuator joint coordinates and (small) elastic deformations. Next, the two-step perturbation method is applied. In the first step the nominal (desired) motion of the rigidified manipulator is computed. In the second step the deviations from this nominal motion are modelled as first-order perturbations. The results from the latter simulations are compared with those from the very time-consuming non-linear simulations.